July 2014

NASA/TM2014-218505/Volume I (Corrected Copy) 

NESC-RP-13-00852 

Evaluation of Agency Non-code Layered 

Pressure Vessels (LPVs) 

William H. Prosser/NESC 

Langley Research Center, Hampton, Virginia 
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA scientific and technical information (STI) 

program plays a key part in helping NASA maintain 

this important role. 

The NASA STI program operates under the 

auspices of the Agency Chief Information Officer. 

It collects, organizes, provides for archiving, and 

disseminates NASA’s STI. The NASA STI 

program provides access to the NASA Aeronautics 

and Space Database and its public interface, the 

NASA Technical Report Server, thus providing one 

of the largest collections of aeronautical and space 

science STI in the world. Results are published in 

both non-NASA channels and by NASA in the 

NASA STI Report Series, which includes the 

following report types: 

 TECHNICAL PUBLICATION. Reports of

completed research or a major significant phase

of research that present the results of NASA

Programs and include extensive data or

theoretical analysis. Includes compilations of

significant scientific and technical data and

information deemed to be of continuing

reference value. NASA counterpart of peerreviewed formal professional papers, but

having less stringent limitations on manuscript

length and extent of graphic presentations.

 TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or of

specialized interest, e.g., quick release reports,

working papers, and bibliographies that contain

minimal annotation. Does not contain extensive

analysis.

 CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

 CONFERENCE PUBLICATION.

Collected papers from scientific and

technical conferences, symposia, seminars,

or other meetings sponsored or cosponsored by NASA.

 SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

 TECHNICAL TRANSLATION.

English-language translations of foreign

scientific and technical material pertinent to

NASA’s mission.

Specialized services also include organizing 

and publishing research results, distributing 

specialized research announcements and feeds, 

providing information desk and personal search 

support, and enabling data exchange services. 

For more information about the NASA STI 

program, see the following: 

 Access the NASA STI program home page

at http://www.sti.nasa.gov

 E-mail your question to help@sti.nasa.gov

 Fax your question to the NASA STI

Information Desk at 443-757-5803

 Phone the NASA STI Information Desk at

443-757-5802

 Write to:

STI Information Desk

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076-1320
National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 23681-2199 

July 2014 

NASA/TM2014-218505/Volume I (Corrected Copy)

NESC-RP-13-00852 

Evaluation of Agency Non-code Layered 

Pressure Vessels (LPVs) 

William H. Prosser/NESC 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320

443-757-5802 

The use of trademarks or names of manufacturers in the report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration.
ERRATA

NASA/TM‒2014-218505/Volume I

Issue Date: 08/25/2014 

Corrected two table titles (Table 7.2.1.2-2 page 202 and Table 7.2.1.2-3 page 204) and updated 

List of Tables on page 7. 

Changed Version number on pages 1-193 from 1.0 to 1.1. 
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESC-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

1 of 193

Evaluation of Agency Non-code Layered 

Pressure Vessels (LPVs)

June 26, 2014

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

2 of 193

Report Approval and Revision History

NOTE: This document was approved at the June 26, 2014, NRB. This document was submitted 

to the NESC Director on July 21, 2014, for configuration control. 

Approved: Original Signature on File 7/21/14

NESC Director Date

Version Description of Revision Office of Primary 

Responsibility Effective Date

1.0 Initial Release William H. Prosser, 

NASA Technical Fellow 

for Nondestructive 

Evaluation

6/26/14 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

3 of 193

Table of Contents

Technical Assessment Report

1.0 Notification and Authorization ..................................................................................................... 8

2.0 Signature Page ................................................................................................................................ 9

3.0 Team List ...................................................................................................................................... 10

4.0 Executive Summary ..................................................................................................................... 12

5.0 Assessment Plan ........................................................................................................................... 14

6.0 Problem Description and Proposed Approach .......................................................................... 17

6.1 History of LPVs ................................................................................................................ 17

6.1.1 Original Patents, Manufacturers, and Benefits and Limitations of Construction 

Technique.......................................................................................................................... 17

6.1.2 ASME Boiler and Pressure Vessel Code .......................................................................... 19

6.1.3 Additional Risk Factors for NASA Non-code LPVs ........................................................ 23

6.1.4 LPV Failure History .......................................................................................................... 24

6.1.5 LPV Reliability Evaluation ............................................................................................... 28

6.2 Problem Statement ............................................................................................................ 29

6.3 Planned Tasks ................................................................................................................... 29

7.0 Data Analysis ................................................................................................................................ 30

7.1 Task 1 Summary ............................................................................................................... 30

7.1.1 Center Request for Information (RFI) Responses............................................................. 30

7.1.2 Summary of Previous or Ongoing LPV Related Work Identified at NASA Centers ....... 39

7.1.3 External User Survey ........................................................................................................ 59

7.1.4 Risk Assessment for Different LPV Regions ................................................................... 64

7.2 Task 2 – Limited Scope Testing and Analysis .................................................................. 74

7.2.1 LPV Material Evaluations Conducted at MSFC ............................................................... 75

7.2.2 WRS Modeling of LPVs ................................................................................................. 110

7.2.3 Evaluation of Two Digital Image Correlation Techniques to Detect LPV 

Deformations during Hydrotest ...................................................................................... 123

7.2.4 AE Testing of a Multilayer Pressure Vessel ................................................................... 141

7.2.5 Feasibility of using Phased Array Ultrasound to Inspect the Shell-to-Head 

Circumferential Welds in NASA Pressure Vessels ........................................................ 149

7.2.6 AE Laboratory Test Specimen Design ........................................................................... 161

7.3 Assessment Team Comments on Proof Testing of LPVs ............................................... 169

7.3.1 Methods of Proof Test Assessment ................................................................................. 172

7.3.2 Proof Test Conclusions ................................................................................................... 175

7.4 Assessment Team Comments on LPV Replacement Strategy ........................................ 175

8.0 Findings, Observations, and NESC Recommendations.......................................................... 177

8.1 Findings .......................................................................................................................... 177

8.2 Observations ................................................................................................................... 181

8.3 NESC Recommendations ............................................................................................... 181

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

4 of 193

9.0 Alternate Viewpoint ................................................................................................................... 184

10.0 Other Deliverables ..................................................................................................................... 184

11.0 Lessons Learned ......................................................................................................................... 184

12.0 Recommendations for NASA Standards and Specifications .................................................. 184

13.0 Definition of Terms .................................................................................................................... 184

14.0 Acronym List .............................................................................................................................. 185

15.0 References ................................................................................................................................... 187

16.0 Appendices (separate Volume 2) .............................................................................................. 193

List of Figures

Figure 5.0-1. Typical Large Facility LPVs ...................................................................................... 15

Figure 5.0-2. Cross-sectional Diagram of LPV ............................................................................... 15

Figure 6.1.5-1. Notional Bathtub Curve Illustrating Changes in Failure Rate versus Time/Cycles ... 29

Figure 7.1.2.3-1. MSFC Vessel V0256 with Welds Stripped for Inspection ......................................... 47

Figure 7.1.2.3-2. Longitudinal Crack in Outer Layer of V0256 ............................................................ 47

Figure 7.1.2.3-3. PAUT Scan of Head-to-Shell Weld Region ............................................................... 49

Figure 7.1.2.3-4. PAUT Scan of Head-to-Shell Weld Region ............................................................... 49

Figure 7.1.2.3-5. PAUT Scan of Shell-to-Shell Weld Region ............................................................... 50

Figure 7.1.2.3-6. PAUT Scan of Shell-to-Shell Weld Region ............................................................... 50

Figure 7.1.2.3-7. PAUT Scans for Indication in C5 Weld O to N Location, Indicative of Slag ............ 54

Figure 7.1.2.3-8. PAUT Scans for Linear Indication in C5 Weld O to P Location, Indicative of 

Lack of Fusion ............................................................................................................ 55

Figure 7.1.2.3-9. PAUT Scans for Visible Crack in Outer Layer of V0256 .......................................... 56

Figure 7.2.1.1-1. Vessel V0032 before Sectioning, Shown with Sister Vessels V0030 and V0035 ..... 78

Figure 7.2.1.1-2. Vessel V0125 Shown after Sectioning ....................................................................... 78

Figure 7.2.1.2-1. V0032 Gross Sectioning Plan..................................................................................... 90

Figure 7.2.1.2-2. V0125 Sectioning Cut Plan ........................................................................................ 90

Figure 7.2.1.2-3. V0032-1 Sectioned for Metallographic Microstructural Examination for 

Determining Material Orientation .............................................................................. 91

Figure 7.2.1.2-4. V0032-1 Cut Plan for Metallographic Microstructural Evaluation for 

Determining Material Orientation .............................................................................. 92

Figure 7.2.1.2-5. Microstructural Evaluation at 50× Magnification of A225 Material from Vessel 

V0032 showing Banding of Pearlite and Ferrite on Meridional Face ........................ 93

Figure 7.2.1.2-6. Cut Plan for V0032 Charpy Impact Samples ............................................................. 95

Figure 7.2.1.2-7. Charpy V-notch Impact Test Results for V0032 A225 Head Material ...................... 97

Figure 7.2.1.2-8. Partial Cut Plan for Fracture Toughness and Tensile Test Specimens from A225 

Head Material from V0032 Showing Salient Features of Full Cut Plan .................... 98

Figure 7.2.1.2-9. Comparison of Fracture Surfaces for Fracture Toughness Tests Performed in 

C-L Orientation and L-C Orientation ....................................................................... 100

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

5 of 193

Figure 7.2.1.2-10. Master Curve Plot for 1143 Material........................................................................ 105

Figure 7.2.1.2-11. Master Curve Plot for 1146 Material........................................................................ 106

Figure 7.2.1.3-1. Microhardness Traverses Taken across the V0125 Head-to-Shell Weld ................. 108

Figure 7.2.1.3-2. Microhardness Traverses Illustrating Hardness Response Differences in Inner 

Layer and First Thinner Layer in V0125 .................................................................. 108

Figure 7.2.1.3-3. Cross-section for V0032-11, Evaluated by RT for Longitudinal Shell 

Welds Shown ............................................................................................................ 109

Figure 7.2.2-1. Four-layer Tank Considered for Weld Residual Stress Analysis ............................. 111

Figure 7.2.2-2. VFT Weld Modeling System ................................................................................... 113

Figure 7.2.2-3. Axisymmetric Weld Models and Meshes................................................................. 113

Figure 7.2.2-4. Stress Strain Curves Used for Mechanical Portion of Analysis ............................... 115

Figure 7.2.2-5. Hoop WRS Predictions for Layered Shell-to-Shell Weld ........................................ 116

Figure 7.2.2-6. Axial WRS Predictions for Layered Shell-to-Shell Weld ........................................ 117

Figure 7.2.2-7. Hoop WRS Predictions for Layered Shell-to-Head Weld ........................................ 117

Figure 7.2.2-8. Axial WRS Predictions for Layered Shell-to-Head Weld ........................................ 118

Figure 7.2.2-9. Line Path Definitions for Through Thickness Stress Plots ...................................... 119

Figure 7.2.2-10. Shell-to-Head Weld: Line Plot from ID to Outer Diameter (OD) along 

Centerline ................................................................................................................. 119

Figure 7.2.2-11. Shell-to-Head Weld: Line Plot from ID to OD along Layer Path 2-3 ...................... 120

Figure 7.2.2-12. Comparison of API-579 Solutions to Calculated Results for Both Shell-to-Shell 

and Shell-to-Head Tank Cases: Weld Centerline ..................................................... 121

Figure 7.2.2-13. Comparison of API-579 Solutions to Calculated Results for Both Shell-to-Shell 

and Shell-to-Head Tank Cases: Layer 2-3 Path ....................................................... 122

Figure 7.2.3.1-1. Pretest View of PV0236 ........................................................................................... 125

Figure 7.2.3.1-2. NDE Results Summarizing Known Vessel Flaws for Vessel PV0236 .................... 126

Figure 7.2.3.3-1. Test Pressure Vessel Cleaned and Painted White .................................................... 128

Figure 7.2.3.3-2. Application of Vinyl Stencil and Spray-painting Process on Vessel ....................... 129

Figure 7.2.3.3-3. Pressure Vessel with Completed Dot Pattern ........................................................... 129

Figure 7.2.3.3-4. Two Views Depicting Placement of Digital Targets on Pressure Vessel ................. 130

Figure 7.2.3.3-5. Aramis Camera Pair in Test Configuration .............................................................. 131

Figure 7.2.3.4-1. Aramis Three-Dimensional Fringe Plot of Computed Radial Values on 

Pressure Vessel ......................................................................................................... 132

Figure 7.2.3.4-2. Vessel Point Locations and Respective Displacement Plotted versus Stage 

Number ..................................................................................................................... 134

Figure 7.2.3.4-3. Color Fringe Overlay of Axial Displacement (X-direction) at 4,200 psi ................. 135

Figure 7.2.3.4-4. Selected Axial Section Lines on Color Overlay of Axial Displacement 

(X-direction) at 4,200 psi ......................................................................................... 136

Figure 7.2.3.4-5. Average Computed Radius of a Best Fit Cylinder from Aramis Data Results 

from Second Pressure Test ....................................................................................... 137

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

6 of 193

Figure 7.2.3.4-6. Screenshot of Tritop Graphical User Interface Showing Pressure Vessel Point 

Cloud with Best-fit Cylinder .................................................................................... 138

Figure 7.2.3.4-7. Displacement Vectors Overlaid on Coded Target Locations at Pressurized End 

of Vessel ................................................................................................................... 139

Figure 7.2.3.4-8. Displacement Vectors Overlaid on Coded Target Locations at Sealed End of 

Vessel ....................................................................................................................... 139

Figure 7.2.4-1. Drawing Detail of A. O. Smith Vessel PV0236 ....................................................... 142

Figure 7.2.4-2. Pressurization Plan ................................................................................................... 142

Figure 7.2.4-3. Sensor Placement ..................................................................................................... 143

Figure 7.2.4-4. Crack Locations ........................................................................................................ 144

Figure 7.2.4-5. Crack Locations (continued) .................................................................................... 144

Figure 7.2.4-6. Corrosion between Layers on MSFC LPV V-32 ..................................................... 145

Figure 7.2.4-7. Location Event Amplitudes versus Time ................................................................. 146

Figure 7.2.4-8. Location Calibration Plots ........................................................................................ 147

Figure 7.2.4-9. Location Plot ............................................................................................................ 147

Figure 7.2.4-10. Trending Plots .......................................................................................................... 148

Figure 7.2.5.1-1. Examples of Common Operating Modes using a Phased Array Probe .................... 150

Figure 7.2.5.2-1. View from either Side of Calibration Block Showing SDHs ................................... 151

Figure 7.2.5.2-2. View from Underside of Calibration Block Showing the Three RDHs in 

the Weld ................................................................................................................... 152

Figure 7.2.5.2-3. Ultrasonic Velocity Measurement Locations in First Test Block ............................ 152

Figure 7.2.5.3-1. View of the Probe and Wedge used to Conduct Phased Array Half-V Sectorial 

Exams ....................................................................................................................... 154

Figure 7.2.5.3-2. S-scans of Three SDHs on Head Side of Weld and A-scan of Each Hole at 

Angle of Peak Response ........................................................................................... 155

Figure 7.2.5.3-3. S-scans Showing Lower Two SDHs on Multilayered Side of Weld and A-scan 

of Each Hole at Angle of Peak Response ................................................................. 156

Figure 7.2.5.3-4. S-scans of RDH#2 Detected with Probe Flush with Weld and Skewed at 30° ........ 157

Figure 7.2.5.3-5. Sectorial Exam View of Three Radially Drilled Holes in Weld Root and A-scan 

of Each Hole at Angle of Peak Response ................................................................. 158

Figure 7.2.5.3-6. Two S-scans Showing Unidentified Indications Not Associated with Known 

Flaws or Part Geometry ............................................................................................ 159

Figure 7.2.5.4-1. Illustration of Full-V Beam Path to SDH#2 ............................................................. 160

Figure 7.2.6-1. AE Laboratory Specimen to be Tested from V0032 1146a Material ....................... 162

Figure 7.2.6-2. Bar Graph Comparing Measured Signal Energies from Different-sized 

1⁄4-inch-thick Plates ................................................................................................... 164

Figure 7.2.6-3. Early Waveform from Pencil-Lead Break on 36- by 72-inch Aluminum Plate ...... 165

Figure 7.2.6-4. Early Waveform from Pencil-Lead Break on 3- by 6-inch Steel Plate .................... 165

Figure 7.2.6-5. Early Waveform from Pencil-Lead Break on 4- by 8-inch Steel Plate .................... 166

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

7 of 193

Figure 7.2.6-6. Early Waveform from Pencil-Lead Break on 6- by 12-inch Steel Plate .................. 166

Figure 7.2.6-7. Full Waveform from Pencil-Lead Break on 36- by 72-inch Aluminum Plate ......... 167

Figure 7.2.6-8. Full Waveform from Pencil-Lead Break on 3- by 6-inch Steel Plate ...................... 168

Figure 7.3-1. Proof Test Concept ................................................................................................... 171

Figure 7.3-2. Proof Test Concept with Proof and Operational Critical Crack Size Curves ........... 172

List of Tables

Table 7.1.2.3-1. Head-to-Shell Weld .................................................................................................... 52

Table 7.1.2.3-2. Shell-to-Shell Welds................................................................................................... 52

Table 7.1.4-1. LPV Regions or Operating Conditions that Present the Highest Risk for Potential 

Failure for NASA Non-Code LPVs ........................................................................... 66

Table 7.1.4-2. LPV Regions or Operating Conditions with Lower Risk for Potential 

Failure for NASA Non-Code LPVs ........................................................................... 72

Table 7.2.1-1. Definition of Terms Specific to LPV Materials Testing Discussed in this Section ... 75

Table 7.2.1.1-1. Summary of Vessels Evaluated .................................................................................. 79

Table 7.2.1.1-2. Typical Material Properties for LPV Materials .......................................................... 79

Table 7.2.1.1-3. Comparison of Tensile Data Obtained from V0032 against A. O. Smith 

Certification and Lot Acceptance Values ................................................................... 80

Table 7.2.1.1-4. ICP Analytical Chemistry Composition Results for V0032 Inner Shell Material 

Compared with 1146a Specification Requirements ................................................... 82

Table 7.2.1.2-1. Charpy V-notch Impact Energy Results for V0032 A225 Head Material.................. 96

Table 7.2.1.2-2. T0 Transition Reference Temperature Data for 1143 Material from 

V00125 in Bounding Material Orientation ..............................................................102 

Table 7.2.1.2-3. T0 Transition Reference Temperature Data for 1146 Material from 

V00125 in Bounding Material Orientation .............................................................. 104 

Table 7.2.3.2-1. Vessel Stress, Strain, and Deformation Computations at Two Pressure 

States based on Modulus of Elasticity of 2.95 E+07 psi .......................................... 127

Table 7.2.3.4-1. Data-indicated Pressure Values Associated with Stages Recorded in 

Aramis ...................................................................................................................... 133

Table 7.2.4-1. AE System Parameters ............................................................................................. 143

Table 7.2.5.2-1. Wave Velocity and Relative Signal Loss Measurements taken from Test Block .... 153

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

8 of 193

Technical Assessment Report

1.0 Notification and Authorization

In coordination with the Office of Safety and Mission Assurance (OSMA) and the respective 

Center Pressure System Managers (PSMs), the NASA Engineering and Safety Center (NESC) 

was requested to formulate a consensus draft proposal for the development of additional testing 

and analysis methods to establish the technical validity, and any limitation thereof, for the 

continued safe operation of facility non-code (i.e., not in compliance with the American Society 

of Mechanical Engineers Boiler and Pressure Vessel Code, Section VIII, Division 1, in effect at 

the time of construction) layered pressure vessels. 

The NESC Review Board approved the assessment plan on April 18, 2013. Dr. William H. 

Prosser was assigned to lead this assessment. The primary stakeholders for this assessment were

the OSMA, as represented by Owen Greulich, Technical Discipline Manager for Pressure 

Systems, and the respective Centers responsible for the safe operation of these vessels, as 

represented by the PSMs. The PSMs from each NASA Center were asked to participate as part 

of the assessment team by providing, collecting, and reviewing data regarding current operations 

of these vessels. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

9 of 193

2.0 Signature Page

Submitted by: 

Team Signature Page on File 7/25/14 

Dr. William H. Prosser Date

Signatories declare the findings, observations, and NESC recommendations compiled in the 

report are factually based from data extracted from program/project documents, contractor 

reports, and open literature, and/or generated from independently conducted tests, analyses, and 

inspections. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

10 of 193

3.0 Team List

Name Discipline Organization

Core Team

William Prosser NESC Lead; NASA Technical Fellow 

for Nondestructive Evaluation LaRC

Pravin Aggarwal Structures MSFC

Matt Adler Materials Jacobs ESSA (MSFC)

Gilda Battista Materials Jacobs ESSA (MSFC)

Douglas Fraser ARC PSM ARC

Pete Taddie KSC PSM KSC

Ernest Graham MSFC/MAF PSM MAF

Owen Greulich OSMA Discipline Manager for 

Pressure Systems HQ

Maan Jawad LPV Consultant Global Engineering 

and Technology, LLC

Edward Johnson MSFC PSM MSFC

Pat Johnston PAUT NDE LaRC

Kenny Le JPL PSM JPL

Son Le SSC PSM SSC

Max Leuenberger JSC/WSTF PSM WSTF

Nick Noparat GSFC/WFF PSM WFF

Cynthia Null NASA Technical Fellow for Human 

Factors ARC

Carlos Perez-Ramos LaRC PSM LaRC

Al Pulley Pressure Vessels MSFC

Ivatury Raju NASA Technical Fellow for Structures LaRC

Fayssal Safie NASA Technical Fellow for Reliability 

and Maintainability MSFC

Roy Savage MTSO Program Analyst LaRC

Keith Van Tassel JSC PSM JSC

Douglas Wells Materials MSFC

Stephen Wnuk GRC PSM GRC

Calogero Dirienzo Pressure Vessels GRC

Richard Wong AFRC PSM AFRC

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

11 of 193

Name Discipline Organization

Consultants

Alan Puchot PAUT NDE Southwestern 

Research Institute

Charles Duffer PAUT NDE Southwestern 

Research Institute

Bud Brust Weld Residual Stress Analysis

Engineering 

Mechanics 

Corporation of 

Columbus

James Joyce Fracture Mechanics U.S. Naval Academy 

(retired), AMA

Dave Dawicke Structures AS&M

Matthew Melis Photogrammetry GRC

Justin Littell Photogrammetry LaRC

Andrew Ring Photogrammetry Ohio Aerospace 

Institute

Claud Hunter Acoustic Emission Jacobs Technology

James Walker Acoustic Emission MSFC

Eric Madaras Acoustic Emission LaRC

Administrative Support

Linda Burgess Planning and Control Analyst LaRC/AMA

Jonay Campbell Technical Writer LaRC/NG

Tina Dunn-Pitman Project Coordinator LaRC/AMA

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

12 of 193

4.0 Executive Summary 

The NASA Engineering and Safety Center (NESC) was asked to perform an assessment related 

to the approximately 300 older, layered pressure vessels (LPVs) that are in service at a number 

of NASA facilities. Although NASA has not experienced a catastrophic failure of an LPV, the 

high pressures and large volumes of these vessels could produce severe consequences in the 

event of a failure. These vessels were fabricated prior to the adoption of layered fabrication

techniques as an acceptable construction methodology in the American Society of Mechanical 

Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) [ref. 1]. Additionally, there are 

variances in the materials, fabrication, and inspection techniques used in these vessels with 

respect to the BPVC at the time of construction, as well as the later adopted BPVC that provided 

for LPV construction. Thus, these vessels are non-code and, consequently, are not compliant 

with Occupational Safety and Health Administration (OSHA) regulations [ref. 2] as required for 

federal agencies [ref. 3]. Further, these vessels are typically more than 50 years old and, as such,

are at additional risk for age-related degradation, including fatigue, corrosion, and embrittlement, 

which can lead to fracture. 

The scope of the assessment was to formulate a consensus draft proposal for the development of 

additional testing and analysis methods to establish the technical validity, and any limitation 

thereof, of these vessels for continued safe operation. It should be made clear that the 

establishment of the current safety of these vessels, or their safety for continued operations for 

any period into the future, was not within the scope of this assessment. In performance of this 

assessment, three tasks were executed. The first was to review the inventory, usage conditions, 

and ongoing and proposed risk mitigation methods for LPVs at all NASA Centers and for select 

locations at other Government facilities and in private industry. The second task was to conduct 

limited scope testing and analysis necessary to formulate final recommendations. Major 

activities for this task were defined based on the results of the first task. The third and final task 

was to coordinate a consensus draft proposal for the development of additional testing and 

analysis methods to establish the technical validity, and any limitation thereof, of LPVs for 

continued safe operation. The output of this task forms the basis for the recommendations 

contained in this report. 

The assessment team found that the operation of these non-code LPVs does present an elevated 

level of risk compared with conventional code-compliant vessels. However, insufficient 

information is available to quantify that level of increased risk within the scope of this effort. 

The assessment team found that the Agency lacks a consensus approach to addressing the risks 

associated with the operation of these vessels. Also, although near-term actions are available and 

identified to reduce the risk of and consequences from catastrophic failure of LPVs, the level of 

risk reduction for many of these actions cannot be quantified. Longer-term efforts are required

and are identified for characterizing and mitigating the risk for LPVs, and these should be 

focused on the highest risk operating conditions and regions of these vessels identified by the 

assessment team. Several additional discipline-specific findings are provided specific to 

materials testing, structural analysis, and nondestructive evaluation (NDE) technique 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

13 of 193

developments that are applicable to better characterizing and reducing of LPV risks. The 

assessment team also observed that the Center Pressure Systems Managers (PSMs) who

participated on the assessment team demonstrated understanding of the technical issues and 

concerns associated with non-code LPVs and a willingness to engage across Centers to develop 

improved approaches to mitigating risks associated with the continued use of these vessels. The 

assessment team also observed that significant efforts are underway outside this assessment that 

directly relate to the understanding of and continued safe use of LPVs at Centers. 

The assessment team identified two sets of recommendations. The first set of recommendations 

consists of near-term actions that if enacted could reduce either the risk of or consequences from 

catastrophic LPV failure resulting from their continued use. Because of the lack of adequate 

materials property data, inspection methodologies, and analyses methods, the risks associated 

with the continued use of these vessels and the degree to which these recommendations would 

reduce that risk cannot be accurately estimated at this time. In addition, it is recommended that 

the Agency formulate and support an Agency-wide team to continue efforts to assess and reduce 

risks associated with LPVs and to develop a centralized database of information on the design, 

fabrication, materials, operation, inspection, maintenance, and repair of LPVs. 

The second set of recommendations comprises the longer-term activities proposed to improve 

understanding of the risks associated with continued use of LPVs and to develop Agency 

processes to mitigate these risks. The assessment team recommends that the Agency expand 

efforts to gather LPV materials property data. In doing so, it is recommended first that a 

statistical analysis be performed to determine the number of LPVs and associated materials 

testing specimens needed to provide adequate bounding materials property data. To address a 

high-risk operating condition, notably the potential operation of these vessels at temperatures 

where brittle fracture is possible, it is recommended that measurements of the temperatures 

associated with ductile to brittle fracture behavior (initiated in Task 2) be completed. These 

measurements are necessary to establish appropriate minimum design metal temperatures 

(MDMTs) for future operation of LPVs. Additionally, it is recommended that additional 

materials property data be gathered to support analysis methods to evaluate and utilize proof test 

methodologies and to establish critical crack sizes for NDE detection criteria. 

It is further recommended that the Agency expand efforts to develop and validate analysis 

methods to address the identified highest risk conditions and regions of LPVs. Using these 

analysis methods, it is necessary to assess the sensitivity of any results to uncertainties in the 

input variables, such as material properties, structural configurations, and weld residual stress 

conditions. It is recommended that analysis predictions of weld residual stresses for LPVs 

(initiated in Task 2) be completed and measurements made to validate these results. It is

recommended that an analytical framework be established to evaluate proof test logic for these 

vessels and, where proof testing cannot be applied, to analyze the leak-before-break (LBB) 

capability of the LPV system. To aid in NDE development and validation and remaining safe 

life determination, it is recommended that an analytical framework be established to evaluate 

critical initial flaw sizes. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

14 of 193

It is also recommended that the Agency develop and, where possible, validate a number of NDE 

techniques to address the highest risk conditions and regions of LPVs. These include the phased 

array ultrasonic technique (PAUT), which has shown promise for inspecting the critical full 

penetration circumferential welds in LPVs. Other techniques recommended for development and 

validation include photogrammetry, both for validation of analysis methods and potential flaw 

detection in shell sections; acoustic emission (AE) techniques for detection of flaw growth in 

welds, heads, and shell sections; and radiographic and low-frequency electromagnetic methods 

for potential flaw detection in welds, heads, and shell sections.

Lastly, the assessment team entertained significant discussion regarding whether to make any 

recommendations regarding the need for consideration of plans for replacement of these vessels, 

which would cost an estimated $1–2M per vessel and would result in ancillary cost and schedule 

impacts while a vessel undergoing replacement is off-line. In the end, the assessment team

stopped short of such a recommendation for a variety of reasons. However, all team members 

were in agreement with the obvious fact that replacement of these vessels would eliminate the 

additional risks associated with their continued use, in comparison with the better understood and 

more easily mitigated risks associated with monolithic code-compliant replacement vessels.

5.0 Assessment Plan

NASA has an active inventory at nine of its Centers or facilities of approximately 300 old, noncode LPVs in high-pressure service. Many of these are in close proximity to personnel and 

expose personnel and facilities to catastrophic consequences in the event of failure. The 

fabrication and inspection techniques in use at the time of construction for these older vessels 

were not included in the ASME BPVC; thus, these vessels are not compliant with the thencurrent ASME BPVC, or the 1968 ASME BPVC [ref. 4], as is now required by the Federal 

OSHA regulations (29 CFR 1910 and 1960, Occupational Safety and Health Standards) 

[refs. 2 and 3]. OSHA regulations do not provide for “grandfathering” of these older, non-code 

pressure vessels, which would allow these older vessels to be exempt from the existing 

regulations. Additionally, the layered construction technique makes the application of traditional 

NDE techniques, such as ultrasonic shear and straight-beam, radiography, and magnetic-particle 

inspection of the inner layers of these vessels, difficult if not impossible. This difficulty in 

detecting defects is further enforced when considering the types of NDE methods available and 

their detection capabilities at the time when these LPVs were constructed.

Figure 5.0-1 shows examples of typical large facility LPVs. Figure 5.0-2(a) shows a crosssectional diagram of an LPV with a monolithic head, and Figure 5.0-2(b) shows a cross section 

of an actual vessel where the layers, weld, and monolithic head are clearly visible. The head for 

these vessels can be monolithic, as shown, or composed of multiple layers. All of the NASA 

LPVs have monolithic heads. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

15 of 193

Figure 5.0-1. Typical Large Facility LPVs 

(a) (b) 

Figure 5.0-2. Cross-sectional Diagram of LPV ((a) monolithic head 

and (b) cross section of actual vessel) 

Although there has never been a catastrophic failure of this type of vessel at any NASA Center, 

there have been failures in industry (as described in Section 6.1.4) as recently as 2010, including 

at least one newer LPV that was ASME code-stamped. These NASA vessels have been in use 

for decades without an established Agency-wide-consensus technical basis for ensuring their safe 

operation. 

Inner

Shell

Layers

Head

Weld Vent hole

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

16 of 193

The assessment team was requested to formulate a consensus draft proposal for the development 

of additional testing and analysis methods that would be required to establish the technical 

validity, and any limitation thereof, of these vessels for continued safe operation. This 

assessment was to leverage common skills, capabilities, and data from within and outside NASA 

and to consider inspection, analysis, and monitoring techniques that could be applicable to these 

vessels. To accomplish this task, the assessment team was to evaluate failure risks associated 

with the inner shell, outer layers, heads, nozzles, and welds of these vessels. It was expected that 

the results of this assessment and the proposed follow-on testing and analysis could subsequently 

be used as a basis for the development of a future NASA standard that would be submitted to 

OSHA for use as an alternate and supplementary standard, as required by 29 CFR 1960 [ref. 3]. 

An informational presentation to familiarize OSHA with the suggested approaches was a 

possible product of this assessment, with the understanding that this assessment was only a first 

step and that the resulting proposed testing and analysis method development was outside the 

scope of this assessment. Likewise, this assessment did not include the generation, review, and 

Agency approval of any NASA alternate and supplementary standard for OSHA. The tasks for 

this assessment included the following: 

Task 1: Review inventory, usage conditions, and ongoing and proposed risk mitigation methods 

for LPVs at all NASA Centers and at select locations within other Government facilities, as well 

as in private industry. Major activities included: 

• Hold virtual technical interchange meetings (TIMs) with Center PSMs to acquire

available data on NASA usage of LPVs, to include inventory, usage conditions, and

ongoing and proposed risk mitigation methods.

• Research locations and points of contact for external, non-NASA sites where similar

vessels are currently in operation.

• Conduct phone interviews with non-NASA users of LPVs.

• Engage multilayer pressure vessel fabricators and additional experts (e.g., ASME or

American Society for Testing and Materials (ASTM) technical committee members) as

needed.

• Conduct a virtual TIM with the assessment team to collate review results and identify

technical areas requiring further studies, opportunities for limited testing and analysis

efforts to be considered, and preliminary initial recommendations.

Task 2: Conduct limited scope testing and analysis necessary to formulate final 

recommendations and a path forward. Major activities for this task were to be defined based on 

the results of task 1, for example: 

• Proof of concept of NDE and in situ sensing methods.

• Destructive materials properties testing and associated analysis.

• Structural analysis of LPV response.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

17 of 193

Task 3: Coordinate development of a consensus draft proposal for the development of 

additional testing and analysis methods required to establish the technical validity, and any 

limitation thereof, of LPVs for continued safe operation. Major activities for this task included: 

• Conduct final virtual TIM to identify highest priority risk mitigation methodologies and

necessary future testing and/or analyses required to develop, evaluate, and validate these

approaches.

• Develop findings, observations, and NESC recommendations and draft a final report for

peer review and approval.

6.0 Problem Description and Proposed Approach

6.1 History of LPVs 

6.1.1 Original Patents, Manufacturers, and Benefits and Limitations of Construction 

Technique

The methods of construction for LPVs were originally described in the United States (U.S.) in 

patents assigned to the A. O. Smith Corporation in the 1930s and 1940s [refs. 5 and 6]. During 

WWII, Krupp in Germany also developed a layered method of constructing high-pressure 

vessels. Rather than using a single monolithic layer of material, multiple layers are used to 

construct these vessels, as shown in Figure 5.0-2. Although in principle both the shells and the 

heads of cylindrical vessels can be fabricated with layers, most vessels fabricated in this manner, 

including all within NASA, have monolithic heads and layered shells. 

This fabrication approach originated to address the difficulty at the time in building thick 

monolithic steel shells to withstand very high pressures. Thick steel products produced by the 

steel mills at that time had considerable variability in chemical composition and material 

properties and suffered from slag inclusions and poor impact properties; thin-plate materials with 

better and more consistent properties were more readily available. Of course, as stated, NASA’s 

layered vessels still have thick heads and present risks associated with materials variability, 

which is discussed later.

The advantages of the layered vessel construction method for vessels to be used at high pressures 

included: 

1. The high-strength steel necessary for construction was readily available in thin (typically

1⁄4- to 1⁄2-inch) plates, which could be rolled into cylinders for the shell sections.

2. Thin plates could more readily be rolled into cylinders with lighter forming rolls and

without heating.

3. Large overall thicknesses could be fabricated using multiple thin layers.

4. The thin plates generally had superior material properties (e.g., ductility, strength, and

toughness) over equivalent thicker plates.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

18 of 193

5. Vent holes could be drilled through all of the layers except the inner shell of the vessels,

as shown in Figure 5.0-2(a), ensuring that pressure did not build up between layers and

also providing a means of detecting leakage, indicating inner shell failure before

catastrophic rupture of the vessel. (Note that a similar technology was developed in

parallel in Germany in the coal gasification industry for the war effort, where an inner

layer was used as the pressure seal and the outer shell layers were vented).

6. Welding of the thin longitudinal seam joints in the thin plates was more easily

accomplished versus welding joints in thicker plates.

7. Redundancy of layers ensured that even a crack in a layer at a location near a

circumferential weld could not propagate through the full vessel wall without first

propagating to and through the circumferential weld.

8. The fact that such a crack must propagate through the circumferential weld to get to any

layer other than that immediately adjacent to it (i.e., to which it could propagate through

the longitudinal weld fusing the two layers) also ensured that a crack not extending to the

vicinity of the circumferential weld was essentially harmless (barring other lost layers,

which could result in a simple overstress failure).

Disadvantages included: 

1. The intermediate layers of the shell sections are difficult, if not impossible, to reliably

nondestructively inspect after fabrication.

a. Ultrasonic methods could only be used to inspect the outer layer and, if access to

the interior of the vessel was available, the inner shell. Ultrasound is not readily

transmitted to the intermediate layers because of the discontinuity between the

layers.

b. Radiographic techniques typically used for inspecting vessel welds do not work

well because of the presence of the multiple layers. The presence of layers creates

indications in radiographic images that may obscure flaw indications in the

circumferential welds (called layer wash in the ASME BPVC).

c. The flaw sizes detectable using radiography are a function of the total wall

thickness (which is in the 2.5- to 6-inch range for NASA’s vessels), while the

critical flaw size in a layer based on fracture mechanics is a function of the layer

thickness.

d. Ability of radiography to detect cracks is limited if the crack is not closely aligned

with the direction of the radiation (i.e., generally accepted maximum of about

5 degrees). Reference 7 specifies 4.3 degrees, while NASA-STD-5009 [ref. 8]

requires a maximum of 5 degrees).

e. NDE methods and their detection capabilities available at the time of construction

were more limited than those in use today.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

19 of 193

2. Analytical methods used to predict the structural response of layered vessels are

simplified and generally do not consider, for example, the complexity of the geometry,

including the potential for asymmetric gaps between layers or at layer-to-circumferentialweld locations, among other issues.

3. It is difficult to achieve contact between layers across the full surface of the rolled plate

layers, which results in a lower strain transfer efficiency to the outer layers. To ensure

that adequate contact is made, ASME BPVC criteria requires that the measured shell

expansion during an LPV vessel hydrotest for new vessels be at least 50% of that which

would be expected for a monolithic shell of equivalent thickness.

4. The vast majority of NASA’s vessels were manufactured before ASME BPVC rules

existed for such vessels, using high-strength shell layer and nozzle materials that were

never incorporated into the BPVC. In addition, the A225 Grade B head material, which

was listed in the BPVC at the time of construction, was eliminated from the code in the

Summer 1979 Addenda to the 1977 Section VIII Code [ref. 9] for reasons that are

currently not known. Consequently, these vessels are generally noncompliant with

OSHA regulations and/or interpretations (which mandate ASME design and fabrication),

and NASA cannot currently prove they are safe for use for any specific period of time.

LPVs have been fabricated by a number of companies. A. O. Smith fabricated vessels though 

the 1950s under the registered trademark name of “Multilayer Vessels.” They licensed the 

technology in the 1960s to Chicago Bridge & Iron Company, Inc. (CB&I) and Hahn and Clay. 

The Nooter Corporation began manufacturing vessels in the 1960s under a different patent using 

spirally wound cylindrical sections rather than concentrically wound cylinders and used the 

trademark “Plywall Vessels.” Struthers Wells also began fabricating layered vessels in the 

1960s under another patented technique, where thicker cylinder sections were shrink-fitted to 

form the shell, under the name “Multiwall Vessels.” LPVs are also currently manufactured 

overseas, but specifics were not researched for this work. ASME uses the generic name of 

layered vessels to refer to vessels constructed by these various methods. NASA has layered 

vessels from a number of different manufacturers. The greatest portion of NASA vessels were 

fabricated by A. O. Smith, followed by CB&I, with a few by other manufacturers. 

6.1.2 ASME Boiler and Pressure Vessel Code

Section VIII of the ASME BPVC [ref. 1] provides rules for the construction of new pressure 

vessels. Newly constructed pressure vessels that fully comply with and are stamped in 

accordance with ASME BPVC Section VIII are considered to be “code” vessels. A new codestamped vessel having appropriate safety pressure relief protection is normally accepted as 

having sufficient integrity and minimal risk to serve its intended purpose without further 

mitigations against potential failures for its design service life (i.e., barriers, restricted access, 

etc., are generally not required simply because an ASME vessel is in use). It is also the standard 

required by OSHA for both industry and federal workplaces. (In formal letters of interpretation, 

OSHA refers to “nameplate, records, and stamping,”, and concludes that actual ASME BPVC 

stamping, not just “equivalent construction,” is required.) The BPVC is a code for new 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

20 of 193

construction and does not address any degradation or changes in service that might occur after 

completion of construction of a vessel. 

Most of the NASA-owned LPVs are from the 1950s and 1960s. The ASME BPVC did not 

provide for the layered method of construction. Only monolithic vessels were addressed by the 

BPVC. The layered method of construction was added to the ASME BPVC in the Winter 

Edition of 1978 [ref. 10]. Paragraph A-140 of reference 9 includes the following statement: 

“The rules in Section VIII, Division 1 and Division 2 which cover the construction of 

layered vessels have been developed to parallel each other as far as can be done within 

the parameters of each Division. The design criteria may influence the selection of the 

Division.” 

Even the first treatment of layered vessels in the ASME BPVC in 1978 included extensive 

requirements beyond what is included in the balance of the BPVC, addressing material 

requirements, design, welding (including heat treatment), NDE, fabrication, inspection and 

testing, and pressure relief devices.

LPVs constructed prior to 1978 were not ASME “code-stamped” and, thus, are “non-code.” 

Although designed and constructed by reputable companies using documented and formalized 

procedures and generally good practices, these were not consistent with the BPVC at the time of 

manufacture, or with later additions, and generally had lower factors of safety than required by 

ASME for code-stamped vessels. Appendix A shows an example of a procurement specification 

for a NASA LPV in 1961. In section 1-5(d) of Appendix A for design conditions, it specifies 

that the design and fabrication of the vessel shall conform to one of the following: 

1) ASME Code for Unfired Pressure Vessels (1956) [ref. 11] with the following exceptions:

(a) the pressure limitations stated therein shall not apply and (b) the provisions covered

by Code Interpretations Case 1205-3 [ref. 12] shall apply.

2) A. O. Smith Corporation’s specification MLS-30A [ref. 13] and A. O. Smith

Corporation’s specification dated August 27, 1957 [ref. 14], for multi-layer construction

of vessels above 3,000 psi.

ASME Code Case 1205-3 [ref. 12] refers to forged pressure vessels without welds. Since the 

ASME BPVC of 1956 did not allow LPV construction techniques, the resultant pressure vessel, 

in order to meet the terms of the procurement specification, would have to be either a monolithic 

forged vessel complying with the 1956 BPVC and Code Case 1205-3 (which covered such 

forged vessels), or a layered vessel. If layered, it would have been required to conform to the 

A. O. Smith specifications. 

Further, in section 1-5(e) of Appendix A of the specification, it was specified that a safety factor 

of 3 could be used for the LPV being procured. The ASME BPVC at the time required a safety 

factor of 4, except for Code Case 1205-3 for forged vessels without welds, which also allowed 

for a safety factor of 3. Additionally, in section 1-7(a) of Appendix A for materials, it was stated 

that the steel selected for fabrication of the LPV did not necessarily have to be in strict 

accordance with the requirements of the ASME BPVC. Thus, in addition to the fact that the 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

21 of 193

general layered construction method was not covered by the BPVC at the time of construction of 

the NASA LPVs, additional variances with respect to the BPVC were permitted on materials and 

safety factors. There is no information available as to any assessments of the safety of the noncode vessel being procured with respect to these variances from the BPVC at that time. Note 

that government safety regulations were much less prevalent at this time, and OSHA did not 

exist until created by the OSH Act of 1970. 

When layered methods of construction were added into the BPVC, A. O. Smith, as well as 

competitors Nooter Corporation and Hahn and Clay, were heavily involved in the development 

of the rules. The different companies had different fabrication procedures, and the resulting 

ASME rules were developed using a consensus process; thus, the rules that resulted allowed for a 

number of fabrication techniques and did not reflect what any one company had done for 

previously constructed vessels. Thus, it is important also to evaluate how the non-code NASA 

LPVs were constructed relative to the later-introduced versions of the BPVC that permitted their 

construction. There are two aspects to the discussion of equivalence to code: (1) do the vessels 

comply with all of the requirements of the BPVC, and (2) if not, can a level of safety and 

reliability essentially the same as compliant vessels be demonstrated? The following discussion 

relates to the first of these questions; this report as a whole deals with the second. 

Although an exhaustive line-by-line comparison of the fabrication techniques used by the 

different LPV manufacturers was not performed, a review of 21 drawings for vessels constructed 

by A. O. Smith revealed the following discrepancies:

1. Fifteen drawings showed shell nozzles of 2-inch pipe size or less, six did not. None of

the 15 nozzles complies with the configuration required in the 1978 BPVC for such

nozzles.

2. Part ULW-20 in reference 10 states, “When the nondestructive examinations outlined in

ULW-50 through ULW-57 have been complied with, the weld joint efficiency (factor to

allow for possible weld defects) for design purposes shall be 100%.” No alternative to

the 100% joint efficiency is offered, and the requirements in ULW-50 through ULW-57

are all stated as requirements with no alternatives offered. Therefore, the NDE

requirements on the 21 drawings were compared with the ULW-50 through ULW-57

requirements, revealing the following:

a. Part ULW-51 requires that Category A and B joints (see Figure UW-3 in the

BPVC for category definitions) in the inner shells of layered shell sections and in

the inner heads of layered heads...shall be 100% radiographic tested (RT). This

was required in all drawings.

b. Part ULW-52(a) requires 100% magnetic particle testing (MT) inspection of the

longitudinal weld joints in layers welded to the previous surface. Some MT was

specified on every drawing (indicating that any MT specified could be expected to

be found on the drawings rather than in some other associated specification), but

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

22 of 193

none of the drawings reviewed met the 100% MT requirement for longitudinal 

joints. 

c. Part ULW-54(a), “Full Thickness Welding of Solid Section to Layered Sections,”

requires 100% RT inspection of weld-category A, B, and D joints attaching a

solid section to a layered section of any thickness given in ULW-52. Other

radiography was specified on every drawing, while no drawings specified

radiography meeting this requirement. While this is an inspection that can be

performed after the fact, the vessels as constructed did not meet this requirement;

therefore, it may be expected that the weld quality will not consistently

meet the requirement.

3. Any impact testing performed typically consisted of U-notch or keyhole-notch Charpy

specimens, which have a 5-mm-deep notch that has a 1-mm radius at the base. These

were not permitted in the BPVC after 1967, after which Charpy V-notch specimens,

which have a 2-mm-deep notch with a 45-degree angle and a 0.25-mm radius at the base,

were required. The development of the Charpy V-notch specimen occurred as a result of

WW II ship failures caused by high ductile-to-brittle transition temperature steels similar

to A225B. Earlier Charpy notch geometries were not sensitive to this phenomenon.

(The Department of Transportation (DOT) also made this change related to DOT

requirements for gas storage bottles the same year.)

4. One of the later revisions of the ASME BPVC was in 1989 [ref. 15]. This revision added

the UCS-66 curves for determining MDMT. At the time these vessels were constructed,

the MDMT was typically based only on material strength and did not take into account

brittle fracture that may occur at lower temperatures, including the nameplate MDMT.

There is evidence that some A. O. Smith vessels were keyhole impact tested at –40°F,

consistent with code procedures of the day, but the keyhole method used was removed

from the BPVC in the early 1967 and is not acceptable in current practice because it was

shown beginning in the 1950s that brittle failure of ship plates could be correlated with

Charpy V notch, but not with Charpy keyhole notch characteristics [ref. 16]. Thus,

operating at or above the nameplate MDMT for these vessels may not ensure safety with

respect to the potential for brittle failure.

5. Proprietary shell and nozzle materials used were not BPVC listed (approved) materials.

These proprietary materials also failed to comply with the BPVC requirements for

unlisted materials. Thus, complete material fracture characteristics are not readily

available.

6. The head material, ASTM A225B, while included in some prior editions of the BPVC,

was deleted as an acceptable material in the Summer 1979 Addenda of the 1977 Section

VIII, Division 1 Code [ref. 9], and is not currently included in ASME BPVC Section II

(Materials) [ref. 17].

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

23 of 193

7. While the allowable stress basis varied by customer and was typically specified in a

procurement document, the design factor of safety (FS) of these vessels is known in at

least one case to be as low as 1.66 on yield strength (which for these steels results in an

FS of about 2.1 on ultimate tensile strength). Using 60% of yield as the allowable stress

(FS of 1.66) results in overstress at the design pressure based on the BPVC allowable

stresses at the time of construction in 1978 when LPV design was incorporated into the

BPVC, and the current ASME Section VIII, Division 1 [ref. 1] criteria.

It is clear that the NASA LPVs are not only “non-code” with respect to the BPVC in place at the 

time of construction but also “non-code” with the subsequent code that allowed the construction 

method. While some of these variances can be addressed to reduce risk associated with 

continued use of these vessels, they will always remain “non-code” and present some level of 

elevated risk as compared with code vessels. Additionally, the use of “non-code” vessels is not 

compliant with the Federal OSHA regulations (29 CFR 1910, Occupational Safety and Health 

Standards) [ref. 2]. 

To reduce risk, some but not all NASA LPVs have been derated to pressures lower than their 

original nameplate maximum allowable working pressure (MAWP), some to a pressure low 

enough to comply with ASME equivalent allowable stress criteria. However, it is noted that 

derating alone does not make them equivalent to ASME vessels due to the many other code 

compliance factors noted above.

6.1.3 Additional Risk Factors for NASA Non-code LPVs

In addition to the risks associated with being non-code vessels, there are other elements that 

contribute to increased risk for continued usage of these vessels. The first is the age of these 

vessels. Many in use at NASA were fabricated in the 1950s and 1960s. Primary risks associated 

with aging include fatigue, corrosion, and embrittlement. As will be discussed below, a number 

of cracks have been detected in NASA LPVs that may be the result of fatigue. However, it is 

recognized that the NDE techniques available at the time of construction were not as advanced as 

those currently available; therefore, the potential exists that some subsurface defects have existed 

since the time of construction. Corrosion on the interior of NASA LPVs has been a lesser 

concern since, unlike vessels operated in the petrochemical industry that often contain corrosive 

products at elevated temperatures, NASA vessels are largely operated with nonreactive gases 

(except, in some cases, hydrogen) and at ambient temperatures. Corrosion on the exterior is 

more of a concern, as many NASA LPVs are located near the coast. However, exterior corrosion 

is readily detected and can be easily abated. With respect to embrittlement, other than vessels 

that were in hydrogen service, there is little reason to believe that the vessels have changed in 

properties since their initial fabrication, given the operating environment of NASA LPVs. 

Another factor is that in many cases there is limited information about the usage history of these 

vessels, such as the number of pressure cycles, the ranges of operating pressures, and 

temperatures, etc. A number of these vessels were not originally constructed for NASA but were 

put in service for other Agencies or organizations and later obtained by NASA. Service history 

for those vessels prior to arrival at NASA is often nonexistent. Additionally, even for vessels in 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

24 of 193

service within NASA for their entire life, there may be limited information on the usage, 

maintenance, and inspection and repair history, or information may be buried in old written files 

that are difficult to access. This lack of accurate usage information impacts the ability to 

perform an accurate fitness-for-service (FFS) assessment of the vessels. Another identified area 

of concern regarding these vessels within the Agency is the lack of consistency in how they are 

maintained, inspected, and operated. 

6.1.4 LPV Failure History

It is estimated that the total number of LPVs, both code and non-code, that have been fabricated 

is less than 24,000. When A. O. Smith licensed the technology to CB&I in the mid-1960s, they 

estimated 15,000 layered vessels in existence. Production of layered vessels in the U.S. has 

slowed since then and has been about 20 per year since the mid-1960s. The rate of production 

for Nooter Corporation was two to three vessels per year, and these appear to be used mostly in 

the chemical process industry, such as for urea manufacturing.

There have been a number of known catastrophic failures of LPVs, all at chemical process plants

(none at a NASA facility). Vessels that failed in the U.S. prior to 1978 were obviously non-code 

vessels. For some of the more recent failures, as well as failures outside the U.S., it was not 

documented as to whether the vessels were non-code or code. In one case, however, it was a 

newer, code-compliant vessel that failed. Thus, it is possible for both code and non-code LPVs 

to fail catastrophically. Poor maintenance and inspection practices often contributed to missing 

early signs of inner shell failure through the vent holes. 

The following paragraphs describe some of the vessel failures. 

Catastrophic vessel failures:

1. Clinton, Iowa, fertilizer plant, March 30, 1969 [ref. 18]. Failure started in the heataffected zone (HAZ) of a repair weld at a nozzle. Two-thirds of the vessel blew away.

The report noted that “Not withstanding (sic) the general low fracture toughness of the

vessel, it is doubtful that this failure would have occurred if the nozzle had a well

radiused inside corner and if the flow tube had not been welded to this corner.”

2. Cartagena, Colombia, fertilizer plant, December 8, 1977 [ref. 19]. This lead-lined vessel

was constructed by A. O. Smith and operated at 3,600 psi and 400°F. The failure

occurred in the vicinity of the shell-to-shell weld. Indications show that the failure

occurred first longitudinally, then circumferentially, resulting in separation of the vessel

sections. The vent holes were plugged with carbamate (a compound derived from

carbamic acid) and salt (from seawater), and inner shells were corroded where the failure

is believed to have initiated.

3. Lake Charles, Louisiana, fertilizer plant, July 28, 1992 [ref. 20]. Reported by

Occupational Safety and Health Review Commission Docket 93-0628. Technical details

not included.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

25 of 193

4. Hebei Qianan, China, fertilizer plant, October 7, 1995 [ref. 21]. Layered vessel. Ten

urea reactors scrapped in inspection and assessment after an explosion of one reactor.

Li et al. report that 15 or more reactors were investigated due to cracking, some from the

outside in, some from the inside out, and some from intermediate layers. Reason for

cracking is given as stress corrosion cracking (SCC).

5. Pingyin, China, fertilizer plant, March 21, 2005 [ref. 22]. Reported failure due to SCC

caused by forcing steam through vent holes as a leak detection mechanism.

6. Coffeyville Kansas, fertilizer plant, September 30, 2010 [ref. 23]. A 10-year-old ASME

code-stamped vessel constructed by Hahn and Clay. Radial and transverse weld cracking

in the head-to-shell weld, attributed to “environmental stress cracking” caused by the

boiler feed water that was used inside the vessel jacket, resulting in complete failure of

the circumferential weld at the head-to-shell junction. Similar cracking but no failure

was reported in the weld to the bottom head. Both welds were replaced.

7. At least two additional known catastrophic failures, one in Johannesburg, South Africa,

and one in Europe, but no further details were available.

There also have been much larger numbers of vessels that have developed cracks, requiring 

either repair or removal from service. A number of NASA vessels have developed cracks; some 

of these have been removed from service, and others have remained in service when it has been

judged they can be adequately monitored for additional crack growth. Some examples of noncatastrophic vessel cracking are provided below. Where available, the published reports are 

referenced. However, in many cases, formal reports or documentation is not available, and 

reports of the damage observed were obtained from assessment team members who either 

observed the damage or were involved in responding to the cracks. In these cases, the team 

member who provided the information is identified. While the specific details of these events 

were not captured in formal reports, the available anecdotal information is still of interest in 

providing insight into the types of damage that can occur in these vessels. 

1. Leakage: Kennedy Space Center (KSC), October 14, 1980, from a helium vessel

constructed by A. O. Smith in March 1958 and procured by the Department of Defense

(DOD) [refs. 10 and 11]. The crack was in the cylindrical shell along the longitudinal

weld, variously reported as propagating from inclusions in the plate adjacent to the weld

or from weld undercut on the inner diameter (ID). Overall condition of the vessel was

reported as good.

2. Cracking: Ames Research Center (ARC), 1987: ARC salvaged 16 1,750-cubic-foot

A. O. Smith pressure vessels from the DOD Jackass Flats nuclear test site. Pre-service

NDE (visual testing (VT), MT, and RT) showed most vessels (i.e., 15) having BPVC

rejectable indications, including six with cracks in the circumferential or longitudinal

welds. Most rejectable indications, including all cracks, were repaired by CB&I in 1986,

and the remainder were accepted as found. All vessels remain in service to the present.

There have been no subsequent service-related defects discovered in their 26-year

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

26 of 193

operational history at ARC. (Documented in pressure system certification reports at 

ARC.)

3. Cracking: Langley Research Center (LaRC), 1987(?): 12 vessels removed from service

after extensive cracking in outer shell layer adjacent to the head-to-shell weld of one or

more vessels, as well as internal cracking. Blowdown temperatures and shocking were

believed to be involved in the internal cracking. (Witnessed by Greulich.)

4. Cracking: KSC, 2013: Two vessels removed from service with cracks in the center of the

outer shell plate, not adjacent to any welds or other known stress concentrations.

(Observed by Greulich. Date of removal from service is not known.)

5. Cracking: LaRC, 2014: Two vessels removed from methane service at B-1265 due to

external cracking. One vessel had transverse weld cracking in the head-to-shell weld.

One of the three transverse cracks was removed with light grinding, two were reported to

extend 0.87 inches into the weld based on shear wave ultrasonic testing (UT). The other

vessel had a longitudinal crack in the shell adjacent to the shell-to-shell weld.

Photographic verification of magnetic particle inspection indications. (Reported by

Christie Swarts of Jacobs Technology.)

6. Cracking: Marshall Space Flight Center (MSFC) (1990s): A vertical vessel was removed

from service due to cracks suspected in layers based on AE. (Reported by Pulley;

detailed records lacking.)

7. Cracking: MSFC (2011/2012): A large horizontal vessel (V0256) was removed from

service due to a longitudinal crack approximately 2 1⁄4 inches long in the outer layer near

the head-to-shell weld. (Viewed by Greulich and Prosser, August 2013.)

8. Cracking: Glenn Research Center (GRC): GRC reported in the NESC LPV team

teleconference on February 27, 2014, that they have one or more vessels in service with

one or more cracks in an intermediate layer that are being monitored using periodic RT.

(Reported by Wnuk.)

9. Stennis Space Center (SSC) (out of a total of 22 vessels for which history was provided),

reported cracking and other indications in both A. O. Smith and Struthers Wells vessels,

including the following:

a. Four Struthers Wells vessels exhibited a number of cracks that were removed by

grinding at various times. At least two of these vessels later showed additional

cracking, even though one had been derated (it was later “downmoded,”

i.e., removed from service, but not scrapped). In 1988, “Assessment concluded

that previous history of cracking due to high residual stress from welding and not

service related.”

b. One CB&I vessel exhibited cracking that was “removed by light grinding.”

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

27 of 193

c. Four Struthers Wells and one CB&I vessel were reported to have “leakage at vent

holes,” though it is not clear whether action was taken based on these indications.

In one case it was “determined leaks are from air trapped between vessel layers.”

d. In five cases, indications were not described as cracks were removed by grinding.

e. Numerous vessels showed indications under AE testing. Further inspection using

UT or RT (through as much as 11 inches of material) was used to accept these

vessels as having no rejectable indications.

10. All A. O. Smith LPVs at Eastman Chemical Company were repaired and finally removed

from service due to cracking of outer layers, according to a verbal report to Greulich from

a representative of Eastman Chemical.

11. Cracked LPVs are found in industry on a regular basis (numerous times per year) and

removed from service; these are usually repaired and returned to service. (Reported by

Jawad, who is often involved in these repairs due to his association with a company

previously in the business of fabricating layered vessels and now performing repairs and

process industry maintenance.)

Discussion of past failures

The typical degradation mechanism associated with the six confirmed catastrophic failures of 

LPVs in industry has been SCC, and the service typically involves elevated temperature 

operation. One case involved a weld repair. Two cases involved cracking reported to be caused 

by boiler feedwater or steam used external to the vessel and either injected through the vent holes 

for leak detection or through cooling jackets. 

Some vessels at NASA and many in industry have been removed from service due to cracking, 

most often in the vicinity of circumferential welds, usually the head-to-shell welds. The crack in 

the shell-to-shell weld noted in item 5 above at LaRC is stated by Jawad to be highly unusual: it 

was the first longitudinal crack associated with a shell-to-shell weld that he had seen. Similarly, 

the cracks reported in item 4, located in the outer layer away from all welds, are reported to be 

unique. 

Service at NASA is typically at ambient temperatures or below, often with cold service due 

either to ambient conditions in wintertime or blowdown service. A number of vessels have been 

removed from service due to cracking. The cracking in NASA vessels is generally assumed to 

be caused by pressure cycling. The cracks that occur in the shell adjacent to circumferential 

welds appear to be emanating from a region that is expected, based on the current weld residual 

stress study, to be in circumferential compression. While it is possible to repair these cracks, 

which is often done in industry, it is clear that a full understanding of their initiation and growth 

mechanism is lacking. This lack of understanding, combined with the crack in an intermediate 

layer in the GRC vessel reported in item 8 above, lead to the conclusion that until a reliable 

inspection method for intermediate layers is identified, intermediate shell layer cracks cannot be 

ruled out. This raises the possibility that multiple shell layers could be compromised without 

anyone being aware of it.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

28 of 193

Thus far, the authors of this report are not aware of any catastrophic failures outside the fertilizer 

industry. However, based on the number of cracks identified in shell sections and in the 

circumferential welds, the possibilities of either undetected cracks in multiple layers leading to a 

failure due to overstress or of a crack propagating through a circumferential weld in such a way 

as to result in failure appear to be real, with nontrivial likelihood. 

6.1.5 LPV Reliability Evaluation 

In general, quantitative reliability evaluation is accomplished through reliability prediction and 

reliability demonstration.

Reliability prediction is the process of quantitatively or qualitatively estimating system 

reliability using both objective and subjective data at the level for which data are available. 

Reliability prediction is performed either to integrate the reliability into the design up front so as 

to satisfy a certain design reliability requirement or to better understand the failure modes and 

causes associated with an existing system. 

Reliability demonstration provides the basis for quantitatively estimating the reliability from 

objective data provided by the demonstration. It is generally performed after the system/

component is qualified and ready for operation. Reliability demonstration is intended to supply 

additional information for reliability prediction.

Although no well-documented reliability evaluation exists for pressure vessels, according to 

NASA-STD-8719.17, “NASA Requirements for Ground-Based Pressure Vessels and Pressurized 

Systems (PVS)” [ref. 24], and based on literature, the failure rate per year for ASME codestamped pressure vessels constructed after 1988 is expected to be 10–6 or less. Pressure vessels 

constructed before that time are expected to have a higher failure rate, which NASA assumes to 

be in the 10–3 to 10–6 per year range (i.e., one block more frequent in NASA’s risk assessment 

charts), though at the less-frequent end of that block, based on experience. The lower failure rate 

for vessels constructed after 1988 is expected because MDMT determination methods have 

improved since that time. 

For LPVs, even though data exist regarding the numbers of vessels constructed and a number of 

known failures, it is not possible to perform a high-confidence reliability evaluation. Operational 

data for NASA or industry vessels is not well documented in terms of operation time, operation 

cycles, operating environment, current condition, etc. Without this information, any reliability 

evaluation performed would be speculative. This is further complicated by the significant age of 

NASA’s vessels, with unknown accumulated degradation and defects. If the vessels follow a 

notional bathtub curve, as shown Figure 6.1.5-1, which is not certain, then these old vessels may 

also be in the region of escalating failure rate. In this regime, reliability assessments would 

require the use of conditional failure probabilities to account for the LPV wear. To maintain the 

lower constant failure rate suggested by the bathtub curve, LPVs with significant aging would 

need to be replaced if their actual condition could not be adequately assessed for high-confidence 

life extension. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

29 of 193

Figure 6.1.5-1. Notional Bathtub Curve Illustrating Changes in Failure Rate versus Time/Cycles

6.2 Problem Statement

Because of these identified risks and concerns, the NESC, in cooperation with the NASA Office 

of Safety and Mission Assurance (OSMA) and the respective Center PSMs, was requested to 

formulate a consensus draft proposal for the development of additional testing and analysis 

methods that would be required to establish the technical validity, and any limitation thereof, 

of these vessels for continued safe operation. 

6.3 Planned Tasks

To accomplish the objective of developing a proposal for additional testing and analysis methods 

necessary for continued safe operation of LPVs, the assessment team was to evaluate failure 

risks associated with the inner shell, layers, heads, nozzles, and welds of these vessels. It is

expected that the results of this assessment and the proposed follow-on testing and analysis 

subsequently can be used as a basis for the development of a future NASA standard that will be 

submitted to OSHA for use as an alternate and supplementary standard in accordance with 

29 CFR 1960 [ref. 3]. This assessment was to leverage common skills, capabilities, and data 

from within and outside NASA and to consider inspection, analysis, and monitoring techniques 

that could be applicable to these vessels. The planned tasks for this assessment are listed in 

detail in Section 5.0 and included a review of how LPVs are maintained and operated both 

within and external to NASA as well as some limited scope testing and analysis. 

Instanta ne ous Fail ure 

Run Time/Cycles

Failure 

Rate

Constant Failure Rate

(Design Life)

Decreasing Failure Rate 

(Infant Mortality) Escalating Failure Rate 

(Aging)

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

30 of 193

7.0 Data Analysis

7.1 Task 1 Summary

7.1.1 Center Request for Information (RFI) Responses

To better understand the inventory, usage conditions, and ongoing and proposed risk mitigation 

methods for LPVs at all NASA Centers, the assessment team generated a request for information 

(RFI) that was sent to the PSMs at all Centers. A full copy of this RFI is included as 

Appendix B. The RFI requested information in eight general areas. These included detailed 

information about the inventory of LPVs, NDE used on the vessels, current additional risk 

mitigation approaches, previous risk mitigation approaches no longer used, risk mitigation 

approaches under development, recommendations for any additional risk mitigation methods that 

should be considered, special concerns about continued safe operation, and how the risks were 

being communicated to Center management. The detailed responses to the RFI and the

accompanying spreadsheets with vessel information from each Center that has LPVs are also 

included in Appendix B. 

From these responses, it was determined that non-code LPVs were in use at nine NASA Centers 

or facilities. These included ARC, GRC, Wallops Flight Facility (WFF), White Sands Test 

Facility, KSC, LaRC, Michoud Assembly Facility (MAF), MSFC, and SSC. A total of 376 

vessels were reported, with 302 of these in active service and the remaining 74 inactive. There 

was a variety of reasons that vessels were not in active service, ranging from no identified 

current need to the existence of known defects within a given vessel. Depending on the owner 

of the vessel and the reason it was out of service, it was noted that some of these inactive vessels 

could be made available as specimens for future testing efforts. MSFC had the largest number of 

vessels of any Center with a total of 199, 173 of which were in current service. GRC had 

86 with 57 in current service. ARC, KSC, and SSC all had on the order of 20 vessels, with the 

remaining Centers and facilities having smaller numbers. The vast majority of these vessels 

(i.e., 308) were manufactured by A. O. Smith; 40 were manufactured by CB&I; 23, by Struthers 

Wells; and the remaining few by other fabricators.

In reviewing and discussing the RFI responses from the Centers, two important points were 

identified. The first is that much of the important data that are needed to better characterize the 

risks associated with LPVs is in disparate formats, is difficult to access and retrieve, and in some 

cases, has been lost or was never tracked. This includes data such as design records and 

drawings, fabrication records, maintenance and inspection records and data, and operational 

usage data. Vessels not originally fabricated for NASA that were procured from other 

organizations may have minimal data regarding their use prior to acquisition from NASA. Even 

for vessels originally fabricated for NASA, the records may be more than 50 years old and have

been subject to varying degrees of care and retention. The majority of these data exist only in 

paper format, with some documents scanned into local records. However, some design drawing 

documents have been scanned and made accessible Agency-wide.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

31 of 193

Another important point, not unexpected but readily apparent, was that there is not an Agency 

consensus on approaches to certification, recertification, inspection, maintenance, record 

keeping, or usage of these vessels. There is no consensus on what constitutes a certification or 

recertification of an LPV, or when it is required. Some Centers periodically “recertify” these 

vessels with their local procedures, while others do so only when a vessel is repurposed or not at 

all, although such an approach is not consistent with NASA’s PVS requirements documents. 

There is no consensus regarding intervals between required periodic maintenance and

inspections, nor on which methods are required for those inspections. As will be discussed 

below, there is a variety of different inspection and analysis methods that are used across the 

Agency. Further, no consensus exists on the operating conditions under which these vessels are 

allowed to operate, such as the temperature at which they can be used or the safety factor that is 

required. 

7.1.1.1 Summary of NDE or Other Inspections/Analyses Related to Continued Safe Usage 

In discussions with the Centers, it was noted that NDE methods and detectability levels have 

changed over time and that the detection requirements for currently available NDE methods for 

LPVs are not well established. There are no formal defect size requirements established for the 

different regions of an LPV, due to the lack of validated analysis methods and limitations in 

available materials property data. Additionally, the capabilities of some of the methods that are 

used to inspect LPVs are not established or validated for these complex structures. Thus, the 

NDE data are used for “engineering evaluation,” and findings of flaw indications can lead to 

repair actions or removal of a vessel from service. However, the lack of findings of flaw 

indications does not necessarily ensure lack of defects or continued safe operation of LPVs. 

Further, as noted previously, the periodicity at which NDE is applied and which techniques are 

used vary from Center to Center. 

Conventional surface inspection methods such as visual (VT) (including borescope), magnetic 

particle (MT), and dye penetrant (penetrant testing (PT)) methods are widely used at most 

Centers to inspect outer surfaces, exposed weld surfaces, and inner shells when access is 

available. The capabilities for these NDE methods are well established and documented, and 

there are no unique considerations for applying these methods to LPVs. However, their 

application is limited to exposed surfaces of these structures, and these methods are not 

completely effective unless surface coatings are removed.

UT methods are also utilized at some Centers to inspect the monolithic heads and the head-tonozzle welds. UT is often used as a follow-up method to inspect regions of the outer and inner 

(when accessible) shell layers where surface techniques have indicated a flaw. UT in this case

can provide information about the depth of the flaw in these layers, whereas the surface methods 

only indicate the presence of surface-breaking flaws. UT is also used to measure thickness and 

assess any loss of thickness due to corrosion. UT, however, cannot penetrate into the interior 

shell layers. It is not commonly used for complete scanning of the inner or outer shell layers. 

Conventional UT methods have not been successful for inspecting the full penetration 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

32 of 193

circumferential head-to-shell or shell-to-shell welds due to reflections from the layers 

boundaries. 

RT has been used at some Centers to inspect the circumferential head-to-shell and shell-to-shell 

welds, as well as the longitudinal welds in shell layers. The presence of multiple shell layers 

adds irrelevant indications (i.e., layer wash) to radiographic images, which make detection of 

flaws more difficult. Although RT has been used to detect indications of cracks in layered 

sections and welds, some of which have been confirmed by destructive analysis and repair 

operations, the capabilities (i.e., flaw type and minimum detectable flaw size/probability of 

detection) of this method have not been established for application to these structures. Thus, the 

fact that flaw indications have not been found does not provide assurance that no flaws are 

present.

AE is one of the few methods that offers the potential for detecting crack growth within internal 

shell layers or longitudinal welds in these layers. A number of Centers have used AE to evaluate 

LPVs. Although AE methods have not been fully validated for application to LPVs, work in 

China [ref. 22] has shown at least one successful case of the detection of cracking in full 

penetration circumferential welds of an LPV. The crack was confirmed first with PAUT 

techniques and then by destructive material removal and visual examination. 

Historically, there are two types of AE measurement systems. The first, known as parameterbased, or conventional, AE measurement does not actually record the full waveform for an AE 

signal. Instead, when a signal crosses a preset trigger threshold, analog circuits are used to 

rapidly measure certain parameters or features of the signals from each sensor, such as the peak 

amplitude, duration, arrival time, energy, etc. These parameters are then plotted for real-time 

visualization, as well as stored for later, more detailed analysis. Parameter-based AE systems 

were originally developed because high-speed digital waveform acquisition systems were not 

available at the time. The advantages of these systems include high data acquisition rates and the 

ability for near real-time data analysis and visualization. Resonant sensors are most often used 

with parameter-based AE systems as they provide the highest sensitivity response. 

The second type of AE measurement system is known as waveform acquisition. As the name 

implies, this newer type of system digitally acquires from AE sensors each waveform whose 

signals cross the trigger threshold. The advantage of waveform systems is that the entire signal 

is acquired and available for more detailed analysis, which in turn can provide additional and 

more accurate information about the AE signal and its source. As an example, the signal arrival 

time, which is used to triangulate source location, can be more accurately determined. One 

limitation of these systems is that the larger amount of data acquired for each signal leads to a 

significant increase in data storage requirements. This increase in the amount of data for each 

signal can also lead to signals not being acquired at higher AE event rates due to dead time (i.e., 

time during which the system cannot accept new triggers) which occurs during data 

transfer/storage. Also, the capability of near real-time data processing and display during a test 

is more limited. Most often, broadband sensors, which have a lower sensitivity but a higher 

fidelity in reproducing the actual signal, are used with waveform acquisition systems. Some 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

33 of 193

newer systems offer the ability to capture both parameter and waveform data in the same 

acquisition system.

There is also a variety of analysis methodologies for AE data from the two different types of AE 

systems. One particular methodology that has been developed for parameter-based AE data for 

metallic pressure vessels is known as MONPACTM [ref. 25]. The MONPACTM approach was 

developed from the analysis of parameter AE data from testing a large number of storage tanks 

and monolithic pressure vessels. Correlations with certain signal parameters and when they 

occurred in the vessel pressurization profile were developed to discriminate background 

extraneous noise and to identify signals of interest. Signals of interest are graded using intensity 

analysis to determine a course of action that ranges from no action, to performing additional 

NDE, to the immediate shutdown of the vessel with additional NDE. Since the proprietary 

database on which MONPACTM is based is built on the testing of monolithic pressure vessels, its 

applicability to LPVs is not known. 

Another approach to the analysis of waveform AE data is known as Modal AE (MAE). In MAE, 

the waveforms are analyzed based on knowledge of the specific wave modes that will propagate 

in the structure being tested. MAE has been demonstrated to be useful in discriminating 

extraneous noise and in identifying crack sources of interest based on the signal modal content. 

Within NASA, both parameter and waveform AE data have been acquired during testing of 

LPVs. MONPACTM and MAE analysis approaches have been employed, along with other 

analysis methods, such as location analysis. Efforts toward validation of the MAE method 

performed at NASA ARC in 2011 and 2012 [ref. 26] (funded by OSMA) are summarized later in

this report. These efforts have included characterizing wave modes that propagate in these 

vessels, demonstrating signal transmission through the layered sections using simulated AE 

sources, and one test monitoring crack growth from a known crack in a full-scale vessel. While 

signals from the known crack were detected, they were of very small amplitude near the 

threshold for detection. Results from full-scale vessel testing with both the MONPACTM and 

MAE methods have shown that substantial noise is detected in comparison with the testing of 

monolithic vessels, believed to be associated with the relative movement of the shell layers as the 

layers expand and rub against one another. This background noise makes detection of actual 

crack growth much more difficult than for monolithic vessels. Additional efforts are necessary 

to understand and validate the capabilities of AE techniques relative to the application to LPVs. 

In particular, the sensitivity of the AE technique to detect cracks of different sizes relative to 

critical crack sizes will need to be established.

Other testing or measurements reported by Centers included the use of hardness testing, as well 

as precision diameter tape (i.e., Pi Tape) measurements on LPVs for circumferential expansion 

as a function of pressure, consistent with current ASME BPVC Part-ULW requirements. A few 

Centers reported cleaning of vent holes and checking for leaks. However, it was not clear that all 

Centers cleaned or monitored vent holes or, for those Centers that did, how frequently this was 

done. Hydrostatic pressure testing to 1.42 times the MAWP was used by one Center as part of 

their initial certification of vessels. Other Centers have also applied the typical 1.5 times MAWP 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

34 of 193

hydrotest (required by the BPVC at the time of construction) when placing the vessels into 

service at NASA. AE testing has been performed during overpressure testing at a lower level, 

typically 1.10 times MAWP. 

Some Centers reported the use of a number of different analysis approaches to estimate static 

strength (burst pressure) and fatigue life (pressurization cycles to failure), as well as additional 

permissible cycles following proof/overpressure testing of LPVs. In all cases, it was noted that 

many assumptions were necessary for these analyses, especially with regard to material 

properties, flaw size detection capabilities for NDE, and the structural response of the layered 

shells. Further, it was noted that none of the analysis methods have been validated on LPVs. 

Most Centers expressed significant doubt with regard to the validity of these assumptions and, 

thus, the results of the analyses. Nonetheless, overpressure testing may demonstrate that 

additional remaining cyclic life exists and, thus, increase confidence that a near-term failure will 

not occur during normal operations; however, as previously noted, this cannot currently be 

accurately quantified. 

7.1.1.2 Summary of Additional Risk Mitigation Approaches Currently in Use

Other approaches to either mitigate the risk of, or consequence from, catastrophic LPV failures 

that were reported by the Centers included the use of berms or other physical barriers around 

some vessels, as shown in Figure 5.0-1. However, these approaches cannot practically be 

employed uniformly across the Agency. Also, the degree to which any such barriers truly 

mitigate consequences was not investigated in this assessment. Location of the vessels to remote 

sites and limiting personnel access was also reported. Multiple Centers have intentionally 

derated vessel MAWP to reduce risk and increase safety factor, while others are operating below 

nameplate MAWP due to reduced pressure needs of the facility being serviced. In some cases, 

operational pressure cycles are limited and closely tracked. An additional approach employed by 

a couple of Centers is to have more frequent recertification intervals for LPVs, although the 

effectiveness of recertification is an open issue at present. 

7.1.1.3 Summary of Risk Mitigation Methods no Longer Used

The Centers were also asked to report on any risk mitigation methods that were used in the past 

but are no longer used. Most Centers reported none, although a couple of Centers reported that 

the MAWP had been reduced to mitigate risk but was later increased because of operational 

impacts caused by the reduced pressure. Two Centers also reported that the use of AE (both the 

MONPACTM and MAE approaches) had either been eliminated or was under review because of 

questions that recent validation test results had raised with regard to the capability of this 

method. 

7.1.1.4 Summary of Risk Mitigation Methods under Testing and Development

In response to inquiries regarding risk mitigation efforts under testing and development, a 

number of responses were provided by the Centers and are described in this and subsequent 

sections. Details of these activities, which prior to this assessment were sometimes only 

communicated locally within the Centers performing the work, were communicated across the 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

35 of 193

Agency to all Centers. This communication afforded other Centers the opportunity to provide 

input and, in some cases, data or materials that enhanced these ongoing activities. Additionally, 

in some cases these efforts served as a basis for Task 2 activities that were funded within this 

assessment. Communication of these efforts across the Agency and the resulting follow-on 

improvements was perhaps one of the more significant accomplishments of this assessment.

NASA MSFC, operator of the largest number of these vessels across the Agency, initiated 

several activities to address risk associated with LPVs. Efforts were underway in 

characterization of materials properties from a sacrificed vessel, development of new analysis 

methodologies that would include some of the structural details of layered vessel construction, 

and, working with SSC, characterization of a new NDE method, PAUT, for inspection of the full 

penetration circumferential head-to-shell and shell-to-shell welds. Details of some of these 

efforts will be described in a following section. As a result of the communication fostered in this 

NESC assessment, additional materials from other sacrificed vessels at other Centers were 

provided to MSFC to expand the materials property database and evaluate vessel-to-vessel 

variability. Additional efforts in materials characterization, structural analysis, and NDE were 

identified and funded as part of Task 2 of this assessment to augment some of the efforts already 

underway at MSFC. 

MSFC also continues to perform MONPACTM AE testing on field LPV vessels to build a 

database. Primarily, tests to date have used the MONPACTM AE approach, as there is limited 

MAE instrumentation available at MSFC. Plans include procurement of additional MAE

instrumentation to allow future testing to apply both methodologies. As the AE method is still 

not validated for LPVs, the information acquired is for engineering evaluation only. However, it 

is providing useful information on issues related to applying AE in the field on these vessels and 

on the assessment of the background noise produced and how this noise varies across different 

vessels that have undergone different periods of inactivity. No AE indications of crack growth 

have been detected in the recent history of vessel testing, including a vessel with a known crack 

that was tested at GRC as part of the photogrammetry work that was supported by MSFC (see 

Section 7.2.4 of this report). However, a vessel with a known crack (V256) has been removed 

from service, and plans are under development to perform hydrostatic testing while monitoring 

with AE.

NASA ARC had significant efforts completed and underway to work toward validation of MAE 

methods for application to LPVs. ARC has used MAE since 2001 [ref. 27], when a concurrent 

test was performed with both MAE and parametric AE vendors on a single vessel. As a result of 

that work, ARC contracted for MAE testing of all 16 active A. O. Smith vessels in 2002 and 

2008 [refs. 28 and 29]. No relevant crack indications were identified during this testing. During 

this work, ARC demonstrated the transmission of AE signals between layers and from the inner 

layer to the outer layers. For these tests, pencil-lead breaks were used to generate simulated AE 

signals (per ASTM E-1419) in a part-through hole drilled in one vessel and on the inner layer 

surface, which were detected with minimal attenuation by sensors on the outer layers. They also 

demonstrated the ability to detect and accurately locate such pencil-lead breaks over 20 feet from 

the nearest sensor on their large vessels at the specified ASTM threshold sensitivity. However, 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

36 of 193

since no actual crack was ever found (and none are known to exist), the ARC PSM concluded 

that full-scale testing was required to fully validate the method on LPVs. In principle, this could 

be accomplished by MAE monitoring of an LPV under pressure cycling to force crack growth 

from a starter notch. This was proposed as a research project to OSMA in 2010 and accepted as 

2011/2012 work. ARC managed the work through its site wind tunnel engineering and 

operations contractor and the selected MAE vendor. This particular crack growth turned out to 

be harder to detect than expected, and more testing is needed. Discussion of the cyclic test 

results and conclusions are contained in Section 7.1.2.2, Appendix G of this report, and reported 

in reference 26. As part of the cyclic test effort, when cycling was completed, the vessel was 

sent to an independent laboratory for materials property characterization to validate the ability to 

calculate crack growth using newly measured fracture properties and the NASGRO® fracture 

analysis program via comparison to the actual crack extension. This very limited material 

assessment in the vicinity of the crack was supplemented in 2012 with Phase 2 work by the same 

laboratory to more fully characterize shell, head, weld, and HAZ material, with some unexpected 

results but good correlation between the analysis and crack measurements. The materials 

characterization efforts are discussed in Section 7.1.2.2 and Appendix H of this report and 

reported in references 41 and 42. 

Based on discussions with the same testing laboratory, ARC also identified PAUT as a potential 

method for inspecting circumferential welds in these vessels, and the laboratory developed a 

proposal for initial development of this method using a section of the sacrificed test vessel as a 

calibration block. This proposal was accepted and included as part of Task 2 of this assessment, 

with task management transferred to LaRC. Again, additional details on these efforts are 

provided in following sections and appendices. 

Finally, the NASGRO® fracture analysis program (which was originally developed by NASA) 

has never been fully developed for application to ground-based vessels, and the above work 

required simplified geometry and nonstandard use of program features that limit its usefulness to 

the PSMs. Stress intensity factor modeling and failure assessment diagram application 

development are needed to make it fully and efficiently useful to the Centers. Since NASGRO®

is free to all NASA users, the ARC PSM also requested a proposal for that software development 

work from the same independent laboratory mentioned above (which develops NASGRO® under 

a Space Act Agreement). This was provided to ARC, and the proposal was later passed on to 

OSMA and NESC for consideration in future tasks. 

NASA GRC reported on efforts to continue refinement of their customized UT bore probes for 

nozzle and nozzle-to-head weld inspections. This capability was originally developed by GRC, 

and, while it had been discussed to a limited extent among the PSMs under their OSMA working 

group, it was not widely understood and had not been implemented at any other Centers. With 

further validation, it might be an approach that finds application across the Agency. NASA GRC 

also noted some preliminary efforts to apply full-field optical measurement approaches, such as 

photogrammetry, for measurement of LPV surface deformation as a result of pressurization. 

There are two potential applications for such a measurement tool. First, the measured 

deformations, if suitably accurate and sensitive, can in principal provide a basis for validating 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

37 of 193

analysis methods under development. Additionally, if it can be shown that the presence of flaws, 

including manufacturing anomalies and in-service defects, results in significant changes to the 

surface deformation of LPVs under pressure that are greater than the threshold noise/sensitivity 

level of the method, then the technique could possibly be used as a flaw screening method. 

Additional testing with this method was performed again as part of Task 2 and is described in 

more detail in Section 7.2.3 of this report. The ability to measure general surface deformation 

was demonstrated (e.g., vessel elongation and bending due to internal pressure). Efforts in 

detecting deformation variations due to known flaws were not successful, and additional analysis 

tools are needed to understand whether this would be possible, and if so, what size flaws might 

be detectable. Analytical efforts are needed to determine expected surface deformations due to 

various defects, such as major layer weld cracks or layer gaps. Usefulness of the technology will 

depend on whether motion due to defects is sufficient enough to not be masked by other vessel 

physical characteristics.

MAF reported that the operating pressures for their vessels had been reduced unless otherwise 

specifically required for a short-duration test. In such cases, additional requirements for 

operation were introduced, including evacuation of personnel from potential blast zones and 

installation of temporary or permanent berms. It was noted that such higher pressure operations 

typically only occurred one to three times a year and lasted only 1 or 2 days at a time.

7.1.1.5 Summary of Proposed or Recommended Risk Mitigation Methods

The Centers were also asked for input on any proposed or recommended risk mitigation 

methods. A number of responses were received, many related to ongoing work previously 

discussed. One NDE method not previously noted was identified as having potential to inspect 

through layered shell sections. This is the Saturated Low Frequency Eddy Current (SLOFECTM) 

technique, an electromagnetic inspection technique that has been used in efforts to detect 

corrosion in thick-walled steel piping [refs. 30 and 31]. Given that no current methods exist for 

inspecting the shell sections, some initial efforts to evaluate its capabilities for LPVs, at least 

with modeling, and if warranted, laboratory testing, are recommended. 

Multiple Centers recommended additional efforts to validate AE techniques, including full-scale 

vessel testing with known and independently verified measurements of crack growth. Likewise, 

multiple Centers recommended additional efforts to develop and validate the PAUT method for 

inspecting circumferential welds. One Center suggested further work on photogrammetric 

techniques. 

In a different area, one Center recommended the installation of manways to allow internal 

inspections of LPV inner shells. Another Center suggested the redesign of systems to provide 

local storage of low-pressure/high-capacity gas capabilities to reduce the requirements for, and 

risk associated with, high-pressure LPVs and systems currently in use. However, physical 

modification and/or replacement of the vessels was outside the scope of the NESC’s charter, and 

these were not pursued further. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

38 of 193

7.1.1.6 Summary of Special Concerns Noted

The Centers were asked to identify any special concerns they had with respect to LPVs. One 

response highlighted the variability that has been observed in previous testing to characterize the 

material properties of the A225 material used in LPV heads. The variability makes it difficult to 

use generic materials properties for this material in analyses and may lead to requirements to use 

either very low bounding properties or to the development of some methods to assess properties 

specific to individual vessels. The problems associated with the unknown relevance of past 

Charpy U-notch or keyhole data to current fracture toughness methodologies were also 

highlighted. 

Several Centers highlighted concerns over the application of ASME Division 1 versus 

Division 2 methods to rate pressures for these vessels. Some noted that Division 2 was not 

appropriate because these vessels were not fabricated to all Division 2 requirements, while others 

suggested that Division 2 could be used safely. This was identified as an area that needs

resolution, and it is noted that the OSMA PVS lead and at least one PSM strongly disagrees with 

the appropriateness of applying Division 2, as is discussed elsewhere in this report. Also, it was 

questioned as to when updated MAWP calculations should be required. 

Another concern highlighted by multiple Centers was the need to understand the overall safety 

record for these vessels in industry, as well as to better understand the catastrophic failures that 

have occurred. In particular, the importance of understanding how the LPVs that failed were 

used was identified, as well as identifying the similarities and differences as to how NASA uses 

these vessels.

One Center highlighted the lack of internal access for inspection of vessel inner shells as a 

particular concern. 

Another concern identified by multiple Centers was the difficulty that might result from attempts 

at implementing uniform processes across the Agency because of the variety of vessel designs 

and applications. It was noted that as an example, invasive inspection approaches might be 

acceptable for one Center and application but could create excessive problems and possible 

damage from contamination in other Centers and applications. The need for having tailored 

approaches for inspection and hazard mitigations was highlighted. 

7.1.1.7 Summary of Processes/Time Periods for Communicating Risk to Center 

Management 

The Centers were asked to provide insight as to the processes, time periods for reporting risk 

associated with these vessels to Center Management, and, in general, perceptions as to the 

awareness of Center management of the risks. The responses clearly indicated that the frequency 

and methods of reporting LPV risks to Center management and their awareness of the risks 

presented by continued use of these vessels varies significantly from Center to Center. The types 

of reporting and frequency of reporting ranged from quarterly reports to Center management 

with specific inclusion of risks and efforts to reduce risk for LPVs, to no reporting at all. Some 

Centers cited waivers for their operation that were approved at various times in the past, ranging 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

39 of 193

from within the last couple of years to more than a decade ago. Further, it was noted that these 

waivers were not periodically reviewed, updated, or required to be reapproved. Most Centers 

generally reported that there was not a high degree of awareness of potential, albeit undefined, 

risks associated with LPVs at Center management levels, but that the potential risk was better 

known to personnel within the facilities or operations organizations, as well as the safety 

organizations. 

7.1.2 Summary of Previous or Ongoing LPV Related Work Identified at NASA Centers

During the internal review as part of Task 1, it was determined that several Centers had previous 

and ongoing activities to help mitigate risks associated with these vessels. Several examples of 

these efforts are summarized in the following. Although some of these efforts were collaborative 

involving multiple Centers, it was important to note that information about much of the work and 

associated results were not being effectively communicated across the Agency. The lack of an 

Agency team or forum to address this problem sometimes resulted in information about the 

individual efforts being kept local to the Center or Centers performing the work. One significant 

accomplishment of this assessment was that information about these efforts has been 

communicated across the Agency and further collaboration on these efforts has been established. 

7.1.2.1 MSFC Materials and Processes Laboratory LPV Activities

The MSFC Materials and Processing Laboratory conducted several preliminary activities to gain 

an understanding of the materials used in the non-code LPVs. These activities were in the area 

of material properties, NDE, and analysis. Available material properties were reviewed, and 

preliminary mechanical tests and metallurgical evaluations were performed. Several NDE 

inspection methods were investigated to provide a sufficient understanding of their relative 

screening efficacies in order to down-select appropriate NDE methods for future evaluations. 

Stress levels and corresponding limiting flaw sizes for vessels of concern were investigated using 

the ASME FFS [ref. 32] methods. In order to gain a better understanding of the structural 

loading inherent to LPVs, with specific interest in determining the location and sizes of flaws for 

a damage-tolerance-based FFS analysis, an analytical tool was developed to parametrically 

model the weld regions in LPVs. The results from this preliminary effort were used to request 

funding to proceed with an ongoing LPV effort, which was ultimately funded and is described in 

detail in Section 7.2. 

7.1.2.1.1 Materials

Material testing objectives were to develop an initial understanding of material performance of 

LPVs, more specifically, the structurally significant material properties, including tensile and 

fracture mechanics behavior. Representative surplus vessels were identified to collect data to 

develop a materials database. Material sources available at MSFC included a CB&I vessel 

(V0125) and an A. O. Smith vessel (V0032) (see Appendix C). This included 1146a shell and 

A225 head material from V0032, and 1146 and 1143 shell and A225 head material from V0125, 

where the materials were identified from the original manufacturer’s data reports. Both 1143 

and 1146 were A. O. Smith designations, while A225 is an ASTM designation. Based on later 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

40 of 193

testing, which is described in Section 7.2.1, the pedigree of the 1146a inner shell material from 

V0032 was shown not to be in question, but this was unknown at the time of the preliminary 

effort. Both vessels were slated for mechanical testing and NDE to determine if there were any 

meaningful differences between materials based on the vendor or material lot. Existing material 

property data were surveyed and compared with preliminary materials tests. Limited tensile 

properties collected at MSFC for the CB&I vessel did not show the expected margin over 

reported design strengths (see Appendix D), and microstructural tests showed unexpected 

possible material anisotropy.

Fracture toughness as a function of temperature was identified as the most influential material 

property for vessel assessment. The fully ductile fracture toughness (upper shelf) and the 

temperature at which the steel transitions to cleavage fracture (lower shelf) were determined to 

be of critical interest in the assessment of LPVs. The use of the T0 reference temperature 

methodology from ASTM E1921-13, “Standard Test Method for Determination of Reference 

Temperature, T0, for Ferritic Steels in the Transition Range” [ref. 33], was chosen as a method to 

evaluate fracture mechanics toughness data as a function of temperature due to brittle transition 

effects. Cleavage versus ductile in terms of ASTM E1921-13 refers to the fracture mechanism, 

(i.e., cleavage of crystal planes versus ductile crack-growth crack mechanism). A brittle 

structural failure can mean either cleavage or ductile but implies a low toughness controlled 

failure relative to failure due to net-section yielding behavior. The E1921-13 approach was 

expected to minimize the number of test samples required to define the ductile-to-brittle 

transition curve. The E1921-13 method was considered especially advantageous because it uses 

a fracture-mechanics-based weakest link theory in the evaluation to maximize the information 

gained from testing from a limited number of tests and because E1921-13, as a statistical 

method, allows failure probability levels (confidence levels) to be statistically determined. The 

fact that it is a fracture-mechanics-based method rather than a correlation means that it gives 

accurate rather than conservative measurement of the temperature above which cleavage fracture 

is not to be expected, compared with nil-ductility-temperature (NDT) based approaches. This 

method was applied to generate confidence curves for several LPV materials (see Appendix E). 

A consultancy between MSFC and a cleavage fracture specialist was researched and proposed to 

assist in planning the scope and performing the analysis for E1921-13 testing. To date, the 

Master Curve method has predominantly been used to evaluate steels in the nuclear industry, and 

it was expected that an expert consultant could help address some of the unique challenges that 

were anticipated. 

Preliminary examination of several welds was also conducted to identify potential fracture planes 

to investigate. These locations included head-to-shell and shell-to-shell welds. The scope of this 

evaluation was to investigate the varied microstructure around the various welds (see 

Appendix D). 

This initial effort led to the recommendation to procure funding to pursue: 

1. Mechanical testing on LPV A225 base head material, to characterize the ductile-to-brittle

fracture toughness versus temperature response. The initial literature research showed

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

41 of 193

that the A225 material would likely have the most elevated transition temperature and 

may cleave at operating temperatures.

2. Mechanical testing on LPV shell materials including 1143 and 1146, to characterize the

ductile-to-brittle fracture toughness versus temperature response.

3. Preliminary mechanical and microstructural testing in LPV weld materials, including:

a. A225 head-to-shell

b. shell-to-shell

c. longitudinal welds (inner shell and wraps)

d. A225 to 5002 nozzle (circumferential weld at base of nozzle), where 5002 is a

common nozzle steel used in the majority of vessels

These activities are discussed in Sections 7.2.1. 

7.1.2.1.2 AE testing of LPVs

AE testing at MSFC is used as a method of screening LPVs, with the goal of identifying active 

crack growth. The original plan at MSFC was to use the MONPACTM testing procedure to 

screen for crack growth. However, because MONPACTM is primarily intended for structurally 

significant defects (those close to failure), an additional analysis method was added to the 

MONPACTM intensity analysis to allow screening for potentially catastrophic crack growth and 

smaller crack extensions. This approach does not require a MONPACTM procedure change and 

uses three-dimensional and two-dimensional algorithm-based source location software that can 

locate smaller sources. The software accuracy is verified using a pretest calibration consisting of 

lead breaks and/or center punch strikes in known locations. The AE software sound velocity 

setting is then adjusted until the software accurately locates the breaks/strikes. 

MSFC uses a Physical Acoustics Corporation 56 Channel DiSPTM Workstation (with 40 channels 

installed) with the following parameters and capabilities:

• Test parameters currently in use: threshold, 42 dB (decibels), 100–400 kHz frequency

filter

• R15I-AST sensors: mid-range sensors, operating range 80–200 kHz

• WDI-AST sensors: wideband sensors, operating range 200–900 kHz

• Waveform option module: processor, buffer, memory, adjustable sample length and rate

Plans are for procurement of additional MAE instrumentation to allow future testing to apply 

both methodologies. 

The current AE test plan at MSFC is to gather data from approximately 10% of the layered 

vessels in the MSFC active inventory to establish an initial data set for evaluation (17 to 

20 vessels total). Ten layered vessels have been AE tested at MSFC to date. No AE signals that 

had characteristics associated with the propagation of structurally significant defects have been 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

42 of 193

detected. MSFC is also planning to conduct lab-scale AE testing later in fiscal year (FY) 2014, 

as funding allows, using layered vessel coupons to investigate crack extension and overall AE 

test effectiveness and sensitivity. The goal of this testing is to understand the amplitude of AE 

signals that might be produced in a typical AE test on an LPV, which is usually accomplished by 

a 10% overpressure at nominal operating ambient temperature conditions, and to evaluate the 

apparent feasibility of using AE to detect growing cracks in the LPVs during proof testing. 

Accordingly, the data of most interest will be crack growth at room temperature. The samples 

will be as large as can be machined and tested due to the constraints of the thin, curved pressure 

vessel materials. With regard to specimen dimensions, the thickness of the samples is one 

limitation that will affect the signal characteristics significantly; it will change the signal mode of 

propagation from bulk modes to plate modes. However, with the layers present in the tank this 

may occur naturally to some extent for the real signals that occur in the tank. Test specimens 

will be used that maximize the lateral extent since the effect of reflections from these lateral 

boundaries will impede observation of the original arriving mode and make it difficult to assess 

the direct arriving signal amplitude of interest. For these tests, the focus is only whether the 

crack grew and by how much, not necessarily on calculating the fracture toughness, which is to 

be accomplished separately. Detecting cleavage fracture at low temperatures, which would 

likely produce the largest amplitude and most easily detectable AE signals, will not be pursued. 

Since the AE method is still not validated, the information acquired is for engineering evaluation 

only. However, the method is providing useful information on issues related to applying AE in 

the field on these vessels and on the assessment of the background noise produced and how it 

varies between different vessels that have undergone different periods of inactivity. A vessel 

with a known crack (V0256) has been removed from service, and plans are under development to 

perform hydrostatic testing while monitoring with AE. 

7.1.2.1.3 Analysis

The infrastructure LPVs are considered industrial ground support equipment (GSE) for which 

safety rationale based on FFS is being developed. National consensus code approaches are 

customarily used on GSE because these approaches maintain proper conservatisms and 

consistency in the FFS assessment. The current ASME BPVC [ref. 1] and ASME-FFS-1 

(API-579) [ref. 32] and related document are being considered as the FFS anchor for the LPV 

assessments. Even though ASME FFS-1 (API 579) does not specifically address assessment 

methods for layered vessels, Part 9 of this Standard allows flexibility for tailoring and 

establishing an approach for assessing LPVs. 

To establish a viable FFS approach for the LPV fleet requires not only a clear understanding of 

the material behavior fundamentals being investigated concurrently (as discussed in Sections 

7.1.2.1.1 and 7.2.3) but also an understanding of the physical behavior resulting from the layered 

construction. These include the effects of unintended layer gaps, flaws in intermediate layers or 

layer welds, residual stress effects, and the effects of proof tests on the overall state of stress, for 

example. Since a real-time snapshot of the interior or in-between layers of LPVs is not always 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

43 of 193

practical, analytical tools, such as finite element analyses, are being employed to gain an 

understanding of these physical fundamentals. 

As part of this assessment, finite element analyses were conducted to investigate various 

parameters that affect the state of stress of an LPV as compared with the classical thick-walled 

pressure vessel solution. A modeling tool, LPV Analysis Tool (LAPVAT), was developed to 

automatically generate and post-process LPV finite element models using the Abaqus® PythonTM 

interface. LAPVAT enables efficient parametric studies by allowing the user to input the 

dimensions of the vessel, the number of layers, the layer gaps, the material properties, meshing 

parameters, internal pressures, temperatures, angular section size, and other parameters. These 

inputs are used to automatically generate a three-dimensional model of a sector of an LPV 

including a head, the specified number of shell layers, and the head-to-shell weld region with 

contacts between layers. Details of the development of this tool and results may be found in 

Appendix G. 

7.1.2.2 NASA ARC – MAE Validation and Materials Testing Efforts 

AE testing is a method used to detect cracks or other damage sources in structures by detecting 

and analyzing acoustic waves that are produced by growth of the damage or the contact and 

rubbing of crack surfaces as a result of load or pressure being applied to the structure. An array 

of acoustic sensors is placed on the structure to detect AE signals. The detected AE signals are 

analyzed to locate the damage site by triangulation and to assess the damage and discriminate 

extraneous noise by analysis of the signals or features of the signals. A simple analogy of the 

AE technique is that of structural seismology, where the growing damage sources are the 

equivalent of earthquake sources within the structure. 

MAE is an AE approach based on the capture and analysis of complete AE waveforms acquired 

with high-fidelity, broadband acoustic sensors. MAE analysis incorporates a physics-based 

understanding of the modes of acoustic propagation for the structure being tested and the 

relationship between the crack sources of interest and the resulting acoustic modes that are 

generated and propagated [refs. 34–36]. This is in contrast to the resonant sensors and analysis 

of signal features or parameters based on a preexisting database for similar structures, which are 

employed in a competing approach that has also been applied to LPVs, known as MONPACTM

AE [ref. 25]. MAE has the potential for providing more accurate location of AE sources and for 

improved characterization of AE sources and discrimination of extraneous noise as compared 

with conventional AE parameter analysis approaches.

Although MAE has not been previously fully validated for application to LPVs, it has been 

validated for a number of other related applications. One example is the application of MAE to 

retest DOT cylinders and submarine flasks that resulted in the award of a DOT Special Permit 

[ref. 37] and the Navy’s ongoing use of MAE for flask requalification. Previous conventional 

parameter-based AE approaches were demonstrated for detection and location of subcritical and 

critical size cracks in these solid-wall high-strength steel vessels [refs. 38 and 39]. MAE 

improved on these approaches by providing improved source location accuracy and the ability to 

discriminate noise. In these and other applications of MAE, Digital Wave Corporation (DWC) 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

44 of 193

utilizes published ASTM standards [refs. 40–43] as they apply to MAE with respect to 

instrument calibration and system performance testing.

Because of its demonstrated success in detecting crack growth in other related applications 

involving steel pressure vessels, MAE was evaluated for and eventually applied to full-scale 

testing of LPVs at NASA ARC as well as at other NASA Centers. As part of this effort, NASA 

ARC has worked with DWC since 2001 to fully validate MAE for LPVs [refs. 26–29] with 

cyclic testing in 2011 and 2012 funded by the NASA OSMA. These efforts, which are described 

in more detail in Appendix G, have successfully demonstrated many aspects of MAE for LPVs 

including evaluation of modal wave propagation characteristics (i.e., modal content, velocity, 

and attenuation) of signals generated by simulated AE sources (i.e., pencil-lead breaks) in LPVs, 

demonstration of AE signal propagation though the layered shell thickness, and evaluation of 

LPV background extraneous noise. The only aspect of MAE that was not demonstrated in initial 

testing on the large vessels at ARC was detection of actual crack growth in an LPV because it 

was not feasible to achieve verifiable growth of an induced crack on these large vessels.

Even without the full validation of the MAE technique to detect growing known cracks in an 

LPV, it was the engineering judgment of both DWC and NASA PSMs at several Centers that the 

method could be used as a periodic inspection tool to determine whether any relevant, growing 

cracks existed in the vessels (i.e., large cracks growing with each pressure cycle, probably close 

to critical size). This was based on the known success of MAE for monitoring other steel 

pressure vessels and the success in demonstrating MAE propagation in LPVs. Thus, MAE was 

applied for full-scale testing on the ARC vessels with the intent that if MAE indicated the 

presence of any growing cracks, the vessels would be removed from service and efforts made to 

verify the presence of cracks through other forms of NDE. The vessels would then, if possible, 

be repaired. If any indicated cracks were confirmed, this would provide additional validation 

data for the method. However, none of the vessels at ARC exhibited MAE indications of crack 

growth. Thus, ARC concluded that an additional validation test was required in which MAE 

would be applied to monitor an LPV with a known, fully characterized growing crack. 

Attempts at demonstrating MAE detection of crack growth in a LPV were performed using 

cyclic crack-growth tests on a much smaller LPV (44 cubic feet) that was obtained from KSC. 

This work was managed by ARC and their prime contractor, Jacobs Technology, under OSMA 

research funding, and carried out by DWC. The test vessel was pressure cycled 4,688 times, 

about half of which were between its nameplate rating of 6,600 psi and 1,000 psi, and the 

remainder at progressively higher pressures up to yield strength of the outer layer. MAE was 

recorded throughout. This effort was, however, only partly successful in that, while MAE was 

shown to be able to detect the actual growing crack under field test conditions, the crack 

exhibited low acoustic energy release and was essentially at the threshold of detectability for the 

AE system being used. As such, it would likely have been missed in a standard production field 

test. It was later determined that the crack was much smaller than the expected critical size

(approximately 25%) [refs. 26 and 44], which results in lower AE signal amplitude. Incomplete 

knowledge (at the time of the test) of the higher toughness of the material in the vessels 

contributed to using too small an initial crack size in the previous demonstration test. Further, 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

45 of 193

limited funding precluded continuation of the vessel cycling to grow a larger crack closer to 

critical size with (presumed) greater intensity AEs. Tests on vessels with larger cracks (closer to 

the critical crack size) will be necessary to better demonstrate and fully validate the capability of 

MAE for detecting crack growth and to document the relationship between signal amplitude and 

detectable crack size. Details on both the cyclic pressure testing and MAE assessment work 

performed by DWC are contained in reference 26. 

While additional full-scale cyclic testing is required to validate either modal or parametric AE, 

this cyclic test work suggested that research on AE monitoring of laboratory scale specimens 

under cyclic loading could provide important information regarding the absolute intensity of AEs 

from growing cracks as detected by the various approaches to AE. Although this is discussed in 

the literature with mixed results, current techniques could improve upon issues encountered in 

the past. This recommendation was accepted by the NESC and ongoing work at MSFC and 

LaRC in that regard is described in Section 7.2.6 of this report. 

Upon completion of the MAE tests of crack growth on the smaller LPV, the vessel was sent to 

Southwest Research Institute® (SwRI)® for fractographic analysis of the crack growth and

materials characterization efforts to assess some of the relevant properties for the vessel

materials [refs. 44 and 45]. In particular, measurements of fracture toughness and fatigue crackgrowth properties of the principal material components for this vessel were performed. This 

work focused on the inner and outer shell layers, material from one of the heads, and weld and 

HAZ material from one head-to-shell weld. For this limited sampling from this one vessel, 

tensile properties, impact energy, fracture toughness, and fatigue crack-growth properties were 

documented. When SwRI® incorporated these properties into a fracture crack-growth analysis 

(using NASA’s NASGRO® software, for which they are a Space Act Agreement developer) of 

the starter notch that was machined into the outer shell, the calculated crack growth closely 

matched the actual growth experienced in the cyclic field test. Of note, critical fracture 

toughness for both shell and head material was shown to be greater than indicated by prior work 

done at LaRC in the 1970s [ref. 46], although impact energy absorption using the Charpy 

V-notch test (which is required by ASME) was measured to be much less at low ambient 

temperatures. In fact, both head and shell material exhibited brittle fracture characteristics at 

0°F, which can be a significant problem at several Centers. While there are known issues with 

utilizing Charpy impact energy results to statistically predict brittle fracture behavior, further 

work is required to determine the NDT and safe operating temperatures for all of the material 

that comprises these vessels. This effort has begun at MSFC with a program to determine 

transition reference temperature in accordance with ASTM E 

1921-13 [ref. 33] and ASME FFS-1/API-579 standards [ref. 32]. In addition, since destructive 

material testing cannot practically be performed on NASA’s fleet of in-service vessels, a much 

wider sampling of materials from other similar vessels is needed to develop an adequate 

confidence level in applying the SwRI®, MSFC, or any other test data obtained from specific 

surplus vessels to the in-service fleet. Details on both the phase 1 and phase 2 work performed 

by SwRI® are contained in a detailed summary in Appendix H, and the SwRI® reports are 

available in total as NASA Contractor Report documents [refs. 44 and 45].

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

46 of 193

7.1.2.3 SSC/MSFC Evaluation of PAUT Methods for Inspection of Full Penetration 

Circumferential Welds on LPVs

There are currently no NDE techniques available that are validated for volumetric inspection of 

cracks in the full penetration circumferential welds of LPVs (RT limitations were discussed 

previously). PAUT techniques have been identified as a potential method for inspection of these 

welds, and at least one report in the literature [ref. 22] has shown promising results. Preliminary 

evaluation performed for MSFC and SSC also demonstrated the potential of this method. 

Background 

In April 2013, personnel from Davis NDE, located in Hoover, AL, were invited to visit MSFC to 

provide a demonstration of PAUT equipment for potential use as a weld inspection tool for 

LPVs. Davis NDE had previously reported to the SSC PSM some success in detecting flaws in 

layered vessels located overseas using PAUT techniques. In a collaborative effort between SSC, 

MSFC, and GRC, a plan was made to have Davis NDE provide an overview of the PAUT 

technique and operating principles, along with a hands-on demonstration of the PAUT 

equipment. MSFC was chosen as the site for the equipment demonstration due to the close 

proximity of MSFC to the Davis NDE office and the fact that MSFC had an out-of-service vessel 

readily available for the weld scanning demonstration. 

Davis NDE was not under contract with MSFC for any services and the initial visit was intended 

to be an equipment demonstration only, at no cost to the government. However, subsequent to 

the visit, Davis NDE was asked to perform a review and interpretation of the PAUT data and 

provide informal (courtesy) reports to MSFC. It was understood that the PAUT weld scans 

performed were not code-quality scans since there was no calibration standard available for 

PAUT equipment calibration or for code-required weld scan plan development. Therefore, the 

PAUT weld scan data provided in this report are for reference purposes only and may not be 

used to assess vessel V0256 FFS. 

Initial Demonstration of PAUT Equipment

The initial proof-of-concept demonstration by Davis NDE included general setup and operation 

of Olympus Omniscan phased array equipment. MSFC provided a vessel head-to-shell weld 

coupon obtained from layered Vessel V0125. Davis NDE personnel discussed general PAUT 

equipment setup and operation, performed a weld scan of the vessel coupon to demonstrate the 

scanning technique and on-screen data presentation options, and provided a real-time 

interpretation and discussion of the screen data. This first demonstration was focused primarily 

on equipment setup and adjustment, various scanning techniques, transducer options, data 

presentation formats, calibration requirements, and critical variables associated with 

development of weld scan plans. 

PAUT Demonstration on Vessel V0256 

Davis NDE personnel were asked to perform an in situ scan of weld areas on MSFC vessel 

V0256, shown in Figure 7.1.2.3-1, which was previously depressurized and taken out of service. 

V0256 is a 1,250-cubic-foot, 5,000-psig layered vessel built by CB&I in 1963. The vessel was 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

47 of 193

constructed with a monolithic head and a layered shell section consisting of 14 total layers 

(15/32-inch inner layer and 13 overwraps of 9/32 inches each) for a total shell thickness of 

4 1/8 inches. Vessel V0256 also has a readily visible defect in the outer layer base metal, a 

longitudinal crack running perpendicular to the head-to-shell weld, which was originally detected 

during an MT inspection (see Figure 7.1.2.3-2).

Figure 7.1.2.3-1. MSFC Vessel V0256 with Welds Stripped for Inspection 

(A225 head material; MLP 1143 shell material)

Figure 7.1.2.3-2. Longitudinal Crack in Outer Layer of V0256 

Aside: Radiography had recently been performed on the full penetration welds of vessel V0256, 

not only to assess the quality of the vessel welds, but to gain more insight into the challenges of 

performing radiography on large, thick-wall layered vessels and to understand the extent and 

quality of volumetric weld data available from the radiography. Because MSFC has limited 

experience with performing radiography on large, thick-walled vessels, personnel from GRC

traveled to MSFC to assist with the setup and provided setup tools commonly used at GRC. In 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

48 of 193

short, radiography was being considered as another inspection tool for use by MSFC in 

evaluating the integrity of LPVs. 

Vessel V0256 provided an opportunity for demonstration of the PAUT technique and equipment 

under field conditions and on a shell-to-shell weld, which was not available with the vessel 

V0125 coupons. Weld segments on V0256, including both head-to-shell and shell-to-shell weld 

regions, were prepared in advance of the PAUT demonstration by grinding the weld crowns flat 

to allow better coupling of the ultrasonic transducer with the vessel weld surface. Radiography 

was performed on V0256 prior to arrival of Davis NDE but resulted in only one positive 

indication, which could be used for comparison with the PAUT results. 

Davis NDE personnel scanned the areas identified by MSFC, demonstrated various scan 

techniques, provided real-time data interpretation, and conducted hands-on training for the 

MSFC NDT technicians present. The line scan inspections on vessel V0256 were performed by 

setting the PAUT equipment gain to define both the weld root area and the shell section layers. 

Davis NDE indicated that this approach was used because no qualified procedure or calibration 

block was immediately available for this vessel inspection. Davis NDE personnel also scanned 

the longitudinal crack in the outer layer base metal and the adjacent head-to-shell weld in an 

attempt to better characterize the flaw.

Results

Figures 7.1.2.3-3 through 7.1.2.3-6 show PAUT scans of various indications observed in the 

vessel V0256 head-to-shell and shell-to-shell weld volumes. These figures demonstrate the 

PAUT capability to distinguish, to some extent, the shell layer edges from indications within the 

weld volume. In particular, Figure 7.1.2.3-5, which is a shell-to-shell weld scan, shows the 

symmetry of the individual layer tip reflections (indications) on both sides of the weld volume. 

In Figure 7.1.2.3-6, Davis NDE reports a discontinuity extending from the end of a layer into the 

weld volume. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

49 of 193

Figure 7.1.2.3-3. PAUT Scan of Head-to-Shell Weld Region (top portion) 

Figure 7.1.2.3-4. PAUT Scan of Head-to-Shell Weld Region (bottom portion) 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

50 of 193

Figure 7.1.2.3-5. PAUT Scan of Shell-to-Shell Weld Region 

Figure 7.1.2.3-6. PAUT Scan of Shell-to-Shell Weld Region 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

51 of 193

Davis NDE also provided tabular results of the weld scans for specific weld segments on 

vessel V0256 using both time based and linearly encoded scans. Davis NDE used both 

techniques to further demonstrate the scanning and data acquisition capabilities of the 

equipment. Table 7.1.2.3-1 shows the results for head-to-shell weld using linearly encoded weld 

scans. The linear encoding allowed Davis NDE to characterize the indications (i.e., height and 

length) and shows the relative location of the indications along the weld length. Table 7.1.2.3-2 

shows results reported by Davis NDE for the vessel shell-to-shell weld using time-based scans 

rather than linearly encoded scans. Further characterization of the indications from these scans 

was not provided since the data were not encoded for this part of the demonstration. 

The PAUT data interpretation by Davis NDE resulted in observations at each weld location 

scanned. Davis NDE personnel were asked to identify any and all observations, not just those 

that would be BPVC rejectable indications. Weld C3, location O-P is the only weld area of those 

scanned by Davis NDE personnel where radiography also indicated there was any indication 

within the weld volume. The MSFC American Welding Society film interpreter reported a 

BPVC rejectable indication of porosity at this weld location (C3/O-P). 

As noted earlier, the data reviewed were not from code quality scans, which results in less than 

desirable confidence in the accuracy of the reviewed data. The records below are merely

documentation of the indications presented in these scans and do not detail the overall 

component quality. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

52 of 193

Table 7.1.2.3-1. Head-to-Shell Weld

Identification

Meets CodeFails CodeLinear

PlanarRoundedCluster% DAC

Discontinuity

Scan Number

Comments

(Surface/

Height

Subsurface)

LengthSound PathDepth From “A”

Distance From X From Y

Weld C1 

(TOP) X .05” .24” 0” 6.7” Slag

X .18” .35” +.7” 7.9” LOF

X .10” .27” 0” 9.3” Slag

X .15” .26” 0” 9.9” Slag

X .09” .28” +.2” 12.9” Slag

X .28” .37” +1” 13.0” LOF

X .07” .14” +.2” 13.8” Slag

Weld C1 

(BOT) X .59” 2.9” -.1” 7.5” Crack

X .34” .26” +.3” 11.3” Slag

X .07” .15” 0” 12.1” Slag

X .44” .21” +.1” 13.0” Crack

Table 7.1.2.3-1 notes:

1. The head-to-shell data were encoded but scan axis positional information may differ

slightly from the first to the second index position.

2. 100% weld coverage was not obtained in this scan.

Table 7.1.2.3-2. Shell-to-Shell Welds 

Identification

Meets CodeFails CodeLinearPlanarRoundedCluster% DAC

Discontinuity

Scan Number

Comments

Height

(Surface/Subsurface)

LengthSound PathDepth From “A”

Distance From X From Y

Weld C3 (L 

to K) X Slag

X Slag

X Crack-like

Weld C3 (O 

to P) X Crack-like

X Slag

The only weld location scanned by Davis 

NDE which had indication (porosity) 

previously identified by radiography 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

53 of 193

Weld C5 (M to 

N) X Crack-like

X Slag

X Crack-like

X Crack-like

X Slag

Weld C5 (N to 

O) X Slag

X Crack-like

X Slag

X Slag

X Slag

X Crack-like

Table 7.1.2.3-2 notes:

1. The shell-to-shell weld scans were not encoded, which prohibits reporting of any

relatively accurate information pertaining to flaw length.

2. 100% weld coverage was not obtained in this scan.

Figures 7.1.2.3-7 through 7.1.2.3-9 show additional data obtained by Davis NDE during the 

PAUT scan of vessel V0256. These figures provide a comparison of the “A,” “S,” and “B” scan 

data available with the Olympus Omniscan phased array equipment. A-scan is the typical time 

versus signal amplitude plot shown in the upper left of these figures. The S-scan is the sectorial 

scan image shown in the upper right in which the ultrasonic beam is electronically steered 

through a sector of the material while the transducer remains at a fixed location. The B-scan is 

shown in the lower image where the transducer is translated along the weld. Figure 7.1.2.3-7 

shows an indication in the C5 weld O to N location, indicative of slag. Figure 7.1.2.3-8 shows a 

linear indication in the C5 weld O to P location indicative of lack of fusion. The “S” and “B” 

scan data formats provide a far more intuitive presentation of data than that of the “A” scan 

(which is the typical conventional UT data format). It should be noted that these figures are 

screen views, which represent data from only a single location along the length of the weld. 

Phased array data were being recorded at approximately every 0.040 inches along the weld 

length. These screen views are also not representative of all the indications listed in 

Table 7.1.2.3-2 for the C5 weld. 

Figure 7.1.2.3-9 shows the scan data from the visible crack in the outer layer of V0256 (photo of 

crack shown in Figure 7.1.2.3-2). The PAUT data do not provide any significant new 

information beyond what could be determined from radiography or conventional UT but do 

provide better characterization of the crack as it runs through the outer shell. As noted earlier, 

the crack is contained in the outer shell base metal and does not enter the weld. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

54 of 193

Figure 7.1.2.3-7. PAUT Scans for Indication in C5 Weld O to N Location, Indicative of Slag 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

55 of 193

Figure 7.1.2.3-8. PAUT Scans for Linear Indication in C5 Weld O to P Location, Indicative of Lack 

of Fusion

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

56 of 193

Figure 7.1.2.3-9. PAUT Scans for Visible Crack in Outer Layer of V0256 

Observations and Conclusions

The demonstration by Davis NDE of the Olympus Omniscan PAUT equipment provided insight 

into the overall equipment operation and capabilities, as well as the weld scanning techniques. 

The amount of data and detail observed using the PAUT equipment far exceeded the amount of 

information available from the radiography or conventional UT in the specific weld areas 

inspected. The PAUT data shown in this report are only a small sample (screen view snapshots) 

of the data recorded during the V0256 weld inspection demonstration. 

• No direct conclusions can be made from the comparison of PAUT data with radiography

or conventional UT data on Vessel V0256 since there were few indications for

comparison and there were no fully characterized flaws within the weld volumes that

could be used for a quantifiable comparison. The PAUT weld scans of vessel V0256

were the most expedient means of obtaining PAUT data at the time to better understand

the inspection technique and to assess general equipment capabilities. Many of the

indications reported by Davis NDE in Tables 7.1.2.3-1 and 7.1.2.3-2 simply cannot be

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

57 of 193

confirmed at this time since radiography revealed no indications at these locations and no 

weld excavations have been performed.

• The PAUT technique appears to offer an immediate improvement over conventional UT

inspection in the level of detail that can be observed for solid head to layered shell weld

inspection. Although radiography and conventional UT could be used for inspection of

the head-to-shell weld, PAUT offers the benefit of being a nonintrusive technique while

providing beam steering capability to more fully investigate the area of interest, which is

not available with conventional UT. (Beam steering refers to the PAUT capability to

shift the phase of the signals from the pulser array, taking advantage of constructive and

destructive wave interference and, in essence, directing (steering) the sound beam to

specific areas of interest within the weld volume. Conventional UT does not provide this

capability since only a single pulser-receiver is used.)

• The PAUT data appear to distinguish between indications of individual layers and

indications in the weld volume. Further assessment will be needed to determine how

defect orientation, size, and location (especially with respect to the shell layers) will

affect capability and probability of flaw detection.

• The additional data presentations available with PAUT but not with conventional UT—

the “S” and “B” scan data formats—allow a far more intuitive presentation of the weld

volume and facilitate improved characterization and evaluation of flaw data. The

improved data visualization in combination with the beam steering capability offers a

powerful tool for weld inspection and assessment.

• The advantage that PAUT offers over conventional UT is somewhat offset by the

additional training, interpretation skill required by the NDE specialist, and initial cost of

the PAUT equipment. Individual assessment is required to determine whether PAUT

benefits outweigh the associated cost especially in cases where other existing NDE

techniques are already being employed with some success.

• PAUT does not provide a means of inspecting the longitudinal welds within a layered

vessel other than those layers that are physically accessible (i.e., the outer shell and

sometimes the inner shell) as the layer gaps are reflective surfaces and preclude passage

of sound waves. However, PAUT may provide a means of inspecting a short distance

into the layers immediately adjacent to the full penetration welds. Additional evaluation

will be necessary to determine how far inspection capability extends into the layers, if at

all, and how the weld geometry affects the probability of flaw detection.

• The Olympus Omniscan equipment provides capability to digitally store all equipment

parameters and data from the inspection. Once the welds are scanned, the resulting data

files can be reviewed at any time, and these data files provide a permanent record of the

inspection. These data files also facilitate collaboration with others when evaluating and

interpreting the inspection data.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

58 of 193

• PAUT requires a relatively flat weld crown to provide better coupling between the weld

surface and the ultrasonic transducer. This requires time and effort to prepare welds,

especially on larger vessels, but once the weld surfaces are properly prepared subsequent

PAUT inspections can be performed without further weld preparation.

• Other, more general benefits of PAUT include:

o PAUT can be performed with the vessel in service (on-stream), minimizing

impact to ongoing operations.

o Reduced vessel downtime in cases where PAUT is used in lieu of intrusive vessel

inspection techniques. The reduced downtime can be substantial for vessels

containing cryogens, propellants, and high purity media due to both the preinspection and post-inspection activities required to open the vessel, inspect it,

and return it to service (e.g., pre-inspection warming and the rendering of

cryogenic and propellant vessels inert; post-inspection leak check, cleaning,

sampling, inerting, and reestablishing media purity levels).

o No personnel exclusion zones (as are required for radiography) are necessary for

PAUT, allowing inspections to be performed during regular working hours and

concurrent with other unrelated but ongoing operations. RT exclusion zones can

be particularly intrusive to other operations when using a cobalt source, which

would be necessary for radiography of the many heavy walled vessels at MSFC

that are in close proximity to inhabited buildings and test facilities. All costs and

benefits of each inspection method must be included in analyses, including such

items as cost of NASA or NASA contractor personnel who may need to work

overtime to monitor after hours operations.

Future Work

Additional work is planned to further explore the capabilities of PAUT for inspection of LPVs. 

The PAUT work that initially began as a collaborative effort between SSC, MSFC, and GRC 

now includes participation by the NESC. 

A statement of work (SOW) was developed in late FY13 to further investigate and better 

quantify the capabilities and limitations of PAUT for detecting flaws in LPVs. The SOW the 

and associated drawing were provided to Davis NDE to obtain a cost estimate to build a 

performance demonstration block that will simulate a layered vessel full penetration weld and 

will include a number of induced flaws with prescribed size, orientation, and location. ASME 

Code Case 2235 is being used as a basis for flaw size for the demonstration block and has been 

validated by the MSFC Material Laboratory personnel as an acceptable approach for this effort. 

In addition, ASME Section VIII Div I Part ULW [ref. 1] will be used as the basis for 

construction of the layered demonstration block. The primary objectives of this work will be to: 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

59 of 193

• Further quantify the capability of PAUT to characterize known flaws in the weld volume

(accuracy of flaw sizing and locating within the weld volume) and identify variables

associated with flaw characterization.

• Investigate the effect of layer gap reflections in the area adjacent to the full penetration

weld region and the ability of PAUT to distinguish layer gaps (layer tip reflections) from

actual flaw indications in the weld volume.

• Determine whether PAUT can detect flaws that occur in the layers and do not continue

into the circumferential weld, and if so, assess how far into the layers flaws can be

detected.

• Further the development of weld scanning techniques, procedures, and scan plans for

PAUT inspection of head-to-shell and shell-to-shell full penetration welds.

The start date for this follow-on work with Davis NDE is planned for mid-FY14. Additional 

work may be required beyond this planned effort to further assess PAUT capabilities, to address 

specific problem areas that are identified or to quantify the probability of flaw detection. 

Additional efforts to develop a calibration block for PAUT evaluation was included as part of 

Task 2 of this assessment. The results from this activity are described in Section 7.2.5. 

7.1.3 External User Survey

In addition to the internal survey of NASA Centers, select external users of non-code LPVs were 

contacted and questioned about their approaches to operating, inspecting, and maintaining these 

vessels. To assist in this external user survey, a nationally recognized consultant on these 

vessels, Dr. Maan Jawad, was added to the assessment. Dr. Jawad previously worked with a 

company that designed, fabricated, and repaired LPVs. He also has served extensively in 

numerous roles on the ASME Codes Committees responsible for the development and approval 

of codes for pressure vessels, including LPVs. As such, he had numerous contacts in industry 

where these vessels are in use. Additionally, he was able to provide general background 

information on the procedures for non-code commercial LPV certification and inspection. 

In addition to the contacts provided by Dr. Jawad, several other contacts for additional industrial 

and government facility users of these vessels were gathered. In all, responses from seven 

different external LPV users were received. Most agreed to provide information only on the 

condition that their company names would not be included in the final report. Most of the 

external users surveyed were in the chemical processing industry, although some were in 

aerospace. In general, the LPVs used in the chemical processing industry were operated under 

significantly different conditions from those within NASA. They typically contained corrosive 

products and were operated at elevated temperatures. The vessels in use at aerospace facilities 

were operated under similar conditions to NASA LPVs, storing high-pressure noncorrosive gases 

and used under ambient temperature conditions but at times subject to low temperatures 

associated with blowdown operations or unusual weather conditions. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

60 of 193

In the following section, some general information regarding the certification and inspection 

procedures for non-code LPVs is provided. Then, some additional insight from the specific 

external users contacted in this survey is also provided. 

7.1.3.1 General Procedures for Commercial Non-ASME Stamped LPVs

7.1.3.1.1 Certification

The rules for layered vessels were incorporated in the Sections VIII-1 and VIII-2 in 1978. Prior 

to that time, layered vessels were built in accordance with the manufacturer’s in-house standards. 

Each manufacturer had its own construction standard. The design of these layered vessels was, 

as much as possible, in general accordance with VIII-1 [ref. 1]. Because of these variations from 

BPVC requirements, these layered vessels could not be put into service in states requiring code 

construction without first registering them with the state jurisdiction where the vessel was to 

operate. Jurisdictional requirements can vary greatly from state to state. However, the general 

registration process proceeds along the following lines: 

• The manufacturer contacts the Chief Boiler Inspector of the state where the layered

vessel is to operate and requests a “state special.”

• The “state special” certificate is issued by the Board on Boilers and Pressure Vessels for

that state. The Board consists of members who are normally appointed by the Governor

and approved by the senate of that state. The board members represent various “interest”

segments within the state and usually include representatives from manufacturing,

academia, users, labor unions, insurance companies, technical consultants, etc.

• The manufacturer submits a package to the Board prior to their meeting (they typically

meet four times a year) consisting of the following:

o Design report showing the required thicknesses of various components such as

heads, shell, nozzles, skirt, etc.

o Manufacturing drawings showing materials used and details of construction.

o Welding details and procedures in accordance with Section IX of the ASME

BPVC.

o NDE testing in accordance with Section V of the ASME Code.

o Hydrotest information and procedures.

o Any other pertinent information.

• The manufacturer’s request is discussed at one of the meetings of the Board.

Representatives from the manufacturer (typically, the chief engineer and chief

metallurgist) are normally present at the meeting to answer any questions the Board may

have.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

61 of 193

• The Board usually approves the registration request with additional requirements. These

additional requirements vary greatly from state to state and may include changing the

factor of safety, imposing additional fabrication and or testing requirements, etc.

• The Board issues a “State Special” certificate with a registration number. This number

will be stamped on the vessel nameplate prior to operating the vessel.

7.1.3.1.2 In-Service Inspection

Commercial layered vessels are inspected during routine shut downs (normally every 2 to 

5 years). For inspection guidance, the chemical industry normally follows the rules of the 

National Board Inspection Code (NBIC) [ref. 47], and the petroleum industry follows the rules of 

API 579 [ref. 32], although neither of these standards contains guidance specific to LPVs. Some 

large companies have their own proprietary in-house standards to supplement the standards 

mentioned above. Some of the features of layered vessels that are taken into consideration in the 

in-service inspection program in commercial applications are listed here.

• Vent holes: Vent holes are considered one of the most important safety features of

layered vessels. Any cracks that occur in the inner shell during operation will result in

leakage through the vent holes, which is detectable in principle by a number of different

means.

o Many companies, and especially those producing fertilizer, routinely monitor the

depth of vent holes during planned shutdown to make sure they are not plugged

with product (such as carbamate, which changes from liquid to solid immediately

upon exposure to atmospheric pressure).

o Many companies, especially those using lethal gases, connect all vent holes to a

piping system that use sniffers, vacuum sensors, and pressure sensors to detect

leaks caused by cracking of the inner shell.

• Residual stresses in circumferential welds: The ASME BPVC requires carbon steel

welds greater than 1 1⁄2 inches in thickness to be post-weld heat treated (PWHT) in solid

wall construction. Heat treatment accomplishes two purposes. The first is to temper the

welds, and the second is to reduce fabrication residual stresses. Such a requirement for

PWHT is not mandated for layered construction. Thus, the circumferential thick welds

in layered construction tend to have high hardness and high residual stresses, both of

which are detrimental to the welds due to the possibility of both SCC and fatigue.

Accordingly, the inside and outside of these welds are routinely dye-penetrant examined

or magnetic-particle inspected during scheduled shutdowns to detect any possible cracks.

7.1.3.2 Responses from Specific External LPV Users

Three companies were contacted regarding their procedure for inspecting layered vessels. The 

information received is general in nature, with no specifics. This was to be expected due to 

liability issues. Two of the companies used the NBIC, which is for general guidance only. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

62 of 193

API 579 was also mentioned as a reference, although API 579 does not contain specific rules for 

layered vessels. 

Company A (chemical plant in Tennessee)

The company does not have in-house rules for inspection, repair, and alteration. The company 

follows the NBIC and state rules for operations. 

• The company uses NBIC as a basis for all of their pressure-vessel inspection, repair, and

alterations.

• The company supplements NBIC rules with the more rigorous rules of API 579 (when

needed), subject to the state jurisdictional approval.

o API 579 allows operation with full pressure and 90% remaining wall thickness

(10% general corrosion) with appropriate NDE testing.

o API 579 allows temporary operation of a vessel with a crack. The company uses

very conservative KIc and crack-growth values and thorough NDE examination in

order to allow a vessel to operate with a flaw until a scheduled shutdown, during

which the flaw is removed and repaired.

Company B (refinery in California)

• The company does not have in-house rules for inspection, repair, and alteration but rather

follows the NBIC, API 579, and state rules for operations.

• All inspection is done by the inspection department of the company in accordance with

the NBIC.

• If the NBIC does not have rules that address a specific problem, then the inspection

department contacts the engineering department for an evaluation using the rules in

API 579.

• API 579 has three levels of evaluation. Level 1 is the most comprehensive, and level 3 is

the least. The state jurisdiction requires that all repairs and alterations made in

accordance with level 1 must be approved by the Chief Boiler Inspector of the state prior

to implementation.

Company C (polyethylene and polypropylene plant in Texas)

• The company uses layered vessels when a process is highly corrosive and the stainless

steel inner shell is needed for protection, and the vent holes are needed for leak detection.

• Because of the corrosive environment, the company uses AE for monitoring the layered

vessels, which is performed by Stress Engineering (of Houston, Texas) during a hydrotest

using a factor of 1.1 on operational pressure.

• The company encounters more problems with cracking when the layers are made of highstrength steel and the heads of lower strength steel.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

63 of 193

• When cracking is detected, it is at the head-to-shell junction.

• The company inspects the layered vessels inside and out during normal shutdowns.

• The company does not normally use the NBIC since Texas is a non-code state.

Based on the above information, it can be concluded that the commercial segment uses available 

NDE and inspection tools. No particular new methodology or procedure has been developed for 

inspecting and monitoring layered vessels. However, these vessels are regularly inspected 

during planned shutdowns at intervals of 2 to 5 years.

Two other chemical companies provided responses. One noted that their vessels have jackets, 

which is where they have the most problems with cracking due to differential thermal expansion 

at elevated temperatures. Another company noted that they had undergone a process to rerate 

their LPVs to operate at higher pressures a number of years ago. This was based on results from 

analysis and MAE testing under hydrostatic proof testing. Unfortunately, they were not able to 

provide any details of the analysis or test results. Otherwise, both companies reported the use of 

similar inspection methods, including visual inspection, inspection of vent holes, and limited UT. 

Two aerospace LPV users (one commercial company and one government facility) provided 

responses. Both indicated that their vessels were used for high-pressure, noncorrosive gases at 

ambient operating temperatures similar to NASA LPVs. Both also indicated that hydrostatic 

proof testing was used as part of the initial certification of the vessels. They also used visual 

inspections, UT, and radiography during initial certification. One user noted that their vessels 

were last certified for an additional operating period of 20 years and were approaching the end of 

this certification. They stated that they were unsure of what they would do to recertify the 

vessels for continued operation and were interested in the results from this assessment and 

follow-on testing to help in their decisions for recertification. Neither user was doing any 

materials testing work or evaluating any new NDE methods for these vessels, although one noted 

that they had discussed evaluating the applicability of PAUT. 

7.1.3.3 Summary of Key Points from External User Survey 

Although the external review consisted of contact with a limited number of commercial LPV 

users, no significant new approaches to inspection, analysis, and testing of these vessels were 

identified, nor were any efforts to develop materials property databases for the materials used in 

these older vessels. The commercial companies were largely applying hydrostatic proof testing 

and conventional NDE inspection methods such as visual inspection, MT, conventional UT, and 

radiography. The chemical industry performs detailed inspections of these vessels at regularly 

scheduled plant shutdowns every 2 to 5 years. Most admitted that they felt these methods were 

inadequate for key areas of the vessels such as the layered shells and the associated welds and 

the circumferential full penetration welds. As a result, most companies contacted were very 

interested in the results of this NESC assessment and any follow-on testing and highlighted the 

need to have an established standard or procedure for inspection and certification of these 

vessels.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

64 of 193

7.1.4 Risk Assessment for Different LPV Regions

Based on the information obtained in Task 1, the assessment team attempted to identify the 

highest risk operating conditions or regions of NASA LPVs. The purpose of this activity was to 

provide a basis for selection of additional efforts to be funded in Task 2, as well as to provide a 

basis for prioritizing the final team recommendations to be incorporated in this report. 

Information considered included the review of previous LPV catastrophic failures, as well as 

reports of commonly reported noncatastrophic damage; review of how NASA LPVs are 

operated, inspected and maintained in comparison with those in industry and, in particular, in 

comparison with those that have catastrophically failed; and the limitations of available materials 

property data, structural analysis methods, and NDE techniques as applied to LPVs. 

Table 7.1.4-1 provides a summary of the high-risk LPV regions or operating conditions that were 

identified by this assessment team, along with the reasons for the high risk and potential actions 

to understand and/or mitigate the risk. The highest risk for NASA LPVs that was identified was 

the operation of the LPVs below the ductile-brittle transition temperature (i.e., NDT). The 

reason for this concern is that operation below the NDT can result in the propagation of any 

existing cracks by a lower energy brittle fracture mode resulting in an increased potential for 

catastrophic failure. Although the LPVs are typically operated above the MDMT that is either 

cited on the nameplate of the vessel or on the design drawings, as noted previously the MDMT 

values determined for these older, non-code LPVs were generally based on strength only and did 

not account for brittle fracture at lower temperatures of –20°F or greater. Some LPVs with lower 

nameplate MDMTs were keyhole impact tested consistent with BPVC requirements at the time, 

although this is not a code-accepted impact test method today. Further, data on the NDT for 

materials used in these vessels are not available. Thus, it is possible that some NASA LPVs are 

being used at temperatures near or below the NDT. To address this risk, characterization of the 

toughness and NDT is needed for all materials in NASA LPVs, including head, shell, weld, and 

weld HAZ materials. Measurements of these parameters from multiple vessels are needed to 

establish bounding NDT values that can be utilized to define more appropriate MDMT values for 

these NASA LPVs. Once the MDMT values are established, then operations can be restricted to 

ambient temperatures above these limits, or environmental controls such as enclosures or heaters 

can be utilized to maintain temperatures above these limits when ambient temperatures are too 

low. Alternatively, a reduction in operating pressure can be used to allow lower temperature 

operation in accordance with ASME BPVC. It should be noted that data showing previous safe 

operations of these vessels at lower temperatures may be beneficial. However, this does not 

fully mitigate this risk since, if new cracks have formed or cracks have propagated to critical 

sizes in the interval since the last exposure, then the risk of brittle fracture can exist for the next 

exposure to similar low temperatures.

The next high-risk region of NASA LPVs that was identified was that of the circumferential full 

penetration welds at either the head-to-shell, or shell-to-shell junctions of these vessels. This 

was cited to be of high risk because of the history of failures that have initiated in these areas, 

along with the current lack of capability to inspect this region. An additional concern is the lack 

of knowledge as to the stress state of this region, since these welds are not stress relieved in the 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

65 of 193

fabrication process. With respect to the previous non-Agency failures relative to the NASA 

LPVs, it is noted that they occurred in vessels operating at higher temperatures with corrosive 

products, and there may have been environmental influences that contributed to the failures that 

are not applicable to NASA vessels. While possibly enhancing corrosion, however, the elevated 

temperatures reduce the risk seen due to low-temperature operation at NASA. To help mitigate 

risks associated with this region, analytical methods to predict the weld residual stress, as well as

further predict stress states and critical initial flaw sizes, are needed. Evaluation and, if possible, 

validation of NDE methods to inspect this region are also needed. PAUT has shown some initial 

promise, and for this reason was included as one of the funded activities in Task 2. The 

capabilities of radiographic and AE methods for this regions need to be determined also. 

Alternatively, proof test logic may be applicable to reduce risk in this region.

The shell regions present elevated levels of risk in large part because of the lack of ability to 

inspect them except for the outer and inner (when internal access is available) layers. 

Additionally, one of the most common locations where cracks have been detected is in the inner 

or outer shell layers adjacent to the circumferential welds. This is anecdotally attributed to 

complex stress states, including bending where the head transitions into the shell though this 

longitudinal bending fails to explain longitudinal cracks that would normally be caused by 

circumferential stresses. However, currently available analysis models and materials property 

data are inadequate to provide detailed information on the stress states. The longitudinal welds 

in these layers are also a concern due to the lack of ability to inspect them and the varying degree 

to which they were inspected when initially fabricated. As with other high-risk areas, improved 

materials property data and analysis methods are needed to better understand the risk and 

estimate critical flaw sizes. AE is a potential technique for detecting propagating flaws in these 

regions, but this method needs to be further evaluated and, if possible, validated. Low-frequency 

electromagnetic NDE methods should also be evaluated for flaw detection capabilities in the 

shell regions. 

Nozzles and nozzle welds are areas of increased risk, especially those through the shell sections. 

While some UT methods have been developed for nozzles and nozzle welds through the 

monolithic heads, these have not been fully validated and have not been evaluated for nozzles 

through the shell sections. Again, improved analysis capabilities are needed to better understand 

the risk, and development and validation of NDE methods such as UT, AE, or PAUT are needed 

to mitigate risks. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

66 of 193

Table 7.1.4-1. LPV Regions or Operating Conditions that Present the Highest Risk for Potential 

Failure for NASA Non-Code LPVs

LPV Region or Operating 

Condition Reasons for Elevated Risk 

Potential Actions to 

Understand and/or 

Mitigate Risk

1. Operation of vessels

below ductile-brittle 

transition temperature 

(i.e., NDT). 

* Use of pressure vessels

below NDT can lead to 

brittle fracture, resulting in 

catastrophic vessel failure. 

* NDT has not been

established for head, shell, 

and weld materials in 

NASA LPVs.

* NASA LPVs are used at

ambient temperature 

conditions year round, 

which even in southern 

locations can be quite cold. 

* Some NASA LPVs are

used in blowdown 

operational modes, which 

result in reduced vessel 

temperatures. 

* Characterize NDTs for

head, shell, nozzle, and 

weld materials in NASA 

LPVs (including variability 

across multiple vessels). 

* Ensure MDMT is

appropriately established 

relative to the measured 

NDT and restrict operation 

to temperatures above 

established MDMT. 

* Build enclosures with

temperature control around 

vessels or utilize heating 

pads around vessels that 

must be operated when 

ambient temperatures are 

below established MDMT, 

or when operational 

conditions such as 

blowdown could result in 

such temperatures. 

* Reduce operating

pressures to correspond to 

reduced operating 

temperatures in accordance 

with established procedures, 

such as the use of Figure 

UCS-66.1 of the ASME 

Section VIII, Division 1 

BPVC [ref. 1].

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

67 of 193

LPV Region or Operating 

Condition Reasons for Elevated Risk 

Potential Actions to 

Understand and/or 

Mitigate Risk

2. Circumferential full

penetration head-to-shell or 

shell-to-shell welds. 

* Known catastrophic

vessel failures originated at 

head-to-shell or shell-toshell welds. Examples are 

the LPV failures at 

Cartagena, Colombia (shellto-shell weld failure) and 

Kansas (head-to-shell 

failure). Environmental 

effects also played a part in 

these failures. 

* No current validated

NDE methods exist to 

provide adequate inspection 

for these welds. 

* Welds are not stress

relieved, resulting in 

unknown residual stress 

states. 

* Current lack of materials

properties and analysis 

techniques inadequate to 

characterize stress states 

and determine critical initial 

flaw sizes.

* Evaluate and, if possible,

validate PAUT, AE, and/or 

radiographic methods for 

inspection of these welds. 

* Perform external surface

inspections (MT/PT). 

* Reduce inspection

intervals for aging vessels. 

* Investigate analysis

methods for determination 

of residual stresses. 

* Develop materials

property database. 

* Develop and validate

analytical techniques to 

assess stress states, fatigue 

crack growth, and critical 

initial flaw sizes at these 

welds. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

68 of 193

LPV Region or Operating 

Condition Reasons for Elevated Risk 

Potential Actions to 

Understand and/or 

Mitigate Risk

3. Shell sections near headto-shell welds* Most common flaws

found and repaired in these 

vessels are cracks in outer 

and inner shell layers near 

or intersecting head-to-shell 

welds, although some flaws 

have been detected at this 

region in intermediate 

layers especially where the 

longitudinal welds intersect 

the circumferential weld. 

* Anecdotally attributed to

complex stress states in this 

region that includes bending 

at this transition from head 

to shell, although current 

limited materials properties 

and analysis techniques are 

inadequate to provide 

detailed stress information. 

* Cracks at this location

only observed directly at 

outer and inner (when 

accessible) layers. When 

repairs are made, additional 

cracks in interior layers 

adjacent to repaired layer 

have been found. Due to 

lack of NDE methods 

capable of inspecting 

interior layers, it is 

unknown whether similar 

cracks may exist in interior 

layers away from the 

repaired area, although 

some cracks have been 

detected in intermediate 

layers using RT.

* Develop materials

property database. 

* Develop and validate

analytical techniques to 

assess stress states, fatigue 

crack growth, and critical 

initial flaw sizes at these 

locations. 

* Develop improved nontraditional NDE methods 

for inspecting interior shell 

layer when manways are 

not present to allow interior 

access to the inner layer. 

* Investigate capability of

AE methods for detecting 

crack propagation in 

interior layers. 

* Investigate very low

frequency electromagnetic 

NDE methods for detecting 

cracks within interior 

layers. 

* Evaluate and, if possible,

validate radiographic 

methods for inspection of 

these regions. 

* Reduce operating

pressure (also account for 

low operating temperatures 

if below MDMT). 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

69 of 193

LPV Region or Operating 

Condition Reasons for Elevated Risk 

Potential Actions to 

Understand and/or 

Mitigate Risk

4. Longitudinal welds in

shell sections (especially at 

intersection with 

circumferential welds). 

* No current validated

NDE methods for 

inspecting these welds other 

than on outer layer (and 

inner layer if internal access 

is available), although RT 

methods have been 

successful at detecting some 

flaws. 

* Degree of NDE

inspection of these welds at 

time of fabrication is 

unknown or varied by 

manufacturer, and typically 

was not consistent with 

current code requirements. 

In some cases, RT 

inspection of inner shell 

longitudinal weld was 

performed, but no 

inspection of intermediate 

layer longitudinal welds.

* Limited knowledge of

materials properties, as 

constructed structural 

configuration (i.e., size of 

layer gaps, etc.), and 

analysis techniques 

inadequate to provide 

detailed stress analysis and 

establish initial flaw sizes 

(other than those assumed 

by the code) as well as 

critical flaw sizes.

* Develop materials

property database. 

* Develop and validate

analytical techniques to 

assess stress states, fatigue 

crack growth, and critical 

initial flaw sizes at these 

locations. 

* Investigate capability of

AE methods for detecting 

crack propagation in 

longitudinal welds of 

interior layers. 

* Investigate analysis of

flaw tolerance within the 

layered shell and means for 

detecting overstress of outer 

shell via Pi Tape, 

photogrammetry, or other 

means.

* Investigate very low

frequency electromagnetic 

NDE methods for detecting 

cracks within longitudinal 

welds of interior layers. 

* Reduce operating

pressure (also account for 

low operating temperatures 

if below MDMT). 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

70 of 193

LPV Region or Operating 

Condition Reasons for Elevated Risk 

Potential Actions to 

Understand and/or 

Mitigate Risk

5. Drain nozzles or other

penetrations through shells 

and associated welds. 

* No current validated

NDE methods to provide 

adequate inspection for 

these welds. 

* Welds not stress relieved

resulting in unknown 

residual stress states. 

* Current lack of materials

properties and analysis 

techniques inadequate to 

characterize stress states 

and determine critical initial 

flaw sizes. 

* Cross-bore openings are

areas of stress 

concentrations and, thus, 

would have elevated stress 

intensity factors for cracks 

occurring in these regions. 

* Most likely sites for

failure during burst tests

* Greatest potential for

adiabatic cooling and 

operation below MDMT.

* Develop NDE methods

as cited above for 

identification of defects. 

* Reduce inspection

intervals for aging vessels. 

* Develop materials

property database. 

* Develop and validate

analytical techniques to 

assess stress states

(including residual 

stresses), fatigue crack 

growth, and critical initial 

flaw sizes at these welds. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

71 of 193

LPV Region or Operating 

Condition Reasons for Elevated Risk 

Potential Actions to 

Understand and/or 

Mitigate Risk

6. Shell sections in parent

materials away from welds

* Recent discovery of

cracks in exterior layer of 

KSC vessels at locations 

remote from welds – 

unknown what factors 

contributed to occurrence of 

these cracks. 

* No current validated

NDE methods for 

inspecting regions other 

than on outer layer and 

inner layer if internal access 

is available. 

* Lack of materials

properties, as constructed 

structural configuration 

(i.e., size of layer gaps. 

etc.), and analysis 

techniques inadequate to 

provide detailed stress 

analysis and establish initial 

critical flaw sizes. 

* Further develop materials

property database. 

* Develop and validate

analytical techniques to 

assess stress states, fatigue 

crack growth, and critical 

initial flaw sizes at these 

locations. 

* Investigate capability of

AE methods for detecting 

crack propagation in 

interior shell layers. 

* Investigate very low

frequency electromagnetic 

NDE methods for detecting 

cracks within interior shell 

layers. 

* Investigate analysis of

flaw tolerance within the 

layered shell and means for 

detecting overstress of outer 

shell via Pi Tape or 

photogrammetry or other 

means.

* Reduce operating

pressure (also account for 

low operating temperatures 

if below MDMT).

Regions within LPVs that were considered to be of less risk were also identified. These are 

summarized in Table 7.1.4-2 with the basis for this designation along with residual risk 

considerations and possible risk mitigations. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

72 of 193

Table 7.1.4-2. LPV Regions or Operating Conditions with Lower Risk for Potential Failure for 

NASA Non-Code LPVs

LPV Region or Operating 

Condition Reasons for Lower Risk

Residual Risks and 

Potential Methods to 

Mitigate

1. Monolithic vessel heads

(other than when operating 

at temperatures below 

MDMT or at or near headto-shell welds). 

* While what is known of

material fracture properties 

in this region is not 

encouraging, there is better 

capability of NDE methods 

for inspection of monolithic 

vessel heads, although NDE 

methods are not currently 

and consistently used to 

regularly inspect acreage of 

heads. 

* Analysis methods more

adequate for predicting 

stress states and establishing initial critical flaw 

sizes in monolithic heads.

* Material properties not

well characterized for head 

materials – further develop 

materials property database. 

* Quality of thick head

materials likely less than 

thin shell materials – 

develop materials property 

database, ensure critical 

initial flaw sizes established 

and validated inspection 

techniques used to ensure 

flaws that exceed critical 

initial flaw size are not 

present. 

2. Welds at through-head

nozzles or other 

penetrations. 

* Available NDE methods

for inspecting these welds. 

* Analysis methods more

adequate for predicting 

stress states and 

establishing initial critical 

flaw sizes. 

* Material properties not

well characterized for weld 

materials – develop 

materials property database

* Different NDE

methods/procedures in use 

at different Centers – 

determine best approach 

and standardize these 

inspections across the 

Agency.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

73 of 193

LPV Region or Operating 

Condition Reasons for Lower Risk

Residual Risks and 

Potential Methods to 

Mitigate

3. Corrosion on vessel

interior. 

* Unlike LPVs used in

industry that contain 

corrosive products and 

operate at elevated 

temperatures, NASA LPVs 

contain high-pressure gases 

at ambient temperatures and 

are at low risk for internal 

corrosion. 

* For vessels with internal

access, internal corrosion is 

readily detected and can be 

abated.

* Difficult to inspect

interior of vessels with no 

internal access. Use

borescopes and collapsible 

UT probes on best effort 

basis. 

4. Corrosion on vessel

exterior. 

* Proper use of protective

paints and coatings can 

minimize occurrence. 

* Readily detectable and

can be abated. 

* Maintenance and

inspection protocols vary 

from Center to Center – 

develop standardized 

process and ensure adequate 

funds available to inspect 

for exterior corrosion and 

apply protective coatings to 

minimize corrosion.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

74 of 193

LPV Region or Operating 

Condition Reasons for Lower Risk

Residual Risks and 

Potential Methods to 

Mitigate

5. Vent holes. * Safety feature to detect

cracks in internal layers 

resulting in product loss 

through vent holes. 

* Occurrence of corrosion

in the vent hole resulting in 

a plugged vent hole, 

preventing detection of 

internal layer cracks – 

inspect vent holes to ensure 

no blockage. 

* Provides potential path

for moisture intrusion to 

layers, resulting in 

corrosion – evaluate and 

document any corrosion in 

layers around vent holes 

during vessel sectioning. 

* Periodic clean-out of vent

holes, and protection from 

moisture without creating a 

pressure seal.

7.2 Task 2 – Limited Scope Testing and Analysis

As a result of the information and data gathered in Task 1, a number of limited scope testing and 

analysis activities were identified for support as part of Task 2. The purpose of these activities 

was to provide additional data to better formulate the final recommendations for this report. 

Additionally, these efforts provided a mechanism to augment some of the Center-led activities 

that were already ongoing. In addition to the OSMA funds that were provided for this NESC 

assessment, some additional NESC funds available at the end of the fiscal year were added to 

increase the number and scope of the Task 2 activities supported. The following Task 2 efforts 

were conducted and are summarized in the following sections: 

• Characterization of material toughness transition temperature for specimens from head,

inner shell, and shell layer materials.

• Evaluation of the toughness of different microstructural regions of LPV welds (e.g., weld,

fusion zone, heat-affected zone, etc.).

• Weld residual stress (WRS) modeling of LPVs.

• Evaluation of photogrammetric methods for measurement of LPV deformation and

potential flaw detection.

• Evaluation of PAUT methods for inspecting head-to-shell welds.

• AE laboratory specimen design.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

75 of 193

7.2.1 LPV Material Evaluations Conducted at MSFC 

This section describes current materials testing and analysis performed at MSFC related to this 

NESC LPV non-code vessel assessment. Although other LPV related characterization efforts 

independent of this NESC assessment are ongoing at MSFC, the following tasks were defined 

expressly as a part of this assessment effort:

• Task 1: Determine the toughness transition reference temperature T0 per ASTM

E1921-13 for A225 head material.

• Task 2: Determine the toughness transition reference temperature T0 per ASTM

E 1921-13 for cylindrical shell base metals.

• Task 3: Investigate the LPV weld regions microstructurally to develop recommended test

methodologies for future evaluation.

A complete list of terminology and variables used in these sections is provided in Table 7.2.1-1 

to assist the reader.

Table 7.2.1-1. Definition of Terms Specific to LPV Materials Testing Discussed in this Section

Term Definition

1T Syntax used to represent fracture toughness specimen dimensions; general 

form is nT, where the specimen thickness is n inches

a Crack size (length), a lineal measure of the principal planar dimension of a 

crack

ASTM E 399-12 “Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness 

KIC of Metallic Materials”

ASTM E 1820-11 “Standard Test Method for Measurement of Fracture Toughness”

ASTM E 1921-13 “Standard Test Method for Determination of Reference Temperature, T0, for 

Ferritic Steels in the Transition Range;” referred to as the Master Curve 

method

b0 The initial remaining ligament, the distance from the crack tip to the back face 

of the specimen; a critical parameter for determining crack tip constraint

B0 Full thickness of a fracture toughness specimen, ignoring side grooves if 

present

C Used to designate the orientation of the crack or applied load for a toughness 

test specimen, indicating the circumferential direction around a pressure 

vessel

Compliance Used for measuring crack length during fracture toughness testing; the ratio of 

specimen displacement increment to applied force increment (the inverse of 

specimen stiffness)

C(T) Compact-tension specimen, a single-edge notched and fatigue cracked plate 

loaded in tension and used for fracture toughness tests

∆a Change in specimen crack length during test, in this context, due to stable 

tearing

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

76 of 193

Term Definition

δj Kronecker delta from ASTM E 1921-13 analysis, equal to 1 if the datum is 

valid or 0 if the datum is a substitute value related statistical censoring of 

invalid test results

E Elastic modulus

FCC Face-centered cubic crystal system, the only steel crystalline structure to 

which ASTM E 1921-13 is directly applicable

ICP Inductively coupled plasma, a quantitative chemistry technique for 

determining chemical composition

JC Fracture toughness test result measuring the energy of fracture, taken as the 

point of cleavage instability prior to the onset of significant stable tearing 

crack extension, and meeting validity requirements of ASTM E1820-11

JIC Fracture toughness test result measuring the energy of fracture, defined at an 

average of 0.008 inches of stable, ductile crack extension, and meeting 

validity requirements of ASTM E1820-11

KIC Fracture toughness test result for linear-elastic measure of fracture toughness

and meeting validity requirements of ASTM E399

KJC Fracture toughness test result obtained by converting a JC value to a linearelastic equivalent stress intensity factor 

KJc(1T) Fracture toughness test result obtained by adjusting a KJC value to an 

equivalent value from a specimen of size 1T, accounts for statistical size 

effects on cleavage and used as an input to the T0 analysis

KJC(Limit) Maximum allowed KJC capacity of a specimen that will maintain a condition 

of high crack-front constraint at fracture based on specimen size; test results

exceeding this limit must be statistically censored in order to be included in 

the ASTM E 1921-13 T0 analysis

KJ∆a Value of KJ at the crack extension limit; tests that terminate in cleavage in 

which slow stable crack growth exceeds the smaller crack extension limit of 

either 0.05b0 or 0.040 in. must be statistically censored to be used in the 

ASTM E 1921-13 T0 analysis by substituting this value for KJC

L Used for designating the orientation of the crack or applied load for a 

toughness test specimen, indicating the material longitudinal direction or 

pressure vessel longitudinal axis

M Used for designating the orientation of the crack or applied load for a 

toughness test specimen, indicating the pressure vessel meridional axis for 

hemispherical heads

R Used for designating the orientation of the crack or applied load for a 

toughness test specimen, indicating the pressure vessel radial axis

RTNDT An index temperature determined from Charpy V-notch and nil-ductility 

temperature data that provides conservative bounding values of fracture 

toughness versus temperature

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

77 of 193

Term Definition

RTT0 A Master Curve-based index temperature determined from fracture toughness 

data used as an alternative to RTNDT as permitted by ASME Code Cases N-629 

and N-631 that establishes an RTNDT – like quantity from a T0 value via the 

relationship RTT0 = T0 + 35°F.

S Used for designating the orientation of the crack or applied load for a 

toughness test specimen, indicating the material short-transverse direction

SE(B) Single-edge notch bend specimens, see ASTM E1820

σys Yield strength

T Used for designating the orientation of the crack or applied load for a 

toughness test specimen, indicating the material long-transverse direction 

T temperature

T0 Reference transition temperature determined from ASTM E 1921-13 that

represents the temperature at which the statistical median of the KJc

distribution from 1T size specimens equals 100 MPa√m (91.1 ksi√in.) and 

statistically locates the fracture toughness versus temperature curve

T35,50 Transition temperature from Charpy V-notch specimens that exhibit at least 

35 mils of lateral expansion and not less than 50 ft-lbs of absorbed energy per 

ASTM E 23-12c.

TNDT nil-ductility-temperature from drop weight testing from ASTM E 208-06

uTS Ultimate strength

ν Poisson’s ratio

W Width dimension for fracture toughness specimens; other specimen 

dimensions are commonly expressed relative to this width

7.2.1.1 Background on Vessels and Associated Materials

These three tasks are considered introductory in scope. They are intended to provide early 

guidance for evaluating the overall risk posed by the current LPV fleet, as well as to provide 

sufficient guidance to develop meaningful recommendations for future evaluations. These tasks 

do not provide a final answer with regard to the LPV material behavior. For example, the 

important issue of material property variability from lot-to-lot has not been addressed. 

Tasks 1 and 2 involve evaluating the tensile strength and fracture toughness of the LPV base 

materials. These properties, particularly fracture toughness, are the most critical for a proper 

FFS assessment for the LPVs. Because these tests are destructive, the materials testing could 

only be performed on available sacrificial vessels. In situ evaluations on vessels in service are 

generally not practical. As testing for these tasks began, the surplus vessels available for 

sacrificial testing were limited to two MSFC vessels: V0032 (MV-50288-34) and V0125 

(M-117). The parenthetical designations are the manufacturer’s vessel numbers. Additional 

vessel materials were gathered as made available in preparation for future testing but were not 

evaluated as part of these tasks.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

78 of 193

Both V0032 and V0125 were of multilayered, right-cylinder construction manufactured by the 

A. O. Smith Corporation in 1962 and by CB&I in 1963, respectively, as shown in 

Figures 7.2.1.1-1 and 7.2.1.1-2. The body of vessel V0032 was constructed from six total 

layers (shells) with the inner layer nominally 1⁄2-inch-thick and the remaining five layers each 

nominally 1⁄4-inch-thick, for a total nominal wall thickness of 1 3/4 inches. The body of V0125 

was of similar construction, but with only four total layers, with the inner layer nominally 

1⁄2-inch-thick and its remaining three layers each nominally 1⁄4-inch-thick, for a minimum total 

thickness of 1.217 inches. The shells were fabricated from A. O. Smith 1143, 1146, and 

1146a steel, which were proprietary, non-ASME material specifications. The hemispherical 

vessel heads were fabricated from monolithic, A225 Gr. B, a standard ASTM plate material. 

V0032 was 16 feet, 7.5 inches in total length, with an ID of 20 inches. V0125 was 

approximately 12 feet, 6.5 inches in total length, with a 24-inch ID. Table 7.2.1.1-1 summarizes 

the characteristics of these two vessels.

Figure 7.2.1.1-1. Vessel V0032 before Sectioning, Shown with Sister Vessels V0030 and V0035 

Figure 7.2.1.1-2. Vessel V0125 Shown after Sectioning

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

79 of 193

Table 7.2.1.1-1. Summary of Vessels Evaluated

MSFC Vessel ID

Manufacturer Vessel ID

Manufacturer

Manufacture Date

Total Layers

Inner Shell Thickness (in)

Outer Shell Thickness (in)

Head Thickness (in)

Overall Length

Inner Shell Material

Outer Shell Material

Head Material

Nozzle Material

Shell ID (in) 

MAWP (psi)

Purpose

Design Temperature

V0032

MV50288-

34

A. O. 

Smith

Sept. 

1962 6 1⁄2 1⁄4 2-

3/16

16 ft. 

7 1⁄2 in. 1146a 1146a

A225 

Gr. B 

FBX

ASTM 

A105 

Gr II

20 5,500 Gaseous 

Hydrogen Ambient

V0125 M-117 CB&I Sept. 

1963 4 1⁄2 1⁄4 1.056 12 ft. 6 1⁄2 in.

MLP 

1143 

Mod

MLP 

1146

A225 

Gr. B 

FBX

MLF 

5002 

Mod

24 3,500

HighPressure 

Air

120 °F 

7.2.1.1.1 Evaluated Base Materials, Tasks 1 and 2 

Table 7.2.1.1-2 summarizes the primary base metals that are considered representative of the vast 

majority of the LPVs in service throughout the Agency. Each is a low alloy steel. The most 

common material for vessel heads is ASTM A225, Grade B, which is the subject of the Task 1 

evaluations. The shell materials (i.e., 1143, 1146, 1146a) are the subject of Task 2. Though 

clearly important to the thorough and proper assessment of the LPVs, the nozzle materials 

(mainly MLF 5002) were not included in this assessment activity due to limitations in funding. 

For the base metals evaluated in Tasks 1 and 2, a decision was made to focus on only one lot of 

material during this assessment, primarily for the technical reasons that are described in detail in

the discussion of the fracture toughness testing (Section 7.2.1.2). The focus on strength and 

toughness evaluations of base materials during this assessment does not imply that these are 

considered priorities over such testing in the LPV welds. To the contrary, the welds are 

considered the most critical aspect of the LPV material system, based, for instance, on the failure 

history of non-NASA vessels. However, the weld evaluations are considerably more 

complicated than the base metal evaluations. The prudent order of evaluation for this assessment 

was to begin in the relatively simpler base metals, leaving the detailed assessment of the welds as 

required forward work.

Table 7.2.1.1-2. Typical Material Properties for LPV Materials (plate materials are reported for 

thicknesses ranging from 0.180 to 0.375 inches, inclusive; data are taken from original 

manufacturer’s certifications)

Use Common Designation σys

(ksi)

uTS

(ksi)

Elongation 

(%) 

Inner Layer 1143 [ref. 48] 75 90 22.0

Outer layers 1146 [ref. 49] 70 105–135 20.0

Inner/Outer Layers 1146a [ref. 50] 77 105–135 22.0

Heads ASTM A225 Gr. B [ref. 51] 43 75 18.0

Nozzles MLF 5002 Mod [ref. 52] 51 80 18.0

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

80 of 193

7.2.1.1.2 Selected Base Materials for Evaluation 

With the focus on base materials limited to a single lot, the head material from vessel V0032 was 

selected for the Task 1 evaluation of the ASTM A225 Grade B material.

As shown in Table 7.2.1.1-2, the two MSFC sacrificial vessels provide access to three shell 

materials: 1143 and 1146 from V0125, and 1146a from V0032. The 1146 and 1146a materials 

were initially considered to have the same chemistries, but discovery of a CB&I material 

specification [ref. 53] for 1146 suggests that the 1146a material, with increased maximum 

allowable phosphorus and sulfur contents and an increased tensile strength, is a modified version 

of alloy 1146. While increased phosphorous can result in higher strength, it has been known 

since at least 1940 that sulfur and phosphorus can promote hot cracking and lower toughness in 

alloy steel welds [ref. 54], so it is unclear why an increase in the allowed maximum allowable 

phosphorus and sulfur would have been allowed. Considerable inconsistencies were also present 

in the historical 1146 and 1146a specifications available through the period manufacturers, 

which would ultimately require additional effort to settle, although a final, conclusive answer 

may prove elusive [ref. 55]. An evaluation of the constituent chemistry in currently tested and 

future lots of alloy 1146 and 1146a should be performed. 

Though the original A. O. Smith paperwork indicates that 1146a was used to fabricate all V0032 

shells (inner and outer), substantial differences in strength for the inner shell material were

observed in the MSFC tests compared with the specification minimum properties and lot 

acceptance data provided for the V0032 inner shell material. These data are summarized in 

Table 7.2.1.1-3. The data brought the pedigree of the inner layer material into question. 

Replicate testing on the inner shell on multiple sample configurations ruled out that the data were

the result of testing anomalies. Testing of the inner and outer 1⁄4-inch layer materials resulted in 

tensile properties that were consistent with the A. O. Smith certification, indicating that the 

pedigree issue was limited to the 1⁄2-inch inner layer. 

Table 7.2.1.1-3. Comparison of Tensile Data Obtained from V0032 against A. O. Smith 

Certification and Lot Acceptance Values

Specimen ID Location Sample Type Orientation σys

(ksi)

uTS

(ksi)

Elongation* 

(%)

380-11-1A-13 Inner 

Shell

Round, Ø 

0.250 in. L 50.8 76.6 32.3

380-11-1A-14 Inner 

Shell

Round, Ø 

0.250 in. L 50.8 77.1 32.5

380-1 Inner 

Shell

Round, Ø 

0.113 in. C 52.4 80.2 37.4

380-2 Inner 

Shell

Round, Ø 

0.113 in. C 45.3 80.8 33.8

380-3 Inner 

Shell

Round, Ø 

0.113 in. L 51.1 82.0 32.4

380-4 Inner 

Shell

Round, Ø 

0.113 in. L 53.3 82.1 29.9

380-21-33 Inner 

Shell

Flat, 0.250 in.

Gage L 48.1 74.0 34.4

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

81 of 193

380-22-34 Inner 

Shell

Flat, 0.250 in.

Gage L 49.1 74.5 35.5

380-19-31 Shell 

Layer 2

Flat, 0.250 in.

Gage L 93.0 124.1 24.1

380-20-32 Shell 

Layer 2

Flat, 0.250 in.

Gage L 92.5 124.6 24.5

380-40 Outer 

Shell

Flat, 0.250 in.

Gage L 88.5 119.5 24.8

380-41 Outer 

Shell

Flat, 0.250 in.

Gage L 87.1 120.0 24.7

V0032 Lot 

Acceptance 1

Inner 

Shell Not Given Not Given 88.4 114.4 36.0

V0032 Lot 

Acceptance 2

Inner 

Shell Not Given Not Given 91.8 124.8 29.0

V0032 Lot 

Acceptance 3

Inner 

Shell Not Given Not Given 90.7 117.0 28.0

V0032 Lot 

Acceptance 4

Outer 

Shell Not Given Not Given 90.0 116.4 25.0

V0032 Lot 

Acceptance 5

Outer 

Shell Not Given Not Given 93.4 118.7 28.0

A. O. Smith 

Spec

Base 

Metal Not Given Not Given 77.0 105 - 135 22.0

V0032 

Certification

Base 

Metal Not Given Not Given 77.0 Not Given Not Given

* Elongation values referenced in Table 7.2.1.1-3 are provided for reference only, as the

extensometer used for testing was verified to only 8%. 

The strength test results suggested that a potential undocumented substitution of softer material 

or heat treatment modifications to the 1146a may have occurred for the V0032 inner shell during 

construction, possibly to avoid problems with hydrogen embrittlement, since V0032 was 

designed for gaseous hydrogen storage. An industry consultant suggested that based on his past 

experience, a lower strength steel like A-285 or A516 Grade 70 may have been substituted for 

the 1146a [ref. 56]. It may be worthwhile to investigate the inner shells of the available sister 

vessels to V0032 to determine whether their properties also show a discrepancy. In the larger 

context, the discrepancy observed for V0032 indicates that the manufacturer’s data records, the 

originating source of material pedigree information, are not infallible. Uncertainty in the 

pedigree of the material should be recognized as a risk associated with these vintage LPVs. 

Generally, it is not feasible to independently test and verify the material pedigree of the inner 

layer materials in a vessel due to limited physical access. 

Once this material discrepancy was discovered, no further testing was performed on the 1146a 

material from V0032, although the thinner, higher strength 1146a material in the V0032 outer 

layers should be part of future evaluations. The pedigree of the V0032 inner layer material was 

evaluated by chemical analysis. X-ray fluorescence (XRF) analysis was attempted, with the 

hope of differentiating the material from other candidates, such as 516 or A-285, based on the 

presence of vanadium as a discriminator (i.e., 1146 has vanadium, where most potential 

candidates do not). XRF is best used as a comparator against known standards. Without such 

reference standards in this case, the XRF results were simply inconclusive; therefore, inductively 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

82 of 193

coupled plasma (ICP) analytical chemistry tests were performed. ICP is more time consuming

but produces very accurate chemistries, including many trace elements in metals. The ICP 

chemistry results are shown in Table 7.2.1.1-4. 

Table 7.2.1.1-4. ICP Analytical Chemistry Composition Results for V0032 Inner Shell Material 

Compared with 1146a Specification Requirements (this material was designated as 1146a on the 

originating A. O. Smith manufacturer's report) 

Material ICP (Run 1) ICP (Run 2) 1146a Specification Agreement

Manganese (Mn) 1.26 1.29 1.10 to 1.50 Yes

Phosphorus (P) <0.02 <0.02 0.040 max Yes

Silicon (Si) 0.261 0.247 0.20 to 0.35 Yes

Vanadium (V) 0.154 0.156 0.13 to 0.18 Yes

Nickel (Ni) 0.555 0.556 0.40 to 0.70 Yes

Chromium (Cr) 0.065 0.066 Not Given NA

Copper (Cu) 0.048 0.048 Not Given NA

Carbon (C) Not Given Not Given 0.18 to 0.25 Unknown

Sulfur (S) Not Given Not Given 0.05 max Unknown

Note: Specification agreement for chromium and copper is described as not applicable (NA) since these 

elements often existed in steels with a scrap component.

The levels of manganese, phosphorus, silicon, nickel, and vanadium were confirmed to meet the 

1146a specification requirements. Carbon and sulfur were not reported, as the ICP test cannot 

measure those trace elements. A separate, targeted test for carbon was considered but not 

performed due to cost. Chromium and copper were also reported in small quantities. Because of 

these subtle ambiguities, the V0032 inner shell material remains unidentified. Based on the 

strength data and the chemistry results, this soft inner layer material is not expected to be 

representative of the general population of LPV materials in use across the Agency and, thus, 

will likely not be investigated further. The base metals evaluated in Task 2 were, therefore, 

limited to the 1143 and 1146 materials from the V0125 vessel. These materials are considered to 

be generally representative of the nominal LPV materials used for the shells by A. O. Smith 

LPVs in NASA’s infrastructure, with the caveat that the 1146 material is not as strong as 1146a 

and is expected to have slightly greater fracture toughness than 1146a. An evaluation of properly 

representative 1146a material should be performed. 

In reference to Table 7.2.1.1-3, lot acceptance data are provided for the LPV materials that were 

evaluated. Lot acceptance testing for 1143 inner shell material for V0125 demonstrated a 

minimum 70-ksi yield and 91-ksi ultimate strength minimum in one material lot. It appears that 

the 1143 material from V0125 did not meet the yield strength specification minimum of 75 ksi, 

which again underscores the confusion associated with the observed specification 

inconsistencies.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

83 of 193

7.2.1.1.3 LPV Materials Collected for Future Evaluations

An ongoing aspect of this assessment activity has been to identify material resources in the form 

of vessels available across the Agency and elsewhere that may be sacrificed to provide needed 

materials for evaluation. This collection of vessel materials will be critical to fulfill the 

recommended future testing required to evaluate each of the base materials and welds properly. 

Furthermore, a bulk of material from a variety of vessels will be critical to assessing the expected 

material variability present in the LPV fleet across the Agency. A considerable amount of the 

vessel materials have been collected at MSFC and are being carefully retained in long-term 

storage in anticipation of this future testing. As part of this collection effort, an A225 head from 

a multilayer vessel at GRC, designated as GRC Vessel PV0296, was obtained, as well as all of 

the remnant pieces from a multilayer vessel designated by serial number MV-50466-8 that was 

tested at SwRI® as funded by NASA OSMA and managed by ARC [refs. 44 and 45]. Vessel 

MV-50466-8 was fabricated from 1146a and A225. The original manufacturer’s data report for 

the PV0296 vessel was unavailable, but the report for a representative vessel produced and 

delivered at the same time by the same vendor designated as serial number MV-50405 

demonstrates that the PV0296 head material is likely A225 Gr. B. Vessel V0031, a sister vessel 

of similar size, construction, and service history to V0032, was also available. No material 

testing was performed on PV0296, MV-50466-8, or V0031 in this assessment. 

7.2.1.1.4 Material Anisotropy

The correlation between the structural and material orientations must be identified first to orient 

and extract appropriate test specimens. As a general rule for FFS assessments, fracture 

mechanics material property data are generated on the plane within the material that provides the 

least capability. For most thin-plate materials, specimens are extracted such that the crack is 

oriented parallel to the direction of elongated material grains, usually the longitudinal rolling 

direction. This typically produces bounding material properties. This philosophy for generating 

material data on the least capable orientation is required if the material orientation in the vessel is 

not certain. The small selection of sacrificial vessels under consideration in this assessment has 

illustrated that material orientation cannot be assumed to be consistent. While this assessment 

has proceeded mindful of determining bounding fracture properties in-plane, the behavior of 

cracks growing radially through the thickness of the materials has not been evaluated. Tests for 

this behavior in the thin-shell materials would typically use a surface-crack geometry. These 

studies were beyond the scope of the current assessment but should be considered for future 

evaluation. Material orientations are specified relative to the original plate corresponding the 

material longitudinal (L), transverse (T), and short-transverse (S) directions. The structural 

orientations of a vessel are described by the circumferential (C), longitudinal (L), radial (R), 

and meridional (M, spherical head) directions. 

Due to inexperience with these types of pressure vessel steels, significant anisotropy was not 

initially anticipated in the planning stages of this assessment and was not a part of the budgeting 

process when allocating the available assessment funding. This oversight required time and 

testing resources to resolve, hindering the ability to achieve the scope that was anticipated in the 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

84 of 193

testing tasks; thus, the 1143 and 1146 materials were more completely characterized than A225 

due to these complications. Although unfortunately coming to the attention of the planning team 

too late, the material anisotropy was clearly identified in the SwRI® findings resulting from the 

mechanical testing they performed on an A. O. Smith vessel MV-50466-8 for NASA ARC 

[refs. 44 and 45]. 

Due to this anisotropic behavior, the material orientation for each piece of material needed to be 

determined prior to orienting the final set of test specimens. The shell materials proved fairly 

easy to orient, given that they maintained distinct orthogonal rolling directions. The only 

question to be resolved was whether the shell material longitudinal direction was oriented 

circumferentially around the vessel or along the vessel axis. Fracture toughness tests were 

performed in these two orientations (C-L and L-C) to determine the material orientation, which 

was always made clear by a large reduction in toughness in the longitudinal direction of the shell 

material. (Note that in the two-letter fracture toughness orientation designations above and 

elsewhere in this report, the first letter indicates the direction of applied loading, and the second 

letter indicates the direction of crack extension.) Identifying the bounding toughness orientation 

in the A225 head material was considerably more complicated. Both selective microstructural 

investigation and instrumented Charpy impact testing were used to help identify the material 

orientation. The microstructural evaluations were inconclusive, but the Charpy impact testing 

proved informative. Further details on the efforts to identify material orientation are provided in 

the discussions of Task 1 and Task 2 results. 

7.2.1.1.5 Lot Variability

Though the LPV fleet is comprised of nominally the same steels based on the A. O. Smith 

designations previously discussed, the LPV fleet contains vessels fabricated over a span of years 

and by different vendors and the steels may have been processed by different mills using 

different rolling procedures. The LPV steels are expected to contain significant lot-to-lot 

variability; thus, one cannot assume the data obtained in this study are directly applicable to all 

LPVs without a further characterization of this variability. 

The initial plan for this assessment was to test from as many different lots of material as possible 

to diversify the testing and characterize the lot-to-lot variability. This goal shifted primarily due 

to technical reasons and, to a lesser extent, schedule reasons. The technical reason for the shift to 

single material lot evaluation was that the E 1921-13 Master Curve test method used in this 

investigation is best applied to a macroscopically homogeneous material having uniform and 

isotropic strength and toughness properties. Therefore, the proper way to approach the transition 

toughness evaluation is to test a sufficient quantity of specimens to independently evaluate each 

lot. The result from each lot can then be compared for variability. A subsequent analysis 

combining data across lots can then be assessed for comparison.

7.2.1.2 Fracture Toughness Testing

Fracture toughness testing was the focus of this effort because fracture toughness as a function of 

temperature is the most important material property for FFS assessment of the vessels. The fully 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

85 of 193

ductile fracture toughness (upper shelf) and the temperature at the transition to cleavage fracture 

(transition range) were of interest. The Master Curve methodology from ASTM E 1921-13 

[ref. 57] was used to facilitate the assessment of the fracture toughness data and determine the 

fracture toughness versus temperature behavior for the LPV materials. The Master Curve 

method is a robust approach to characterizing the temperature dependent fracture toughness of 

ferritic steels with as few as six test samples. The following background information on the 

Master Curve method will provide a brief introduction to the concept and convey the rationale 

for choosing this method to evaluate the LPV materials instead of using the more traditional 

impact energy (Charpy) methods. 

Other than obvious schedule and cost constraints, the choice of a fracture toughness test method 

is usually dictated by the expected failure mechanism of the material. The two primary 

mechanisms by which fracture occurs in ferritic steels are ductile rupture and cleavage. These

are micromechanisms for fracture that describe the mechanism by which a material fails at the 

microstructural level. Care is needed not to confuse these mechanisms with terms such as 

“ductile” or “brittle,” which are often used to describe the failure behavior of structures or 

specimens. Note that a ductile rupture fracture may very well occur in an unstable, “brittle” 

manner. This is common in many alloys used in the aerospace industry, such as high strength 

aluminum or titanium alloys. Ductile rupture occurs in metals that fail by the growth and 

coalescence of voids initiating from loosened or broken inclusions and second-phase particles. 

Cracks extend through this void coalescence process, driven by plastic deformation, ultimately 

leading to fracture in either a stable or unstable manner. This is the typical failure mechanism 

for structural steels when they are kept sufficiently warm and not loaded in a highly dynamic 

manner. In contrast, cleavage fracture is a much lower energy process that progresses by the 

splitting of atomic planes with little associated plasticity, resulting in a flat and faceted fracture 

surface. The cleavage fracture process is of primary concern in structural steels at reduced 

temperatures or high loading rates [ref. 58]. 

For ferritic body-centered-cubic steels, the fracture mechanism undergoes a demonstrative 

transition from ductile rupture to cleavage as temperature decreases, which leads to a 

concomitant decrease in fracture toughness. This temperature dependency of fracture toughness 

is common to all ferritic steels. The material property of most importance to ferritic structural 

steels in LPV applications, therefore, is the ductile-to-brittle transition temperature curve that 

quantifies the fracture toughness loss with decreasing temperature and identifies the probability 

that a cleavage fracture will occur. Despite the common use of the term “transition temperature” 

that implies a discrete temperature below which steels fail by cleavage fracture, the reality is 

more subtle and complicated. An “upper shelf” exists where temperatures are sufficiently warm 

to ensure fracture progresses by ductile rupture mechanisms. Fracture toughness is greatest on

the upper shelf and is generally consistent and repeatable. As temperatures decrease (or loading 

rates increase), the steel enters into a “transition range” where ductile rupture and cleavage are 

competing fracture mechanisms. In this range, the fracture behavior becomes considerably more 

variable since both mechanisms are operative and fracture toughness values drop below those of 

the upper shelf. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

86 of 193

Determining this temperature range where cleavage fracture becomes probable is critical to 

understanding the reliability of the LPVs. Cleavage fracture events often result in unexpected, 

catastrophic structural failures. Historically, brittle fracture has been characterized by a linearelastic fracture toughness test to determine KIC, which treats the material in front of the crack tip 

as a homogeneous and elastic continuum. This simplistic approach is insufficient to characterize

cleavage where the fracture process is governed by the random distribution of cleavage initiators, 

such as carbide particles or inclusions, in the highly stressed zone of material just in front of the 

crack tip. This makes cleavage initiation a strongly stochastic process that follows a weakest 

link model. Therefore, a statistical approach is needed to adequately characterize cleavage 

fracture. ASTM Standard E 1921-13 [ref. 57] couples modern fracture mechanics and statistical 

methods to define a statistically based curve of fracture toughness versus temperature that is 

derived using only fracture-mechanics-based test data. The statistical nature of the method 

allows confidence bounds to be determined. This elastic-plastic method utilizes the J-integral at 

the point of cleavage instability, JC, which is converted into a stress-intensity equivalent, KJC, 

and uses these data to define a curve of median fracture toughness, KJC versus temperature. 

Performing elastic-plastic J tests and deriving K from J, as opposed to conducting linearly elastic 

K tests directly, was necessitated by specimen size limitations imposed by the thin, cylindrical 

pressure vessel layers. Specimens for J test methods can be as much as 1/40th the size required 

for linear-elastic K tests and still maintain sufficient constraint to produce toughness data that are 

unaffected by sample size [ref. 59]. It would not have been possible to perform linear-elastic 

fracture toughness tests per ASTM E 399-12 due to the thin-layer material constraints in LPV 

shells. Test specimens could not be made large enough to obtain the linear-elastic fracture 

toughness, KIC. 

Though the use of the J-integral method greatly relaxes specimen size requirements for testing 

fracture toughness, it does not altogether remove the effect of specimen size from the process of 

evaluating cleavage. Because cleavage fracture is a stochastic process, the amount of material 

sampled during a test will affect the likelihood of a cleavage event occurring. Therefore, the 

thickness of the specimen brings a size effect into the process by dictating the volume of material 

sampled along the crack front. This size effect is simply the increase in the likelihood of having 

a cleavage initiation particle along the crack front for samples of increased thickness. This 

influence of specimen size is accounted for in the statistical treatment of the test data in ASTM 

E1921-13. E 1921-13 uses a standard thickness of 25.4 mm (1 inch) and provides statistical 

relationships to relate thinner or thicker specimen tests to obtain a standard output. An extension 

of this logic then must be used in the application of the standard result to the structural 

application [ref. 60]. The relatively thin specimens dictated by the shell materials require more 

size correction than those extracted from the comparatively thick head material. These size 

corrections also play a role in the proper application of cleavage fracture toughness data obtained 

from laboratory specimens to the prediction of fracture behavior in real structures like the LPVs. 

The E1921-13 approach is referred to as the Master Curve method because it has been 

demonstrated that the fracture toughness versus temperature behavior for most ferritic steels 

conforms to one universal curve shape [ref. 60]. The Master Curve method is empirically 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

87 of 193

derived and supported by a wealth of data that includes thousands of tests generated over more 

than three decades [ref. 61].

The universal Master Curve is made specific for a given material by being positioned on a plot of 

KJC versus temperature by means of T0, the reference transition temperature. ASTM E 1921-13 

prescribes both experimental and computational procedures that are used to determine the 

reference temperature T0. This one value, the T0 reference temperature, is the only unknown that 

needs to be determined from the test data to fully establish the Master Curve for the subject 

material. T0 is defined as the temperature at which a set of data obtained with 1-inch-thick 

specimens will have a median KJC of 100 MPa√m based on testing six or more specimens at a 

single temperature. While this definition of T0 may seem somewhat arbitrary, because the shape 

of the Master Curve has been empirically predetermined, the T0 definition simply serves as a 

standardized anchor for positioning the Master Curve on the temperature axis. It is important to 

realize that T0 is not the same as the historically common NDT, TNDT, though the two values are 

easily reconciled. 

T0 and TNDT can be reconciled by utilizing available ASME code cases, which are official 

responses from ASME to the user community when meaningful inquiries are raised about the 

ASME code so that the knowledge base can be expanded, the code adapts, and repetitive 

inquiries are avoided. Relevant to the Master Curve method are ASME code cases N-629 and 

N-631, which establish the basis for using a Master Curve-based index temperature derived from 

T0 as an alternative to TNDT. These code cases directly permit use of the Master Curve index 

temperature: 

RTT0 = T0 + 35°F (Eq 7.2.1) 

This is an alternative to RTNDT, where RTNDT is related to the NDT from ASME Section III and 

ASME NB-2331. For pressure retaining materials other than bolting applications, ASME 

NB-2331 requires that the reference NDT RTNDT is established:

RTNDT = MAX{TNDT, T35/50 – 60} (in °F) 

where TNDT is the NDT from drop-weight testing from ASTM E 208-06 and T35,50 is the 

transition temperature from Charpy V-notch specimens that exhibit at least 35 mils of lateral 

expansion and not less than 50 ft-lb of absorbed energy per ASTM E 23-12c. The use of RTNDT

was intended to account for heat-to-heat differences in fracture toughness transition temperature 

and collapse the data more completely into a single curve, but it does not do so as well as the 

Master Curve. 

The assumption in using the Master Curve index temperature is that it should provide the same 

implicit margin between KIC and measured fracture toughness data as the accepted RTNDT

approach. While the fundamental relationship between T0 and RTNDT has unfortunately not been 

derived, comprehensive statistical analyses against existing fracture toughness data has shown 

that the relationship between RTT0 and T0 used in the ASME code cases N-629 and N-631 

(i.e., RTT0 = T0 + 35°F) bounds 97.5% of existing fracture toughness data [ref. 61]. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

88 of 193

RTNDT is considered conservative compared to the fracture toughness versus temperature 

relationship determined by T0. The two methods differ in their approach. T0 is a statistically

based fracture mechanics method that allows for a direct determination of the transition 

temperature, whereas TNDT is defined by a relatively simple bounding method where the impact 

energy for fracture of blunt-notched specimens drops below a given threshold. 

Defining RTNDT from T0 is straightforward as given by the ASME code cases. On the other hand, 

defining the proper MDMT for the LPV fleet from this value will require additional study. The 

use of a Master Curve derived T0 to define the RTNDT (and subsequently an MDMT) rather than a 

TNDT based on an impact energy threshold is expected to reduce conservatism in that value. 

(Note: the Master Curve was born out of necessity to reduce known conservatisms in the 

traditional RTNDT method, so the nuclear power industry could demonstrate adequate safety in 

aging structures in use beyond their original design service objectives.) The key distinction to 

underscore is that the Master Curve is consistent with a risk-informed framework for decision 

making that provides best estimates of fracture toughness by means of an explicit description of 

uncertainty. The Master Curve provides two key features: a statistical description of fracture 

toughness at a single temperature and the relationship between fracture toughness and 

temperature. 

The process used by E1921-13 to establish the Master Curve indexing temperature T0 is 

essentially a four-step procedure: 

1. Six to eight fracture toughness specimens are tested as near to the estimated T0 as

possible as long as fracture occurs due to cleavage1

. Estimates of T0 can be made from

statistical offsets based on Charpy impact tests, if available. The elastic-plastic fracture

toughness at cleavage instability, JC, is determined and converted to its corresponding

elastic fracture toughness KJC. Test data must then be statistically censored according to

two primary censoring limits before the data can be used in the T0 analysis. Any

specimens exceeding a ductile crack extension limit in which the slow stable crack

growth before cleavage fracture is greater than 5% of the initial remaining ligament or

that experience a loss of constraint are regarded as invalid and must be statistically

censored before being included in the analysis. Any specimens that experience a loss of

constraint must also be statistically censored. This statistical censoring, performed in

accordance with ASTM E1921-13, means that the measured value is replaced with a

smaller value close to JIC, the ductile initiation toughness as near to the test temperature

as possible. Censoring is also necessary if the elastic-plastic fracture toughness exceeds a

small-scale deformation limit KJC(limit) defined in E 1921-13 that depends on the specimen

dimensions and the material’s yield strength. For the steels tested here, this limit was not

1 The fracture toughness tests are performed in accordance with ASTM E 1820-11, according to the basic test method from sections 

8.1.2 and A2.4.2.1 without compliance unloading. As the basic test method is used, only this cleavage instability JC is determined from a 

given test as opposed to a full J-R curve, which would be typical of unloading compliance fracture tests. That only JC is determined as

compared to the complete J-R curve is sufficient for determining T0. The basic method facilitates fracture testing when cleavage 

instability is expected, since unloads may occur in the vicinity of a cleavage instability and obfuscate the ability to determine cleavage 

initiation.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

89 of 193

approached, and this censoring was not required. Compact tension C(T) specimens are 

preferred over single-edge notch bend SE(B) specimens because T0 differs for 

measurements made with C(T) and SE(B) specimen geometries due to differing 

constraints. C(T) specimens have higher constraint and produce higher (more 

conservative) T0 temperatures. All test data generated for purposes of determining T0 in 

this investigation were measured from C(T) specimens2 [ref. 49]. All specimens must be 

tested in the crack orientation corresponding to the lowest toughness in the material. 

2. The KJC data are statistically corrected based on specimen thickness to the equivalent of a

1-inch-thick specimen, referred to as 1T. This correction reflects the fact that cleavage is

a stochastic process, dependent on the length of the crack front. Inherent to this

conversion is the assumption that there is a lower shelf at which fracture energy reaches

an asymptotic limit and is independent of temperature. The conversion of data to the

1T equivalence is necessary because the Master Curve method physically assumes that

the cleavage fracture mode is modeled by a weakest link process by which the fracture

process is determined by the failure of a single cleavage initiation site and assumes that a

random distribution of cleavage initiation sites exists through a given sample width.

There is no special incentive for the choice to normalize to the 1T thickness, other than

that provided a single, convenient, and common specimen width around which to develop

a set of closed-form equations for T0.

3. The temperature T0 at which the 1T equivalent KJC is 100 MPa√m is determined. The

evaluation of T0 from the test data inherently assumes that the toughness data are well

described by the Master Curve. (This assumption can be confirmed if a sufficient

quantity of data is generated, as was generally the case in this assessment.) Toughness

data that are obtained closer to the T0 temperature are preferentially weighted, and all

tests must be within 50°C of T0 to be considered valid. The 50°C window around

T0 exists because the weakest link assumptions become invalid as the upper and lower

shelf temperatures are approached [ref. 60]. Some iteration on temperature is to be

expected to determine T0; thus, a provisional estimate of T0 is needed after the first three

to four fracture tests are completed to ensure test temperature remains within the 50°C

window. The combination of a low upper shelf toughness KJIC that is below 100 MPa√m

and the thin specimens available required testing close to the T0 – 50°C limit, which in

turn required the testing of 8 to 10 samples rather than the minimum of 6. The extra data

did, however, permit a better assessment of the E 1921-13 standard for the case of

interest and as discussed below.

4. Based on the statistical procedures in ASTM E 1921-13, the median KJC of the 1T data

population corresponding to a 50% cumulative probability for fracture is determined and

plotted. This curve is referred to as the Master Curve for the subject material. The

2 A SE(B) fixture was manufactured, and a procedure for using the fixture for performing fracture tests on precracked Charpy V-notch 

samples was developed during this effort but was not used for measuring fracture data to be used in determining T0. This fixture and 

procedure, however, are available for future testing, as necessary.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

90 of 193

Master Curve statistical model provides the ability to define probability levels other than 

the median, such as 95% and 5% probabilities of cleavage; thus, upper and lower 

statistically bounding confidence intervals are also calculated and plotted. The 

distribution of data with respect to the Master Curve should be evaluated to ensure that 

the curve-fit is loyal to the original data and described well by the Master Curve.

This four-step procedure was used for each of the base metal evaluations discussed in the 

following sections. 

7.2.1.2.1 Task 1: A225 Head Material Fracture Toughness Reference Transition 

Temperature

In order to perform mechanical tests on the pressure vessels, they were coarsely sectioned into 

smaller segments to better accommodate machining test specimens. These cut plans are shown 

below in Figures 7.2.1.2-1 and 7.2.1.2-2. 

Figure 7.2.1.2-1. V0032 Gross Sectioning Plan (drawing number CP-380) 

Figure 7.2.1.2-2. V0125 Sectioning Cut Plan (drawing number CP-344) 

Task 1 of the materials characterization effort was to evaluate T0 for the A225 head material. 

The A225 material testing was afforded highest priority based on historical accounts and 

anecdotal evidence that the A225 material may be the highest risk to cleavage at common vessel 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

91 of 193

operational temperatures. The A225 testing initially focused on the heads from V0032 and the 

head material provided from the GRC PV0296 vessel. The decision to reduce focus to single 

lots for base metal testing led to using only V0032 head material in the current assessment. As 

mentioned in the prior discussion of anisotropy, the biggest challenge of the A225 material 

involved identifying the lowest toughness orientation in the head. At the time of this writing, the 

A225 T0 test series is still ongoing. This section discusses the considerable efforts invested in 

determining microstructural orientation. The plan to determine microstructural orientation began 

with a traditional metallographic approach. When metallographic methods proved inconclusive, 

a series of Charpy impact tests were used to provide further quantitative insight.

Metallographic microstructural evaluations at 50× and 100× magnifications were performed on 

A225 material from V0032 and GRC V0296 for determining material orientation3

. (Note: 

testing of the V0296 head was still ongoing when this report was prepared.) Nine total samples 

in three orthogonal planes (C-M, R-C, and M-R) were taken at three equidistant circumferential 

locations around each head with 45° spacing between samples. Three samples were taken as 

close as possible to each nominal circumferential location as possible to expedite the 

metallurgical evaluation, instead of evaluating a single sample. Evaluation of a single sample 

would have required its polishing mold to be broken in order to evaluate a different sample 

face. As such, a small tradeoff in accuracy was accepted for expediency. Photographs and cut 

plans of the location in which blanks were evaluated from the V0032 head are provided in 

Figures 7.2.1.2-3 and 7.2.1.2-4. 

Figure 7.2.1.2-3. V0032-1 Sectioned for Metallographic Microstructural 

Examination for Determining Material Orientation

3 Performed by Element in Houston, TX.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

92 of 193

Figure 7.2.1.2-4. V0032-1 Cut Plan for Metallographic Microstructural Evaluation for 

Determining Material Orientation 

The 0° location was chosen arbitrarily since there was no unique feature on the axisymmetric 

head that would indicate how the head was fabricated with respect to the parent plate. Each 

dimension of the metallurgical blanks was unique to maintain traceability of the macro 

orientations. The macros revealed no discernable microstructural evidence of the plate 

orientation prior to forming. For V0032, the microstructure appeared to be independent of the 

circumferential location with a uniform grain structure generally following the shape of the head. 

Although the same Nital etching process (3% nitric acid, balance methanol) was followed for the 

samples from both heads, the grain orientation for GRC V0296 was much less apparent in the 

microstructural samples and even less useful in determining material orientation.

The V0032 microstructure was dominated by what appeared to be considerable banding of 

pearlite and ferrite, which is undesirable (see Figure 7.2.1.2-5). The dark bands are pearlite, and 

the lighter bands are ferrite. This banding is possibly a result of microsegregation of alloying 

metals during the solidification of the original ingot, which was not ideally homogenized during 

the subsequent heat treatment. Alloying elements always segregate to some extent during the 

solidification of steel. Elements that are especially prone to segregation are carbon, phosphorus, 

sulfur, silicon, and manganese [ref. 62]. Manganese is especially problematic because it lowers 

the chemical activity of carbon in austenite, from which pearlite is formed. The manganese-rich 

areas are, thus, the last to transform and are mostly pearlite [ref. 63]. During the microstructural 

evaluations, the banding features were observed regardless of the sample orientation or angular 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

93 of 193

location. These microstructural features likely obscured any visible anisotropy introduced into 

the material from the original plate production.

Figure 7.2.1.2-5. Microstructural Evaluation at 50× Magnification of A225 Material from Vessel 

V0032 showing Banding of Pearlite and Ferrite on Meridional Face (both radial and 

circumferential faces demonstrated similar features)

These inconclusive microstructures, showing either no orientation or spherical banding following 

the head, were not expected. There was an expectation that the prior plate microstructure would 

remain dominant. Although exact historical records are incomplete, information on the typical 

head fabrication process was obtained from an industry consultant4. For forming a head such as 

V0032, the steel mill would start with a thick slab of commercial A225 and roll the plate at a 

temperature close to 1,000°F between two rolls to reduce the thickness. In this process, one 

primary direction would be stretched the most and become dominant—the plate longitudinal (L) 

direction. The orthogonal direction in the flat plane of the plate is identified as the transverse (T) 

direction. Based on the desired surface area of the hemisphere, the head manufacturer would cut 

an appropriately sized disk out of the plate. The head manufacturer would usually heat the blank 

disk to around 800 to 900°F and press it between male and female dies in small increments in 

order to attain the desired head dimensions and form. 

Based on this manufacturing process, the head forming operations would not be expected to 

eliminate the prior plate microstructure, but rather the plate material orientation was projected 

over the head while retaining much of the original orthogonality. If the prior plate 

4 Information provided by Maan Jawad, Consultant.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

94 of 193

microstructure remains, then the microstructure would continuously change circumferentially 

around the head, lining up with a prior plate orientation every 90°. The lack of such structure 

implies that the head material may have experienced an intermediate thermal treatment to 

eliminate the plate microstructure. Determining grain orientation by macros is commonly 

difficult in steels, particularly when the macros are arbitrarily oriented, as in this case. Given the 

inconclusive nature of these microstructural studies, a more quantitative approach with Charpy 

specimens was undertaken.

Instrumented Charpy V-notch tests according to ASTM E 2298-13 [ref. 64] were performed as a 

quick, relatively inexpensive yet quantitative way to determine the material orientation in the 

A225 head material from V0032. Due to deciding to focus on one lot of material, impact tests on 

the GRC V0296 head were not performed under this assessment. Instrumented Charpy tests 

utilize strain gages to capture the load versus displacement record at specimen impact. It is a 

more robust method than traditional Charpy tests, even if the load versus displacement is used 

only qualitatively as an aid to evaluate the fracture mechanism between a brittle and ductile 

failure. The instrumented test is also preferred for its precision in measuring fracture energy; it 

is more precise than monitoring pendulum height after impact. This precision also serves to 

lower the minimum range of impact energies that can be reliably measured, which can be 

important with cleavage. Given cost considerations, samples were limited to two at each of the 

three circumferential locations in two orthogonal orientations, M-C and C-M. Note that the 

direction of loading and crack extension is the same for Charpy specimens; therefore, in the twoletter orientation designation for the Charpy specimen, the first letter indicates the direction of 

the long axis of the specimen, and the second letter indicates the direction of crack extension. 

The cut plans for these samples are shown in Figure 7.2.1.2-6. Sample size was ASTM E23-12c 

type A, with a 10-mm by 10-mm square cross section and 55 mm long—the standard specimen 

used for ferrous metals. 

The results of these Charpy tests are shown in Table 7.2.1.2-1 and Figure 7.2.1.2-7. All tests 

were conducted at 0°F. Regarding the test temperature, these tests were not intended to 

determine the transition temperature but were simply used to reveal the least tough 

microstructural orientation in the material for subsequent fracture testing. The prior work by 

SwRI® indicated that 0°F would be a good choice for determining anisotropy for this material 

because orientation impact energy differences appeared to be largest at 0°F in their tests [ref. 44]. 

More informative fracture toughness tests were considered for determining the material 

orientation, but due to the required quantity the Charpy tests were more financially viable. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

95 of 193

Figure 7.2.1.2-6. Cut Plan for V0032 Charpy Impact Samples

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

96 of 193

Table 7.2.1.2-1. Charpy V-notch Impact Energy Results for V0032 A225 Head Material 

Note: Testing at 0 degrees F was to establish materials properties and should not be interpreted 

as a basis to safely operate LPVs at this temperature.

The Charpy impact results showed that the C-M orientation produced significantly lower impact 

energies than the M-C orientation with invariance to the sample circumferential location. This 

matches the observed orientation of the microstructural banding in the V0032 head material. It is 

possible that the material anisotropy effects in the V0032 head are being dominated by this 

banding structure. Future tests on head material without the banded microstructure may yield 

different results. It is suggested that all future efforts to identify the material orientation for 

pressure vessel heads predominantly utilize Charpy impact testing. Having attempted 

microstructural orientation evaluation in the A225 head material in the SwRI® work, the GRC 

PV0296 head material, and the V0032 head, none of these investigations has provided the clarity 

of the Charpy impact data. Despite this finding, as future A225 material is evaluated it is 

suggested that the basic microstructure of the head material be documented for features such as 

banding. 

Sample ID Vendor Test 

ID

Temp 

(F)

Theta 

(deg)

Meridional 

Location Orientation Impact Energy (ft-lbs)

Lat Exp 

(mils) % Shear

CP-380-17 U09577 0 0 1 C-M 73.88 39 80

CP-380-18 U09578 0 0 1 C-M 66.83 34 40

CP-380-19 U09579 0 0 1 M-C 84.60 42 40

CP-380-20 U09580 0 0 1 M-C 90.85 44 40

CP-380-21 U09581 0 45 1 C-M 39.36 20 30

CP-380-22 U09582 0 45 1 C-M 34.28 20 30

CP-380-23 U09583 0 45 1 M-C 142.86 71 90

CP-380-24 U09584 0 45 1 M-C 135.21 71 80

CP-380-25 U09585 0 90 1 C-M 43.74 23 50

CP-380-26 U09586 0 90 1 C-M 42.60 22 30

CP-380-27 U09587 0 90 1 M-C 123.06 57 80

CP-380-28 U09588 0 90 1 M-C 111.80 55 70

CP-380-45 U33589 0 60 2 C-M 36.03 19 20

CP-380-46 U33590 0 60 2 C-M 36.91 18 20

CP-380-47 U33591 0 60 2 M-C 136.89 66 80

CP-380-48 U33592 0 60 2 M-C 186.76 81 100

CP-380-49 U33593 0 75 2 C-M 47.24 24 30

CP-380-50 U33594 0 75 2 C-M 44.66 26 35

CP-380-51 U33595 0 75 2 M-C 168.58 72 100

CP-380-52 U33596 0 75 2 M-C 163.52 75 100

CP-380-65 U47089 0 45 2 C-M 43.89 27 20

CP-380-66 U47090 0 45 2 C-M 39.18 19 20

CP-380-67 U47091 0 45 2 M-C 167.64 74 80

CP-380-68 U47092 0 45 2 M-C 200.16 72 100

CP-380-69 U47093 0 90 2 C-M 44.31 23 30

CP-380-70 U47094 0 90 2 C-M 44.50 27 40

CP-380-71 U47095 0 90 2 M-C 177.24 78 100

CP-380-72 U47096 0 90 2 M-C 162.75 67 90

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

97 of 193

Figure 7.2.1.2-7. Charpy V-notch Impact Test Results for V0032 A225 Head Material 

(note strong separation between C-M and M-C orientations without dependence 

on circumferential angle) 

Based on the Charpy impact results, a cut plan was developed for A225 material from V0032, 

which is shown in Figure 7.2.1.2-8. All fracture toughness specimens are to be tested in the C-M 

orientation, which was determined as the lowest toughness orientation. Based on the unexpected 

delays caused by the time required for identification of material anisotropy, these fracture 

toughness specimens are still in process at the time of this writing. Due to time constraints, a 

survey through the thickness of the A225 head was also not performed to determine the 

bounding radial location for the samples. Accordingly, fracture samples are planned to be tested 

at two radial locations as shown in the cut plan. The feasibility of combining these two radial 

location data sets will be investigated in future work. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

98 of 193

Figure 7.2.1.2-8. Partial Cut Plan for Fracture Toughness and Tensile Test Specimens from A225

Head Material from V0032 Showing Salient Features of Full Cut Plan 

(the full cut plan shows detailed views of all specimens, but those views are self-similar

to the views currently shown and are omitted for simplicity) 

7.2.1.2.2 Task 2: Shell Materials Fracture Toughness Reference Transition Temperature

The objective of Task 2 of the materials characterization effort was to determine the toughness 

transition reference temperature T0 per ASTM E 1921-13 [ref. 33] for the LPV shell materials. 

This study was limited to 1143 and 1146 shell materials. Absent from this study was the 1146a 

material, which was initially based on the assumption that 1146 and 1146a were expected to 

produce nearly equivalent properties. The 1143 material was tested from the inner shell of vessel 

V0125. Where possible, inner shell material was preferred for testing because the greater 

thickness meant that thicker fracture toughness specimens could be machined and less of a 

statistical size correction would need to be made to account for the weakest link cleavage 

process. The 1146 material was not available from an inner shell and was tested from the outer 

layers of V0125. In reference to the master cut plan for V0125, the 1143 and 1146 samples were 

all taken from section 4. This selection was arbitrary but helped minimize the costs of 

performing additional machining operations on large vessel sections. Testing of these materials 

was confined to single vessels and layers so that the material would be representative of a single 

material lot. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

99 of 193

Prior to testing the shell materials, as with A225 head material, material orientation needed to be 

determined with respect to the vessels such that samples could be machined from the material in 

the lowest toughness orientation. Based on the assumption of logical manufacturing processes 

where the shells are rolled from thin sheets and from prior SwRI® testing results [ref. 44], this 

task was inherently much more direct for the shells than the head because there were only two 

possible complementary layouts for the shell materials:

1. The longitudinal (L) direction of the plate material was coincident with the longitudinal

(L) direction of the vessel. This would be evaluated with C-L fracture samples.

2. The longitudinal (L) direction of the plate material was coincident with the

circumferential (C) direction of the vessel. This would be evaluated with L-C fracture

samples.

To determine the material orientation, two standard ASTM E 1820-11 [ref. 65] fracture 

toughness tests were each performed at room temperature in the C-L and L-C orientations for 

each material and vessel section tested. These data were used to determine the material 

orientation, which would allow for test samples to be produced for determining T0 from the 

Master Curve method. 

For the 1143 material, fracture toughness tests performed at room temperature on the inner shell 

of section 4 from V0125 showed that the initiation fracture toughness was approximately 

40% lower for the C-L orientation than for L-C. Fracture surfaces are shown in Figure 7.2.1.2-9. 

Neither of these tests produced a fully valid JIC or KJ value, but the results were sufficiently 

advisory on material anisotropy. Given this outcome, all specimens for determining T0 from the 

Master Curve method for 1143 V0125 section 4 materials were obtained in the C-L orientation. 

Inspection of the fracture surfaces for the two orientations confirmed the results, as the L-C 

surfaces were much more ductile than the C-L surfaces. Since the C-L orientation represents that 

the specimen loading direction was in the vessel circumferential direction with a crack plane in 

the vessel longitudinal direction, then, given the observed anisotropy, this inner shell was 

oriented in the lowest fracture toughness orientation with respect to the hoop loading direction in 

the vessel. This suggests that material anisotropy was not a consideration in the original vessel 

design, further confirming the need to use the lowest toughness orientation throughout any FFS

evaluations of the LPV fleet. However, the results from the current study may not apply 

universally to all vessels; thus, in conducting further fracture toughness testing on materials from 

different lots and different manufacturers, it will be necessary first to determine the weakest 

orientation. 

For the 1146, material taken from the outer shells of section 4 from V0125 showed that the 

initiation fracture toughness was approximately 45% lower for the C-L orientation than L-C; 

thus, all 1146 specimens were also made in the C-L sample orientation. 

It is important to note that, to reduce material and labor costs, plates were sometimes rolled into 

cylinders with the plate transverse direction in the vessel longitudinal direction and sometimes in 

the vessel circumferential direction.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

100 of 193

Figure 7.2.1.2-9. Comparison of Fracture Surfaces for Fracture Toughness Tests Performed in 

C-L Orientation (upper image) and L-C Orientation (lower image)

A total of 18 basic method fracture toughness tests were performed on C-L 1143 C(T) specimens 

over the range of –20 to –180°F. This range was required because the test temperatures were 

continually decreased until the samples failed due to cleavage, a requirement for the Master 

Curve method. The initial temperature selected for testing was –20°F based on the available 

Charpy impact energy data obtained by SwRI® for the LPV materials [refs. 44 and 45] and the 

procedure from ASTM E 1921-13, Section 8.4.1 [ref. 25], but significantly colder temperatures 

were required for cleavage.

The fact that colder temperatures than initially predicted were needed to produce cleavage in the 

1143 material is advantageous for the FFS outcome of LPVs fabricated with 1143 material. 

However, this meant that more tests than initially planned were required to produce a large 

enough data set to evaluate T0 per ASTM E 1921-13. In total, one fracture toughness test was 

performed at –20°F, one at –70°F, four at –120°F, five at –170°F, and four at –180°F, for a total 

of 15 tests. The data are shown in Table 7.2.1.2-2. Of the 15 tests, 9 were fully valid and 6 have 

not yet been included in the current analysis due to failing certain data qualification limits5. The 

Kronecker delta shown in the table indicates whether the KJC data point is a fully valid data point 

5 The six tests that are currently excluded are not unconditionally excluded. In fact, ASTM E 1921-13 explicitly contains a 

special censoring process for such data so that they may be included in the analysis. This censoring procedure was attempted but 

is currently incomplete because i) there is a difficulty with applying the censoring procedure for the LPV materials that exhibited 

a very flat crack resistance R-curve (indicating that the fracture resistance did not monotonically increase with growing crack 

size) and ii) even though the E1820-12 Basic Method is advantageous for measuring cleavage initiation fracture toughness, the 

Basic Method is not conducive to the censoring procedure.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

101 of 193

or whether a data qualification limit was exceeded. When δj = 0 at the jth KJC value, then a data 

qualification check has failed and that data point must be censored in order to be included in the 

analysis. Data are used unconditionally when δj = 1. Censoring, as defined by E1921-13, is a 

specific, conservative substitution process to ensure the data are well described by cleavage. The 

term itself may be somewhat misleading since censoring often means deleting or removing, but 

in the context of E1921-13 censoring does not mean that the data are eliminated from the 

analysis. Instead, if certain qualification limits are not satisfied, then in order to be included in 

the analysis, statistical censoring requires that a conservative estimate for KJC must be substituted 

for the KJC datum that was obtained. This statistical censoring is necessary since the method 

applies a standard statistical analysis to measured data. Statistical censoring is an accepted 

method with a long history, and E 1921-13 has simply adapted it for its purposes. The two 

qualification limits for censoring are:

• The loss of plastic constraint

• Excessive ductile crack growth before the onset of cleavage

First, the plastic constraint condition requires that the specimen remaining ligament b0 = W – a0

have sufficient size to maintain a condition of high crack-front constraint at fracture. This 

constraint is the three-dimensional stress state at the crack tip, and a high constraint condition is 

required to conservatively measure cleavage fracture toughness. The maximum KJC capacity of

a specimen is given by E 1921-13 as 

KJC(Limit)=

Eb0σys

30(1 − v2)

KJC data that exceed this requirement must be first used in a data censoring procedure to be 

included in the analysis. The censored data must be replaced in the analysis by the KJC(limit)

value. The argument is that while the value of KJ at the onset of cleavage is unknown for this 

specimen, it exceeds KJC(limit), and this information should be included in the statistical analysis 

of the data set. In the case for the steels investigated, none of the tests failed this censoring 

requirement.

Second, a KJC datum is also invalid if too much slow, stable crack growth occurs before 

cleavage failure. The limit allowed for this stable portion of crack growth is the smaller of either 

0.05 b0 or 0.040 inches (b0 is the length of the remaining uncracked ligament after precracking 

but prior to testing). This latter condition is necessitated because the E1921-13 method is 

premised on a physical model that assumes a cleavage failure mechanism, and the volume of 

material sampled grows dramatically as the crack moves through the material. For the size of the 

C(T) specimens tested in this effort, this meant that ductile crack growth was not allowed to 

exceed approximately 0.025 inches before the sample failed due to cleavage to be considered a 

valid test, which is determined by post-test measurements of the fracture surface. This post-test 

measurement is a standard procedure in fracture testing. As shown in Table 7.2.1.2-2, six tests 

failed this data qualification limit and could not be used in the T0 analysis without censoring. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

102 of 193

Table 7.2.1.2-2. T0Transition Reference Temperature Data for 1143 Material from 

V00125 in Bounding Material Orientation 

Sample B0,

in

T, 

°F

JC, 

in-lbf/in2

KJC, 

ksi√in

KJC (1T), 

ksi√in δj

344-4-38 0.3738 –179.8 60.1 43.7 38.1 1

344-4-41 0.3753 –180.1 69.2 46.9 40.6 1 

344-4-42 0.3743 –170.5 93.9 54.6 46.7 1 

344-4-44 0.3735 –170.8 100.3 56.4 48.1 1 

344-4-46 0.3735 –180.2 106.7 58.2 49.5 1 

344-4-24 0.3748 –170.1 117.6 61.1 51.8 1 

344-4-39 0.3742 –179.9 199.5 79.6 66.2 1

344-4-23 0.3748 –120.2 220.0 83.6 69.4 0 

344-4-25 0.3762 –170.1 227.3 85.0 70.5 1 

344-4-28 0.3742 –171.5 234.2 86.2 71.4 1 

344-4-22 0.3752 –121.5 241.6 87.6 72.5 0 

344-4-17 0.3770 –20.1 334.9 103.1 84.7 0 

344-4-20 0.3757 –121.6 353.2 105.9 86.9 0

344-4-18 0.3767 –70.2 403.0 113.1 92.6 0 

344-4-19 0.3743 –120.3 405.5 113.5 92.7 0 

A separate validity criterion requires that the test temperature be within ±50°C of T0. If this 

condition is not met, then the data do not describe cleavage behavior well in the transition 

regime. In this case, no censoring procedure is appropriate, and the data must be excluded from 

the T0 analysis per ASTM E 1921-13. This requirement can be procedurally difficult to meet 

since T0 is not known a priori and is, in fact, what is being measured. Fortunately, while many 

tests were close to violating this condition, none actually did so. 

Sufficient data were produced for calculating T0, even with the interim exclusion of the censored 

data. Based on the nine valid tests, T0 was determined to be –71.0°C (–95.8°F). This should be 

considered a provisional but generally representative T0 since the censoring process has not been 

performed and the inclusion of censored data is expected to change T0 slightly. This provisional 

T0 is expected to be conservative compared with the fully qualified T0 determined with the 

censored data included. 

The reference transition temperature T0 is not equivalent to TNDT, but a Master Curve-based index 

temperature RTT0 can be determined, which can be used as a substitute for RTNDT. This can be 

determined based on equation (7.2.1) to be RTT0 = T0 + 35°C = –71.0°C + 35°C = –36.0°C 

(–32.8°F) for the 1143 material. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

103 of 193

It is noteworthy to consider that because T0 was not known before completing the tests and 

analysis, it is not surprising that some tests did not fail due to cleavage before the ductile crack 

extension limit was reached as test temperatures were iteratively decreased. Historical testing of 

nuclear grade materials, which are much tougher than the materials tested for the LPV program, 

conventionally violate the plastic constraint data qualification limit. Due to considerably lower 

toughness, this was not of concern for the LPV materials; no tests came close to violating this 

limit. Conversely, there were many tests on the LPV materials that failed the ductile crack 

extension limit, which upon thorough analysis revealed some difficulties in censoring based on 

this limit. Censoring is required in the case of excessive ductile crack growth before fracture 

because if the sample experienced an appreciable amount of ductile crack growth before failure, 

then the fracture is not described as cleavage. In such cases, the measured fracture toughness 

following considerable crack extension needs to be bounded by a lower K value in order for the 

cleavage fracture assumption to be valid. This lower K value is determined as the lesser of either 

(1) KJIC at the coldest possible temperature at which it can be measured or (2) the value of KJ∆a,

which is the value of K in the specimen at the point in the test where the 0.025-inch ductile crack 

extension limit, was violated. 

In regard to the LPV material tests, the issue is that these limiting K values are difficult to

determine given the LPV material behavior and the test method that was used. Determining the

KJ∆a value requires knowledge of the ductile J-integral resistance curve as the crack grows, 

which requires crack length data as it grows throughout the test. The test method used in this 

assessment does not provide that feedback. This choice involves a trade between running tests 

inexpensively with some that may not be valid, versus a more time intensive and complicated 

test that provides crack length feedback. At the beginning, the choice was to utilize the simpler 

method, and this approach has been sufficient. However, this choice has also left a number of 

tests lingering with the ductile crack extension invalidity. It remains undetermined whether a 

change to the more complicated test is warranted to avoid these invalid tests. 

The other means of getting a proper censoring reference test is to determine the coldest value of 

JIC feasible to provide the KJIC. A few tests were attempted for the 1143 material at –120°F but 

were unsuccessful. During the tests, increases in displacement occurred without a corresponding 

change in load, indicating a test issue such as ice buildup on the measurement gage or possible 

carbide or manganese locking [ref. 66]. These tests will be further pursued to obtain a proper 

censoring KJIC. As an alternative in the absence of such data, ASTM E 1921-13, Section 10.2.2, 

requires the use of the highest uncensored KJc(1T) in the data. In the 1143 data set, this 

corresponds to specimen 344-4-25, with a value of 70.5 ksi√in. This exceeds the KJc(1T) value for 

only 344-4-23 and, thus, does not make a reasonable censoring value for that specimen. 

However, this value would be acceptable in the absence of a better alternative to use as a 

censoring value for the five other censored tests. At the time of this writing, this procedure has 

not yet been applied. 

It was determined from this test program is that testing at colder temperatures and then testing

warmer, as needed, is likely a more effective method than testing in a decreasing temperature 

pattern. The former method is likely to converge more quickly on the temperatures required for 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

104 of 193

cleavage. One caveat to this is the cautionary note that testing at exceptionally cold temperatures 

is not recommended because the test temperature must be within 50°C of T0 for it to be useable 

in the T0 analysis. It is noted that for both the 1143 and the 1146 material the testing 

temperatures ultimately required to produce cleavage pushed close to the extent of this allowable 

testing window. Too much weight should not be placed on Charpy impact methods, which can 

be overly conservative. 

A total of 17 basic method fracture toughness tests were performed on C-L 1146 material over 

the range 32°F to –60°F. To date, 1 test was performed at 32°F, 1 at –20°F, 2 at –50°F, and 13 at 

–60°F. The data are shown in Table 7.2.1.2-3. Of the 17 total tests, 11 were fully valid and

6 will require censoring and have not been included in the current analysis. When considering 

only the fully valid data, sufficient data were measured for calculating T0. This provisional T0

was determined to be –2.6°C (27.3°F). Based on Equation (7.2.1), RTT0 is determined to be 

–2.6°C + 35°C = 32.4°C (90.3°F). The 1146 material clearly has a much higher T0 than the 1143

material, which was not predicted based on the strength properties shown in Table 7.2.1.1-3. 

This provisional T0 is considered generally representative but is expected to change when the 

censored data are included in the analysis. Censoring the 1146 data according to ASTM 

E 1921-13, Section 10.2.2, is not possible because the highest uncensored KJc(1T) in the data 

corresponds to specimen 344-4-30, with a value of 68.1 ksi√in. This exceeds the KJc(1T) value for 

all of the uncensored specimens and, thus, is not a reasonable censoring value. 

Table 7.2.1.2-3. T Tr 0 ansition Reference Temperature Data for 1146 Material from 

V00125 in Bounding Material Orientation 

Sample B0,

in

T, 

°F

JC,

in-lbf/in2

KJC,

ksi√in

KJC (1T),

ksi√in δj

344-4-49 0.191 –59.8 94.6 54.9 42.4 1 

344-4-51 0.1905 –60.8 116.7 61.0 46.4 1 

344-4-14 0.1995 –60.8 142.8 67.5 51.0 1 

344-4-29 0.2453 –60.1 140.6 66.9 52.4 1 

344-4-35 0.2442 –60 141.3 67.1 52.5 1 

344-4-52 0.1901 –59.8 164.3 72.4 53.9 1 

344-4-50 0.1906 –59.7 165.0 72.5 54.0 1 

 344-4-15 0.1978 –60.1 179.9 75.7 56.5 1 

344-4-34 0.244 –60.3 179.8 75.7 58.5 1 

344-4-48 0.1901 –60.5 209.9 81.8 60.1 0 

344-4-33 0.2435 –60 192.6 78.4 60.4 1 

344-4-16 0.1985 32 221.4 84.0 62.0 0 

344-4-12 0.1962 –50.3 238.7 87.2 64.0 0 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

105 of 193

344-4-13 0.1993 –50.8 257.3 90.5 66.4 0 

344-4-11 0.1968 –20 268.3 92.5 67.6 0 

344-4-32 0.2435 –59.8 249.5 89.2 67.9 0 

344-4-30 0.245 –60.9 250.4 89.3 68.1 1

All specimens tested to determine T0 for both the 1143 and the 1146 material were of C(T)

geometries, with the specimen thicknesses maximized given the constraints of the thin shell 

layers. The cut plans specified cutting the specimens to the full thickness with minimal cleanup. 

Multiple nominal thicknesses were tested for the 1146 material for no other reason than that the 

machinists became more adept with experience at maximizing the specimen thickness. 

Master Curve plots for the 1143 and 1146 materials are shown as the red lines in 

Figures 7.2.1.2-10 and 7.2.1.2-11, respectively. Ninety-five-percent confidence bounds have 

been determined according to E 1921-13 and are also shown as the black dashed lines. 

Figure 7.2.1.2-10. Master Curve Plot for 1143 Material 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

106 of 193

Figure 7.2.1.2-11. Master Curve Plot for 1146 Material 

Note that certain details of determining T0 have been purposely omitted from this report, as they 

were not considered germane to the principal LPV scope. Except for the use of only noncensored data, the analysis for T0 was done according to ASTM E 1921-13. Since not using 

censored data removes the highest toughness data from the analysis, any revised T0 will be 

higher and, hence, less conservative than the values reported here. These details and all 

supporting data are planned to be completely provided in a separate NASA Technical 

Memorandum. 

7.2.1.2.3 Use of Fracture Toughness Values from T0

The work described in the previous sections for generation of T0 values for the head and shell 

materials marks the first step in the process of determining the performance of these materials in 

the presence of defects as a function of the use temperature and material thickness (as related to 

potential crack front length). It must be clear that the T0 value is only a reference value anchored 

at a toughness of 100 MPa√m for an equivalent 1-inch-thick specimen. The 1-inch-thick 

reference specimen size establishes the 1-inch crack front length as the standard sampling 

volume of material relative to randomly occurring cleavage initiators, such as carbide particles. 

The T0 value must not be confused with the MDMT used to limit vessel temperature in use. The 

T0 value should be considered only a convenient way to express the fracture toughness of the 

material as a function of temperature with a statistically substantiated model of the data. There 

are a number of ways future work may utilize these T0 values to arrive at a MDMT for a given 

vessel. First, as discussed in Section 7.2.1.1.5, it is important to recognize that the fracture 

-150 -100 -50 0 50

0

50

100

150

200

Temperature, C

KJ1T, MPa-sqrt(m)

Master Curve

E1921 95% Confidence Bounds

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

107 of 193

toughness testing performed to date represents the performance of only a single lot of the 

1143 and 1146 alloys. Additional evaluation of T0 across many lots, including all alloys and 

welds, representing the larger LPV fleet is needed before obtaining T0 values that may be 

considered representative of the entire LPV population. Once fully representative values for 

T0 are produced, there are two likely ways that the T0 value will be used to evaluate an MDMT. 

The first is to use the T0 model for thickness correction to develop a set of curves for the LPV 

materials along the lines of the Charpy exemption curves shown in Section VIII, part UCS-66 

[ref. 1]. This family of curves, covering a variety of specific pressure vessel steels, has 

classically been used to set the MDMT as a function of material thickness for vessels fabricated 

from the specified materials or by the bounding curve (A) for general carbon steels. This 

approach is consistent with the traditional Section VIII design criteria and provides a rationale 

for MDMT independent of the actual defect state within the vessels; however, its safe application 

to older, non-code vessels requires further evaluation. A second approach to using T0 to develop 

an MDMT is to use a vessel-specific FFS logic, using rationale based on fracture mechanics 

assessment in accordance with ASME FFS-1 [ref. 32]. This approach has the advantage of 

adapting to a given vessel’s use conditions (i.e., pressure and temperature), so it may provide 

additional flexibility. Although this approach is perhaps more appropriate to the older, non-code 

LPVs because it starts with the assumption that the vessel includes crack-like defects, the need 

for this assumption is also the distinct disadvantage of this method. As discussed in detail in this 

report, the LPV design currently precludes quantitative NDE. This leaves the FFS approach 

potentially unanchored in many sections of the LPV. Regardless of approach, there remains a 

considerable body of research to property define the MDMT for the LPV fleet. The currently 

reported T0 values are merely the beginning of the process.

7.2.1.3 Evaluation of Weld Microstructural Regions

The third task of this assessment involved investigating various representative welds of the LPV 

system to begin the process of specifying a method for fracture toughness testing in the weld 

regions. The welds are significantly more likely to contain defects that cause concern for the 

FFS of the LPV fleet; therefore, the toughness of the welds is of considerable interest. The 

evaluation of the weld material is complicated by the presence of nonuniform microstructure and 

residual stress. The toughness of the weld can vary significantly within the same joint, such as 

the weld nugget, the fusion lines, or the HAZs on either side. A common method to begin the 

process of understanding the propensity for toughness debits across a weld is to look at the local 

hardness in the microstructure across the various zones of the weld. Frequently, the zone with 

the lowest toughness will present itself with the highest local hardness. To evaluate this, each of 

the representative welds in the V0125 vessel was cross-sectioned and mounted for 

microstructural evaluation. A microhardness traverse was done across representative sections of 

the weld. One example is shown in Figure 7.2.1.3-1 for the head-to-shell weld of V125. The 

small white marks are reflections from the dents made by the microhardness indenter. 

Figure 7.2.1.3-2 shows the results of three of the hardness traverses. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

108 of 193

Figure 7.2.1.3-1. Microhardness Traverses Taken across the V0125 Head-to-Shell Weld

Figure 7.2.1.3-2. Microhardness Traverses Illustrating Hardness Response Differences in 

Inner Layer and First Thinner Layer in V0125 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

109 of 193

The microhardness profiles are well behaved, showing gradual changes across the weld zones 

and no isolated regions of particularly high hardness. Because the LPV welds do not receive a 

post-weld heat treatment, it was expected that the hardness would vary more than observed in 

these data. This is a preferred result from the perspective of the weld performance—there do not 

appear to be high-strength/low-toughness zones within the welds, but reducing the burden for 

testing the various zones across the welds does not help as was hoped. Though of largely 

uniform hardness, the microstructures (and residual stresses) do vary across the weld. The weld 

location of least toughness will have to be determined through toughness testing. Though not 

accomplished as part of this assessment, preliminary weld region toughness tests are currently in 

planning. 

As another means of evaluating weld quality while helping to determine any systematic 

occurrence of weld process defects, a radiographic inspection of 1146a wrapping-layer 

longitudinal-seam welds was performed on approximately 50 inches of weld from section 11 of 

V0032 after the vessel section was dissected to reveal the welds [ref. 67]. For this vessel, each 

layer is joined to the adjacent layer by a single longitudinal seam weld, as shown in Figure 

7.2.1.3-3. The five longitudinal welds in the 10-inch-long section comprised the 50 inches that 

were inspected. Sampling the quality of these seam welds is important because there is currently 

no NDE method available for the longitudinal shell welds for intact LPVs and because the 

original LPV mechanical drawings typically specified only limited NDE inspection, including 

RT of inner shell longitudinal welds and some MT of longitudinal shell welds. Data from these 

inspections are not available. 

Figure 7.2.1.3-3. Cross Section for V0032-11, Evaluated by RT for Longitudinal 

Shell Welds Shown

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

110 of 193

The inspections performed in the current evaluation were conducted on dissected sections 

comprised of two layers with an individual longitudinal weld. The goal was to determine the 

typical weld quality, including the location, type, size, and number of defects per linear foot of 

the welds, and whether a pattern emerges in the nature of these observed defects. Pattern refers 

to any outstanding characteristic or feature in the observed defects, with special interest in 

features that might impact the final FFS rationale. RT was a first attempt at characterizing the 

longitudinal shell welds, with the understanding that one of the limitations of RT is that it does 

not reliably detect cracks that are not open and are not optimally oriented with respect to the 

direction of the beam. 

The V0032 section 11 results showed considerable porosity and lack of fusion, with all welds 

inspected failing the inspection criteria per ASME Section VIII [ref. 1], Appendices 4 and 12. 

Appendix 4 is specific to rounded RT indications. Appendix 12 is referenced so that lack of 

fusion can be included as a rejectable indication, as the BPVC does not include this callout under 

RT. Given this unfavorable inspection result, five additional longitudinal seam welds from 

V0032 section 8 were inspected utilizing the same inspection method [ref. 68]. Section 8 was a 

19-inch-long segment, which at the time of this writing brings the total inspected length of 

V0032 longitudinal welds to approximately 145 inches. All of the longitudinal seam welds 

inspected in V0032 to date have failed to meet ASME Section VIII due to lack of fusion and 

porosity. 

An additional set of RT inspections was performed on ring 4 from V0125 to determine possible 

vessel-to-vessel longitudinal weld quality differences [ref. 69]. As a four-layer vessel, this 

included the inspection of three welds, each approximately 8 inches in length. In contrast to 

V0032 (an A. O. Smith vessel), no rejectable conditions were found in the longitudinal seam 

welds of V0125 (constructed by CB&I). Overall, the quality issues that were observed (lack of 

fusion and porosity) are probably more indicative of weld processing issues than material quality 

issues. Additional RT inspection is needed to build a sufficient family of data to provide a 

broader assessment. This work is currently ongoing with additional evaluations planned for 

V0032 and V0125 and other available vessel materials. 

7.2.2 WRS Modeling of LPVs

Introduction

Remaining life assessment of the aging NASA LPV vessels requires that a fatigue and fracture 

mechanics FFS assessment be performed. The WRS caused during LPV fabrication can strongly 

affect both fracture response and fatigue life6

. The purpose of this effort is to perform weld 

analyses of several different typical layered vessels to determine the WRS fields in the layered 

shells. Two NASA vessels were chosen for WRS analysis: (1) a small, four-layer vessel and 

(2) a large, 14-layer vessel. This report summarizes the results for the small four-layer vessel; 

the results for the larger vessel will be reported later. These analyses will be compared with 

those of monolithic shells to determine the layering effect on WRS fields. If the layering effect 

6 WRS can affect corrosion growth, but this is not considered a problem in the NASA LPV tanks.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

111 of 193

is found to be small, it is possible that the WRS fields for use in fracture assessment may be 

obtained from monolithic vessels, which permit much easier analysis. In addition, if the layering 

effect is small, the use of solutions that appear in the API-579 code [ref. 32] may be permissible.

Geometry

Only the results for the four-layer tank circumferential weld are provided here. The smaller 

vessel was evaluated first because of its simpler construction (fewer layers) so that the layered 

vessel weld analysis procedure could be refined before addressing a larger vessel. The 

description of this tank is discussed in [ref. 70]. A schematic of the weld geometry as modeled 

can be seen in Figure 7.2.2-1. 

Figure 7.2.2-1. Four-layer Tank Considered for Weld Residual Stress Analysis

As seen in Figure 7.2.2-1, a shell-to-shell and a head-to-shell weld were considered. This 

represents a 3,500-psig, 609.6-mm (24-inch) diameter, and 3098.8-mm (122-inch) long vessel of 

vessel type M117. This vessel has an inner shell thickness of 12.7 mm (0.5 inches) and three 

layers of 6.35-mm (0.25-inch) thickness, for a total thickness of 31.75 mm (1.25 inches). In 

addition, the shell-to-shell and head-to-shell welds were also modeled as monolithic vessels 

(without the layers) in order to compare the layered analysis results with the monolithic results. 

Note that the four-layer model had a head the same size as the shell. However, many heads have 

smaller diameters than the shell, which is often the case with NASA’s fleet of LPVs since 

primary stress in the hemispherical heads is only one-half that in the cylindrical shells. The 

14-layer tank being analyzed now includes the taper transition. The taper transition between 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

112 of 193

such sections, including the severity of the taper, has a yet-to-be-determined effect on the stress 

results and should be considered in the future. 

Weld Modeling Procedure

The Computational Weld Model: The analysis was performed using the Virtual Fabrication 

Technology (VFTTM) computational weld modeling code [ref. 71]7. As seen in Figure 7.2.2-2, 

VFT has three modules: 

1. GUI (graphical user interface), which is used to define weld passes, material data, weld

sequence, etc. (i.e., GUI in Figure 7.2.2-2).

2. Thermal module, which calculates the temperature time histories at each nodal point as

each pass is deposited. Here, the possibility of gap resistance between the plate layers is

ignored since the plates are generally assumed to be tight against each other. Based on

discussions with a former layered vessel manufacturer8

, the thermal conductance in the

radial direction at the layered regions was modeled as being 25% of the thermal

conductance in the other directions.

3. The structural solution is performed using Abaqus® with a weld-specific material user

routine (labeled “UMAT” in Figure 7.2.2-2), which is written to handle the unique

aspects of welding such as material melting/resolidification, annealing caused by material

heating above the phase transformation temperature, temperature-dependent hardening up

to melting (800°C to 900°C suffices for ferritic steels), etc. The possibility of phase

transformation plasticity effects is ignored since data are not available.

Finite Element Weld Model – The finite element weld models are shown in Figure 7.2.2-3, where 

the upper left model is layered shell to monolithic head, the upper right is layered shell to layered 

shell, the lower left is monolithic shell to monolithic head, and the lower right shows the 

monolithic shell-to-shell weld. The layers are identified via the different colors, as are the weld 

grouping elements. A total of 14 passes are modeled. Note that the simplified model using 

“rectangular” weld passes has been shown to have little effect on WRS results, as long as the 

relative pass sizes are realistic. For the structural solution, there were convergence difficulties 

caused by the use of contact9 between the layers. This was mainly caused during deposition of 

each pass when the material properties in the weld being deposited and near the interface contact 

zones were very low (low stiffness). However, the convergence difficulties were overcome by 

using restart analyses where solution parameters were continually modified.

7 Numerous publications of the theory and examples of weld residual stress and distortion calculations are available 

(for example, see references 67 through 70, and the many references cited therein, for more model details). Emc2 

developed and uses VFT for many problems for weld modeling analysis, fatigue, and corrosion assessment of 

welded structures. In addition, VFT is leased by a number of organizations including Caterpillar, Knowls Atomic 

Power Laboratory (KAPL), U.S. Nuclear Regulatory Commission (NRC), Babcock and Wilcox (B&W), and Rolls 

Royce (United Kingdom).

8 Maan Jawad, private discussions.

9 Frictionless contact was used.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

113 of 193

Figure 7.2.2-2. VFT Weld Modeling System

Figure 7.2.2-3. Axisymmetric Weld Models and Meshes

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

114 of 193

Material isotropic hardening was initially used for the weld analyses. With classical plasticity 

theory used here, the stress state must be on the yield surface during plastic straining. With 

isotropic hardening, the yield surface expands during this plastic straining due to hardening. 

With kinematic hardening, the yield surface moves rigidly during plastic straining with the size 

of the yield surface remaining the same as the original size (i.e., the yield stress). Mixed 

hardening permits both translation and expansion of the yield surface and is considered most 

accurate. Isotropic hardening tends to produce upper bound results in WRS predictions. The 

material data used for the weld metal should be obtained in the annealed condition because the 

shrinkage during solidification is modeled. Here, the annealed data are not available. Therefore, 

the analyses were performed using linear kinematic hardening10. It is believed that use of 

kinematic hardening might be most appropriate here since the as-welded material data were used. 

However, this is only an experience-based guess at this point. Both results are presented here 

since isotropic hardening results should produce conservative predictions of FFS. It is noted that 

material data are not available to model mixed hardening. 

An accurate analysis requires material data for each material from room temperature up to 

melting. As seen in Figure 7.2.2-3, the materials in the four-layer tank are 1146a for the shell 

and A225 Grade B for the head. The weld metal also was assumed to be 1146a. Since stressstrain data were not available for these materials, the properties used were obtained by scaling 

known stress strain curves for another similar pressure vessel steel (A516) based on the actual 

yield stress data tested by SwRI® [ref. 45]. Based on prior work in the nuclear industry, it is 

believed that this approach produces estimates of the stress strain response for the layered tank 

materials that are quite reasonable. The curves used to perform the weld analysis are shown in 

Figure 7.2.2-4. Since actual properties for the weld material were not known, any weld 

overmatch was not modeled. If it exists, it would result in increases in the WRS fields roughly 

by the ratio of the yield stresses between the base and weld materials.

10 Mixed (Chaboche) hardening would be best to use since cyclic loading does occur as each weld pass is deposited 

near prior passes. However, the material data necessary for this are not available.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

115 of 193

Figure 7.2.2-4. Stress Strain Curves Used for Mechanical Portion of Analysis

WRS Results

While no direct validation of the residual stress predictions was made here, the predictions of 

radial distortion and the corresponding gapping seen near the welds were qualitatively predicted. 

However, as mentioned in the previous section, extensive validation of the procedures and 

predictions of WRS and distortions have been made as indicated in references 72 through 76, the 

references sited therein, and user and validation manuals. It turns out that the hydrotest pressure 

of the vessel (38.06 MPa/5,250 psi), which was performed at a pressure of 1.5 times the 

nameplate design pressure of 3,500 psi (24.13 MPa), tends to relax the as-welded WRS fields, so 

this process was modeled as well. This hydropressure induces plasticity in and near the weld, 

which has the effect of relaxing the WRS magnitudes. This result was not initially anticipated 

based on experience with modern code vessels. However, since this layered vessel was 

fabricated from higher strength steel than is common today (105 ksi tensile strength, 75 ksi yield) 

and was designed with a significantly lower FS on shell tensile strength (2.86 FS) than code 

vessels at the time (i.e., 4) or modern BPVC vessels (i.e., 3.5), the stress due to pressure loading 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

116 of 193

is much higher than exists in code vessels. In hindsight, it is not surprising that a 

1.5× overpressure test would cause more inelastic strain than anticipated with resulting residual 

stress relaxation.

Figure 7.2.2-5 shows contour plots of hoop WRS fields in the layered shell-to-shell 

circumferential weld. The upper illustration shows isotropic hardening, and the bottom shows 

linear kinematic hardening results. As discussed above, it is believed that the linear kinematic 

hardening results are most appropriate since the as-welded properties for the weld metal had to 

be used. However, results are shown using both hardening laws since isotropic hardening 

provides the most conservative predictions for FFS assessments. For the fracture assessment, it 

may be desirable to use the most conservative predictions if they are not overly conservative. 

Figure 7.2.2-5. Hoop WRS Predictions for Layered Shell-to-Shell Weld

Figure 7.2.2-6 shows contour plots of axial WRS fields in the layered shell-to-shell 

circumferential weld. The upper illustration shows isotropic hardening, and the bottom shows 

linear kinematic hardening results. Somewhat higher axial stresses are seen near the ID and 

alternate between tension and compression at each layer. This is clearly different behavior 

compared with a monolithic analysis (which was performed but not reported in this high-level 

summary). Figures 7.2.2-7 and 7.2.2-8 show similar contour plots for hoop and axial stress, 

respectively, for the shell-to-head weld. For the most part, the WRS in the shell-to-head weld 

are slightly higher than for the shell-to-shell weld. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

117 of 193

Figure 7.2.2-6. Axial WRS Predictions for Layered Shell-to-Shell Weld

Figure 7.2.2-7. Hoop WRS Predictions for Layered Shell-to-Head Weld

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

118 of 193

Figure 7.2.2-8. Axial WRS Predictions for Layered Shell-to-Head Weld

WRS Line Plots and Comparison to Monolithic Results – It is useful to compare line plots of the 

WRS fields from the layered shell cases to monolithic vessel cases to determine the importance 

of including the layers in the analysis. As discussed above, solutions using the layers with 

contact are more challenging and can lead to convergence difficulties. Moreover, if the layer 

effect can be neglected, then solutions from API-579 (or other sources) can be used to estimate 

the stress intensity factors using the weight function approach in Appendix C of API-579. In 

addition, it is often easier to compare the stress fields directly with line plots.

As seen in Figures 7.2.2-5 through 7.2.2-8, the WRS distributions vary spatially in the weld 

region, as well as in the HAZs. For fracture mechanics assessment it is most proper to include 

this variation when calculating fracture parameters (e.g., the stress intensity factor, K) by using 

(for instance) the finite element alternating method (FEAM)11 to insert cracks at various 

locations within the WRS field and then determining the K value. However, API-579 

conservatively permits the use of a through-thickness line definition of WRS (for use with 

Appendix C stress intensity factor weight function tabulations), which is assumed to be invariant 

throughout the weld and the HAZ12. 

11 FEAM is a convenient method for quickly obtaining stress intensity factors within WRS fields.

12 Of course, this assumption is overly conservative since the WRS field clearly varies throughout the weld and 

HAZ, as can be seen in Figures 7.2.3-5 through 7.2.3-8.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

119 of 193

In this section, line plots are provided and compared. Line plots of stress are provided along 

three paths, as illustrated in Figure 7.2.2-9. One represents a plot through the weld centerline, 

one intersects layer 1 with the weld (path 1), and one intersects layers 2 and 3 and the weld (path 

2-3). Figure 7.2.2-10 illustrates line plots of axial and hoop WRS for the shell-to-shell case. The 

solid black and solid red lines compare results between isotropic and kinematic hardening. It is 

seen that the axial stress differences are relatively small except at the ID of the vessel. It is noted 

that the differences between isotropic and kinematic hardening are much greater in the aswelded condition prior to application of the hydrotest. The dashed lines in Figure 7.2.2-10 are 

for the monolithic case. The monolithic shell results do not compare well for the axial stresses 

and are conservative for hoop stresses. 

Figure 7.2.2-9. Line Path Definitions for Through Thickness Stress Plots

Figure 7.2.2-10. Shell-to-Head Weld: Line Plot from ID to Outer Diameter (OD) along Centerline

Figure 7.2.2-11 illustrates the line plots of the axial and hoop WRS fields along the layer 

path 2-3 (see Figure 7.2.2-9) for the shell-to-head case. The axial stresses are highest at the ID 

for both the isotropic and kinematic hardening cases. For low toughness materials, this may lead 

to a fully circumferential crack that, if undetected, could possibly grow through-wall with cyclic 

pressure loadings and eventually lead to a head blowout, although this was not specifically 

modeled and evaluated and would require crack-growth rate analyses and loading history 

assumptions that are well outside the scope of this study. The axial stresses for the monolithic 

case are somewhat similar to the layered case here. On the other hand, the monolithic shell-toCenterline Path Path 1 Path 2-3

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

120 of 193

head case produces higher hoop stresses than the shell case. One can also see that the stresses 

have “jumps” near the layer-to-weld intersections where a natural crack exists. Other plots like 

this show similar trends and suggest that modeling the actual shell behavior is more important 

than using the monolithic results. The effect of tapered head-to-shell transitions must also be 

determined.

Figure 7.2.2-11. Shell-to-Head Weld: Line Plot from ID to OD along Layer Path 2-3 

Comparison to API-579 Solutions 

One of the goals of this effort was to determine whether the procedure in Appendix E of 

API-579 [ref. 32] for estimating WRS fields in pipes and vessels could apply to NASA LPVs. 

The API-579 estimates are meant to be upper bound generic solutions used in lieu of finite 

element WRS-based solutions13. Figure 7.2.2-12 compares the WRS analyses for both the 

layered cases for the shell-to-shell (top) and shell-to-head (bottom) cases, respectively. The use 

of the API-579 solution for both axial and hoop stresses (for evaluating circumferential and axial 

cracking, respectively) appears to be conservative for most points through the thickness14. For 

the shell-to-shell case, the isotropic hardening axial stresses near the ID are highest. However, if 

the API-579 solutions are used for cracks near the OD of the vessels, then fracture predictions 

will likely be too conservative as axial stresses approach 700 MPa and hoop stresses 800 MPa 

from the API solutions. Note that the API-579 estimated solutions for the shell-to-head case use 

the material yield stress for the shell material (layer side of weld).

As discussed above, the API-579-estimated WRS fields assume that the stresses are constant 

spatially throughout the weld. This is not true based on Figures 7.2.2-7 and 7.2.2-8 contour 

plots. Therefore, it is also useful to examine line plots at a location away from the weld 

centerline. Figure 7.2.2-13 illustrates the WRS fields along the layer path 2-3 (see 

Figure 7.2.2-9), which is along the line near the top of the vessel where “natural cracks” exist 

due the structure of the layered vessel. It is seen that the API-579 solutions for hoop stress are 

conservative throughout the thickness and probably too conservative from midway through the 

13 Users of API-579 always have the option of developing their own WRS fields from welding.

14 It is noted that the API-579 solutions are not developed for the shell-to-head case, but it can be assumed to apply 

regardless.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

121 of 193

thickness to the OD. The API-579 axial stresses are conservative throughout except right at the 

ID and somewhat near mid thickness. Again, the API-579 solutions near the OD are probably 

overly conservative.

Figure 7.2.2-12. Comparison of API-579 Solutions to Calculated Results for Both Shell-to-Shell and 

Shell-to-Head Tank Cases (axial, left; hoop, right): Weld Centerline

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

122 of 193

Figure 7.2.2-13. Comparison of API-579 Solutions to Calculated Results for Both Shell-to-Shell and 

Shell-to-Head Tank Cases (axial, left; hoop, right): Layer 2-3 Path 

Measurement of Weld Residual Stress Fields

It may be useful to obtain a sample of WRS measurements in order to validate the predictions 

since full material test data are not available. There are several state-of-the-art methods that are 

often used in order to obtain through-thickness WRS fields, including neutron diffraction, deep 

hole drilling, and the contour method (see reference 73 for a summary of all three methods). 

Because of the nature of LPVs it appears that deep hole drilling might be most useful here. It is 

noted, however, that there are often scatter in measurements and differences in measurements 

between the different methods. Therefore, it is often useful to obtain measurements using two 

different methods.

Summary and Conclusions

From the analyses of the four-layer vessels, the following conclusions are drawn: 

• WRS predictions using isotropic hardening are almost always highest in absolute value

for hoop stresses (versus other hardening models), while axial stresses are quite similar

except near the ID. Because there is a lack of material property data, both sets of results

are presented at this time.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

123 of 193

• Hoop WRSs are conservatively predicted using a monolithic geometry, neglecting the

layers for both shell-to-head and shell-to-shell welds. However, predictions may be

overly conservative from an FFS standpoint.

• The use of API-579 WRS solutions will likely be overly conservative, and calculated

WRS fields where the correct hydrotest pressure is applied can be used if the API

solutions for FFS predictions are overly constraining.

• Axial and hoop WRS tend to be maximum along path 2-3, which is near the edges of the

weld near the region of layer-to-weld convergence. Axial and hoop WRS spikes occur

near the region where the path intersects layer/weld interfaces due to the sharp crack-like

interface. This could possibly be a crack initiation location for both circumferential and

axial cracks.

• Results for a 14-layer vessel are nearly complete and will be presented soon.

• Calculations of stress intensity factors in these WRS fields need to be made for cracks

positioned at different locations near the weld in order to perform FFS and remaining life

calculations.

• A second hydrotest pressure of 1.5 times design load was applied, and this had little

effect on reducing the WRSs. However, when a second hydrotest pressure 10% higher

(41.86 MPa rather than 38.06 MPa) the WRSs were reduced with a maximum reduction

of about 5% (hoop stresses) and 7% (axial stresses). This might be considered for life

extension if the additional higher-pressure hydrotest did not cause damage.

7.2.3 Evaluation of Two Digital Image Correlation Techniques to Detect LPV

Deformations during Hydrotest

NASA has a long history of safe pressure vessel operation that is largely attributable to a 

rigorous vessel certification program. Vessel certification entails inspections to detect and assess 

the effect of damage such as corrosion and cracking. Damage to solid wall vessels is readily 

detectable through standard NDE inspection techniques. However, damage to the shell sections 

of LPVs is hard to detect using standard NDE inspection techniques. For example, a crack 

within an intermediate layer is generally confined to the affected layer and cannot be detected 

using visual or surface NDE inspection techniques. Such cracks could propagate undetected into 

critical head-to-shell welds, leading to catastrophic vessel failure without warning. This type of 

intermediate layer damage could partially compromise shell integrity and might produce local 

shell deformations that could be detectable via photogrammetric techniques. The ability to 

detect defects using this approach could be compromised by the effect of layer gaps, which result 

in a reduced outer shell deformation. For example, ASME requires that an LPV have a 

minimum of only 50% of the outer shell deformation as measured by Pi Tape, compared with a 

monolithic shell. 

Full-field optical deformation techniques, which include scanning vibrometers, various types of 

interferometry techniques, and digital image correlation have become more commonplace within 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

124 of 193

the past decade. Digital image correlation applications have vastly expanded due to the 

advancements in sensor hardware capability and computer processing power needed for the 

pattern recognition and point tracking algorithms. In recent years, the testing paradigm has 

dramatically shifted from the use of physical instrumentation gages to optical techniques to 

capture deformation and motion. 

NASA has previously used commercial digital image correlation systems in a number of areas, 

including the Space Shuttle Program. For the Columbia Accident Investigation and the Return to 

Flight Program, deflections of reinforced carbon-carbon test panels from impact tests were 

captured and used to validate analysis models for flight rationale. In the current timeframe, 

optical metrology is used at nearly every NASA field center on a daily basis. References are 

provided to illustrate the wide range of research and engineering applications of optical 

metrology in areas of material characterization [refs. 77–80], structural analysis [refs. 80–83], 

and biomechanics [refs. 84–87].

The test in this report used two photogrammetric techniques to measure deformations of a scrap 

LPV during hydrotest pressure excursion from 0 to 4,200 psig (150% of vessel design pressure). 

The objective of the test was to document gross vessel deformations, as well as specific surface 

locations where underlying flaws were known to exist via previous radiographic inspection. The 

hydrostatic test was conducted at GRC from August 20 to 23, 2013, and consisted of four total 

excursions from 0 to 4,200 psig. As part of this exercise, the team from MSFC conducted an AE 

survey on the first excursion with the intent to identify the applicability of AE to find internal 

flaws in pressure vessels. Results of the AE effort are presented in Section 7.2.4 of this NESC 

report. 

7.2.3.1 Test Article

The test article, shown in Figure 7.2.3.1-1, was a scrap A. O. Smith LPV MV-50405A-31 (GRC 

vessel PV0236) with a 52-inch internal diameter, 24 feet long, with an internal volume of 

306 cubic feet. The vessel was fabricated with two solid heads and a shell section consisting of 

an inner shell with seven overwrap layers. Manufacturer design pressure was 2,800 psig. The 

hydrotest pressure was 4,200 psig (150% vessel design pressure), which was the original 

hydrotest pressure of the vessel at manufacture in 1959. Prior to testing, the vessel was dormant 

for roughly 20 years due to internal crack-like defects located within vessel shell layers near the 

head-to-shell welds. These flaws were discovered by radiographic inspection in the early 1990s. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

125 of 193

Figure 7.2.3.1-1. Pretest View of PV0236 (vessel measures 52-inches ID × 24 feet long, has an 

internal volume of 306 cubic feet and a design pressure of 2,800 psig, has a layered shell 

construction, and was fabricated in 1959) 

Prior to testing, the vessel was re-inspected with radiography and ultrasound techniques to better 

quantify internal flaws for correlation with both the acoustic and optical measurement methods. 

Radiography was performed on both vessel head-shell welds, and an ultrasonic bore-probe 

inspection was performed on both nozzle welds. The radiographic inspection confirmed several 

crack-like indications in the vessel shell near both head-shell welds. Ultrasonic inspection 

revealed lack of fusion in the front fusion zone of both nozzle welds. These NDE findings are 

summarized pictorially in Figure 7.2.3.1-2. Referring to the figure, there are three crack-like 

indications in the vessel shell near head-shell weld C-1, measuring between roughly 0.25 inches

and 0.56 inches, and one 0.27-inch crack-like indication in the vessel shell near head-shell weld 

C-2. The depth of these flaws within the vessel shell is unknown. There is a lack of fusion in 

the front fusion zones of both nozzle welds consistent around the entire weld circumference.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

126 of 193

Figure 7.2.3.1-2. NDE Results Summarizing Known Vessel Flaws for Vessel PV0236

7.2.3.2 Digital Image Correlation Methods 

Digital image correlation is the acquisition of engineering data from digital images of an object 

from different perspectives for determination of object shape and deformation at different states 

or loading conditions. Two different commercially available measurement systems were 

employed on this test: Aramis and Tritop, developed by the German company, GOM mbH.

Aramis computes full-field three-dimensional surface deformation and strains from stereo image 

sets taken at any selected time interval throughout a test sequence. These image sets are 

acquired by two image sensors fixed to a rigid beam apart from one another and focused on the 

object of interest being tested. Aramis utilizes pattern mapping/tracking algorithms to determine 

object deformations in the image pairs between discrete time intervals, and typically requires a 

high-contrast dot or speckle pattern to be applied to the test object of interest to ensure highfidelity data capture. Post-test, the Aramis measurements can be viewed as full threedimensional color contour images or plotted against load, time, or other desired values for 

engineering analysis.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

127 of 193

Tritop is a quasi-static photogrammetry technique that generates shape and deformation results 

from sets of photographs, taken from arbitrary points of view, of an object with a handheld, highquality, 12-megapixel digital single-lens reflex camera. Tritop computes the spatial centroidal 

locations of dots and specialized “coded targets” placed at selected locations on the surface of a 

test article to generate a three-dimensional point cloud. Using multiple sets of photographs, one 

or more point clouds of the test article can be generated at various test static states and then 

compared with the baseline data. From these comparisons, Tritop calculates highly accurate 

three-dimensional positional changes for each dot and target, enabling effective visualization of 

structural motion between states. Typical data reduction from Tritop shows vector plots resolved 

into a defined coordinate system representing the motion of each of the points in the point cloud. 

To calibrate both of these optical systems, a calibration object is required to orient the camera 

spatial locations and establish a scale to ensure the software uses accurate dimensions. For this 

test sequence, a 2-meter carbon fiber cross, with digital targets placed at known dimensional 

locations, was used for calibration. For Aramis, calibration of the stereo camera set takes place 

prior to the actual test by recording a few dozen images of the cross located at several different 

locations in the cameras field of view. With Tritop, the calibration artifact remains in the field of 

view for a number of the handheld images, enabling the software to perform calibration at the 

same time it is calculating the spatial locations of the targets.

A preliminary study was performed to establish that both Aramis and Tritop were capable of 

resolving the expected deformations for the vessel. Hand calculations were performed to 

establish theoretical vessel stress, strains, and deformations under 2,800 and 4,200 psig loading 

pressures, for a fully intact vessel, a vessel with one layer lost, and a vessel with two layers lost. 

These parameters are summarized in Table 7.2.3.2-1. At the 4,200 psig state expected results 

were nominally 1,692 microstrain circumferentially (hoop strain) and an axial vessel elongation 

of about 0.185 inches. 

Table 7.2.3.2-1. Vessel Stress, Strain, and Deformation Computations at Two Pressure States based 

on Modulus of Elasticity of 2.95 E+07 psi 

AT DESIGN MAWP (2,800 PSIG) AT 150% MAWP (4,200 PSIG)

All 

layers 

intact

One 

layer 

lost

Two 

layers 

lost

All 

layers 

intact

One 

layer 

lost

Two 

layers 

lost

Nominal Vessel Thickness (in) 2.1875 1.9375 1.6875 2.1875 1.9375 1.6875

Ave Hoop Stress (PSIG) (1) 33280 37574 43141 49920 56361 64711

Ave Hoop Strain, με 1128 1274 1462 1692 1911 2194

Circumferential Vessel Growth (in) 0.200 0.224 0.254 0.300 0.335 0.382

Axial Shell Growth (in) 0.124 0.139 0.160 0.185 0.209 0.240

The Aramis displacement resolution is linearly dependent on the physical length captured by 

each pixel on the sensor. This means that the smaller the length that Aramis is measuring, the 

more accurately it can record deformation. Thus, tradeoffs are commonly made when trying to 

optimize the combination of sensor distance to the test object and focal length of the lens 

(representing the size of the Aramis field of view) to establish the theoretical optimal accuracy of 

the test setup. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

128 of 193

It was determined that one-half of the vessel would be imaged for each pressure cycle. This was 

a balance between capturing a large overall view of the vessel, while maintaining acceptable 

displacement resolution. The 5-megapixel sensors capture an image array of 2,448 pixels wide 

by 2,050 pixels high. With the field of view being about 14 feet, or 168 inches, this would yield 

an effective pixel width of 168 inches/2,448 pixels, which equals 0.0686 inches per pixel. 

GOM mbH publishes a theoretical accuracy to 1/30th of a pixel (0.0686/30), resulting in a 

±2.3 thousandths of an inch theoretical maximum accuracy displacement for the 14-foot field of 

view on the vessel. Referring back to Table 7.2.3.2-1, this value is well below the expected 

vessel expansion of 0.300 inches and axial growth of 0.185 inches at 4,200 psi. This basic 

analysis calculation provided the confidence that Aramis would theoretically be able to 

accurately capture the vessel deformations throughout the planned test. 

7.2.3.3 Vessel Preparation

As mentioned in the earlier section, Aramis correlates stereo image pairs through sophisticated 

pattern recognition and tracking. In all cases of Aramis use, a high-contrast stochastic black and 

white pattern must be applied to the surface for measurements to be made. The stochastic 

pattern, in many cases, is created by preparing the surface to be white. A black pattern is then 

applied over the white base. The pattern can be accomplished using airbrushes, artificial 

patterned stamps, stencils, or, in many cases, a large number of circular dots applied at random. 

For the dot technique, each dot typically must be on the order of five to seven pixels in diameter. 

Using these criteria for the established field of view established a nominal dot diameter from 

0.34 to 0.48 inches. The dot patterns were applied using a precut stencil. As can be seen in

Figure 7.2.3.1-1, the vessel surface paint was in poor condition and required significant 

preparation to ensure an optimal pattern for Aramis. The vessel was striped, cleaned, and 

repainted entirely white in preparation for a black dot pattern application, as shown in 

Figure 7.2.3.3-1. 

Figure 7.2.3.3-1. Test Pressure Vessel Cleaned and Painted White (black squares on the vessel are 

unpainted areas to accommodate AE sensors)

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

129 of 193

For application of the black dots, 60-inch-wide vinyl stencils, cut on a Graphtec vinyl cutter, 

were adhered to the vessel. The large size of the stencils required multiple individuals to assist 

with the application process. The vinyl adhesive enabled easy removal and reuse on another 

section of the vessel. With the stencils in place, the vessel was spray painted. Figure 7.2.3.3-2 

depicts the stencil application and spray painting of the vessel in progress; Figure 7.2.3.3-3 

shows the tank with the completed dot pattern. 

Figure 7.2.3.3-2. Application of Vinyl Stencil (left) and Spray-painting Process (right) on Vessel 

Figure 7.2.3.3-3. Pressure Vessel with Completed Dot Pattern

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

130 of 193

As mentioned in the earlier discussion on Tritop, digital or coded targets, printed on adhesive 

paper, are used to establish unique point locations on a test article between two states. Prior to 

running the final pressure cycle, the coded targets were adhered to the vessel at various locations. 

Particular locations of interest were the end caps of the pressure vessel and several 

circumferential lines along the axis of the vessel. Target size required for any given test was 

established in a similar manner as dot size was for Aramis. In order for Tritop to generate an 

accurate point cloud from the targets, an adequate number of targets must be common in 

overlapping images, which can be used to “stitch” the points together from image to image. This 

requirement can sometimes result in a greater number of targets being affixed to the test article 

than needed for successful or desired data capture on a test, which was the case with this 

investigation. The primary interest with Tritop was to identify circumferential and axial 

deformation of the vessel between the 0- and 4,200-psi endpoints, resulting in targets applied in 

concentrations on the end caps and on circumferential lines. The length of the vessel created the 

need for many “nonessential” targets to be added to the vessel to ensure proper stitching from 

end to end. Figure 7.2.3.3-4 shows two images depicting digital target placement on the pressure 

vessel. In both images, the carbon fiber calibration artifact is visible. Figure 7.2.3.3-5 shows the 

Aramis camera pair in testing configuration on beam for the first pressure cycle. 

Figure 7.2.3.3-4. Two Views Depicting Placement of Digital Targets on Pressure Vessel

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

131 of 193

Figure 7.2.3.3-5. Aramis Camera Pair in Test Configuration (note acoustic sensors on vessel)

7.2.3.4 Experimental Results

Three pressure cycles were documented with Aramis, and one was documented using Tritop. 

The scope of this exercise was to verify whether Aramis and Tritop could capture deformations 

of the pressure vessel at levels of fidelity that would be useful in future engineering assessments 

of pressure vessels throughout the Agency. 

Aramis Results

Aramis offers a wide variety of techniques for post-processing the image correlation data. Color 

contour overlays of deformations and strains in user-defined coordinate systems can be 

generated, as well as X-Y plots of the data as a function of any desired variables, such as time, 

load, etc. 

Upon completing the image correlation process, Aramis provides a numerical indicator called an 

intersection deviation, which establishes a level of confidence that the cameras remained 

calibrated throughout the entire image capture sequence. In all three of the pressurization cycles, 

the Aramis intersection deviations indicated that the cameras remained calibrated. The 

correlated data sets yield a spatial, three-dimensional representation of the pressure vessel. An 

example is shown in Figure 7.2.3.4-1, which shows the computed surface radius value (from the 

computed centroidal axis of the vessel) at every location of the pattern. A close inspection of the 

figure reveals the weld lines on the saddle strap and where the axial shell meets the end cap.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

132 of 193

Figure 7.2.3.4-1. Aramis Three-Dimensional Fringe Plot of Computed 

Radial Values on Pressure Vessel

A useful feature provided by Aramis is the ability to generate a variety of geometric primitives 

from the computed three-dimensional shapes, including cylinders, which enables users to 

conduct further comparisons, define coordinate systems, and check Aramis data against known 

physical quantities. One item that was investigated using the Aramis data was analysis of the 

overall radius of the vessel. This was completed by creating a best-fit cylinder averaged from 

the Aramis data. From this cylinder, a Cartesian coordinate system was defined along the 

X-axis, representing the axial centroid of the vessel. In addition, the diameter of the cylinder 

created represented an average measured diameter of the vessel surface. These values yielded an 

average radius of 28.131 inches, or a diameter of 56.263 inches. From Table 7.2.3.2-1, adding 

the nominal thickness of the vessel wall of 2.188 inches to the nominal 52-inch ID results in a 

vessel OD of 56.375 inches, nominally within about one-tenth of an inch of the measured data. 

With the stated vessel measurements not accounting for layers of paint on the vessel and 

manufacturing irregularities (which can be seen on the vessel with the naked eye), it is fair to 

assume that Aramis is accurately capturing the spatial dimensions of the vessel. 

During all of the tests, five sets of images were taken at each pressure level of interest. This was 

done for two reasons: it helped identify the true “noise floor,” or the amount of uncertainty in

the data throughout the test, and at the same time enabled averaging of the data afterward to 

obtain more representative plots of the physical deformation. Aramis saves the images as time 

histories, called “stages,” such that the number of stages in the data file is the number of image 

pairs recorded during that particular test. Graphically, data sets are presented below using stages 

for the X-axis to enable a broader visualization of the data. Table 7.2.3.4-1 is for reference in 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

133 of 193

interpreting these figures. Single stages were recorded at 100-psi increments as the vessel was 

vented back to 0 psi. These stages were not included in the table to optimize space for this 

report.

Table 7.2.3.4-1. Data-indicated Pressure Values Associated with Stages Recorded in Aramis

After detailed analysis of the data, it was discovered that the camera system was potentially 

compromised because its view had slightly shifted multiple times during the first and third 

pressure tests. Several factors may have contributed to this, but it is believed that the wind and 

saturated ground may have played a primary role in these induced motions. The large, 8-foot 

camera beam may have been susceptible to the moderate gusts that were experienced during the 

first excursion. Although the camera pair remained calibrated (as determined from the 

intersection deviation), shifted camera view angles created somewhat nonsensical deformation 

results. Aramis does employ algorithms that are able to remove some of the nonsensical motion; 

however, without tracking a fixed reference system (i.e., the background fence or the ground) the 

motion only due to the physical shifts of the cameras could not be decoupled from potential 

vessel motion due to the pressurization. Consequently, results from the first and third pressure 

tests were not presented in this report. 

Note that the boundary conditions on this vessel were not completely understood. Both saddle 

bands were stitch-welded to the vessel, and the saddle supports sat, unconstrained, on the 

concrete support pad. As a consequence, drawing conclusions from displacement measurements 

on just one-half of the vessel was much more difficult without knowledge of how the saddle 

supports were displacing on the concrete pad throughout the test. During the development of the 

photogrammetry test plan, inadequate focus was placed on this subject, which will be examined 

in our conclusions. The surface strain analysis below, however, compensates for some of the 

ambiguity in the deformation data.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

134 of 193

Predicted axial deformations and hoop strain will be the primary values used for comparative 

purposes with Aramis measurements. In post analysis, measured data can be taken from any 

desired point location within the data field. The left-hand image in Figure 7.2.3.4-2 depicts an 

Aramis image for the second excursion with the relative locations on the vessel where 

displacement data were taken, along with the positive direction of the vessel cylindrical axis. 

The graph on the right-hand side of the figure plots the axial deformation of those points as a 

function of stage number. Refer to Table 7.2.3.4-1 for correlation of pressure to stages. From 

the graph in Figure 7.2.3.4-2, the relative noise can be estimated to have an average on the order 

of 2.5- to 3.0-thousandths of an inch, which is in line with the expected 2.3 × 10–3-inch system 

capability. 

Figure 7.2.3.4-2. Vessel Point Locations and Respective Displacement Plotted versus Stage Number

The displacement curves in the graph show a somewhat intuitive deformation profile. The 

largest magnitude displacements are at the “vessel end” point, with the other points deforming 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

135 of 193

less as their locations move in the positive axial direction of the vessel. Note that the “vessel 

middle” point actually begins to deform in the positive axial direction, indicating it to be near the 

“midpoint,” at which point the vessel expands away in both axial directions. The maximum 

displacement at the vessel end was measured to be on the order of 0.05 inches. The prediction 

from Table 7.2.3.2-1 calls for a total axial shell displacement of 0.185 inches at 4,200 psi. At 

this point in the analysis, no conclusions can be drawn between Aramis and predicted 

displacements, as data do not exist to indicate how the other half of the vessel is deforming. This 

would be required for a meaningful comparison to the predicted value. The Tritop survey did, 

however, include the entire vessel; those results are presented below. 

As stated earlier, Aramis provides the ability to overlay color fringe plots onto the actual test 

images. Figure 7.2.3.4-3 shows the axial deformation overlay at the maximum 4,200-psi 

pressure load, providing validation of a uniform continuous expansion of the vessel. Note that 

the bottom of the vessel expands slightly more than the top, which may possibly be due to 

interaction with the saddle bands or manufacturing irregularity. The large gap in the data in the 

lower right corner of the vessel results from data being disregarded because of coverage issues 

during the start of the test.

Figure 7.2.3.4-3. Color Fringe Overlay of Axial Displacement (X-direction) at 4,200 psi 

In addition to axial displacements, axial strains were also of interest for comparison to expected 

values. Section lines are created in Aramis at several selected locations and directions on the 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

136 of 193

Aramis data field to obtain the strains along that line. Aramis measures the difference in 

displacement between the two endpoints of the section line, effectively creating an optical 

extensometer over its length. Three section lines were selected along the axial direction of the 

vessel to generate strain data. Figure 7.2.3.4-4 shows the vessel location of two “medium” and 

one “large” length section lines, with their respective strain plots as a function of stage number. 

Note that the longer section line is slightly less noisy due to the greater averaging that results 

over a longer physical length on the vessel. This longer section line is more representative of the 

overall growth of the vessel during pressurization. 

Figure 7.2.3.4-4. Selected Axial Section Lines on Color Overlay of Axial Displacement 

(X-direction) at 4,200 psi

The strain-stage plot in Figure 7.2.3.4-4 shows a nominal range of vessel strain from 380 to 

500 microstrain. Assuming a uniform expansion of the vessel and applying these strains to the 

initial axial shell length of 223 inches yields a resultant expansion of 0.085 to 0.111 inches

computed by Aramis. Referring to Table 7.2.3.2-1, the predicted expansion of the vessel can be 

seen to be 0.185 inches, about 67% greater than the Aramis measurement.

A final measurement to be computed was the hoop strain of the vessel. This was accomplished 

by computing the average outside diameter of a best-fit cylinder calculated from the Aramis data 

at both the 0 and 4,200 psig states. Figure 7.2.3.4-5 shows a graph depicting the growth of the 

best-fit cylinder radius as a function of stage number. From the Aramis data, the average outside 

vessel radius was 28.1313 and 28.1986 inches at 0 and 4,200 psig, respectively. Calculating the 

circumferential strain from this value ((28.1986 – 28.1313)/28.1313) yields 2,392 microstrain. 

This value is about 40% higher than the calculated value taken from Table 7.2.3.2-1, presuming 

all shell layers were intact and uncompromised. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

137 of 193

Figure 7.2.3.4-5. Average Computed Radius of a Best Fit Cylinder from Aramis Data Results from 

Second Pressure Test

Tritop Results

A Tritop survey of the vessel was conducted at 0 and 4,200 psig pressure conditions in an 

attempt to identify the differences in vessel deformations between those two states. The effort 

was successful in obtaining Tritop measurements at several locations on the vessel but was 

limited in terms of the number of data points from which measurements could be taken. This 

was largely due to operator inexperience with understanding the software requirements for 

adequate digital target coverage on a large structure such as the pressure vessel. Figure 7.2.3.4-6 

depicts a snapshot from the Tritop GUI depicting the generated point cloud from targets on the 

pressure vessel with a best-fit cylinder created from the data points. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

138 of 193

Figure 7.2.3.4-6. Screenshot of Tritop Graphical User Interface Showing Pressure Vessel Point 

Cloud with Best-fit Cylinder

Tritop computes vector displacements between the two pressure states and represents their 

magnitude and direction at each point in the cloud where the data were considered valid. In 

addition, color assignment representing displacement magnitude is included in this figure. Note 

that there are many points in this plot that do not have associated vectors placed upon them. The 

lack of data at these points is a due to a combination of an inadequate density of digital targets 

placed on the vessel and an inadequate number of photographs taken to get a complete data 

analysis of all points on the vessel. 

Displacement results can be plotted virtually on any photograph included in the Tritop data set in 

any coordinate system the user chooses to define. Figures 7.2.3.4-7 and 7.2.3.4-8 are examples 

showing vector displacements of targets at both ends of the pressure vessel. Note that not all of 

the coded targets in these photographs have associated displacement vectors, again due to the 

lack of targets/photographs required in this survey. In both figures, the leftmost photograph 

shows the resultant displacement vector directions and magnitudes, which are coordinate-system 

independent. The rightmost photos depict the axial deformation vectors of the vessel plotted 

along the axis of the best-fit cylinder computed from the data targets.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

139 of 193

Figure 7.2.3.4-7. Displacement Vectors Overlaid on Coded Target Locations at Pressurized End of 

Vessel

Figure 7.2.3.4-8. Displacement Vectors Overlaid on Coded Target Locations 

at Sealed End of Vessel

As intuitively expected, the vessel expands outward at both ends. Close inspection of the data in 

the Tritop database showed that targets placed on the flanges on each end of the vessel expanded

outward about 0.05 inches on the feed line end and about 0.04 inches on the sealed end. 

Comparing the Aramis and Tritop deformations of points on the endcap weld lines showed 

general agreement. To get a direct comparison to the axial shell growth prediction, deformations 

were taken from two targets placed near the endcap welds at each end of the vessel along 

the same relative axial location. The straight-line distance calculated was 221.4734 and 

221.5536 inches at respective vessel pressures of 0 psi and 4,200 psig, yielding a difference of 

0.082 inches axial shell growth. This is less than half of the 0.185-inch growth prediction but 

closer to the predictions made from measurements with the Aramis system of 0.111 inches. An 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

140 of 193

adequate number of data points were not available in the Tritop survey at the 4,200-psig pressure 

state to compute a reliable best-fit cylinder. Consequently, we were unable to extract a reliable 

value for hoop strain to compare with the Aramis results and the predicted values. The Tritop 

survey was of limited success due to a less-than-comprehensive data point set from which to 

analyze. This was confirmed by a Tritop error-predictor function, which indicated a lessaccurate survey. Lessons learned outlined in the conclusion will ensure better results with 

potential future investigations using this tool.

7.2.3.5 Conclusions and Future Efforts

Results from the photogrammetry surveys produced measurement data that, in part, agreed with 

predictions and with one another. Due the lack of understanding of the true structural 

configuration of the vessel, along with manufacturing irregularities, it was impossible to 

conclude the true accuracy of either the predictions or the photogrammetry measurements. 

Regrettably, resources were not available to allow physical strain or displacement measurements 

to be made for this test to provide further validation of the predictions and photogrammetry. 

Lessons learned and conclusions are presented below. 

Aramis Results

The quality Aramis data taken from the second pressure cycle excursion were considered 

acceptable and accurate to about 0.003 inches, based on both data analysis and the fact that 

internal Aramis error parameters were within acceptable limits for the data set. 

• The strain and displacements in the region of known flaws were examined. No

significant variations beyond the background noise of the systems were observed at these

locations. However, it was anticipated that Aramis would not be effective in identifying

such with the field of view used for this test series. Strain gages may provide the

necessary sensitivity for detecting deformation response to small internal cracks or

defects; however, a cost-benefit analysis and practicality assessment would have to be

made.

• Environmental factors played a more significant role than anticipated in the stability of

the Aramis camera system; wind, saturated soil, thermal shifts, and changes in lighting all

presented challenges. These factors should be carefully considered and anticipated prior

to future testing efforts.

• Including stationary points on the concrete pad to account for rigid body motion of the

camera system may have salvaged the data from the first and third excursions. This

should be a mandatory practice in future test efforts.

• Inadequate time was spent considering the potential deformation and translation

behaviors of the vessel during a pressure cycle. Consequently, certain conclusions could

not be drawn from the Aramis data, which were limited to a field of view of only half of

the vessel during any particular excursion.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

141 of 193

• A second Aramis system would have been of high value for this effort in order to capture

the complete behavior of one side of the vessel. This would be of particular value for

future testing efforts since Aramis facilitates the merging of data sets taken from multiple

camera systems. Additional systems would enable capturing deformation of both sides of

a pressure vessel.

• Aramis has the potential for detecting other types of potentially dangerous flaws: layer

gaps, long cracks propagating within intermediate layers, etc. However, this was not

demonstrated in this testing, and further analysis and testing is required to establish this

capability.

Tritop Results

• The quality of the Tritop data was inadequate to conclusively assess its potential for

evaluating deformations on large pressure vessels. This was due to the inexperience of

the operators in using this technique, as well as time/schedule constraints associated with

the survey.

• Careful advance consideration of the application of targets to the test article, along with

the photo-capture strategy, is mandatory to acquiring a comprehensive, accurate

deformation survey.

• Tritop is a substantially easier and more forgiving photogrammetry technique to use than

Aramis to identify accurate quasi-static deformations at discrete points on a structure.

Based on known commercial successes and use of both the Tritop and Aramis products, it is fair 

to assume that both could have valuable applicability for quick and cost-effective deformation of 

LPVs during pressurization. Such data could have application for validating LPV deformation 

analysis techniques under development. Further, if it can be demonstrated that subsurface flaws 

generate measurable surface deformation disturbances, then these techniques might also have 

applicability for screening LPVs for flaws. 

7.2.4 AE Testing of a Multilayer Pressure Vessel

In addition to the photogrammetry measurements performed during the pressurization of the 

LPV at GRC, AE testing was also performed during the first pressurization cycle. The MSFC 

Certification Team provided AE test support for this activity. As described previously, cracks in 

or near the head-to-shell welds on this vessel were previously identified using radiography; thus, 

the vessel afforded an excellent opportunity to acquire AE data from a vessel with known flaws. 

An excerpt of the A. O. Smith construction drawing for this vessel (PV0236) is shown in 

Figure 7.2.4-1. 

The MAWP of PV0236 is 2,800 psig, and the hydrostatic test was conducted at 4,200 psig 

(2,800 psig × 1.5). A pressurization plan was developed and implemented based on this test 

pressure. A chart of the pressurization plan is shown in Figure 7.2.4-2. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

142 of 193

Figure 7.2.4-1. Drawing Detail of A. O. Smith Vessel PV0236 

Figure 7.2.4-2. Pressurization Plan

The AE equipment used for this test was a 56-channel DISP56 AE workstation manufactured by 

Physical Acoustics Corporation. The vessel was instrumented with 19 AE R151 sensors having 

a resonant frequency of 150 kHz, per an AE sensor layout plan developed by the MSFC 

Certification Team. Three sensors were placed evenly on each circumferential weld, and the 

sensors on adjacent welds were installed 120° apart. Two additional sensors were installed on 

each head. The sensor layout plan is shown in Figure 7.2.4-3, and the AE system parameters are 

provided in Table 7.2.4-1.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

143 of 193

Figure 7.2.4-3. Sensor Placement

Table 7.2.4-1. AE System Parameters

AE Parameters Peak definition time (PDT) 200 μs

Hit definition time (HDT) 800 μs

Hit lockout time (HLT) 1,000 μs

Pre-amp gain 40 dB

Threshold 45 dB

External Parameters Parametric multiplier None

Location Parameters Wave speed 200,000 inch/sec

Event definition value 200 inches

Event lockout value 400 inches

Hits/events Min = 6

Max = 8

Max Iteration = 256

Evaluation threshold (dB) 70

Three cracks were known to exist in the vessel at the locations shown in Figures 7.2.4-4 and 

7.2.4-5. Crack growth was not measured following the test. Worst-case expected crack growth 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

144 of 193

was estimated to be on the order 0.0001 inches by GRC Engineering (based on API 579, part 9, 

analysis over four overpressure excursions [ref. 32]). The locations of these cracks were not 

revealed to the MSFC AE test team until after the test was completed.

Figure 7.2.4-4. Crack Locations 

Figure 7.2.4-5. Crack Locations (continued)

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

145 of 193

General Analysis 

During the AE test of PV0236, the extraneous noise levels were very high, and under normal 

circumstances, the test would have been terminated no later than 2 minutes into the first hold 

period. Using the criteria found in ASTM E569/E569M-13 [ref. 88], paragraph 5.5.6, 

background noise must be fully investigated and minimized before any AE monitoring can 

begin. However, since photogrammetry testing was being performed in parallel and this test was 

not intended to assess the structural integrity of the vessel, pressurization was continued. A 

major limitation for using AE testing for crack extension detection is extraneous noise. 

Extraneous noise levels must be controlled and minimized, when possible, to increase the 

probability for a crack extension to be detected. 

It is believed but not confirmed that the extraneous noise was due to corrosion between the 

layers. PV0236 was stored unprotected from weather for more than 20 years. Similar AE 

background noise was also observed during a recent test of MSFC multilayered vessel V-32, 

which also was stored outside and unprotected for over 20 years prior to the test. 

The pictures in Figure 7.2.4-6 are coupons cut from V-32 that reveal corrosion between the 

layers. During pressurization, it is presumed that the corrosion, in combination with the layer 

expansion and movement (layer rubbing), resulted in intense extraneous noise. V-32 was 

manufactured by A. O. Smith and has a 1⁄2-inch-thick inner shell and five 1⁄4-inch-thick layers.

Figure 7.2.4-6. Corrosion between Layers on MSFC LPV V-32 

The AE sensor locations on PV0236 were based on earlier investigations at MSFC related to 

overall detection sensitivity versus sensor spacing. This earlier work was performed using 

multilayered MSFC vessel V0343 with lead breaks performed on the inner wall of the vessel and 

the sensors located on the outer wall. A sensor spacing of 4.54 feet was used on PV0236. It is 

estimated that a 0.3-mm lead break at any point on the vessel inner wall would result in at least a 

75-dB signal at one of the AE sensors. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

146 of 193

Location Analysis 

The location algorithm was calibrated by means of a single set of five center-punch events 

located near sensor 7, performed pre- and post-test. Figure 7.2.4-7 shows a plot of amplitude 

versus time for each calibration run. The main strike can be clearly seen for each event, along 

with the subsequent ringing in the structure from that event. Also, note the high degree of noise 

between the center-punch events when there should be low levels of acoustic activity. Figure 

7.2.4-8 locates the events on the vessel above sensor 7, where the punches took place. Utilizing 

the location calibration and applying the data from the pressure test, it can be seen that there is 

no significant clustering near known defect locations and the located events appear to be 

somewhat random across the acreage (reference Figure 7.2.4-9). There was no significant 

correlation with emissions from the saddle or nozzle areas. 

Figure 7.2.4-7. Location Event Amplitudes versus Time

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

147 of 193

Figure 7.2.4-8. Location Calibration Plots

Figure 7.2.4-9. Location Plot 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

148 of 193

Trending Analysis

As can be seen in the amplitude-versus-time plot (Figure 7.2.4-10A), there were many significant 

(>80 dB) signals during ramp, as well as during hold periods. The amplitudes drop in magnitude 

during the hold, indicating some drop in AE severity, but were still very high. If these were

inter-laminar rubbing noises, then they would mask any real crack-growth activity. Overall, 

there was significant low-level activity (<70 dB) during most of test.

The pressure data recorded for this test were logged manually from a calibrated pressure gage. 

It is difficult to obtain any quantitative data from the hits versus time plots in Figure 7.2.4-10 

(B and D) without more accurate “real time” pressure data from a transducer. Here, only a 

qualitative assessment can be made from the data. The graphs show significant activity during 

each of the holds, approaching but not reaching a point where the activity stops. Ideally, the 

system should become completely quiet after 2 minutes of hold time. The activity, however, 

continued at about the same rate during each hold (parallel purple lines with approximately the 

same slope) and increased with each pressure ramp, as given by the increase in slope (green 

lines), which indicates that the vessel was becoming more acoustically active.

A measure of the “goodness” of the AE timing parameters can be obtained using the counts 

versus duration plot, shown in Figure 7.2.4-10C. For resonant sensors, it should be a single line, 

following the resonant frequency of the sensors. 

In general, the activity is so numerous and covers such a large dynamic range that if these were 

inter-laminar rubbing noises, as assumed, they would mask any real crack-growth activity.

Figure 7.2.4-10. Trending Plots

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

149 of 193

Conclusions

The amplitude-versus-time and hits-versus-time plots reveal that there was significant low- and 

high-level activity throughout the test and that during hold periods the acoustic activity never 

returned to zero. This, coupled with the random nature of the location plots, tends to indicate 

that the activity resulted from inter-laminar rubbing. Also, considering that theoretical crack 

growth was estimated by GRC engineers to be only 0.0001 of an inch, the emissions from the 

inter-laminar rubbing and corrosion would likely mask any true crack-growth activity. 

Therefore, the results of the AE test results are inconclusive. 

Further consideration will be given to the use of broadband sensors and a more rigorous pre-test 

location calibration procedure to determine if improvements can be made in differentiating flaw 

growth from elevated levels of background noise on multilayered vessels. MSFC is also 

preparing to perform lab-scale AE testing, which should help to answer questions related to 

sensor density and predicted energy levels from flaw extension in multilayered vessels. 

7.2.5 Feasibility of using Phased Array Ultrasound to Inspect the Shell-to-Head 

Circumferential Welds in NASA Pressure Vessels

Due to the age and operating history of LPVs, it is possible that cracks have developed over time 

and could provide a potential failure mechanism during future operation. Of particular interest 

are the circumferential shell-to-head welds, as one such weld has already failed catastrophically 

in Kansas. The shell-to-head weld joins the domed head of the vessel to a cylindrical body that 

is composed of concentric plate layers that have been welded together. To ensure the safe future 

operation of these vessels, it is necessary to develop an inspection procedure to detect potential 

crack-like defects in the shell-to-head weld regions. 

In a prior investigation conducted at SwRI®, the feasibility of conducting ultrasonic inspection of 

the shell-to-head weld was investigated using standard single element transducers. In this 

preliminary investigation, SwRI® found that ultrasonic waves entering through the multi-layered 

body of the vessel tended to remain trapped within the outermost layer and could not effectively 

propagate to the shell-to-head weld. Alternatively, SwRI® attempted to direct sound into the 

shell-to-head weld via an ultrasonic beam entering the vessel through the head at an angle. The 

results of this work indicated that an angle beam inspection from the head side merited further 

investigation.

Given the relatively complex geometry of the part and the thickness of the weld, an effective 

inspection of the weld using conventional single element ultrasonic probes would require twoaxis scanning of the part to sweep the beam through the area of interest. Furthermore, inspection 

may require several probe angles to provide full coverage of the weld. These factors would lead 

to an involved inspection and analysis procedure. Instead, a phased array probe may be used to 

provide more extensive coverage of the weld using a limited number of line scans around the 

vessel’s circumference. In the interest of developing a relatively straightforward inspection 

method for the shell-to-head welds, SwRI® conducted several phased array exams on shell-toNESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

150 of 193

head weld samples with machined flaws to determine the feasibility of developing an ultrasonic 

phased array inspection procedure. 

7.2.5.1 PAUT Background 

UT phased array is based on electronically controlling the excitation and reception characteristics 

of a multi-element probe. Although there are several different phased array probe 

configurations, the most common phased array probe is a linear array composed of several 

narrow ultrasonic transducers that are spaced at some regular interval. During ultrasonic 

transmission, each element of the probe is excited with a phase delay relative to the other 

elements, where the phase delays of all elements are calculated to produce an ultrasonic beam 

with the desired directivity and focal characteristics. If a feature is present at the desired focal 

point, then the feature reflects a portion of the transmitted wave and the reflection returns to the 

probe. The received signal is captured and recorded by each element. Calculated phase shifts 

and gains are applied to each waveform, and then all waveforms are summed to produce an 

A-scan. The gains and phase delays for each element are calculated to enhance signals captured 

at the desired focal point and suppress signals originating from other locations within the part. 

By rapidly adjusting the firing sequence, gain, and phase delay of each element within the probe, 

the ultrasonic beam can be made to dynamically sweep or shift through the inspected part. 

Figure 7.2.5.1-1 shows the some common operating modes of a phased array probe in which 

varying the timing of the excitation of the individual elements according to different focal laws 

results in different effects on the resulting ultrasonic beam, such as focusing and scanning. 

Figure 7.2.5.1-1. Examples of Common Operating Modes using a Phased Array Probe 

(used by permission from Olympus)

Phased array ultrasound inspection has several potential advantages over conventional ultrasound 

inspection. To test or interrogate a large volume of material, a conventional probe must be 

physically scanned over the area of interest. In contrast, the beam from a phased array probe can 

be moved electronically along one inspection axis without having to move the probe, effectively 

eliminating one scan axis in an inspection. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

151 of 193

Another key advantage of phased array is the data visualization. In a conventional ultrasonic 

exam, the probe must be scanned over some area of the part (often in an X-Y grid), and an 

A-scan, or a time-amplitude plot of the reflected ultrasound, must be recorded at each probe 

location. These A-scans must then be post-processed to present the data in a visualization of the 

inspected part. This task can be time consuming and often requires off-site manipulation of the 

exam data. In a phased array system, the data from a scan can be immediately processed and 

displayed. 

7.2.5.2 Inspection Feasibility Study

SwRI® conducted a series of ultrasonic tests to evaluate the feasibility of detecting several flaws 

distributed throughout the weld. The study was conducted using an Omniscan MX2, a 

commercially available instrument produced by Olympus NDT. Inspections were conducted 

with two 60-element 1-mm-pitch linear array phased array probes with frequencies of 3.5 and 

7.5 MHz.

Test Blocks

Two test blocks were manufactured from samples cut out of a multi-layer, A. O. Smith pressure 

vessel MV-50466-8, the same vessel that was tested in references 40 through 42. Each test block 

measured 6 inches in width. They included roughly a 7-inch span of the vessel head and a 

2.75-inch span of the multi-layer plate structure joined together by the shell-to-head weld. In the 

first block, a total of nine flat-bottom holes (FBHs) were drilled into the weld at varying depths 

and locations. An illustration of the six side-drilled holes (SDHs) in the test block is shown in 

Figure 7.2.5.2-1, and an illustration of the three radially drilled holes (RDHs) is shown in 

Figure 7.2.5.2-2. 

Figure 7.2.5.2-1. View from either Side of Calibration Block Showing SDHs

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

152 of 193

Figure 7.2.5.2-2. View from Underside of Calibration Block Showing the Three RDHs in the Weld

No flaws were added to the second block. At a later point, electronic discharge machine (EDM) 

notches that behave similar to crack-like flaws will be added to the second block. In the work to 

date, the second block was used to compare the phased array response to a specific location 

within the block with and without a known flaw. 

Ultrasonic Velocity Measurements

Prior to conducting phased array inspections, the longitudinal wave and shear wave velocities 

were measured in the part at various frequencies. In addition to velocity, the relative signal level 

at each frequency and location was compared against a baseline signal level taken at 1 MHz 

through the width of the block. These measurements were taken in three locations in the first test

block, as shown in Figure 7.2.5.2-3, and the measurement results are provided in Table 7.2.5.2-1. 

Figure 7.2.5.2-3. Ultrasonic Velocity Measurement Locations in First Test Block

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

153 of 193

Table 7.2.5.2-1. Wave Velocity and Relative Signal Loss Measurements taken from Test Block

Shear Wave Longitudinal Wave

Measurement 

location 

Frequency 

(MHz)

Velocity at 

0° (mm/μs)

Velocity at 

90° (mm/μs)

Velocity 

(mm/μs)

Signal loss relative 

to 1 MHz at 

location A (dB)

A 1 5.92 0.0

A 2.25 5.91 –1.7

A 5 3.24 3.25 5.92 –3.8

A 10 5.92 –5.3

B 1 5.89 –2.7

B 2.25 5.87 –1.3

B 5 3.21 3.21 5.88 –1.5

B 10 5.87 –3.1

C 1 5.99 –0.1

C 2.25 5.99 –0.8

C 5 3.28 3.29 5.99 –3.2

C 10 5.98 –7.4

Based on the relative signal strength measured at each frequency, there does not appear to be a 

significant difference in attenuation over the tested frequency range that would encourage the use 

of one inspection frequency over another. For the purpose of the phased array investigation, the 

average longitudinal velocity is assumed to be 5.90 mm/μs and the average shear velocity is 

assumed to be 3.23 mm/μs.

Initial Observations

The first phased array tests were conducted using the 3.5-MHz and 7.5-MHz probes with a 0° 

Lucite® wedge that was machined to have a concave contact surface that matched the vessel 

head’s radius of curvature. In addition to serving as a coupling surface between the probe and 

the part, the wedge serves as a medium for the ultrasonic beam to develop before entering the 

part, which improves the focusing performance of the probe. Initial tests were conducted both as 

sectorial exams and as fixed angle linear electronic scans. 

The initial exams showed that at both frequencies, significant signal loss was observed within the 

part, and high gains were needed to resolve the drilled holes. Although the attenuation at the two 

frequencies is not significant, the overall signal loss at both frequencies will be a limiting factor 

to detection sensitivity. At high gains, background noise caused by the part became significant 

enough to interfere with the flaw signal responses, and the lower frequencies tended to have 

much higher sensitivity to background noise. Based on this observation, the majority of later 

testing was conducted with the 7.5-MHz probe. 

Many of the flaws were detectable in both the linear exams and the sectorial exams, but the 

linear exams tended to exhibit a greater number of extraneous signals and focal law artifacts. 

Furthermore, the sectorial exams were capable of covering a greater extent of the weld (from cap 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

154 of 193

to root) from one fixed location. Therefore, later tests tended to focus on the development of 

sectorial exams. Finally, the 0° curved surface wedge proved less effective than originally 

anticipated. In later exams, this wedge was replaced with a wedge with a flat coupling surface 

and an 18° incidence angle. 

7.2.5.3 Exams Using Half-V Path

Testing demonstrated that eight of the nine drilled holes in the first test block could be detected 

with sectorial exams using a half-V path, which is illustrated in Figure 7.2.5.3-1. The only hole 

not seen was the uppermost SDH (SDH#4) on the multi-layered plate side the weld. This hole 

was located above the uppermost coverage of the sectorial beam sweep and, therefore, could not 

be detected with the half-V sectorial exam.

Figure 7.2.5.3-1 shows an image of the probe and wedge setup on the test block for the half-V 

sectorial exam. The exam coverage across the weld is highlighted in green on the side of the 

block. Due to the radius of curvature of the vessel head, the weld appears slightly canted toward

the probe position. In the half-V exam, the leading edge of the wedge is allowed to ride flush 

along the shell-to-head weld. As the probe and wedge is pulled back from the weld, the effective 

orientation of the weld with respect to the probe changes. This is important because the 

estimated depths of the detected flaws are dependent on the assumed position of the weld. The 

acquisition software of the Omniscan MX2 assumes that the weld is always normal to the beam 

entry surface, as indicated by the white lines in Figure 7.2.5.3-1. 

Figure 7.2.5.3-1. View of the Probe and Wedge used to Conduct Phased Array 

Half-V Sectorial Exams

Figure 7.2.5.3-2 shows the sectorial exam views (S-scan) of the three SDHs located on the head 

side of the weld. The uppermost flaw (SDH#1) was detected with a peak response of 78% full 

screen height (FSH) at a depth of 1.20 inches below the sound entry surface. SDH#2 was 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

155 of 193

detected with a peak response of 80% FSH at a depth of 1.90 inches, and SDH#3 was detected 

with a peak response of 74% at a depth of 2.55 inches. All flaws were detected within 0.2 inches

of their true locations. Faint echoes from several of the plate layers (labeled C) can be seen in 

Figure 7.2.5.3-2. Signal artifacts caused by interactions between the wedge and the part (labeled 

A and B in the figure) are also observable in the S-scans. Each SDH echo is also accompanied 

by a second reverberation, which appears slightly below and behind the main echo. 

Figure 7.2.5.3-2. S-scans of Three SDHs on Head Side of Weld and 

A-scan of Each Hole at Angle of Peak Response

Figure 7.2.5.3-3 shows the S-scans of the lower two SDHs located on the multilayered plate side 

of the weld. SDH#5 was detected with a peak response of 26% FSH at a depth of 

1.35 inches, and SDH#6 was detected with a peak response of 28% at a depth of 2.10 inches. 

Signal artifacts caused by interactions between the wedge and the part (labeled A and B) are also 

observable in the S-scans. At low angles, a reverberation (labeled D) is observable. This 

reverberation is produced by an abrupt change in thickness in the vessel head at the inner surface 

of the part. Both flaw signals are preceded by one or more signals that have not been identified 

(labeled E and F). These signals are noted to be unrelated to the flaw signals because their 

circumferential extents across the part do not coincide with the circumferential extents of the 

SDH signals.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

156 of 193

Figure 7.2.5.3-3. S-scans Showing Lower Two SDHs on Multilayered Side of Weld and 

A-scan of Each Hole at Angle of Peak Response

Due to their orientation, the RDHs exhibited a fainter response than the side drilled holes. 

Furthermore, due to their location within the part, signals from the RDHs tended to interfere with 

signals from the plate layers and with other extraneous signals detected throughout the test block, 

making detection of these holes difficult. As a result, exams for the RDHs were conducted both 

with the probe flush with the weld as in the previous exams, and with the probe skewed by some 

angle (usually 30° to 40°). Figure 7.2.5.3-4 shows S-scans of RDH#2 detected using both probe 

orientations. As the probe is skewed, the metal path to the hole increases, causing its apparent 

depth to increase. This can be corrected by updating the position of the probe on the part within 

the instrument. Unfortunately, there is no convenient way to notate the probe skew angle within 

the instrument.

By skewing the probe, fewer signals from the plate layers were detected, but the gain required to 

detect the holes increased, making the exam more sensitive to background noise. As a result, the 

S-scans tend to appear more cluttered with extraneous signals, but the extraneous signals appear 

over much shorter circumferential spans and are easier to discern from the flaws. Given the 

number of interfering signals detected when scanning flush with the weld and the amount of 

background noise detected when scanning at some angle, it may be necessary to conduct 

inspections around the weld using two probe orientations. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

157 of 193

Figure 7.2.5.3-4. S-scans of RDH#2 Detected with Probe Flush with Weld (left) and 

Skewed at 30° (right)

Figure 7.2.5.3-5 shows the S-scans of the RDHs acquired with the probe skewed at a 30° angle. 

RDH#1 was detected with a peak response of 10% FSH and a depth of 2.34 inches

(2.70 cos(30)), RDH#2 was detected with a peak response of 9% FSH and a depth of 1.73 inches

(2.00 cos(30)), and RDH#3 was detected with a peak response of 10% FSH and a depth of 

1.26 inches (1.45 cos(30)). A 4-dB increase in the instrument gain was required to capture these 

signals.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

158 of 193

Figure 7.2.5.3-5. Sectorial Exam View of Three Radially Drilled Holes in Weld Root and 

A-scan of Each Hole at Angle of Peak Response

Although the half-V sectorial exam was effective at detecting most of the included holes within 

the test block, it also detected several extraneous signals within the part that cannot be associated 

with the part geometry or with the included flaws. Figure 7.2.5.3-6 shows two S-scans with 

example extraneous signals. In the left view, a low-amplitude indication (labeled G) is detected 

close to the weld root. This S-scan also shows a clear presentation of the echoes originating 

from the various layered plates (labeled C). In the right view, two relatively high amplitude 

indications (labeled H and I) are detected close to the weld cap. Both of these S-scans were 

captured in supposedly defect-free regions of the block. Throughout both test blocks, several of 

these indications have been identified. An attempt to correlate the unidentified indications in one 

block with those in the other block showed that the unidentified indications were not present at 

the same positions in both blocks. The presence of these unidentified indications in both blocks 

and the fact that these indications appear in different locations in each block imply that they are 

not artifacts of the exam settings and that they are not secondary indications of the included 

flaws within the first test block. Evaluation with other NDE methods and/or destructive analysis 

will be required to determine whether these indications correlate to real flaws. Until further 

work is conducted to determine the source of these unknown indications, it will be difficult to 

develop this exam approach into an inspection procedure. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

159 of 193

Figure 7.2.5.3-6. Two S-scans Showing Unidentified Indications Not Associated with Known Flaws 

or Part Geometry

7.2.5.4 Full-V Inspection

Attempts were made to develop a full-V sectorial exam that would provide inspection coverage 

over the upper portion of the weld that cannot be inspected using the half-V exam. The full-V 

inspection approach relies upon reflecting the ultrasonic beam off the inner surface of the vessel 

so that the beam can insonify the part material that is located above the area that can be directly 

interrogated by the transmitted wave. To focus the beam at the correct location within the part, 

the phased array instrument attempts to calculate a focal point at some virtual depth that is 

greater than the thickness of the part. An illustration of this approach showing the full-V beam 

path to SDH#2 is shown in Figure 7.2.5.4-1. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

160 of 193

Figure 7.2.5.4-1. Illustration of Full-V Beam Path to SDH#2

The full-V beam path from the probe to SDH#2 is shown in green. An idealization of the beam 

focus is shown as yellow rays. A unique characteristic of this exam is that each indication can be 

potentially observed twice. It appears at lower apparent depth but at greater inspection angle 

along the half-V path (shown in blue) and then at greater apparent depth but lower inspection 

angle in the full-V path. The obvious disadvantage to this exam approach is that the probe must 

be further offset from the weld, meaning the metal path to the inspected region must be greater. 

The increased metal path results in greater attenuation, lower position accuracy, and greater 

difficulty in correctly focusing the transmitted beam at the intended location. 

Unfortunately, while the full-V exam was demonstrated to work in principle, it was shown to be 

highly inconsistent and difficult to repeat. In addition to the added challenges related to the 

increased metal path, the irregular curvature of the cap inner surface tended to scatter the 

reflected beam, resulting in unreliable focusing. As a result, inspection using a full-V sectorial 

exam is not recommended. It is possible that the performance of the full-V exam can be 

improved by converting the exam from a sectorial exam to a linear electronic exam. A linear 

exam could be made to produce a wider focal spot that would be less sensitive to scattering after 

reflection from the inner surface. At present, this exam has not been attempted. 

7.2.5.5 Conclusion 

Based on the investigative work conducted to date, it appears that all but the uppermost 

0.6 inches (as measured from the shell head side) of the shell-to-head weld can be inspected with 

phased array half-V sectorial exams. The remaining portion of this weld configuration (extreme 

taper) cannot be insonified with a half-V sectorial sweep, so flaws close to the weld cap must be 

detected with another inspection approach. Attempts were made to insonify the uppermost 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

161 of 193

region of the weld using a full-V sectorial sweep exam, but this approach has not proven 

successful. In future work, SwRI® plans to attempt full-V linear electronic exams to inspect this 

region of the weld. It is believed that the larger focal spot produced by linear electronic exams 

will be less susceptible to the issues encountered in full-V sectorial exams. 

All of the FBHs located within the region of the weld that can be interrogated with the half-V 

sectorial exam have been demonstrated to be reliably and repeatedly detected, although not 

without some difficulty. Due to the signal loss within the part, the background noise is often 

comparable to the signal response of the included flaws. Additional investigation into 

appropriate instrumentation, including the probe, the wedge, and the driving electronics, may be 

needed to improve flaw signal-to-noise ratio. Furthermore, several extraneous indications have 

been detected throughout both test blocks. Because these indications are present regardless of 

the presence of the flaws and because the locations of these indications do not coincide between 

the two blocks, it is believed that these indications are not related to either the weld geometry or 

the flaws included in the first test block. Until the causes of these extraneous indications are 

better understood (perhaps through destructive testing of a test weld sample), it will be difficult 

to use PAUT as an inspection procedure for LPVs. 

In the remaining period of performance, SwRI® will add several EDM notches to the second test 

block. The successful phased array examination approach developed previously will be tested on 

these notches to verify the method’s capability to detect crack-like flaws within the weld. SwRI® 

will also continue to pursue methods to detect flaws in the uppermost portion of the weld. 

7.2.6 AE Laboratory Test Specimen Design

Based on the results from the ARC validation activities, where smaller than expected AE signals 

were detected from known crack growth, it was decided by the assessment team that laboratory 

scale testing of AE from crack growth in LPV materials was preferred as the next step, prior to 

further full-scale validation efforts. Such testing could be performed at lower expense and under 

more controlled conditions, with better documentation of actual crack growth to correlate with 

observed AE signals. Further, tests could easily be performed on different material components 

of these vessels (i.e., head parent material, shell parent material, and weld and HAZ materials) to 

assess whether these different material components more preferentially generated AE. Results 

from such tests might define the applicability of AE across the different vessel regions and target 

regions where it might be more successfully validated and applied. 

As such, a low-level Task 2 activity was defined to develop designs for laboratory test coupons. 

The design effort was based on a plan to utilize materials from the MSFC sacrificed vessel 

V0032, which has been used for ongoing materials characterization activities. Considerations for 

the design of the AE laboratory specimens included maximizing the specimen lateral dimensions 

to minimize the influence of AE signal reflections from the lateral boundaries against a number 

of constraints that limited the specimen size. These constraints included the curvature of the 

vessel, thickness of the layers, load capabilities of test load frames to be used, and fracture 

mechanics considerations for predicting and measuring crack growth. The resulting planned 

specimen size was defined to be a dog bone specimen 18 inches in length by 3.5 inches wide in 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

162 of 193

the gage section. This sample is shown in Figure 7.2.6-1. Curvature was retained in the 

specimen to be as representative as possible of the LPV, which requires custom backing plates. 

A large center crack 2a of 0.75 inches is designed to produce failure in the gage section. Even 

with this compromise in specimen dimensions, reflections of AE signals generated by crack 

growth will quickly reflect and superimpose on the original AE signal. Hamstad [ref. 89] 

modeled this effect, showing the resultant increase in measured signal amplitude for a particular 

finite-dimensioned laboratory specimen. This is a low-constraint sample, which will need to be 

taken into account when applying these data to the structure, but the purpose of this test is to 

assess AE detection capability and not to directly determine fracture properties. It is planned to 

run a simple finite element analysis model of this test first to estimate the stress state at tearing 

and evaluate the feasibility of the sample for the test. The sample will be loaded monotonically 

at room temperature until crack tearing is observed. While performing the test at cold 

temperatures with expected cleavage failure would be more distinct and more easily measured by 

AE, the purpose of this test is to be representative of a proof test on an actual vessel. 

Figure 7.2.6-1. AE Laboratory Specimen to be Tested from V0032 1146a Material

To understand the amplitude enhancement factor that might also occur for the specimens 

designed for this testing, an additional laboratory investigation of signals from simulated AE 

signals (generated by pencil-lead breaks) was performed. Specimens of the same crack-growth 

specimen design were compared with increasingly large plate specimens, and the relative 

amplitudes of simulated AE signals were compared. Efforts will be made to simulate the same 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

163 of 193

AE signal mode of propagation as might be expected for cracks to better replicate the observed 

effect. 

The additional laboratory investigation involved testing on three steel plates, 1⁄4-inch-thick, with

different dimensions: 3 by 6 inches, 4 by 8 inches, and 6 by 12 inches. In addition, a 1⁄4-inchthick aluminum plate, 36 by 72 inches, was used as a reference for a plate because reflections 

would be more spread out due to the larger lateral dimensions. Pencil-lead breaks were 

performed on the surface of each plate, using a Hsu-Nielsen source (0.3-mm leads). Pencil-lead 

breaks have been demonstrated to provide a good simulation of the time and frequency response 

of cracks in metals. However, they provide a much more energetic source and, thus, result in 

much larger detected amplitude signals than are typical for small increments of fatigue crack 

propagation. However, as this study was only to detect the relative amplitude differences for 

different geometries and not the absolute amplitude for a real crack source, the use of pencil-lead 

breaks as a simulated source was deemed appropriate.

For the steel plates, a wide-band AE sensor (DWC model B225.5 (bandwidth 40–330 Khz)) was 

placed at each plate’s center, and the lead breaks were located midway on the long edge at the 

edge. For the aluminum plate, the transducer was placed 6 inches from the long edge and 

18 inches from the short side. The pencil-lead breaks were performed 6 inches from the sensor 

at the long edge of the plate. The signals were recorded with a 16-channel DWC model FM1 

signal conditioner and a dedicated signal acquisition system/computer, also from DWC. The 

signals were amplified with 42 dB of gain. The data were band pass filtered from 20 to 

750 KHz. The system was set to digitize at 2 MSamples/sec and recorded 16,438 data points 

(8.219 ms), with 1,024 points before the trigger. Triggering was accomplished by utilizing a 

second transducer (B1025) located at the pencil-lead break site with a simple threshold trigger. 

The signal energy (SE) of each recorded waveform was then computed with the following 

formula:

tVSE

N

i

∑ i ∆=

2

where Vi is the voltage at time i and Δt is the time between measurement samples. Since pencillead breaks are manually done, the lead break signals can vary slightly from test to test, so the 

triggering sensor signal was used to select equivalent signals. 

For the four different plates, Figure 7.2.6-2 shows the signal energies (gain corrected) that were 

measured.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

164 of 193

Figure 7.2.6-2. Bar Graph Comparing Measured Signal Energies from 

Different-sized 1⁄4-inch-thick Plates

Logically, the smaller plate should suffer more reflections per unit time, which in this example 

generated the greatest signal energy, followed by the 4- by 8-inch, then the 6- by 12-inch, and 

then the 36- by 72-inch plate. The smallest plate recorded more than an order of magnitude 

greater signal energy that the largest plate. Figure 7.2.6-3 shows the initial waveform for the 

large plate. In the figure, the trigger signal is at about 510 μs. The first arrival signal at the 

sensors is a symmetric plate wave that arrives at about 535 μs and has an initial amplitude of 

about 30 to 40 mV. A second plate wave, a flexural wave, follows at about 550 μs. Its 

amplitude is about 10 times greater. The echoes from the nearest plate edge do not show up until 

after 800 μs. For contrast, Figure 7.2.6-4 shows the waveform from a similar pencil-lead break 

on the smallest plate. In that figure, the trigger again occurs at about 510 μs. Because of the 

small plate size, the first arrivals of the symmetric and flexural plate modes arrive almost 

simultaneously just after 510 μs. In this specific case, the waveform seems to indicate a phase 

reversal from the largest plate as the echoes rapidly interfere. In Figures 7.2.6-5 and 7.2.6-6, one 

can see this progression of echo interference moving to earlier and earlier times as the echoes are 

arriving earlier and earlier. The results of the echoes are constructive interferences that will 

increase and make the waveform appear more energetic.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

165 of 193

Figure 7.2.6-3. Early Waveform from Pencil-Lead Break on 

36- by 72-inch Aluminum Plate

Figure 7.2.6-4. Early Waveform from Pencil-Lead Break on 3- by 6-inch Steel Plate

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

166 of 193

Figure 7.2.6-5. Early Waveform from Pencil-Lead Break on 4- by 8-inch Steel Plate

Figure 7.2.6-6. Early Waveform from Pencil-Lead Break on 6- by 12-inch Steel Plate

By comparing the full 8 ms of data from the smallest and the largest samples, one can understand 

the extent of the impact of this phenomenon on the signal energy measurement. Figure 7.2.6-7 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

167 of 193

shows the full waveform for the largest plate tested. After about 900 μs, a small echo is evident, 

and then the signal settles down to a relatively constant level of noise. In contrast, Figure 7.2.6-8 

shows the signal immediately interfering and growing in magnitude. It is not until after 4 ms that 

the amplitude of that signal declines to less than the largest plate’s signal.

Figure 7.2.6-7. Full Waveform from Pencil-Lead Break on 36- by 72-inch Aluminum Plate

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

168 of 193

Figure 7.2.6-8. Full Waveform from Pencil-Lead Break on 3- by 6-inch Steel Plate

Based on this testing, we can expect that the crack-growth signals measured in a modest-sized 

load frame sample will be 5 to 10 times greater than the signals that might be detected near a 

crack site on a larger-sized LPV.

The crack-growth plan for the specimens is to insert a starter notch and then grow the starter 

notch in fatigue to produce a sharp crack that well represents an actual crack. The load will 

monotonically increase until crack growth occurs while monitoring with AE. The driving force 

for crack growth is the stress-intensity factor which is a function of the load, crack length, and 

specimen geometry. Accordingly, an overload cycle representative of the overload pressure 

applied during AE monitoring of an LPV in the field is not directly relevant, since the 0.75-inch 

crack will not produce an equivalent K at a given load as a 0.75-inch crack would in a given 

layer (such as the outer shell) of an LPV. This difference is an issue since the two processes can 

be easily reconciled by fracture mechanics keeping in mind that the goal of this laboratory-scale 

AE effort is to determine the efficacy of AE for measuring structurally significant crack growth 

in LPV materials. Both resonant and broadband sensors that are employed by the different AE 

approaches commonly used for LPV testing (i.e., MONPACTM and MAE) will be used in this 

testing to provide additional data relative to comparison of the two techniques. Crack growth 

will be verified with standard materials testing NDE (potential difference) and post-test 

measurements after the sample is broken open. Again, tests are planned on the different material 

components of these vessels including head and shell parent materials, as well as weld and HAZ 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

169 of 193

materials. The expected results are an indication of the signal amplitude as a function of the 

amount of crack growth under the given stress intensity conditions. Results from the signal 

amplification study will be used to remove the effect of lateral boundaries on the measured 

signal amplitudes. These results will provide a basis for evaluating the potential for AE for 

detecting crack growth in the various materials and regions of LPVs and a basis for decisions on 

further validation efforts on larger subscale and, eventually, full-scale vessels. 

To date, the specimen designs have been completed and are undergoing evaluation by the MSFC 

materials community and are awaiting specimen fabrication. A block of the V0032 vessel has 

been designated for this purpose. AE instrumentation and personnel at MSFC and LaRC have 

been identified to support this testing effort. 

7.3 Assessment Team Comments on Proof Testing of LPVs

Proof testing of pressure vessels (i.e., pressurizing a vessel to a level higher than the intended 

operating pressure) is a long-standing acceptance test procedure following vessel fabrication. 

Proof testing subsequent to fabrication is expected to ensure the quality and safety of the vessel 

for its design use and carries additional benefits, such as reducing weld or shell-section residual 

stresses through cyclic shakedown. The objectives of the proof test procedure commonly fall 

into two categories: (1) to provide a general screen on the quality of materials and workmanship 

present in the vessel (most common) and (2) to provide a quantifiable engineering rationale for 

safe operation of the vessel over a number of pressure cycles based on the stress in the vessel at 

proof pressure, serving as a sieve to identify flaws greater than a critical size, whereby fracture 

mechanics principles may be used to evaluate a minimum remaining cyclic life beginning at that 

proof-critical flaw size (uncommon). The first approach is often referred to as a “workmanship” 

proof test, and the second, as a “fracture mechanics” based proof test. Both share the goal of 

providing rationale for the safety of the vessel; the difference is in the quantifiable nature of the 

latter, which leads to a much clearer understanding of the vessel and its associated structural 

risks.

All known proof testing performed to date on LPVs has been of the workmanship variety. The 

acceptance proof test (or “hydrotest”) to 1.5× MAWP performed at vessel fabrication or 

following certain vessel repair activity provides an unquantified confidence in the integrity of the 

vessel. This concept fits with the general ASME code approach of establishing rigorous design, 

fabrication, and inspection quality requirements, which result in a code-approved vessel in an 

assumed “undamaged” state. This approach has served the industry well for decades. For older 

vessels, those with known defects that violate the assumed “undamaged” state, or in the subject 

case, vessels of non-code construction, rationale for the FFS of the vessel is required. In simple 

cases, this may involve repair of the vessel to an equivalent undamaged state or, in the case of 

non-code vessels, may require an alternative engineering rationale. The proof test, either 

workmanship or fracture-mechanics-based, often plays a key role in this alternative logic. For 

cases such as the LPVs, the geometric limitations of layered construction reduce the 

effectiveness of quantifiable NDE throughout the vessel, making the standard NDE-based 

analytical methods of FFS less practical. The lack of comprehensive quantifiable data from the 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

170 of 193

LPV NDE process emphasizes the benefits of quantifiable results from a proof test by enabling a 

FFS rationale that moves beyond mere risk assessment toward a more anchored and quantified 

engineering rationale. While the layered construction should not hinder the physics behind the 

proof logic, as with NDE the layered construction makes quantification of the proof test efficacy 

considerably more complicated. 

Figure 7.3-1 illustrates the basic idea of a fracture-mechanics-based proof test to provide a 

quantifiable expectation of reliable cyclic life. The curve in the axes reflects the general 

relationship between critical crack size and pressure in the vessel, showing increasing critical 

crack size as pressure decreases. The position and shape of the critical crack size curve is a 

function of the crack geometry (e.g., through crack, surface crack, or embedded crack) and the 

material toughness, which will vary by material (head, shell, nozzle, or weld) and temperature. 

The most important aspect to quantifying the cyclic life assured by proof test is the ability to 

assess accurately the critical flaw size in the vessel at the proof pressure condition and at the 

maximum operating pressure condition. The ratio of these stress states (σproof/σoper) defines the 

proof factor. For most common pressure vessels, this ratio of stress states is equivalent to the 

ratio of internal pressures at these conditions because pressure is the primary source of stress. 

The higher pressure at the proof test condition makes the critical crack size at proof smaller than 

the critical crack size in operation (acrit.pr < acrit.op); therefore, following a successful proof test, 

there is a range of crack growth of length ∆a = acrit.op – acrit.pr reserved for cyclic growth in 

operation. The number of operational pressure cycles available within Δa may be determined 

through fracture mechanics methods for fatigue crack growth, such as predicted by crack growth 

relations like such that ( ) da n

C K

dN

= ∆ such that 1

( )

op

pr

a

n

a

N da

C K = ∆ ∫ , where N is the number of 

cycles and ΔK is the cyclic range of the crack’s stress intensity factor. While simple in concept, 

an accurate assessment of the Δa (or cycle life N) assured by the proof test is complicated. As 

illustrated in Figure 7.3-1, the value of the proof factor will be the most direct influence, but the 

shape and consistency of the critical crack size curve is also extremely crucial.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

171 of 193

Figure 7.3-1. Proof Test Concept

Considering the concept illustration of Figure 7.3-1, an accurate definition of the critical crack 

size curve is fundamental to the proof test logic. In this figure, a single critical crack size curve 

is shown for simplicity. In reality, the two critical events in the proof logic (i.e., proof test and 

reaching the operational critical crack size) are well separated in time, perhaps by years. The key 

aspects that define the curve (i.e., crack geometry and material performance) may not be the 

same over time. For example, as a crack grows over the operational service life, the shape of the 

flaw may evolve from an embedded crack to through-wall crack, or the temperature of the vessel 

in operation may differ from that during proof test. Considering this, it is more appropriate to 

consider a version of the conceptual figure that has a critical crack size curve for both the proof 

condition and the operational condition, as illustrated in Figure 7.3-2. In this context, the life 

assured through the proof test, Δa, can appropriately consider the limitations imposed by the 

proof and operational conditions of the vessel. The life assured through proof is maximized by 

maximizing Δa. This implies screening for the smallest possible acrit.pr and allowing the largest 

operational crack size acrit.op. However, from a safety perspective, the proof test must be 

assessed based on the minimum Δa that may exist between proof and operational failure. This 

requires considering the largest crack that may escape proof test and the smallest that may cause 

failure, which minimizes Δa. To do this, the toughness of the material during proof must be 

considered at its upper bound, and worst-case performance must be assumed regarding toughness 

and material variability during operation. For materials with consistent ductile fracture 

properties (always upper shelf), the upper bound toughness assumed in proof can often 

reasonably be assumed to be present in operation as well. This reduces some of the pessimism in 

the assessment. However, for these LPV ferritic steels, which cleave through a stochastic 

sampling of crack front cleavage initiators such as carbide particles, the assumption of consistent 

upper bound toughness values being present for the operational condition are not justified 

because the crack will have advanced through the material during operational cycles and will not 

be sampling the same crack front material later in the operational life. At this point for the crack, 

it is a new draw from the random deck. 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

172 of 193

Figure 7.3-2. Proof Test Concept with Proof and Operational Critical Crack Size Curves

For more detail on proof test considerations, see the thorough review of the fundamentals of 

proof testing provided in reference 90. 

7.3.1 Methods of Proof Test Assessment

The efficacy of a proof test may be quantified analytically, experimentally, or through a 

combination of these. Proper analytical assessments of proof test efficacy are difficult. The use 

of linear elastic fracture mechanics (LEFM) to determine acrit.pr and acrit.op with the desired 

accuracy is generally not feasible and could likely lead to nonconservative results. LEFM is 

generally more robust in evaluating cyclic life in fatigue than in accurately predicting true 

fracture failure, particularly for ductile materials like the LPV steels. An analytical assessment 

of the bounding crack sizes for proof and operation may be performed more robustly with a 

J-integral based failure assessment diagram, such as described in API-579 [ref. 32]. In this case, 

the main variables affecting the outcome of the Δa assessment will be the material toughness 

values chosen to represent the two conditions and the choice of assumed crack geometries. For 

the case of the LPV problem, the latter represents the larger challenge. The material toughness 

and its expected variability can be assessed and reasonably understood through evaluations, such 

as those described elsewhere in this report (e.g., Section 7.2.1). Choosing representative and 

appropriate hypothetical crack geometries to assess the proof test is complicated somewhat by 

the layered construction relative to traditional solid-wall vessels. There are more potential 

scenarios to consider, and the load cases are more complicated to assess. The prediction of 

crack-shape evolution during the operational life is also more complicated. This does not imply 

that the problem is intractable, but the time required to assess an LPV for proof will be longer 

than for a typical solid-wall vessel. Fortunately, for structural assessment considerations, an 

analysis that is reasonably suited to provide a conservative estimate of proof stress conditions at 

a location in an LVP can generally also be used for the assessment of the operational stress state 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

173 of 193

given that most aspects of the problem governing stress states remain consistent between the two 

conditions. The only changes from the proof assessment to the operational assessment are 

typically the applied pressure, the evolved crack shape, and the material properties.

Though more costly, the experimental approach to assessing proof test efficacy is, in principal, 

more robust, although its practicality for application to NASA’s LPV fleet has not been assessed. 

The issues of elastic-plastic fracture mechanics, material property changes, crack-shape 

evolution, and fatigue-crack-growth predictions can be rolled into a single test result that requires 

less specific knowledge about the contributions and interactions of all these complicated 

behaviors. The experimental methodology commonly used for such an assessment is referred to 

as an “imminent failure simulated service test.” In this method, a crack is fatigued into a 

representative test specimen. The specimen may simply be a section of the LVP base material or 

the specimen may be a more complicated, configurational analog of the LPV structure, including 

layers or welds. The specimen and crack are sized such that the crack will be near failure at a 

stress level representative of the LPV proof stress. Under environmental conditions 

representative of the proof test, the specimen is loaded in a test frame to a point of “imminent 

failure” (i.e., a point where it is anticipated that any additional load would cause failure). The 

proof load is then removed and the specimen cycled under environmental conditions 

representative of the operational stress level (i.e., peak load is reduced by the proof factor ratio) 

and the specimen cycled to failure. This experiment provides a direct measure of the cyclic 

capability of a crack that barely survives the proof test. Much of the complicated physics that 

confound the analytical assessment is handled cleanly in the experiment, such as plasticity, crack 

extension by tearing during proof, fatigue crack-shape evolution, and final failure. A larger 

number of tests would be needed for cleavage-prone materials such as appear to exist in NASA’s 

LPV fleet than for other more consistently ductile fracture materials due to the stochastic nature 

of the cleavage mechanism. The variability this may impose on the experimental assessment of 

proof test efficacy is not clear at this time. The issues of geometric differences between vessels, 

material property and toughness variations, different manufacturers and manufacturing 

processes, etc., of NASA’s LPV fleet are factors that would further complicate development of 

an experimental approach. Reference 91 provides a wealth of information regarding proof test 

assessment.

There are numerous other influences that require investigation related to the proof test 

assessment in LPV systems. Some effects may be negligible, some helpful, others detrimental. 

Each should be studied to ensure it does not present an unreasonable limitation or reduction in 

confidence in the proof test assessment and its influence on perceived vessel reliability. The 

following are a few items for future consideration: 

• Choice of proof factor: To be effective as a screen for additional safe operating cyclic

life, the proof factor must be sufficient to overcome the variability present in the system.

As previously described, the material behavior with respect to cleavage failure has a

considerable stochastic nature. The need to be conservative in material properties for

both acrit.pr (upper bound) and acrit.op (lower bound) can greatly reduce the actual

“effective” proof factor. This material variability alone may drive the need for

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

174 of 193

substantial proof factors. The proof factor level should be an integral part of the vessel 

proof assessment, taking into account system variability and the desired proof-assured 

life. 

• Proof test temperature: The need to utilize upper bound toughness properties for the

proof evaluation of acrit.pr makes it particularly appealing to mitigate that effect by

minimizing the material toughness during proof. This is accomplished by lowering the

proof test temperature to the lowest practical degree. It is preferable that the proof test

temperature at least bound the allowable minimum operational temperature. Methods to

effectively implement such a test in field conditions may require special development.

• Residual stress state: Residual stress in and around welds may have a significant effect

on the driving force for cracks and failure due to cleavage. Changes in residual stress due

to cyclic shakedown of the materials may also influence the material response. An

understanding of how these stresses likely evolved from the original vendor proof test to

those currently present or following additional proof cycles may be important.

• Crack-shape evolution: As previously discussed, the larger the difference between

acrit.pr and acrit.op, the longer the proof-assured life will be; however, large Δa also means

an increased likelihood that the fundamental crack shape will change between the time of

proof testing and operational failure. The layered structure offers additional

complications. Experiments or simulations may be used to determine likely crack-shape

evolution paths. Fracture mechanics tools, most notably LEFM boundary element

methods, have matured sufficiently to provide good first-order estimates of this crackshape evolution behavior. An understanding of these trends in LPVs will be important to

an accurate general assessment of LPV proof efficacy.

• Tearing, sharpening, retardation, (i.e., nonlinear effects): Tearing and sharpening of

defects may be aspects of vessel behavior that limit the effectiveness, or reduce the

reliability, of the proof test. Stable crack tearing is most likely to occur during or at the

peak of the proof loading cycle. Unless reasonably represented in the analytical or

experimental assessment of the proof loading cycle, the tearing adds to the length of

acrit.pr, reducing the available Δa for operation. If tearing occurs and is undetected or

unanticipated, the proof test may not be effective. Unanticipated crack tearing may be

mitigated by understanding toughness R-curve behavior, representing the tearing in the

simulated service tests, listening for it during proof with acoustic emission, or conducting

more robust screening via multi-cycle proof testing. Sharpening of defects in proof is

also a concern. In this case, initial defects that are not fully crack-like, such as a welding

lack of fusion or chain of porosity, is torn to a crack-like condition and then sharpened

during the unloading cycle of the proof test. The most effective screen for this

consideration is the multi-cycle proof test. A third likely influence of proof test

assessment is retardation of fatigue-crack-growth. Unlike tearing and sharpening,

retardation effects may tend toward enhancing the realized proof life by increasing the

number of cycles required to grow the crack by Δa. This effect is a strong function of the

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

175 of 193

applied proof factor and occurs due to well-characterized crack closure effects following 

an overload cycle, where the crack opening is suppressed in subsequent loading cycles by 

the residual compression caused plastic wake following the overload. Each of these 

phenomena can complicate the proof test assessment by adding what are essentially 

nonlinear effects to the crack behavior.

• Multi-cycle proof testing: The multi-cycle proof test concept involves cycling the

vessel to the proof condition a number of times (five is common) to improve the

reliability of the proof screen. This does not provide a smaller value of acrit.pr and, thus,

does not alter the assured Δa, but rather improves the likelihood that any defects close to

the acrit.pr size or evolving to that size through tearing or sharpening mechanisms will be

screened. Probabilistically, the effectiveness of the proof test in assuring vessel

reliability is improved. The additional cycles at proof stress levels are a cost in

accumulated damage, but are not likely significant in the overall cyclic life of such

vessels. Details can be found in reference 92.

• Other effects, for example, costs and facility threats: A reliance on proof testing of

the LPV fleet to provide service life reliability may overcome many of the inspection and

analytical challenges inherent to these vessels, but such testing comes with significant

costs and presents potential hazards. The direct costs of implementing a proof test on any

given vessel may be significant, especially if there are infrastructure developments

needed to provide a cold, hydrostatic condition for proof testing, or vessel cleaning costs

post-proof are substantial. Additionally, there will often be costs associated with either

moving a vessel to a safe location or providing some other form of protection from

collateral damage should a vessel fail in proof. The potential energy release from large

vessels can be significant, even in hydrostatic testing, and hazard assessment and

mitigation must be taken seriously. Though expensive, these costs are a small fraction of

typical replacement costs. Though the loss of a vessel by failure in proof can be

considered a substantial cost, a preferred point of view is that the proof test was

successful at removing a dangerous vessel from the fleet under controlled circumstances.

While undesirable, vessel failure in proof is vastly preferable to failure in service,

provided the potential hazards due to such failure have been adequately mitigated.

7.3.2 Proof Test Conclusions

Systematic use of proof testing for FFS assurance in the LPV fleet will require the issues 

discussed in this section be investigated such that the benefits and risks of proof testing these 

vessels are better understood. When proof testing is performed, it must be assumed that, 

although highly unlikely, sudden catastrophic failure could occur, and hazard mitigations must 

be implemented to protect personnel and, if appropriate, facilities, from such failures. 

7.4 Assessment Team Comments on LPV Replacement Strategy

In light of the elevated and indeterminate risks associated with the continued use of aging, noncode LPVs, the assessment team had considerable discussions as to whether to include any 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

176 of 193

recommendations regarding replacement of these vessels. In the end, such a recommendation

was not included for the following reasons: 

• The defined scope of the assessment was to identify work that was required to continue to

use these vessels safely. Thus, recommending replacement, although it would eliminate

the additional risk associated with these LPVs, is inconsistent with the assessment scope.

It is noted that even the likely replacement (i.e., single-walled ASME vessels) are not risk

free but have a lower risk that is acceptable based on ASME BPVC and OSHA

regulations.

• Although this report must not be taken out of context to indicate that these vessels are

entirely safe to operate or will continue to be so for any future period, there were no

findings that warranted a recommendation to begin immediate planning for replacement

of these vessels.

• The report instead identifies that there is a lack of information with which to adequately

assess the immediate risks associated with these vessels or to determine the remaining

safe life of any particular vessel in its specific service. Characterization of those risks

will be critical information needed to guide replacement decisions. Thus, the

recommended longer-term activities are necessary not only to develop a process to ensure

continued safe operations but also to provide essential information to support eventual,

orderly replacement decisions.

• Further, as replacement of these vessels is an expensive option (i.e., an estimated average

cost of around $2 M per vessel), such decisions regarding replacement should be driven

by a risk analysis that incorporates an understanding of costs and benefits associated with

available options. Obviously, replacing these vessels with single-walled ASME code

vessels eliminates the additional risks associated with non-code LPVs, but at a substantial

cost. Analysis of costs and benefits in terms of risk reduction for other options needs to

be considered as part of the replacement decisions. The risks of thick-walled single-layer

ASME code vessels are generally better understood and more manageable through a

variety of validated analysis and NDE methods, but these risks are not zero. Estimates of

costs will need to include costs for the recommended efforts that are identified in this

report to support continued use, as well as costs that might occur from additional

inspections, operational constraints, etc., that will likely originate as a consequence of the

results of this work. Thus, again, it is highlighted that the activities recommended in this

final report are critical not only for assurance of continued safe operations but also for

input into decisions on eventual replacement of these vessels.

• Lastly, as the costs of wholesale replacement of all NASA LPVs is not affordable to the

Agency under current budget constraints, such decisions will need to be made on a caseby-case basis, driven by the individual circumstances for specific vessels.

However, all assessment team members were in agreement with the obvious fact that 

replacement of these vessels eliminates the risks associated with their continued use, albeit 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

177 of 193

substituting for that risk the better understood risks of thick-walled single-layer vessels. Thus, 

where there are opportunities and funding available to allow replacement of LPVs, this action

could be advisable, although an assessment of the particular circumstances would still be 

required. Further, while a general recommendation for replacement is not being made, 

identification of certain defects may require removal from service of individual vessels.

8.0 Findings, Observations, and NESC Recommendations 

8.1 Findings

General

F-1. Non-code LPVs are in use at nine NASA Centers and facilities, with 376 vessels 

reported, 302 of which are in active service. 

F-2. Operation of NASA non-code LPVs presents an elevated level of risk compared with

conventional code vessels.

• Non-code materials, process controls, and inspection methods used during original

vessel fabrication.

• Uncertain minimum safe-operating temperatures, with indications of likely operation

within the transition region to brittle fracture behavior.

• Inability to currently perform adequate inspections or assessments to ascertain

remaining safe life, with no specific LPV guidance provided in the principal

post-construction pressure-vessel codes NB-23 and ASME FFS-1/API-579.

• Potential aging effects that could include fatigue, corrosion, and embrittlement.

o Noncatastrophic cracks documented in industry and NASA vessels, including

those found recently in LaRC high-pressure methane and KSC nitrogen LPVs.

o Internal corrosion of NASA LPVs not known to be an issue due to operation

with noncorrosive gases at ambient temperatures.

o Some NASA LPVs subject to external corrosion due to proximity to ocean.

o No known data on embrittlement of NASA LPV materials, although some

vessels used for gaseous hydrogen service.

• At least nine catastrophic LPV failures in industry.

o Occurred under different usage conditions than NASA LPVs (elevated

temperatures with corrosive products).

o Two occurred after evidence of inner shell failure was observed through

product loss at vent holes.

F-3. The level of increased risk associated with non-code LPVs and the overall reliability of 

these vessels cannot be quantitatively estimated.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

178 of 193

• Lack of historical data associated with design, fabrication, operation, inspection,

maintenance, and repair.

• Available data in disparate formats and locations and difficult to retrieve.

• Original materials property data in some cases acquired with methods not consistent

with current code; cannot be correlated with current methods.

• Recently acquired materials property data from NASA sacrificed vessels are limited,

do not provide sufficient sampling to adequately represent potential fleet variability

(material lot to lot and within lot), and in some cases show lower than expected

property values or possible undocumented material substitutions.

• Currently available structural analysis approaches for LPVs are idealized and

simplistic and do not accurately reflect some of the key details of construction.

• Uncertainty in the identification of probable failure modes and associated critical

initial and final flaw sizes currently precludes the identification of NDE detection

requirements.

• Currently no validated NDE methods available to inspect certain critical regions of

LPVs (e.g., full penetration circumferential welds, shell longitudinal welds, and

interior shell layers).

F-4. The Agency lacks a consensus approach to addressing the risks associated with these 

vessels.

• Center PSMs have not reached consensus on certification, recertification, inspection,

maintenance, or usage of LPVs.

• Frequency and methods of reporting LPV risks to Center management, and their

awareness of the risks presented by continued use of these vessels, varies from Center

to Center.

• Previous and ongoing efforts at specific Centers to develop data and methods to

mitigate risks associated with LPVs have not always been effectively communicated

across the Agency.

• No consistent methodology identified in industry survey.

F-5. Near-term actions (identified in R-1 through R-7) are available to reduce the risk of or 

consequences from catastrophic failure of LPVs. However, for many of these actions, the 

associated degree of risk reduction cannot be quantified. 

F-6. Longer-term efforts will be required to characterize and mitigate risks for LPVs. 

Emphasis should be placed on vessel features and operating conditions that pose the 

highest risks, accompanied by a focus on the most promising technologies to mitigate 

these risks.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

179 of 193

• Load sharing, interlayer friction, and other interactions from the layered construction

complicate the application of the traditional fracture mechanics approach for LPVs in

their entirety.

• Proof test methodologies may be able to address risk in regions where NDE is

inadequate, provided that bounding material properties are available and appropriate

analysis methods are developed and validated.

• Operation of NASA LPVs at ambient temperatures in cold weather is a significant

concern.

o MDMT stamped on the nameplate or shown on the design drawings for

non-code LPVs was determined via methods seen as inadequate today and

does not account for brittle fracture potential at temperatures below the NDT.

o During recent record cold temperatures (i.e., January through March 2014), an

attempt was made to gather pressure and temperature data for a number of

NASA LPVs at some Centers.

• Identified high-risk regions of NASA LPVs include circumferential full penetration

welds, shell sections near full penetration welds and along longitudinal welds, and

welds for drain nozzles through shell sections.

• NDE methodologies that have the potential to detect relevant defects have not been

satisfactorily validated.

Specific Discipline Findings

Materials

F-7. Strength evaluations of LPV steels and associated welds reflect properties mostly 

consistent with material records from the time of fabrication. In some cases, strength 

values have been nearer to assumed design values than typically would be expected. One 

vessel material tested (i.e., inner layer of MSFC vessel V0032) was of significantly lower 

strength than the certification records indicate. Uncertainty in the pedigree and 

consistency of materials in the LPV fleet is recognized as a risk associated with these 

vessels.

F-8. Inconsistencies are present in the historical material specifications from the various LPV 

manufacturers, including the requirements for chemistry and strength. 

F-9. Material anisotropy is relatively mild for strength properties but significant with respect 

to fracture toughness across the head and shell materials evaluated. Anisotropy is not 

always readily apparent in metallographic evaluations; therefore, toughness or Charpy 

testing is best used to identify the material orientation of minimum toughness. Materials 

may not be oriented favorably in vessels with respect to material anisotropy in fracture 

toughness, so bounding values are necessary.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

180 of 193

F-10. Initial single-lot measurements of toughness in the transition regime have been made for 

two-shell-layer materials (i.e., alloys 1143 and 1146) from sacrificed vessels. The 

toughness measurements were developed in terms of the T0 parameter per 

ASTM E1921-13, providing the fracture toughness versus temperature relationship in the 

transition toughness regime. Once fully developed, the T0 values for the steels will 

provide rationale for the allowable service temperatures for the LPV fleet. 

F-11. The LPV steels tested are less tough than the more modern pressure vessel steels used in 

the development of the Master Curve method prescribed by ASTM E1921-13. The LPV 

material toughness presents difficulties in meeting the current requirements and 

assumptions of the test standard; however, the temperature-dependent material toughness 

generated by the method is expected to be directly applicable to FFS assessments. 

F-12. Radiographic comparison of selected sections of longitudinal layer welds for two 

sacrificial vessels revealed markedly different internal weld qualities between the two 

vessels, an indicator of potential quality variations that exist within the LPV fleet.

Structural Analysis

F-13. The diversity of pressure vessels across the Agency will require unique assessments 

based on individual vessels or vessel types. 

F-14. Preliminary models have been developed to estimate residual stresses in LPV full 

penetration circumferential welds, which were not stress relieved in the fabrication 

process. The initial results show high residual stresses, which could negatively affect 

safe life calculations.

Nondestructive Evaluation 

F-15. Preliminary testing of PAUT methods has demonstrated some potential for volumetric 

inspection of full-penetration circumferential welds.

F-16. Photogrammetric techniques were used to provide LPV full-field surface deformation 

measurements, which could be useful for validating analysis models. The capability of 

photogrammetry to detect subsurface flaws will be a function of the magnitude of the 

change in surface deformation produced by those flaws. It will also require an 

understanding of the surface deformation produced by other LPV features (e.g., layer 

gaps, welds, support structure, etc.). 

F-17. AE has potential for detecting crack growth in internal layered shell sections and welds if 

cracks can be shown to produce detectable AE in the presence of high background noise 

associated with layer rubbing. 

F-18. The capabilities for RT as applied to crack detection in LPVs have not been established. 

• The presence of multiple shell layers adds non-relevant indications (i.e., layer wash)

to radiographic images that make detection of flaws more difficult.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

181 of 193

• The flaw detection capability for RT is a function of the overall thickness being

inspected, while the critical flaw size for a layer will be based on the layer thickness.

F-19. Low-frequency electromagnetic inspection methods have been developed to detect wall 

thinning in thick carbon steel pipes. These methods may also have applicability to the 

detection of flaws in LPVs. 

8.2 Observations

O-1. The Center PSMs have demonstrated understanding of the technical issues and concerns 

associated with non-code LPVs and a willingness to engage across Centers to develop 

improved approaches to mitigating risks associated with the continued use of these 

vessels. 

O-2. A significant amount of pertinent, valuable testing and development that directly relates 

to the understanding of and continued safe use of LPVs at NASA is being performed by 

NASA Centers outside this NESC assessment. 

8.3 NESC Recommendations

Near-term Recommendations 

In drafting recommendations to the Agency and Centers that operate and maintain LPVs, an 

attempt was made to provide some near-term recommendations that could, if enacted, reduce the 

risk of or consequences from catastrophic LPV failure resulting from their continued use. It was 

recognized that due to the lack of adequate materials property data, inspection methodologies, 

and analyses methods, the risks associated with the continued use of these vessels and the degree 

to which these recommendations would reduce that risk cannot be accurately estimated. It was 

also recognized that these recommendations cannot be uniformly adapted or applied across the 

Agency because of the unique applications/requirements and local site conditions for specific 

vessels. These recommendations are as follows:

R-1. Agency ensure continued-use rationale is documented for all in service LPVs. 

(F-2, F-3)

R-2. Centers inspect LPV vent holes to ensure that they are not corroded, blocked, or plugged, 

and periodically monitor vent holes for product loss that may indicate inner shell failure. 

(F-2, F-5) 

R-3. Centers consider imposing service restrictions (e.g., reduced operating pressures or 

operation at temperatures where material toughness is in the upper shelf region) to reduce 

the risk of catastrophic failure. (F-2, F-5, F-6) 

• When data necessary to establish appropriate MDMT are not available, set minimum

allowable usage temperature at conservative levels above historical cold usage

temperatures where such data are available.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

182 of 193

R-4. Centers consider the implementation of physical barriers (e.g., berms) or operational 

controls (e.g., minimized personnel access) where practical to mitigate the consequences 

of vessel failure. (F-2, F-5) 

R-5. Agency develop and implement a consistent program of minimum maintenance and 

vessel inspection requirements. (F-2, F-4, F-5) 

R-6. Centers remove from service any LPV with crack-like indications in critical areas until 

acceptance rationale (e.g., repair, proof testing, periodic inspection, physical barriers, 

operational constraints, and/or analysis) and safety/risk assessment are approved by 

Center management. (F-2, F-3) 

R-7. Agency formulate and support an Agency-wide team to continue efforts to assess and 

reduce risks associated with LPVs, with the goal of developing a standard Agency 

process for their continued usage. 

• Team regularly report to Agency and Center management baseline LPV risks and

results to reduce risk. (F-2, F-4, F-5)

R-8. Agency develop a centralized database that documents, to the extent practical, LPV 

design, fabrication, materials, operation, inspection, maintenance, and repair data. 

(F-3, F-5) 

Longer-Term Recommendations

The following are discipline-specific recommendations to develop improved understanding of 

the risks associated with the continued use of LPVs and to lead to the development of Agency 

processes to mitigate these risks.

R-9. Agency expand efforts to gather LPV weld and parent materials property data.

• Perform statistical analysis to determine number of LPVs and associated materials

testing specimens needed to provide adequate bounding materials property data.

(F-2, F-3, F-6)

• Provide for a complete characterization of fracture toughness as a function of

temperature for each of the steels used in LPV construction, including welds. The

expected variability in fracture toughness across the LPV fleet should be sufficiently

characterized to enable developing meaningful MDMT values. Studied influences

should include variables such as the steel producer, vessel vendor, manufacturing

time period, steel product size, and other known manufacturing process dependencies.

(F-2, F-3, F-6, F-10)

• Perform failure analysis to include metallurgical examination and materials property

characterization for specimens collected from cracked LPVs. (F-2, F-3, F-6)

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

183 of 193

• Collect available and future material property data into a central and complete

database sufficient to support all forms of LPV FFS evaluations, proof testing

methodology and efficacy evaluations, and critical crack size determinations for NDE

detection criteria. Establish and maintain this database in an openly accessible

location, such as maptis.nasa.gov. (F-2, F-3, F-6)

R-10. Agency expand efforts to develop and validate analysis methods to address identified 

highest risk conditions and regions of LPVs. 

• Assess sensitivity of results to uncertainties in input variables (e.g., material property

values, structural configurations, and weld residual stress states). (F-3)

• Complete development of analytical models that incorporate key structural details of

LPVs. Validate models where possible against deformation measurements from

pressurized LPVs. (F-3)

• Complete analytical predictions of residual stresses in LPV circumferential fullpenetration welds and obtain a sample of WRS measurements to validate predictions.

(F-14)

• Establish an analytical framework to evaluate proof test logic. (F-6)

• In cases where proof testing cannot be applied, establish an analytical framework to

evaluate the LBB capability of the system. (F-3)

• To aid in NDE development and validation, establish an analytical framework to

evaluate critical initial flaw sizes. (F-3)

• Predict surface deformations associated with critical flaw sizes to assess the

capability of photogrammetry methods for flaw detection in shell sections. (F-16)

• Evaluate friction effects between intermediate layers of shell sections. (F-3)

R-11. Agency develop and validate the following NDE techniques to address the highest risk 

conditions and regions of LPVs.

• PAUT for inspection of LPV full-penetration circumferential welds. (F-15)

• Photogrammetric techniques for validating analysis methods and assess capability for

detecting flaws in shell sections. (F-16)

• AE techniques for application to LPVs. (F-17)

• RT methods for detecting cracks in shell sections and welds. (F-18)

• Low-frequency electromagnetic methods for the detection of flaws in shell sections of

LPVs. (F-19)

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

184 of 193

9.0 Alternate Viewpoint 

There were no alternate viewpoints identified during the course of this assessment by the NESC 

team or the NESC Review Board quorum. 

10.0 Other Deliverables

No unique hardware, software, or data packages, outside those contained in this report, were 

disseminated to other parties outside this assessment.

11.0 Lessons Learned

No applicable lessons learned were identified for entry into the NASA Lessons Learned 

Information System as a result of this assessment. 

12.0 Recommendations for NASA Standards and Specifications

No recommendations for NASA standards and specifications were identified as a result of this 

assessment.

13.0 Definition of Terms 

Corrective Actions Changes to design processes, work instructions, workmanship practices, 

training, inspections, tests, procedures, specifications, drawings, tools, 

equipment, facilities, resources, or material that result in preventing, 

minimizing, or limiting the potential for recurrence of a problem. 

Finding A relevant factual conclusion and/or issue that is within the assessment 

scope and that the team has rigorously based on data from their 

independent analyses, tests, inspections, and/or reviews of technical 

documentation. 

Lessons Learned Knowledge, understanding, or conclusive insight gained by experience 

that may benefit other current or future NASA programs and projects. 

The experience may be positive, as in a successful test or mission, or 

negative, as in a mishap or failure. 

Observation A noteworthy fact, issue, and/or risk, which may not be directly within the 

assessment scope, but could generate a separate issue or concern if not 

addressed. Alternatively, an observation can be a positive 

acknowledgement of a Center/Program/Project/Organization’s operational 

structure, tools, and/or support provided. 

Problem The subject of the independent technical assessment. 

Proximate Cause The event(s) that occurred, including any condition(s) that existed 

immediately before the undesired outcome, directly resulted in its 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

185 of 193

occurrence and, if eliminated or modified, would have prevented the 

undesired outcome. 

Recommendation A proposed measurable stakeholder action directly supported by specific 

Finding(s) and/or Observation(s) that will correct or mitigate an identified 

issue or risk. 

Root Cause One of multiple factors (events, conditions, or organizational factors) that 

contributed to or created the proximate cause and subsequent undesired 

outcome and, if eliminated or modified, would have prevented the 

undesired outcome. Typically, multiple root causes contribute to an 

undesired outcome. 

Supporting Narrative A paragraph, or section, in an NESC final report that provides the detailed 

explanation of a succinctly worded finding or observation. For example, 

the logical deduction that led to a finding or observation; descriptions of 

assumptions, exceptions, clarifications, and boundary conditions. Avoid 

squeezing all of this information into a finding or observation 

14.0 Acronym List

AE Acoustic Emission

AFRC Armstrong Flight Research Center (formerly Dryden Flight Research Center)

AMA Analytical Mechanics Associates, Inc.

ARC Ames Research Center

ASME America Society of Mechanical Engineers

ASTM American Society for Testing and Materials

B&W Babcock and Wilcox 

BPVC Boiler and Pressure Vessel Code

C Circumferential (structural orientation)

C(T) Compact Tension 

CB&I Chicago Bridge & Iron Company

dB Decibel

DOD Department of Defense

DOT Department of Transportation 

DP Dye Penetrant

DWC Digital Wave Corporation 

EDM Electronic Discharge Machine

FBH Flat-bottom Hole 

FEAM Finite Element Alternating Method

FFS Fitness for Service

FS Factor of Safety

FSH Full Screen Height

FY Fiscal Year

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

186 of 193

GRC Glenn Research Center

GSE Ground Support Equipment 

GUI Graphical User Interface

HAZ Heat-Affected Zone

HDT Hit Definition Time

HLT Hit Lockout Time

Hz Hertz

ICP Inductively Coupled Plasma 

ID Inner Diameter

JPL Jet Propulsion Laboratory 

JSC Johnson Space Center

KAPL Knolls Atomic Power Laboratory 

KSC Kennedy Space Center 

L Longitudinal (material or structural orientation)

LAPVAT Layered Pressure Vessel Analysis Tool

LaRC Langley Research Center

LBB Leak before Burst 

LEFM Linear Elastic Fracture Mechanics

LPV Layered Pressure Vessel

M Meridional, spherical head (structural orientation) 

MAE Modal AE (Acoustic Emission) 

MAF Michoud Assembly Facility 

MAWP Maximum Allowable Working Pressure 

MDMT Minimum Design Metal Temperature

MSFC Marshall Space Flight Center

MT Magnetic Particle Testing

MTSO Management and Technical Support Office 

NBIC National Board Inspection Code 

NDE Nondestructive Evaluation 

NDT Nil Ductility Temperature

NESC NASA Engineering and Safety Center

NG Northrop Grumman Corporation 

NRC U.S. Nuclear Regulatory Commission 

OD Outer Diameter

OSHA Occupational Safety and Health Administration

OSMA Office of Safety and Mission Assurance

PAUT Phased Array Ultrasonic Technique 

PDT Peak Definition Time

PSM Pressure Systems Manager

PT Penetrant Testing

PWHT Post-Weld Heat Treated

R Radial (structural orientation) 

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

187 of 193

RDH Radially Drilled Hole

RFI Request for Information 

RT Radiographic Testing

S Short-transverse (material orientation)

SCC Stress Corrosion Cracking 

SDH Side-drilled Hole

SE Signal Energy

SE(B) Single-edge Notch Bend

SLOFECTM Saturated Low Frequency Eddy Current 

SOW Statement of Work

SSC Stennis Space Center

SwRI Southwest Research Institute 

T Transverse (material orientation)

TIM Technical Interchange Meeting

U.S. United States

UT Ultrasonic Testing

VFT Virtual Fabrication Technology

VT Visual Testing

WFF Wallops Flight Facility

WRS Weld Residual Stress

WSTF White Sands Test Facility

XRF X-ray Fluorescence

15.0 References 

1. “ASME Boiler and Pressure Vessel Code, Section VIII – Pressure Vessels,” Division 1,

current edition.

2. “Occupational Safety and Health Standards,” 29 CFR 1910, OSHA, U.S. Department of

Labor.

3. Basic Program Elements for Federal Employee Occupational Safety and Health Programs

and Related Matters: Subpart 1 for Recordkeeping and Reporting Requirements,” 29 CFR

1960, OSHA, U.S. Department of Labor, November 26, 2004.

4. “ASME Boiler and Pressure Vessel Code, Section VIII – Pressure Vessels,” Division 1,

1968 edition.

5. U.S. Patent 1,925,118, “Pressure Vessel and Method of Fabricating It,” Stresau, Richard,

Assigned to A. O. Smith Corporation, September 5, 1933.

6. U.S. Patent 2,332,463, “Multilayer Pressure Vessel,” Nilson, E. H., Assigned to A. O. Smith

Corporation, October 19, 1943.

7. Gilbertson, M. and Benac, D. J., “Multi-layered Secondary Urea Reactor Failure,” AICHE

Ammonia Safety Symposium, Chicago, IL, Paper 1C, September 9–13, 2012.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

188 of 193

8. “Nondestructive Evaluation Requirements for Fracture Critical Metallic Components,”

NASA-STD-5009, April 7, 2008, para. 4.2.4.4.3.

9. ASME BPVC Code, Summer 1979 Addenda to the 1977 Section VIII.

10. “Requirements for Pressure Vessels Fabricated by Layered Construction,” Part UWL, ASME

BPVC Code, Winter Edition of 1978 (Section VIII, Divisions 1 and 2).

11. ASME Code for Unfired Pressure Vessels (1956).

12. ASME Code Interpretations Case 1205-3.

13. A. O. Smith Corporation’s specification MLS-30A.

14. A. O. Smith Corporation’s specification dated August 27, 1957.

15. “ASME Boiler and Pressure Vessel Code, Section VIII – Pressure Vessels,” Division 1,

1989 edition.

16. Gross, J. H., “The Effect of Strength and Thickness on Notch Ductility,” presented at

Symposium on Testing by Impact at ASTM Annual Meeting, Atlantic City, NJ,

June 25, 1969.

17. “ASME Boiler and Pressure Vessel Code, Section II – Materials,” current edition.

18. Cloud, R. L. and Spiering, G. A., “Failure of a High Pressure Vessel in Ammonia Service,”

Joint Conference of The Pressure Vessels and Piping, Materials, Nuclear Engineering, and

Solar Divisions, June 21-25, 1981, Denver, CO, ASME 81-PVP-8, pp. 1–15.

19. Jojima, T., “Urea Reactor Failure,” Ammonia Plant Safety, Vol. 21, 1979, pp. 111–118.

20. Occupational Safety and Health Review Commission Docket 93-0628, 2004, last accessed on

April 28, 2014, URL: http://www.oshrc.gov/decisions/html_2004/93-0628.html.

21. Li et al., “Analysis on Layer Cracking of Urea Reactors,” Paper No. PVP2010-25855,

ASME 2010 Pressure Vessels and Piping Division/K-PVP Conference, Bellevue, WA,

July 18–22, 2010. ISBN: 978-0-7918-4924-8, pp. 265–274.

22. Song, M., Wang, W., Zhao, Y., and Cui, Y., “Urea Reactor Integrity Evaluation Based on

Failure Analysis,” Transactions of the ASME, Vol. 129, November, 2007, pp. 744–753.

23. Gilbertson, M. and Benac, D. J., “Multi-layered Secondary Urea Reactor Failure,” AICHE

Ammonia Safety Symposium, Paper 1C, Chicago, IL, September 9–13, 2012.

24. “NASA Requirements for Ground-Based Pressure Vessels and Pressurized Systems (PVS),”

NASA-STD-8719.17.

25. Fowler, T. J., Blessing, J. A., Conlisk, P. J., and Swanson, T. L., “The MONPAC System,”

Journal of Acoustic Emission, Vol. 8, No. 3, pp. 1–8.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

189 of 193

26. Ziola, S. M., “Cyclic Crack Growth Testing of an A. O. Smith Multilayer Pressure Vessel

with Modal Acoustic Emission Monitoring and Data Assessment,” NASA/CR 2014-218156,

January 2014.

27. Ziola, S. M., “AE Qualification Test of HPADS Storage Vessel #16 at N250A,” Ames

Research Center, Moffett Field, CA, Digital Wave Corp., October 10, 2001.

28. Ziola, S. M., “AE Qualification Test of HPADS Storage Vessels 21–36 at N250A,” Ames

Research Center, Moffett Field, CA, Digital Wave Corp., October 29, 2002.

29. Ziola, S. M., “AE Qualification Test of HPADS Storage Vessels 1–16 at N250A,” Ames

Research Center, Moffett Field, CA, Digital Wave Corp., August 12, 2009.

30. “SLOFECTM Fast Corrosion Screening Technique,” Innospection, Ltd., 2004, last accessed

on April 28, 2014, URL: http://www.innospection.com/pdfs/SLOFEC%20Technique.pdf.

31. “Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in

the Transition Range,” ASTM E 1921-13, ASTM International, West Conshohocken, PA.

32. “Recommended Practice for Fitness-For-Service,” ASME-FFS-1 (API-579), American

Petroleum Institute, Washington, D. C., 2008.

33. “Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in

the Transition Range,” ASTM E 1921-13, ASTM International, West Conshohocken, PA.

34. Graff, K., Elastic Wave Motion in Solids, Ohio State University Press, Columbus, 1976.

35. Pao, Y. H., “Theory of Acoustic Emission, Elastic Waves and Nondestructive Testing of

Materials,” ASME Pub. AMD-Vol. 29, ASME, New York, 1978, pp. 107–128.

36. Gorman, M. R., “Plate Wave Acoustic Emission,” Journal of the Acoustic Society of

America, July 1991, pp. 358–364.

37. “Use of Modal Acoustic Emission and Ultrasonic Examination (AE/UE) in lieu of

Hydrostatic Pressure Test and Internal Visual Examination,” DOT SP-15322, Digital Wave

Corp., April 26, 2012.

38. Blackburn, P. R. and Rana, M. D., “Acoustic Emission Testing and Structural Evaluation of

Seamless, Steel Tubes in Compressed Gas Service,” ASME Journal of Pressure Vessel

Technology, 1985.

39. Hudak, Connolly, Hanley, and Lukezich, “A Damage Tolerance Approach for Recertification

of Submarine High-Pressure Flasks,” SwRI Project No. 06-4557, Southwest Research

Institute, March 31, 1995.

40. “Standard Practice for Acoustic Emission Monitoring of Structures during Controlled

Stimulation,” ASTM E-569, ASTM International, West Conshohocken, PA.

41. “Standard Test Method for Examination of Seamless, Gas Filled Pressure Vessels using

Acoustic Emission,” ASTM E-1419–02b, ASTM International, West Conshohocken, PA.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

190 of 193

42. “Standard Practice for Continuous Monitoring of Acoustic Emission from Metal Pressure

Boundaries,” ASTM E-1139, ASTM International, West Conshohocken, PA.

43. “Standard Terminology for Nondestructive Examinations,” ASTM E-1316, ASTM

International, West Conshohocken, PA.

44. Cardinal, J. W. and Popelar, C. F., “Multilayer Pressure Vessel Materials Testing and

Analysis (Phase 2),” NASA/CR 2014-218158, January 2014.

45. Cardinal, J. W., Popelar, C. F., and Page, R. A., “Multilayer Pressure Vessel Materials

Testing and Analysis (Phase 1),” NASA/CR 2014-218157, January 2014.

46. Mike Hudson et al., “An Investigation of Fracture Toughness,” NASA TM X-3316,

December 1975.

47. ANSI/NB-23 “National Board Inspection Code,” National Board of Boiler and Pressure

Vessel Inspectors, 2013 Edition, 2013.

48. “Specification for High Strength Low-Alloy Steel Plates (1143),” Standard 626-1143,

Distribution 7, Chicago Bridge and Iron Company, released November 1, 1963.

49. A. O. Smith, “Specification for High Strength Loy-Alloy Steel Plates (1146),” Standard

626-1146, published November 8, 1963, Rev. March 1968.

50. A. O. Smith, “Specification for 1146-a, High Tensile Strength – Low Alloy Steel Flange

Quality Plates,” Gauges 0.180 in. to 0.580 in., dated April 20, 1956.

51. ASME SA225 Gr. B, 1956 ASME Code Section II.

52. “Materials Standards, Material Specification for MLF-5002 Modified* High Tensile

Strength, Low Alloy Steel Forgings for Fusion Welded Pressure Vessels,” Chicago Bridge

and Iron Company, March 14, 1963.

53. “Specification for High Strength Low-Alloy Steel Plates (1146),” Standard 626-1146,

Distribution 7, Chicago Bridge & Iron Company, released November 8, 1963.

54. Kenyon, N. and Epstein, A. L., “Technical Note: Sulfur and Phosphorus in Low Alloy Steel

Welds Containing Up to 6% Nickel,” Welding Journal, Vol. 37, No. 11, November 1958,

Research Supplement, pp. 481s–492s.

55. Sharp, D., General Physics Corporation Contact Report, Submitted to B. Howel and

B. Hower of Nooter Corp., St. Louis, MO, November 28, 1989.

56. Private e-mail communication between Maan Jawad (consultant) and Matthew A. Adler,

Ph.D. (MSFC, Jacobs ESSSA) on January 15, 2014.

57. “Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in

the Transition Range,” ASTM E 1921-13, ASTM International, West Conshohocken, PA.

58. Broek, D., The Practical Use of Fracture Mechanics, Kluwer Academic Publishers, 1989,

pp. 10–12.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

191 of 193

59. McCabe, D. E., Merkle, J. G., and Wallin, K., “An Introduction to the Development and Use

of the Master Curve Method,” ASTM International, West Conshohocken, PA, 2005, p. 6.

60. Wallin, K., Fracture Toughness of Engineering Materials, EMAS Publishing, 2011,

pp. 115–190.

61. Joyce, J. A. and Tregoning, R. L., “Investigation of Specimen Geometry Effects and Material

Inhomogeneity Effects in A533B Steel, European Transactions,” European Conference on

Fracture 14, Crakow, Poland, 2002.

62. Fletcher, F. B., “Properties and Selection: Irons, Steels, and High-Performance Alloys,

Carbon and Low-Alloy Steel Plate,” ASM Handbook Volume 1, pp. 227–239.

63. http://www.georgesbasement.com/Microstructures/LowAlloySteels/Lesson1/Specimen01.htm.

64. “Standard Test Method for Instrumented Impact Testing of Metallic Materials,”

ASTM E 2298-13, ASTM International, West Conshohocken, PA.

65. “Standard Test Method for Measurement of Fracture Toughness,” ASTM E 1820-11, ASTM

International, West Conshohocken, PA.

66. Private phone communication between Jim Joyce (Consultant) and Matthew A. Adler, Ph.D.

(MSFC, Jacobs ESSSA) on April 9, 2014.

67. Palmer, C., “NASA MSFC Radiography Test Report,” EM20 NDE Work Order No.

2014-0011, January 13, 2014.

68. Palmer, C., “NASA MSFC Radiography Test Report,” EM20 NDE Work Order No.

2014-0276, April 9, 2014.

69. Palmer, C., “NASA MSFC Radiography Test Report,” EM20 NDE Work Order No.

2014-0162, March 6, 2014.

70. Vessel V125 Data Package Index, Chicago Bridge and Iron Company, 1963.

71. “VFTTM Manual,” Engineering Mechanics Corporation of Columbus, Emc2, June 2013.

72. Cheston, D., Piascik, R., Larsen, C., Knight, N., Brust, F. W., Womack, J.,

Phillips, D. R., Raju, I. S., Russel, S., Hudak, S., Reuter, W., Dawicke, D., and

Hayes, M., “Independent Evaluation of the Critical Initial Flaw Sixe for Ares I-X (AIX)

Upper Stage Simulator (USS) Common Segment to Flange-to-Skin Welds,” NESC Technical

Report Document RP-08-09, February 26, 2008.

73. Raju, I. S., Brust, F. W., Phillips, D. R., and Cheston, R., “Ares I-C Upper Stage Simulator

Residual Stress Analysis,” NASA/TM-2008-215339 (NESC-RP-08-09/06-081-E),

August 2008.

74. Brust, F. W., Dawicke, D., Raju, I. S., and Cheston, D., “Residual Stresses and Critical Initial

Flaw Size Analyses of Welds”, Proceedings of the 50th AIAA/ASME/ASCE Structures,

Structural Dynamics, and Materials Conference, Palm Springs, CA, May 2009.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

192 of 193

75. Brust, F. W., Raju, I., Cheston, D., Dawicke, D., and Phillips, D., “Weld Residual Stress and

Distortion Analysis of the ARES I-X Upper Stage Simulator,” Proceedings of the ASME

Pressure Vessels & Piping PVP 2008 Conference, PVP 2008-61247, Chicago, IL,

July 27–31, 2008.

76. Broussard, J., and Crooker, P., “NRC/EPRI Residual Stress Validation Program Phase 1:

Experimental Specimen Modeling and Measurement,” Proceedings of ASME Pressure

Vessel and Piping Conference, Paper No. PVP2011-57677, Baltimore MD, July 2011.

77. Schmidt, T., et al., “Performance Verification of 3D Image Correlation Using Digital HighSpeed Cameras,” Proceedings of 2005 SEM Annual Conference and Exposition, Portland,

OR, June 7–9, 2005.

78. Melis, M., et al., “Impact Testing on Reinforced Carbon-Carbon Flat Panels with BX-265

and PDL 1034 External Tank Foam for the Space Shuttle Return to Flight Program,”

NASA TM 2009-213642/REV1, 2009.

79. Ambur, D. R., et al., “Progressive Failure Studies of Stiffened Panels Subject to Shear

Loading,” Composite Structures, Vol. 65, 2004, pp. 129–142.

80. Littell, J. D., et al., “Measurement of Epoxy Resin Tension, Compression and Shear Stress

Strain Curves over a Wide Range of Strain Rates Using Small Test Specimens,”

J. Aerosp. Eng., Vol. 21, 2008, pp. 162–173.

81. Amsterdam, E., et al., “Failure Mechanisms of Closed Cell Aluminum Foam under

Monotonic and Cyclic Loading,” Acta Materialia, Vol. 54, 2006, pp. 4465–4472.

82. Lesser, W. P., et al., “Fatigue Crack Closure Analysis Using Digital Image Correlation,”

NASA TM 2010-216695, 2010.

83. Chevalier, L., et al., “Digital Image Correlation used to Analyze Multiaxial Behavior of

Rubber-like Materials,” European Journal of Mechanics A/Solids, Vol. 20, 2001,

pp. 167–187.

84. Revilock, D. M., et al., “Three-Dimensional Digital Image Correlation of a Composite

Overwrap Pressure Vessel during Hydrostatic Pressure Tests,” NASA TM 2007-214938,

2007. 

85. Thornburgh, R. P. and Hilburger, M. W., “Longitudinal Weld Land Buckling in Compression

Loaded Orthogrid Cylinders,” NASA TM 2010-216876, 2010.

86. Tyson, J., et al., “Biomechanics Deformation and Strain Measurements with 3D Image

Correlation Photogrammetry,” Experimental Techniques, Vol. 26, No. 5, 2002, pp. 39–42.

87. Zhang, D., et al., “Evaluating the Material Behavior of Arterial Tissue using Digital Image

Correlation,” Experimental Mechanics, Vol. 42, No. 2, 2002, pp. 409–416.

88. “Standard Practice for Acoustic Emission Monitoring of Structures during Controlled

Stimulation,” ASTM E569/E569M-13, ASTM International, West Conshohocken, PA.

NESC Request No.: TI-13-00852
NASA Engineering and Safety Center

Technical Assessment Report

Document #:

NESCP-RP13-00852 

Version:

1.1

Title:

Evaluation of Agency Non-code LPVs

Page #:

193 of 193

89. Hamstad, M. A., O’Gallagher, A., and Gary, J., “Effects of Lateral Plate Dimensions on

Acoustic Emission Signals from Dipole Sources,” J. of Acoustic Emission, Vol. 19, 2001,

pp. 258–274.

90. Chell, G. G., McClung, R. C., Russell, D. A., Chang, K. J., and Donnelly, B., “Significant

Issues in Proof Testing: A Critical Appraisal,” NASA/CR-4628, 1994.

91. Chell, G. G., McClung, R. C., Kuhlman, C. J., Russell, D. A., Garr, K., and Donnelly, B.,

“Guidelines for Proof Test Analysis,” NASA/CR-1999-209427, 1999.

92. McClung R. C., Chell, G. G., Millwater, H. R., Russell, D. A., and Orient, G. E., “A

Comparison of Single-Cycle to Multi-Cycle Proof Testing Strategies,” NASA/CR-1999-

209426, 1999.

16.0 Appendices (separate Volume 2) 

Appendix A. Example LPV Procurement Specification

Appendix B. Center RFI Summaries

Appendix C. MSFC Sacrificed Vessel General Information

Appendix D. Current Considerations for the MSFC Non-Code Pressure Vessel Material 

Properties Assessment Presentation, February 28, 2014 

Appendix E. LPV TIM – September 18, 2013: Current Considerations for the MSFC NonCode Layered Pressure Vessel Material Testing Tasks Presentation, February 28, 

2014 

Appendix F. MSFC LPV Analysis Activities/LAPVAT Presentation, 

March 17, 2014 

Appendix G. ARC MAE Validation Effort Summary

Appendix H. SwRI® Testing Summary

NESC Request No.: TI-13-00852
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE

Technical Memorandum

4. TITLE AND SUBTITLE

Evaluation of Agency Non-code Layered Pressure Vessels (LPVs)

5a. CONTRACT NUMBER

6. AUTHOR(S)

Prosser, William H.

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PERFORMING ORGANIZATION

REPORT NUMBER

L-20446 NESC-RP-13-00852

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 39 Structural Mechanics

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

In coordination with the Office of Safety and Mission Assurance and the respective Center Pressure System Managers (PSMs), 

the NASA Engineering and Safety Center (NESC) was requested to formulate a consensus draft proposal for the development 

of additional testing and analysis methods to establish the technical validity, and any limitation thereof, for the continued safe 

operation of facility non-code layered pressure vessels. The PSMs from each NASA Center were asked to participate as part of 

the assessment team by providing, collecting, and reviewing data regarding current operations of these vessels. This report 

contains the outcome of the assessment and the findings, observations, and NESC recommendations to the Agency and 

individual NASA Centers.

15. SUBJECT TERMS

Layered pressure vessel; NASA Engineering and safety Center; Nondestructive Evaluation; Boiler and Pressure Vessel Code; 

Phased array ultrasonic technique

18. NUMBER

OF

PAGES

198

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF

ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

April 2013 - June 2014

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

869021.03.07.01.08

11. SPONSOR/MONITOR'S REPORT

NUMBER(S)

NASA/TM-2014-218505/Volume I

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 07 - 2014