January 2014 

NASA/CR2014-218156 

 

Cyclic Crack Growth Testing of an 

A.O. Smith Multilayer Pressure Vessel with 

Modal Acoustic Emission Monitoring and 

Data Assessment 

Steven M. Ziola 

Digital Wave Corporation, Centennial, Colorado 
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January 2014 

NASA/CR2014-218156 

 

Cyclic Crack Growth Testing of an 

A.O. Smith Multilayer Pressure Vessel with 

Modal Acoustic Emission Monitoring and 

Data Assessment 

Steven M. Ziola 

Digital Wave Corporation, Centennial, Colorado 
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. 
iii 

Executive Summary 

Digital Wave Corp. (DWC) was retained by Jacobs ATOM at NASA Ames Research Center to 

perform cyclic pressure crack growth sensitivity testing on a multilayer pressure vessel 

instrumented with DWC’s Modal Acoustic Emission (MAE) system, with captured wave 

analysis to be performed using DWCs WaveExplorerTM software, which has been used at Ames 

since 2001 [Refs. 13 - 15]. The objectives were to document the ability to detect and 

characterize a known growing crack in such a vessel using only MAE, to establish the sensitivity 

of the equipment vs. crack size and / or relevance in a realistic field environment, and to obtain 

fracture toughness materials properties in follow up testing to enable accurate crack growth 

analysis. Although the wave propagation theory and acoustic science behind MAE is well 

established, no prior crack-correlation validation testing of this nature had been attempted on 

multilayer vessels, which exhibit significant noise emissions under loading and which are thicker 

walled with tougher material than the vessels on which the method was originally validated. 

From DWC’s own prior work and that of others, such as documented in Ref. 10 by Southwest 

Research Institute (SwRI), it was also known that stable crack growth does not emit strong AE 

signals, and they likely would not become so until the crack was near critical size. Using 

toughness properties provided by NASA from work performed in 1975 [Ref. 11], it was believed 

that the crack induced in this vessel would be closer to critical size than, in fact, it was based on 

later material property testing and analysis by SwRI [Ref 9] that revealed the shell material to be 

much tougher than originally indicated by NASA. 

Crack growth starter notches were milled into the outer shell layer of a surplus A.O. Smith 

multilayer pressure vessel obtained from Kennedy Space Center, serial number MV50466-8. 

The vessel was pressure cycled 4,688 times at stress levels between 1/2 of yield and nominal 

yield strength of the shell material. A visible crack indication was seen in the center-shell crack 

under a video magnifier after about two thousand cycles, but its depth could not be determined 

during testing. No clear MAE signals were obtained real-time during this phase of crack growth, 

but some were revealed in later examination. Due to funding limitations, the test was ultimately 

terminated when several cycles of yield-level stress were applied to force crack extension. It was 

later determined in post-processing that a low level MAE crack signal was captured during these 

final cycles. In general, the MAE response was less than anticipated, and it is possible that this 

crack, if hidden, would not have been identified with a normal test level-of-effort. However, the 

refinements in filtering made possible by this test will improve future field assessments, although 

the sensitivity and crack size correlation to signal strength were not clearly established since the 

crack was at the threshold of detectability for this tough material. Future cyclic testing should 

focus on welds and weld heat affected zone which are more likely to crack in service, and are 

also more likely to emit strong AE signals assuming they are lower toughness material. 

The vessel was subsequently sent to Southwest Research Institute (SwRI) on a subcontract for 

fractographic analysis of the center crack and for testing of strength, fracture toughness, and 

fatigue crack growth properties to facilitate correlation of the actual crack with fracture 

mechanics calculations, which were reported in Ref. 7. This materials testing work demonstrated 

that the crack grew 0.064 inches before the final, rapid through-wall extension under yield level 

stress, and provided extensive fracture toughness data. The photomicrographs also showed that 

the crack growth was not uniform across the crack front, and was highly plastic, which also had 

an unanticipated negative effect on the ability to identify the crack using MAE because of the 
iv 

more diffuse plastic energy release mechanism. In retrospect, a smaller, elastic mode crack 

extension may have been easier to detect. Nonetheless, the filtering developed to evaluate these 

results will significantly improve the data assessments in future testing. 

Revision 1 of this test report is issued to incorporate findings from Phase 2 materials testing 

performed by SwRI [Ref. 9] and to make various editorial changes, more complete reference 

citations in the text, and typographical corrections. 
v 

Table of Contents 

Executive Summary ....................................................................................................................... iii

Section 1. Testing Outline and Objectives ...................................................................................... 1

Test Objectives.................................................................................................................... 2

Section 2. A.O. Smith Pressure Vessel and Vessel Preparation ..................................................... 2

Section 3. Testing Instrumentation and Hardware .......................................................................... 7

Strain Gages and Strain Instrumentation ............................................................................ 7

Modal Acoustic Emission Instrumentation ......................................................................... 9

Cyclic Pressurization of A. O. Smith Vessel .................................................................... 10

Section 4. Pressure and Strain Data .............................................................................................. 11

Section 5. Modal Acoustic Emission – Review, Theory and Practice .......................................... 15

Historical Review of Conventional Acoustic Emission Analysis for Pressure Vessels ... 15

Overview of Modal Acoustic Emission ............................................................................ 16

Source Orientation Discrimination ............................................................................. 16

Noise Identification and Rejection.............................................................................. 17

Theoretical Predictions ..................................................................................................... 17

Section 6. Fatigue Testing of the A. O. Smith Vessel and MAE Results ..................................... 19

MAE Sensor Locations ..................................................................................................... 19

Fracture Mechanics ........................................................................................................... 20

Crack Growth Rates .................................................................................................... 20

Crack Growth Sensitivity ............................................................................................ 21

Pencil Lead Breaks (PLB’s).............................................................................................. 22

Wave Mode Formation ..................................................................................................... 22

PLB Location Accuracy .................................................................................................... 23

Test Results ....................................................................................................................... 25

Section 7. Discussion of Test Results ........................................................................................... 30

Prior DOT and ASME Pressure Vessel Testing ............................................................... 32

Section 8. Conclusions .................................................................................................................. 34

Section 9. Follow-on Testing ........................................................................................................ 35

Section 10. References .................................................................................................................. 37

Appendix A. Table of Test Files ................................................................................................... 38

Appendix B. Modal Acoustic Emission Detection Sensitivity .................................................... 42

Appendix C. WaveExplorerTM Digital Filter Settings ................................................................. 52
vi 

List of Figures 

Figure 1 - Drawing of A. O. Smith MV50466-8 pressure vessel. .................................................. 3

Figure 2 - Cross section of the head to shell weld of the A. O. Smith pressure vessel MV50466-8 

(section provided by SwRI) .......................................................................................... 4

Figure 3 - Nameplate on the MV50466 A. O. Smith pressure vessel ............................................. 5

Figure 4 - Milling of the flaws into the vessel. ............................................................................... 5

Figure 5 - Vessel Preparation for Testing ....................................................................................... 6

Figure 6 - Strain gages mounted on the head and near the head-to-shell weld of the vessel ......... 7

Figure 7 - Strain gage mounted on the vessel head ........................................................................ 8

Figure 8 - Strain gages at the head-to-shell weld location .............................................................. 8

Figure 9 - Strain gages mounted at the center of the vessel............................................................ 9

Figure 10 - MAE Equipment Setup ................................................................................................ 9

Figure 11 - Strain vs. Pressure, 7260 psig, head-to-shell weld ..................................................... 11

Figure 12 - Strain vs. Pressure, 7260 psig, spherical head ........................................................... 12

Figure 13 - Strain vs. pressure, 7260 psig, shell center ................................................................ 12

Figure 14 - Strain vs. Pressure, 14,000 psig, head-to-shell weld .................................................. 13

Figure 15 - Strain vs. Pressure, 14,000 psi, spherical head .......................................................... 13

Figure 16 - Strain vs. Pressure, 14,000 psig, center of vessel ...................................................... 14

Figure 17 - Cyclic pressurization cycles for 1000-8400 psi. ........................................................ 14

Figure 18 - Extensional Mode....................................................................................................... 17

Figure 19 - Flexural Mode ............................................................................................................ 17

Figure 20 - Theoretically extensional mode ................................................................................. 18

Figure 21 - Theoretically predicted flexural mode ....................................................................... 18

Figure 22 - Sensor locations for cycles 1 - 3291 .......................................................................... 19

Figure 24 - da/dN curve based on data from NASA Technical Memorandum X-3316 [Ref 11] . 21

Figure 25 - Captured lead break waveform with extensional mode ............................................. 22

Figure 26 - Extensional and Flexural mode dispersion curves for 3.125 inch thick steel plate ... 23

Figure 27 - Lead break source locations on the test vessel with head sensors ............................. 24

Figure 28 - Lead break source locations on the test vessel without head sensors ........................ 24

Figure 29 - Vessel with Video Microscope Images ...................................................................... 25

Figure 30 - Crack growth event plots, cycles 2462-2509 ............................................................. 27

Figure 31 - FFT of crack growth signal ........................................................................................ 28

Figure 32 - Typical FFT of noise. ................................................................................................. 29

Figure 33 - Crack growth signal at 13.710 psi .............................................................................. 30

Figure 34 - Expanded view of Channel 8 showing the extensional mode formation ................... 30
vii 

Figure 35 - Photograph of One Face of the As-Opened Crack (credit SwRI, Ref 7) ................... 31

Figure 36 - MAE monitoring of a Type 2 COPV with 1 inch liner vessel ................................... 33

Figure 37 - FFT of the signals in Figure 36 .................................................................................. 34

Figure A-1. Crack growth per cycle for 3AAX steel. ................................................................... 44

Figure A-2. Schematic of crack growth. ....................................................................................... 47

Figure A-3. Crack growth signal from a 3AAX tube. The waveform from channel 1 is shown in 

blue, and channel 2 is shown in red. ........................................................................... 48

Figure A-4. Sensor spacing as a function of waveform energy (y-axis) and crack growth area. 49

List of Tables 

Table A-1. Theoretical calculated and experimentally measured wave energy. ......................... 48
Section 1. Testing Outline and Objectives 

The objective of this testing was to determine whether non-critical fatigue crack growth 

detection in an actual, previously in-service, non-ASME Code multi-layered pressure vessel 

under realistic cyclic pressure loading was possible using modal acoustic emission (MAE) 

waveform assessment methodology, and to attempt to determine the sensitivity limits of such 

detection. It has been demonstrated unequivocally that energy releases from Pentel 2H 0.3 mm 

lead breaks (as per ASTM E-1419) can be detected for ranges well over 20 feet from inner and 

outer surfaces and from intermediate layers of these vessels, and was done so again in this test. 

However, it is believed that the energy from the lead breaks is so large it is useful only to 

establish the presence of a large growing crack, likely near-critical size. In order to improve the 

usefulness of the MAE method for fitness for service and life extension, a more detailed 

understanding is required of the actual crack sizes (sub-critical) that can be detected in the 

presence of the considerable non-relevant noise in tests of these vessels (due to layer wash at 

welds, corrosion flakes breaking, layer relative movement, etc.). Towards this end, the following 

steps were taken. 

1. NASA provided the test vessel to be used for testing, A. O. Smith pressure vessel, 

MV50466-8, which was located at the Kennedy Space Center facility. This vessel and a 

backup were shipped by DWC to our Centennial, CO headquarters. To ensure safe 

operations, a bunker was excavated in the soil in the back field area such that the vessel 

was entirely below the surface, and the trench walls were reinforced with wood and a 

roof cover was constructed. This also protected the vessel from the elements, and 

minimized nonrelevant environmental noise sources. 

2. The vessel was prepped for fatigue testing and monitoring using Digital Wave 

Corporation’s WaveExplorerTM waveform analysis modal acoustic emission data 

acquisition hardware and software analysis techniques. 

3. Preparation and pre-inspection for safety assessment included x-ray inspection of the 

head-to-shell welds, magnetic particle inspection of the outer long seam weld, and 

borescope inspection of the internal geometry of the head-to-shell welds. There were no 

unacceptable defects identified. 

a. The vessel outer diameter was also measured to 0.001” accuracy at both zero and 

6600 psi pressure in accordance with ASME Section VIII, Div 1, part ULW to 

assess the extent of gaps between layers. 

b. A one-inch diameter part-through hole was machined through 7 of the 12 shell 

layers in order to provide accessible intermediate layer surfaces for performing 

lead break calibrations and coupling studies. 

4. Axial flaws were machined by a milling cutter into the vessel near both head-to-shell 

welds, and at the center of the vessel. Due to schedule and budget limitations as well as 

physical constraints with the vessel, a more precise EDM notch could not be applied. 

5. Strain gages were mounted on the vessel head, near the head-to-shell weld and at the 

center of the vessel to monitor the strain during the cycling and to be able to correlate 

actual stresses experienced to theoretical, given the potential for gaps in the layers. 

6. The vessel was then hydrostatically cycled to induce flaw growth at the notches. 
2 

Test Objectives 

1. Document the ability to detect modal acoustic emission events from fatigue crack growth 

under realistic field testing condition 

2. Monitor strain readings during the cyclic testing to ensure that the vessel was not 

plastically deforming and to have strain data if stress calculations need to be performed at 

a later date. 

3. Perform fractographic analysis of the crack growth and ASTM tensile, fracture 

toughness, stress intensity and fatigue crack growth (da/dn) testing of the material 

properties of the vessel. 

4. Correlate actual measured incremental crack growth with MAE signals to establish 

energy release sensitivity and nonrelevant noise discrimination in order to improve 

multilayer pressure vessel structural integrity assessments using the MAE waveform 

assessment methodology. 

Section 2. A.O. Smith Pressure Vessel and Vessel Preparation 

Figure 1 shows the drawing of A. O. Smith pressure vessel MV50466-8, used in the testing. The 

vessel consisted of 11 layers of approximately 0.25 inch thick wraps, with a 0.375 inch thick 

liner (12 total layers). This resulted in a vessel with an approximate wall thickness of 3.125 

inches, a 30 inch ID, and a 36.25 inch OD. The spherical head wall thickness was a minimum of 

2.5 inches. 

Figure 2 shows a picture of the cross section of the head-to-shell weld of the vessel that was 

obtained by SwRI after the vessel was sectioned, Ref [7]. In the photograph the layers of the 

vessel can be clearly seen. Except for the outer layer in this small section of the vessel, the 

layers are in intimate contact when the vessel is in use, which was also demonstrated by the 

measured stress levels and diameter expansion measurements. Because the wavelengths of the 

plate waves are so long, they do not interact with the layers, unlike typical frequencies used in 

ultrasonic inspections. This was also separately verified and documented in testing performed by 

DWC at ARC in 2001 and 2002 [Refs. 13 and 14]. 

The nameplate is shown in Figure 3, and contains pertinent information about the vessel. 
3 

Figure 1 - Drawing of A. O. Smith MV50466-8 pressure vessel. 
4 

Figure 2 - Cross section of the head to shell weld of the A. O. Smith pressure vessel MV50466-8 

(section provided by SwRI) 

The eleven one-quarter inch layers and the single three-eighths inch inner layer can be clearly 

seen in the photo. This sample was prepared after the cycling was finished and while the 

metallurgical analysis was being performed. Other samples showed that he gap between the 

outer layers was a local artifact, and not representative of the entire vessel. 
5 

Figure 3 - Nameplate on the MV50466 A. O. Smith pressure vessel 

Flaws were machined into the vessel using a 6 inch diameter, 600

 milling cutter. This was done 

on a large milling machine, Figure 4. 

Figure 4 - Milling of the flaws into the vessel. 

Longitudinal flaws were machined at both ends 0.5 inches in from the edge of the head-to-shell 

welds, and one in the center of the vessel. All flaws were inline, at the top of the vessel, due to 

the constraints of the mill. 
6 

The flaws were machined to a depth of 0.175 inches into the outer wrap, and by using the large 

diameter cutter created a radiused notch in the outer layer, approximately 2 inches long. Electro 

discharge machining for the notches was considered, but due to the short time frame to get the 

testing started, it was not possible to find a vendor that could machine the notches in the time 

frame needed. 

Figure 5 below shows a montage of pictures as the vessel was prepped for testing. 

 

 

 

Figure 5 - Vessel Preparation for Testing 

This grouping of pictures above shows the preparation of the vessel for the fatigue testing. 

Starting at the upper left and going clockwise shows, the cutter and the notch; the relationship of 

the notch to the edge of the head-to-shell weld; the hole drilled into the side of the vessel through 

seven layers; the seven layers numbered by layer, the vessel (the hole can be seen at the right 

side of the vessel); and the length of the notch (this is the notch at the center of the vessel). 

Hole 
7 

Section 3. Testing Instrumentation and Hardware 

Strain Gages and Strain Instrumentation 

Strain gages were mounted on the vessel to provide strain readings on the vessel spherical head, 

head-to-shell weld and at the center of the weld. Figure 6 shows the gages on the head and the 

head-to-shell weld. National Instruments strain gage instrumentation was used to perform these 

measurements. The strain gages used were from Micro-measurements. The gages were wired 

using a three wire configuration to account for wire resistance. 

Strain Gages: Micro-measurements strain rosette, CEA-06-125UR-120 

Strain Gage Instrumentation: National Instruments cDAQ-9181 CompactDAQ 1-slot Ethernet 

chassis connected to a National Instruments 9235 8-channel quarter-bridge strain gage module. 

System control was done through a National Instruments USB-6008 12-bit, 10 kS/s 

multifunction DAQ module. 

Figure 6 - Strain gages mounted on the head and near the head-to-shell weld of the vessel 

The National Instruments system was capable of 8 channels of acquisition. Since the strain field 

at the center of the vessel should be well described by classical thin wall pressure vessel theory, 

only the axial and hoop gages were acquired (not the 450

). All three of the gages on the head and 

head-to-shell strain gage rosettes were acquired so that the full strain field could be calculated, if 

needed. 

Figures 7-9 show close-ups of all the strain gages. 
8 

Figure 7 - Strain gage mounted on the vessel head 

This photograph shows the strain gage mounted on the head of the vessel. The gage was a 

triaxial gage, with one gage along the axial direction, one along the circumferential direction and 

one at 450

. 

Figure 8 - Strain gages at the head-to-shell weld location 
9 

Figure 9 - Strain gages mounted at the center of the vessel 

Modal Acoustic Emission Instrumentation 

The instrumentation used for the monitoring of the cyclic test vessel was the Digital Wave 

Corporation WaveExplorerTM MAE system, Figure 10. This is a broadband system with full 

high-speed wave capture. It has been proven capable and qualified for measuring multilayer 

pressure vessels through work done at several NASA research centers, as well as for retesting 

and requalification of DOT containers (for example, see DWC’s DOT Special Permit 15332, 

Ref. 12). The exact components used for the tests are given below. 

Figure 10 - MAE Equipment Setup 

This photograph shows the FM1, video microscope monitor (on top of the FM1), and the 

monitors for the modal AE and pump control and pressure and strain data acquisition. The 
10 

settings shown here for the filters are for the analog portion of the hardware. Values shown in 

Appendix C are for the software digital filters. 

Hardware

Sensors: Digital Wave B225-5 IPP 

Preamplifiers: Digital Wave P20 

Signal Conditioning: Digital Wave FM1 

Software 

Data Acquisition: Digital Wave WaveExplorerTM software 

Data Acqusition System Settings 

A/D Rate: 2MHz 

Total System Gain: 68 dB 

Highpass Filter: 20 kHz

Lowpass Filter: 750 kHz 

Points per Waveform: 4096 

Pretrigger Points: 1024 

The MAE system threshold used for the tests was 32 dBAE. This threshold is specified in ASTM 

E 1419 [Ref. 16], and has been generally shown to be adequate for AE testing, although ASTM 

E 1419 itself is only applicable to seamless single layer vessels. Further assessment of this 

parameter will be performed based on the results of this test on a multilayer vessel. That the -

02b edition referenced above is cited as mandatory in DWC’s DOT special permit for cylinder 

requalification, Ref. 12. 

Cyclic Pressurization of A. O. Smith Vessel 

The pumps used for the testing were pneumatically operated, high pressure pumps. These pumps 

are relatively robust and inexpensive, and are typically used for hydraulic pressurization of high 

pressure bottles. Each system was fitted with high pressure stainless steel tubing and fittings, a 

mechanical gage, and a 0-10 volt pressure transducer so that pressure readings could be digitized 

and stored for each mechanical and MAE test. Pressure transducers were calibrated against the 

mechanical gage for each pressure system. An infrared temperature transducer was also 

incorporated into each pressure system so that water temperature could be monitored, digitized 

and stored. The components in each system are listed below. 

0-10,000 psi Cyclic Pressurization: 

Pump: 

Haskel

Model #: GSF-60 

Pressure Range: 7500 psi 

Pressure Gage: 

ENFM

Pressure Range: 0-10,000 

psi

Pressure Transducer: 

WIKAI

Model #: A-10 

Pressure Range: 0-10,000 

psi

Voltage Range: 0-10 volts 

Temperature Transducer: 

Omega

Model #: OS 136-1-V 

Temperature Range: 0-

4000 F 

Voltage Range: 0-10 volts 
11 

14,000 psi Pressurization: 

Pump: 

Sprague Products, CurtissWright Flow Control 

Group

Model #: S216J300

P/N: 77895-51001 

Pressure Range: 33,500 psi 

Pressure Gage: 

McDaniel Controls, Model 

G, 30,000 psi

Pressure Transducer: 

Omegadyne Inc.

Model #: PX02S1-

30KG10T

Pressure Range: 0-30,000 

psi

Voltage Range: 0-10 volts 

Temperature Transducer: 

Omega

Model #: OS 136-1-V 

Temperature Range: 0-

4000 F 

Voltage Range: 0-10 volts 

Section 4. Pressure and Strain Data 

Representative strain data is shown in the Figures 11-13 below. The first set of three figures 

show the strain readings for a 0-7260 psi pressure cycle. The vessel was rated by A.O. Smith for 

a pressure of 6600 psi, with a factor of safety of 2 on yield stress. It can be seen from these plots 

that the strain remained linear throughout the cycle, and thus no plastic deformation is occurring. 

The next set of Figures 14-16 shows the pressure-strain curves for the 14,000 psi load (2.2 over 

operating pressure). It can be seen from these plots that at this pressure, the vessel began to 

deform plastically, as the strain began to roll over. This is clearly seen in the shell plots. 

Figure 11 - Strain vs. Pressure, 7260 psig, head-to-shell weld 

Figure 11 shows the strain readings for the circumferential (hoop), 450

 and axial strains for a 

pressurization of 7260 psi for the strain gages mounted at the head-to-shell weld. 
12 

Figure 12 - Strain vs. Pressure, 7260 psig, spherical head 

Figure 12 shows the strain readings for the circumferential (hoop), 450

 and axial strains for a 

pressurization of 7260 psi for the strain gages mounted on the spherical head. 

Figure 13 - Strain vs. pressure, 7260 psig, shell center 

This plot (Figure 13) shows the strain readings for the circumferential (hoop), and axial strains 

for a pressurization of 7260 psi for the strain gages mounted on the shell at the center of the 

vessel. 
13 

Figure 14 - Strain vs. Pressure, 14,000 psig, head-to-shell weld 

Figure 14 shows the strain readings for the circumferential (hoop), 450

 and axial strains for a 

pressurization of 14,000 psi for the strain gages mounted at the head-to-shell weld. The data 

below each of the pressure-strain plots is the offset in the strain due to the plastic deformation of 

the vessel. 

Figure 15 - Strain vs. Pressure, 14,000 psi, spherical head 
14 

Figure 15 shows the strain readings for the circumferential (hoop), 450 and axial strains for a 

pressurization of 14,000 psi for the strain gages mounted on the spherical head. 

Figure 16 - Strain vs. Pressure, 14,000 psig, center of vessel 

This plot shows the strain readings for the circumferential (hoop) and axial strains for a 

pressurization of 14,000 psi for the strain gages mounted on the shell at the center of the vessel. 

Figure 17, below, shows the cyclic pressurization plot for 12/28/2011, for cycles from 1000-8400 

psi. This is shown as a representative example of the cycling performed. All test files have the 

pressurization files digitized and stored in electronic format. The maximum and minimum 

cycling pressures used for each days testing are shown in the table in Appendix A. Table of Test 

Files. 

Figure 17 - Cyclic pressurization cycles for 1000-8400 psi. 
15 

Section 5. Modal Acoustic Emission – Review, Theory and Practice 

Historical Review of Conventional Acoustic Emission Analysis for Pressure Vessels 

For much of the history of acoustic emission testing, it was thought that the wave propagation 

due to flaw growth in structures was too complicated to understand. This led to parameter based 

analysis techniques, such as scatter plots of amplitude, duration, counts, and other single 

parameter analyses. The first mistake in this approach is thinking that these are independent 

parameters – they are not. If this is thought of in terms of a bell, if the bell is struck hard, a long 

(duration) and loud (amplitude) tone will result, and vice versa. Correlating these two against 

each other does not provide unique information about the bell. This, noise and flaw growth 

discrimination is not be possible with this approach. 

However, even with this limitation, AE had useful qualities. The main one was that event 

locations could be remotely determined. By measuring the difference in arrival times of the 

wave at two or more sensors, the location of the event could be calculated. This was taken 

advantage of in the testing of DOT high strength tube trailer cylinders. The low mechanical noise 

environment of this testing, combined with the ability to perform ultrasonic examination for flaw 

characterization, eliminated the need for source characterization through analysis of the AE 

results, and the analysis was reduced to determining the given number of events at a single 

location. Specifically, if five events were located in the same location, then follow-up ultrasonic 

inspection was required to verify the flaw and flaw size. This test method was developed in the 

early 1980’s by Blackburn and Rana, Ref. 1, and then ASTM E 1419 was written around this 

work. This work also set the threshold sensitivity of the AE systems (32 dBAE) for the detection 

of flaws in these high strength steel pressure vessels. This sensitivity could be used with the 

existing equipment available at that time (early 1980’s), because there were no interfaces in the 

tubes to create noise or to prevent ultrasonic inspection, and the systems were filtered to remove 

signal frequencies below 100 kHz and above 300 kHz (narrowband filtering). Thus, event rates 

were low, and the equipment could process the data without saturating. Plus, determination of 

flaw existence was done with a follow-up method, e.g., ultrasound. Thus, the false calls 

typically associated with AE could be eliminated. 

However, in moving on to multi-layered vessels, this approach breaks down. Why? Because of 

the way that multi-layered vessels are manufactured, when they are pressurized they emit 

significant amounts of noise over the entire length of the vessel. As pressurization occurs, the 

layers rub against each other, as well as weld slag and other impurities, and create noise sources. 

Tens of thousands of events per cycle, at the 32 dBAE threshold, are not unusual. Parameter 

based approaches will not work, and source location will not discriminate possible relevant flaw 

growth events from non-relevant noise in this environment. More sophisticated analysis 

techniques are required to sort these large numbers of events at the 32 dBAE threshold setting. 

In conventional AE around this time, “expert systems” were developed to perform the analysis of 

AE data on structures that did require source discrimination. Dr. Timothy Fowler of the 

Monsanto Corporation approached fiberglass tank failures by creating an acoustic emission 

database of signal parameters from tanks that he had tested, and then visually inspected to 

determine if they were good or bad. AE signal parameters from follow-on tests were compared 

against the information in the database, and then rated the tanks from good to “failed”. This 
16 

approach was highly successful for Monsanto since the database correlated directly to the tanks 

under test, and there were a large number of these tanks. However, using this database to 

analyze data from high pressure vessels would result in the passing of bad vessels, and the failure 

of good vessels. Why? Because the database was developed for atmospheric tanks that had 

large mechanical failure sources, such as mixers, that caused large events. In this case, large 

energy events were bad. If we look at the failure mechanism in high pressure/high strength steel 

vessels, the large amplitude/large energy failure assumption is most definitely not the case unless 

a crack is already at critical size. Pressure vessels in almost all common services fail by fatigue 

crack growth. If one looks at the energy release in this mechanism for the very ductile, tough 

steels in use in these vessels, it is very small in the stable fatigue crack growth regime. In this 

case, small amplitude, high frequency events, with specific wave modes are “bad” events, and 

large amplitude events are typically from events not related to flaw growth, and are usually 

caused by rubbing of supports or fracturing of brittle corrosion products. 

While acoustic emission has always held promise, due to its remote monitoring capabilities, it 

was historically plagued by its inability to sort noise source from true flaw growth. This is due 

to the simplistic nature of many of the analysis approaches that have been taken in this field due 

to the lack of wave propagation knowledge of the practitioners. Blindly applying non-applicable 

criteria will result in false calls. If a database approach is used, the analysis in the database must 

be known and understood in order to for the user to determine whether the analysis is truly 

analyzing the vessel under test with the correct physics. Analysis techniques, when possible, 

must be based on well understood engineering principles, and be able to withstand an 

engineering audit. If not, a false sense of security is created, and the safety goals cannot be met. 

Overview of Modal Acoustic Emission 

Modal AE analysis is based on well documented ultrasonic wave modes that propagate in 

engineering structures [2 - 3]. The analysis is very similar to that of seismology, where primary 

waves (p-waves) and secondary waves (s-waves) are analyzed to determine the source type, 

magnitude and location of earthquakes. In plate-like structures (for purposes of MAE analysis, 

pressure vessels are rolled-up plates) the wave modes of interest are the extensional mode (inplane stretching and compressing of the plate) and the flexural mode (bending of the plate). 

Source Orientation Discrimination 

When a defect (e.g., a crack) grows it has depth (it is not a surface source) into the vessel wall, 

and it releases its energy into the plane of the wall, and creates a propagating extensional mode 

(compression-tension motion). Sources such as corrosion and rust on the exterior surface of the 

vessel, or vessels contacting each other, release the energy out of the plane of the plate, and 

cause the plate to bend (much like striking the head of a drum) as the wave propagates. This 

source type creates large flexural modes. The two modes have very distinct characteristics (see 

the figures below), and can be identified. They also have different propagation velocities that 

can be accurately predicted from acoustic dispersion curves for a given plate. 
17 

Noise Identification and Rejection 

By analyzing mode shapes, the source can be more readily identified with MAE than with 

conventional AE techniques that use qualitative analyses to try to identify sources. This is 

important since this approach has the potential to eliminate the false calls that are so familiar in 

conventional AE testing. 

Low High

Frequency goes from low to high

Figure 18 - Extensional Mode 

High Low

Frequency goes from high to low 

Figure 19 - Flexural Mode 

Theoretical Predictions 

Since Modal AE is based on first principles, theoretical predictions can be made prior to testing 

to guide the tester in analyzing the data. Figures 20 and 21 below show the theoretical 

predictions using DWC’s WavePredictorTM software for the experimental data shown in Figures 

18 and 19 above. 
18 

-0 .015

-0 .01

-0 .005

0

0.005

0.01

0.015

0.02

0 50 100 150

Amplitude

Time, seconds

Figure 20 - Theoretically extensional mode 

Figure 21 - Theoretically predicted flexural mode 

As stated above, Modal AE analyzes the wave modes produced by the source orientation and 

material geometry to detect flaw growth. Conventional AE (e.g., MONPACTM) uses simplified 

parameter-based analyses, based on parameters obtained from resonant sensors such as signal 

duration, amplitude, hit-counts, and energy. These parameters are not unique parameters for a 

given source. As Dr. Goranson, former Chief of Structures at Boeing states in [5], ““Classical” 

AE data consist of various parameters intended to represent the AE waveform. The 

parameters characterizing a given waveform are not necessarily unique to that waveform and 

could be the same for other waves generated by other types of sources.” Thus, it is very 

difficult to characterize various signals according to their source if the parameter approach is 

taken. Furthermore, in conventional AE, the signal (waveform) is not stored and the only 

information the tester is left with is a few parameters stripped from the signal - with no wave 

mode or frequency content information available. Even if the signal were to be stored, the mode 

information is lost due to the resonant sensor and narrowband filtering inherent in these systems. 

Another approach that has come into use, capturing the waves digitally but then categorizing 

them using such classical parameters as amplitude, duration, energy and counts by stripping the 
19 

information from the wave, is also not modal acoustic emission testing or assessment because the 

wave shape and frequency content are still not considered. The approach taken in this testing 

was true Modal AE as described above. 

Section 6. Fatigue Testing of the A. O. Smith Vessel and MAE Results 

MAE Sensor Locations 

Fifteen sensors were arrayed on the vessel. Two array configurations were used, and are shown 

In Figures 22 and 23. At cycle 3292 the sensor from the heads were moved onto the shell to aid 

in source location at the center notch. Appendix A has the date, file name and cycle information 

listed. 

Figure 22 - Sensor locations for cycles 1 - 3291 

Notch locations are shown as red dashes. The rough outline of the unwrapped vessel is shown. 

The top of the vessel is at y = 0, and the bottom of the vessel is at approximately +/-60 inches. 
20 

Figure 23 - Sensor locations for cycles 3292 - 4688 

Notch locations are shown as red dashes. The rough outline of the unwrapped vessel is shown. 

The top of the vessel is at y = 0, and the bottom of the vessel is at approximately +/-60 inches. 

Fracture Mechanics 

Crack Growth Rates 

From NASA Technical Memorandum X-3316, December 1975 [Ref 11], the da/dN curve for the 

A. O. Smith vessel steel was claimed to be represented by the classical Paris equation with the 

following constants: 

Rearranging gives: 

Integrating this equation results in: 

2

0.5 10 3 3 1.5 (0.5)(3.6 10 )( )

1 »

1⁄4

o «

¬

a

 u ' '   a M N

a

i

f V S

Assuming an elliptical crack (Barsom, Ref 6, with M=1, and a flaw depth of 0.175 inches and an 

operating stress of 35.2 ksi gives the following curve, Figure 24, for the crack growth per cycle. 

10 3 (3.6x10 )( K) dN

da ' 

> @ 3

u '

 

f

i

a

a M a

da N 3 10 3.6 10 V S
21 

Figure 24 - da/dN curve based on data from NASA Technical Memorandum X-3316 [Ref 11] 

The crack depth used in this analysis was chosen based on the initial three month test time. 

Assuming 30 cycles per day (based on the rate of pressurization of the pump), this would result 

in approximately 900 cycles per month. Over this time frame (2700 cycles), and an initial crack 

depth of 0.175 inches, the crack would grow approximately 0.055 inches over this time frame. 

Since the referenced 1975 NASA report identified the critical toughness for the shell material to 

be about 80 ksi√inch, which was also approximately the applied ∆K during this test, it was felt 

that enough fracture of the shell layer wall should occur to produce unambiguous MAE events 

above the threshold setting. Subsequent SwRI material testing proved that the actual critical 

fracture toughness was much higher (in the range of 170 ksi√inch, Ref [9], and hence that the 

crack was still in the stable crack growth regime when the test was terminated. Therefore, the 

actual low level emissions captured and evaluated on reanalysis were consistent with results 

from prior experience. 

Crack Growth Sensitivity 

Using the analysis outlined in Appendix B, it was calculated that the crack growth per cycle, due 

to the cyclic fatigue loading, would produce events of sufficient energy to be detected for the 

given sensor spacing. However, this assessment was based on elastic energy release for lower 

toughness material rather than elastic-plastic as was determined to be the actual case, and the 

Appendix B assessment was not shown to be applicable in the data obtained. Further work is 

needed in this area. 
22 

Pencil Lead Breaks (PLB’s)

Pencil lead breaks (Pentel 2H 0.3 mm) were performed inside each of the notches. This was 

done for several reasons, which are listed below. 

1. To see if all the sensors were coupled and had similar detection sensitivities. 

2. To see if the wave modes were being developed as expected. 

3. To see if the location accuracy of the PLB source could be maintained. 

Wave Mode Formation 

Below is a representative waveform, Figure 25, from a lead break at the center notch. Channel 8 

has been expanded to show the mode formation more clearly. From this the extensional and 

flexural modes can be observed forming, thus the plate wave analysis will be suitable for this 

vessel for source identification. 

Figure 25 - Captured lead break waveform with extensional mode 

The above figure shows a waveform from a lead break at the center notch. The wave modes are 

labeled. It can be seen that they have the characteristics shown in Figures 18 and 19. 

The wave modes have specific frequency-velocity relationships based on the material and 

thickness of the plate. Figure 26 shows this relationship in the form of a dispersion curve for the 

A.O. Smith pressure vessel used in this testing (see reference 5) for more information on 

dispersion curves). It can be seen that at the lower frequencies, the extensional mode propagates 

at a higher velocity than the flexural mode. That is why the extensional mode arrives first in the 

waveforms. When performing source location, it is important to know which wave modes are 

being generated, so that the correct velocity is used in the source location algorithms. 

Extensional Mode 

Flexural Mode 
23 

Figure 26 - Extensional and Flexural mode dispersion curves for 3.125 inch thick steel plate 

In this figure, the 21 kHz location for the extensional mode is marked. The extensional mode 

velocity at this point is approximately 4400 m/s. The dispersion curve plot was generated by the 

WaveExplorerTM software. 

PLB Location Accuracy 

The accuracy of the 2-D source location algorithms were checked using the lead breaks in the 

notches. Since the waveforms have good extensional mode formation, the extensional mode 

velocity was used for the source location. The software calculates the differences in arrival times 

and from those can determine the location of the source. From Figures 27 and 28 it can be seen 

that both sensor arrays resulted in source location well within six inches of the lead break 

location. This provides adequate accuracy for follow up inspections, if needed. 
24 

Figure 27 - Lead break source locations on the test vessel with head sensors 

Lead break locations as found with the WaveExplorerTM software for lead breaks in the three 

notches machined into the vessel are shown in the above figure. The blue circles are sensor 

locations, the red lines are the flaw locations, and the yellow diamonds are the lead break 

locations. 

Figure 28 - Lead break source locations on the test vessel without head sensors 

Lead break locations as found with the WaveExplorerTM software for lead breaks in the three 

notches machined into the vessel, without AE sensors located on the heads. The blue circles are 

sensor locations, the red lines are the flaw locations, and the yellow diamonds are the lead break 

locations. 
25 

Test Results 

The table in Appendix A shows the number of cycles and the pressure range over which the 

cycles were performed. The data files from each test are shown as well. The comments to the 

right were made during the testing so that any changes in sensor locations or test parameters 

could be noted. 

The vessel and the notches at the welds were remotely monitored using digital video cameras, 

Figure 29. The center notch had a video microscope mounted above it to monitor the crack at 

that notch. A pan and zoom video camera was mounted in the test bunker to monitor the pump 

and test fixtures so that entry into the bunker during pressurizations was not required. 

 

Figure 29 - Vessel with Video Microscope Images 

This array of photographs, starting in the upper left corner and proceeding clockwise shows; the 

vessel with sensors attached (magnetically held in place) and the video cameras monitoring the 

notches at the welds; the video microscope mounted from the ceiling of the bunker monitoring 

the crack in the center flaw; a picture of the monitor for the video microscope showing the crack 

in the center flaw at 14,000 psi; and a picture of a pair of calipers held under the video 

microscope, opened to 0.005”. From this it can be seen that the crack opened slightly under 

0.005” at 14,000 psi.

The WaveExplorerTM system was set to the parameters outlined in Section 3, and the vessel was 

monitored during the cycles. The acquisition settings on the WaveExplorerTM system were set 

conservatively. This resulted in large data sets, but since the data was digitized and stored, it was 

0.005”

Crack Opening 

Video Microscope 

Video Cameras 
26 

felt that it was better to acquire extra data and not miss flaw growth events, than to limit the data 

acquisition and possibly miss crack growth. 

Along with the modal AE data, pressure, strain and temperature data were acquired and stored. 

After the data was digitized and stored, it was then sorted using algorithms that analyzed the 

number of channels that crossed the 32 dBAE threshold, saturation of the signals, energy level of 

the signals, pretrigger noise, and frequency content. A short explanation of each filtering step is 

given below. 

Threshold: This was set so that at least three channels had to cross the threshold. This was 

required so that the 2-D source location could be performed. This insured that the source of the 

event was the notches. 

Energy: Small amplitude signals will cross the threshold settings, but still not have enough wave 

mode definition to be accurately identified as noise or flaw growth signals. This setting will 

eliminate signals that cannot be properly identified via modal content. 

Saturation: If the signals are so large that they saturate the analog amplifiers or the A/D, then 

there is no useful information in these signals. This filter removes these signals from the data 

set. As a note, fatigue crack growth signals during stable fatigue crack growth are typically 

small amplitude (see Appendix B), while noise sources, such as mechanical rubbing of saddles, 

surface corrosion, weld slag, etc., are much larger energy sources. Thus, when the gain is set to 

the ASTM E1419 standard, noise sources are so large they saturate the system amplifiers. 

Pretrigger Noise: If the front end of the signal is not clearly defined, either due to flow noise or 

mechanical noise, the signal cannot be analyzed for modal characteristics, nor can the location of 

the source be determined. These signals are removed. 

Frequency Content: Flaw growth signals contain higher frequency content, due to the short rise 

time of the fracture source. Noise signals do not have this characteristic. Signals not meeting 

the flaw growth signal frequency requirements are removed. 

The data files were first analyzed manually until flaw growth data from the center crack location 

was observed. This data was then used to set the filtering parameters described above. 

Appendix C contains the filter settings. The results of this filtering resulted in removal of at least 

95% of the waveforms, and in many instances almost 99%. This reduced the number of events 

per file from 50,000 to 100,000 events, to approximately 1000 to 5000 events. The remaining 

events were then manually reviewed for modal characteristics to determine if they were from 

flaw growth. 

Shown in Figure 30 is a waveform from earlier in the testing that was located at the center notch. 

Prior experience has shown that heuristically the approximate first arriving frequency in the 

extensional mode can be estimated using O=c/f, where O the wall thickness multiplied by four 

(4), c is the extensional mode propagation velocity, and f is the frequency. Assuming a wall 

thickness of approximately 3 inches (recall that the plate layers are tightly coupled acoustically) 

and an extensional mode velocity of 185,000 in/s (as measured from lead breaks), results in a 

frequency of 15.4 kHz. It should be noted that this is a quick “rule-of-thumb” approach, based 
27 

on data from wave propagation measurements in varying thicknesses of steel and aluminum 

plates and then manually measuring the frequency of the first cycle of the extensional mode, and 

then applying a “fit” to the data, not waveform theory (see dispersion curve discussion in Section 

6, Wave mode formation). Variations could occur based on the frequency content of the source, 

or the source orientation. With this said, comparing this 15.4 kHz estimated frequency to the 

measured frequency of 21 kHz (see Fig. 30 below, which correlates exactly with the dispersion 

curve calculation shown in Fig. 26), shows the usefulness of the simplified method, and proves 

that the wave was propagating through the entire wall of the vessel, not just in the outer layer. If 

the wave were propagating only in the outer layer, the frequency of the extensional mode would 

have been much higher. As a final note, plate waves can take anywhere from 30 to 100 plate 

thicknesses to fully form (reference 4, Weaver & Pao), which for this vessel would be 90 to 300 

inches from the source, although wave attenuation effects at the longer distances make detection 

problematic, particularly low energy signals in multilayer and thick single vessels. Sensor 15 

was approximately 60 inches from the source, so full mode formation may not have occurred, 

thus the frequency may not be entirely representative. There were few of these events observed 

in the data, and the reasons for this are discussed later in the paper.

Figure 30 - Crack growth event plots, cycles 2462-2509 

For the events shown above, the simultaneous arrival at sensors 8 and 9 are the quick indicators 

of the location. This event occurred at a pressure of 6933 psi, near the peak pressure of the 

cycle, 7260 psi. Channel 15 has been expanded to show the front end frequency of the 

waveform. This was measured at 21 kHz. 

Other events with source locations at the center notch were also seen in the data, with front end 

frequencies of the waveforms in the range of 30-40 kHz, slightly higher than the expected 

frequency of 15.4 kHz for this thickness of vessel. These may be examples of how the source 
28 

function (crack growth) may not contain the sufficient frequency content due to non-brittle 

fracture, as discussed later. 

These waveforms were also useful in that they showed that the frequency content of the crack 

growth signals resulted in greater frequency content over 200 kHz, than just noise signals, Figure 

31. This information was used to filter the data by looking at the percentage of energy in the 

signals over 200 kHz, and eliminating signals that did not have enough content in this frequency 

range. See Figure 32 for a typical noise signal Fast Fourier Transform (FFT). 

Figure 31 - FFT of crack growth signal 

Note the frequencies above 200 kHz in channels 8 and 9. This frequency is an indicator of possible crack growth. 
29 

Figure 32 - Typical FFT of noise. 

Note the lack of higher frequency content in the noise signal. 

The final portion of the testing (completion was forced due to budget limitations) was an overpressure of the vessel to 14,000 psi, which was intended to put the outer layer near nominal yield 

stress to force crack growth since it was uncertain at the time how deep the visible crack was. 

The working pressure of the vessel was 6600 psi, with a factor of safety of 2 on nominal yield 

stress. By pressuring to 14,000 psi, the vessel was pressurized to 2.12 times working pressure, 

6% over its factor of safety design pressure. Many safety precautions were taken throughout this 

testing, and four of these cycles were completed without incident before the last remaining 

hydraulic pump failed and the test was terminated after consultation with NASA. 

It was also determined that pressurizing to this level did put the vessel outer wall into some 

plastic deformation (see Figures 14, 15 and 16 showing the Pressure-Strain curves), and thus 

plastically deformed the notch. This was done in order to see if a large enough energy release 

could be obtained for crack growth detection. During this pressurization, there were several 

small events from the notch that were detected. However, there was one larger energy event, 

with mode formation, that was captured, and is shown in Figure 33 (an expanded view of 

channel 8 is shown in Figure 34 showing the wave mode details). This particular event, while 

emanating from the notch, did not meet the extensional mode front end frequency calculation 

described earlier, of 15.4 kHz. In hindsight, this is not unexpected since the source for this 

waveform would definitely not have been brittle fracture. Thus, most likely the source did not 

contain the low frequency components that would form the front end of the extension mode. 
30 

Figure 33 - Crack growth signal at 13.710 psi 

The crack growth signal during over-pressurization shown above occurred at a pressure of 

13,710 psi. Note the nearly simultaneous arrival times at sensors 2, 8, 9 and 14. This would be 

observed for flaw growth from the center notch, for the sensor arrangement in Figure 28. 

Figure 34 - Expanded view of Channel 8 showing the extensional mode formation 

The trailing wake in the event shown above is due to waves propagating around the vessel. The 

x-axis divisions are 100 Pseconds. 

Section 7. Discussion of Test Results 

In the testing of the vessel, the crack growth did not result in brittle fracture of the fatigue crack. This is discussed in 

the following section. 

In determining the depth of the notch, the main driver was the amount of time available for the testing. The original 

time frame for the fatigue testing was a three month period. The Haskel pump that was used for the pressurization 

took approximately 45-60 minutes for each cycle. That meant approximately 30 cycles per day, accounting for 

maintenance and repair down time. Based on these constraints, it was decided to machine the notches in the outer 

Extensional Mode 

Flexural Mode 
31 

wrap to a depth of 0.175 inches, which from the facture mechanics should result in approximately 0.055 inches of 

crack growth over the three month period, Figure 24. It was felt that this was a reasonable amount of crack growth 

over this shortened time frame to capture fatigue crack growth data, without the crack going through-wall. 

However, the MAE data from the cyclic fatigue testing was not as definitive as hoped. Except 

for a few events, the wave propagation characteristics were not well defined. After review of the 

notch geometry, it may have been machined too deep in an attempt to insure that the crack grew 

in the first few hundred cycles. At the 0.175 inch depth, the stress at the notch would have been 

in the range of 100 ksi. This would have been beyond the elastic limit of the steel, per NASA 

TM X-3316 [Ref. 11] measurements. Fracture of the notch would have not been brittle fracture 

(plane strain), but plastic fracture. Brittle fracture releases the energy in a much shorter time 

frame, creating the frequency content needed for clear mode formation. This can be thought of 

as the difference between a dry twig and a green twig fracturing. The dry twig snaps with a 

sharp, loud sound and a clean break. The green twig bends, deforms, and then fails over multiple 

surfaces, with little sound. If the fatigue fracture was elastic-plastic in the vessel, then the energy 

release was either out-of-plane or too slow, resulting in poor mode formation and energy release 

below the theoretical calculated sensitivity as shown in Appendix B, which assumes brittle 

fracture. 

A photograph of the crack surface that was obtained by Southwest Research Institute on 

subcontract as reported in Ref. 7 is shown in Figure 35. The various facture surfaces are shown 

in the photograph. 

Figure 35 - Photograph of One Face of the As-Opened Crack (credit SwRI, Ref 7) 

The photograph shows the cross section of the notch after the layer was fractured open. The dark gray area is the 

machined notch, the lighter gray area underneath the notch is the fatigue crack, the darker area under that is the rapid 

fracture area due to the overload near the end of the test (Ref 7, Southwest Research Institute). 

Machined Notch 

Fatigue Crack Growth 

Rapid Fracture Crack Growth Due to Over 

Pressure 
32 

The elastic-plastic analysis is supported by the follow-on fracture mechanics and SEM images of 

the fracture surface (Ref. 7, SwRI). The photographs show both transgranular and a significant 

amount of intergranular fracture surfaces, along with secondary (out of plane) cracking and 

dimpled cup and cone associated with ductile fracture. Since transgranular failure is associated 

with brittle fracture, while intergranular is normally associated with ductile (overload) failure 

(Janssen, et. al., Fracture Mechanics, Ref. 8), it is concluded that the loading for the vessel at the 

notch was in the elastic-plastic regime, which was borne out by their assessment that the actual 

stress intensity factor at the crack (K1) during these cycles was 85.6 ksi√inch, nearly equal to 

the average plane stress fracture toughness (Kc) of 90 ksi√inch obtained in their material testing 

(and which was revised upward to 170 ksi√inch in their Phase 2 report, Ref [9] . This likely lead 

to the sensitivity and mode formation loss observed in the waveform data due to the out-of-plane 

tearing of the material, as opposed to the transgranular in-plane brittle fracturing. 

Prior DOT and ASME Pressure Vessel Testing 

Digital Wave Corporation has performed fatigue crack growth testing on high strength steels used for DOT and 

ASME specification pressure vessels that operate at high (1800-5000 psi) pressures. This testing has shown that 

shown that brittle fatigue results in waveforms with definite modes in these monolithic (non-layered) vessels. 

A comparison of the data from the A.O. Smith pressure vessel to the data captured during the 

testing phase to develop an ASME Code Case (Case 2390-5, Composite Reinforced Pressure 

Vessels, Section VIII, Division 3, now incorporated in KF-1216 and KE-5 of Sect. VIII, Div. 3) 

will be shown in this section. This Code Case is for the use of acoustic emission for the 

certification of the laminate, but the steel liner was also monitored during the test. The vessels 

for this Code Case were Type 2 construction vessels, with a one inch thick steel liner and a one 

inch thick fiberglass composite overwrap, with exposed domes. During the cyclic testing of the 

vessels, sensors were mounted on the exposed steel domes to determine if modal acoustic 

emission could detect fatigue cracking in the long seam weld of the steel liner. Channels 1-4 

where on one dome, channels 5-8 were on the opposite dome. The vessel was 20 feet long. In 

one of the test vessels, an interior shelf, or lip, occurred in the weld, and fatigue cracking began 

to occur at this location. Modal AE was able to discriminate between the cracking occurring in 

the composite from that in the weld. Figure 36 shows a crack growth signals from the long seam 

weld. The same gain settings were used for this testing as for the A. O. Smith pressure vessel. 

The crack growth signals captured from the fatigue crack growth in these vessels were similar in 

energy to the signals captured during the A. O. Smith pressure vessels tests. 
33 

Figure 36 - MAE monitoring of a Type 2 COPV with 1 inch liner vessel 

The above figure shows fatigue crack growth waveforms from a long seam weld in a type 2 

vessel. Channel 3 has been expanded to show the mode formation. 

The frequency content of the signals is shown in Figure 37. From this it can be seen that the 

crack growth signals from this vessel contain frequencies above 200 kHz, just like those from the 

A. O. Smith pressure vessel. Thus, this discriminator, along with extensional mode formation, 

can be used to filter data in high strength steel pressure vessels. 
34 

Figure 37 - FFT of the signals in Figure 36 

This figure shows the FFT (frequency content) of the signals in Figure 36. The x axis is 

frequency, in kHz, and the y axis is amplitude. The main feature to note is the signal content 

over 200 kHz. This corresponds closely with the data from the fatigue testing of the A. O. Smith 

pressure vessel. 

Section 8. Conclusions 

The following are the principal conclusions from the MAE cyclic crack growth testing. 

1. Crack growth was detected – Crack growth signals were captured from the center notch, 

and identified via frequency content and source location. There were not many 

detectable events, which was likely due to elastic-plastic fracture of the steel. Due to 

time and budget limitations, and the amount of data captured during the testing, crack 

growth data from the other notches were not analyzed. 

2. Wave Modes – Extensional mode formation was observed in some of the events, but not 

in others. This was likely an indication that the failure mode of the vessel was not brittle 

fracture, but elastic-plastic fracture. This would change the expected frequency content 

of the signal, as well as the mode shape, since the source would not be in-plane with the 

large extensional modes. 

3. Frequency content can be used as discriminator/filter – One of the main results to come 

out of this test is the frequency content of the crack signals. Crack signals showed higher 

frequency content above 200 kHz. This aided greatly in removing noise signals from the 

data, reducing data by 95-99%. This reduction then allows manual discrimination of the 

remaining data based on wave mode formation. 

4. Data matches that from other pressure vessel fatigue tests – The crack growth data was 

similar to that from fatigue tests on DOT ad ASME high strength pressure vessels. 
35 

5. Crack growth was elastic-plastic – The SEM photographs of the fracture surfaces 

provided by SwRI in Ref. 7 clearly showed the failure of the steel was in the elasticplastic zone (e.g., intermittent and even infrequent striations, and clear physical 

indications of plastic tearing), and thus the expected brittle fracture behavior of the vessel 

(due to its relatively high tensile strength below ambient test temperatures) was not 

achieved during this testing. 

6. Data Analysis – From this testing, the difficulties in sorting crack growth signals from 

noise are clearly evident. Due to the construction of the vessels, noise sources are 

abundant and varied. The ability to detect these sources requires detailed knowledge of 

the fracture mechanics and the resulting wave propagation due to the fracture of the 

material. Simple analysis routines based on gross waveform parameters cannot be used 

to analyze these complex waveforms. Extensive post-test analysis by the investigator 

showed that filters could be useful in further refinement of data for identification of a 

relevant source, but this would likely need to be validated in further testing to gain 

general acceptance. 

7. Correlation with physical results – The SwRI data [Ref 7 ]shows that crack grew 

downward, through the thickness, but not lengthwise, and that the average depth 

extensions (da’s) per cycle (dN’s) were in the range of 7.2E-6 (early growth) to 1.8E-5 

inches per cycle near the end of the test. Cumulatively, about 0.064 inches of growth was 

achieved with the pressure cycling prior to the rapid growth with the yield stress level 

cycles at the end. 

8. It was widely expected by all involved that this would be an easy crack to locate and 

characterize with the MAE method, but for the reasons discussed, it proved to be 

possible, but very difficult. 

Section 9. Follow-on Testing 

The following are logical objectives for future testing, if possible. 

1. Review of data from the other notches - existing data from the other notches (near each 

head) have not been reviewed due to funding and schedule constraints. A first step would 

be to analyze the data from these stress risers. 

2. Stress analysis of head-to-shell weld - The most likely point of failure for these vessels 

based on industry experience is the failure of the head-to-shell weld, and then the head 

blowing off during pressurization, or nozzle failures. Notches were not machined in the 

head-to-shell welds or nozzles because the stress was unknown in this region (due to 

unknown weld residual stresses) and because the lower level of cyclic stress in the 

longitudinal direction would likely have required many more cycles to grow a crack 

significantly, and there was insufficient funding to support that amount of cycling. 

Because of this, it was not felt that a reasonable machined notch depth could be 

determined, and this could lead to either catastrophic failure or no crack growth at all. 

Either result was not acceptable for the test objectives. Thus, a thorough stress analysis 

would be very useful for determining the required parameters of a machined 

circumferential flaw that would be safe and achieve growth in a reasonable number of 

cycles. This approach will need to incorporate any weld geometry variations observed in 

these vessels, as well as welding variabilities. 
36 

3. Determine notch depth and shape for circumferential notch in head-to-shell weld at heat 

affected zone (HAZ) – Once the stress analysis is performed, then a notch depth that 

would still result in a 5-10 year life at the cyclic intervals of pressurization could be 

machined into the HAZ at the head-to-shell weld. This would result in crack growth 

more typical of the failures expected in the A. O. Smith pressure vessels. Then perform 

additional cyclic fatigue testing and MAE – Repeat the test program, but with the headto-shell weld notches. Such notches are more representative of flaws that what would 

likely exist in these vessels, and they would be in thick material with a much lower 

applied delta stress intensity factor. Consequently, their crack growth would be in the 

brittle crack growth regime, with extensional wave formation that would be more readily 

detectable by the MAE method. 

4. Determine notch size for elastic fracture in parent material in outer wrap – If failure in the 

parent material in the outer wrap is of interest (since individual layer failure can be 

tolerated, and inner layer cracking can be detected with leak monitoring, this question 

should be considered), then a smaller EDM notch depth for earlier-life elastic fracture 

should be calculated and machined in the vessel so that a more likely, realistic correct 

crack growth acoustic signal is developed. Include this notch in the repeat test described 

above. 

5. Perform laboratory testing on samples from wrap layers from A. O. Smith vessel –

Fatigue test specimens from vessel MV50466-8 could be machined and fatigue tested in 

the lab using a servo-hydraulic test machine. These specimens could be instrumented 

with MAE sensors, and crack growth monitored during rapid cycling to determine fatigue 

crack growth energies with direct correlation to captured MAE waveforms and FFT 

assessments. 
37 

Section 10. References 

1 Blackburn, P.R., Rana, M.D., “Acoustic Emission Testing and Structural Evaluation of 

Seamless, Steel Tubes in Compressed Gas Service,” Journal of Pressure Vessel 

Technology, May 1986, vol. 108, pp. 234-240. 

2 Gorman, M.R., “Plate Wave Acoustic Emission,” Journal of the Acoustical Society of 

America, 1991, vol. 90, no. 1, pp. 358-365. 

3 Gorman, M.R., Prosser, W.H., “AE Source Orientation by Plate Wave Analysis,” Journal 

of Acoustic Emission, 1991, vol. 9, no. 4, 283-288. 

4 Weaver, R. L., Pao, Y.H., Axisymmetric Elastic Waves Excited by a Point Source in Plate, 

Journal of Applied Mechanics, December, 1982, Vol. 49, pp. 821-836

5 Graff, K. F., “Wave Motion in Elastic Solids,” Dover Publications, 1975.

6 Barsom, J.M., Rolfe, S. T., “Fracture and Fatigue Control in Structures, Applications of 

Fracture Mechanics,” Second Edition, Prentice-Hall, 1987, p. 48 

7 Cardinal, J.W., Popelar, C. F., and Page, R.A., Revised Final Report, Southwest Research 

Institute (SwRI) Project No. 17408 “Multilayer Pressure Vessel Materials Testing and 

Analysis (Phase I)”, Cardinal, Joseph W, Revision 1, November 6, 2012

8 Janssen, Zuidema, Wanhill, Fracture Mechanics, VSSD, Second Edition, 2002-2006, pp. 

286, 294-295 

9 Cardinal, J.W. and Popelar, C. F., Multilayer Pressure Vessel Materials Testing and 

Analysis (Phase 2), Southwest Research Institute, SwRI® Project No. 18.17633, Final 

Report (Rev 01), September 2013 

10 Hudak, Connolly, Hanley & Lukezich, A Damage Tolerance Approach for 

Recertification of Submarine High-Pressure Flasks, SwRI Project No. 06-4557, 

Department of the Air Force, Sacramento Air Logistics Center, Contractual Engineering 

Task #TIE-91-335, Southwest Research Institute, March 31, 1995 

11 Hudson, M.C., Newman, J.C., & Lewis, P.E., An Investigation of Fracture Toughness, 

Fatigue-crack Growth, Sustained Load Flaw Growth, and Impact Properties of Three 

Pressure Vessel Steels, NASA Technical Memorandum X-3315, Langley Research 

Center, December, 1975 

12 DOT SP-15322 – Digital Wave Corp, Use of Modal Acoustic Emission and ultrasonic 

examination (AE/UE) in lieu of hydrostatic pressure test and internal visual examination, 

April 26, 2012. 

13 Ziola, S. M., AE Qualification Test of HPADS Storage Vessel #16 at N250A, Ames 

Research Center, Moffett Field, CA, Digital Wave Corp., 10/10/2001 

14 Ziola, S. M., AE Qualification Test of HPADS Storage Vessels 21 - 36 at N250A, Ames 

Research Center, Moffett Field, CA, Digital Wave Corp., 10/29/2002 

15 Ziola, S. M., AE Qualification Test of HPADS Storage Vessels 1 - 16 at N250A, Ames 

Research Center, Moffett Field, CA, Digital Wave Corp., 8/12/2009 

16 ASTM E-1419 – 02b – Standard Test Method for Examination of Seamless, Gas Filled 

Pressure Vessels using Acoustic Emission 
38 

Appendix A. Table of Test Files 

Total 

Number 

of 

Cycles Date 

Cycles 

per 

File Filename Notes 

10 9/23/11 10 aosmith MV50466-8 VH cycles 1-10.Wave 3000-6600 psi 

15 9/26/11 5 aosmith MV50466-8 VH cycles 11-15.Wave 3000-6600 psi 

19 9/26/11 4 aosmith MV50466-8 VH cycles 16-19.Wave 3000-6600 psi 

29 9/26/11 10 aosmith MV50466-8 VH cycles 20-29.Wave 3000-6600 psi 

31 9/26/11 2 aosmith MV50466-8 VH cycles 30.Wave 3000-6600 psi 

40 9/27/11 9 aosmith MV50466-8 VH cycles 31-40.Wave 3000-6600 psi 

45 9/27/11 5 aosmith MV50466-8 VH cycles 41-45.Wave 3000-6600 psi 

65 9/27/11 20 aosmith MV50466-8 VH cycles 46-65.Wave 3000-6600 psi 

95 9/28/11 30 aosmith MV50466-8 VH cycles 66-95.Wave 3000-6600 psi, lead breaks at each notch 

114 9/29/11 19 aosmith MV50466-8 VH cycles 96-114.Wave 3000-6600 psi 

131 9/29/11 17 aosmith MV50466-8 VH cycles 115-131.Wave 3000-6600 psi 

144 9/30/11 13 aosmith MV50466-8 VH cycles 132-144.Wave 3000-6600 psi 

146 9/30/11 2 aosmith MV50466-8 VH cycles 145-146.Wave 3000-6600 psi 

157 9/30/11 11 aosmith MV50466-8 VH cycles 147-157.Wave 3000-6600 psi 

196 10/3/11 39 aosmith MV50466-8 VH cycles 158-196.Wave 

3000-6600 psi, 3 cycles ran before AE 

started recording 

200 10/4/11 4 aosmith MV50466-8 VH cycles 197-200.Wave 3000-6600 psi, gauge leaking 

202 10/4/11 2 aosmith MV50466-8 VH cycles 201-202.Wave 3000-6600 psi, changed sensor 3 

219 10/5/11 17 aosmith MV50466-8 VH cycles 203-219.Wave 2500-6600 psi 

245 10/6/11 26 aosmith MV50466-8 VH cycles 220-245.Wave 

2500-6600 psi, one 7000 pressurization 

cycle 

256 10/6/11 11 aosmith MV50466-8 VH cycles 246-256.Wave 

2500-6600 psi, one 7300 pressurization 

cycle 

257 10/6/11 1 aosmith MV50466-8 VH cycles 257-257.Wave 3000-6500 psi 

270 10/7/11 13 aosmith MV50466-8 VH cycles 258-270.Wave 2500-6600 psi 

271 10/8/11 1 aosmith MV50466-8 VH cycles 258-270.Wave 2500-6600 psi 

272 10/8/11 1 aosmith MV50466-8 VH cycles 271-271.Wave 2500-6600 psi, Strain gages lost 

276 10/10/11 4 aosmith MV50466-8 VH cycles 272-275.Wave 2500-6600 psi 

305 10/11/11 29 aosmith MV50466-8 VH cycles 276-305.Wave 500-6600 psi 

330 10/12/11 25 aosmith MV50466-8 VH cycles 306-330.Wave 500-6600 psi 

358 10/12/11 28 aosmith MV50466-8 VH cycles 331-358.Wave 500-6600 psi 

390 10/13/11 32 aosmith MV50466-8 VH cycles 359-390.Wave 500-6600 psi 

413 10/13/11 23 aosmith MV50466-8 VH cycles 391-413.Wave 500-6600 psi 

446 10/14/11 33 aosmith MV50466-8 VH cycles 414-446.Wave 500-6600 psi 

449 10/14/11 3 aosmith MV50466-8 VH cycles 447-449.Wave 500-6600 psi 

542 10/16/11 93 aosmith MV50466-8 VH cycles 450-542.Wave 1000-6600 psi 

580 10/17/11 38 aosmith MV50466-8 VH cycles 543-580.Wave 1000-6600 psi 

602 10/17/11 22 aosmith MV50466-8 VH cycles 581-602.Wave 1000-6600 psi 

634 10/18/11 32 aosmith MV50466-8 VH cycles 603-634.Wave 1000-6600 psi 

646 10/18/11 12 aosmith MV50466-8 VH cycles 635-646.Wave 

1000-6600 psi, Strain gages back on - all 

0.25" rosettes 

691 10/19/11 45 aosmith MV50466-8 VH cycles 647-690.Wave 1000-6600 psi 

754 10/20/11 63 aosmith MV50466-8 VH cycles 691-754.Wave 1000-6600 psi 
39 

804 10/21/11 50 aosmith MV50466-8 VH cycles 755-804.Wave 1000-6600 psi 

844 10/22/11 40 aosmith MV50466-8 VH cycles 805-844.Wave 1000-6600 psi 

869 10/23/11 25 aosmith MV50466-8 VH cycles 845-868.Wave 1000-6600 psi 

914 10/24/11 45 aosmith MV50466-8 VH cycles 869-913.Wave 

1000-6600 psi, check 27048, cycle 42 PTE 

turned off 

959 10/25/11 45 aosmith MV50466-8 VH cycles 914-958.Wave 1000-6600 psi 

1002 10/26/11 43 aosmith MV50466-8 VH cycles 959-1001.Wave 1000-6600 psi 

1047 10/27/201 45 aosmith MV50466-8 VH cycles 1002-1046.Wave 1000-6600 psi 

1097 10/28/11 50 aosmith MV50466-8 VH cycles 1047 - 1097.Wave 1000-6600 psi 

1157 10/29/11 60 aosmith MV50466-8 VH cycles 1098-1157.Wave 

1000-6600 psi, scribed righthand side of 

notch, cycles 40-41, time 65,750 sec. 

1207 10/30/11 50 aosmith MV50466-8 VH cycles 1158-1207.Wave 1000-6600 psi 

1245 10/31/11 38 aosmith MV50466-8 VH cycles 1208 - 1245.Wave 1000-6600 psi, Scribed all notches 

1293 11/1/11 48 aosmith MV50466-8 VH cycles 1246-1293.Wave 1000-6600 psi 

1341 11/2/11 48 aosmith MV50466-8 VH cycles 1294-1341.Wave 1000-6600 psi 

1388 11/3/11 47 aosmith MV50466-8 VH cycles 1342-1388.Wave 1000-6600 psi 

1437 11/4/11 49 aosmith MV50466-8 VH cycles 1389-1437.Wave 1000-6600 psi 

1496 11/5/11 59 aosmith MV50466-8 VH cycles 1438-1495.Wave 1000-6600 psi 

1529 11/6/11 33 aosmith MV50466-8 VH cycles 1496-1528.Wave 1000-6600 psi 

1583 11/7/11 54 aosmith MV50466-8 VH cycles 1529-1583.Wave 1000-6600 psi 

1631 11/8/11 48 aosmith MV50466-8 VH cycles 1584-1631.Wave 1000-6600 psi 

1680 11/9/11 49 aosmith MV50466-8 VH cycles 1632-1680.Wave 1000-6600 psi, lead breaks in notches 

1728 11/10/11 48 aosmith MV50466-8 VH cycles 1681-1728.Wave 1000-6600 psi, lead breaks in notches 

1776 11/11/11 48 aosmith MV50466-8 VH cycles 1729-1776.Wave 1000-6600 psi 

1797 11/12/11 21 aosmith MV50466-8 VH cycles 1777-1797.Wave 1000-6600 psi 

1833 11/13/11 36 aosmith MV50466-8 VH cycles 1798-1832.Wave 1000-6600 psi 

1881 11/14/11 48 aosmith MV50466-8 VH cycles 1833-1880.Wave 1000-6600 psi 

1929 11/15/11 48 aosmith MV50466-8 VH cycles 1881-1929.Wave 1000-6600 psi 

1978 11/16/11 49 aosmith MV50466-8 VH cycles 1930-1978.Wave 1000-6600 psi 

2027 11/17/11 49 aosmith MV50466-8 VH cycles 1979-2027.Wave 1000-6600 psi 

2074 11/18/11 47 aosmith MV50466-8 VH cycles 2028-2074.Wave 1000-6600 psi, run waveutility 

2140 11/20/11 66 aosmith MV50466-8 VH cycles 2075-2140.Wave 1000-6600 psi, notches polished 

2192 11/21/11 52 aosmith MV50466-8 VH cycles 2141-2192.Wave 1000-6600 pis 

2240 11/22/11 48 aosmith MV50466-8 VH cycles 2193-2240.Wave 1000-6600 pis 

2279 11/23/11 39 aosmith MV50466-8 VH cycles 2241-2279.Wave 1000-6600 pis 

2329 11/29/11 50 aosmith MV50466-8 VH cycles 2280-2328.Wave 1000-6600 pis 

2368 11/30/11 39 aosmith MV50466-8 VH cycles 2329-2367.Wave 

1000-6600 psi, replaced release valve, flow 

adjusted 

2371 11/30/11 3 aosmith MV50466-8 VH cycles 2368-2370.Wave 

pressurized to 7260, PTE filter off over 

6200 psi 

2418 12/1/11 47 aosmith MV50466-8 VH cycles 2371-2418.Wave 1000-7260 psi 

2461 12/2/11 43 aosmith MV50466-8 VH cycles 2419-2461.Wave 1000-7260 psi 

2510 12/3/11 49 aosmith MV50466-8 VH cycles 2462-2509.Wave 1000-7260 psi 

2547 12/4/11 37 aosmith MV50466-8 VH cycles 2510-2546.Wave computer locked up, need file fixed 

2594 12/5/11 47 aosmith MV50466-8 VH cycles 2547-2594.Wave 1000-7260 psi 

2642 12/6/11 48 aosmith MV50466-8 VH cycles 2595-2642.Wave 1000-7260 psi 

2687 12/7/11 45 aosmith MV50466-8 VH cycles 2643-2687.Wave 1000-7260 psi 

2734 12/8/11 47 aosmith MV50466-8 VH cycles 2688-2734.Wave 

hard disk full, WE locked up, no idea for 

number of cycles 
40 

2771 12/9/11 37 aosmith MV50466-8 VH cycles 2735-2771.Wave 1000-7260 psi 

2808 12/10/11 37 12-10-2011 37 cycles.Wave 1000-7260 psi 

2856 12/11/11 48 12-11-2011 48 cycles.Wave 1000-7260 psi 

2906 12/12/11 50 12-12-2011 50 cycles.Wave 1000-7260 psi 

2957 12/13/11 51 12-13-2011 51 cycles.Wave 1000-7260 psi 

3003 12/14/11 46 12-14-2011 46 cycles.Wave 1000-7260 psi 

3049 12/15/11 46 12-15-2011 46 cycles.Wave 1000-7260 psi 

3092 12/16/11 43 12-16-2011 43 cycles.Wave 1000-7260 psi 

3137 12/17/11 45 12-17-2011 45 cycles.Wave 1000-7260 psi 

3183 12/18/11 46 12-18-2011 46 cycles.Wave 1000-7260 psi 

3210 12/19/11 27 12-19-2011 27 cycles.Wave 1000-7260 psi 

3211 12/19/11 1 12-19-2011 1 cycle 7920 psi.Wave 0-7920 psi 

3253 12/20/11 42 12-20-2011 42 cycles.Wave 1000-7920 psi 

3292 12/21/11 39 12-21-2011 39 cycles.Wave 1000-7920 psi, new sensor locations 

3328 12/22/11 36 12-22-2011 36 cycles.Wave 1000-7920 psi 

3363 12/23/11 35 12-23-2011 35 cycles.Wave 1000-7920 psi 

3402 12/24/11 39 12-24-2011 39 cycles.Wave 1000-7920 psi 

3448 12/25/11 46 12-25-2011 46 cycles.Wave 1000-7920 psi 

3494 12/26/11 46 12-26-2011 46 cycles.Wave 1000-7920 psi 

3520 12/27/11 26 12-27-2011 26 cycles.Wave 1000-7920 psi 

3551 12/28/11 31 12-28-2011 26 cycles.Wave 1000-8400 psi 

3584 12/29/11 33 12-29-2011 26 cycles.Wave 1000-8400 psi 

3615 12/30/11 31 12-30-2011 26 cycles.Wave 1000-8400 psi 

3646 12/31/11 31 12-31-2011 26 cycles.Wave 1000-8400 psi 

3673 1/1/12 27 1-1-2012 27 cycles.Wave 1000-8400 psi 

3699 1/2/12 26 1-2-2012 26 cycles.Wave 1000-8400 psi 

3728 1/3/12 29 1-3-2012 29 cycles.Wave 100-8400 psi 

3756 1/4/12 28 1-4-2012 28 cycles.Wave 1000-8400 psi 

3782 1/5/12 26 1-5-2012 26 cycles.Wave 1000-8400 psi 

3804 1/6/12 22 1-6-2012 22 cycles.Wave 

1000-8400 psi. Drilled holes in long seam 

weld and in outer wrap 

3832 1/7/12 28 1-7-2012 28 cycles 1000-8400 psi 

3858 1/8/12 26 1-8-2012 26 cycles 1000-8400 psi 

3884 1/9/12 26 1-9-2012 26 cycles 

1000-8400 psi. Drilled starter holes/scored 

holes in long seam weld and wrap 

3910 1/10/12 26 1-10-2012 26 cycles 1000-8400 psi 

3936 1/11/12 26 1-11-2012 26 cycles 1000-8400 psi 

3951 1/12/12 15 1-12-2012 15 cycles 1000-8400 psi. Pump died 

3994 1/20/12 43 1-20-2012 43 cycles 1000-6600 psi cycles 

4046 1/21/12 52 1-21-2012 52 cycles 1000-8400 psi cycles from now on 

4100 1/22/12 54 1-22-2012 54 cycles 1000-8400 psi 

4144 1/23/12 44 1-23-2012 44 cycles 1000-8400 psi 

4187 1/24/12 43 1-24-2012 43 cycles 1000-8400 psi 

4231 1/25/2012 44 1-25-2012 44 cycles 1000-8400 psi 

4238 1/26/2012 7 1-26-2012 7 cycles 1000-8400 psi. Pump died 

4286 1/27/2012 48 1-27-2012 48 cycles 1000-8400 psi 

4342 1/28/2012 56 1-28-2012 56 cycles 1000-8400 psi 

4395 1/30/2012 53 1-30-2012 53 cycles 1000-8400 psi 
41 

4431 1/31/2012 36 1-31-2012 36 cycles 1000-8400 psi 

4486 2/2/2012 55 2-2-2012 55 cycles 1000-8400 psi 

4530 2/3/2012 44 2-3-2012 44 cycles 1000-8400 psi 

4535 2/4/2012 5 2-5-2012 5 cycles 1000-8400 psi 

4548 2/21/2012 13 2-21-2012 13 cycles new pump, 2000-8400 psi cycles 

4560 2/22/2012 12 2-22-2012 12 cycles 2000-9500 psi cycles 

4561 2/22/2012 1 2-22-2012 1 cycles 

first 10,000 psi cycle - strain not turned on 

in pressure software so stopped test 

4573 2/23/2012 12 2-23-2012 23 cycles 2000-10,000 psi cycles 

4583 2/24/2012 10 2-24-2012 10 cycles 2000-10,000 psi cycles 

4603 2/26/2012 20 2-26-2012 20 cycles 2000-10,000 psi cycles 

4617 2/27/2012 14 2-27-2012 14 cycles 2000-10,000 psi cycles 

4639 3/3/2012 22 3-3-2012 22 cycles 

2000-10,000 psi. Pressure transducer 

changed from 10,000 psi/10 v to 30,000 

psi/10 v, WE software not changed 

4661 3/5/2012 22 3-5-2012 22 cycles 

4662 3/5/2012 1 3-5-2012 1 cycles 

14,000 psi run. Correction in We software 

for voltage conversion made 

4683 3/7/2012 21 3-7-2012 21 cycles 2000-10,000 psi cycles 

4688 3/8/2012 5 3-8-2012 5 cycles 6000-14,000 psi cycles 
42 

Appendix B. Modal Acoustic Emission Detection Sensitivity 

Theoretical Calculation of Modal Acoustic Emission Sensor Spacing Using 

Fracture Mechanics 

Since the early 1980’s, acoustic emission (AE) has been used for the examination of seamless 

steel tubes used in tube trailers (high strength modified AISI 4130 and 4140). Blackburn and 

Rana [1] used fracture mechanics to determine the maximum permissible existing flaw sizes that 

were required to be found by AE detection and ultrasonic inspection (UT) measurement based on 

the mandatory DOT 5-year inspection interval to ensure that an existing flaw would not grow to 

critical size between inspections. Based on this work, an ASTM standard, E 1419, Standard Test 

Method for Examination of Seamless, Gas-Filled, Pressure Vessels using Acoustic Emission, was 

written that outlines the steps in examining seamless tubes using the AE equipment that was 

commercially available at the time. 

While this work gave a fracture mechanics basis for the flaw size detectability requirement so 

that a flaw would not grow to a critical size before the specified examination interval, it gave no 

guidance on the AE detectability, e.g., sensor spacing and energy release (amount of incremental 

crack extension area) that could be detected for a given material. It gave a standard sensor 

placement, instrument settings and sensor response which were based on empirical knowledge at 

the time, and correlated the AE findings to UT flaw characterization measurements with good 

results. Significantly, it required the use of UT examination to characterize the emission sources, 

which is problematic for many vessel configurations, including tube trailers. The ASTM E 1419 

standard is being extended by testers to other pressure vessel configurations and materials (e.g., 

stainless steel tanks) for which this test method may or may not be valid. For this standard to be 

extended to other types of vessels and materials a fracture mechanics based analysis of the vessel 

material, design and flaw geometry must be developed to determine the required sensitivity of 

the system. Using these calculations, the examiner can set the correct sensitivity of the system, 

either by increasing system gain or increasing the number of transducers used, so that a valid test 

can be performed. 

The ability to perform AE testing by capturing and evaluating individual acoustic wave modes 

using broadband sensors was developed in the late 1980’s. Modal Acoustic Emission (MAE) 

lends itself to the detection, identification and fracture mechanics analysis for flaw detection. 

The wave mode identification capabilities of MAE has resulted in the ability to sort noise events 

from true flaw growth events, eliminating the problem associated with conventional AE of false 

positives due to the non-independent variable analysis [2-4]. The energy calculations required 

for a fracture mechanics analysis can easily be performed on the digitized waveforms captured 

using MAE as well. This paper presents an approach using fracture mechanics analysis to 

predict the energy release due to cyclic fatigue in 4130X steel. Data from flaw growth in a 

seamless steel pressure vessel is then compared to the theoretical fracture mechanics calculations 

to determine if the method can be used to aid in determining system sensitivities, e.g., sensor 

spacing, for flaw growth detection. 

Waveform Capture and Analysis 

A short discussion of data capture is presented here. Modal acoustic emission requires the use of 

a broadband system to properly capture the surface displacement from the propagating wave for 
43 

analysis. The strength of the analysis is that it does not rely on empirical relationships between 

parameters such as counts, amplitude, energy and duration. Parameters such as these are not 

independent (e.g., higher amplitude means greater energy, longer duration and more counts). 

Secondly, wave propagation effects, such as geometric attenuation, are not accounted for in 

parameter based AE, e.g., an event detected at two different distances from the source, will have 

two different energies, even though the source was the same. Thus, distribution plots, such as 

energy, amplitude, and duration will not take the attenuation into account, and are often seen as 

meaningless [5]. Using MAE eliminates these issues since the wave propagation analysis 

accounts for these effects. This paper will not cover the science of wave propagation in thin 

walled media, and the reader is directed to the many readily available references in this subject. 

Fracture Mechanics Energy Analysis for Crack Growth 

The focus of this paper is to develop a theoretical basis for determining the detectability of crack 

growth using MAE. From ASTM 1419, the system threshold for an examination using that 

standard is 32 dBAE. For most systems, this requirement is near the noise floor of the electronics, 

and no further sensitivity can be achieved by increasing the gain, or amplification, of the system. 

Many systems apply a narrow band filter to the signals to reduce noise, but when this is done, 

wave mode information is lost, and the ability to sort noise signals from flaw growth signals is 

lost as well. This results in false positives, and perfectly good pressure vessels either failing the 

examination, or inspector dispositions that require the use of follow up NDE – dispositions that 

are either not acceptable or helpful when follow up NDE is not feasible, e.g multi-layered 

pressure vessels. MAE overcomes this by identifying relevant, flaw based emissions, which can 

be evaluated on the basis of actual energy content. This section outlines the steps required to 

calculate the energy release due to crack growth in a given material and structural geometry due 

to cyclic fatigue. This value is then used to determine the sensor spacing required to detect crack 

growth. 

Step 1. Theoretical energy released by cyclic fatigue crack growth 

The first step in determining if crack growth in a given material is detectable is to calculate the 

theoretical energy release rate, G, of the material for Mode I crack growth. From [6], 

  2

2

 1Q

E

K G I

 

Where G (units of lb/in) is the energy release rate, i.e., rate of change in potential energy with the 

crack area (a.k.a., crack extension force and crack driving force, KI is the Mode I stress intensity 

factor for the given geometry, E is Young’s modulus, and Q is Poisson’s ratio. If these values are 

known for a given material, the energy release rate can be calculated. When this value is 

multiplied by the crack growth area, the total theoretical energy released by the crack growth is 

calculated. However, when this is calculated for a material such as steel, the energy is much 

greater than the energy calculated from a waveform captured due to flaw growth. The reasons 

for this are discussed in detail in the following sections, but a quick overview is provided below. 

1. Most of the energy from crack growth is released due to free surface formation of the 

crack, and very little of the energy is transformed into wave propagation. 

2. The sensor only captures the wave displacement directly under its face. As the wave 

propagates away from the source, the sensor only detects a very small portion of the wave 

energy. 
44 

The following steps describe how to account for the effects described above. 

Step 2. Theoretical Cyclic fatigue crack growth 

The second step in determining system sensitivity is to calculate the theoretical crack growth per 

fatigue cycle for a given material and crack configuration. This is typically done by integrating 

the da/dN curve for a given material from experimental data. The data used in this paper was for 

a DOT 4301X specification steel. Reference [1] gives the Fatigue crack growth (FCG) rate 

equation for this steel: 

   8 2.25 0.66 10 KI dN

da u ' 

, at R=0,

where: 

Q

a KI M S V

KI is the stress intensity factor, M is the correction factor for membrane stress, Q is the flaw 

shape parameter and a is the crack depth. For this work, Q=1.0. 

When integrated, this gives the following equation for the amount of crack growth per cycle as: 

8

0.125 8 2.25 2.25 1.125 (0.125)(0.66 10 )( )

1 »

1⁄4

o «

¬

a

 u ' '   a M N

a

i

f V S

Where ai is the initial crack size, af is the final crack growth, and 'V is the difference between 

the maximum and minimum cyclic stress. 

In reference [1], the value used for M was 1.5 and the cyclic stress to which the test cylinder was 

subjected was 34 ksi. Plotting this equation using an initial crack depth of 0.1 inches (about 1⁄4 

critical size at the specified conditions) results in the following plot, Figure A-1. 

Figure A-1. Crack growth per cycle for 3AAX steel. 
45 

The plot in Figure A-1 is for the initial crack growth, and results in a conservative crack 

extension per cycle of approximately 0.00001 inches per cycle with an applied membrane stress 

of 34 ksi. 

It is noted that these results are for incremental fatigue crack growth behavior and not for ductile 

tearing that would occur during rapid fracture near critical, end of life crack growth. 

Step 3. Calculation of the detected energy at the transducer 

The next step is to determine how much of the energy of the propagating wave created by crack 

opening is detected at the AE transducer. As the wave propagates, it spreads out in a ring. As it 

propagates further from the source, there is less energy per unit length since the circumference of 

the wave front is expanding while the total energy remains constant. This effect is known as 

geometric dispersion. The fraction of wave energy that the sensor detects is due to this effect is: 

R

D

Energy Sensor

Wave 2S 

Where EnergyWave is the fraction of wave energy detected, DSensor is the diameter of the sensor, 

and 2SR is the circumference of the wave front due to the distance of the sensor from the source, 

R. For a source 10 feet from a 0.5 inch diameter transducer, this would result in approximately 

0.0007 of the total wave energy being detected by the sensor. 

Step 4. Calculation of energy converted to wave motion 

Reference [7] discusses in detail the experimental method used to measure the energy conversion 

from G, the energy release rate, to GMAE, the energy release rate in the modal acoustic emission 

(MAE) wave. A short summation is provided here. 

A sensor is coupled to an aluminum plate (7075-T6) of given dimensions [7]. To impart a 

known energy into the plate, an incline of known length and height is placed against the edge of 

the aluminum plate. The height of the incline is adjusted so a ball of known diameter impacts 

the edge of the plate normally, at the centerline of the plate and at the centerline of the ball. The 

transducer is placed a known distance away from the point of impact and acoustically coupled to 

the plate. The energy of the rolling ball impact (RBI) can be calculated using E = mgh. The 

resulting waveform from the impact is captured using a high speed digitizer, and the energy in 

the first half cycle of the extensional mode calculated by integrating V2

 over time, as shown in 

the equation below, and then dividing by the input impedance to the preamplifier. 

U V dt Z

T

/

0

2

3 

Where U is the wave energy, in Joules, V is the signal amplitude in volts and Z is the impedance 

at the preamplifier in Ohms. 

The scaling factor is then calculated by dividing the wave energy by the energy of the rolling ball 

impact, mgh. For the electronics and sensor used for this testing, this scaling factor was 

calculated to be 2.63x10-7. From this number it can be seen that the amount of energy released in 

the form of a propagating wave is very small compared to the initiating event. The 

correspondingly much smaller energy released and detected from creation of a typical free crack 

surface is determined below. 
46 

Step 5. Determining theoretical crack growth detectability from cyclic crack growth 

The final step is to take the information from Steps 1-4, and calculate the energy of the wave 

from the cyclic fatigue crack growth to determine the theoretical sensor spacing required for 

crack growth detection. The steps are: 

1. Calculate G for the given material 

2. Multiply G by the wave scaling factor to get GMAE

3. Calculate the geometric attenuation factor for the distance of propagation, EnergyWave and 

multiply GMAE by this factor to get GMAE

Attn to get the fully scaled energy release for both 

the wave energy and the geometric attenuation energy 

4. Multiply GMAE

Attn by the crack growth area, A to get the total energy released by the 

crack growth that results in wave propagation. 

Example for 4130X steel for theoretical crack growth wave energy release 

Assumed values for calculations 

V = 34 ksi 

a = 0.1 in 

Q = 0.3 

E = 30x106 psi 

Propagation Distance = 50 inches 

Sensor Diameter = 0.25 inches 

GMAE scaling Factor = 2.63x10-7

To calculate G, the following equation from reference [6] is used: 

  2

2

 1Q

E

K G I

; I M K Q

a K 1.12V S x

From [6], KI is the stress intensity factor for a part through crack in a plate. MK is approximately 

1.0 for the crack geometry in this example, Q = 1.1. 

Using the above values and equations, the following values were calculated: 

Geometric Attenuation Correction for 50 inches = 0.25 in/(2Sx50 in) = 0.0008 

KI = 20.35 (ksi)(in)1/2

G = 12.6 lb/in 

GMAE = G x 2.63x10-7 = 3.31x10-6 lb/in 

GMAE

Attn = GMAE x EnergyWave = 3.31x10-6 lb/in x .0008 = 2.65x10-9 lb/in 

To calculate the amount of crack growth area, A, the following rationale was used. Shown in 

Figure A-2 is a schematic of an elliptical crack in the sidewall of a pressure vessel. The 

assumption is a 4:1 ratio for c to a, from reference [1]. The figure shows the initial crack (dotted 

line) and the crack after it has grown a distance 'a, due to a fatigue cycle. If the initial value of 

a is 0.1 inches, then c is 0.4 inches. From the crack growth per cycle calculation, Figure A-2, 'a

is 0.00001 inches, so then 'c must be 0.00004 inches to maintain the 4:1 aspect ratio. From 
47 

prior testing experience, the MAE source location shows that typically only one side of the notch 

grows during fatigue testing, not the entire length of the elliptical notch. Therefore, to calculate 

the area of crack growth, the area of the initial full ellipse (if it were an embedded flaw) is 

calculated from A=Sac, and the area of the crack growth full ellipse is calculated, 

A=S(a+'a)(c+'c), and the two areas subtracted to get the difference in areas, 'A. The 'A is 

then divided by four to get the area of one side of the notch growth. The squares in Figure A-2 

below show the proposed area of crack growth. Doing these calculations results in an increase in 

notch area of 6.3x10-6 in2

, due to a crack depth increase of 0.00001 inches. 

'c 2c 'c

'a

a

Figure A-2. Schematic of crack growth. 

Using these values, the theoretical corrected wave energy release is: 

GMAE

Attn

 x A= (2.65x10-9 lb/in)( 6.3x10-6 in2)(1/12 ft/in)(1.356 J/ft lb) = 1.89x10-15 Joules 

This energy is the baseline energy for cyclic flaw growth in 4130X steel at the stated conditions. 

This number will be used to set the system sensitivity for crack growth detection. 

Experimental Modal Acoustic Emission Data 

A waveform from crack growth in a pressure vessel manufactured from 4130X steel is shown in 

Figure A-3. The data shown was from a test performed to the ASTM E 1419 standard where two 

sensors were acoustically mounted to the tube, approximately 400 inches apart, and the vessel 

pressurized to 110% of operating pressure. The source of the waveform was located 

approximately 50 inches from sensor 1, and 350 inches from sensor 2. The location of the flaw 

was verified with follow up ultrasonic inspection. The system gain was set to 68 dB (x2511). 

Using the waveforms in Figure A-3, the energy contained in the signal can be calculated. Only 

the energy of the direct wave is used (reflections are not included in the calculation, as that 

would add energy that was not in the initial source). The signal between the vertical bars on 

each signal was used for the energy calculation. The voltage for each digitized point was 

squared, multiplied by the 1/sampling rate, divided by the gain squared (25112

), and then divided 

by the input impedance of the preamplifier (10,000 :). This gave an energy value for Channel 1 

of 8x10-16 J, and for Channel 2, 1.28x10-16 J. 
48 

Figure A-3. Crack growth signal from a 3AAX tube. The waveform from channel 1 is shown in 

blue, and channel 2 is shown in red. 

A greater signal length was used for sensor 2 due to the dispersion of the wave as it propagated a 

longer distance. The amount of propagation time for the lower frequencies contained in the 

flexural mode of the signal was calculated from the dispersion curve for this material to be about 

3300 Pseconds (using a propagation velocity of 106,000 in/sec). Adding this to the initial trigger 

point in sensor 1 of approximately 800 Pseconds, gave a time of 4100 Pseconds for the arrival of 

the lower frequencies in the flexural mode that travel at much slower velocities than those in the 

extensional mode. 

If we compare the theoretically calculated energy values to the experimentally determined values 

for each channel, we have the following results shown in Table A-1. 

Table A-1. Theoretical calculated and experimentally measured wave energy. 

Theory Experiment % Difference 

Ch 1 Energy – 50” 1.89e-15 J 8.0e-16 J 42 

Ch 2 Energy – 350” 2.67e-16 J 1.28e-16 J 48 

This result is well within experimental error for these types of calculations, and likely can be 

improved with better sensor calibration measurements and fracture mechanics values. Most 

likely, the largest error in the calculation is the theoretical crack growth area, since that is the 

greatest unknown in the measurement. 

Usage of waveform energy release in MAE testing to determine sensor spacing 

Sensor spacing in acoustic emission and MAE testing has been based on testing experience and 

development of empirical standards. ASME E 1118, Acoustic Emission Examination of 

Reinforced Thermosetting Resin Pipe (RTRP), has a sensor spacing requirement based on 

attenuation of signals in a lead plate. While this provides the novice examiner with a guideline, 

it is impractical to carry a lead plate of the required dimensions to test sites. 

The work in this paper allows the examiner to set conservative a sensor spacing that is based on 

fracture mechanics testing and material values for the material the vessel was manufactured 

from. The spacing should be on the conservative side since it is based on the cyclic fatigue of 
49 

the vessel at operating loads, and a much smaller initial crack size than what would be required 

for the flaw to reach critical crack size during the recertification period. MAE testing uses a 

110% of operating overstress to exercise the crack in a structure. The increased pressure and a 

larger initial crack depth would result in a greater energy release, resulting in larger energy 

signals. 

Figure A-4 shows a way that the fracture mechanics calculations could be used to develop sensor 

spacing curves for a given material. For a given crack growth area, the energy release is 

calculated, and then plotted for a sensor spacing. The Figure A-4 log-log plot shows the 

waveform energy (y-axis) and crack growth area (x-axis) plotted as a function of sensor distance 

from the source (colored lines), for the 4130X steel used in the example in this paper. Using 

steps 1-5 outlined previously in the paper, one can determine the proper sensor spacing for the 

data acquisition system used. 

Figure A-4. Sensor spacing as a function of waveform energy (y-axis) and crack growth area. 

For example, the energy noise floor for the system used in the experimental data is 

approximately 1x1017 J for a quiescent signal (no MAE signal in the data). If a rule of thumb of 

a signal-to-noise ratio of two is used to set the minimum signal sensitivity, this would result in a 

floor of 2x10-17 Joules. That would mean for our example, a crack growth area of 6.3x10-6 in2 

and an energy release of 1.89x10-15 Joules, that a sensor spacing of up to 40 feet could be used 

for the detection of signals of this size. Based on experience and data from 1.1 inch thick and 

5/8 inch thick steel cylinders that have been MAE tested, this is typically the maximum distance 
50 

that can be reliably detected for cyclic flaw growth. Please note that the noise floor of the 

instrumentation must be measured to determine the minimum signal level detection. 

Also note that, as previously mentioned, this detection capability is based on stable fatigue crack 

growth, not ductile tearing associated with near-critical rapid flaw growth. That regime is not 

addressed in this paper. 

Discussion 

While the theory presented in this paper is verified on a very limited sample of actual data, it 

does provide a baseline rationale for the placement of sensors when performing MAE testing. 

Sensitivity of the technique has always been an issue, and this analysis approach can provide 

insight to the sizing of flaw growth using MAE (i.e., what amount of crack growth is detectable) 

– note that this is different from determining the physical size of the flaw. Flaw size, or depth, is 

answered not by this work, but by observing the extensional and flexural mode amplitudes to 

determine approximate crack depth [3], or by using a secondary inspection method, such as 

ultrasound, when possible. 

Obviously, actual fracture/waveform data is needed to fully verify and validate this approach, but 

it is based on science and well established plate wave propagation characteristics, and for 

metallic materials that have well documented fracture mechanics material data associated with 

them, this gives the examiner and owner a framework for helping to ensure that the testing is 

valid and the parameters that are being measured meet the intended goals of the test. 

Furthermore, the fracture mechanics analysis will rule out the use of acoustic emission testing on 

materials that will not have enough energy release to create detectable events, e.g., low strength 

stainless steel vessels. 

Conclusions 

A method has been presented to theoretically calculate the energy released due to crack growth 

in 4130X steel as it relates to acoustic wave detection and assessment. The purpose of the 

method is to provide examiners using modal acoustic emission a science based method to assist 

in determining maximum sensor spacing to ensure that crack growth signals can be reliably 

detected on their equipment. The theoretical calculations were compared to crack growth data in 

a single wall forged pressure vessel manufactured from 4130X steel. The results were: 

1. An approach that correlates the energy release calculated using fracture mechanics to the 

energy contained in captured modal acoustic waveforms has been developed. 

2. The calculated energy from fracture mechanics can be used to develop maximum sensor 

spacing limitations for MAE sensors to ensure that relevant flaw growth is detected. 

This approach should allow the extension of acoustic emission testing standards such as ASTM 

E 1419 to other materials and give the examiner confidence that the sensitivity of the system is 

meeting the test objectives. 

References 

1 Blackburn, P.R., Rana, M.D., “Acoustic Emission Testing and Structural Evaluation of 

Seamless, Steel Tubes in Compressed Gas Service,” Journal of Pressure Vessel Technology, 

May 1986, vol. 108, pp. 234-240. 
51 

2 Gorman, M.R., “Plate Wave Acoustic Emission,” Journal of the Acoustical Society of 

America, 1991, vol. 90, no. 1, pp. 358-365. 

3 Gorman, M.R., Prosser, W.H., “AE Source Orientation by Plate Wave Analysis,” Journal of 

Acoustic Emission, 1991, vol. 9, no. 4, 283-288. 

4 Ziola, S.M., “Digital Signal Processing of Modal Acoustic Emission Signals,” Journal of 

Acoustic Emission, 1996, vol. 14, no. 3-4, pp. S12-S18. 

5 Goransen, “Jet Transport Structures Performance Monitoring,” Structural Health 

Monitoring, Current Status and Perspectives, Proceedings of the International Workshop on 

Structural Health Monitoring, Stanford University, CA, September 18-20, 1997. 

6 Barsom, J.M., Rolfe, S. T., “Fracture and Fatigue Control in Structures, Applications of 

Fracture Mechanics,” Second Edition, Prentice-Hall, 1987. 

7 Gorman, M.R., “Modal AE Analysis of Fracture and Failure in Composite Materials, and the 

Quality and Life of High Pressure Composite Pressure Vessels,” Journal of Acoustic 

Emission, 2011, vol. 29, pp. 1-28. 
52 

Appendix C. WaveExplorerTM Digital Filter Settings 

These are the digital filter settings used in the software for the data analysis. 

Filter Qualifier Value Number of Channels to 

Pass On 

Threshold ≥ 100 mV 0 

Parametric ≥ 2.5 volts NA 

Pretrigger (PTE) ≤ 0.4 V2-msec 16 

Saturation (Satur.) ≤ 1 % 

Confidence (Conf.) ≥ 10 % 

Energy ≥ 4 V2-msec 4 

Frequency (Freq) ≥ 15 kHz NA 

Input screen for the confidence filter. 
REPORT DOCUMENTATION PAGE Form Approved

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Contractor Report

 4. TITLE AND SUBTITLE

Cyclic Crack Growth Testing of an A.O. Smith Multilayer Pressure Vessel 

with Modal Acoustic Emission Monitoring and Data Assessment

5a. CONTRACT NUMBER

NNA09DB39C

 6. AUTHOR(S)

Ziola, Steven M.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

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Publication of this report was requested by the NESC. This task was funded by OSMA in with funds transferred to ARC for program execution. Doug Fraser was the NASA Project Manager at ARC. Jacobs ATOM managed the project through their contracts office under NASA contract NNA09DB39C. Owen Greulich and Doug Fraser were the NASA Technical Monitors.

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14. ABSTRACT

Digital Wave Corp. (DWC) was retained by Jacobs ATOM at NASA Ames Research Center to perform cyclic pressure crack 

growth sensitivity testing on a multilayer pressure vessel instrumented with DWC's Modal Acoustic Emission (MAE) system, 

with captured wave analysis to be performed using DWCs WaveExplorerTM software, which has been used at Ames since 

2001. The objectives were to document the ability to detect and characterize a known growing crack in such a vessel using 

only MAE, to establish the sensitivity of the equipment vs. crack size and / or relevance in a realistic field environment, and to 

obtain fracture toughness materials properties in follow up testing to enable accurate crack growth analysis. This report 

contains the results of the testing.

15. SUBJECT TERMS

Modal Acoustic Emission; Crack Growth Rate; Pencil Lead Break; Pressure Vessel

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