DOT/FAA/AR-05/30 

Office of Aviation Research 

Washington, D.C. 20591

Inspection Development for 

Titanium Billet—Engine Titanium 

Consortium Phase II 

September 2005 

Final Report 

This document is available to the U.S. public 

through the National Technical Information Service (NTIS), 

Springfield, VA 22161. 

U.S. Department of Transportation 

Federal Aviation Administration
NOTICE 

This document is disseminated under the sponsorship of the U.S. 

Department of Transportation in the interest of information exchange. The 

United States Government assumes no liability for the contents or use 

thereof. The United States Government does not endorse products or 

manufacturers. Trade or manufacturers names appear herein solely 

because they are considered essential to the objective of this report. This 

document does not constitute FAA certification policy. Consult your local 

FAA aircraft certification office as to its use. 

This report is available at the Federal Aviation Administration William J. 

Hughes Technical Center's Full-Text Technical Reports page: 

actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF). 
 Technical Report Documentation Page

1. Report No.

DOT/FAA/AR-05/30

2. Government Accession No. 3. Recipient's Catalog No.

5. Report Date 

 September 2005

 4. Title and Subtitle 

INSPECTION DEVELOPMENT FOR TITANIUM BILLET—ENGINE TITANIUM 

CONSORTIUM PHASE II 6. Performing Organization Code

7. Author(s)

Mike Keller1, Thadd Patton1, Andrei Degtyar2, Jeff Umbach2, Waled Hassan3, 

Andy Kinney3

, Ron Roberts4, Frank Margetan4, and Lisa Brasche4

8. Performing Organization Report No. 

10. Work Unit No. (TRAIS) 

 

9. Performing Organization Name and Address

1

General Electric Company 

Cincinnati, Ohio 45215 

2

Pratt & Whitney 

East Hartford, CT 

3

Honeywell Engines, Systems & Services 

Phoenix, AZ 

4

Iowa State University 

Ames, IA

11. Contract or Grant No.

 

 DTFA0398FIA029 

13. Type of Report and Period Covered

 Final Report

12. Sponsoring Agency Name and Address

U.S. Department of Transportation 

Federal Aviation Administration

Office of Aviation Research 

Washington, DC 20591

14. Sponsoring Agency Code

 ANE-110

15. Supplementary Notes 

The FAA William J. Hughes Technical Center Technical Monitors were Rick Micklos and Cu Nguyen.

16. Abstract 

The Engine Titanium Consortium (ETC) is comprised of Iowa State University; General Electric; Honeywell Engines, Systems & 

Services; and Pratt & Whitney. The ETC Phase I program began in 1993 with a focus on improved inspection of titanium billet 

used in the production of jet engines. The Phase I program completed in 1998 included the development and evaluation of two 

zoned approaches to billet inspection, namely, multizone and phased array inspections. The Phase II program began in 1999 and 

focused on further sensitivity improvements to titanium billet using the multizone approach. The goal of the Phase II effort was to 

achieve a #1 flat-bottom hole sensitivity for 10′′ diameter billet and assess the impact of attenuation compensation procedures. This 

report documents the results for 5′′, 10′′, and 14′′ diameter billets using calibration standards in a laboratory setting. 

17. Key Words 

Titanium billet, Ultrasonic inspection, Probability of detection 

18. Distribution Statement 

This document is available to the public through the National 

Technical Information Service (NTIS) Springfield, Virginia 

22161.

19. Security Classif. (of this report) 

 Unclassified

20. Security Classif. (of this page) 

 Unclassified

21. No. of Pages 

 151 

22. Price

Form DOT F1700.7 (8-72) Reproduction of completed page authorized 
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY xiii 

1. INTRODUCTION 1 

1.1 Purpose 1 

1.2 Background 1 

1.3 Program Objectives 4 

2. APPROACH 4 

3. DISCUSSION OF RESULTS 5 

3.1 Validation and Use of Transducer Design Models 5 

3.2 Transducer Design and Procurement 8 

3.2.1 Transducer Manufacturing Parameter Tolerance Analysis 12 

3.2.2 Transducer Characterization 12 

3.3 Transducer Evaluations 24 

3.3.1 Evaluation of F/10 Transducers 24 

3.3.2 Pulse Volume Calculations 28 

3.3.3 Phased Array Evaluation 28 

3.3.4 Evaluation of F/8 Transducers 31 

3.3.5 Explanation for Zones 6a and 6b 36 

3.3.6 Sensitivity Curves 38 

3.4 Summary of Laboratory Evaluation for 10′′ Diameter Billet 41 

3.5 Attenuation Compensation Procedures 41 

3.6 Banding Phenomena and UT Property Variations 45 

3.6.1 Attenuation Compensation Research Plan 48 

3.6.2 Current Billet Compensation Procedures 50 

3.6.3 Test Billet and Transducer Selection 51 

3.6.4 Back-Wall Variations and Near-OD FBH Placements 53 

3.6.5 Correlation Between Back-Wall and Near-OD FBH Amplitude 

Variations 56 

3.6.6 Straw Man Procedures for Global Attenuation Compensation 67 

iii
3.6.7 Proposed Straw Man Global Attenuation Compensation Procedure 67 

3.7 Fabrication of a Test Specimen Containing Near-Center FBHs 69 

3.7.1 Tests of the Global Attenuation Compensation Straw Man 

Procedure 73 

3.7.2 Local Attenuation Adjustments and Their Computation 79 

3.8 Summary and Recommendations for Attenuation Compensation 98 

3.9 Assessment of Large-Diameter Billet 102 

3.9.1 Chord Block Fabrication 103 

3.9.2 Baseline CV Inspection 104 

3.9.3 Transducer Design 106 

3.9.4 Transducer Characterization 112 

3.9.5 Chord Block Measurements 118 

3.10 Summary of 14′′ Chord Block Evaluation 120 

4. SUMMARY AND CONCLUSIONS 121 

5. REFERENCES 122 

APPENDICES 

 A—Major Accomplishments of the ETC Phase 1 Program

B—Description of Multizone Inspection for Billet 

iv
LIST OF FIGURES 

Figure Page 

1 Summary of Transducer Performance Measurements on Billet Standard 7 

2 Results of Sensitivity Analysis for Zone 4 F/10 Transducer Design 12 

3 Voltage Response V(z) for Manufacturer B Curved Element Transducer 13 

4 (a) Billet Inspection Geometry Showing the Beam Coordinate System and (b) The 

Four Parameters Used to Characterize the Focal Properties of a Transducer 14 

5 Three Measurement Geometries for Acquiring Data for Transducer 

Characterization 15 

6 Function That is Minimized to Determine the Best Fit Transducer Characteristics 16 

7 Measurement of the Time Offset (0.81 μsec) for the UT System Used for the 

BMIW Method 17 

8 The BMIW Method (a) Water Paths Used For 2D Scans and (b-d) Comparison of 

Fitted and Measured Beam Profiles for Three of the Water Paths 18 

9 The BMIW Method Fitted and Measured Beam Profiles at 5.08 MHz for Each of 

the Ten Water Paths 18 

10 The BMIS Method (a) Data Acquisition Details and (b-e) Fitted and Measured 

Beam Profiles at 5.08 MHz for Each of the Four FBHs 19 

11 V(z) Method (a) Data Acquisition Details and (b) Measured and Fitted 

Amplitude Profiles at 5.08 MHz 19 

12 Deduced Focal Parameters for Each Method When all Four Parameters are 

Allowed to Vary 21 

13 Comparison of V(z) Curves Calculated Using Two Different Beam Models 23 

14 Percentage Error Resulting From Using The Paraxial MGB Model Rather Than 

the Green’s Function Formalism to Deduce Probe Focal Parameters From 

Synthetic V(z) Data 24 

15 C-Scan of Targets in ETC 10′′ Diameter Calibration Standard Using the Zone 2 

Transducer 25 

16 Noise and Target Amplitude Responses, Zone 2 Transducer 27 

17 Noise and Target Amplitude Responses, Zone 4 Transducer 27 

v
18 Comparison of Modeled Beam Diameters for the F/8 and F/10 Transducer 

Designs 29 

19 Results of Phased Array Evaluation of the ETC Calibration Standard on the 3.15′′

Deep Calibration Target 30 

20 Comparison of the Measured and Projected Peak Noise Levels vs the sqrt pv for 

the F/10, Phased Array, and Projected F/8 Transducers 31 

21 F/8, Elliptical Elements, 7.5 MHz, Zones 5 and 6 Transducers 31 

22 Zone Sensitivity Model Predictions for the F/8 Transducers 32 

23 Set of Six F/8 7.5-MHz Transducers and Table of Dimensions 33 

24 Comparison of Pulse Volumes as Determined by the ISU Model and the Pulse 

Volumes of the Delivered Transducers 35 

25 Zone 6 Divided Into Two Zones, 6a and 6b, to Improve SNR 36 

26 Comparison of Sensitivity Variation Throughout the Inspection Zone for the F/10 

and F/8 Transducer Designs 36 

27 Pulse Volume Calculations for the Zone 6 Transducer 37 

28 Percent Noise Peak vs the sqrt pv Original Curve Based on F/10 Zone 4 Results 38 

29 C-Scan of ETC Calibration Standard Showing the Five FBHs for Zones 6a and 6b 38 

30 Sensitivity Graph Using the ETC 10′′ Diameter Calibration Standards 39 

31 Sensitivity Graph Using the RDB 10′′ Diameter Specimen 39 

32 #1 FBH Inspection Sensitivity for Zones 1-3 on the ETC Calibration Standard 40 

33 Zone 2 #1 FBH Sensitivity on the RDB 40 

34 Billets Awaiting Inspection 42 

35 Conventional and MZ Billet Inspections Use Calibration Standards Containing 

FBH Reflectors 42 

36 Calibration Standard for an MZ Inspection of a 6′′ Diameter Billet 43 

37 Ultrasonic Billet Inspection (Idealized at Depth Z) 43 

38 The Standard Method for Attenuation Compensation Makes Use of Average 

Back-Wall Amplitudes 44 

vi
39 Banding Patterns Seen During the Inspection of a 14′′ Diameter Ti-6-4 Billet 

Specimen 46 

40 Noise Banding and UT Property Variations Seen in a 6′′ Diameter Ti-6-4 Billet 47 

41 Correlation Between Backscattered Noise FOM and Attenuation for Six Coupons 

From a 6′′ Diameter Ti-6-4 Billet 48 

42 Areas Typically Scanned to Acquire Back-Wall Amplitude Data for Attenuation 

Compensation 51 

43 Model Calculations of Beam Properties for Two 5-MHz MZ Transducers 52 

44 Five-MHz Phased Array Transducer, Operated in Standard Focusing Mode to 

Simulate Inspections With Fixed-Focus Probes 53 

45 Back-Wall C-Scan Images of 6′′ Diameter CBS Billet (B2W1A) Acquired Using 

Two MZ Transducers 54 

46 Back-Wall Amplitude Statistics for Four Regions of the 15′′ Long Billet Section 54 

47 Possible Schemes for Positioning Near-OD FBHs Relative to the Back-Wall 

Amplitude Pattern 55 

48 (a) 6′′ Diameter Ti-6-4 Test Specimen Containing Near-OD #4 FBHs and 

(b) C-Scan Image Showing the FBH Enumeration Scheme 56 

49 Measurements on a Ti-6-4 Billet Segment Using a 5-MHz, 6′′ MZ Probe Focused 

Near the Billet Center 58 

50 Measurements on a Ti-6-4 Billet Segment Using a 5-MHz, 13′′ MZ Probe 

Focused Near the Billet Back Wall 59 

51 Measurements on a Ti-6-4 Billet Segment Using a 5-MHz Phased Array 

Transducer Focused Near the Billet Center 60 

52 Measurements on a Ti-6-4 Billet Segment Using a 5-MHz Phased Array 

Transducer Focused Near the Billet Back Wall 61 

53 Measurements on a Ti-6-4 Billet Segment Using a Phased Array Transducer 

Focused Near the Billet Center 62 

54 Measurements on a Ti-6-4 Billet Segment Using a Phased Array Transducer 

Focused Near the Billet Back Wall 63 

55 Normalized FBH Amplitudes and Nearby Back-Wall Amplitudes Seen in the Six 

Measurement Trials 64 

vii
56 Flat-Bottom Holes and (Nearby) Back-Wall Amplitude Fluctuations Seen in 

Six Measurement Trials Using a 15′′ Long Ti-6-4 Billet Containing 13 

Near-OD FBHs 65 

57 Correlation Between Back-Wall and FBH Amplitudes as a Function of Beam Size 

at the FBH Depth 66 

58 Fabrication of Specimens to Test Straw Man Procedures for Attenuation 

Compensation 69 

59 Back-Wall C-Scans of the Candidate Billet for the Straw Man Test 69 

60 Mean, Maximum, and Minimum Back-Wall Amplitudes for Various Regions of 

the C-Scan Images Shown in Figure 59 70 

61 Comparison of Mean Amplitudes of Back-Wall Echoes for the Six Regions 

Indicated in Figures 59 and 60 70 

62 Six-in.-Diameter, 18-in.-Long Ti-6-4 Test Specimen Showing How #2 FBHs 

Were Drilled at a Depth of 3.5′′ From the Inspection Surface 71 

63 Six-in.-Diameter, 18-in.-Long Ti-6-4 Test Billet Showing Drilled and 

Plugged Holes 71 

64 Numbering Scheme for FBH Locations in all C-Scans of the 18′′ Long Test Billet 72 

65 (a) Layout of the Deep-Zone (Zone 4) Portion of the 6′′ Diameter MZ Calibration 

Standard and (b) C-Scan of the Deep-Zone Region of the Calibration Standard 72 

66 Back-Wall C-Scans of the Calibration Standard and Test Billet Acquired With the 

Beam Focused Near the 6′′ Diameter Billet Centerline (Zone 4) 73 

67 Back-Wall C-Scans of the Calibration Standard and Test Billet Acquired With the 

Beam Focused Near the 13′′ Diameter Billet Back Wall (Zone 7) 74 

68 Example Attenuation Compensation Calculation 74 

69 (a) C-Scan Images From Zone 4 Inspections of the Calibration Standard and Test 

Billet and (b) Raw Peak Amplitudes of the 17 #2 FBHs in the Test Billet 76 

70 Peak Amplitudes of the 17 #2 FBHs in the Test Billet When no Attenuation 

Compensation is Used 77 

71 (a) Output Generated When Automatic S/N Software Was Applied to the As-Run 

Zone 4 C-Scan of the Test Billet and (b) As-Run Peak Amplitudes of the 17 

#2 FBH Defects With S/N Ranges Indicated 78 

72 Peak Amplitudes of the 17 #2 FBHs in the Test Billet Using the Standard 

Compensation Procedure and no Attenuation Compensation 80 

viii
73 Results of Applying Three Global Attenuation Compensation Procedures to the 

Test Billet Containing 17 #2 FBH Defects, Zone 4 81 

74 Results of Applying Three Global Attenuation Compensation Procedures to the 

Test Billet Containing 17 #2 FBH Defects, Zone 7 82 

75 Summary of FBH Amplitude Statistics for the Six Global Compensation 

Procedures 84 

76 Zone 3 C-Scan of the 18′′ Long Test Billet Showing Analysis Boxes for the 

Computation of Noise Statistics 86 

77 Noise Statistics at the Axial and Hoop Locations of the 17 FBH Defects 87 

78 Raw and Normalized Average Gated-Peak Noise Amplitudes Seen in the MZ 

Inspection of the Ti-6-4 Test Billet 87 

79 (a) Methods of Choosing the Analysis Region Used to Compute the Average 

Back-Wall Amplitude in the Vicinity of Each Defect and (b) Analysis Regions for 

the Back-Wall C-Scan of the Test Billet Made With the Beam Focused Near the 

Billet Back-Wall 88 

80 Raw Average Back-Wall Amplitudes in the Vicinities of the 17 FBH Defects in 

the Test Billet 89 

81 Comparison of Measured and Predicted FBH Amplitudes of the 17 #2 FBH 

Defects in the Test Billet 91 

82 Correlation Between Predicted and Measured FBH Amplitudes in the Ti-6-4 Test 

Billet 92 

83 Results of Applying Local Attenuation Compensation Procedures to FBH 

Amplitudes in the Ti-6-4 Test Billet 93 

84 Results of Applying Local Attenuation Compensation Procedures to FBH 

Amplitudes in the Ti-6-4 Test Billet, Standard Global Compensation 94 

85 Results of Applying Local Attenuation Compensation Procedures to FBH 

Amplitudes in the Ti-6-4 Test Billet, Semiconservative—Global Compensation 95 

86 Combinations of Global and Local Attenuation Compensation Procedures 

Considered in This Report 96 

87 Chord Block Schematic 103 

88 Noise Scan of the Billet From Which Chord Block Was Made 104 

89 Scans of the Chord Block With a Transducer Used in a CV Inspection 105 

ix
90 Scans of the Chord Block With an MZ Transducer Designed for the Inspection of 

the Central Zone of 13′′ Diameter Billet 106 

91 Schematic of an Elliptical Element, Bicylindrically Focused Transducer 107 

92 Model-Predicted on-Axis (Radial Direction) Response Within Center Zone of 14′′

Diameter Ti Billet for Various Transducer Designs 109 

93 Model-Predicted Beam Size Within Center Zone of 14′′ Diameter Ti Billet for 

Various Transducer Designs 110 

94 Predicted Beam Area Within the Central Zone of 14′′ Diameter Billet for 

Transducers 1 and 2 111 

95 Transducer 2 112 

96 V(z) Method for Transducer Characterization 113 

97 Waveforms Around the First Focus of Transducer 1 113 

98 V(z) at (a) 4.69 MHz and (b) 6.08 MHz for Transducer 1 114 

99 V(z) at (a) 4.69 MHz and (b) 6.08 MHz for Transducer 2 114 

100 Reconstruction Results for GFL in (a) Circumferential, Fx, and (b) Axial, Fy, 

Directions at Various Frequencies for Transducer 1 115 

101 Reconstruction Results for GFL in (a) Circumferential, Fx, and (b) Axial, Fy, 

Directions at Various Frequencies for Transducer 2 116 

102 Measured and Simulated C-Scans of 5/16′′ Diameter Steel Ball Target in Water 

for Transducer 1 117 

103 Measured and Simulated C-Scans of 5/16′′ Diameter Steel Ball Target in Water 

for Transducer 2 117 

104 Scans of the Chord Block With Different Water Paths 118 

105 Analysis of One of the Chord Block Scans (Water Path = 2.5′′) Using Automatic 

SNR Calculation Feature of GE-Multizone Software 119 

106 Waveforms With Responses From #2 FBHs in Chord Block Water Path = 2.7′′ 120 

x
LIST OF TABLES 

Table Page 

1 First-Generation Transducer Design Parameters 9 

2 Designs for F/8 and F/10 Transducers for 10′′ Diameter Billet 10 

3 Predicted Beam Diameters and Signal-to-Noise Improvement of Proposed 

Transducer Designs 11 

4 Values of Effective Diameters and GFL Deduced Using Various Fitting Methods 22 

5 Preliminary Evaluation Results for Zone 2 Transducer 24 

6 Preliminary Evaluation Results for Zone 4 Transducer 25 

7 Frequency Measurements for Zone 2 Transducer 26 

8 Frequency Measurements for Zone 4 Transducer 26 

9 Details of the Beam Dimensions and the Resultant Signal-to-Noise Values for 

One Set of Parameters 30 

10 Summary of the Transducer Beam Characteristics for Pulse Volume Calculations 

and Results of the Noise Statistics for the ETC and Random Defect Billets 35 

11 Computed Attenuation Compensation Values in dB/inch of Depth in the Test 

Billet 75 

12 Flat-Bottom Hole Amplitudes for the Ti-6-4 Test Billet After Adjustment 83 

13 Local Attenuation Adjustment Factors at the 17 FBH Sites Determined From 

Local Back-Wall and Grain Noise Amplitudes 90 

14 Statistics of Adjusted FBH Amplitude Variations in the 18′′ Long Test Billet for 

Each of the Attenuation Compensation Procedures Considered 97 

15 Zoning Scheme for MZ Inspection of 14′′ Diameter Billets 103 

16 Specifications of Transducers 1 (F/10) and 2 (F/7) Intended for the Inspection of 

the Central Zone of 14′′ Diameter Ti Billet as Well as F/8 Design and Existing 

Transducer Intended for Inspection of Center Zone of 13′′ Diameter Billet 109 

17 Results of V(z) Reconstruction for Transducers 1 and 2 116 

xi
LIST OF ACRONYMS 

2D Two-dimensional 

BMIS Beam mapping in solid 

BMIW Beam mapping in water 

BTBI Billet to be inspected 

CBS Contaminated Billet Study 

CV Conventional 

DAC Distance-amplitude correction 

GEAE General Electric Aircraft Engines 

ETC Engine Titanium Consortium 

FAA Federal Aviation Administration 

FBH Flat-bottom hole 

FOM Figure of merit 

FSH Full screen height 

GE General electric 

GFL Geometrical focal length 

ID Inside diameter 

ISU Iowa State University 

MGB Multi-Gaussian Beam 

MZ Multizone 

Ni Nickel 

OD Outside diameter 

OEM Original Equipment Manufacturer 

PoD Probability of Detection 

POP Pulse-on-position 

RAM Random access memory 

RDB Random defect billet 

ROI Region of interest 

SNR Signal-to-Noise ratio 

TRCRT Titanium Rotating Components Review Team 

UT Ultrasonic 

WORM Write-once-read-many 

xii
EXECUTIVE SUMMARY

A critical element in ensuring overall flight safety is the performance of the propulsion system, 

in particular, the safe operation of the rotating components. In data analysis performed by the 

Federal Aviation Administration (FAA) and industry in the 1990s, the need for reduction in 

engine incidents caused by failure of disks was identified. In response to this need, the Engine 

Titanium Consortium (ETC) carried out several efforts to improve inspection of jet engine 

components and the materials used in their manufacture. Much of the effort has focused on the 

detection of hard alpha, a melt-related defect that can occur in the manufacture of titanium (Ti) 

ingots and inspection of billets and forging. This report, however, is limited to details of the Ti 

billet inspection activity. 

The objective of this research was to evaluate means to improve the ultrasonic inspection 

sensitivity for Ti cylindrical billet. The specific goals were: 

• To achieve #1 flat-bottom hole (FBH) sensitivity in Ti billet up to 10 inches in diameter. 

• To improve means of accounting for attenuation in the billet, thus making the inspection 

more uniform. 

• To assess the inspection capability in billet diameters greater than 10 inches in diameter. 

Highlights of each are provided here with full details in the report: 

• Achieving #1 FBH Sensitivity: Multizone technology is currently used to inspect a 

10-inch-diameter billet to 7/8 of a #2 FBH sensitivity, with the reject amplitude set at 

87.5% of calibration. Setting a goal of #1 FBH would improve this sensitivity by 4 times 

over the calibration sensitivity. To meet this goal, an inspection was designed based on 

extensive use of model calculations to design improved transducers. Not all of the 

7.5-MHz, F/8 transducers in the new set meet ETC specifications. In spite of this 

problem, the #1 FBH goal was nearly achieved or exceeded in 100% of the material 

volume, FBH sensitivities varying between #0.9 FBH and #1.1 FBH in the most difficult 

to inspect (3′′ to 5′′ depth) center zones. Sensitivities in the shallower zones (to ~3′′

depth) varied between #0.6 FBH and #0.8 FBH, thus exceeding the program goal. 

Current transducer manufacturing limitations prevented achievement of this goal at the 

deeper sections of the billet but did demonstrate improved inspection sensitivity by 2.7 

times over existing sensitivity levels. 

• Attenuation Compensation Procedures: Several means of accounting for sound 

attenuation in the billet were evaluated, ranging from less to more conservative. These 

can be applied globally to the inspection data to enhance sensitivity in the areas of 

highest attenuation. Alternate postinspection evaluation of attenuation using the C-scan 

images is another possibility. This approach would apply compensation locally in the 

billet and might reduce the areas of overinspection, unlike the global applied procedure. 

Impacts of these compensation procedures have not been fully evaluated in a production 

environment and could have substantial effects on the billet inspection. 

xiii
• Assessment of Large Diameter Billet: Inspection sensitivity improvements were assessed 

for billets up to 14′′ in diameter. Research showed that inspection sensitivity could be 

improved by use of focused multizone transducers in place of the conventional 

transducers. It was demonstrated, using 14′′ diameter chord blocks in the laboratory, that 

a #2 FBH sensitivity could be achieved at the center of the billet using the multizone 

inspection system. This represents more than a 2 times improvement in sensitivity 

through application of the multizone system. 

xiv
1. INTRODUCTION. 

1.1 PURPOSE. 

Titanium (Ti) alloys were selected for jet engine applications because of their strength-to-weight 

ratio and ability to operate in harsh environments. The performance demands for Ti jet engine 

components can be quite stringent which, in turn, necessitates high-sensitivity inspection of the 

product forms that lead to the final engine component. Typical Ti material used in production of 

commercial jet engine disks underwent ultrasonic (UT) inspection at the billet and forging 

stages. The Engine Titanium Consortium (ETC) focused much of its Phase I work on Ti billet 

inspection. 

The goal of this research was to build on the Phase I progress to improve Ti billet inspection and 

to evaluate further multizone (MZ) improvements that could lead to improvements in sensitivity 

of 10′′ diameter billets and explore potential sensitivity improvements that could be realized 

through the application of MZ techniques on larger (14′′) diameter billets. Improvements in 

sensitivity should lead to enhanced detection of deleterious defects such as hard alpha, reducing 

the risk of catastrophic failure and improving flight safety. An evaluation was also performed on 

the 5′′ diameter nickel (Ni) and Ti billet. Those results are reported in the Ni billet inspection 

report [1]. 

1.2 BACKGROUND. 

In 1989, an in-flight separation of a Ti fan disk led to substantial loss of an aircraft and 111 lives 

in Sioux City, Iowa. The failure was attributed to the occurrence of hard alpha in the Ti-6Al-4V 

disk, a material defect caused by localized concentrations of interstitial impurities, such as 

nitrogen, oxygen, or carbon. The Federal Aviation Administration (FAA) established the 

Titanium Rotating Components Review Team (TRCRT), which was chartered to review the 

design, manufacturing, inspection, and life management procedures of rotating parts used in all 

types of commercial aircraft turbine engines and to make recommendations for improvement of 

their structural integrity [2]. While the occurrence and consequences of the Sioux City crash are 

tragic, they have served as catalysts in shoring up the safety of the commercial aviation engine 

community. 

The FAA is working with the materials, lifing, and inspection communities to address 

recommendations for the TRCRT. The ETC was established in 1993 to address the inspection 

recommendations of the FAA TRCRT report [2] and has as its objective to provide reliable and 

cost-effective new methods and improvements in mature methods for detecting cracks, 

inclusions, and imperfections in materials or components used in engine applications. Starting in 

1992, working with the FAA William J. Hughes Technical Center and New England Directorate 

staff; Iowa State University (ISU); General Electric Aircraft Engines (GEAE); Honeywell 

Engines, Systems & Services (formerly AlliedSignal); and Pratt & Whitney have addressed highpriority inspection issues related to critical rotating components. Other efforts related to lifing 

and design issues are being addressed by the Rotor Integrity Subcommittee and FAA-funded 

efforts as part of the Turbine Rotor Materials Design program at Southwest Research Institute. 

The Jet Engine Titanium Quality Committee and Special Metals Processing Consortium are 

addressing issues related to melt quality. 

1
The conventional rotor life management methodology used in the commercial industry is known 

as “safe life,” where components are designed and substantiated to have a specified service life, 

which is stated in operating cycles and operation hours, during which no detectable cracks will 

occur. Continued safe operation, up to the stated life limit, is not contingent upon interim 

inspections, and when a component reaches the published life limit, it is retired from service. 

Under the safe-life concept, 100 percent of the units are expected to be capable of reaching the 

published life limit. Under nominal conditions, the methodology provides a structured process 

for the design and life management of high-energy rotors, which results in assurance of structural 

integrity throughout the life of the rotor. Because the safe-life approach was founded on a 

database of nominal material and manufacturing conditions, the approach does not address the 

occurrence of material and manufacturing anomalies that can potentially degrade structural 

integrity, e.g., hard alpha in Ti. Undetectable material and manufacturing anomalies, therefore, 

represent a departure from the assumed conditions and necessitate a potential change in the 

maintenance, inspection, and operation of engine components. 

As a result of the Sioux City crash in 1989, the FAA requested through the Aerospace Industries 

Association that the industry review available techniques to determine if a damage tolerance 

approach to engine life management could be introduced and thereby reduce the rate of 

uncontained rotor events [3]. Note that engine uncontained rotor events, in their own right, 

should not result in catastrophic accidents, but have occasionally done so in conjunction with 

other adverse circumstances. Advisory material has been published that presents a generic 

damage tolerance approach that can be readily integrated with the existing safe-life process for 

high-energy rotors to produce an Enhanced Life Management Process [4]. In response to an 

FAA request for prioritization, the industry working group focused its efforts on subsurface 

defects in Ti materials. As efforts were completed on subsurface defects in Ti, focus was turned 

to other alloy systems, i.e., Ni, and surface-related anomalies. It should also be noted that there 

was no intention of using these changes to extend the life of existing engines, but rather the 

intention was to reduce the risk within the existing safe-life approach. Given the focus of these 

industry and regulatory efforts on subsurface flaw detection, the ETC undertook efforts to 

improve the inspection of Ti billets and forgings. 

Many of the recommendations of the TRCRT report addressed the need for improvements in the 

inspection of Ti. Phase I of the ETC Production Inspection Task concentrated on improved 

inspection of Ti billet. Two approaches to zoned inspection, MZ [5] and phased array [6], were 

evaluated and improved with an objective of providing uniform sensitivity inspection at all 

depths. Model-based approaches developed at ISU were used to enhance the inspection, 

including transducer design optimization for both MZ and phased array. Phased array and MZ 

systems were demonstrated to the Ti melters using facilities at RMI Titanium in Niles, Ohio. 

Currently, five MZ systems are operating in a production environment. Phased array systems are 

also being used in evaluation of billet finds by some melters. In a related effort, a program that 

made use of extensive naturally occurring hard alpha defects was completed as part of the ETC 

Phase I and Phase II programs. The program known as the Contaminated Billet Study (CBS) 

provided an unprecedented amount of data in the public domain on the morphology of hard alpha 

and the changes it undergoes throughout the processing and service life of Ti components. 

Acoustic, metallographic, and chemical analysis were completed on ten representative hard alpha 

defects, and the results were provided to the Ti community [7]. Significant improvements were 

2
made in inspection sensitivity with a thorough, quantitative assessment occurring in the 

reliability and CBS programs of ETC. A list of the ETC Phase I accomplishments are provided 

in appendix A. 

Progress made in Phase I on transducer design modeling [8] and transducer characterization 

established a foundation for increasing the sensitivity significantly beyond that achieved by 

current MZ and phased array systems. An extensive fundamental understanding was developed 

for inspection properties of billets and led to identification of significant limitations in the 

methods currently used to measure and compensate for attenuation [9]. Improved methods for 

attenuation compensation were developed in Phase II and are reported in section 3.5 of this 

report. 

The TRCRT report recommended that engine manufacturers should require billets to be 

inspected to the highest standard (smallest flat-bottom hole (FBH) or equivalent) practicable in 

the industry for the size of the part being inspected. The report stated that the levels considered 

to be practicable were a #2 FBH from 6′′ to 10′′ diameter billet and #3 FBH for diameters greater 

than 10′′. Phase I met or exceeded the practicable FBH sensitivities for all diameters, 

demonstrating approximately a four-fold increase in sensitivity. The lifing community requested 

that further improvements be made in inspection sensitivity, particularly for larger diameter 

billets. A goal of #1 FBH sensitivity was established for billets up to and including 10′′ in 

diameter, with an additional need to improve and quantify the inspection sensitivity for >10′′ to 

14′′ diameter billet, all without significant reduction in productivity. ETC Phase I provided 

improved transducer design models, transducer parameters, signal processing approaches, and 

calibration standards with potential to further improve the sensitivity through the application of 

these results. 

The results of Phase I indicated that a sensitivity improvement of 4-6 dB is needed to achieve 

#1 FBH sensitivity in billets up to 10′′ in diameter, with the largest improvement needed in the 

center region of the billet. Several potential approaches were identified, though none 

individually was expected to provide the required improvement in isolation. The potential 

approaches were as follows: 

• Improved specification of transducer parameters through improved, model-based design 

and manufacturing, applying techniques developed in Phase I are expected to yield 2 dB 

sensitivity improvement. 

• The use of electronic distance-amplitude correction (DAC), which is not part of the 

current MZ systems and should provide a 1 to 2 dB sensitivity gain. 

• Material processing approaches to yield lower-noise Ti billets are being pursued outside 

the ETC program and are expected to yield significant sensitivity improvements on the 

order of 2 dB. 

Other approaches considered were use of denser sampling (not pursued because it would reduce 

productivity) and signal processing (not pursued because of concern over production readiness). 

Larger-diameter (13′′ and 14′′) billets cannot be inspected to as high a sensitivity because of 

3
coarser material structure (due to less thermomechanical work) and because of the increased 

difficulty of producing well-focused sound beams through a greater thickness of material. The 

same approaches described above for 10′′ diameter billet were expected to yield significant 

improvement in sensitivity for larger-diameter billets as well, but there may be a falloff in 

sensitivity with increasing diameter. 

Models developed in Phase I were valuable in improving the design and performance of fixedfocus transducers and in understanding the operation of phased arrays. However, even with the 

use of design models, it appeared that transducers were not achieving the performance for which 

they were theoretically capable. An assessment of the discrepancies between the predicted and 

actual performance was completed. 

Measurement of, and compensation for, attenuation differences between billet and calibration 

standards were a source of potential variation between true inspection sensitivity and the FBH 

sensitivity assumed from the calibration standard. This problem has received very little attention 

in the past, so the magnitude of errors due to attenuation compensation is largely unknown. 

Initial approaches to this problem were considered in Phase I and further developed in Phase II. 

1.3 PROGRAM OBJECTIVES. 

The following were the specific program objectives: 

• To demonstrate the UT equipment and techniques required to inspect Ti alloy billets to 

#1 FBH sensitivity for 10′′ diameter and smaller billet. 

• To suggest procedures to account for attenuation effects so that the variation between 

calibration and inspection sensitivity is minimized. 

• To provide an initial assessment of sensitivity at diameters greater that 10′′. 

2. APPROACH.

The approach for this research effort included the following steps. 

Transducer design models and characterization tools were developed and used to optimize 

bicylindrically focused transducers. A comparison of predicted to actual performance for several 

transducers revealed manufacturing variabilities. 

• Laboratory Demonstration on 10′′ Diameter Billet: The results from the Phase I 

measurements (at 2.25′′ and 4.05′′ depths) were used to identify the best combination of 

focal spot diameter and frequency, and this information was used to design a complete set 

of transducers [10]. Phase I work indicated that frequency and bandwidth should be 

increased from the current production transducers, which are 5 MHz frequency and 

approximately 50% bandwidth. Transducer evaluations were performed on the ETC 10′′

diameter standards. The FBH amplitudes, noise, and signal-to-noise ratios (SNR) were 

evaluated. A determination was made of whether the sensitivity level met the #1 FBH 

goal in all regions of the billet. 

4
• Small-Diameter Billet Assessment: Honeywell also performed a sensitivity assessment 

on smaller 5′′ diameter billets. Technology developed in Phase II for larger Ti billet was 

applied to improve Ti billet inspection of interest to small engine manufacturers. A 

feasibility demonstration was conducted in the laboratory environment with small Ti 

alloy billet using MZ transducers developed for the smallest of the large-diameter billets. 

The ETC also chose to evaluate phased array for the small-diameter billets. A sensitivity 

analysis (#FBH) was conducted for the 5′′ billets and a comparison was made with the 

conventional spherical focus approach. 

• Assessment of Large-Diameter Billet (>10′′ diameter): Assessment of the sensitivity at 

larger billet diameters used 14′′ chord blocks. Existing 13′′ calibration standards were 

used, and a 14′′ chord block standard with near-centerline targets was designed and built. 

Initial assessments were compared to baseline conventional (CV) inspection sensitivity 

using transducers borrowed from suppliers. 

• Attenuation Compensation Procedures: The current procedures used to measure and 

compensate for material attenuation were evaluated and improved. The current 

procedure for the MZ inspection uses a preinspection of four short sections (1′′ long) of 

the billet to obtain an average back-wall echo amplitude. This is compared with an 

average back-wall amplitude measured on the calibration standard, and the difference is 

used to calculate an attenuation compensation factor in decibels per inch. A transducer 

focused at the billet center is used to make the measurements. The drawback of this 

method is that it ignores the effects of distortion of the UT beam during propagation 

through the metal microstructure. Phase I work has shown that beam distortion can be a 

major contributor to the response from flaws and back walls. Even when beam distortion 

effects are minor and energy loss dominates, the material attenuation is found to vary 

significantly with position in a given billet in conjunction with noise banding. The 

effects of noise banding and nonuniform attenuation can lead to an incorrect 

measurement of flaw response, and a true inspection sensitivity different from that 

assumed from calibration. The current attenuation compensation technique was 

evaluated by drilling a number of FBHs into several billet sections and comparing the 

measured amplitudes of those holes to those expected from the attenuation analysis. 

Improvements focused on selecting a transducer that minimized the effects of beam 

distortion and provide an attenuation estimate, which enables good prediction of the hole 

echo amplitudes. 

3. DISCUSSION OF RESULTS. 

3.1 VALIDATION AND USE OF TRANSDUCER DESIGN MODELS. 

Computer models were developed at ISU in ETC Phase I and transitioned to the original 

equipment manufacturers (OEM) for use as a design aid to optimize transducer performance, 

which could lead to time and money savings in transducer procurement. At the close of the 

Phase I program, the models were used in procurement of several transducers. Discrepancies 

were found between the predicted and actual performance. The first task of Phase II was to 

identify and resolve any existing discrepancy between experimentally observed and 

5
model-predicted transducer performance. In the Phase I work, it was observed that transducers 

fabricated according to the specifications generated from the models were consistently focusing 

short of the model prediction. The purpose of this task was to establish whether or not such a 

discrepancy existed with the computer models at their present stage of development. If so, a 

careful examination of the various physical assumptions underlying the present computer models 

would occur to determine where the discrepancy lies and enable necessary modifications to the 

code. 

The approach taken to test the model was to specify a transducer design using the computer 

codes, have the transducer fabricated by a manufacturer, and then perform a comparison of 

model predictions and measured performance to determine if a significant discrepancy existed. 

Should a significant discrepancy be observed, the study would then turn to an examination of the 

functioning of the various components of the transducer to determine which of the components 

were being improperly modeled. 

Procurement specifications for the transducers were established using computer models 

employed in the ETC work, which are based on the Gauss-Hermite beam transmission model [1]. 

A different computer model was used in the Phase I procurement activity based on the Green’s 

function transmission model [11]. However, ISU experience to date indicated that, in the case of 

billet geometry, the two computer models yielded nearly identical predictions. To test this 

observation, a current version of the Green’s function transmission model was transferred to 

General Electric (GE) from ISU, and GE personnel performed a model comparison. This 

comparison confirmed past observations that no appreciable difference was observed in model 

predictions. However, in performing this comparison, it was determined that the version of the 

Green’s function model software code used in the Phase I procurement did not include 

provisions to account for acoustic attenuation. This was a provision that was added at later 

stages of the Phase I activity. It is well understood at this point in time that neglect of 

attenuation will indeed result in an erroneous prediction of focal depth, possibly accounting for 

the consistent discrepancy observed in the Phase I activity. 

ETC was able to take advantage of an ongoing, internal GE-funded program to procure MZ 

transducers for 10′′ diameter billets from three transducer suppliers. Under this activity, 

purchase orders were placed with three transducer manufacturers requiring each to make two 

identical transducers according to the current design provided by the Gauss-Hermit model. All 

transducers were built to the same design goal (focus at 3.1′′ depth in 10′′ diameter billet). Two 

of the manufacturers produced transducers having lens configurations, and one transducer 

manufacturer provided transducers to their own design using a Fermat transducer surface without 

a lens. Upon receipt of the sets of transducers, a careful comparison of model predictions and 

measured performance of the transducers was made. 

The comparisons showed that one manufacturer produced transducers that focused short of the 

model predictions. A second manufacturer produced transducers that focused slightly deeper 

than the model predictions. A third manufacturer produced a transducer that was in close 

agreement with model predictions. Figure 1 summarizes some performance measurements from 

the three pairs of transducers. 

6
0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Manufacturer

beam area near (sq in)

A B C

Model

prediction

0.053 sq in

-3 dB Beam Area (Near end of Zone)

-1

0

1

2

3

4

5

6

Manufacturer

amplitude difference (near-far) dB

A B C

Model

prediction

1.17 dB

Zone Balance (dB difference from near

to far target in zone)

 (a) (b) 

FIGURE 1. SUMMARY OF TRANSDUCER PERFORMANCE MEASUREMENTS ON 

BILLET STANDARD 

The model predictions were made using the Gauss-Hermite model and the specified design 

parameters. Manufacturer B used the curved element approach and designed the transducers 

using an alternative model. Comparison of the alternative model with the Gauss-Hermite design 

showed no significant difference in element surface profile. It can be seen that performance of 

Manufacturer B transducers is very close to the model prediction and that their two transducers 

perform almost identically. This confirms the expectation that curved element transducers will 

perform more predictably because the variations associated with lens manufacture are 

eliminated. 

Manufacturers A and C used a lens-focused design with a flat, piezoelectric element. In each 

case, the pair of transducers from each manufacturer did not perform consistently with each 

other. This was not unexpected, as the addition of a lens introduces potential variation in lens 

velocity and possible variation in element flatness, which is not measurable because of the added 

lens. Figure 1 also shows that performance of the lens-focused transducers does not agree as 

well with the model. 

To accelerate the transducer evaluation, it was agreed by the ETC team to participate in a series 

of face-to-face meetings to formulate approaches to accommodate variability in manufacturing 

when determining procurement specifications. At the first meeting, follow-on action items were 

identified and preliminary conclusions were drawn as follows: 

• There can be significant variation in performance between transducers made by the same 

manufacturer. 

• Lens-focused transducers showed variation within and between manufacturers as well as 

variation from the model predictions. 

7
• The curved element transducers with Fermat surface had good consistency within 

manufacturers and good agreement with model predictions. 

A second transducer meeting was held and included presentation of additional data, which 

supported the idea that manufacturing variation is the source of performance/model 

disagreement. The consensus of the ETC team members was that the discrepancy that exists in 

transducer performance was not a result of the design model but rather inconsistencies in the 

transducer manufacturing process. Some sources of the inconsistencies included variation in 

lens velocity, curvature variations, element flatness, and frequency variation. With the action to 

resolve model discrepancies completed, there was the confidence to proceed with the design of 

the transducers for the #1 FBH sensitivity inspection of the 10′′ diameter billet using the ISU 

models. The work did point out the need for additional transducer studies to further improve 

their manufacturing processes. 

3.2 TRANSDUCER DESIGN AND PROCUREMENT.

The plan to design the first prototype transducer included the following steps: 

• Design using a similar design approach to the one used by GE in 1993 for the original 5 

MHz transducers. 

• Team review of assumptions and resulting first design. 

• Use Gauss-Hermite model and iterative procedure to calculate lens radii and predict 

performance. 

• Team review of model results. 

• Finalize the design and procure transducers. 

As a first step, prior experience in designing MZ transducers was used in the first transducer 

design. The general framework for the design was reported by Howard, Copley, and Gilmore 

[12]. The following parameters were considered: 

• Frequency: SNR improves with increasing frequency and with decreasing beam 

diameter. Experimental data supported the original design approach developed by GE of 

maximizing SNR by minimizing the pulse volume. Experience indicates that 7.5 MHz is 

the highest useful frequency for a 10′′ billet. Higher frequencies up to 10 MHz would 

have too much attenuation and would not have enough signal to the billet center. 

• Zones: Continue with the current zones of 0.8′′ to 0.9′′. This is consistent with using an 

eight-channel system for billets up to 13′′ in diameter. Current hardware is limited to an 

eight-channel system. The 14′′ diameter billet would require the zones be increased to 

1.1 inches. 

• Depth-of-Field: The original transducer design was based on a 3 dB sensitivity variation 

within the zone. This was designed for instrumentation without DAC. DAC was to be 

8
operational for the next generation of MZ and for evaluation of the new transducers. The 

decision was made to purchase two sets of transducers, one set of transducers to meet 

similar criteria (3 dB depth of field at 0.8′′) and another more highly focused set with a 

shorter depth of field. The final selection was based on model predictions of beam cross 

section at the ends of the zone. 

• Shape of Element: Utilize elliptical element that most accurately accounts for billet 

curvature. The same F number will be maintained. 

• Calculations: Approximate formulae used for the original design were as follows: 

− Depth of field = 4λ (F/D)2

− Beam diameter = 1.03λ (F/D) 

Where F is the frequency, D is the transducer diameter, and λ is the wavelength. 

• Designs: Approximate calculations from the above formulae were completed for two F 

numbers and resulted in the designs listed in table 1. 

TABLE 1. FIRST-GENERATION TRANSDUCER DESIGN PARAMETERS 

Series 

Frequency 

(MHz) F/D 

Cone Half Angle 

(degrees) 

Beam diameter 

(in.) 

Depth-of-Field 

(in.) 

1 7.5 10 11.6 0.080 0.75 

2 7.5 8 14.6 0.064 0.48 

With the general guidelines described above, two series of elliptical element transducers were 

designed using the iterative design model incorporating the Gauss-Hermite algorithm. Both sets 

of transducers were 7.5 MHz frequency and were based on the current zoning scheme with six 

zones to cover from 0.2′′ to 5.5′′ in depth. One set has an F/8 aperture, producing a focal spot 

diameter around 0.065′′, and approximately 6 dB sensitivity variation within the zone. The other 

set, with a smaller aperture of F/10, was predicted to give 3 dB sensitivity variation within the 

zone and a focal spot diameter around 0.085′′. Table 2 lists the transducer design parameters. 

The zone 1 transducers were spherically focused rather than the bicyclindrical design used for 

the other zones to address potential near-surface resolution issues. 

Calculations were made to estimate whether the proposed transducer would improve signal-tonoise levels enough to achieve the #1 FBH sensitivity goal. An improvement of 4 to 6 dB (equal 

to a factor of 1.6 to 2.0) is needed in SNR. The calculation was made for zone 4 transducers 

using the current production design as a baseline and used the rule-of-thumb that SNR is 

inversely proportional to the square root of the pulse volume. Table 3 shows the estimated 

relative improvement in SNR for the new design transducers, both in the focal plane and at the 

extremes of the inspection zone. These estimated improvements vary from 1.67 times to 2.4 

times, which compares favorably with the 1.6 to 2.0 times improvement required to achieve the 

#1 FBH sensitivity. 

9
TABLE 2. DESIGNS FOR F/8 AND F/10 TRANSDUCERS FOR 10′′ DIAMETER BILLET 

Series 1, F/8 design 

Zone Focus Depth Diameter x Diameter y 

Radius of 

Curvature x 

Radius of 

Curvature y 

1 0.5 Spherical Focus to be Calculated 

2 1.3 1.33 1.03 6.273 8.773 

3 2.2 2.00 1.49 6.939 12.846 

4 3.1 2.67 1.95 7.427 16.501 

5 4.0 3.34 2.41 7.774 20.125 

6 5.0 4.08 2.91 8.024 24.041 

Series 2, F/10 design 

Zone Focus Depth Diameter x Diameter y 

Radius of 

Curvature x 

Radius of 

Curvature y 

1 0.5 Spherical Focus to be Calculated 

2 1.3 1.08 0.83 6.519 10.483 

3 2.2 1.62 1.20 7.073 14.096 

4 3.1 2.16 1.57 7.535 17.440 

5 4.0 2.70 1.94 7.821 21.084 

6 5.0 3.30 2.36 8.115 23.923 

Based on these results, the decision was made to purchase one set of transducers of each of the 

F/8 and F/10 configurations, with transducers for zone 2 (0.9′′ to 1.8′′ depth) and zone 4 (2.7′′ to 

3.6′′ depth) to be ordered initially. It was also agreed to purchase curved element designs, which 

eliminates two of the critical parameters found in lens-focused designs (lens velocity and 

element deviation from flatness). This reduced the critical parameters to radius of curvature in 

the x and y directions (geometrical focal lengths (GFL)x and GFLy) and frequency. 

Because the transducer price was much higher than expected, it was decided to buy only the F/10 

transducers and evaluate them on the ETC 10′′ diameter calibration standards. If the results 

indicated that the F/10 design was adequate for the #1 FBH sensitivity, then the ETC team would 

proceed to buy the additional four transducers to complete the set. If it appeared that the F/8 

transducers were needed to achieve the sensitivity, then they were to be purchased and evaluated. 

There was some additional modeling effort by GE to support the purchase of the two F/10 

transducers. The transducers were designed using an optimization routine based on the ISU 

Gauss-Hermite model. The transducer supplier also performed a design, using the CIVA [13] 

model, resulting in a slightly different curvature and some differences in predicted performance. 

Discussions with the supplier led to the conclusion that the source of the differences was that the 

ETC design incorporated the effects of material attenuation, whereas the supplier design did not. 

The predicted differences were consistent with this explanation, and the supplier was instructed 

to manufacture the transducers according to the ETC design. 

10
TABLE 3. PREDICTED BEAM DIAMETERS AND SIGNAL-TO-NOISE IMPROVEMENT OF PROPOSED 

TRANSDUCER DESIGNS 

Transducer 

Type Depth 

Beam Diameter x 

(in.) 

Beam Diameter y

(in.) 

Beam Area 

(sq. in.) 

Relative Pulse 

Length* 

Relative Pulse 

Volume** 

Relative 

SNR*** 

Current (5 MHz, 1.75′′diameter) 

At focus 0.143 0.115 0.0129 1.5 1.000 1.000 

Proposed 7.5 MHz F/10 

Zone start 0.0803 0.08 0.0050 1 0.260 1.960 

Proposed 7.5 MHz F/10 

At focus 0.086 0.085 0.0057 1 0.296 1.837 

Proposed 7.5 MHz F/10 

Zone end 0.0929 0.095 0.0069 1 0.358 1.672 

Proposed 7.5 MHz F/8 

Zone start 0.072 0.07 0.0040 1 0.204 2.212 

Proposed 7.5 MHz F/8 

At focus 0.066 0.065 0.0034 1 0.174 2.398 

Proposed 7.5 MHz F/8 

Zone end 0.078 0.08 0.0049 1 0.253 1.988 

11

*Used arbitrary values for pulse length assuming pulse length proportional to 1/frequency. **Ratio of the pulse volume to the pulse volume of the current transducer. ***Calculated as 1/(square root of relative pulse volume) 
In an attempt to get further agreement between the CIVA model and the ISU transducer design 

model, the UT attenuation coefficients for Ti were provided to the manufacturer for 

incorporation with their model. After some additional effort, the manufacturer achieved 

agreement in lens designs with the ISU Gauss-Hermite model. 

3.2.1 Transducer Manufacturing Parameter Tolerance Analysis. 

A sensitivity analysis was completed to gain a better understanding of how the element 

curvatures and frequency affect the transducer performance and to help define a manufacturing 

tolerance on these parameters. The analysis was performed for each of the proposed transducer 

designs. The inputs for radii of curvature were the design value and values 1% above and below 

the design value. The inputs for frequency were the design value (7.5 MHz) and values 0.5 MHz 

above and below this. For each transducer, the Gauss-Hermite model was used to predict the 

location of the focal point and the value of the zone balance (amplitude difference between 

targets at the near and far ends of the zone) for all combinations of the input parameters (a zone 

balance limit of 3 dB is currently used for acceptance of production transducers). The output of 

these calculations was used to generate a fit of predicted zone balance versus the deviation of the 

radii and frequency from their nominal values. 

Figure 2 shows an example of fitted data for the zone 4 F/10 transducer and predicted change in 

zone balance (in dB) for variation of ±1% in radius of curvature normal to billet axis (DGFLX) 

and variation of the ±6.7% (±0.5 MHz) in frequency (DFREQ). As shown in the plot, a shift in 

frequency of -0.5 MHz and a shift in radius of curvature of -1% would result in a 2 dB change in 

the zone balance. This analysis was completed for each of the transducer designs to define 

tolerance limits for the purchase specification. 

FIGURE 2. RESULTS OF SENSITIVITY ANALYSIS FOR ZONE 4 

F/10 TRANSDUCER DESIGN 

0.05

0.00

-0.01

-3

-2

dfreq

-1

0

1

2

0.00 -0.05

zb

0.01 dgflx

3.2.2 Transducer Characterization. 

In addition to model validation activities, the ETC team began to look at simple ways the 

transducer manufacturers might have of verifying the performance of the transducers without the 

need for costly reference blocks. The current evaluation of billet transducers requires a 

12
full-round billet standard and the means to rotate and scan it; this is not a practical arrangement 

for a transducer manufacturer. Three different approaches were considered. The approach 

deemed easiest to implement involved mapping the V(z) amplitude profile against a flat surface 

reflector [14]. The test performs a simple V(z) measurement, that is, the voltage in response to 

reflection from a planar surface is recorded as a function of distance, z, from the surface. A 

model was developed to predict the V(z) response for the billet transducers. The predicted V(z) 

response was compared to the measured V(z) response for three transducers. The agreement 

observed in these tests were consistent with the agreement previously observed when comparing 

performance using the FBH billet standard. Specifically, the transducer that showed good 

agreement in the billet standard also showed good agreement in the V(z) measurement. The 

other transducers displayed a discrepancy similar to that observed in the billet standard. It was 

noted that the V(z) measurement allows an independent check of the focal lengths in the 

circumferential and axial directions. Figure 3 shows this agreement for manufacturer B. The 

lens-focused transducers displayed a discrepancy similar to that observed in the billet standard. 

This observation gave further insight into the underlying cause of the anomalous performance 

seen in the billet standard. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000

Reflection amplitude 

from front surface 

Reflection amplitude 

from front surface 

(model prediction) 

Transit time (us) 

FIGURE 3. VOLTAGE RESPONSE V(z) FOR MANUFACTURER B CURVED 

ELEMENT TRANSDUCER 

Tests and modeling were performed to show that the V(z) method is sensitive to errors in the 

axial and circumferential focusing radii and can predict the focusing performance in the billet. 

This was done by using the tolerance analysis to estimate how much perturbation would be 

needed in each of the design parameters in order to cause a 1 dB shift in the zone balance. These 

same perturbations were then used to calculate V(z) curves for the nominal and perturbed 

conditions. The robustness of the method was investigated relative to pulse shape and alignment 

of the transducer during the test. This study indicated that it was a viable transducer evaluation 

process that the supplier might use for an acceptance test. 

MZ transducers used for billet inspections are typically bicylindrically focused to compensate for 

the curved sound entry surface of the billet. The two distinct focal zones in water coalesce to a 

single focal zone in metal after transmission through the billet surface, and the focusing of the 

13
beam in the metal tends to improve the SNRs. Prior to certifying transducers for service, it is 

useful to determine how accurately their actual focal characteristics match their intended design 

values. The focal characteristics are also necessary inputs to inspection simulation models used 

by ETC to estimate the probability of detecting billet defects of various types, sizes, and 

locations. The term characterizing a transducer is used to mean providing a method for 

describing the radiation pattern broadcast by the transducer. 

The radiation pattern of a transducer is determined both by its physical geometry and the manner 

in which it is excited. A broadband sonic pulse can always be decomposed into its Fourier 

(single-frequency) spectral components, and thus, it is sufficient to provide a means for 

describing the radiation pattern at each frequency within the transducer’s operating bandwidth. 

As a working hypothesis, it was assumed that a given commercial transducer behaves 

approximately like an ideal, focused piston probe, i.e., one in which the entire face oscillates 

with uniform amplitude and generates (if focused) curved initial wave fronts in water. The 

characterization problem then reduces to determining, at each frequency of interest, the four 

geometrical parameters shown in figure 4(b): two diameters (Dx and Dy) defining the effective 

lateral size of the elliptically shaped probe cross section and two GFLs (Fx and Fy) describing 

the bicyclindrical curvature of the initial wave fronts in water. These four parameters are 

understood as being properties of the transducer alone, independent of the method in which it is 

excited. Different excitation methods, i.e., the use of different UT pulsers, will produce different 

sonic pulses with different spectral mixtures, but at a given frequency, all would have the same 

radiation pattern to within a constant factor. 

x

y

z

probe billet

Fx Fy

Dy Dx

(a) (b)

FIGURE 4. (a) BILLET INSPECTION GEOMETRY SHOWING THE BEAM COORDINATE 

SYSTEM AND (b) THE FOUR PARAMETERS USED TO CHARACTERIZE THE FOCAL 

PROPERTIES OF A TRANSDUCER 

Three approaches for characterizing MZ transducers were explored prior to selecting the V(z) 

method. Details of all three methods are described below. 

a. The three experiment geometries considered for probe characterization are shown in 

figure 5. In the Beam Mapping in Water (BMIW) method, echoes from a small 

hemispherical target are acquired as the probe is scanned in the two lateral dimensions. 

Several such scans are made with different water paths, bracketing the two focal lengths. 

Although the measurements are easy to perform, the BMIW method generally leads to 

single-frequency images of reflected amplitude, which are relatively complicated 

14
interference patterns at all water paths. This is a consequence of having two, often 

widely separated, focal zones in water. 

b. Since the probe is designed to project a single, well-defined focal zone within a billet, 

simpler images of reflected amplitude can be obtained using the Beam Mapping in Solid 

(BMIS) method. There, echoes are similarly acquired by scanning the probe above a 

small reflector, typically an FBH, located within a metal specimen having a cylindrically 

curved surface. The sound entry surface should have approximately the same radius of 

curvature as the billet the probe was designed to inspect. To map out different portions 

of the beam in metal, several FBHs located at different depths are scanned. The hole 

depths should bracket the focal zone in the metal. In the BMIS method, two methods of 

scanning the transducer above the target can be used: a simple two-dimensional (2D) 

planar scan (in the x y plane of figure 4(a)) or a surface-following rotational scan. There 

are inherent difficulties with using a rotational scan for deep-zone transducers. An FBH 

target located near the billet center is seen over a wide angular range, and hence, the size 

and shape of the amplitude image are primarily determined by the scanning geometry 

rather than the beam spot size. This makes it more difficult to estimate focal parameters 

from the FBH images. This method also requires access to standards for the range of 

billet diameters of interest, which presents a difficulty for the transducer manufacturer. 

c. When using the V(z) method, one orients the transducer at normal incidence to a flat 

reflecting surface and measures the amplitude (V) of the front-wall echo as a function of 

water path (z). Data are acquired at 100 or so equally spaced water paths covering the 

two focal zones. Unlike BMIW and BMIS, the V(z) measurement does not provide 

direct information about transverse beam cross sections. However, it is much simpler to 

apply, and was found to yield accurate estimates of focal characteristics. 

Beam Mapping in Water Beam Mapping in Solid V(z) Method

FBH

scan scan scan

Hemispherical

target

2D scan over target

at each of several

water paths.

No special specimen

needed. Get direct

info about beam

widths, but beam

profiles are

somewhat complex.

2D scan over target

for each of several

FBH depths.

Special specimen

needed. Get direct

info about beam

widths in the actual

billet geometry.

1D scan over flat

surface (typically >

100 water paths)

Simplest experiment.

No direct information

about beam widths in

the two lateral

directions, but probe

char.’s can be inferred.

(a) (b) (c)

FIGURE 5. THREE MEASUREMENT GEOMETRIES FOR ACQUIRING DATA FOR 

TRANSDUCER CHARACTERIZATION 

15
The estimation of focal parameters proceeds similarly for the three measurement geometries. In 

each case, UT A-scans are acquired and stored at each transducer position in the scan pattern. 

Fast Fourier transform operations are then used to produce reflected amplitude-versus-position 

maps at each discrete frequency of interest. These maps are a sequence of 2D images for the 

BMIW and BMIS methods, and a one-dimensional (1D) curve for the V(z) method. The model 

parameters are then adjusted to optimize the agreement between the measured and predicted 

maps. 

There are two general philosophies for the parameter adjustment or fitting procedure. When 

circular, spherically focused, commercial transducers are approximated as ideal piston probes. 

Past studies have shown that the best choice for the effective diameter is usually very near to or 

slightly smaller than the nominal diameter. However, the effective focal length can be quite 

different than the nominal value. Thus, the effective diameter is usually not in question, but the 

focal length is. Assuming that this will also be the case for MZ transducers, one can choose to 

fix Dx and Dy near their nominal values, and then adjust only Fx and Fy. This greatly speeds 

and simplifies the optimization procedure and helps to limit the multiple minima problem that 

occurs when differing sets of {Dx, Dy, Fx, Fy} values are found for which theory and 

experiment are in similar agreement. Rather than fixing Dx and Dy, one can, of course, choose 

to simultaneously fit all four model probe parameters. Both the two- and four-parameter fits 

were considered in the exploratory work. 

Under either fitting philosophy, the optimal probe parameters are determined by using a 

numerical search to minimize the function, shown in figure 6, which quantifies the difference 

between the measured and predicted amplitudes. For the V(z) method, the attenuation of water 

must be known. For BMIW and BMIS, probe characterization can be done even if the 

attenuation values of the water and solid media are unknown. In such cases, the fitting 

philosophy is to adjust the probe parameters so that the measured and predicted 2D amplitude 

images are as similar as possible in shape, i.e., to within an amplitude scale factor that is 

different for each image. Thus, amplitude differences from image to image are ignored. For 

BMIW and BMIS, both the with attenuation (absolute amplitude fitting) and without amplitude 

(profile shape fitting) approaches were considered. 

Chi = w (A - A ) Σ Exp. Thry

j j

2

j

2

Measured amplitude

Predicted amplitude

(depends on Dx, Dy, Fx, Fy)

Sum over

probe positions

j

Weighting factor

FIGURE 6. FUNCTION THAT IS MINIMIZED TO DETERMINE THE BEST FIT 

TRANSDUCER CHARACTERISTICS 

To test the three characterization methods, they were applied in turn to a typical 5-MHz MZ 

transducer. This commercially manufactured probe had a curved element rather than a focusing 

lens. It was designed to focus 3.15′′ deep in the 10′′ diameter Ti alloy billet when operated at 

16
normal incidence with a 3′′ water path (i.e., zone 4 for the 10′′ billet). The nominal design 

parameters were {Dx, Dy, Fx ,Fy}={1.75′′, 1.75′′, 7.5′′, 18.5′′}. 

By chance, each of the three characterization methods was performed using a different inspection 

system, i.e., different pulser and scanning bridge. Since inspection water paths were inferred 

from sound arrival times, for each UT system, it was necessary to determine the sound speed in 

water (v0) and the time offset (t0). The latter appears in the equation t = t0 + 2z0/v0, which 

specifies the arrival time (t) of an echo from a small reflector located at a water path z0 along the 

beam axis. It represents the time delay between the triggering of the oscilloscope trace (when an 

electrical impulse is sent to the probe) and the arrival time of an echo from a target in contact 

with the probe. As illustrated in figure 7, t0 is determined by measuring echo arrival times for a 

series of known water paths and extrapolating to z = 0. To do this, a small spherical target is 

placed in contact with the center of the transducer face, and the water path is defined to be z = 0 

for this setup. The probe is then moved away from the target, with the target remaining on the 

beam axis. The water path is increased until the target echo cleanly separates from the main 

bang, and then data acquisition commences; this occurred for a water path of about 0.13′′ in 

figure 7. Water paths and echo arrival times are then recorded, and a linear fit yields an estimate 

of both the time offset and the sound speed in water. Note that when a focal length in water is 

deduced by any of the methods, that focal length is always measured from the center point on the 

face of the transducer. 

y = 34.215x + 0.8134

R2

 = 1

5.00

6.00

7.00

8.00

9.00

10.00

11.00

12.00

0.10 0.15 0.20 0.25 0.30 0.35 0.40

Water Path from Front-Dead-Center (inches)

Echo Arrival Time (usec) .

Measured

Linear (Measured)

v =0.1485 cm/usec

FIGURE 7. MEASUREMENT OF THE TIME OFFSET (0.81 μsec) FOR THE 

UT SYSTEM USED FOR THE BMIW METHOD 

(The time offset is obtained by extrapolating the line to zero water path.) 

Typical measured amplitude profiles are compared with their fitted model counterparts in figures 

8, 9, 10, and 11. Details about the experimental setups used to acquire probe characterization 

data are shown in figures 8(a), 10(a), and 11(a) for the BMIW, BMIS, and V(z) methods, 

respectively. For the BMIW method, a 1/8′′ diameter glass rod having a hemispherical cap was 

used as the reflector, and 2D scans were performed at ten water paths. For the BMIS method, a 

cylindrical chord block of fine-grained Ni alloy containing FBHs at various depths was used. 

The chord block was one-half of the Inconel (IN)718 10′′ diameter billet calibration standard that 

was fabricated as part of the ETC effort in Ni billet inspection development [1]. As indicated in 

17
figure 10(a), 2D scans were made over four of the holes, three of which lie in the depth zone that 

the transducer was designed to inspect. BMIS data was also acquired for a shallower hole (at 

2.25′′ depth), but that data from the near field of the transducer was quite distorted and deemed 

unusable. For both the BMIW and BMIS methods, each 2D scan typically contained about 80 by 

80 probe positions with about 40 by 40 of these in the region where the reflected amplitude was 

at least 10% of its peak value. For the V(z) method, A-scan data was acquired at 100 equally 

spaced water paths, as indicated in figure 11(a). For all three methods, A-scans were digitized at 

a 100-MHz sampling rate. 

scan

Hemispherical

target

150 usec; 4.36”

200 usec; 5.82”

260 usec; 7.57”

300 usec; 8.74”

350 usec; 10.20”

400 usec; 11.66”

450 usec; 13.12”

500 usec; 14.58”

550 usec; 16.04”

600 usec; 17.50”

Near

shallow

focus

Near

deep

focus

echo

arrival

time

water

path

260 usec (near shallow focus)

600 usec (near deep focus)

350 usec (“equiaxed” beam)

(a)

(b)

(c)

(d)

FIGURE 8. THE BMIW METHOD (a) WATER PATHS USED FOR 2D SCANS AND 

(b-d) COMPARISON OF FITTED (LEFT) AND MEASURED (RIGHT) BEAM PROFILES 

FOR THREE OF THE WATER PATHS 

(Results for the 5.08-MHz spectral component are shown.) 

150 usec 200 usec 260 usec

300 usec 350 usec 400 usec

450 usec 500 usec

550 usec 600 usec

0.5” x 0.5”

FIGURE 9. THE BMIW METHOD FITTED (LEFT) AND MEASURED (RIGHT) BEAM 

PROFILES AT 5.08 MHz FOR EACH OF THE TEN WATER PATHS 

(Images have true aspect ratios and a common size scale, indicated at upper right.) 

18
scan

Chord block

from 10”-dia.

Cylinder of

IN718 alloy.

#1 FBHs

(1/64” dia.)

Depth of holes

beneath entry

surface:

 2.70”

 3.15”

 3.60”

 4.05”

Depth zone # 4

 in 10” billet

3” water path

at top dead

center

Four FBH’s were used for BMIS.

(a)

(e) 4.05”-deep FBH

(d) 3.60”-deep FBH

(c) 3.15”-deep FBH

(b) 2.70”-deep FBH

FIGURE 10. THE BMIS METHOD (a) DATA ACQUISITION DETAILS AND (b-e) FITTED 

(LEFT) AND MEASURED (RIGHT) BEAM PROFILES AT 

5.08 MHz FOR EACH OF THE FOUR FBHs 

(Images have true aspect ratios and a common size scale. Each image measured 0.20′′ by 0.43′′, 

with the long dimension parallel to the billet axis.) 

scan

Used 100 equally-spaced water

paths ranging from 0.7” - 30.5”

(or 24 - 1040 usec arrival time).

Reflector was

front wall of a

2”-thick block

of fused

quartz.

Beam was

normally

incidence on

the reflecting

surface

Front-Wall Echo From Flat Block

Frequency = 5.08 MHz

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 200 400 600 800 1000

FS Echo Arrival Time (usec)

V(z) [Exp. Peak = 1] .

 V(z) Exp. 

 V(z) Thry. 

Dx = 1.711"

Dy = 1.729"

Fx = 7.544"

Fy = 18.137"

(a) (b)

FIGURE 11. V(z) METHOD (a) DATA ACQUISITION DETAILS AND (b) MEASURED 

AND FITTED AMPLITUDE PROFILES AT 5.08 MHz 

19
The good overall agreement between the measured and predicted profiles adds credibility to the 

ideal piston probe approximation that was used. Note the relative complexity of the BMIW 

images compared to those for BMIS. As pointed out earlier, this is a consequence of having a 

single focal zone in the metal, but two widely separated focal zones in water. Thus, for BMIW, 

all measurements are effectively being made in the near field of the transducer. As the frequency 

increases, the BMIW images tend to become more complicated, with a higher density of relative 

minima and maxima. The same is true for BMIS images when the reflector is positioned outside 

the focal zone in metal. The images shown in figure 9(b) for the BMIS method have the correct 

aspect ratio; the image elongation in the axial direction is primarily due to the refraction of the 

central ray by the curved entry surface during scanning. For a fixed transducer position, the 

beam cross section in the metal has a much more circular appearance. For the BMIW images 

shown in figure 8(b), false aspects ratios have been used to make it easier to visually compare the 

measured and predicted amplitude profiles. BMIW images with correct aspect ratios and relative 

sizes are shown in figure 9. There, one can more clearly see the focusing in the horizontal 

direction (x in figure 4(a)) at short water paths, and the focusing in the vertical direction y at long 

water paths. 

For the V(z) method, the two focal points in water manifest themselves as separate maxima 

located at echo arrival times near 250 and 600 μsec in figure 11(b). As the frequency increases, 

these two principal maxima tend to sharpen, and the smaller, secondary peaks (near 300-500 

μsec in figure 11(b)) tend to increase in number. 

Focal parameters deduced by the fitting process when {Dx, Dy, Fx, Fy} are all simultaneously 

varied are shown in figure 10 as functions of frequency for each method. For BMIW, the fourparameter fits results are independent of frequency to within about ±1% in each parameter when 

the full complement of data is used (i.e., 2D images at ten water paths). When a much smaller 

subset of the BMIW data is used, the deduced focal parameters typically display a more marked 

dependence on frequency. As can be seen in figure 12 for BMIW, using all ten water paths led 

to fitted parameters with the least overall dependence on frequency. V(z) was a close second, 

and BMIS was a more distant third. For BMIS, the results are shown (1) using absolute 

amplitude data and available attenuation values and (2) using only relative amplitude data within 

each 2D image. The results for BMIW used only relative amplitude data. 

A commercial transducer will not behave exactly like the ideal piston transducer assumed by the 

model. Thus, there will always be differences between the measured and predicted responses, 

and the precise model parameter set deemed to produce the best fit will depend to some extent on 

details of the fitting procedure. For example, in the BMIW and BMIS methods, an algorithm is 

used to align the centers of the measured and predicted amplitude images, and all measured 

amplitudes in an image below some cutoff value are excluded from the fit. Typical cutoff values 

were 10% of peak amplitude for BMIW and 30%-50% for BMIS. Modifying the alignment and 

cutoff choices can change the best-fit parameters, typically by several tenths of 1% in this 

example. In addition, different choices of the weighting factors (the wj in figure 6) can influence 

the results. Various methods for choosing the weighting factors were explored, and the 

following choices seemed to perform well: 

• For V(z), all weights were chosen equal. 

20
• For BMIW and BMIS, all weights were equal within a given 2D scan, but different 

weights were assigned to each scan; these weights were chosen so that each 2D scan 

contributed about the same amount to the chi-squared function in figure 6. 

• For BMIW, neglecting the absolute amplitudes of each 2D scan and, hence, concentrating 

only on the shapes of the 2D profiles seem to produce the most stable fits. 

• For BMIS, which had relatively few 2D scans as input, using absolute amplitude and 

attenuation data, seemed to improve the fits. 

The metal attenuation value used in the fitting procedure was based on measurements of coupons 

from a different 10′′ diameter IN718 billet that had been Gesellschaft fur Maschinenbau-undFertigungstechik (GFM)-forged like the chord block. GFM employs multiple hammers oriented 

at 90 degrees to one another that pound the ingot while it rotates. Those attenuation 

measurements were made under the ETC Ni billet fundamental studies efforts [15]. 

Three Methods Compared

All Focal Parameters Varied

1.2

1.4

1.6

1.8

2.0

2345678

Frequency (MHz)

Probe Dia. Dx (inches) .

Dx via V(z)

Dx via BMIW

Dx via BMIS

Dx via BMIS w atten

Three Methods Compared

All Focal Parameters Varied

1.2

1.4

1.6

1.8

2.0

2345678

Frequency (MHz)

Probe Dia. Dy (inches) .

Dy via V(z).

Dy via BMIW

Dy via BMIS

Dy via BMIS w atten

Three Methods Compared

All Focal Parameters Varied

14

15

16

17

18

19

20

21

22

23

2345678

Frequency (MHz)

Probe GFL, Fy (inches) .

Fy via V(z)

Fy via BMIW

Fy via BMIS

Fy via BMIS w atten

Three Methods Compared

All Focal Parameters Varied

6.0

6.5

7.0

7.5

8.0

8.5

9.0

2345678

Frequency (MHz)

Probe GFL, Fx (inches) .

Fx via V(z)

Fx via BMIW

Fx via BMIS

Fx via BMIS w atten

FIGURE 12. DEDUCED FOCAL PARAMETERS FOR EACH METHOD WHEN ALL FOUR 

PARAMETERS ARE ALLOWED TO VARY 

The variation of the deduced {Dx, Dy, Fx, Fy} with frequency is one indicator of the uncertainty 

in the measurement procedure. The mean and standard deviation of the fitted focal parameters, 

computed for the 3-7 MHz frequency range, are listed in table 4 for various fitting trials. The 

mean and standard deviation of the single-frequency estimates are shown for the 3-7 MHz 

frequency range. Included are cases where Fx and Fy alone have been varied, with Dx and Dy 

held fixed at 99% of their common nominal value (i.e., approximately the average of the fitted 

21
diameters for the BMIW and V(z) methods). Note that a coordinate measurement machine 

(CMM) was used to map out the physical surface of the transducer. Bicylindrical functions were 

fitted to the CMM data, with both local and global fits being made. These CMM results appear 

in the lower portion of table 4. CMM values for the focal lengths were obtained by fitting a 

bicylindrical surface to the physical transducer surface as mapped out using a coordinate 

measurement machine. The local CMM fit is to the central region of the probe surface only. 

TABLE 4. VALUES OF EFFECTIVE DIAMETERS AND GFL DEDUCED USING 

VARIOUS FITTING METHODS 

Method 

Dx 

(in.) 

Dy 

(in.) 

Fx 

(in.) 

Fy 

(in.) Notes 

V(z) 1.72 ±0.02 1.73 ±0.03 7.54 ±0.02 17.96 ±0.10 A 100 A-scans 

V(z) 1.73 (fixed) 1.73 (fixed) 7.56 ±0.02 17.97 ±0.06 A 100 A-scans 

BMIW 1.74 ±0.01 1.75 ±0.01 7.60 ±0.01 18.03 ±0.17 Scans at 10 water paths 

BMIW 1.72 ±0.04 1.75 ±0.02 7.56 ±0.05 18.28 ±0.58 Scans at 3 water paths 

BMIW 1.73 (fixed) 1.73 (fixed) 7.59 ±0.02 18.16 ±0.24 Scans at 10 water paths 

BMIW 1.73 (fixed) 1.73 (fixed) 7.58 ±0.03 18.10 ±0.44 Scans at 3 water paths 

BMIW 1.73 (fixed) 1.73 (fixed) 7.55 ±0.02 18.66 ±0.42 A Scans at 10 water paths 

BMIS 1.69 ±0.17 1.62 ±0.10 7.98 ±0.52 20.14 ±1.83 Scans of 4 FBHs 

BMIS 1.63 ±0.05 1.67 ±0.13 7.70 ±0.22 18.59 ±0.71 A Scans of 4 FBHs 

BMIS 1.73 (fixed) 1.73 (fixed) 7.91 ±0.38 19.02 ±1.15 Scans of 4 FBHs 

BMIS 1.73 (fixed) 1.73 (fixed) 7.57 ±0.22 18.34 ±0.53 A Scans of 4 FBHs 

Design 1.75 1.75 7.50 18.54 Design specifications 

CMM 7.66 17.83 Local fit to surface 

CMM 7.66 18.35 Global fit to surface 

A = Absolute amplitude and attenuation data used during fitting. 

For the three characterization methods, the differences in the mean values of {Dx, Dy, Fx, Fy} 

are typically within the listed standard deviations. Notice that the uncertainties for BMIS are 

much larger than for the other two methods. The BMIS method is conceptually inviting because 

the sonic beam is focused in the chord block in the same manner as in an actual billet inspection. 

In practice, however, there seem to be two shortcomings that limit its accuracy: (1) available 

chord blocks typically contain FBHs at only a few depths within the designed inspection zone, 

thus limiting the data input to the fitting algorithm and (2) alignment difficulties and the metal 

microstructure act to distort the FBH images, as shown in figure 10(b). 

As shown in table 4, the most trustworthy of the characterization methods (BMIW and V(z)) 

yielded probe diameters in good agreement with the nominal 1.75′′ value when all four 

parameters were varied. This assumes that the effective diameters of curved element MZ 

transducers are at or near the design diameters, and then fitting only the focal lengths. This not 

only speeds the fitting process, but also leads to results that are much more stable with respect to 

changes in the fitting process (i.e., changes in the cutoff values, weights, or profile-centering 

algorithm). 

22
Of the three methods studied, V(z) was by far the quickest, both for data acquisition and for the 

fitting analysis. It yielded average focal length estimates in good agreement with BMIW, about 

midway between the design values and those estimated from the CMM data. Thus, V(z) 

appeared to be the leading candidate for use by OEMs and transducer manufacturers for 

transducer characterization. 

Note that the fitting process required model calculations of the pressure fields generated by ideal 

piston transducers. In this work, the paraxial Multi-Gaussian Beam (MGB) model was used for 

such calculations, since it affords both rapid computation and reasonable accuracy. In the MGB 

model [16], the sonic field emitted by the transducer is represented as a summation of Gaussian 

beams, and in this case, a 15-Gaussian representation was used. Questions arose about the 

accuracy of the paraxial MGB model for calculating the front-wall echoes needed for the V(z) 

method. To check the accuracy, V(z) curves calculated using the MGB model were compared to 

those calculated using a more exact model based on a Green’s function formulation [11]. 

Comparisons were made for four probe designs being considered for 7.5-MHz MZ inspections of 

10′′ billet. Good agreement was seen between the two model calculations, as illustrated in figure 

13 for two typical cases. In addition, the V(z) curves generated by the exact model were used as 

the experimental input into the MGB-based fitting procedure. As shown in figure 14, the 

deduced probe parameters were in good agreement with the correct values, with the differences 

generally being much less than the variations between model and experiment seen when fitting 

actual data from a commercial transducer. The results are shown for four proposed probe 

designs for 7.5-MHz Ti billet inspections. Thus, errors resulting from using the MGB model to 

calculate V(z) curves will likely have little practical impact on the accuracy but have the 

advantage of being a faster calculation. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12 14

Water Path (inches)

V(z) Magnitude

RR_Z2F10_7.2

MGB-15G

Case 3: Zone 4, F8 Transducer

0

1

2

3

4

5

6

0 5 10 15 20 25

Water Path (inches)

V(z) Magnitude

RR_Z4F8_7.2

MGB-15G

Dx and Dy : 1.08” 0.83” 

Fx and Fy : 6.519” 10.483”

Dx and Dy : 2.67” 1.95”

Fx and Fy : 7.427” 16.501”

Case 2: Zone 2, F10 Transducer

7.2 MHz 7.2 MHz

FIGURE 13. COMPARISON OF V(z) CURVES CALCULATED USING TWO DIFFERENT 

BEAM MODELS—THE GREEN’S FUNCTION-BASED FORMALISM AND THE 

PARAXIAL MULTI-GAUSSIAN BEAM MODEL 

23
% Error in Fitted Fx

(Dx, Dy fixed at nominal)

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

Z2F8 Z2F10 Z4F8 Z4F10

Case

Error from Nominal (%)

Fx

% Error in Fitted Fy

(Dx, Dy fixed at nominal)

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

Z2F8 Z2F10 Z4F8 Z4F10

Case

Error from Nominal (%)

Fy

Average

over freq.

Variation

with freq.

FIGURE 14. PERCENTAGE ERROR RESULTING FROM USING THE PARAXIAL MGB 

MODEL RATHER THAN THE GREEN’S FUNCTION FORMALISM TO DEDUCE PROBE 

FOCAL PARAMETERS FROM SYNTHETIC V(z) DATA 

3.3 TRANSDUCER EVALUATIONS. 

3.3.1 Evaluation of F/10 Transducers. 

Using the model-based design approach, the first two F/10 transducers were ordered. Once the 

two transducers for the 10′′ diameter Ti billet were received, a preliminary evaluation was 

performed. The evaluation consisted of measuring beam diameters and zone balance and taking 

scans of FBH targets in the ETC 10′′ diameter standards. A summary of the results is shown in 

tables 5 and 6. Note that the center frequency and bandwidth measurements shown are those 

reported by the manufacturer. The zone balance is defined as the difference in amplitude 

response between targets at the near and far end of the inspection zone. 

TABLE 5. PRELIMINARY EVALUATION RESULTS FOR ZONE 2 TRANSDUCER 

-6 dB Beam Diameter 

Center (Axial x Circular) 

Frequency 

(MHz) 

Bandwidth 

(%) 

Zone Start 

(0.9′′ depth) 

Zone End 

(1.8′′ depth) 

Zone 

Balance 

(dB) 

Measured 6.33 68 0.13′′ x 0.10′′ 0.10′′ x 0.11′′ -5.0 

Specified 7.5 ±0.5 ≥ 60 

Calculated 

from model 

0.086′′ x 0.08′′ 0.10′′ x 0.10′′ 0 

24
TABLE 6. PRELIMINARY EVALUATION RESULTS FOR ZONE 4 TRANSDUCER 

-6 dB Beam Diameter 

Center (Axial x Circular) 

Frequency 

(MHz) 

Bandwidth 

(%) 

Zone Start 

(2.7′′ depth) 

Zone End 

(3.6′′ depth) 

Zone 

Balance 

(dB) 

Measured 6.02 72 0.09′′ x 0.09′′ 0.09′′ x 0.10′′ -0.5 

Specified 7.5 ±0.5 ≥ 60 

Calculated 

from model 

0.080′′ x 0.080′′ 0.093′′ x 0.095′′

at 3.6′′ depth 

-0.5 

The preliminary results indicated that the frequency was lower than the range specified, which 

was a concern. Replacements were to be requested if the performance did not fall within the 

specification. The beam balance and beam diameter results for the zone 2 transducer also 

showed significant discrepancy from the model predictions. Some C-scans were performed 

using these transducers on the ETC 10′′ diameter Ti-6-4 calibration standards, which contain 

FBH targets of diameters ranging from 0.4 mm (#1) to 0.8 mm (#2) at depth locations 

corresponding to the start, middle, and end of each zone. Figure 15 shows the results of the scan 

using the zone 2 transducer, imaging FBH targets at 0.9′′, 1.35′′, and 1.8′′ depth. All the targets, 

including the 0.4 mm, can be discerned in the image. 

0.8 mm dia 

(2/64”) 

0.7 mm dia 

0.6 mm dia 

0.5 mm dia 

0.4 mm dia 

(1/64”) 

0.9” 

deep 

1.35” deep 1.8” 

deep 

FIGURE 15. C-SCAN OF TARGETS IN ETC 10′′ DIAMETER CALIBRATION 

STANDARD USING THE ZONE 2 TRANSDUCER 

The two 7.5-MHz F/10 aperture elliptical transducers were evaluated in more detail. Frequency 

measurements were made using the echoes from #2 FBH targets in 10′′ diameter calibration 

standards. Tables 7 and 8 compare these frequency measurements with those supplied by the 

manufacturer, which had been measured on a flat brass target, located at the near focal point, in 

water. 

25
TABLE 7. FREQUENCY MEASUREMENTS FOR ZONE 2 TRANSDUCER 

Specified 

Reported by 

Manufacturer 

Measured on #2 

FBH, 1.35′′ Deep 

in IN718 

Center frequency (MHz) 7.5 ±0.5 6.33 7.1 

Lower—6 dB frequency 

(MHz) 

 5.4 

Upper—6 dB frequency 

(MHz) 

 8.8 

Bandwidth (MHz) 3.4 

Bandwidth (%) ≥ 60 68 48 

Pulse duration (μsec) 0.21 

TABLE 8. FREQUENCY MEASUREMENTS FOR ZONE 4 TRANSDUCER 

Specified 

Reported by 

Manufacturer 

Measured on 

2/64′′ FBH, 

3.15′′ deep in 

IN718 

Measured on 

2/64′′ FBH, 

3.15′′ deep in 

Ti-6-4 

Center frequency (MHz) 7.5 ±0.5 6.02 6.35 6.25 

Lower—6 dB frequency 

(MHz) 

 4.9 4.7 

Upper—6 dB frequency 

(MHz) 

 7.8 7.8 

Bandwidth (MHz) 2.9 3.1 

Bandwidth (%) ≥ 60 72 46 50 

Pulse duration (μsec) 0.25 0.31 

To assess the performance relative to the #1 FBH sensitivity program goal, a number of scans 

were made using the 10′′ diameter Ti-6-4 ETC calibration standards, which contain targets of 

various diameters down to 0.4 mm (1/64′′). These scans were done as closely as possible to 

production inspection conditions, with the exceptions that the scan index was reduced to 0.01′′ to 

capture the peak signals, and the gate length was reduced where necessary to avoid back surface 

signals in the gate. For the zone 2 transducer, targets at 0.9′′, 1.35′′, and 1.8′′ (start, center, and 

end of zone) were analyzed from the C-scan data. For the zone 4 transducer, targets were at 

3.15′′ and 3.6′′ (center and end of zone). In each case, there was some lateral banding of the 

noise pattern, so regions of noise in the highest and lowest bands were analyzed and are shown 

separately on the plots. Figures 16 and 17 show the target amplitudes and noise amplitudes for 

zones 2 and 4. Note that the zone 4 transducer was not capable of performing an inspection of 

the material in the calibration standard to a 1/64′′ FBH sensitivity. The noise level in the highnoise band was marginally above the amplitude of the 0.4-mm (1/64′′) target at the deep end of 

the zone. A 3 dB margin is required between the noise and the target amplitude to achieve the 

inspection sensitivity. Review of the measured frequency performance, and the manufacturer’s 

26
reported measurements also showed that the transducer had frequency significantly lower than 

specified (measured at 6.02, 6.25, and 6.35 MHz versus the lower specification limit of 7.0 

MHz). The transducer was returned to the manufacturer. 

The zone 2 transducer appeared marginally capable of meeting the required sensitivity. It 

showed a difference of 2.8 dB between the noise and the lowest target signal versus the 3 dB 

requirement. There is a concern that the measured bandwidth was lower than specified (48% 

versus the lower specification limit of 60%), and the manufacturers reported measurement of 

frequency was only 6.33 MHz. 

The smaller F/10 transducers reported above were purchased with the intent of making the final 

decision on aperture size based on the results of their evaluation. Based on the data of figures 16 

and 17, a decision was made to pursue development of transducers designed with the larger F/8 

aperture. 

FIGURE 16. NOISE AND TARGET AMPLITUDE RESPONSES, 

ZONE 2 TRANSDUCER 

FIGURE 17. NOISE AND TARGET AMPLITUDE RESPONSES, 

ZONE 4 TRANSDUCER 

27
3.3.2 Pulse Volume Calculations. 

As part of the decision process to compare F/10 and F/8 transducer parameters, the pulse volume 

concept was used [17]. Based on ETC Phase I activities and analysis by ISU, acoustic noise was 

found to be roughly proportional to the square root of the UT pulse volume. The pulse volume 

in this case is defined as the 6 dB beam area times the 6 dB pulse width. To achieve #1 FBH 

sensitivity, the principle direction of the transducer development was to reduce the pulse volume 

in the focal zone of the transducer, thus reducing the material noise while maintaining a strong 

signal from the calibration target. Decreasing the pulse volume can be accomplished by several 

means: 

• Reducing the focal diameter either by increasing the transducer frequency or increasing 

the aperture 

• Increasing the bandwidth of the transducer, thus shortening the pulse width 

• Combination of both these techniques 

The elliptical design and the increase in frequency from 5 to 7.5 MHz decreases the pulse 

volume by a third at comparable bandwidth over the same transducer at 5 MHz. The F/10 zone 4 

transducer designed to focus 3.15′′ deep produced a peak noise signal in the C-scan image of 

80% compared to the #1 FBH calibrated at 80%. In other words, a SNR of 1. As agreed earlier 

by the ETC, a minimum SNR of 1.4 or 3 dB is required to claim achievement of target inspection 

sensitivity. By knowing the pulse volume and using that transducer to measure the peak noise in 

the billet, one can project the pulse volume required to achieve a signal to noise of 3 dB. 

Although the zone 4 transducer was returned to the manufacturer for frequency adjustment and 

subsequently returned to ETC for evaluation, projections of meeting the required 1.4 SNR would 

not be met with the F/10 design. 

The fixed-focus 7.5 MHz, F/10 elliptical element produced a pulse volume of about 360,000 

mils3

 (600 square root pulse volume (sqrt pv)) at the transducer focal point and a pulse volume of 

577,600 mils3 (760 sqrt pv) at the far end of the inspection zone. The projected pulse volume for 

the F/8 transducer was about 250,000 mils3 (500 sqrt pv) at the far end of the zone, which was 

projected to be very close to the 3 dB SNR requirement. These results are shown in figure 18. 

3.3.3 Phased Array Evaluation. 

To further determine the projected pulse volume required for achieving the #1 FBH sensitivity, 

the ETC 10′′ diameter calibration standard #2 FBH was sent to Honeywell for phased array 

evaluation. The goal here was to capitalize on the phased array capability to vary the focal spot 

size to evaluate pulse volumes that would achieve the required sensitivity. The transducer 

available for this evaluation was a 7.5 MHz annular sectorial array designed for an 8′′ diameter 

billet. 

28
Table 9 shows the details of the beam dimensions and the resultant signal to noise values for one 

set of parameters. The evaluation (figures 19 and 20) of the phased array volume equal to 

311,724 mils3

 (sqrt pv) of 558 produced a SNR of about 2.3, exceeding the 1.4 SNR 

requirement. 

 F/8 

 F/10

FIGURE 18. COMPARISON OF MODELED BEAM DIAMETERS FOR THE F/8 AND 

F/10 TRANSDUCER DESIGNS 

(F/10 = 588 sqrt pv at 3.6′′ depth and F/8 = 495 sqrt pv at 3.6′′ depth. Pulse width at 60% 

bandwidth is 50 mils.) 

29
TABLE 9. DETAILS OF THE BEAM DIMENSIONS AND THE RESULTANT SIGNAL-TONOISE VALUES FOR ONE SET OF PARAMETERS 

Scan Parameters: 

Probe: 7.5 MHz annular-sectorial array 

Focusing: 3.15′′ in emission 

3.15′′ in reception 

Gain: 20 dB 

Water path: 0.5′′

Beam width: 63 mils (1.6 mm) 

Defect 

Size 

(mm) 

Defect 

Depth 

(in.) 

Peak 

Signal 

(%FSH) 

Peak 

Noise 

(%FSH) 

Mean 

Noise 

(%FSH) SNR 

SNR 

(dB) 

-6 dB Beam 

Width 

Along Scan 

Direction 

(deg) 

-6 dB Beam 

Width 

Along Scan 

Direction 

(mils) 

-6 dB Beam 

Height Along 

Index 

Direction 

(mils) 

0.8 3.15 96.5 38.4 20.1 4.17 12.41 3.6 116 100 

0.7 3.15 100 33.7 20.3 5.95 15.49 4.05 131 100 

0.6 3.15 76.5 30.6 19.1 4.99 13.96 4.5 145 100 

0.5 3.15 52.5 26.3 15.7 3.47 10.81 2.7 87 100 

0.4 3.15 41.2 26.3 14.9 2.31 7.26 5.4 174 100 

6.00

8.00

10.00

12.00

14.00

16.00

18.00

0.4 0.5 0.6 0.7 0.8

FBH Diameter [mm]

SN R [dB]--

40

50

60

70

80

90

100

110

120

0.4 0.5 0.6 0.7 0.8

FBH Diameter [mm]

Amplitude [%fsh]-- 

FIGURE 19. RESULTS OF PHASED ARRAY EVALUATION OF THE ETC 

CALIBRATION STANDARD ON THE 3.15′′ DEEP CALIBRATION TARGET 

(A 0.4-mm target equal to #1 FBH produced an SNR of 2.3 or 7.23 dB.) 

30
Peak Noise Zone 4

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0 100 200 300 400 500 600 700 800 900

Sqrt(pulse vol in mils)

Peak noise %

F/10 3.6 in deep

F/10 3.15 in deep

Projected F/8 3.6 in deep

Required Noise Level

Phased Array 3.15 in deep

FIGURE 20. COMPARISON OF THE MEASURED AND PROJECTED PEAK 

NOISE LEVELS VS THE SQRT PV FOR THE F/10, PHASED ARRAY, AND 

PROJECTED F/8 TRANSDUCERS 

(A sqrt pv of between 450 and 500 should meet the 3 dB noise limit.) 

3.3.4 Evaluation of F/8 Transducers. 

Based on the projected pulse volume required to meet the sensitivity requirement, an order was 

placed to construct two F/8 transducers for zones 5 and 6, which are the more difficult and 

deepest zones in the billet. In the event that these transducers met the performance specifications 

but failed to meet the 3 dB SNR requirement on the ETC calibration standard, other options such 

as phased array technology were to be explored as a contingency in the original program. 

Further efforts to reduce the pulse volume on fixed-focus transducers will have to be weighed 

carefully. Reducing the F number to less than an F/8 would necessitate more zones and is likely 

to have diminishing improvement on the SNR. A redesign of the calibration standard is also 

probable because of the increased number of zones. These actions would increase the inspection 

time considerably as smaller scan and pulse increments would be required. 

GE received the two F/8 design transducers in November 2002. These are 7.5 MHz elliptical 

elements designed to focus at 4′′ and 5′′ deep, respectively, and are shown in figure 21. 

FIGURE 21. F/8, ELLIPTICAL ELEMENTS, 7.5 MHz, ZONES 5 AND 6 TRANSDUCERS 

31
Based on transducer modeling, it was expected that there would be about a 7 dB of sensitivity 

difference between the center of focus and the two extreme targets located at the zone beginning 

and end. This sensitivity range will require the use of an instrument that has DAC and a 

calibration standard that has a target positioned in the center of each zone. Figure 22 shows the 

model prediction for these two zones. 

FIGURE 22. ZONE SENSITIVITY MODEL PREDICTIONS FOR THE F/8 TRANSDUCERS 

32
The transducer evaluation was completed using the ETC set of four 10′′ diameter Ti-6-4 

calibration standards, which contain a center hole in each zone that is useful in calibrating a 

DAC. A commercially available instrument was selected for the measurements because of its 

DAC capability, and it could easily be transported and adapted to the factory site for later 

evaluation. The gated output of the commercial instrument was modified for compatibility to the 

MZ data acquisition system for C-scan generation. This was necessary because the MZ 

hardware is not currently capable of DAC. 

The F/8 transducer focal properties were measured in water and found to be well within the 

purchase specifications. Following some initial evaluation with the transducers on the ETC 

standards, an order was placed for the four remaining transducers covering zones 1 through 4 of 

the 10′′ diameter billet. The four remaining F/8 transducers were received in June 2003. 

The complete set of six 7.5-MHz F/8 transducers were evaluated on the ETC Ti-6-4 calibration 

standards and the random defect billet to determine sensitivity curves. Detailed beam 

measurements and pulse widths were taken for each transducer from which to calculate a pulse 

volume. 

Figure 23 shows the six transducers that comprise the 10′′ diameter billet set and a table of the 

element and lens designs for each transducer. 

Z1 Z2 Z4Z3 Z5 Z6

Focus 

Depth 

(in.) 

GFLX 

(in.) 

(circular) 

GFLY 

(in.) 

(axial) 

DIAX 

 (in.) 

(circular) 

DIAY 

(in.) 

(axial) Zone 

GFLX 

(model 

design) 

GFLY 

(model 

design) 

0.5 4.58 5.08 0.74 0.62 1 5.506 5.671 

1.3 5.97 8.42 1.33 1.03 2 6.273 8.773 

2.2 6.83 12.17 2.00 1.49 3 6.939 12.846 

3.1 7.36 15.92 2.67 1.95 4 7.427 16.501 

4.0 7.72 19.67 3.34 2.41 5 7.774 20.125 

5.0 8.00 23.84 4.08 2.91 6 8.024 24.041 

FIGURE 23. SET OF SIX F/8 7.5-MHz TRANSDUCERS AND TABLE 

OF DIMENSIONS 

33
Once the transducers were received, the following procedure was used to calibrate each zone of 

the ETC calibration standards: 

• Initially establish a 3′′ water path. 

• By a combination of water path adjustment and achieving the maximum response from 

the target holes in each zone, the transducer to billet orientation was established. Adjust 

the water path within the range of ±0.5′′ of 3′′ to obtain the best balance between the 

shallowest and deepest holes for each zone. 

• Record the amplitude of all three holes in the zone without DAC. 

• Adjust the greatest FBH signal close to 80% full screen height (FSH), which was the 

center hole for each zone, then create a DAC to bring each FBH to 80%. 

• Generate a C-scan of the calibration holes using a 0.020′′ index, adjusting the gate during 

scanning so the back wall from adjacent holes is not gated. 

• Record the maximum amplitude in the C-scan using the full zone gate to get a peak noise 

figure. 

• Determine the 6 dB pulse width using the A-scan from the zone’s center hole target. 

• Measure the 3 and 6 dB axial and circumferential dimensions of the transducer beam on 

the shallow center and deep holes for each zone. 

• Calculate the 6 dB pulse volume using the pulse width and beam diameters and take the 

square root of this figure. 

Table 10 summarizes the results of these measurements for each of the six transducers. 

The pulse volumes were calculated using the 6 dB beam dimensions and pulse width (the results 

are in cubic mils). The largest pulse volume of the three target holes in each zone were 

compared to the largest volume derived from a model calculation of the F/8 transducers. These 

comparisons along with the F/10 model predictions are shown in figure 24. The model assumes 

a constant 6 dB pulse width of 200 nsec or about 0.050′′ of material. 

Comparing the delivered large element transducers and the predicted pulse volumes highlight 

some of the current issues with transducer manufacturing. Although progress is being made in 

transducer fabrication, there are still parameters not tightly controlled, mainly the frequency and 

bandwidth. These two transducer characteristics have a direct impact on the desired pulse 

volume. 

34
TABLE 10. SUMMARY OF THE TRANSDUCER BEAM CHARACTERISTICS FOR 

PULSE VOLUME CALCULATIONS AND RESULTS OF THE NOISE STATISTICS FOR 

THE ETC AND RANDOM DEFECT BILLETS 

Transducer Beam Characteristics ETC 

Random 

Defect Billets 

Water 

Path 

FBH 

Depth 

No 

DAC 

Amp. 

Axial 6 

dB 

(in.) 

Axial 3 

dB 

(in.) 

Cir 

(deg.) 

Cir 

(in.) 

Pulse 

Width 

(nsec) 

Sqrt 6 dB 

Pulse 

Volume 

C-Scan 

Max. 

C-Scan 

Mean 

C-Scan 

Max. 

C-Scan 

Mean 

Z1 102 μsec 0.2 31 0.065 0.6 0.050 354 33 18 31 15 

0.55 80 0.04 0.029 0.5 0.039 200 244 23 10 29 9 

0.9 31 0.058 0.8 0.057 357 

Z2 105 μsec 0.9 30 0.092 1 0.072 539 

1.35 80 0.055 0.037 0.95 0.061 230 383 41 16 54 17 

1.8 29 0.081 1.4 0.078 528 

Z3 105 μsec 1.8 30 0.087 1.05 0.059 442 

2.25 82 0.056 0.039 1.1 0.053 200 337 42 15 43 16 

2.7 30 0.061 1.85 0.074 417 

Z4 103 μsec 2.7 30 0.097 1.6 0.064 535 

3.15 58 0.07 0.048 1.8 0.058 240 433 62 26 72 29 

3.6 32 0.071 2.6 0.064 456 

Z5 104 μsec 3.6 36 0.098 2.75 0.067 615 

4.05 80 0.07 0.056 5.2 0.086 300 589 64 23 71 24 

4.5 34 0.085 9.5 0.083 637 

Z6a 103 μsec 4.5 80 0.115 5.4 0.047 499 50 17 55 20 

5 80 240 

Z6b 96 μsec 5 80 8.4 0.073 475 64 27 70 29 

5.5 80 0.067 240 

200

300

400

500

600

700

800

1 2 3 4 5 6a 6b

Zone

Sqrt Pulse Volume (mils)

F10 Model

F8 Model

F8 Delivered

FIGURE 24. COMPARISON OF PULSE VOLUMES AS DETERMINED BY THE ISU 

MODEL AND THE PULSE VOLUMES OF THE DELIVERED TRANSDUCERS 

35
3.3.5 Explanation for Zones 6a and 6b. 

Following numerous attempts without success to achieve adequate signal to noise from the zone 

6 transducer on the ETC calibration standard, a second approach was evaluated. This involved 

dividing zone 6 into two smaller zones, referred to as 6a and 6b. Zone 6a extends from 4.5′′ to 

5′′, and zone 6b extends from 5′′ to 5.5′′. The goal was to decrease the maximum pulse volume 

by shortening the inspection zone that resulted in improved SNR. Figure 25 illustrates this 

principle. 

4.5

5.0

5.5

1”zone 6

4.5

5.0

5.5

Conventional Zone 6

.5”zone 6a

.5”zone 6b

Zone 6A and Zone 6B Zone 6a and Zone 6b 

FIGURE 25. ZONE 6 DIVIDED INTO TWO ZONES, 6a AND 6b, 

TO IMPROVE SNR 

To achieve a balance between FBH targets in each zone, the transducer water path was changed 

slightly. Adjusting a bicyclindrical transducer off its designed operating water path will of 

course distort the beam near its focus and is not as straight forward as for a spherical transducer 

on a flat entry surface; however, the water path adjustments are small (< 0.25′′). Figure 26 

shows the intended effect of shortening the inspection zone to 0.5′′. 

 F/8 F/10 

-8

-7

-6

-5

-4

-3

-2

-1

0

2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6

Metal Travel Depth (inch)

UT Amplitude (dB)

CV10" (dB)

-8

-7

-6

-5

-4

-3

-2

-1

0

2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6

Metal Travel Depth (inch)

UT Amplitude (dB)

CV10" (dB)

.5 inch zone

FIGURE 26. COMPARISON OF SENSITIVITY VARIATION THROUGHOUT THE 

INSPECTION ZONE FOR THE F/10 AND F/8 TRANSDUCER DESIGNS 

36
The sensitivity loss across the zone drops to less than 3 dB, as highlighted by the red dashed 

lines in figure 26. Based on earlier measurements of the F/10 zone 4 transducer and the 

respective material noise from the ETC calibration standards, predictions were made of the 

performance of the F/8 transducers. The pulse volume predicted by the ISU models indicated 

that the F/8 transducers would be close to meeting the noise criteria but perhaps just short of the 

goal. Projections from the zone 4 F/10 transducer indicate that a sqrt pv of about 430 or smaller 

would achieve the 3 dB SNR. The pulse volume was calculated for zones 6a and 6b using the 

measured beam areas and pulse length. 

The results of these measurements produced sqrt pv’s of 500 for zone 6a and 476 for zone 6b, as 

shown in figure 27. Peak noise values were then measured on both the ETC calibration standard 

and the random defect billet (RDB). Physical complexities of the ETC standard that contains 

pilot holes and split lines limit the areas to measure noise, so the RDB was used to supplement 

noise data using a full gate and without artifact signals. These were then plotted against the 

predicted pulse volumes for the F/8 transducer, as shown in figure 28. The actual performance 

of the transducer using the smaller zones was in good agreement with earlier predictions of pulse 

volume capability. Inspection of zone 6a on the RDB and ETC calibration standards indicates 

that the #1 FBH sensitivity could be achieved if the shorter zone were used. The same 

evaluation using zone 6b fell short of the 3 dB peak noise requirement. The C-scan in figure 29 

shows the results of 6a and 6b zoning and the five FBHs in the ETC standard. 

Zone 6a

4.5′′ deep hole 3.0-inch water path 

Axial dia. = 0.115′′

Cir. dia. = 0.047′′ (5.4 deg) 

Pulse width = 0.24 μsec. @ .244′′/μsec = 0.059′′

Square root pulse volume = 500

Zone 6b

5.5′′ deep hole 2.8-inch water path 

Axial dia. = 0.067′′

Cir. dia. = 0.073′′ (8.4 deg) 

Pulse width = 0.24 μsec. @ .244′′/μsec = 0.059′′

Square root pulse volume = 476

FIGURE 27. PULSE VOLUME CALCULATIONS FOR THE ZONE 6 TRANSDUCER 

37
Peak Noise Zone 4, relative to #1fbh at 80%

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0 100 200 300 400 500 600 700 800 900

Sqrt(pulse vol in mils)

Peak noise %

Data from current 

F10 transducer

3.6” depth

3.15” depth

Projection for F/8 at 

3.6” deep (65% noise)

Where we need to be

(57% noise)

x

x

x F8 measured pulse 

Volume – zone 6

FIGURE 28. PERCENT NOISE PEAK VS THE SQRT PV ORIGINAL 

CURVE BASED ON F/10 ZONE 4 RESULTS 

4.5 inches 5.0 inches 5.5 inches 

FIGURE 29. C-SCAN OF ETC CALIBRATION STANDARD SHOWING THE 

FIVE FBHs FOR ZONES 6a AND 6b 

(FBH sizes range in size from 0.4 to 0.8 mm (#1 to #2).) 

3.3.6 Sensitivity Curves. 

Two sensitivity curves were produced, one using the ETC 10′′ diameter Ti standards (figure 30) 

and the second using the RDB (figure 31). There is some difference between the two due to 

different peak and mean noise values identified in the C-scans. 

38
Elliptical Element, F8, 7.5 MHz Transducer Capability

on 10′′ Diameter Ti6-4 ETC Standard (3 dB)

0.0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5 6

Depth Beneath Surface (Inches)

Test Sensitivity (FBH#) 

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Zone 6a

Zone 6b

#1 FBH Sensitivity Goal

Current Production Multi zone Amplitude Reject #1.87

3.18 X Sensitivity Improvement

FIGURE 30. SENSITIVITY GRAPH USING THE ETC 10′′ DIAMETER 

CALIBRATION STANDARDS 

Elliptical Element, F8, 7.5 MHz Transducer Capability

on 10′′ Diameter Ti6-4 Random Defect Billet (3 dB)

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6

Depth Beneath Surface (Inches)

Test Sensitivity (FBH#) 

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Zone 6a

Zone 6b

#1 FBH Sensitivity Goal

Current Production Multi zone Amplitude Reject #1.87

2.8 X Sensitivity Improvement

FIGURE 31. SENSITIVITY GRAPH USING THE RDB 10′′ DIAMETER SPECIMEN 

These sensitivity curves break the capability down on a zone-by-zone basis, and although the 

intent of the MZ inspection approach was to maintain the same pulse volumes in each zone, this 

was not always achieved. In addition, the microstructure characteristics from the outside 

diameter (OD) to the inside diameter (ID) of the billet change. Grain size is typically larger near 

the ID and will tend to create added noise. Although it might be possible to maintain the same 

pulse volume at each depth, the peak noise will change due to grain size and microstructure, 

resulting in a change in sensitivity. An examination of both curves shows the sensitivity 

decreasing with depth. 

39
The values used in the graphs were obtained by taking the greatest noise value identified in each 

C-scan and determining the FBH number that would provide a response that is 3 dB greater than 

the noise. This is given by the expression: 

FBH# = [FBH#cal ] [√ Npk+3 dB / Scal] (1) 

Where FBH#cal is the size of the calibration target in terms of FBH number, Npk+3 dB is the peak 

noise value in the C-scan image at calibration plus 3 dB, and Scal is the amplitude of the 

calibration signal (usually 80%). As an example, figure 32 shows the C-scan image for the first 

three zones. The maximum noise values were taken from these images for the ETC sensitivity 

graphs. As part of the study, the RDB was inspected at the sensitivity level achieved with the #1 

FBH calibration. Zone 2 from the RDB is presented in figure 33. 

FIGURE 32. #1 FBH INSPECTION SENSITIVITY FOR ZONES 1-3 ON THE ETC 

CALIBRATION STANDARD 

FIGURE 33. ZONE 2 #1 FBH SENSITIVITY ON THE RDB 

40
3.4 SUMMARY OF LABORATORY EVALUATION FOR 10′′ DIAMETER BILLET. 

Current inspection of 10′′ diameter Ti billet was to a #2 FBH sensitivity using six 5-MHz F/10 

transducers and six zones. Moving to a #1 FBH or 3.5 times increase in sensitivity using fixedfocus transducers was demonstrated in a laboratory environment but would have some 

challenging obstacles ahead for practical implementation. 

Maintaining transducer to billet orientation would be one critical factor. In conducting the 

laboratory evaluation, it became apparent that the response from the calibration targets was very 

sensitive to transducer alignment and water path. This is particularly true for deep zones 5 and 6. 

Very small changes in either transducer angle or water path produced large differences in target 

response. The surface finish of the billet would also have to be monitored more closely. 

Achieving a sqrt pv of 450, particularly with the large element transducers, also presents 

difficulties for the transducer manufacturer. Although it might be possible to occasionally make 

a transducer that meets the specifications, consistently being able to reproduce and reliably 

manufacture sufficient quantities for industrial use is at issue. 

The companion attenuation compensation study discussed below reveals the role this variable 

plays in achieving uniform inspection. Based on the location of the calibration targets, there will 

be an impact on inspection sensitivity. Efforts to produce more uniform billet microstructure and 

reduce this variability would lead to improved Ti billet inspection. Much of the difficulty in 

inspecting the Ti billet is the variation in the attenuation and noise associated with the billet 

conversion practices. Reducing this variation would simplify the UT inspection as would any 

effort to refine the grains near the billet center. 

Figures 30 and 31 show that #1 FBH sensitivity at the 3 dB level was accomplished for three of 

the six zones. In the three deepest zones, the # 1 FBH target was missed by 10% or a sensitivity 

of #1.1 FBH. Based on the peak noise that was observed in both the ETC calibration standards 

and the RDB, a pulse volume of 202,500 mils3 or a sqrt pv of 450 should achieve the required 

sensitivity for all zones. It was not clear whether transducer suppliers could successfully or 

consistently deliver on the required transducer specifications for the larger elements. One 

alternative approach would be to divide the length of zone 6 into two zones, as was done in this 

study. This would have the undesirable result of increasing either the number of transducers or 

the scan time. This would also necessitate the creation of new zoning schemes and redesign of 

the calibration standards from 1′′ to 0.75′′ or 0.5′′ zones. Another alternative would be the 

development of phased array inspection practices. 

3.5 ATTENUATION COMPENSATION PROCEDURES. 

One goal of the ETC Phase II program was to critically examine existing methods for attenuation 

compensation during UT inspections of Ti alloy billets and, if necessary, to recommend 

improvements. This section documents those research efforts and resulting recommendations. 

As illustrated in figure 34, the billets to be inspected have a cylindrical geometry, with diameters 

ranging from 5′′ to 14′′ and lengths up to 20 ft. The inspection procedure is depicted 

schematically in figure 35. Two inspection methods are commonly used, CV and MZ, 

41
respectively [17 and 18]. In the former, one cylindrically focused transducer with the focus 

located just below the billet surface is generally used to cover the depth range from the outer 

surface to just beyond the billet center, and a depth varying gain (or DAC) is applied so that the 

echo amplitudes from identical targets were independent of depth. For larger-diameter billets, a 

CV inspection may employ a second transducer focused deeper in the billet. For MZ 

inspections, a larger number of transducers were used, ranging from 4 to 7, depending on billet 

diameter, with each transducer designed to focus the sound beam within a given depth zone; the 

gain setting for each transducer is independently set, and DAC is generally not used within a 

given depth zone. Details of this method are provided in appendix B. For either method, the 

inspection procedure makes use of a calibration standard of the same diameter that contains FBH 

reflectors of a fixed size located at a series of depths (one FBH per depth). The MZ calibration 

standards containing the FBHs generally have a full-round geometry, with the holes themselves 

drilled into internal cavities. For CV inspections, the FBH reflectors are usually contained in 

chord blocks. For both types of inspections, an unaltered full-round section of the calibration 

billet material was available for back-wall signal acquisition. In this report, extensive use was 

made of a 6′′ diameter, Ti-6-4 MZ calibration standard as illustrated in figure 36. 

FIGURE 34. BILLETS AWAITING INSPECTION 

Typically

set FBH 

echoes to 

80% of full 

screen.

Conventional Inspection

billet

rotate

FBHs in

cal. standard

( to OD )

water

transducer

FBHs in

cal. 

standard

Multi-Zone Inspection

billet

rotate

water

transducers

Typically

set FBH 

echoes to 

80% of full 

screen.

Conventional Inspection

billet

rotate

FBHs in

cal. standard

( to OD )

water

transducer

FBHs in

cal. 

standard

Multi-Zone Inspection

billet

rotate

water

transducers

Typically

set FBH 

echoes to 

80% of full 

screen.

Conventional Inspection

billet

rotate

FBHs in

cal. standard

( to OD )

water

transducer

Conventional Inspection

billet

rotate

FBHs in

cal. standard

( to OD )

water

transducer

FBHs in

cal. 

standard

Multi-Zone Inspection

billet

rotate

water

transducers

FBHs in

cal. 

standard

Multi-Zone Inspection

billet

rotate

water

transducers

FIGURE 35. CONVENTIONAL AND MZ BILLET INSPECTIONS USE CALIBRATION 

STANDARDS CONTAINING FBH REFLECTORS 

42
Holes are 2/64 inch diameter.

Dimensions are in inches.

0.

20

0.

80

Nominal Inspection Zones:

Zone 1: 0.2′′ to 0.8′′ Zone 2: 0.8′′ to 1.7′′

Zone 3: 1.7′′ to 2.6′′ Zone 4: 2.6′′ to 3.5′′ (Inspection gates are slightly larger to allow overlap)

0.20

0.80

FIGURE 36. CALIBRATION STANDARD FOR AN MZ INSPECTION OF A 

6′′ DIAMETER BILLET 

The basic inspection task is illustrated in figure 37. The calibration standard was first examined 

using the set of transducers. For either a MZ or CV billet inspection, the measurement system 

amplification (gain) was adjusted so that FBHs in the calibration standard produce sonic echoes 

of a specified amplitude, typically 80% FSH. It was desired that similarly sized FBH reflectors, 

if present in the billet to be inspected, would produce echoes of similar amplitude. In particular, 

as depicted in figure 37, if a reflector at depth z in the calibration standard produces an amplitude 

of 80% FSH, then similar reflectors in the test billet at the same depth but arbitrary axial and 

circumferential positions should all produce amplitudes greater than or near 80% FSH. 

Depth

z

Amp = 80%

Depth

z

Calibration Standard.

#2 FBH in calibration standard

at depth z set to 80% FSH

All Amps ≥ 80%

Billet Under Inspection.

#2 defects anywhere at same depth

should produce amplitude ≥ 80%

RF signal

FIGURE 37. ULTRASONIC BILLET INSPECTION (IDEALIZED AT DEPTH Z) 

43
The amplitudes of echoes seen in such pulse/echo inspections were determined in part by the UT 

attenuation of the billet material. Because the attenuations of the calibration standard and test 

billet may be different, the gain settings required for the test billet inspection may be different 

than those set using the calibration standard. The adjustment of inspection gain to account for 

attenuation differences is referred to as compensation. 

In current practice, attenuation compensation values (when used) were generally determined by 

comparing the average amplitudes of back-wall echoes seen in the calibration standard and the 

test billet, respectively. This is shown in figure 38, with the back-wall data for both specimens 

gathered prior to the final inspection using the same transducer and same gain. 

Calibration Standard

billet

rota et

water

transducer

<A cal >

Average BW

amplitude

Test Billet

billet

rota et

water

transducer

<A test >

Average BW

amplitude

Calibration Standard

billet

rota et

water

transducer

<A cal >

Average BW

amplitude

Calibration Standard

billet

rota et

water

transducer

<A cal >

Average BW

amplitude

Test Billet

billet

rota et

water

transducer

<A test >

Average BW

amplitude

Test Billet

billet

rota et

water

transducer

<A test >

Average BW

amplitude

FIGURE 38. THE STANDARD METHOD FOR ATTENUATION COMPENSATION 

MAKES USE OF AVERAGE BACK-WALL AMPLITUDES 

For MZ inspections, back-wall amplitudes were generally measured using the deep-zone 

transducer, which was focused near the billet center. For CV inspections employing more than 

one transducer, the deeper-zone probe was used to acquire back-wall echoes. The calibration 

standard typically contained a full-round segment without interior cavities or FBHs; this was 

used for back-wall echo acquisition. Since the test billet was also free of cavities, any 

representative portion of its length could be used for back-wall echo acquisition. 

Let <Acal> and <Atest> denote the average back-wall amplitudes seen in C-scans of the 

calibration standard and test billet, respectively. For the subsequent inspection of the test billet, 

the gain adjustment used to compensate for attenuation at depth z is generally taken to be 

Gain adjustment (in dB) at depth z = 

D

z

A

A

test

cal ⎥

⎦

⎤ ⎢

⎣

⎡

><

><

10 log20 (2) 

where D is the billet diameter. For example, if the billet diameter is 10′′ and the back-wall 

amplitude difference is 15 dB, the compensation value would be 1.5 dB/inch of depth in the test 

billet. The additional gain applied during the inspection of the test billet, as calculated using 

equation 2, would range from 0 dB at the OD to 7.5 dB at the billet center. Such compensations 

based on average back-wall amplitudes will be referred to as the standard method. The standard 

method, and several variations, will be discussed in section 3.6.6. 

44
3.6 BANDING PHENOMENA AND UT PROPERTY VARIATIONS. 

Before proceeding with the critique of the standard method, it is useful to recall several facts 

concerning Ti alloy billets. A typical billet begins as a cast ingot, which is worked by 

thermal/mechanical processing to reduce its cross-sectional area and to achieve a cylindrical 

geometry. The final microstructure can be quite complex, possessing identifiable features on 

several length scales ranging from centimeters for the large-scale macrostructure to microns or 

tens of microns for the individual metal grains, which are often clustered into colonies of 

intermediate size. Typical microstructures of Ti alloy billets were documented in the ETC 

Phase I report. 

Although the billet shape has cylindrical symmetry, the billet microstructure and the associated 

UT properties were not cylindrically symmetric. This was because asymmetries were introduced 

by the mechanical working process, which usually involves rotating the billet through large 

angles between hammer strikes. The so-called banding patterns commonly seen in C-scan 

images of backscattered grain noise and back-wall amplitude were clear evidence of the 

deviation of billet microstructures from cylindrical symmetry. Typical examples of banding 

patterns are shown in figure 39. These images resulted from the inspection of a length of 14′′

diameter Ti-6-4 billet intended for use in the fabrication of a calibration standard. The billet was 

inspected using a deep-zone MZ probe designed to focus near the billet center. The left-hand 

image in figure 39 shows backscattered grain noise amplitudes seen using an inspection gate 

extending from 6.4′′-7.5′′ deep in the billet. The right-hand image shows amplitudes of backwall echoes when the inspection gate is narrowed and centered on the back wall. Both images 

have the same registration, meaning that corresponding points in the two images have the same 

axial and circumferential position on the billet surface. One sees that bands of low back-wall 

amplitude (high attenuation) match with bands of low backscattered noise, a common 

phenomenon for Ti alloy billets [19]. The banding patterns seen in figure 39 have an 

approximate four-fold rotational symmetry in the hoop direction. Four-fold or two-fold banding 

symmetries are often seen for Ti alloy billets. Note that throughout this report, C-scan images 

such as those in figure 39 always display rectified gated-peak amplitude versus scan position. 

As might be expected on the basis of figure 39, different regions of a given Ti alloy billet can 

have significantly different UT properties. This was documented during Phase I of the ETC 

program [19]. One case study, using a 6′′ diameter Ti-6-4 billet, is summarized in figure 40. 

Panel (a) displays the backscattered grain noise pattern that was seen when an axial-cylindrical 

scan of the specimen was made using a 5-MHz bicylindrically focused MZ transducer, and the 

resulting gated-peak noise amplitude was displayed as a function of scan position [19]. 

Prominent bands of high and low backscattered noise amplitude are seen. As shown in panel (b), 

two sets of rectangular coupons were cut from the billet for property measurements. Each set of 

three coupons was cut along a radius of the billet, one set from a region having a low-noise 

amplitude near the OD (at 1′′ depth), and the other set having a high-noise amplitude near the 

OD. For sonic beam propagation in the radial direction, figure 40(c) shows measured 

longitudinal wave speeds, attenuations, and figure of merit (FOM) values for each coupon. The 

FOM is a measure of the noise generation capacity of the microstructure that (ideally) is a 

property of the microstructure alone and independent of the details of the measurement 

45
procedure [19, 20, and 21]. One sees that the UT properties vary significantly throughout the 

billet, and that the differences are greatest near the OD. Rotation Angle

Low BW

Amplitude

High BW

Attenuation

Low Noise

Amplitude LOW HIGH

Axial position

scan

rotate

billet

Center Zone Grain Noise: 6.4′′-7.5′′ BW Amplitude 

FIGURE 39. BANDING PATTERNS SEEN DURING THE INSPECTION OF A 14′′

DIAMETER Ti-6-4 BILLET SPECIMEN 

It is also important to note in figure 40 that changes in velocity and attenuation values are well 

correlated with changes in backscattered noise capacity. For Ti alloy billets, one generally finds 

an inverse relationship between backscattered noise and effective attenuation. This is shown in 

figure 41, which displays the results of measurements performed on the same set of six billet 

coupons shown in figure 40(b). For each coupon, measurements of UT attenuation and 

backscattered grain noise capacity were made in the radial, hoop, and axial directions [22]. The 

results at one frequency of interest (15 MHz) are displayed in figure 41, and the trend line 

through the data indicates that low values of grain noise are correlated with high values of 

attenuation. This inverse relationship is believed to be due to large-scale macrograins present in 

the cast Ti ingot, which become elongated in the axial direction during the working of the billet 

[22]. Knowledge of FOM variations is required to draw conclusions about attenuation levels 

from noise banding patterns. 

46
Mask

Scan Region : 6” Diameter

11” Height

(Ti 6-4 billet) 

Probe : 5-MHz GE Multizone ;

1” diameter

“High Noise”

Region

“Low Noise”

Region

Gated Peak Noise in a 4-μs-wide gate centered 1” deep (a)

Mask

Scan Region : 6” Diameter

11” Height

(Ti 6-4 billet) 

Probe : 5-MHz GE Multizone ;

1” diameter

“High Noise”

Region

“Low Noise”

Region

Gated Peak Noise in a 4-μs-wide gate centered 1” deep (a)

(b)

883588 B3W3B

Billet

Axis

“Low

Noise”

“High

Noise”

1”

2”

Coupons

2.25” thick

along billet

axis.

LN3 LN2 LN1

Serial Nos. for low-noise coupons are LN1, LN2, LN3.

Serial Nos. for high noise coupons will have HN3, HN2, HN1.

(On the axial furthest from the billet serial number).

Three coupons from each region. Each about 1” x 2” x 2.25”

“CL” “Mid” “OD”

(b)

883588 B3W3B

Billet

Axis

“Low

Noise”

“High

Noise”

883588 B3W3B

Billet

Axis

“Low

Noise”

“High

Noise”

1”

2”

Coupons

2.25” thick

along billet

axis.

LN3 LN2 LN1

Serial Nos. for low-noise coupons are LN1, LN2, LN3.

Serial Nos. for high noise coupons will have HN3, HN2, HN1.

(On the axial furthest from the billet serial number).

Three coupons from each region. Each about 1” x 2” x 2.25”

“CL” “Mid” “OD”

1”

2”

Coupons

2.25” thick

along billet

axis.

LN3 LN2 LN1

Serial Nos. for low-noise coupons are LN1, LN2, LN3.

Serial Nos. for high noise coupons will have HN3, HN2, HN1.

(On the axial furthest from the billet serial number).

Three coupons from each region. Each about 1” x 2” x 2.25”

“CL” “Mid” “OD”

(c)

0.000

0.020

0.040

0.060

0.080

0.100

OD Mid CL

Coupon Location

 Noise FOM (cm**-0.5) 

At 15 MHz using

0.5" F=90mm probe

(1/2 of measured α)

"Low Noise"

Region

"High Noise"

Region

0.618

0.620

0.622

0.624

0.626

0.628

0.630

0.632

OD Mid CL

Coupon Location

L-wave speed (cm/μs)

"Low Noise"

Region

"High Noise"

Region

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

OD Mid CL

Coupon Location

Attenuation (N/cm)

Average α at 

15.2 MHz.

Planar probe.

"Low Noise"

Region

"High Noise"

Region

10

 9

 8

 7

 6

 5

 4

 3

 2

dB/in

L-wave Speed

Varied by ~ 2 %

Attenuation

Varied by factor of ~ 2.5

Noise FOM

Varied by factor of ~ 2

(c)

0.000

0.020

0.040

0.060

0.080

0.100

OD Mid CL

Coupon Location

 Noise FOM (cm**-0.5) 

At 15 MHz using

0.5" F=90mm probe

(1/2 of measured α)

"Low Noise"

Region

"High Noise"

Region

0.618

0.620

0.622

0.624

0.626

0.628

0.630

0.632

OD Mid CL

Coupon Location

L-wave speed (cm/μs)

"Low Noise"

Region

"High Noise"

Region

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

OD Mid CL

Coupon Location

Attenuation (N/cm)

Average α at 

15.2 MHz.

Planar probe.

"Low Noise"

Region

"High Noise"

Region

10

 9

 8

 7

 6

 5

 4

 3

 2

dB/in

0.000

0.020

0.040

0.060

0.080

0.100

OD Mid CL

Coupon Location

 Noise FOM (cm**-0.5) 

At 15 MHz using

0.5" F=90mm probe

(1/2 of measured α)

"Low Noise"

Region

"High Noise"

Region

0.618

0.620

0.622

0.624

0.626

0.628

0.630

0.632

OD Mid CL

Coupon Location

L-wave speed (cm/μs)

"Low Noise"

Region

"High Noise"

Region

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

OD Mid CL

Coupon Location

Attenuation (N/cm)

Average α at 

15.2 MHz.

Planar probe.

"Low Noise"

Region

"High Noise"

Region

10

 9

 8

 7

 6

 5

 4

 3

 2

dB/in

L-wave Speed

Varied by ~ 2 %

Attenuation

Varied by factor of ~ 2.5

Noise FOM

Varied by factor of ~ 2

FIGURE 40. NOISE BANDING AND UT PROPERTY VARIATIONS SEEN IN A 

6′′ DIAMETER Ti-6-4 BILLET (a) NOISE BANDING PATTERN, 

(b) RECTANGULAR COUPON LOCATIONS, AND (c) UT PROPERTY 

VARIATIONS FOR RADIAL SOUND PROPAGATION 

47
0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 2 4 6 8 10

Attenuation (dB/inch))

Noise FOM (cm ^ - 0.5) .

Axial

Radial

Hoop

Trendline

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 2 4 6 8 10

Attenuation (dB/inch))

Noise FOM (cm ^ - 0.5) .

Axial

Radial

Hoop

Trendline

FIGURE 41. CORRELATION BETWEEN BACKSCATTERED NOISE FOM AND 

ATTENUATION FOR SIX COUPONS FROM A 6′′ DIAMETER Ti-6-4 BILLET 

In summary, the banding patterns visible in figures 39 and 40 resulted from variations in the 

billet microstructure, which led, in turn, to variations in ultrasonic velocity, attenuation, and 

grain noise capacity. Because the UT properties vary with position, one might suspect that 

identical reflectors located at different positions in a billet (but at the same depth) would give rise 

to different signal amplitudes upon inspection. This is indeed the case, and it has important 

implications for defect detection. 

3.6.1 Attenuation Compensation Research Plan. 

To investigate billet attenuation differences and to critique the standard compensation procedure, 

the ETC team developed a plan in accordance with that envisioned in the original ETC Phase II 

proposal: 

“The current attenuation compensation technique will be evaluated by applying it 

to several billet segments, then drilling a number of flat-bottomed or side-drilled 

holes into the sections, and comparing the measured amplitudes of those holes to 

those expected from the attenuation analysis. Improvements will focus on 

selection of a transducer which will minimize the effects of beam distortion (on 

the back-wall echo)...” [23] 

As noted in section 3.5, the general goal of an attenuation compensation procedure was to 

determine how FBH amplitudes (or defect echo amplitudes) would vary with depth in a billet 

under inspection and to make appropriate gain adjustments to approximately equalize those 

amplitudes. The most direct way of doing this would be to place FBH or other reflectors at 

various depths into one portion of each billet under inspection (say near a butt end) and then use 

the echo amplitudes to set the inspection gains. Such a direct approach was impractical due to its 

expense. Instead, efforts to develop an alternative compensation procedure, which uses readily 

available echoes, such as those reflected by the billet back wall, were pursued. As demonstrated 

in figures 39 and 40, back-wall amplitudes in Ti alloy billets will vary with axial and 

circumferential position. These variations arise from two sources. First, large-scale systematic 

variations of UT properties with position, like those documented in figure 40(c), will naturally 

48
lead to large-scale systematic variations in back-wall amplitude (banding). Second, on a smaller 

scale, say within a region of low back-wall amplitude, finer variations can be seen that are 

sometimes referred to as speckle. The speckle pattern results from distortions of the amplitude 

and phase profiles of the beam by the metal microstructure. Examples of such distortions are 

shown in references 24 and 25. Even if the microstructure is uniform on a large scale, the 

precise patterns of grain sizes and orientations will be different at different locations. These 

differences led to differing beam distortion effects at different positions and, hence, to different 

reflected amplitudes from identical defects or surfaces. 

The effect of beam distortion on surface echoes can be very different from the effect on FBH 

echoes [26]. This is because surface echoes are quite sensitive to phase-front distortions 

(particularly when the beam is not focused on the reflecting surface), while FBH echoes are more 

sensitive to amplitude distortions. Consider focusing the beam first on an FBH at some depth 

and then on a back-wall surface at the same depth. In the limit where the diameter of the focused 

beam approaches the diameter of the FBH reflector, average attenuation values deduced from 

FBH and surface echoes would be identical if the modest back-wall curvature is neglected. 

Although this limit will not be reached in typical MZ billet inspections, moving toward it (by 

focusing the beam near the back wall) will likely improve the accuracy of compensation 

measurements. Also note that past experiments have shown that microstructure-induced signal 

fluctuations are smallest when the beam is focused on or near the reflector [27]. This is true for 

both FBH and surface echoes. Thus, the speckle component of the wide variations often seen in 

billet back-wall amplitude C-scans would likely be reduced if one were to focus on or near the 

back wall. These past findings motivated the investigation of different focusing schemes when 

obtaining back-wall amplitudes for attenuation compensation. 

The detailed research plan eventually adopted by the ETC team had the following nine major 

steps. 

1. Assemble a summary of the current billet compensation procedure, as practiced during 

MZ and CV inspections. 

2. Locate a billet section that has a significantly lower average back-wall amplitude than the 

billet calibration standard of the same diameter. This test billet should also show 

appreciable back-wall banding. 

3. Machine ≥10 FBH targets through the outside surface of the test billet, with the drilled 

holes only a few tenths of an inch deep, i.e., close to the back wall. Holes should be 

drilled into regions of high and low back-wall amplitude. 

4. Make C-scan recordings of the back-wall echo and the FBH echo amplitudes and 

evaluate the results to see whether the amplitude variations for the two types of echoes 

are related. Investigate at least two focusing schemes: sound beam focused near the 

billet center and sound beam focused near the billet back wall. 

5. In another portion of the test billet, drill other FBH targets (with long pilot holes) to be 

located near the billet center, i.e., near the deepest depth considered during a typical 

49
inspection of a billet of that diameter. At least ten such holes should be drilled, 

distributed among regions of high and low back-wall amplitude. 

6. Use the back-wall amplitude patterns of the test billet and calibration standard to 

calculate global attenuation compensation values. (In global compensation procedures, 

the same attenuation adjustment in dB/inch units is applied uniformly at all axial and 

circumferential positions.) In addition to the standard method based on comparing 

average back-wall amplitudes, consider alternative approaches using other attributes of 

the back-wall amplitude patterns. 

7. Inspect the interior of the test billet (containing ≥10 FBH targets) in the usual manner, but 

with a fixed inspection gain. Postprocess the resulting C-scan data to determine the 

adjusted amplitudes of the FBH echoes that would result for each of the global 

compensation schemes considered in step 6. Compare the resulting FBH amplitudes to 

the ideal target level (80% FSH). 

8. Investigate the use of local attenuation compensation procedures in which the gain 

adjustment (in dB/inch units) is different at different axial and circumferential positions. 

Use the variations seen in back-wall or backscattered grain noise C-scan images as the 

basis for the local adjustments. Apply each local compensation procedure considered to 

the FBH C-scan data acquired in step 7. Compare the adjusted FBH amplitudes to the 

80% FSH target level. 

9. Summarize the findings and make recommendations for improving inspections. 

3.6.2 Current Billet Compensation Procedures. 

Attenuation compensation during billet inspection, when performed, generally uses the standard 

method described earlier. Average back-wall amplitudes for the unaltered full-round segment of 

the calibration standard and a representative portion of the test billet are compared, and equation 

1 was used to compute the gain adjustment. For either a CV or MZ inspection, the deepest focus 

transducer being used in the inspection of the billet interior was also used for back-wall echo 

acquisition. When gathering back-wall amplitude data from the test billet, the entire length of 

the test billet was not generally scanned. Rather, as illustrated in figure 42, it is common for 

local rotational/axial C-scans to be performed at several widely spaced locations along the billet, 

and the average back-wall amplitude is then determined from this collection of strip C-scans. If 

the average back-wall amplitude of the test billet is appreciably below that of the calibration 

standard, then gain adjustments computed using equation 1 are positive, and the amplification 

level for the inspection of the test billet must be boosted above that needed to adjust FBH 

amplitudes in the calibration standard to 80% FSH. Such positive gain boosts are generally 

implemented. If, on the other hand, the average back-wall amplitude of the test billet is above 

that of the calibration standard, a negative adjustment would be computed from equation 1. Such 

negative adjustments are usually not implemented, making the billet inspection somewhat more 

conservative than it would otherwise be. 

50
scan

rotate

Cal Standard

scan

rotate

Test Billet

Back-wall scans

performed

Portion w/o FBHs

or internal cavities

scan

rotate

Cal Standard

scan

rotate

Test Billet

Back-wall scans

performed

Portion w/o FBHs

or internal cavities

FIGURE 42. AREAS TYPICALLY SCANNED TO ACQUIRE BACK-WALL AMPLITUDE 

DATA FOR ATTENUATION COMPENSATION 

3.6.3 Test Billet and Transducer Selection. 

Steps 2-8 of the research plan outlined in section 3.6.1 could have been carried out using a test 

billet of any diameter for which a suitable calibration standard was available. One constraint was 

the need to perform measurements in which the sonic beam would be focused near the back wall. 

For any given billet diameter, it is possible to design an elliptical element, bicylindrically 

focused transducer that would focus the beam near the billet back wall. For 10′′ diameter billet, 

such a transducer was in fact designed at GE Corporate Research and Development (CRD) using 

beam models developed at ISU. The 5-MHz probe was specifically designed to focus near the 

back wall (i.e., at 20′′ depth) with a focal spot size near that achieved within each zone of a 

typical MZ inspection of 10′′ diameter Ti alloy billet. 

It was realized that existing MZ transducers could be used for back-wall focusing if the billet 

diameter was smaller. For example, the deep-zone (zone 7) transducer for 13′′ diameter billet 

was designed to focus 6.5′′ deep (in 13′′ diameter billet) when operated at a water path of 3.5′′. 

By increasing the water path to 6.6′′, a beam spot of suitable area could be formed at the back 

wall of the 6′′ diameter billet. Figure 43 shows the model calculations for two transducers, 

comparing the beam spot sizes at the back wall of the 6′′ diameter billet. The results in the upper 

portion of the figure were used to select the 6.6′′ water path subsequently used for the zone 7 

transducer. 

51
Beam Evaluations

Zone 7 transducer of 13" diameter billet

• Designed to focus at 6.5" depth in 13" billet 

• with 3.5" water path

• Element diameter 2.35" (circular)

• Frequency 5MHz

• GFL(circumferential)=10.20”, GFL(axial)=30.59”

( Lens velocity 0.1134 in/microsec;

Lens radii 14.87" ax, 4.96" circ )

Zone 4 transducer of 6" diameter billet

• Designed to focus at 3" depth in 6" billet 

• with 3" water path

• Element diameter 1.75" (circular)

• Frequency 5MHz

• GFL(circumferential)=6.05”, GFL(axial)=16.55”

Use of Zone 7 transducer of 13"Dia billet for 6"Dia billet backwall attenuation studies 

WP (in)

Focal Depth 

(in)

3dB DOF 

(in)

Z_left 

(in)

Z_right 

(in)

AMP 

@focus

6dB BD_ax 

@focus (in)

6dB BD_circ 

@focus (in)

6dB BD_ax 

@6"BW (in)

6dB BD_circ 

@6"BW (in)

7 4.4 2.35 3.25 5.6 0.0013023 0.165 0.293215309 0.17 0.408407038

6.8 4.55 2.1 3.55 5.65 0.0013805 0.155 0.292168112 0.17 0.387463087

6.7 4.5 2.05 3.6 5.65 0.0013806 0.15 0.287979322 0.17 0.387463087

6.6 4.5 2 3.65 5.65 0.0013617 0.15 0.282743334 0.165 0.397935063

6.5 4.5 1.95 3.7 5.65 0.0013212 0.145 0.282743334 0.165 0.397935063

6.4 4.65 1.9 3.75 5.65 0.0012554 0.14 0.299498495 0.165 0.408407038

6 4.75 1.85 3.9 5.75 0.0008331 0.13 0.378736441 0.165 0.555014693

5.5 4.95 1.85 4.15 6 0.0003454 0.13 1.041437947 0.15 1.235693089

Use of Zone 4 transducer of 6"Dia billet (w/ 3"WP) for 6"Dia billet backwall attenuation studies 0.595 0.303687285

Current method

various 3”

6”-dia.

billet

6”-dia.

billet

Deep-zone 

probe for 13”-

dia. billet

Deep-zone 

probe for 6”-dia. 

billet

Beam Evaluations

Zone 7 transducer of 13" diameter billet

• Designed to focus at 6.5" depth in 13" billet 

• with 3.5" water path

• Element diameter 2.35" (circular)

• Frequency 5MHz

• GFL(circumferential)=10.20”, GFL(axial)=30.59”

( Lens velocity 0.1134 in/microsec;

Lens radii 14.87" ax, 4.96" circ )

Zone 4 transducer of 6" diameter billet

• Designed to focus at 3" depth in 6" billet 

• with 3" water path

• Element diameter 1.75" (circular)

• Frequency 5MHz

• GFL(circumferential)=6.05”, GFL(axial)=16.55”

Use of Zone 7 transducer of 13"Dia billet for 6"Dia billet backwall attenuation studies 

WP (in)

Focal Depth 

(in)

3dB DOF 

(in)

Z_left 

(in)

Z_right 

(in)

AMP 

@focus

6dB BD_ax 

@focus (in)

6dB BD_circ 

@focus (in)

6dB BD_ax 

@6"BW (in)

6dB BD_circ 

@6"BW (in)

7 4.4 2.35 3.25 5.6 0.0013023 0.165 0.293215309 0.17 0.408407038

6.8 4.55 2.1 3.55 5.65 0.0013805 0.155 0.292168112 0.17 0.387463087

6.7 4.5 2.05 3.6 5.65 0.0013806 0.15 0.287979322 0.17 0.387463087

6.6 4.5 2 3.65 5.65 0.0013617 0.15 0.282743334 0.165 0.397935063

6.5 4.5 1.95 3.7 5.65 0.0013212 0.145 0.282743334 0.165 0.397935063

6.4 4.65 1.9 3.75 5.65 0.0012554 0.14 0.299498495 0.165 0.408407038

6 4.75 1.85 3.9 5.75 0.0008331 0.13 0.378736441 0.165 0.555014693

5.5 4.95 1.85 4.15 6 0.0003454 0.13 1.041437947 0.15 1.235693089

Use of Zone 4 transducer of 6"Dia billet (w/ 3"WP) for 6"Dia billet backwall attenuation studies 0.595 0.303687285

Current method

various 3”

6”-dia.

billet

6”-dia.

billet

Deep-zone 

probe for 13”-

dia. billet

Deep-zone 

probe for 6”-dia. 

billet

FIGURE 43. MODEL CALCULATIONS OF BEAM PROPERTIES FOR 

TWO 5-MHz MZ TRANSDUCERS 

(The rightmost two columns list beam diameters (-6 dB points for pressure squared) in the axial 

and circumferential directions at the back wall of 6′′ diameter Ti-6-4 billet.) 

Several lengths of 6′′ diameter Ti-6-4 billet stock were located that had the desired features: 

large back-wall amplitude variations and a significantly larger mean attenuation than an available 

calibration standard of the same diameter. The material was part of the CBS heat that was 

purchased by the ETC during Phase I [7]. It was decided that this material would serve well for 

the attenuation compensation study. For measurements in which the beam was to be focused 

near the billet center, an MZ probe designed for the deep zone of 6′′ diameter billet was used. 

For measurements in which the sonic beam was to be focused near the back wall, an MZ 

transducer designed for the deep zone of the 13′′ diameter billet was used and operated at a 6.6′′

water path. The characteristics of these two MZ probes are summarized in the bottom portion of 

figure 43. It was decided that these primary measurements would be supplemented by data 

acquired using a phased array transducer, operated to simulate a fixed-focus probe whose beam 

was focused either near the billet center or back wall. The 5-MHz phased array transducer in 

question was supplied to Honeywell (Phoenix) by R/D Tech and is described in figure 44. 

Although designed for 8′′ diameter billet, the focusing scheme for the array could be altered to, 

approximately, focus the beam at the desired depths in 6′′ diameter billet. 

52
FIGURE 44. FIVE-MHz PHASED ARRAY TRANSDUCER, OPERATED IN STANDARD 

FOCUSING MODE TO SIMULATE INSPECTIONS WITH FIXED-FOCUS PROBES 

3.6.4 Back-Wall Variations and Near-OD FBH Placements. 

A 40′′ long section of 6′′ diameter CBS billet, denoted B2W1A, was selected for use in the initial 

investigations. Back-wall C-scans of this section acquired using the two MZ transducers are 

shown in figure 45. One sees that the overall pattern of high- and low-amplitude banding is 

similar for both transducers, indicating that the pattern results primarily from billet 

microstructure variations. The black rectangle in figure 45 locates a 15′′ long subsection, which 

was subsequently cut from the larger piece and into which near-OD FBHs were later drilled. For 

all of the transducers used in the study, significant variations in back-wall amplitude were seen in 

the test billet. This is illustrated in figure 46, which shows an annotated back-wall C-scan of the 

15′′ length subsection acquired using one of the MZ transducers. Average and peak back-wall 

amplitudes are shown for four regions. The mean amplitudes of these regions differ by as much 

as a factor of 46/19=2.4 (or 7.7 dB), indicating mean attenuation differences as high as 

1.1 dB/inch of billet depth. 

Standard Focusing 

Standard focusing is a focusing method for which we apply the same group of delays to 

the elements in transmission and in reception to focus at a certain point. The result is a 

narrow beam at the focusing depth and a divergent beam at deeper depths. 

• 5 MHz 114 elements segmented elliptic probe designed for a nominal 8′′ diameter billet and 

pre-focused at 4.5′′. The layout of the elements is shown below. 

53
13”-billet deep-zone

transducer.

6.6” water path.

Spot size at BW =

0.165” x 0.398”

= .0258 sq. in

6”-billet deep-zone

transducer.

3” water path.

Spot size at BW =

0.595” x 0.304”

=.142 sq. in

Rotation angle

Axial position 15”

13”-billet deep-zone

transducer.

6.6” water path.

Spot size at BW =

0.165” x 0.398”

= .0258 sq. in

6”-billet deep-zone

transducer.

3” water path.

Spot size at BW =

0.595” x 0.304”

=.142 sq. in

Rotation angle

Axial position 15”

FIGURE 45. BACK-WALL C-SCAN IMAGES OF 6′′ DIAMETER CBS BILLET (B2W1A) 

ACQUIRED USING TWO MZ TRANSDUCERS 

FIGURE 46. BACK-WALL AMPLITUDE STATISTICS FOR FOUR REGIONS OF 

THE 15′′ LONG BILLET SECTION 

(C-scan image acquired using the 6′′ diameter billet, zone 4 transducer focused near 

the billet center.) 

Low Atten. 

Region 

Max 85 

High Atten. Region.

15”

Max 43 

Mean 21

Max 49 

Mean 31

Max 41 

Mean 19

54
Figure 47 shows two schemes that were considered for drilling shallow FBHs into the back wall 

(OD) of the test billet. In this figure, the underlying back-wall C-scans were acquired using the 

phased array transducer with the beam focused near the back wall. Again, the pattern of highand low-amplitude banding is similar to that shown in figure 46, although the color scheme and 

angular registration are different. The drilling scheme shown in the left-hand panel of figure 47 

locates holes primarily in high- or low-amplitude regions and is expected to produce the 

maximum variation of FBH amplitudes if those amplitudes mirror the back-wall variations. The 

scheme shown in the right-hand panel uses a regular array of FBHs and would likely be more 

useful for a rigorous, statistical probability of detection (PoD) analysis. The regular array covers 

approximately 90o of circumferential arc and 6′′ of axial length. Given the limited budget 

available for drilling holes, it was decided that achieving maximum variation in FBH amplitudes 

was of primary importance, and that this could be most effectively accomplished using the first 

drilling scheme. 

Phased-array probe focused on BW Phased-array probe focused on BW

Low High

Phased array probe focused on back wall Phased array probe focused on back wall 

FIGURE 47. POSSIBLE SCHEMES FOR POSITIONING NEAR-OD FBHs RELATIVE TO 

THE BACK-WALL AMPLITUDE PATTERN 

In accordance with the desire to compare back-wall echoes with those of nearby FBHs, certain 

conditions were imposed on the holes. The holes needed to be shallow, but drilled deeply 

enough to be easily resolved from the back-wall echo for all transducers used. The hole diameter 

needed to be as small as possible (#2 FBHs are used in the associated calibration standard), but 

large enough to be easily visible with all transducers. Finally, the holes needed to be drilled in 

such a manner that the inspecting beam did not have to pass through one hole on its way to a 

target hole on the opposite side. This latter requirement is most easily met by confining all holes 

to a region with less than 180° of circumferential arc. 

55
A few FBHs having different diameters and depths were drilled near one end of the 15′′ long test 

billet to examine resolvability issues. From C-scans of these holes, it was decided that #4 FBHs 

of 0.3′′ depth would serve. Thirteen such holes were then drilled at the approximate locations 

shown in the left-hand panel of figure 47. For easy reference, the holes were numbered 1-13 

from top-to-bottom and left-to-right as they appear in C-scan images. Figure 48 shows the 

finished test specimen and a C-scan image (gated at the FBH depth) of the specimen with the 

individual FBH images numbered. 

# 1

# 2

# 3

# 4

# 5

# 6

# 7

# 8

# 9

# 10

# 11

# 12

# 13

“practice” holes

13”-billet deep-zone probe focused near BW

(a) (b)

Holes

# 1

# 2

# 3

# 4

# 5

# 6

# 7

# 8

# 9

# 10

# 11

# 12

# 13

# 1

# 2

# 3

# 4

# 5

# 6

# 7

# 8

# 9

# 10

# 11

# 12

# 13

“practice” holes

13”-billet deep-zone probe focused near BW

(a) (b)

Holes

FIGURE 48. (a) 6′′ DIAMETER Ti-6-4 TEST SPECIMEN CONTAINING NEAR-OD 

#4 FBHs AND (b) C-SCAN IMAGE SHOWING THE FBH ENUMERATION SCHEME 

3.6.5 Correlation Between Back-Wall and Near-OD FBH Amplitude Variations. 

The 15′′ long section of 6′′ diameter Ti-6-4 billet with its 13 shallow #4 FBHs was inspected 

using several focusing schemes having different beam sizes near the back wall. The objective 

was to determine the severity of the back wall and FBH fluctuations and to study the correlation 

between the two types of amplitudes at nearby locations. Six measurement trials were conducted 

using 5-MHz focused transducers: 

1. Using an MZ probe designed for the deep zone of 6′′ diameter billet (beam focused near 

center of test billet). 

2. Using an MZ probe designed for the deep zone of 13′′ diameter billet with the water path 

modified to approximately focus the beam at the back wall of test billet. 

3. Using the 114-element phased array probe to approximately focus the beam near the 

center of the test billet. 

56
4. Using the phased array probe to approximately focus the beam at the back wall of the test 

billet. 

5. Like 3 above, but with an alternate phased array focusing algorithm. 

6. Like 4 above, but with an alternate phased array focusing algorithm. 

The alternate focusing algorithm for the phased array measurements was designed to better 

correct for small fabrication errors in the positioning of array elements along the transducer 

surface. For each trial, beam widths in the axial and circumferential direction at the -6 dB level 

were estimated from the FBH images. 

The results of the six measurement trials are shown in figures 49 through 54. C-scan images 

resulting from the trials are shown in the upper panels of the figures. The upper left panel of 

each figure shows the rectified peak amplitude of the back-wall echo as a function of position for 

a full-round inspection of the 15′′ long billet section. The upper right panel displays images of 

the FBHs, obtained by performing a second scan at increased gain, using a time gate that 

enclosed the FBH echoes but excluded the back-wall echoes. In all cases, the horizontal 

coordinate is parallel to the billet axis and the vertical coordinate is proportional to the billet 

rotation angle. Annotations on the upper panels list the peak FBH amplitudes and the average 

back-wall amplitudes in small regions centered about each FBH location. One expects the FBH 

to shadow the back wall immediately below it, thus altering the local back-wall amplitude. 

However, the back-wall echoes were very strong compared to the FBH echoes, and no 

significant effects of shadowing were seen. Thus, local back-wall amplitudes were measured in 

a simple fashion without regard to FBH shadows. In all cases, the listed amplitudes are the 

percentage of FSH. One notices in each figure that the back-wall and FBH amplitudes vary 

considerably, and that large (small) FBH amplitudes are usually associated with large (small) 

back-wall amplitudes at similar positions. The correlation between FBH and back-wall 

amplitudes is further illustrated in the lower panels of figures 49 through 54. For each of the 

four inspections, the absolute amplitudes of the back-wall and FBH signals have been adjusted to 

have a mean value of 50% FSH for the suite of FBH targets. These rescaled amplitudes are 

shown in the lower left panel, with the principal FBH targets numbered 1-13, as described in 

section 3.6.4. The lower right panels display correlation plots of the rescaled FBH amplitudes 

versus the rescaled back-wall amplitudes. If there were perfect correlation between the two 

types of echo amplitudes, the plotted points would all fall along a straight line passing through 

the origin and having a slope of unity. In each case, a best-fit line through the origin is shown, 

and its slope is seen to be close to the ideal value of 1 in each case. 

57
59%

73%

24%

30%

38%

34%

93%

55%

65%

68%

80%

75%

66%

39%

68%

21%

20%

20%

51%

13%

47%

49%

7%

23%

20%

61%

59%

65% 64%

47%

22% 12%

Comparison of Normalized 

BW and FBH variations

(6" Multizone Probe)

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW 6"MZ

FBH 6"MZ

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(6" Multizone Probe)

y = 1.0243x

R2

 = 0.565

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Trendline thru origin

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

59%

73%

24%

30%

38%

34%

93%

55%

65%

68%

80%

75%

66%

39%

68%

21%

20%

20%

51%

13%

47%

49%

7%

23%

20%

61%

59%

65% 64%

47%

22% 12%

59%

73%

24%

30%

38%

34%

93%

55%

65%

68%

80%

75%

66%

39%

68%

59%

73%

24%

30%

38%

34%

93%

55%

65%

68%

80%

75%

66%

39%

68%

21%

20%

20%

51%

13%

47%

49%

7%

23%

20%

61%

59%

65% 64%

47%

22% 12%

21%

20%

20%

51%

13%

47%

49%

7%

23%

20%

61%

59%

65% 64%

47%

22% 12%

Comparison of Normalized 

BW and FBH variations

(6" Multizone Probe)

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW 6"MZ

FBH 6"MZ

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(6" Multizone Probe)

y = 1.0243x

R2

 = 0.565

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Trendline thru origin

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

FIGURE 49. MEASUREMENTS ON A Ti-6-4 BILLET SEGMENT USING A 5-MHz, 6′′ MZ 

PROBE FOCUSED NEAR THE BILLET CENTER (a) C-SCAN OF BACK-WALL 

AMPLITUDE, (b) C-SCAN GATED ON SHALLOW #4 FBH TARGETS DRILLED INTO 

THE BILLET OD, (c) SCALED BACK-WALL AND FBH AMPLITUDES, AND (d) 

CORRELATION BETWEEN BACK-WALL AND FBH AMPLITUDES 

(Beam diameters (-6 dB) near the back wall are approximately 250 mils by 610 mils 

(axial by hoop).) 

58
57%

87%

79%

66% 94%

64% 70%

27%

31%

42%

22%

79%

82%

64% 91%

48%

19%

22%

20%

49%

58% 59%

33% 33%

13%

18%

20%

15%

28%

31%

48% 48%

Comparison of Normalized 

BW and FBH variations

(13" Multizone Probe)

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW 13"MZ

FBH 13"MZ

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(13" Multizone Probe)

y = 0.9909x

R2

 = 0.6086

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Trendline thru origin

Back-Wall Scan FBH scan

FIGURE 50. MEASUREMENTS ON A Ti-6-4 BILLET SEGMENT USING A 5-MHz, 13′′ MZ 

PROBE FOCUSED NEAR THE BILLET BACK WALL 

(Beam diameters (-6 dB) near the back wall are approximately 150 by 540 mils (axial by hoop).) 

(a) (b)

(c) (d)

57%

87%

79%

66% 94%

64% 70%

27%

31%

42%

22%

79%

82%

64% 91%

57%

87%

79%

66% 94%

64% 70%

27%

31%

42%

22%

79%

82%

64% 91%

79%

66% 94%

64% 70%

27%

31%

42%

22%

79%

82%

64% 91%

48%

19%

22%

20%

49%

58% 59%

33% 33%

13%

18%

20%

15%

28%

31%

48% 48%

48%

19%

22%

20%

49%

58% 59%

33% 33%

13%

18%

20%

15%

28%

31%

48% 48%

Comparison of Normalized 

BW and FBH variations

(13" Multizone Probe)

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW 13"MZ

FBH 13"MZ

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(13" Multizone Probe)

y = 0.9909x

R2

 = 0.6086

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Back-Wall Scan FBH scan

) (b)

(c) (d)

(a

Trendline thru origin

59
Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 1)

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_middle

FBH PA-foc_middle

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 1)

y = 0.9903x

R2

 = 0.4444

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

FBH Amplitude (% FSH)

BW Amplitude (% FSH)

Trendline thru origin

42%

58%

45%

45%

49%

47%

47%

42%

46%

37%

42%

26%

30%

34%

34%

35%

35%

89%

86%

97%

90%

90%

79%

64%

73%

92%

80%

79%

60%

60%

60%

55%

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

FIGURE 51. MEASUREMENTS ON A Ti-6-4 BILLET SEGMENT USING A 5-MHz 

PHASED ARRAY TRANSDUCER FOCUSED NEAR THE BILLET CENTER 

(Beam diameters (-6 dB) near the back wall are approximately 400 by 600 mils (axial by hoop).) 

Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 1)

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_middle

FBH PA-foc_middle

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 1)

y = 0.9903x

R2

 = 0.4444

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

FBH Amplitude (% FSH)

BW Amplitude (% FSH)

Trendline thru origin

42%

58%

45%

45%

49%

47%

47%

42%

46%

37%

42%

26%

30%

34%

34%

35%

35%

42%

58%

45%

45%

49%

47%

47%

42%

46%

37%

42%

26%

30%

34%

34%

35%

35%

89%

86%

97%

90%

90%

79%

64%

73%

92%

80%

79%

60%

60%

60%

55%

89%

86%

97%

90%

90%

79%

64%

73%

92%

80%

79%

60%

60%

60%

55%

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

60
Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 1)

y = 1.005x

R2

 = 0.7744

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Trendline thru origin

Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 1)

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_at_back

FBH PA-foc_at_back

Average for all sites = 50%

63%

58%

59%

60%

57%

42%

68%

61%

47%

57%

56%

37%

37%

35%

33%

36%

40%

74%

64%

71%

64%

68%

55%

66%

69%

64%

66%

66%

38%

49%

46%

48%

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

63%

58%

59%

60%

57%

42%

68%

61%

47%

57%

56%

37%

37%

35%

33%

36%

40%

74%

64%

71%

64%

68%

55%

66%

69%

64%

66%

66%

38%

49%

46%

48%

FIGURE 52. MEASUREMENTS ON A Ti-6-4 BILLET SEGMENT USING A 5-MHz 

PHASED ARRAY TRANSDUCER FOCUSED NEAR THE BILLET BACK WALL 

(Beam diameters (-6 dB) near the back wall are approximately 200 by 380 mils 

(axial by hoop).) 

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 1)

y = 1.005x

R2

 = 0.7744

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Trendline thru origin

Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 1)

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_at_back

FBH PA-foc_at_back

Average for all sites = 50%

63% Back-Wall Scan FBH scan

(a) (b)

(c) (d)

58%

59%

60%

57%

42%

68%

61%

47%

57%

56%

37%

37%

35%

33%

36%

58% 40%

59%

60%

57%

42%

68%

61%

47%

57%

56%

37%

37%

35%

33%

36%

40%

74%

64%

71%

64%

68%

55%

66%

69%

64%

66%

66%

38%

49%

46%

48%

61
56%

42%

43%

49%

46%

43%

54%

44%

48%

40%

32%

32%

19%

19%

16%

24%

43%

84%

58%

73%

73%

69%

61%

77%

74%

73%

81%

72%

44%

46%

44%

44%

Back-Wall Scan FBH scan

(a) (b)

56%

FIGURE 53. MEASUREMENTS ON A Ti-6-4 BILLET SEGMENT USING A PHASED 

ARRAY TRANSDUCER FOCUSED NEAR THE BILLET CENTER (ALTERNATE 

FOCUSING SCHEME) 

(Beam diameters (-6 dB) near the back wall are approximately 400 by 

600 mils (axial by hoop).) 

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 2)

y = 1.0117x

R2

 = 0.6684

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Trendline thru origin

Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 2)

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_middle

FBH PA-foc_middle Average for all sites = 50%

(c) (d)

42%

43%

49%

46%

43%

54%

44%

48%

40%

32%

32%

19%

19%

16%

24%

43%

56%

42%

43%

49%

46%

43%

54%

44%

48%

40%

32%

32%

19%

19%

16%

24%

43%

84%

58%

73%

73%

69%

61%

77%

74%

73%

81%

72%

44%

46%

44%

44%

84%

58%

73%

73%

69%

61%

77%

74%

73%

81%

72%

44%

46%

44%

44%

Back-Wall Scan FBH scan

(a) (b)

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 2)

y = 1.0117x

R2

 = 0.6684

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

BW Amplitude (% FSH)

FBH Amplitude (% FSH)

Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused in Middle; Trial 2)

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

Trendline thru origin

BW PA-foc_middle

FBH PA-foc_middle Average for all sites = 50%

(c) (d)

62
Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 2)

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_at_back

FBH PA-foc_at_back

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 2)

y = 0.9937x

R2

 = 0.2844

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

FBH Amplitude (% FSH)

BW Amplitude (% FSH)

Trendline thru origin

63%

55%

55%

54%

52%

56%

61%

56%

62%

42%

32%

33%

25%

23%

13%

25%

46%

83%

61%

82%

88%

84%

58%

75%

74%

77%

95%

89%

48%

49%

53%

54%

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

FIGURE 54. MEASUREMENTS ON A Ti-6-4 BILLET SEGMENT USING A PHASED 

ARRAY TRANSDUCER FOCUSED NEAR THE BILLET BACK WALL (ALTERNATE 

FOCUSING SCHEME) 

(Beam diameters near the back wall are approximately 200 by 340 mils 

(axial by hoop).) 

The following points summarize the results of the UT inspections conducted on the 15′′ long Ti6-4 billet specimen that contained 13 near-OD FBH targets. 

• Both the FBH and nearby back-wall amplitudes varied significantly within the billet. For 

FBHs, the ratio of maximum/minimum amplitudes ranged from 2-4, depending on the 

inspection details. This FBH variability was large enough that it should be specifically 

accounted for during inspections via some sort of gain adjustment procedure. 

• FBH and back-wall amplitudes tended to be fairly well correlated: sites with low (high) 

back-wall amplitudes tended to have low (high) FBH amplitudes as well. However, there 

were exceptions: e.g., sites where the local back-wall amplitude was smaller than 

average, while the nearby FBH amplitude was larger than average. Thus, it was wisest to 

use the back-wall fluctuation level as a general indicator of the level of fluctuations for 

Comparison of Normalized 

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 2)

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW PA-foc_at_back

FBH PA-foc_at_back

Average for all sites = 50%

Correlation Between Norm'ed

BW and FBH variations

(5-MHz Phased Array; Focused on Backwall; Trial 2)

y = 0.9937x

R2

 = 0.2844

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

FBH Amplitude (% FSH)

BW Amplitude (% FSH)

Trendline thru origin

63%

55%

55%

54%

52%

56%

61%

56%

62%

42%

32%

33%

25%

23%

13%

25%

46%

83%

61%

82%

88%

84%

58%

75%

74%

77%

95%

89%

48%

49%

53%

54%

63%

55%

55%

54%

52%

56%

61%

56%

62%

42%

32%

33%

25%

23%

13%

25%

46%

63%

55%

55%

54%

52%

56%

61%

56%

62%

42%

32%

33%

25%

23%

13%

25%

46%

83%

61%

82%

88%

84%

58%

75%

74%

77%

95%

89%

48%

49%

53%

54%

Back-Wall Scan FBH scan

(a) (b)

(c) (d)

63
FBH signals, rather than inferring that a site with low (high) back-wall amplitude will 

necessarily have a low (high) FBH amplitude. It was also noted that the amplitude 

fluctuations from the mean tended to be higher for back-wall signals than for FBH 

signals. This indicated that using back-wall variations to estimate FBH variations would 

likely provide a conservative estimate of the latter. 

• The fluctuation levels about the mean, and the degree of correlation between back-wall 

and FBH amplitudes, depend somewhat on the inspection setup. For the fixed-focus 

inspections, the back-wall and FBH fluctuation levels were more similar to one another 

(and the individual amplitudes more correlated) when the beam was focused near the 

back wall. The same was generally true for the phased array inspections, with the 

exception of trial 6. 

The above points are further illustrated in figures 55 through 57. For all six measurements, 

figure 55 shows the 13 measured FBH amplitudes (left panel) and 13 associated (locally 

averaged) back-wall amplitudes at the same positions (right panel). As before, the amplitudes 

for each trial have been rescaled to have a mean value of 50% FSH for the collection of 13 

measurement sites. One sees that sites with low (or high) back-wall or FBH amplitudes tend to 

have low (or high) amplitudes for all (or most) measurement setups. In addition, the back-wall 

amplitudes are fairly well correlated with the FBH amplitudes. For example, sites 8-11 that have 

lower than average back-wall amplitudes also have lower than average FBH amplitudes. 

However, the fluctuations about the mean are shown to be smaller, on average, for FBHs than for 

back-wall signals. 

Comparison of Normalized BW variations

(Six Measurement Trials)

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

BW 6"MZ BW 13"MZ

BW PA-foc_middle_1 BW PA-foc_at_back_1

BW PA-foc_middle_2 BW PA-foc_at_back_2

Comparison of Normalized FBH variations

(Six Measurement Trials)

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Measurement Site

Amplitude (%FSH)

FBH 6"MZ FBH 13"MZ

FBH PA-foc_middle_1 FBH PA-foc_at_back_1

FBH PA-foc_middle_2 FBH PA-foc_at_back_2

FIGURE 55. NORMALIZED FBH AMPLITUDES (LEFT) AND NEARBY BACK-WALL 

AMPLITUDES (RIGHT) SEEN IN THE SIX MEASUREMENT TRIALS 

(Each curve has a mean of 50% FSH.) 

The fact that the curves in figure 55 have similar overall shapes indicates that amplitude 

fluctuations are primarily due to actual differences in attenuation along different billet diameters. 

64
The fact that the normalized curves are not identical indicates that the apparent attenuation along 

a given diameter depends, to some extent, on the transducer used in the measurement, a fact well 

established during Phase I [26]. 

Figure 56 shows two dimensionless measures of the amplitude fluctuations evident in figure 55. 

The first measure is simply the maximum amplitude divided by the minimum amplitude. The 

second is the standard deviation of the amplitudes divided by the mean amplitude. One sees that 

(1) the fluctuation level depends somewhat on the inspection setup; and (2) for a given setup, the 

fluctuation level tends to be smaller for FBHs than for back walls. Figure 57 shows the degree 

of correlation between the back-wall and FBH amplitudes as a function of the beam diameter at 

the FBH depth. The correlation measure used here is the correlation coefficient (R-squared) 

resulting from the linear fit to the FBH amplitude versus back-wall amplitude data, as illustrated 

in figures 49 through 54. Overall, the correlation between FBH and back-wall amplitude 

variations tended to improve when the beam diameter decreased. For the second phased array 

trial, however, focusing near the back-wall led to a lower R-squared value than focusing near the 

billet center. This may be partially due to the fact that the spread in back-wall amplitudes (about 

the mean value of 50% FSH) is significantly larger than the spread in FBH values for some cases 

(e.g., trial 6). This tends to lower the R-squared value for those cases. It is also likely that the 

precise values of FBH and back-wall amplitudes (and hence their correlation) is influenced by 

various inspection parameters such as the degree to which the incident beam is perpendicular to 

the billet specimen. There may have been some nonideality in the setup for trial 6, which 

explains the sizable difference in R-squared values for trials 4 and 6, both of which used the 

phased array probe with the beam focused near the billet back wall. The influence of setup 

parameters on back-wall and FBH amplitudes would no doubt be an illuminating topic for study, 

but it is beyond the scope of the present work. 

Fluctuation Ratio #1: Max / Min

0

1

2

3

4

5

6

7

8

9

10

6"MZ

13"MZ

PA-foc_middle_1

PA-foc_at_back_1

PA-foc_middle_2

PA-foc_at_back_2

Fluctuation Ratio

BW

FBH

Fluctuation Ratio #2: Std.Dev / Average

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

6"MZ

13"MZ

PA-foc_middle_1

PA-foc_at_back_1

PA-foc_middle_2

PA-foc_at_back_2

Fluctuation Ratio

BW

FBH

FIGURE 56. FLAT-BOTTOM HOLES AND (NEARBY) BACK-WALL AMPLITUDE 

FLUCTUATIONS SEEN IN SIX MEASUREMENT TRIALS USING A 15′′ LONG 

Ti-6-4 BILLET CONTAINING 13 NEAR-OD FBHs 

65
Correlation between BW and FBH Amplitudes

as a Function of Beam Size

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5

Beam Area at FBH depth (sq. in.)

Correlation Coefficient (R^2)

trendline

outlier (PA-foc_at_back_trial 2) 

4

2

5

3

6

1

(mils) (mils) (sq. in.) 

trial probe D-axial D-hoop Beam Area Correl. Coeff.

1 6"MZ 250 610 0.1198 0.565

2 13"MZ 150 537 0.0633 0.609

3 PA-foc_middle_1 1000 600 0.4712 0.444

4 PA-foc_at_back_1 200 380 0.0597 0.774

5 PA-foc_middle_2 400 600 0.1885 0.668

6 PA-foc_at_back_2 200 340 0.0534 0.284

FIGURE 57. CORRELATION BETWEEN BACK-WALL AND FBH AMPLITUDES AS A 

FUNCTION OF BEAM SIZE AT THE FBH DEPTH (APPROXIMATELY 5.7′′) 

(Measured beam diameters at the -6 dB level are listed beneath the graph for each 

of the six measurement trials.) 

In summary, the above investigation (section 3.6) indicates that back-wall amplitude banding is 

associated with actual variations in the UT attenuation along different billet diameters. When 

identical FBHs are drilled a short distance into the billet OD, their echo amplitudes fluctuate in 

much the same manner as those of back-wall echoes at nearby positions. This suggests that 

back-wall echoes can be used to deduce attenuation differences that will also approximately 

describe the behavior of small-defect (e.g., FBH) amplitudes. When compensating for 

attenuation differences, one must be aware that identical reflectors at a given depth in either the 

calibration standard or the test billet can have significantly different amplitudes. For example, 

the amplitude of the reflected signal from a given calibration hole (used to set the preliminary 

inspection gain) will depend, to some extent, on whether that hole was drilled into a high- or 

low-attenuation band of the calibration standard. Thus, a robust compensation procedure will 

account not only for gross differences between the average attenuations of the calibration 

standard and test billet, but also for attenuation variations within both specimens. 

66
3.6.6 Straw Man Procedures for Global Attenuation Compensation. 

Recall that the adjective global was used to refer to attenuation compensation procedures in 

which a fixed gain adjustment in dB/inch units was applied uniformly at all axial and 

circumferential positions of the billet under inspection. Such adjustments are the simplest to 

implement in practical inspections, and the existing standard compensation procedure is of this 

nature. Based on the study of the 15′′ long Ti-6-4 billet specimen, a basic global straw man 

compensation procedure having six variants, one being the standard approach, was proposed. 

Again, it was decided that 6′′ diameter CBS billet material would be used to construct test 

specimens to test the variants. 

3.6.7 Proposed Straw Man Global Attenuation Compensation Procedure. 

1. Assume that the target inspection sensitivity is a #2 FBH. Also, assume that before any 

compensation adjustments are made, gains are adjusted to bring #2 FBHs in the 

calibration standard to 80% FSH. 

2. Assume that the purpose of the compensation adjustments is to ensure (to some level of 

certainty) that a #2 FBH or equivalent reflector located anywhere within the billet to be 

inspected would generate an amplitude of 80% FSH or higher. 

3. Using a suitable transducer, gather back-wall C-scan data from representative lengths of 

the calibration standard and billet to be inspected (BTBI) (two focusing schemes to be 

considered). 

4. Calculate the amplitude difference in dB between some features of the back-wall C-scans 

of the calibration standard and BTBI (three combinations of features to be considered). 

5. Divide the dB difference by the billet diameter to obtain an attenuation compensation 

value in dB/inch. Use this value to determine the gain adjustments needed for each 

inspection (depth) zone of the BTBI. 

The choice of a suitable transducer in step 3 is open to interpretation. Back-wall fluctuations are 

likely to be more representative of FBH fluctuations if the transducer has a beam spot size on the 

back wall that is similar to the spot sizes of the transducers used in the actual inspection within 

their respective depth zones. However, a standard deep-zone MZ transducer (focused near the 

billet center) may work sufficiently well for surveying back-wall variations. It was proposed that 

both focusing schemes (focus near billet center; focus near billet back wall) be tested. As in the 

earlier measurements, the deep-zone MZ transducers designed for 6′′ and 13′′ diameter billet 

would be used to achieve the two proposed focusing conditions. 

Different choices in step 4 of the back-wall C-scan features being compared will naturally lead to 

different dB/inch values. If it is known a priori that the FBHs in the calibration standard have 

been drilled into high- or low-attenuation bands, that information can be used when computing 

compensations. For existing calibration standards, it was believed that no consideration was 

given to attenuation banding when the holes were drilled. Moreover, the construction of the 

standards precludes viewing back-wall echoes from the billet OD in the vicinities of the 

67
calibration holes. Each MZ calibration standard contains two parts, which abut one another in 

the axial direction: one is a full-round portion without alteration that is used to acquire back-wall 

echoes and the other contains the FBHs and internal cavities that allow the FBHs to be drilled. 

The two portions are screwed together but their original angular registration was generally not 

maintained. For CV inspections, the angular registration between the chord blocks and fullround specimen is also generally not known. For some individual calibration holes, it would 

likely be possible to estimate whether they were located in high- or low-attenuation bands by 

examining the local backscattered grain noise pattern or the local C-scan image of the echo from 

the associated cavity wall. In any given existing calibration standard, it was likely that some 

FBHs were drilled into high-attenuation bands and others into low-attenuation bands. 

Rather than attempt to decipher and use detailed FBH location information, it was agreed that the 

location of any given FBH hole within the banding pattern of the calibration standard is 

unknown. Under this view, it was proposed that three back-wall amplitude feature combinations 

be considered in step 4, distinguished by how conservative one wishes to be in the compensation 

procedure: 

• Standard: To deduce the attenuation difference, one compares average back-wall 

amplitudes in the calibration standard and the test billet. This corresponds to assuming 

that the calibration hole is located in an average attenuation region in the standard, and 

that the target flaw is located in an average attenuation region of the test billet. This may 

be regarded as the current standard practice for attenuation compensation. 

• Semiconservative: Computes both the average and the standard deviation of the backwall amplitude seen in the calibration standard and the test billet. One then compares the 

average plus one standard deviation in the calibration standard to the average minus one 

standard deviation) in the test billet. This corresponds to assuming that the calibration 

hole is in a somewhat low-attenuation region (one standard deviation above the average 

back-wall amplitude) of the calibration standard, and the target flaw is in a somewhat 

high-attenuation region of the test billet. 

• Conservative: Compares the back-wall amplitude seen in the brightest region of the 

calibration standard C-scan to the back-wall amplitude in the dimmest region of the test 

billet C-scan. This corresponds to assuming that the calibration hole is in the lowest 

attenuation region of the standard and that the target hole is in the highest attenuation 

region of the test billet. 

To test the six variations of the straw man compensation procedure, a suitable specimen had to 

be fabricated. It was desired that the test specimen contain numerous identical reflectors, all 

located at a depth where attenuation adjustments would be maximized. Since the 6′′ diameter 

billet was routinely inspected over the depth range from 0′′ to 3.5′′, it was decided to place all 

reflectors (#2 FBHs) 3.5′′ below the entry surface. A suitable calibration standard containing 

(among others) a #2 FBH located 3.5′′ deep was available at GE. 

As shown in figure 58, a candidate billet was to be located and cut into two sections with similar 

levels of back-wall amplitude variation. One section (B) would be used to fabricate the test 

68
specimen (C) containing inferior FBHs. The other section (A) would be available for gathering 

back-wall amplitude data if the construction of specimen C precluded such measurements. 

+

A B

C #2 FBH

reflectors

6”-diameter

Ti 6-4 billet

+

A B

C #2 FBH

reflectors

6”-diameter

Ti 6-4 billet

FIGURE 58. FABRICATION OF SPECIMENS TO TEST STRAW MAN 

PROCEDURES FOR ATTENUATION COMPENSATION 

3.7 FABRICATION OF A TEST SPECIMEN CONTAINING NEAR-CENTER FBHS. 

A suitable candidate billet section was located, namely, a 72′′ long piece of 6′′ diameter Ti-6-4 

CBS stock bearing the designation B3W1BA. Back-wall amplitude C-scans made using the two 

deep-zone MZ transducers are shown in figure 59. The figure shows the upper scan using the 

zone 4 MZ transducer for 6′′ diameter billet follows the current MZ practice for sampling 

attenuation differences. The lower scan using the zone 7 transducer for 13′′ diameter billet and a 

6.6′′ water path produces a smaller focal spot on the back wall. Inspection gains were adjusted to 

yield similar average back-wall amplitudes. Note that the pattern of amplitude variations is very 

similar for both inspections. This similarity is further demonstrated in figures 60 and 61, where 

the average amplitudes within the small boxed regions of the C-scans are compared. In 

accordance with figure 58, two 18′′ long sections were cut from one end of the 72′′ piece, as 

indicated in figure 59, to serve as test specimens. 

CBS Billet Section #B3W1BA

72 “

Using zone-4 MZ 

probe for 6”-billet. 

3” water path.

Using zone-7 MZ 

probe for 13”-billet. 

6.6” water path.

Section A Section B/C

18 inches 18 inches

CBS Billet Section #B3W1BA

72 “

Using zone-4 MZ 

probe for 6”-billet. 

3” water path.

Using zone-7 MZ 

probe for 13”-billet. 

6.6” water path.

Section A Section B/C

18 inches 18 inches

FIGURE 59. BACK-WALL C-SCANS OF THE CANDIDATE BILLET FOR THE 

STRAW MAN TEST 

69
72 “

CBS Billet Section #B3W1BA

23

55

2

22

53

3

57

89

28

60

89

25

35

63

14

55

85

26

60

84

36

38

71

19 12

33

2

61

83

35

22

57

3

59

85

27

Mean

Max

Min

Mean

Max

Min

Using zone-4 MZ 

probe for 6”-billet. 

3” water path.

Using zone-7 MZ 

probe for 13”-billet. 

6.6” water path.

72 “

CBS Billet Section #B3W1BA

23

55

2

22

53

3

57

89

28

60

89

25

35

63

14

55

85

26

60

84

36

38

71

19 12

33

2

61

83

35

22

57

3

59

85

27

Mean

Max

Min

Mean

Max

Min

Using zone-4 MZ 

probe for 6”-billet. 

3” water path.

Using zone-7 MZ 

probe for 13”-billet. 

6.6” water path.

FIGURE 60. MEAN, MAXIMUM, AND MINIMUM BACK-WALL AMPLITUDES FOR 

VARIOUS REGIONS OF THE C-SCAN IMAGES SHOWN IN FIGURE 59 

(Units are % FSH) 

Mean Amplitude 

Mean Amplitude 

0

10

20

30

40

50

60

70

0 2 4 6 8

Region of Interest

Amplitude 

Series1

Series2

FIGURE 61. COMPARISON OF MEAN AMPLITUDES OF BACK-WALL 

ECHOES FOR THE SIX REGIONS INDICATED IN FIGURES 59 AND 60 

(For series 1 (6′′ diameter, zone 4 probe), the beam is focused near the billet center. 

For series 2 (13′′ diameter, zone 7 probe), the beam is focused near the back wall.) 

The manner in which #2 FBHs were drilled into the 18′′ long test billet is illustrated in figure 62. 

Pilot holes of 205 mil diameter and 1.25′′ length were first drilled, and 1.25′′ length #2 FBHs 

(31.25 mil diameter) were then drilled into the bottoms of the pilot holes. In all, 17 #2 FBHs 

were placed at various axial and circumferential positions, with each hole located 3.5′′ below the 

inspection entry surface on the opposite side of the billet. Eleven of the hole locations were 

70
chosen using the back-wall amplitude patterns shown in figure 59, and six were chosen using the 

backscattered MZ noise pattern for the deepest zone. Locations were chosen to span the widest 

possible range of back-wall and noise amplitudes, with the presumption that this would lead to 

the widest possible variation of FBH amplitudes. Figure 63 shows the completed test specimen 

after the near-OD portion of each hole was plugged to prevent water infiltration. 

Note: There is a 

calibration hole 

located 3.5” deep in 

the MZ calibration

standard for 6”-

diameter billet.

CL

3.5”

Holes in low

attenuation region

Holes in high

attenuation region

#2 FBH’s are 

3.5” below the 

inspection 

surface 3.5”

Pilot holes

(0.205” Dia.)

#2 FBH

(1.25” deep)

Note: There is a 

calibration hole 

located 3.5” deep in 

the MZ calibration

standard for 6”-

diameter billet.

CL

3.5”

Holes in low

attenuation region

Holes in high

attenuation region

#2 FBH’s are 

3.5” below the 

inspection 

surface 3.5”

Pilot holes

(0.205” Dia.)

#2 FBH

(1.25” deep)

CL

3.5”

Holes in low

attenuation region

Holes in high

attenuation region

#2 FBH’s are 

3.5” below the 

inspection 

surface 3.5”

Pilot holes

(0.205” Dia.)

#2 FBH

(1.25” deep)

FIGURE 62. SIX-in.-DIAMETER, 18-in.-LONG Ti-6-4 TEST SPECIMEN SHOWING HOW 

#2 FBHs WERE DRILLED AT A DEPTH OF 3.5′′ FROM THE 

INSPECTION SURFACE 

Holes

FIGURE 63. SIX-in.-DIAMETER, 18-in.-LONG Ti-6-4 TEST BILLET SHOWING DRILLED 

AND PLUGGED HOLES 

All C-scan images of back-wall and interior (noise + defect) echoes seen during inspections of 

the test billet have the same axial and circumferential registration and, hence, the locations of the 

FBH defects are the same for all images. These locations and their enumeration are shown in 

figure 64. 

71
1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17

FIGURE 64. NUMBERING SCHEME FOR FBH LOCATIONS IN ALL C-SCANS OF 

THE 18′′ LONG TEST BILLET 

Echoes from the 17 #2 FBHs in the test billet will be compared with the echo from a #2 FBH at 

the same depth in the 6′′ diameter MZ calibration standard. The design of the deep-zone portion 

of the calibration standard, containing #2 holes at depths of 2.6′′ and 3.5′′ is shown in figure 

65(a). A typical C-scan of that portion of the standard is shown in figure 65(b). 

2.6” 3.5”

Calibration Standard Design

(Zone 4 of 6”-diameter billet)

CL

#2 FBHs

saw cuts

2.6” deep

FBH 3.5” deep

FBH

FBH side

Saw cut

Circumferential

FBH side

Axial (a) (b)

2.6” 3.5”

Calibration Standard Design

(Zone 4 of 6”-diameter billet)

CL

#2 FBHs

saw cuts

2.6” 3.5”

Calibration Standard Design

(Zone 4 of 6”-diameter billet)

CL

#2 FBHs

saw cuts

2.6” deep

FBH 3.5” deep

FBH

FBH side

Saw cut

Circumferential

FBH side

Axial (a) (b)

FIGURE 65. (a) LAYOUT OF THE DEEP-ZONE (ZONE 4) PORTION OF THE 6′′

DIAMETER MZ CALIBRATION STANDARD AND (b) C-SCAN OF THE DEEP-ZONE 

REGION OF THE CALIBRATION STANDARD 

(Beam focus near billet center) 

72
3.7.1 Tests of the Global Attenuation Compensation Straw Man Procedure. 

Back-wall C-scans of the test specimen and the full-round portion of the calibration standard 

were made to gather the data required to compute attenuation adjustments. In each case, as noted 

above, two transducers were used: 

• Zone 4 MZ transducer for 6′′ diameter billet at a 3′′ water path (beam focused near billet 

center). 

• Zone 7 MZ transducer for 13′′ diameter billet at a 6.6′′ water path (beam focused near 

billet back wall). 

The resulting C-scans are shown in figures 66 and 67, respectively, along with annotations listing 

back-wall amplitude statistics within selected regions. As expected from the earlier scans of the 

72′′ long billet section, the patterns of back-wall amplitude variations in the test billet are similar 

for the two transducers. Also notice that shadows of the drilled holes in the 18′′ long test 

specimen can be seen. These occur when the center of the sonic beam propagates along a drilled 

hole. For each drilled hole, two shadows are expected: one at the location shown in figure 64 

and one on the opposite side of the billet (i.e., rotated by 180°). The appearance of the shadows 

was deemed to have little influence on the specific back-wall amplitude statistics listed in figures 

66 and 67. 

FIGURE 66. BACK-WALL C-SCANS OF THE CALIBRATION STANDARD (LEFT) AND 

TEST BILLET (RIGHT) ACQUIRED WITH THE BEAM FOCUSED NEAR THE 6′′

DIAMETER BILLET CENTERLINE (ZONE 4) 

p g ( )

Calibration CBS Billet: att 43 dB, mean amp 39%

Standard: 

att 46.5 dB 

mean 63%

Note: dB values are attenuator settings (inverse gain).

From top-to-bottom statistics are min, max, average, std. dev.

Cal. Std. Test Billet

47

81

68

6

1

42

10

4

41

88

70

6

Big

area

31

81

63

7

Big

area

0

89

39

17

38

68

55

6

p g ( )

Calibration CBS Billet: att 43 dB, mean amp 39%

Standard: 

att 46.5 dB 

mean 63%

Note: dB values are attenuator settings (inverse gain).

From top-to-bottom statistics are min, max, average, std. dev.

Cal. Std. Test Billet

47

81

68

6

1

42

10

4

41

88

70

6

Big

area

31

81

63

7

Big

area

0

89

39

17

38

68

55

6

73
FIGURE 67. BACK-WALL C-SCANS OF THE CALIBRATION STANDARD (LEFT) AND 

TEST BILLET (RIGHT) ACQUIRED WITH THE BEAM FOCUSED NEAR THE 13′′

DIAMETER BILLET BACK WALL (ZONE 7) 

From the back-wall amplitude statistics listed in figures 66 and 67, attenuation compensation 

values for the six variations of the straw man procedure can be calculated using equation 2. An 

example of such a calculation is shown in figure 68 for the standard procedure, requiring a gain 

boost of 4.48 dB for a target located 3.5′′ deep in the test billet. 

FIGURE 68. EXAMPLE ATTENUATION COMPENSATION CALCULATION 

47

83

68

6

12

27

19

3

26

66

54

5

28

57

44

5

Calibration CBS Billet: att 40 dB, mean amp 43%

Standard: 

att 46.0 dB 

mean 50%

Note: dB values are attenuator settings (inverse gain).

From top-to-bottom statistics are min, max, average, std. dev.

Cal. Std. Test Billet

big

area

0

84

43

16

big

area

26

66

50

5

47

83

68

6

12

27

19

3

26

66

54

5

28

57

44

5

Calibration CBS Billet: att 40 dB, mean amp 43%

Standard: 

att 46.0 dB 

mean 50%

Note: dB values are attenuator settings (inverse gain).

From top-to-bottom statistics are min, max, average, std. dev.

Cal. Std. Test Billet

big

area

0

84

43

16

big

area

26

66

50

5

“Standard” Attenuation Compensation Adjustment

(Using BW C-scan data acquired with beam focused near billet center)

Cal Std BW: attenuator setting = 46.5 dB; Mean amplitude = 63%

Test Billet BW: attenuator setting = 43.0 dB; Mean amplitude = 39%

Mean amplitudes at same gain: 63% and 26% @ 46.5 dB

dB difference between Std. and test billet = 20log[63/26] = 7.68 dB

Divide by billet diameter (6”) 1.28 dB/in. 

At 3.5 inch depth, amplitude adjustment is 4.48 dB or factor of 1.68

74
The resulting global attenuation adjustments for each of the six variations are listed in figure 68. 

These values must be multiplied by 3.5′′ to obtain gain corrections (boosts) in dB for the FBHs in 

the test billet. The standard and semiconservative procedures use the back-wall amplitude 

average and standard deviation values listed in table 11. For the conservative procedure, the 

average back-wall amplitudes within a small high-amplitude region of the calibration standard 

and a small low-amplitude region of the CBS test billet was used. These extreme average 

amplitudes are estimated as (68=cal, 10=CBS) for centerline focusing and (60=cal, 19=CBS) for 

back-wall focusing, at the gains used for the raw back-wall C-scans shown in figures 66 and 67. 

TABLE 11. COMPUTED ATTENUATION COMPENSATION VALUES IN dB/INCH OF 

DEPTH IN THE TEST BILLET 

Standarda Semiconservativeb Conservativec

Focus at centerline 1.28 2.26 3.36 

Focus at back wall 1.22 2.03 2.66 

a

 Use average back-wall amplitudes for calibration standard and test billet. 

b

 Use (average + σ) for calibration standard and (average - σ) for test billet. c

 Use average of maximum amplitude zone for calibration standard and average of minimum amplitude zone 

for test billet. 

After completing the back-wall measurements and analyses, the interior of the test billet was 

inspected. The usual procedure for an MZ inspection was followed; however, only the zone 4 

inspection was performed. Zone 4 nominally extends from 2.6′′-3.5′′ deep, but a slightly larger 

acquisition gate is typically used so that there is some overlap between neighboring zones. The 

water path was set to 3′′, and the calibration standard was scanned using the zone 4 transducer to 

locate the #2 FBH calibration holes at the ends of the zone. The gain was adjusted so that the 

amplitudes from both holes exceeded 80% FSH. The two echo amplitudes were sufficiently 

similar (1.6 dB difference) that water path adjustment was not necessary under standard 

inspection protocols. The calibration standard was then replaced by the test billet, and zone 4 

was again scanned using a 3′′ water path. A slightly higher gain setting (+2 dB) was used for the 

test billet inspection. After the measurements, the FBH amplitudes in the test billet were 

adjusted to compensate for attenuation differences. Since six variants of the straw man 

compensation procedure were being considered, it was most efficient to inspect the test billet 

once and then to make all gain adjustments during postprocessing. The results of the zone 4 

inspections of the calibration standard and test billet are shown in figure 69. As expected, 

substantial variations were seen in the peak amplitudes of the 17 nominally identical reflectors in 

the test billet. The raw amplitudes varied from 38% to 96% FSH, or by about a factor of 2.5. 

The FBH amplitude variations are believed to be due to variations in the test billet microstructure 

and its associated attenuation. 

75
100% 83% Calibration +2 dB gain

3.5”-deep #2 calibration 

hole at 83% FSH

17 #2 FBHs in test billet 

are each 3.5” deep.

Cal. Std. Test Billet

(a)

100% 83% Calibration +2 dB gain

3.5”-deep #2 calibration 

hole at 83% FSH

17 #2 FBHs in test billet 

are each 3.5” deep.

Cal. Std. Test Billet

(a)

65 42

54

53

80

82

59

82

38

74

49

96

70

49

69 46

74

#2 FBH Amplitudes in the

18”-long test billet as run.

(At 83% calibration + 2 dB 

added gain.)

All values 

(b) in % FSH

65 42

54

53

80

82

59

82

38

74

49

96

70

49

69 46

74

65 42

54

53

80

82

59

82

38

74

49

96

70

49

69 46

74

#2 FBH Amplitudes in the

18”-long test billet as run.

(At 83% calibration + 2 dB 

added gain.)

All values 

(b) in % FSH

FIGURE 69. (a) C-SCAN IMAGES FROM ZONE 4 INSPECTIONS OF THE CALIBRATION 

STANDARD (LEFT) AND TEST BILLET (RIGHT) AND (b) RAW PEAK AMPLITUDES OF 

THE 17 #2 FBHs IN THE TEST BILLET 

Figures 70 and 71 show the results of adjusting the FBH amplitudes in various ways to 

compensate for attenuation differences between the billet specimen and the 6′′ diameter MZ 

calibration standard that was used as a reference. Figure 70 displays the amplitudes that result 

when no compensation is made, i.e., when the test billet is inspected at the same gain required to 

bring the 3.5′′ deep #2 FBH in the calibration standard to 80% FSH. All 17 FBHs in the test 

billet are shown to have amplitudes below the 80% target threshold, with values ranging from 

29%-74% FSH. Although none of the 17 defects would have been rejectable on the basis of 

peak amplitude ≥ 80% if no compensation were performed, several would have been rejected on 

the basis of a commonly used signal/noise (S/N) criterion, namely 

2.5 S Flaw Signal Amplitude Average Noise Amplitude

N Peak Noise Amplitude Average Noise Amplitude

− ≡ ≥ − (3) 

76
Automatic image analysis software developed by GEAE was applied to the as-run zone 4 C-scan 

of the test billet shown in figure 69(a). The software identified potential defects and calculated 

peak amplitudes and S/N for each, with the results shown in figure 71. Thirteen of the seventeen 

FBH defects were identified by the software, with one FBH (whose C-scan image had two lobes) 

being identified twice. Seven of the thirteen defects were determined to have S/N ≥ 2.5; these 

were the FBH defects having the seven highest amplitudes, namely, those enumerated 2, 3, 7, 10, 

11, 13, and 14 in figure 64. 

50 32

41

41

61

63

45

63

29

57

38

74

54

38

53 35

57

Amp < 80%

Amp > 80%

Amplitudes normalized to #2 in cal std. @ 80%.

No attenuation compensation.

50 32

41

41

61

63

45

63

29

57

38

74

54

38

53 35

57

Amp < 80%

Amp > 80%

Amplitudes normalized to #2 in cal std. @ 80%.

No attenuation compensation.

FIGURE 70. PEAK AMPLITUDES OF THE 17 #2 FBHs IN THE TEST BILLET WHEN NO 

ATTENUATION COMPENSATION IS USED (IN % FSH) 

Figure 72 summarizes the results of applying the standard global compensation procedure to the 

test billet. The adjusted FBH amplitudes are shown for both focusing schemes used to acquire 

the back-wall C-scans on which the adjustments are based. If one assumes that the average 

back-wall amplitude is a good indicator of average FBH amplitude, then the standard 

compensation procedure is designed such that average amplitudes would be the same for FBHs 

in the calibration standard and test billet (if each specimen contained many identical reflectors at 

the same depth). Of course, the calibration standard contains only a single #2 FBH at 3.5′′ depth. 

If one assumes that this particular calibration reflector is located near a region of average 

attenuation, then the average FBH amplitude in the test billet, after the standard compensation 

adjustment, should be near 80% FSH. This is seen in figure 72 to be approximately the case for 

either back-wall focusing method. Also, as might be expected, about half of the compensated 

amplitudes are above 80% and about half are below 80%. None of the FBHs having 

compensated amplitudes below 80% have S/N ≥ 2.5. Thus, none of these would have been 

rejected using the usual signal-to-noise criterion. Figure 72 indicates that when the standard 

compensation procedure is used, some regions of the test billet are being inspected to higher than 

77
#2 FBH sensitivity, while other regions are being inspected to lower than #2 FBH sensitivity. To 

inspect the entire test billet to #2 FBH or better sensitivity (based on signal amplitude), the 

inspection gain must be boosted. 

The same 

FBH

4

(a)

(b)

65 42

54

53

80

82

59

82

38

74

49

96

70

49

69 46

74

< 2.5 snr > 2.5 snr (As run: 83% Calibration + 2 dB)

The 7 targets found to have 

S/N > 2.5 were the targets 

with the 7 highest absolute 

amplitudes.

(b)

65 42

54

53

80

82

59

82

38

74

49

96

70

49

69 46

74

< 2.5 snr > 2.5 snr (As run: 83% Calibration + 2 dB)

The 7 targets found to have 

S/N > 2.5 were the targets 

with the 7 highest absolute 

amplitudes.

FIGURE 71. (a) OUTPUT GENERATED WHEN AUTOMATIC S/N SOFTWARE 

WAS APPLIED TO THE AS-RUN ZONE 4 C-SCAN OF THE TEST BILLET AND 

(b) AS-RUN PEAK AMPLITUDES OF THE 17 #2 FBH DEFECTS WITH 

S/N RANGES INDICATED 

78
The results of applying the standard, semiconservative, and conservative compensation 

procedures to the test billet inspection are compared in figures 73 and 74. Adjustments made for 

figure 73 were based on back-wall C-scans acquired with the beam focused near the centerline, 

while those for figure 74 had the beam focused near the back wall for back-wall C-scan 

acquisition. The adjusted amplitudes of the individual FBHs shown in figure 74 are listed in 

table 12. Figure 75 then provides a brief comparison of the key results from the six variations of 

the basic global attenuation adjustment procedure. Note that as the compensation procedure 

becomes more conservative, the gain applied during the test billet inspection increases (see 

table 12) and the measured FBH amplitudes rise. For example, at 3.5′′ depth where the FBHs are 

located, the semiconservative procedure in figure 73 applies 3.44 dB more gain than the standard 

procedure, and the conservative procedure applies an additional 3.84 dB of gain. As might be 

expected, all the FBHs in the test billet were found to have amplitudes greater than 80% FSH 

when the conservation compensation procedure was used. This was true for both of the focusing 

methods used to acquire back-wall C-scans; however, the focus at back-wall approach 

accomplished this at lower total inspection gain and, thus, is likely to lead to fewer false calls. 

3.7.2 Local Attenuation Adjustments and Their Computation. 

Thus far, only global compensation procedures have been discussed for which a fixed dB/inch 

adjustment is applied uniformly at all locations along the billet surface. Making the so-called 

local adjustments in which the applied gain (again in dB/inch units) varies with axial and 

circumferential position will now be considered. It is likely impractical at present to acquire raw 

UT data in which the gain varies with surface position. Thus, the local adjustment procedures to 

be discussed are intended to be used as postinspection tools, which can be used to adjust the 

amplitudes of suspected defect echoes to account for local variations in the attenuation of the test 

billet. 

Several approaches for applying local adjustments were considered and tested using the backwall C-scan pattern and the backscattered grain noise patterns in the various inspection zones as 

the basis for the adjustments. A number of methods were tested using different combinations of 

back-wall data and accumulated grain noise data for the inspection zones prior to and including 

the defects. In the end, it was found that the simplest methods, using back-wall data alone or 

backscattered noise data alone (from the center zone of the billet), worked as well as any, and the 

detailed discussions that follow will be confined to these. As before, measuring back-wall 

amplitudes using two different focusing schemes were considered. This led to a total of three 

local compensation procedures to be discussed based, respectively, on: 

• Deep-zone backscattered gated-peak grain noise patterns acquired with the beam focused 

near the billet centerline. 

• Back-wall gated-peak amplitude patterns acquired with the beam focused near the billet 

centerline. 

• Back-wall gated-peak amplitude patterns acquired with the beam focused near the billet 

back wall. 

79
80

FIGURE 72. PEAK AMPLITUDES OF THE 17 #2 FBHs IN THE TEST BILLET USING THE STANDARD COMPENSATION 

PROCEDURE (RIGHTMOST TWO PANELS) AND NO ATTENUATION COMPENSATION (LEFTMOST PANEL) 

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at billet centerfor back-wall echo acquisition

No Compensation

Standard Compensation

(Attenuation correction deduced from BW 

echoes with beam focused at billet center)

All 17 FBHs in test billet below 80% amplitude.Range: 29 - 74Average: 48.7

8 FBHs in test billet below 80% amplitude.Range: 49 - 123Average: 81.5

8 FBHs in test billet below 80% amplitude.Range: 48 - 120Average: 79.6

Standard Compensation

(Attenuation correction deduced from BW 

echoes with beam focused near BW)

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at billet centerfor back-wall echo acquisition

No Compensation

Standard Compensation

(Attenuation correction deduced from BW 

echoes with beam focused at billet center)

All 17 FBHs in test billet below 80% amplitude.Range: 29 - 74Average: 48.7

8 FBHs in test billet below 80% amplitude.Range: 49 - 123Average: 81.5

8 FBHs in test billet below 80% amplitude.Range: 48 - 120Average: 79.6

Standard Compensation

(Attenuation correction deduced from BW 

echoes with beam focused near BW)

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL
81

FIGURE 73. RESULTS OF APPLYING THREE GLOBAL ATTENUATION COMPENSATION PROCEDURES TO THE TEST 

BILLET CONTAINING 17 #2 FBH DEFECTS, ZONE 4 

(Back-wall C-scans on which the adjustments are based were acquired with the sonic beam focused near 

the centerline (zone 4 probe for 6′′ diameter billet; 3′′ water path).) 

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at billet centerfor back-wall echo acquisition

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

After "Conservative" Global 

Attenuation Compensation Adjustment

0

50

300250200150100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Beam focused at billet centerfor back-wall echo acquisition

Beam focused at billet centerfor back-wall echo acquisition

Standard Compensation Semi-Conservative Compensation Conservative Compensation

IDEAL

IDEAL

8 FBHs in test billet below 80% amplitude.Range: 49 - 123Average: 81.5

1 FBH in test billet below 80% amplitude.Range: 72 - 183Average: 121

0 FBHs in test billet below 80% amplitude.Range: 113 - 284Average: 189
82

FIGURE 74. RESULTS OF APPLYING THREE GLOBAL ATTENUATION COMPENSATION PROCEDURES TO THE TEST 

BILLET CONTAINING 17 #2 FBH DEFECTS, ZONE 7 

(Back-wall C-scans on which the adjustments are based were acquired with the sonic beam focused near 

the back wall (zone 7 probe for 13′′ diameter billet; 6.6′′ water path).) 

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Conservative" Global 

Attenuation Compensation Adjustment

0

50

300250200150100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

2 FBHs in test billet below 80% amplitude.Range: 66 - 167Average: 110

8 FBHs in test billet below 80% amplitude.Range: 48 - 120Average: 79.6

0 FBHs in test billet below 80% amplitude.Range: 85 - 215Average: 143

Standard Compensation Semi-Conservative Compensation Conservative Compensation

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

After "Conservative" Global 

Attenuation Compensation Adjustment

0

50

300250200150100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Beam focused at back wallfor back-wall echo acquisition

Beam focused at back wallfor back-wall echo acquisition

IDEAL

IDEAL

8 FBHs in test billet below 80% amplitude.Range: 48 - 120Average: 79.6

2 FBHs in test billet below 80% amplitude.Range: 66 - 167Average: 110

0 FBHs in test billet below 80% amplitude.Range: 85 - 215Average: 143

Standard Compensation Semi-Conservative Compensation Conservative Compensation
TABLE 12. FLAT-BOTTOM HOLE AMPLITUDES FOR THE Ti-6-4 TEST BILLET AFTER ADJUSTMENT 

Attenuation Compensated FBH Amplitudes 

FBH No. 

FBH Amplitude 

As-Run 

FBH 

Amplitude at 

Calculated 

Gain 

Centerline

Focus 

Standard 

Centerline 

Focus 

Semiconservative

Centerline 

Focus 

Conservative 

Back-Wall 

Focus 

Standard 

Back-Wall 

Focus 

Semiconservative 

Back-Wall 

Focus 

Conservative 

1 65 49.8 83.3 123.7 192.6 81.3 112.8 145.6 2 96 73.5 123.0 182.6 284.4 120.1 166.5 215.1 3 70 53.6 89.7 133.2 207.4 87.6 121.4 156.8 4 69 52.8 88.4 131.3 204.4 86.3 119.7 154.6 5 49 37.5 62.8 93.2 145.2 61.3 85.0 109.8 6 46 35.2 58.9 87.5 136.3 57.5 79.8 103.0 7 74 56.7 94.8 140.8 219.3 92.6 128.4 165.8 8 38 29.1 48.7 72.3 112.6 47.5 65.9 85.1 9 49 37.5 62.8 93.2 145.2 61.3 85.0 109.8 

10 74 56.7 94.8 140.8 219.3 92.6 

83128.4 165.8 

11 82 62.8 105.0 156.0 243.0 102.6 142.3 183.7 12 59 45.2 75.6 112.2 174.8 73.8 102.4 132.2 13 82 62.8 105.0 156.0 243.0 102.6 142.3 183.7 14 80 61.2 102.5 152.2 237.0 100.1 138.8 179.2 15 53 40.6 67.9 100.8 157.0 66.3 91.9 118.7 16 54 41.3 69.2 102.7 160.0 67.6 93.7 121.0 17 42 32.2 53.8 79.9 124.4 52.5 72.9 94.1 

average = 63.6 48.7 81.5 121.1 188.6 79.6 110.4 142.6 

minimum = 38 29.1 48.7 72.3 112.6 47.5 65.9 85.1 

maximum = 96 73.5 123.0 182.6 284.4 120.1 166.5 215.1 

Gain Adjustment in dB/inch: 0.00 1.28 2.26 3.36 1.22 2.03 2.66 Gain Adjustment at 3.5′′ dB: 0.00 4.47 7.91 11.75 4.27 7.11 9.33 Amplitude multiple factor at 3.5′′: 1.00 1.67 2.49 3.87 1.63 2.27 2.93 

Results are listed for each of the six global compensation procedures. 
2 FBHs in test billet below 

80% amplitude.

Range: 66 - 167

Average: 110

8 FBHs in test billet below 

80% amplitude.

Range: 48 - 120

Average: 79.6

0 FBHs in test billet below 

80% amplitude.

Range: 85 - 215

Average: 143

1 FBH in test billet below 

80% amplitude.

Range: 72 - 183

Average: 121

8 FBHs in test billet below 

80% amplitude.

Range: 49 - 123

Average: 81.5

0 FBHs in test billet below 

80% amplitude.

Range: 113 - 284

Average: 189

Attenuation 

correction 

deduced from 

BW echoes with 

beam focused at 

billet center:

Standard

Compensation

Semi- Conservative

Compensation

Conservative 

Compensation

Attenuation 

correction 

deduced from 

BW echoes with 

beam focused at 

back wall:

Comparison of the Six “Global” Attenuation Compensation Schemes

2 FBHs in test billet below 

80% amplitude.

Range: 66 - 167

Average: 110

8 FBHs in test billet below 

80% amplitude.

Range: 48 - 120

Average: 79.6

0 FBHs in test billet below 

80% amplitude.

Range: 85 - 215

Average: 143

1 FBH in test billet below 

80% amplitude.

Range: 72 - 183

Average: 121

8 FBHs in test billet below 

80% amplitude.

Range: 49 - 123

Average: 81.5

0 FBHs in test billet below 

80% amplitude.

Range: 113 - 284

Average: 189

Attenuation 

correction 

deduced from 

BW echoes with 

beam focused at 

billet center:

Standard

Compensation

Semi- Conservative

Compensation

Conservative 

Compensation

Attenuation 

correction 

deduced from 

BW echoes with 

beam focused at 

back wall:

Comparison of the Six “Global” Attenuation Compensation Schemes

FIGURE 75. SUMMARY OF FBH AMPLITUDE STATISTICS FOR THE SIX GLOBAL 

COMPENSATION PROCEDURES 

As before, for 6′′ diameter test billets, the zone 4 transducer for the 6′′ diameter billet was used to 

gather noise or back-wall data for the scenario where the beam was focused near the centerline, 

and the zone 7 transducer for the 13′′ diameter billet was used to gather back-wall data with the 

beam focused near the back wall. 

It was agreed that local adjustments will likely be made after global compensation adjustments 

have already been applied. The global adjustment (if any) will have determined the fixed 

inspection gain at a given depth and, hence, determined the average amplitude for a collection of 

(many) reflectors (defects) located at that depth. Under this view, a local gain adjustment is 

computed relative to a billet-wide average, and describes the extra gain at a given position above 

and beyond that required for the average location for that depth. 

The basic approach to making local adjustments is straightforward. When analyzing either the 

noise or back-wall amplitude pattern, it was assumed that the amplitude variations seen are due 

solely to attenuation variations. Let A(x,θ, zA) denote the measured amplitude (in % FSH units) 

in the test billet at axial position x, circumferential position θ, and one-way depth zA. In 

particular, A(x,θ, zA) here will represent either a back-wall amplitude (zA=billet diameter) or 

backscattered noise amplitude measured from an inspection zone centered near the billet 

centerline (zA=billet radius). Also, let <A(zA))> denote the billet-wide average of A(x,θ, zA) 

over all axial and hoop positions. Within the test billet, the difference between the local 

attenuation along the billet diameter at position (x,θ) and the billet-wide average (in dB/inch-ofdepth units) is written as 

( , ) 20log[ ( , , ) / ( ) ]/ [ / ] A AA Δ = −< >= α αθ α x A x z A z z dB inch θ < > (4) 

84
In a similar manner, FBH(x,θ,z) denotes the amplitude from a FBH located at position (x,θ,z) in 

the test billet, and <FBH(z)> denotes a billet-wide average amplitude for a large population of 

identical reflectors at the same depth. To estimate the amplitude difference between an FBH at 

location (x,θ,z) and the average FBH at the same depth, the attenuation difference calculated 

from equation 4 is used. 

/ 20 / ( , , ) / ( ) 10 [ ( , , ) / ( ) ] A z z z FBH x z FBH z A x z A z A A

α θ θ Δ • < >= = < > (5) 

In this manner, the effect of local attenuation variations within the test billet can be estimated 

from measured back-wall or grain noise data. 

Note that the backscattered noise level observed at a given depth depends on both the average 

attenuation to that depth and the backscattering capacity (FOM value) of the local 

microstructure. Thus, in writing equation 4 for the noise case, it is tacitly assumed that the grain 

noise capacity of the microstructure (i.e., the FOM value) is approximately uniform along the 

centerline and independent of the approach angle there. This approximation is supported by 

measurements summarized in figure 39, where billet properties near the centerline are similar to 

one another whether the centerline is approached through a high- or low-noise band. Thus, 

differences in backscattered noise levels from the billet center are assumed to be primarily due to 

differences in average attenuation values along different billet radii. Note, however, that for 

backscattered noise from a depth other than the billet centerline, the FOM value is expected to 

vary with surface position and, hence, equation 4 cannot be readily justified. 

It is reiterated that the approach being taken here must be regarded as an approximation because 

it is tacitly assumed that the attenuation is uniform along a given radius or diameter specified by 

(x,θ). For example, when using back-wall amplitudes to make the adjustments, it is assumed that 

the average attenuation in dB/inch computed along a full diameter is equal to the average 

attenuation computed, say, over the first 2 inches of depth. But it was found in earlier studies 

(e.g., figure 39) that billet attenuation varies with depth. Thus, this approach is likely more 

accurate for deeply positioned defects than for those located near the OD, since the path lengths 

zA and z will be more similar. Also, when using noise amplitudes to make the adjustments, the 

analysis assumes that the effective attenuation for noise is the same as that for FBH echoes. It 

may be that attenuation for noise (due to energy loss alone) is lower than that for FBHs (due to 

combined energy loss + beam distortion). Some evidence of this was seen in billet property 

measurements carried out during ETC Phase I [25]. Thus, an adjustable scaling factor > 1 could 

be added when translating from noise attenuation to FBH attenuation. However, such fine tuning 

was not attempted for the present analysis. 

Finally, note that the presence of the defect itself influences the noise or back-wall amplitude 

pattern used to compute the local adjustment. For this reason, when measuring the local 

amplitude A(x,θ,zA), the noise or back-wall amplitude is averaged over a small area centered on 

the defect coordinates, and a smaller region in the immediate shadow of the defect is excluded. 

85
Although only the deep-zone (zone 4) noise C-scan was eventually used for local attenuation 

adjustments, a full four-zone MZ inspection was carried out on the test billet to obtain noise data. 

The resulting C-scan for one of the zones is shown in figure 76 together with boxes marking the 

axial and hoop locations of the (deeper) 17 FBH defects. A similar C-scan for zone 4 was shown 

in figure 69. Within each box, noise statistics were calculated (minimum, maximum, mean, and 

standard deviation) and their values are listed in figure 77 for each inspection zone. For zone 4, 

the images of the FBHs also appear in the C-scan and must be excluded when calculating noise 

statistics. This was done using software designed by GE for calculating SNRs. The software 

determined mean and peak noise levels in the vicinity of each defect, and these values are listed 

in figure 77. 

FIGURE 76. ZONE 3 C-SCAN OF THE 18′′ LONG TEST BILLET SHOWING ANALYSIS 

BOXES FOR THE COMPUTATION OF NOISE STATISTICS 

The noise amplitudes at corresponding axial and hoop positions in different zones are somewhat 

correlated. This is indicated by figure 78 where the measured mean noise amplitudes at the 17 

locations of interest are displayed. Noise amplitudes within each zone are normalized to an 

average value of 100 for the set of 17 measurements and generally range between 70 and 130. 

Discounting zone 1, which shows relatively little variation, normalized noise values for the other 

three zones tend to rise or fall together as the axial and hoop position is varied. Only the local 

mean noise amplitudes for zone 4 are used to make local attenuation adjustments. When using 

equations 3 and 4, the local noise mean, A(x,θ, zA), as listed in figure 77, is compared to the fullscan average noise amplitude estimated to be <A(zA)>=17 on the same (raw) scale. 

Calibration 

Standard 

@ 30.5 dB 

78% 84%

Test Billet at 3 dB additional gain

FBH amplitudes at zone ends of cal standard

6 

19 

11 

2

6

30

13

3

Min: 

Max: 

Mean: 

Std. Dev:

6

15

6

2

6 

25 

13 

3

86
Raw Backscattered Noise Statistics for the 6"-diameter 18"-long Ti 6-4 Test Billet (gated-peak noise amplitudes in % FSH)

Zone 1 Zone 2 Zone 3 Zone 4

Location Mean Min Max Std.Dev Mean Min Max Std.Dev Mean Min Max Std.Dev Mean Max

1 15 12 26 2 13 5 28 4 14 7 24 3 19 33

2 20 29 11 3 15 5 31 4 14 6 24 3 19 34

3 20 13 34 3 9 4 18 2 9 4 19 2 15 28

4 20 11 30 3 11 4 25 3 11 5 22 2 18 30

5 15 12 26 2 10 5 18 2 8 4 15 2 14 25

6 18 11 28 2 13 4 35 4 10 4 20 3 14 24

7 18 11 28 2 15 5 35 5 10 4 19 2 16 32

8 17 9 28 3 11 2 27 3 9 3 20 2 12 25

9 19 9 29 3 16 7 38 4 13 7 25 3 16 29

10 19 9 28 3 13 4 34 5 13 7 24 3 18 30

11 19 12 31 3 13 6 24 3 13 6 24 2 19 36

12 18 12 26 2 10 4 20 2 9 4 20 2 14 28

13 19 11 30 3 16 11 30 3 14 8 26 3 21 38

14 19 11 30 3 17 7 35 4 13 7 29 3 19 35

15 18 11 27 2 15 7 30 4 12 6 21 2 16 31

16 20 13 100 3 13 5 33 3 9 4 18 2 18 32

17 16 11 24 2 12 4 28 3 11 4 25 3 17 37

For Zone 4, the full-area mean noise amplitude is 17% FSH

1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17

Locations in-scans

 Depths of Inspection Zones 

Zon Start End

1 0.2 0.8

2 0.8 1.7

3 1.7 2.6

4 2.6 3.5

Raw Backscattered Noise Statistics for the 6"-diameter 18"-long Ti 6-4 Test Billet (gated-peak noise amplitudes in % FSH)

Zone 1 Zone 2 Zone 3 Zone 4

Location Mean Min Max Std.Dev Mean Min Max Std.Dev Mean Min Max Std.Dev Mean Max

1 15 12 26 2 13 5 28 4 14 7 24 3 19 33

2 20 29 11 3 15 5 31 4 14 6 24 3 19 34

3 20 13 34 3 9 4 18 2 9 4 19 2 15 28

4 20 11 30 3 11 4 25 3 11 5 22 2 18 30

5 15 12 26 2 10 5 18 2 8 4 15 2 14 25

6 18 11 28 2 13 4 35 4 10 4 20 3 14 24

7 18 11 28 2 15 5 35 5 10 4 19 2 16 32

8 17 9 28 3 11 2 27 3 9 3 20 2 12 25

9 19 9 29 3 16 7 38 4 13 7 25 3 16 29

10 19 9 28 3 13 4 34 5 13 7 24 3 18 30

11 19 12 31 3 13 6 24 3 13 6 24 2 19 36

12 18 12 26 2 10 4 20 2 9 4 20 2 14 28

13 19 11 30 3 16 11 30 3 14 8 26 3 21 38

14 19 11 30 3 17 7 35 4 13 7 29 3 19 35

15 18 11 27 2 15 7 30 4 12 6 21 2 16 31

16 20 13 100 3 13 5 33 3 9 4 18 2 18 32

17 16 11 24 2 12 4 28 3 11 4 25 3 17 37

For Zone 4, the full-area mean noise amplitude is 17% FSH

1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17 1

2

3

4

11

5 12

6 10

16

7

8

9

15

14

13

17

Locations in-scans

 Depths of Inspection Zones 

Zon Start End

1 0.2 0.8

2 0.8 1.7

3 1.7 2.6

4 2.6 3.5

FIGURE 77. NOISE STATISTICS AT THE AXIAL AND HOOP LOCATIONS OF 

THE 17 FBH DEFECTS 

Raw Average Noise Levels Test Billet Containing 17 #2 FBHs at 3.5" Depth

Normalized Noise Variations

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Measurement Site

Normalized Amplitude (100 = Ave)

Z1 Noise (norm'ed)

Z2 Noise (norm'ed)

Z3 Noise (norm'ed)

Z4 Noise (norm'ed)

Location Zone 1 Zone 2 Zone 3 Zone 4

1 15 13 14 19

2 20 15 14 19

3 20 9 9 15

4 20 11 11 18

5 15 10 8 14

6 18 13 10 14

7 18 15 10 16

8 17 11 9 12

9 19 16 13 16

10 19 13 13 18

11 19 13 13 19

12 18 10 9 14

13 19 16 14 21

14 19 17 13 19

15 18 15 12 16

16 20 13 9 18

17 16 12 11 17

Ave = 18.24 13.06 11.29 16.76

To normalize such that average = 100 in each zone

multiply the above values by:

Zone 1 Zone 2 Zone 3 Zone 4

5.484 7.658 8.854 5.965

Raw Average Noise Levels Test Billet Containing 17 #2 FBHs at 3.5" Depth

Normalized Noise Variations

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Measurement Site

Normalized Amplitude (100 = Ave)

Z1 Noise (norm'ed)

Z2 Noise (norm'ed)

Z3 Noise (norm'ed)

Z4 Noise (norm'ed)

Location Zone 1 Zone 2 Zone 3 Zone 4

1 15 13 14 19

2 20 15 14 19

3 20 9 9 15

4 20 11 11 18

5 15 10 8 14

6 18 13 10 14

7 18 15 10 16

8 17 11 9 12

9 19 16 13 16

10 19 13 13 18

11 19 13 13 19

12 18 10 9 14

13 19 16 14 21

14 19 17 13 19

15 18 15 12 16

16 20 13 9 18

17 16 12 11 17

Ave = 18.24 13.06 11.29 16.76

To normalize such that average = 100 in each zone

multiply the above values by:

Zone 1 Zone 2 Zone 3 Zone 4

5.484 7.658 8.854 5.965

FIGURE 78. RAW AND NORMALIZED AVERAGE GATED-PEAK NOISE AMPLITUDES 

SEEN IN THE MZ INSPECTION OF THE Ti-6-4 TEST BILLET 

87
The back-wall C-scans used to make local attenuation adjustments are shown in figures 66 and 

67, respectively, for the two focusing schemes. For each C-scan, average back-wall amplitudes 

in the (axial and hoop) vicinity of the defects were measured using the two methods illustrated in 

figure 79(a). In the first method, amplitudes at each site were computed for an area consisting of 

a rectangle with a smaller centered rectangle removed, the center being large enough to fully 

cover the shadow cast by the defect. Special image analysis software was written for this 

purpose. To test the software, back-wall amplitude statistics were also computed using a second 

method in which the area used in the analysis was the union of two rectangles located on either 

side of the defect shadow. As shown in figure 80, the two methods yielded similar estimates of 

the local average back-wall amplitude in the vicinity of each defect. For the later local 

attenuation adjustments, method 1 values were used. 

(a)

(b)

Method 1

Method 2

Defect

Shadow

Defect

Shadow

Analysis regions 

(shaded) used for 

back-wall amplitude 

statistics

(a)

(b)

Method 1

Method 2

Defect

Shadow

Defect

Shadow

Analysis regions 

(shaded) used for 

back-wall amplitude 

statistics

Method 1

Method 2

Defect

Shadow

Defect

Shadow

Analysis regions 

(shaded) used for 

back-wall amplitude 

statistics

FIGURE 79. (a) METHODS OF CHOOSING THE ANALYSIS REGION USED TO 

COMPUTE THE AVERAGE BACK-WALL AMPLITUDE IN THE VICINITY OF EACH 

DEFECT AND (b) ANALYSIS REGIONS (WHITE/BLACK SQUARES) FOR THE 

BACK-WALL C-SCAN OF THE TEST BILLET MADE WITH THE BEAM 

FOCUSED NEAR THE BILLET BACK-WALL (USING METHOD 1) 

88
Raw BW Amplitude Fluctuations

(Focus at Center Line)

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

Local Mean BW Amplitude

Method 1

Method 2

Raw BW Amplitude Fluctuations

(Focus at Back Wall)

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

Local Mean BW Amplitude

Method 1

Method 2

(a)

(b)

 Focus at BW

Location Method 1 Method 2

1 51.9 62.5

2 61.4 62.5

3 29.9 23.5

4 51.4 62.0

5 27.9 27.0

6 32.1 40.0

7 32.1 31.5

8 19.9 20.0

9 21.8 19.5

10 46.2 54.5

11 66.7 66.5

12 31.1 32.0

13 61.2 62.0

14 53.9 63.5

15 33.1 35.5

16 49.3 47.5

17 33.9 42.0

ave = 41.4 44.2

 Focus at CL

Location Method 1 Method 2

1 48.0 52.0

2 50.7 55.5

3 22.6 14.0

4 46.6 55.0

5 22.5 21.0

6 30.1 40.0

7 36.7 37.0

8 19.7 23.0

9 21.1 20.5

10 43.4 57.0

11 62.3 63.5

12 26.7 25.5

13 64.4 67.0

14 47.8 62.5

15 29.6 33.0

16 48.5 52.0

17 28.4 36.5

ave = 38.2 42.1

Raw BW Amplitude Fluctuations

(Focus at Center Line)

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

Local Mean BW Amplitude

Method 1

Method 2

Raw BW Amplitude Fluctuations

(Focus at Back Wall)

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

Local Mean BW Amplitude

Method 1

Method 2

(a)

(b)

 Focus at BW

Location Method 1 Method 2

1 51.9 62.5

2 61.4 62.5

3 29.9 23.5

4 51.4 62.0

5 27.9 27.0

6 32.1 40.0

7 32.1 31.5

8 19.9 20.0

9 21.8 19.5

10 46.2 54.5

11 66.7 66.5

12 31.1 32.0

13 61.2 62.0

14 53.9 63.5

15 33.1 35.5

16 49.3 47.5

17 33.9 42.0

ave = 41.4 44.2

 Focus at CL

Location Method 1 Method 2

1 48.0 52.0

2 50.7 55.5

3 22.6 14.0

4 46.6 55.0

5 22.5 21.0

6 30.1 40.0

7 36.7 37.0

8 19.7 23.0

9 21.1 20.5

10 43.4 57.0

11 62.3 63.5

12 26.7 25.5

13 64.4 67.0

14 47.8 62.5

15 29.6 33.0

16 48.5 52.0

17 28.4 36.5

ave = 38.2 42.1

FIGURE 80. RAW AVERAGE BACK-WALL AMPLITUDES IN THE VICINITIES OF THE 

17 FBH DEFECTS IN THE TEST BILLET (a) BEAM FOCUSED NEAR BILLET 

CENTERLINE AND (b) BEAM FOCUSED NEAR BILLET BACK WALL 

From the measured local zone 4 noise or back-wall amplitudes, local attenuation adjustment 

factors can be computed using equation 4. The results are listed in table 13. The first three 

columns list the (raw) mean noise and back-wall amplitudes at each measurement site 

[A(x, θ, zA)] and the full-scan average [<A (zA)>]. The last three columns evaluate the rightmost 

term in equation 4, which is the predicted ratio of the local FBH amplitude at position (x,θ, z) to 

the full-scan average (assuming many identical FBH defects at depth z). In applying equation 4, 

one takes z = 3.5′′ as the depth of the defects, used zA = 6′′ for back-wall amplitudes, and zA = 3′′

for noise amplitudes. The latter value was chosen because grain noise versus depth curves 

acquired without DAC tend to peak near the focal depth, which was nominally 3′′ for the zone 4 

C-scan used to gather the noise data in question. For a gated-peak noise C-scan, the peak noise 

voltage observed at a given pixel location could occur at any depth within the inspection gate; 

however, the peak voltages tend to be clustered near the focal depth. 

89
TABLE 13. LOCAL ATTENUATION ADJUSTMENT FACTORS AT THE 17 FBH SITES 

DETERMINED FROM LOCAL BACK-WALL AND GRAIN NOISE AMPLITUDES 

Raw Amplitudes Used to Compute Adjustments Computed Local Adjustment Factors at 3.5′′ Depth 

Location 

Back Wall 

(Focus at CL) 

Back Wall 

(Focus at CL) 

Zone 4 Noise

(Focus at CL) 

Back Wall 

(Focus at CL) 

Back Wall 

(Focus at CL) 

Zone 4 Noise 

(Focus at CL) 

1 48.0 51.9 19 1.13 1.12 1.14 

2 50.7 61.4 19 1.17 1.23 1.14 

3 22.6 29.9 15 0.73 0.81 0.86 

4 46.6 51.4 18 1.11 1.11 1.07 

5 22.5 27.9 14 0.73 0.78 0.80 

6 30.1 32.1 14 0.86 0.84 0.80 

7 36.7 32.1 16 0.97 0.84 0.93 

8 19.7 1909 12 0.67 0.64 0.67 

9 21.1 21.8 16 0.70 0.67 00.93 

10 43.4 46.2 18 1.07 1.04 1.07 

11 62.3 66.7 19 1.31 1.29 1.14 

12 26.7 31.1 14 0.80 0.83 0.80 

13 64.4 61.2 21 1.34 1.23 1.28 

14 47.8 53.9 19 1.13 1.14 1.14 

15 29.6 33.1 16 0.85 0.86 0.93 

16 48.5 49.3 18 1.14 1.08 1.07 

17 28.4 33.9 17 0.83 0.87 1.00 

Full C-Scan 

Average: 39 43 17 

Notes: 

CL = centerline 

A factor of 1.13, for example, indicates that the FBH amplitude at that location is expected to be 13% larger than the 

billet-wide average due to lower than average attenuation. 

Predicted FBH amplitude fluctuations are compared with measured fluctuations in figure 81. 

Recall that equation 4 predicts the amplitude of a defect at a specific location relative to that of a 

(hypothetical) billet-wide average amplitude for many similar defects at the same depth. Of 

course, one does not have a full billet-wide population of #2 FBHs at 3.5′′ depth, but rather 17 

specific members of that population. To construct figure 81, a value of 100 was assigned to the 

billet-wide defect amplitude when making the predictions. To get an average value of 100, the 

measured amplitudes of the 17 real defects were normalized. The average of the 17 predicted 

amplitudes for each local adjustment procedure is typically a few percent below 100, perhaps 

suggesting that the 17 chosen locations tend to be in preferentially low-amplitude zones. As 

figure 81 shows, the predicted pattern of amplitude fluctuations is very similar for the three local 

adjustment procedures. Each predicted fluctuation pattern is also fairly similar to the measured 

pattern, but there are a few noteworthy exceptions: for defects 2 and 3, the measured amplitudes 

are significantly above the predictions, and for defects 16 and 17, the measured amplitudes are 

significantly lower than predicted. 

90
0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Site

Normalized Amplitude (100 = Ave)

Est. from Zone 4 Noise

Est. from BW (focus at CL)

Est. from BW (focus at BW)

FBH measured

FIGURE 81. COMPARISON OF MEASURED AND PREDICTED FBH AMPLITUDES OF 

THE 17 #2 FBH DEFECTS IN THE TEST BILLET (NORMALIZED TO 

AN AVERAGE OF 100) 

The relationship between the measured and predicted FBH amplitudes at the 17 sites in the test 

billet is further explored in figure 82. There, the data shown in figure 81 have been replotted in a 

different way. For each of the three procedures used to estimate local FBH amplitudes, the 

measured amplitude of each defect has been plotted against the predicted amplitude. Again, the 

17 measured amplitudes have been normalized to an average of 100, and the predictions assume 

an average of 100 for a large (billet-wide) population of defects. If there were a perfect 

correspondence between the measured and predicted amplitudes, the plotted points would lie 

along a straight line of unit slope and the correlation parameter R2 for the best-fit line would 

equal 1. For each of the three local estimation procedures, similar levels of scatter about the 

best-fit line are seen. R2

 values are somewhat larger (better correlation) for estimates made from 

back-wall amplitudes as opposed to backscattered noise. Also, for estimates made from backwall amplitudes, the correlation is best when back-wall amplitudes are measured with the beam 

focused near the back wall. 

If one estimates, on the basis of back-wall or grain noise C-scans, that a defect at a specific axial 

and hoop location will have an amplitude above or below average due to local attenuation 

variation, then the inspection gain used at that specific location can, in principle, be adjusted. 

The aim of such local gain adjustments is to equalize the echo amplitudes of identical defects 

located at the same depth but different lateral positions in the billet. The gain multiplication 

factor required to equalize amplitudes (in linear rather than dB units) is the inverse of the term on 

the far right-hand side of equation 4. At the sites of the 17 #2 FBHs in the test billet, the 

required gain factors for the three local adjustment procedures under consideration are, thus, the 

inverses of the numbers listed in the last three columns of table 13. 

91
Correlation Between Measured

and Predicted FBH Amplitudes

y = 0.8956x + 12.972

R2

 = 0.5606

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Estimated FBH Amplitude

Measured FBH Amplitude

Predictions made from BW pattern

(beam focused near centerline)

Correlation Between Measured

and Predicted FBH Amplitudes

y = 0.998x + 3.8041

R2

 = 0.6117

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Estimated FBH Amplitude

Measured FBH Amplitude

Predictions made from BW pattern

(beam focused near back wall)

Correlation Between Measured

and Predicted FBH Amplitudes

y = 1.1335x - 11.735

R2

 = 0.5159

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Estimated FBH Amplitude

Measured FBH Amplitude

Predictions made from zone-4 noise pattern

(beam focused near centerline)

Amplitudes of 17 #2 FBH 

targets in the 18”-long test 

billet, normalized to an 

average of 100. These graphs 

compare measured 

amplitudes with those 

predicted from back-wall or 

noise C-scans. 

Correlation Between Measured

and Predicted FBH Amplitudes

y = 0.8956x + 12.972

R2

 = 0.5606

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Estimated FBH Amplitude

Measured FBH Amplitude

Predictions made from BW pattern

(beam focused near centerline)

Correlation Between Measured

and Predicted FBH Amplitudes

y = 0.998x + 3.8041

R2

 = 0.6117

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Estimated FBH Amplitude

Measured FBH Amplitude

Predictions made from BW pattern

(beam focused near back wall)

Correlation Between Measured

and Predicted FBH Amplitudes

y = 1.1335x - 11.735

R2

 = 0.5159

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Estimated FBH Amplitude

Measured FBH Amplitude

Predictions made from zone-4 noise pattern

(beam focused near centerline)

Amplitudes of 17 #2 FBH 

targets in the 18”-long test 

billet, normalized to an 

average of 100. These graphs 

compare measured 

amplitudes with those 

predicted from back-wall or 

noise C-scans. 

FIGURE 82. CORRELATION BETWEEN PREDICTED AND MEASURED FBH 

AMPLITUDES IN THE Ti-6-4 TEST BILLET 

Local attenuation adjustments can be made either on their own or in combination with one of the 

global compensation procedures discussed earlier. The data resulting from the MZ inspection of 

the Ti-6-4 test billet has been postprocessed to show how the amplitudes of the 17 defects would 

appear if (1) local compensation was used alone, (2) local compensation was applied following 

the standard global compensation procedure; or (3) local compensation was applied following 

the semiconservative global compensation procedure. For each of the three combinations there 

are several cases, depending on whether local compensation made use of grain noise or backwall amplitudes and on the choice of focusing method used to gather back-wall C-scans. The 

results are shown in figures 83 through 85. In a given figure, a single global adjustment 

procedure has been applied, e.g., the standard procedure for figure 84. For the upper three panels 

of each figure, the global adjustment (which is always based on back-wall amplitudes) has been 

made using data acquired with the beam focused near the centerline. For the lower three panels, 

the global adjustment is based on back-wall data with the beam focused on the back wall. From 

left to right in each figure, the result of ignoring local adjustments or applying local adjustments 

based on back-wall or noise amplitudes are displayed, respectively. For cases in which both the 

global and local adjustments are based on back-wall data, the same focusing scheme is assumed 

to have been used when acquiring that back-wall data. 

92
Local Adjustments Via 

BW echoes

No Local Adjustments Local Adjustments Via 

Zone-4 Noise

Top row: beam focused at CL when gathering BW data.Bottom row: beam focused at BW when gathering BW data.

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

93

FIGURE 83. RESULTS OF APPLYING LOCAL ATTENUATION COMPENSATION PROCEDURES TO 

FBH AMPLITUDES IN THE Ti-6-4 TEST BILLET 

(No global compensation has been used.) 

IDEAL

Local adjust. via BW pattern.Beam focused at billet centerfor back-wall echo acquisition

No Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via BW pattern.Beam focused at back wallfor back-wall echo acquisition

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via Noise pattern.

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via Noise pattern.

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local Adjustments Via 

BW echoes

No Local Adjustments Local Adjustments Via 

Zone-4 Noise

Top row: beam focused at CL when gathering BW data.Bottom row: beam focused at BW when gathering BW data.

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via BW pattern.Beam focused at billet centerfor back-wall echo acquisition

No Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via BW pattern.Beam focused at back wallfor back-wall echo acquisition

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Local adjust. via Noise pattern.

IDEAL IDEAL

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

No Global Attenuation Compensation

(Gain set to Bring #2 FBH in Cal Std to 80%)

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Local adjust. via Noise pattern.

IDEAL IDEAL
Local Adjustments Via 

BW echoes

No Local Adjustments Local Adjustments Via 

Zone-4 Noise

Top row: beam focused at CL when gathering BW data.Bottom row: beam focused at BW when gathering BW data.

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

94

FIGURE 84. RESULTS OF APPLYING LOCAL ATTENUATION COMPENSATION PROCEDURES TO FBH 

AMPLITUDES IN THE Ti-6-4 TEST BILLET, STANDARD GLOBAL COMPENSATION 

(Here, local compensation is used in combination with the standard global compensation procedure.) 

IDEAL

Beam focused at billet centerfor back-wall echo acquisition

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via BW pattern.Beam focused at billet centerfor back-wall echo acquisition

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via BW pattern.Beam focused at back wallfor back-wall echo acquisition

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via noise pattern.Beam focused at billet centerfor back-wall echo acquisition

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via Noise pattern.Beam focused at back wallfor back-wall echo acquisition

Local Adjustments Via 

BW echoes

No Local Adjustments Local Adjustments Via 

Zone-4 Noise

Top row: beam focused at CL when gathering BW data.Bottom row: beam focused at BW when gathering BW data.

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at billet centerfor back-wall echo acquisition

After "Standard" Global 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Local adjust. via BW pattern.Beam focused at billet centerfor back-wall echo acquisition

Local adjust. via noise pattern.Beam focused at billet centerfor back-wall echo acquisition

IDEAL IDEAL

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

After "Standard" Global + Local 

Attenuation Compensation Adjustment

0

80604020

140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Local adjust. via Noise pattern.Beam focused at back wallfor back-wall echo acquisition

Local adjust. via BW pattern.Beam focused at back wallfor back-wall echo acquisition

IDEAL IDEAL
95

Local Adjustments Via Local Adjustments Via 

BW echoes

No Local Adjustments

Zone-4 Noise

Top row: beam focused at CL when gathering BW data.Bottom row: beam focused at BW when gathering BW data.

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

FIGURE 85. RESULTS OF APPLYING LOCAL ATTENUATION COMPENSATION PROCEDURES TO FBH 

AMPLITUDES IN THE Ti-6-4 TEST BILLET, SEMICONSERVATIVE—GLOBAL COMPENSATION 

(Here, local compensation is used in combination with the semiconservative global compensation procedure.) 

IDEAL

Beam focused at billet centerfor back-wall echo acquisition

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Semi-Conservative Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Nu ber

FBH Amplitude (% FSH)

"

m

IDEAL

Local a just. via BW pattern.Beam focused at billet centerfor back-wall echo acquisition

d

After "Semi-Conservative Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Nu ber

FBH Amplitude (% FSH)

"

m

IDEAL

Loc adjust. via BW pattern.Beam focused at back wallfor back-wall echo acquisition

al

After "Semi-Conservative" Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via Noise pattern.Beam focused at billet centerfor back-wall echo acquisition

After "Semi-Conservative" Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Local adjust. via Noise pattern.Beam focused at back wallfor back-wall echo acquisition

Local Adjustments Via 

BW echoes

No Local Adjustments Local Adjustments Via 

Zone-4 Noise

Top row: beam focused at CL when gathering BW data.Bottom row: beam focused at BW when gathering BW data.

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at billet centerfor back-wall echo acquisition

After "Semi-Conservative" Global 

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH) IDEAL

Beam focused at back wallfor back-wall echo acquisition

After "Semi-Conservative Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Nu ber

FBH Amplitude (% FSH)

"

m

After "Semi-Conservative" Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Local a just. via BW pattern.Beam focused at billet centerfor back-wall echo acquisition

d Local adjust. via Noise pattern.

Beam focused at billet centerfor back-wall echo acquisition

IDEAL IDEAL

After "Semi-Conservative Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Nu ber

FBH Amplitude (% FSH)

"

m

After "Semi-Conservative" Global + Local

Attenuation Compensation Adjustment

0

80604020

200180160140120100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

FBH Number

FBH Amplitude (% FSH)

Local adjust. via Noise pattern.Beam focused at back wallfor back-wall echo acquisition

Loc adjust. via BW pattern.Beam focused at back wallfor back-wall echo acquisition

al

IDEAL IDEAL
The various combinations of global + local attenuation adjustments considered in figures 83 

through 85 are summarized in figure 86, and key statistics of the adjusted FBH amplitudes are 

listed in table 14. The tabulated statistics include the range of FBH amplitudes, the mean 

amplitude, the minimum and maximum amplitudes, the ratio of the standard deviation to the 

mean (which serves as a measure of the severity of the fluctuations), and the number of 

amplitudes below the target level of 80% FSH. A study of figures 83 through 85 and table 14 

reveals that local adjustments based on equations 3 and 4 tend to reduce the fluctuations of the 

FBH amplitudes about their mean without a significant change in the mean amplitude. The 

standard deviation of the fluctuations is reduced by about 40% for local adjustments based either 

on back-wall amplitudes or center zone grain noise. When the standard global procedure is 

combined with the different local adjustment methods, about half of the FBH amplitudes remain 

below 80% FSH. However, when local adjustments are combined with the semiconservative 

global procedure, all adjusted FBH amplitudes (with one exception) were greater than 80% FSH. 

That one exception occurred for FBH #17 (see figure 64 for FBH #17 location) when 

backscattered noise from zone 4 was used to make the local adjustments. In the zone 4 C-scan 

image, just below and right of FBH #17 (see figure 69), heightened noise amplitudes are seen, 

which may actually result from a defect in that vicinity. These heightened signals boost the 

assigned mean noise level near FBH #17 when the automatic analysis software is used. The 

larger mean noise amplitude then results in a smaller local attenuation adjustment than would 

otherwise be the case. Thus, the fact that the adjusted amplitude for FBH #17 is slightly below 

80% (using the semiconservative global + noise-based local procedures) may in fact be due to 

the presence of defects. Hard alpha defects are known to exist in the CBS billet heat from which 

the test specimen was fabricated [7]. 

Global Adjustment Local Adjustment

x

In addition, back-wall echoes used for making either “global” or 

“local” gain adjustments were acquired using two focal conditions:

Based on back-wall variations

in the test billet

Based on grain-noise variations

near center of the test billet

None

Focus Near Centerline Focus Near Back Wall

Fixed gain adjustment in dB/inch

applied at all axial/hoop positions

Gain adjustment in dB/inch is

different at each axial/hoop position

“Standard”

“Semi-Conservative”

“Conservative”

None

Global Adjustment Local Adjustment

x

In addition, back-wall echoes used for making either “global” or 

“local” gain adjustments were acquired using two focal conditions:

Based on back-wall variations

in the test billet

Based on grain-noise variations

near center of the test billet

None

Focus Near Centerline Focus Near Back Wall

Fixed gain adjustment in dB/inch

applied at all axial/hoop positions

Gain adjustment in dB/inch is

different at each axial/hoop position

“Standard”

“Semi-Conservative”

“Conservative”

None

FIGURE 86. COMBINATIONS OF GLOBAL AND LOCAL ATTENUATION 

COMPENSATION PROCEDURES CONSIDERED IN THIS REPORT 

96
TABLE 14. STATISTICS OF ADJUSTED FBH AMPLITUDE VARIATIONS IN THE 18′′

LONG TEST BILLET FOR EACH OF THE ATTENUATION COMPENSATION 

PROCEDURES CONSIDERED 

Attenuation Compensation Method Details Statistics of 17 Adjusted FBH Amplitudes in the Test Billet 

Global Procedure 

Local 

Procedure 

Beam Focus 

for Back Wall

C-Scan 

Average 

(% FSH) 

Range 

(% FSH) Max/Min 

Std. 

Dev./Avg. 

Number 

Below 

80% 

None None N/A 48.7 29-74 2.53 0.26 17 

None Via back wall 

C-scan 

At billet 

centerline 

50.5 36-74 2.02 0.18 17 

None Via back wall 

C-scan 

At billet back 

wall 

50.7 37-67 1.82 0.17 17 

None Via zone 4 

noise 

N/A 49.3 32-65 2.01 0.18 17 

Standard None At billet 

centerline 

81.5 49-123 2.53 0.26 8 

Standard None At billet back 

wall 

79.6 48-120 2.53 0.26 8 

Standard Via back wall 

C-scan 

At billet 

centerline 

84.4 61-123 2.02 0.18 6 

Standard Via back wall 

C-scan 

At billet back 

wall 

82.8 60-110 1.82 0.17 9 

Standard Via zone 4 

noise 

At billet 

centerline 

82.4 54-108 2.01 0.18 8 

Standard Via zone 4 

noise 

At billet back 

wall 

80.5 53-106 2.01 0.18 8 

Semiconservative None At billet 

centerline 

121.1 72-183 2.53 0.26 1 

Semiconservative None At billet back 

wall 

110.4 66-167 2.53 0.26 2 

Semiconservative Via back wall 

C-Scan 

At billet 

centerline 

125.4 91-183 2.02 0.18 0 

Semiconservative Via back wall 

C-scan 

At billet back 

wall 

114.8 84-152 1.82 0.17 0 

Semiconservative Via zone 4 

Noise 

At billet 

centerline 

122.5 80-160 2.01 0.18 0 

Semiconservative Via zone 4 

Noise 

At billet back 

wall 

111.7 73-146 2.01 0.18 1 

Conservative None At billet 

centerline 

188.6 113-284 2.53 0.26 0 

Conservative None At billet back 

wall 

142.6 85-215 2.53 0.26 0 

Conservative Via back wall 

C-scan 

At billet 

centerline 

195.3 141-284 2.02 0.18 0 

Conservative Via back wall 

C-scan 

At billet back 

wall 

148.2 108-197 1.82 0.17 0 

Conservative Via zone 4 

noise 

At billet 

centerline 

190.7 124-250 2.01 0.18 0 

Conservative Via zone 4 

noise 

At billet 

centerline 

144.2 94-189 2.01 0.18 0 

97
In general, local attenuation adjustments based on back-wall amplitudes were found to be 

slightly superior to those based on backscattered noise, and when back-wall amplitudes were 

used, adjustments computed with the beam focused on the back wall were slightly superior to 

those computed with the beam focused at the billet centerline. 

3.8 SUMMARY AND RECOMMENDATIONS FOR ATTENUATION COMPENSATION. 

Cylindrical billets of Ti alloy are ultrasonically inspected prior to use in fabricating rotating jet 

engine components. Although each billet has a cylindrical geometry, its UT properties are not 

cylindrically symmetric due to asymmetries in the process used to produce the billet from the 

cast ingot. In the inspection process, a calibration standard of the same diameter containing FBH 

reflectors is used to set the initial inspection gain (i.e., the signal amplification level). If the UT 

attenuation of the billet to be inspected differs significantly from that of the calibration standard, 

or if attenuation varies significantly with position within either the standard or test billet, then the 

inspection gain should be adjusted to achieve the desired defect detection sensitivity. 

Implementing such gain adjustments is referred to as attenuation compensation. Several 

procedures for determining attenuation-compensated gains were investigated and tested using 6′′

diameter Ti-6Al-4V billet into which many FBH were drilled. 

Two billet specimens were fabricated for the study. The first, 15′′ long, contained 13 shallow #4 

FBHs drilled 0.3′′ into the billet OD at various locations spanning a wide range of back-wall 

amplitudes. The purpose of this specimen was to determine whether the large back-wall 

amplitude variations evident in C-scan images implied that echoes from a collection of small, 

identical defects (FBH reflectors) would display similar large variations. FBH amplitudes and 

back-wall amplitudes at nearby locations were measured using both fixed-focus and phased array 

transducers. Two basic focal procedures (local and global) were investigated, having the sonic 

beam focused near the billet centerline and back wall, respectively. Six measurement trials were 

conducted, and for each trial, the FBH and back-wall amplitudes were found to vary significantly 

within the billet. These amplitude variations were believed to result from variations of the local 

UT attenuation within the test billet. For FBHs, the ratio of maximum-to-minimum amplitudes 

ranged from 2-4, depending on the inspection details. This FBH variability was large enough 

that it should be specifically accounted for during inspections via some type of compensation 

procedure. FBH and back-wall amplitudes in the first test specimen were found to be fairly well 

correlated: sites with low (high) back-wall amplitudes tended to have low (high) FBH 

amplitudes as well. The fluctuation levels about the mean, and the degree of correlation between 

back-wall and FBH amplitudes, depended somewhat on the inspection setup. The back-wall and 

FBH fluctuation levels were more similar to one another (and the individual amplitudes more 

correlated) when the beam was focused near the back wall. Although nearby FBH and back-wall 

amplitudes were reasonably well correlated, a few exceptional cases were seen, i.e., sites at 

which the local back-wall amplitude was smaller than average while the nearby FBH amplitude 

was larger than average. Thus, it is wisest to use the back-wall fluctuation level as a general 

indicator of the level of expected FBH fluctuations, rather than inferring that a site with a low 

(high) back-wall amplitude will necessarily have a low (high) FBH amplitude as well. 

The measurements on the first test specimen indicated that back-wall echo variations can be used 

to monitor attenuation variations within a billet, i.e., that defect amplitudes (in this case FBH 

98
amplitudes) vary with lateral position in a similar manner to back-wall amplitudes. The 

measurements also indicated that attenuation compensation procedures should ideally account 

for both (1) the average attenuation difference between the calibration standard and test billet and 

(2) local attenuation variations within either the standard or the test billet. 

To test various procedures for using back-wall amplitudes to compensate for attenuation, the 

second test specimen was fabricated. This 18′′ long specimen contained 17 #2 FBHs located 

3.5′′ deep, i.e., at the far end of the depth range considered in either an MZ or CV inspection of 

6′′ diameter billet. The FBH sites were chosen on the basis of the back-wall and backscattered 

noise C-scan patterns to span the widest possible amplitude range. A standard MZ inspection of 

the second test specimen was carried out without attenuation compensation, and the amplitudes 

of the 17 FBH targets were measured relative to that of a similarly sized FBH located 3.5′′ deep 

in the calibration standard. The data was then postprocessed to determine the effect of applying 

various attenuation compensation procedures. In the simulated inspections, the amplitude of the 

3.5′′ deep #2 FBH in the calibration standard was set to 80% FSH; and it was desired that all of 

the #2 FBHs in the test billet produce adjusted amplitudes of 80% FSH or higher and, hence, be 

identified as rejectable defects. 

The gain adjustments considered fall into two broad categories: 

• Global attenuation compensation procedures in which a fixed gain adjustment (in dB/inch 

units) is applied at all axial and hoop locations in the test billet. These adjustments are 

based on the back-wall C-scan patterns of the calibration standard and the test billet. 

Current inspection practice uses only global procedures. 

• Local attenuation compensation procedures in which the gain adjustment varies with 

axial and hoop location in the test billet. These adjustments are based on C-scan patterns 

of back-wall amplitude or backscattered grain noise in the test billet. 

Three global compensation procedures were considered distinguished by how conservative one 

wishes to be in the adjustment procedure. These procedures are referred to as 

• Standard: To deduce the attenuation difference, one compares average back-wall 

amplitudes in the calibration standard and the test billet. This corresponds to assuming 

that the calibration hole is located in an average attenuation region in the standard and 

that the target flaw is located in an average attenuation region of the test billet. This may 

be regarded as the current standard practice for attenuation compensation. 

• Semiconservative: One compares (avg. + std. dev. of back wall) in the calibration 

standard to (avg.-std. dev. of back wall) in the test billet. This corresponds to assuming 

that the calibration hole is located in a somewhat low-attenuation region (one standard 

deviation above the average back-wall amplitude) of the calibration standard and that the 

target flaw is located in a somewhat high-attenuation region of the test billet. 

• Conservative: One compares the back-wall amplitude seen in the brightest region of the 

calibration standard C-scan to the back-wall amplitude in the dimmest region of the test 

99
billet C-scan. This corresponds to assuming that the calibration hole is located in the 

lowest attenuation region of the standard and that the target hole is in the highestattenuation region of the test billet. 

Two fixed-focus transducers were used to acquire the back-wall C-scans on which the global 

(and some local) adjustments were based, one with the beam focused near the billet centerline 

and one with the beam focused near the billet back wall. Various combinations of global and 

local adjustments were considered in the study, as indicated in figure 86, and the results of 

applying each combination are summarized in table 14. 

The first step consisted of global adjustments alone. It was found that the standard global 

attenuation compensation procedure accounts well for the overall attenuation difference between 

the calibration standard and the test billet (as it is designed to do), but does not treat local 

attenuation variations in either the standard or the test billet. When the standard compensation 

procedure was used, only about half of the target defects in the test billet were rejectable based 

on having a reflected amplitude exceeding the 80% target threshold. Moreover, those FBH 

defects having low amplitudes were not rejectable based on either amplitude or a commonly used 

signal-to-noise criterion (S/N > 2.5). The standard procedure was judged to be inadequate for 

either focusing method used to acquire back-wall echoes for attenuation compensation. The 

standard compensation procedure essentially overinspects low-attenuation bands in the test billet 

while underinspecting high-attenuation bands. 

The semiconservative and conservative global compensation procedures apply higher inspection 

gains and consequently cause a larger number of the test defect amplitudes to exceed the 80% 

FSH threshold and, hence, be identified as rejectable. The additional gain (above that required 

by the standard procedure) was approximately 1 dB/inch for the semiconservative procedure and 

2 dB/inch for the conservative scheme. Only one or two of the target FBHs had amplitudes 

below 80% FSH when the semiconservative adjustment procedure was used, depending on the 

focusing method used to acquire back-wall C-scan data. For the conservative procedure, all 

adjusted amplitudes were greater than 80%; however, the focus at back-wall approach 

accomplished this at lower inspection gain and, thus, is likely to lead to fewer false calls in noisy 

billets. 

Although global attenuation compensation is easiest to accomplish at the inspection facility, 

local compensation procedures were also studied. The local compensation approach is regarded 

as a possible postprocessing tool to adjust amplitudes of candidate defects identified in the 

primary inspection that uses global compensation. Local compensation, as defined here, does 

not change the average amplitude of a large collection of defects distributed throughout the test 

billet, but does boost the amplitudes of defects located in high-attenuation regions and reduce the 

amplitudes of defects located in low-attenuation regions. Regions of low (high) attenuation are 

identified by higher (lower) than average back-wall amplitude or center zone grain noise. Local 

adjustments based on earlier equations 4 and 5 were found to reduce the fluctuations of FBH 

amplitudes in the test billet without a significant change in the mean amplitude. The standard 

deviation of the fluctuations was reduced by about 40% for local adjustments based either on 

back-wall amplitudes or center zone grain noise. When the standard global procedure is 

combined with the different local adjustment methods, about half of the FBH amplitudes still 

100
remained below the 80% FSH target level. This results in an average sensitivity equal to a 

#2 FBH, with about 50% of FBH detected and 50% undetected. However, when local 

adjustments are combined with the semiconservative global procedure, virtually all adjusted 

FBH amplitudes were greater than 80% FSH. Local attenuation adjustments based on back-wall 

amplitudes were found to be slightly superior to those based on backscattered noise, and when 

back-wall amplitudes were used, local adjustments computed with the beam focused on the backwall were slightly superior to those computed with the beam focused at the centerline. 

Based on the above findings, several tentative recommendations can be made to improve 

inspection practice. It was noted in the test billet that FBH amplitudes at a given depth varied 

significantly with axial and hoop position, due to local attenuation variations. These variations 

will cause echo amplitudes from some defects (i.e., those located in high-attenuation bands) to be 

significantly below expected values, and such low-amplitude defects will not necessarily be 

rejected based on typical S/N criteria. Similar amplitude variations are also expected within the 

calibration standards themselves. Such variations would likely be obvious if the calibration 

standards contained several identical FBHs at a given depth, rather than the single hole typically 

used. If it is known that the FBHs in a given calibration standard have been drilled into high- or 

low-attenuation bands, that information can be used during attenuation compensation. 

Unfortunately, calibration holes in existing standards were drilled in random positions relative to 

attenuation banding patterns, and internal cavities within the standards usually preclude the 

measurement of back-wall echoes from the OD or backscattered grain noise from the centerline 

region. Thus, when using existing standards, it would be safest to adopt the conservative 

approach, i.e., to assume that FBHs in the calibration standard could have been drilled into lowattenuation bands, and that flaws in the billet may exist in high-attenuation bands. This will 

require the inspection gain to be boosted beyond that used for the standard compensation 

procedure currently being used for many inspections. Such gain boosts will necessarily increase 

absolute grain noise levels, particularly near the billet centerline, and may result in the deep zone 

regions of some billets being deemed uninspectable. Alternatively, the semiconservative 

adjustment approach could be used if combined with local compensation based on either backwall amplitudes or centerline grain noise. However, attenuation compensation procedures based 

on global + local combinations may be too complicated at the present time for practical 

application at the factory. 

When new calibration standards are made, the luxury exists of placing the FBH reflectors at axial 

and hoop locations of the owner’s choosing. It is recommended that for new standards (1) FBHs 

should be drilled into high-attenuation bands, and (2) when these calibration standards are used 

in the inspection of other billets, one should compare zones of low back-wall amplitude in both 

the calibration standard and the test billet when deducing global attenuation adjustments. This 

will be equivalent to the conservative global compensation procedure. The adjustment procedure 

will be very similar to that currently being used at the factory; the only change is using low backwall amplitudes rather than average back-wall amplitudes to compute gain adjustments. Again, 

inspection gains will effectively be boosted by both the increased gain needed to set these 

weaker FBH echoes in the calibration standard to 80%, and by the increased gain associated with 

computing adjustments using low-amplitude back-wall bands in the test billet. The 

recommended approach of drilling calibration holes into high-noise bands has been followed for 

101
the fabrication of a new 14′′ diameter Ti-6-4 billet standard constructed as part of the 

ETC Phase II program. 

Finally, note that this study was based on specimens constructed from a single heat of Ti-6-4 

billet produced several years ago, and that billet fabrication procedures evolved over time. 

Although the inspection procedure followed the MZ approach, it is believed that the basic 

findings would apply to CV inspections as well, since the findings follow from an inherent 

characteristic of the billets themselves, namely, the variation or banding of UT properties. The 

gain differences that separate the standard, semiconservative, and conservative attenuation 

compensation procedures depend on the severity of the banding. Anecdotal evidence indicates 

that some billet fabricators have been adjusting production procedures to limit noise banding, 

and this should serve to limit attenuation variations as well. As billet properties become more 

rotationally symmetric, the gain adjustment needed to implement the conservative compensation 

procedure will approach that of the current standard procedure. Increasing the inspection gain to 

implement a more conservative inspection will necessarily increase backscattered noise levels 

from the deeper zones where the gain adjustments are largest. In some cases, the increased noise 

levels may lead to undesirable false-call levels. For a given billet, the effect of such a change on 

peak and average noise levels can be readily calculated if back-wall and interior C-scan data are 

available. Those OEMs who have access to such archival data should use them to assess the 

effect on interior noise levels of adopting the recommended compensation procedures. If 

archival data is not available, attempts should be made to gather and store back-wall C-scan and 

interior noise data for current and future inspections. Such data will enable banding severity to 

be assessed and allow the effects of proposed modifications in inspection procedures to be 

determined. 

3.9 ASSESSMENT OF LARGE-DIAMETER BILLET. 

One of the goals of the program was to assess the sensitivity of MZ inspection for large (>10′′) 

diameter Ti billets. The largest billet diameter used to make rotating parts of jet engines is 14′′. 

Before the start of the program, 13′′ was the largest billet diameter for which MZ transducers had 

been manufactured. Since an MZ transducer set did not exist for 14′′ diameter billets, the 

approach was to concentrate exclusively on the inspectability of the region near the centerline of 

the billet. This was believed to be the most difficult region to inspect because of long metal 

travel and potential billet microstructure differences that could exist from the OD to centerline. 

This section describes the design and evaluation of the performance of transducers intended for 

the inspection of the region near the centerline of 14′′ Ti billets. 

The zoning scheme for MZ inspection of 14′′ diameter billets is given in table 15. It is based on 

guidance in Aerospace Material Specification (AMS) 2628. The number of zones is dictated by 

the number of available channels of an MZ production system, which has a total of eight 

channels. One channel is reserved for monitoring the back-wall signal. Thus, the number of 

available channels for inspection is seven. The zone size for all zones, except the first two, 

is 1.1′′. In this study, the concentration was on the inspection of the center zone of the billet, 

which is referred to as zone 7 in table 15. It spans from 6.4′′ to 7.5′′ and covers the centerline of 

the billet. 

102
TABLE 15. ZONING SCHEME FOR MZ INSPECTION OF 

14′′ DIAMETER BILLETS 

Zoning Scheme for 14′′ Diameter Billets 

Zone Start End Center 

1 0.2 1.0 0.60 

2 1.0 2.0 1.50 

3 2.0 3.1 2.55 

4 3.1 4.2 3.65 

5 4.2 5.3 4.75 

6 5.3 6.4 5.85 

7 6.4 7.5 6.95 

3.9.1 Chord Block Fabrication. 

A chord block was designed to assess inspection sensitivity for zone 7 using both CV and MZ 

inspections. A schematic of the block is shown in figure 87. 

#2 FBH

2.5”

7.5” 7.5” 7.0” 6.4”

2.0” 2.0” 2.0” 2.5”

#3 FBH

8”

C/L

5.0”

#2 FBH

2.5”

7.5” 7.5” 7.0” 6.4”

2.0” 2.0” 2.0” 2.5”

#3 FBH

8”

C/L

5.0”

2.5”

7.5” 7.5” 7.0” 6.4”

2.0” 2.0” 2.0” 2.5”

#3 FBH

8”

C/L

5.0”

FIGURE 87. CHORD BLOCK SCHEMATIC 

A 27′′ long piece of 14′′ diameter Ti-6-4 billet was acquired to make the chord block. The intent 

was to select a high-noise region from which to cut the chord block. To identify high-noise 

regions, the billet was scanned using a bicyclindrically focused transducer designed for the 

inspection of the central zone of 13′′ diameter billet. The zone position for the noise scan is from 

6′′ to 7′′. Calibration was performed using #3 FBH at 6′′ and 7′′ of a 13′′ diameter Ti-6-4 

calibration standard. The minimum of the two FBH responses was set to 80% FSH and an 

additional 7.5 dB was added to the gain to increase noise content of the C-scan image. The 

resulting C-scan of the billet is shown in figure 88 and reveals that noise is nonuniform through 

103
the billet. The chord block was cut from the high-noise location marked with a black box. The 

final block has three #2 FBHs (0.03125′′ diameter) at 6.4′′, 7.0′′ and 7.5′′ depths and a #3 FBH 

(0.0469′′ diameter) at 7.5′′. The holes are spaced 2′′ apart. The length of the block in the axial 

direction is 11′′; the width of the block is 5′′. Note that the noise is nonuniform within the 

sample. Higher noise is exhibited towards the right end of the billet. The shallower holes were 

placed in this region. In Ti billets, higher noise is associated with lower attenuation. Thus, the 

attenuation is lower towards the end of the chord block where the 6.4′′ deep hole is placed. 

27”

0 100

axial direction

hoop direction

axial direction

hoop direction

FIGURE 88. NOISE SCAN OF THE BILLET FROM WHICH CHORD BLOCK WAS MADE 

(Transducer used for inspection of central zone of 13′′ diameter billet was used, gate: 6.0′′-7.0′′. 

Setup: Ti #3 FBH 80% + 7.5 dB. The location of chord block is marked with a black box.) 

In addition to the chord block, a 10′′ long, full-round piece was cut for attenuation compensation 

measurements. The purpose of this piece was to compare attenuation between the material of the 

chord block and the material of the billet to be inspected. 

3.9.2 Baseline CV Inspection.

First, the chord block was scanned with a transducer used in CV inspection. The transducer was 

made available by a billet supplier. This was a 5-MHz, cylindrically focused transducer with 

focal length of 7′′. The transducer had a rectangular element with a length of 0.75′′ in the axial 

direction of the billet and 1.0′′ in the circumferential direction. The entire chord block was 

scanned with a 2.2′′ water path. The resulting C-scan image is shown in figure 89(a). One can 

see that only the #3 FBH is detectable in this case. After that, the area containing #3 FBH was 

rescanned with a water path of 3.0′′. This water path is used in the production inspections with 

104
this transducer by a supplier. Figure 89(b) shows the resulting image. Although the amplitude 

reflected from #3 FBH signal was reduced, the SNR stayed approximately the same. SNR was 

calculated using the formula used by the MZ inspection system: 

nn

s

 - P

 - P

 = 

μ

μn SNR

nn

ns

P

P SNR

μ

μ

−

− =

where is the peak signal from the defect of interest, is the peak noise, and is the mean 

noise in the surrounding area. In both scans, the SNR was approximately 1.5. Note, that this is a 

conservative way to evaluate inspection sensitivity, because the scans were collected with 

constant gain, selected such that #3 FBH at 7.5′′ depth gives a signal with amplitude equal to 

80% of FSH. In actual measurements, DAC is used and such a high gain is only applied to the 

7.5′′ deep FBH signal. The conclusion is that the CV inspection, which uses a single transducer 

to inspect the entire volume of 14′′ diameter Ti billet, can resolve #3 FBH near the center of the 

billet but cannot resolve #2 FBHs. Also, SNR ratio for #3 FBH is low. But because of the large 

beam footprint, this inspection is not highly sensitive to transducer misalignment and billet 

geometry imperfections. 

Ps Pn μn

#3 FBH

@ 7.5”

0 100

axial

hoop

2.2”

3.0”

#3 FBH

@ 7.5”

0 1

(a) 

(b) 

0 100

axial

hoop

2.2”

3.0”

FIGURE 89. SCANS OF THE CHORD BLOCK WITH A TRANSDUCER USED IN A CV 

INSPECTION (a) WATER PATH = 2.2′′ AND (b) WATER PATH = 3.0′′

(The location of #3 FBH is marked with a rectangle.) 

After completion of the CV inspection, an MZ transducer originally designed to inspect the 

center zone (6′′-7′′) of 13′′ diameter billet was used to scan the chord block. This transducer is a 

master standard used for internal GE activities for the production inspection of 13′′ diameter 

billets with a 3′′ water path. It has a circular element of 2.35′′ diameter. Its focus in the hoop 

105
direction is 10.0′′ and in the axial direction is 30.6′′. The chord block was scanned with 3′′ water 

path, generating the image shown in figure 90(a). In addition to the #3 FBH, it was also possible 

to resolve #2 FBHs at 6.4′′ and 7.0′′ depths. Only the #2 FBH at 7.5′′ depth remained 

undetectable. An attempt was made to focus deeper by shortening the water path to 2.5′′. But 

the #2 FBH at 7.5′′ depth still remained undetectable. The resulting C-scan image is shown in 

figure 90(b). As expected, the MZ transducer provided better sensitivity than the conventional 

transducer. The expectation was to achieve further sensitivity improvement by designing a 

transducer that specifically targets the inspection of the central zone (6.4′′-7.5′′) of 14′′ diameter 

billet. 

 (a) 

 (b) 

0 100

axial

hoop

#2 FBH

@ 6.4”

#2 FBH

@ 7.0”

#2 FBH

@ 7.5”

#3 FBH

@ 7.5”

3.0”

2.5”

0 100

axial

hoop

#2 FBH

@ 6.4”

#2 FBH

@ 7.0”

#2 FBH

@ 7.5”

#3 FBH

@ 7.5”

3.0”

2.5”

FIGURE 90. SCANS OF THE CHORD BLOCK WITH AN MZ TRANSDUCER DESIGNED 

FOR THE INSPECTION OF THE CENTRAL ZONE OF 13′′ DIAMETER BILLET 

(5 MHz, 2.35′′, FOCUS 6.45′′ IN 13′′ BILLET) (a) WATER PATH = 3.0′′ AND 

(b) WATER PATH = 2.5′′

3.9.3 Transducer Design. 

It was assumed that the desired transducer has an elliptical element and bicyclindrical focus and 

its center frequency is 5 MHz and bandwidth is 60%. Such a transducer is characterized by four 

parameters, as shown in figure 91 and discussed in section 3.2.2. There are two dimensions: 

diameter in circumferential direction ( ) and diameter in axial direction ( ), and two 

geometrical focal lengths: circumeferential ( ) and axial ( ). 

Dx Dy

Fx Fy

106
Fy

Dy

Fx

Dx

FIGURE 91. SCHEMATIC OF AN ELLIPTICAL ELEMENT, BICYLINDRICALLY 

FOCUSED TRANSDUCER 

Ray-tracing calculations were performed. Assume that one wants to design a bycylindrically 

focused transducer that focuses at depth, F1, in a billet of radius, R, made of material with 

longitudinal wave speed, V1. 

Geometrical focal lengths are defined by the following formulae: 

⎟

⎠

⎞ ⎜

⎝

⎛ −−

+=

+=

R

F

V

V

R

F

F

wpF

V

V FwpF

y

x

1

1

1 0

1

0

1

1

1

where is the desired water path and is the wave speed in water. Transducer dimensions are 

given by the formulae below: 

wp V0

⎥

⎦

⎤ ⎢

⎣

⎡

+⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ = −+

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

1

1

0

11

1

1

1

0

1

2 1

2

F

R

F

V

V

R

F D wp

F

V

V D wp

y

x

θ

θ

where 

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎝

⎛

= −

1

0

1

1

 

5.0

tan

V

V

D

F θ

107
D

F is called the F number of the transducer. It defines the size of the beam at focus as well as 

the depth of the field as follows: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =

=

1

00

0

4

03.1

V

V

D

F

f

V Depth of field

f

V

D

F Beam Diameter

Performing this calculation for the focus at the center ( = 7′′) of the 14′′ diameter billet 

(

F1

R = 10

D

F = 8

D

F = 7′′) for and one obtains the F/8 and F/10 (transducer #1) transducer designs 

in table 16. For optimal transducer performance, it is desirable to maximize the SNR. SNR is 

inversely proportional to the square root of pulse volume. Pulse volume is equal to the product 

of pulse duration and beam diameter. It was assumed that the transducer would have a central 

frequency of 5 MHz and, thus, pulse duration is fixed. In this case, in order to maximize SNR, 

one needs to minimize the beam diameter through the zone of interest. To determine beam 

diameter of a bicyclindrical transducer at a given depth, the UT model developed by ISU was 

used. To obtain the optimal transducer design, the beam size within the zone was minimized: 

[ ] ′′ 7.5 to6.4 ′′ ,,,

min Maximum beam diamter within th me zone fro

DyDxFyFx

The direct search complex algorithm [28] was used to perform this minimization. The resulting 

transducer design is given in table 16 as optimal (transducer 2). As one can see, the transducer 

designed using this approach has an F value equal to 7. 

Thus, there were three candidate designs: F/10 and F/8 that were obtained using the ray-tracing 

approach and the optimal design that was obtained by minimizing the beam diameter over the 

zone of interest. The next step was to model the performance of each of the designs and select 

which transducers to order. Figure 92 shows the comparison of on-axis profiles within the zone 

of interest for the above mentioned designs as well as for the central zone of the 13′′ diameter 

billet transducer. Figure 93 shows beam diameters for the same four designs. As expected, the 

optimal design has the smallest beam diameter within the zone, but it has more variation in the 

responses from the middle and ends of the zone and would require the use of the DAC. 

108
TABLE 16. SPECIFICATIONS OF TRANSDUCERS 1 (F/10) AND 2 (F/7) INTENDED FOR 

THE INSPECTION OF THE CENTRAL ZONE OF 14′′ DIAMETER Ti BILLET AS WELL 

AS F/8 DESIGN AND EXISTING TRANSDUCER INTENDED FOR INSPECTION OF 

CENTER ZONE OF 13′′ DIAMETER BILLET 

F/8 

Zone 

Zone 

Start 

Zone 

End 

Approximate 

Focus Depth Aperture 

Diameter 

X 

Diameter 

Y 

Radius of 

Curvature 

X 

Radius of 

Curvature 

Y 

Center 6.4′′ 7.5′′ 6.9′′ F/8 5.10′′ 3.93′′ 10.04′′ 32.85′′

F/10 (Transducer 1) 

Zone 

Zone 

Start 

Zone 

End 

Approximate 

Focus Depth Aperture 

Diameter 

X 

Diameter 

Y 

Radius of 

Curvature 

X 

Radius of 

Curvature 

Y 

Center 64′′ 7.5′′ 6.9′′ F/10 4.12′′ 3.18′′ 10.06′′ 33.29′′

Optimal (Transducer 2) 

Zone 

Zone 

Start 

Zone 

End 

Approximate 

Focus Depth Aperture 

Diameter 

X 

Diameter 

Y 

Radius of 

Curvature 

X 

Radius of 

Curvature 

Y 

Center 6.4′′ 7.5′′ 6.9′′ F/7 6.08′′ 4.65 10.00′′ 32.24′′

Center Zone Transducer of 13′′ Diameter Billet 

Zone 

Zone 

Start 

Zone 

End 

Approximate 

Focus Depth Aperture 

Diameter 

X 

Diameter 

Y 

Radius of 

Curvature 

X 

Radius of 

Curvature 

Y 

Center 6.0′′ 7.0′′ 6.5′′ F/10 2.35′′ 2.35′′ 10.12′′ 33.15′′

FIGURE 92. MODEL-PREDICTED ON-AXIS (RADIAL DIRECTION) RESPONSE 

WITHIN CENTER ZONE OF 14′′ DIAMETER Ti BILLET FOR VARIOUS 

TRANSDUCER DESIGNS 

109
FIGURE 93. MODEL-PREDICTED BEAM SIZE WITHIN CENTER ZONE OF 14′′

DIAMETER Ti BILLET FOR VARIOUS TRANSDUCER DESIGNS 

The program included design and manufacture of two MZ transducers for the inspection of the 

zone around the centerline of 14′′ diameter billet. The transducers were used to evaluate the 

chord block described above. The team selected two different designs: F/10 and the optimal 

design, which happened to be F/7. The F/10 transducer will be referenced as transducer 1 and 

the optimal design transducer as transducer 2. The transducers had the following requested 

specifications: 

• Center frequency: 5.0 ±0.5 MHz 

• Bandwidth: ≥ 60% 

• Element material: Piezo-composite 

• Focusing method: Curved element 

• Excitation pulse: 300V exponential pulse with half-amplitude pulse width approximately 

100 nsec 

• Impedance matching: 50 ohms 

• Receiver bandwidth: 25 MHz 

• Operating water path: 3.0′′

110
• Element profile is a bicylindrical surface (surface of a toroid), defined by two principal 

radii of curvature 

• Element size and focal parameters are provided in table 16 with the following definitions: 

− x direction perpendicular to billet axis 

− y direction along billet axis 

The two transducers were delivered as curved element transducers made of piezo-composite 

material. For curved element transducers, radii of curvatures are very close to geometrical focal 

lengths (GFL), which are used in the transducer modeling. Essentially, specifying the radii of 

curvatures was equivalent to specifying GFLs. Figure 94 shows the modeled beam area of these 

two transducers within the zone of interest. Figure 95 shows transducer 2. 

FIGURE 94. PREDICTED BEAM AREA WITHIN THE CENTRAL ZONE OF 

14′′ DIAMETER BILLET FOR TRANSDUCERS 1 AND 2 

111
FIGURE 95. TRANSDUCER 2 

3.9.4 Transducer Characterization. 

Several steps have been performed to characterize the transducers. First, the transducer focal 

parameters were determined using the V(z) method [14] discussed in section 3.2.2. Second, a 

ball target in water was scanned using different water paths to assess the symmetry of the sound 

field. Third, measurements on the chord block were performed to assess the potential inspection 

sensitivity. The results are discussed with more details below. 

The intent of the V(z) method for bicyclindrical transducers is to determine GFLs of the 

manufactured probe and to conclude if the transducer meets the specifications. This transducer 

characterization uses only a flat reflector and, thus, does not require calibration blocks. As 

discussed in section 3.2.2, it could potentially be done by the transducer manufacturer. An 

aluminum plate that was designed to be aligned with the UT tank rails was used as a reflector in 

these measurements. The water path was varied from 8′′ to 36′′ with 0.2′′ step increments. The 

general setup arrangement for V(z) measurements is shown at figure 96. A front wall waveform 

was acquired at each position with 0.02 μsec resolution. Figure 97 shows acquired waveforms 

for several water paths for transducer 1. 

112
FIGURE 96. V(z) METHOD FOR TRANSDUCER CHARACTERIZATION 

scan

z

FIGURE 97. WAVEFORMS AROUND THE FIRST FOCUS OF TRANSDUCER 1 

Waveform at water path = 9.74"

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 9.94"

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 10.54"

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 8.34"

-1.5

-1

-0.5

0

0.5

1

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 9.74"

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 9.94"

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 10.54"

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0123456

Time, usec

Amplitide, Volts

Waveform at water path = 8.34"

-1.5

-1

-0.5

0

0.5

1

1.5

0123456

Time, usec

Amplitide, Volts

Spectral components of acquired waveforms are shown in figures 98 (transducer 1) and 99 

(transducer 2) with circles for frequencies 4.69 and 6.08 MHz. One can see that there is a sharp 

peak at around the 10′′ water path, which corresponds to circumferential focus and a smaller and 

wider peak at around 32′′, corresponding to the axial focus. Given these results, it was assumed 

that actual element diameters are close to the designed values. 

113
0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in

V(z

) 

[Ex

p. 

pea

k =

1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 9.92

 Fy = 31.59

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in

V(z

) 

[Ex

p. 

pea

k =

1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 9.93

 Fy = 31.73

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in 

V(z) [Exp. peak = 1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 9.92

 Fy = 31.59

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in

(a) 

V(z) [Exp. peak = 1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 9.93

 Fy = 31.73

(b) 

FIGURE 98. V(z) AT (a) 4.69 MHz AND (b) 6.08 MHz FOR TRANSDUCER 1 

(Spectrum range used in reconstruction: 3.5-7 MHz. Reconstructed values for 

Fx and Fy are given.) 

FIGURE 99. V(z) AT (a) 4.69 MHz AND (b) 6.08 MHz FOR TRANSDUCER 2 

(Spectrum range used in reconstruction: 3.5-7 MHz. Reconstructed values for 

Fx and Fy are given.) 

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in

V(z) [Exp. peak = 1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 10.06

 Fy = 32.83

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in

V(z) [Exp. peak = 1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 10.06

 Fy = 32.89 a)

b)

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Wate ath, in

V(z) [Exp. peak = 1]

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40

Water Path, in

V(z) [Exp. peak = 1]

 V(z) Exp. 

 V(z) Thry. 

 Fx = 10.06

 Fy = 32.83

r P

 Fx = 10.06

 Fy = 32.89 a)

b)

 V(z) Exp. 

 V(z) Thry. 

(a) 

(b)

114
The two parameters determined by the inversion procedure, which minimizes the difference 

between measured and calculated V(z) curves, were GFLs in circumferential (Fx) and axial (Fy) 

directions. The reconstruction is performed for each frequency in the 3.5-7.0 MHz range. The 

solid line in figures 98 and 99 show the best fit for the theoretical prediction. At 4.69 MHz, the 

reconstructed value for Fx and Fy are 9.93′′ and 31.73′′, respectively. At 6.08 MHz, Fx = 9.92′′

and Fy = 31.59′′. Figure 100 shows the reconstructed values of Fx and Fy as a function of 

frequency. 

31.00

31.50

32.00

32.50

33.00

33.50

34.00

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Frequency, MHz

Fy

, 

in

9.910

9.915

9.920

9.925

9.930

9.935

9.940

9.945

9.950

9.955

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Frequency, MHz

Fx

, 

in

31.00

31.50

32.00

32.50

33.00

33.50

34.00

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Frequency, MHz

Fy, in

9.910

9.915

9.920

9.925

9.930

9.935

9.940

9.945

9.950

9.955

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Frequency, MHz

Fx, in

(a)

(b)

FIGURE 100. RECONSTRUCTION RESULTS FOR GFL IN (a) CIRCUMFERENTIAL, Fx, 

AND (b) AXIAL, Fy, DIRECTIONS AT VARIOUS FREQUENCIES FOR TRANSDUCER 1 

(Spectrum range used in reconstruction: 3.5-7 MHz.) 

The average reconstructed value for Fx = 9.93′′ and for Fy = 31.41′′. They are very close to the 

nominal values of Fx = 10.06′′ and Fy = 33.29′′. The second set of V(z) measurements was 

performed with 0.02′′ water path increments. The reconstructed values of Fx = 9.92′′ and Fy = 

31.80′′ were obtained form this data. One can see that reducing the step increment by a factor of 

10 (0.02′′ versus 0.2′′) gave a very similar result. This indicated that the coarser step increment 

of 0.2′′ was adequate. Figure 101 shows similar results for transducer 2. The reconstructed 

values versus nominal values for both transducers are given in table 17. One can see that the 

reconstructed values are close to the nominal ones. 

115
9.990

10.000

10.010

10.020

10.030

10.040

10.050

10.060

10.070

10.080

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Frequency, MHz

Fx, in

32.20

32.30

32.40

32.50

32.60

32.70

32.80

32.90

33.00

33.10

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Frequency, MHz

Fy, in

(b)

(a)

FIGURE 101. RECONSTRUCTION RESULTS FOR GFL IN (a) CIRCUMFERENTIAL, Fx, 

AND (b) AXIAL, Fy, DIRECTIONS AT VARIOUS FREQUENCIES FOR TRANSDUCER 2 

(Spectrum range used in reconstruction: 3.5-7 MHz.) 

TABLE 17. RESULTS OF V(z) RECONSTRUCTION FOR TRANSDUCERS 1 AND 2 

Transducer 1 (F/10) Transducer 2 (F/7) 

Fx

(in.) 

Fy

(in.) 

Fx

(in.) 

Fy

(in.) 

Nominal 10.06 33.29 10.00 32.24 

Reconstructed 9.92 31.80 10.06 32.85 

Fx, Fy reconstructed from V(z) curves. 

To complete transducer characterization, the efficiency factor of the two transducers needed to 

be calculated. This was done by acquiring a reference waveform from 1′′ deep #1 FBH in an 

IN100 calibration block with a water path of 5.15′′. The water path was selected such that the 

transducer focused on the hole. To check for beam symmetry, a 5/16′′ diameter steel ball target 

was scanned in water. Scans with different water paths were performed. Figure 102 shows the 

116
comparison between measured and modeled scans for water paths of 9.91′′, 11′′, and 12′′ for 

transducer 1. One can see that the transducer produces a symmetrical field and the model 

predicts well the empirical patterns. Similar results for transducer 2 are shown in figure 103. 

measured

simulated

wp = 9.91” wp = 11” wp = 12”

2.6”

0.08”

1.6”

0.8”

0 100

measured

simulated

wp = 9.91” wp = 11” wp = 12”

2.6”

0.08”

1.6”

measured

simulated

wp = 9.91” wp = 11” wp = 12”

2.6”

0.08”

1.6”

0.8”

0 100

FIGURE 102. MEASURED AND SIMULATED C-SCANS OF 5/16′′ DIAMETER STEEL 

BALL TARGET IN WATER FOR TRANSDUCER 1 

(Ball target is near first focus of the transducer.) 

wp =7.2” wp =10” wp =12” wp =14”

2”

FIGURE 103. MEASURED AND SIMULATED C-SCANS OF 5/16′′ DIAMETER STEEL 

BALL TARGET IN WATER FOR TRANSDUCER 2 

(Ball target is near first focus of the transducer.) 

117
From the described results, it was concluded that the transducers are well-behaved, i.e., they 

have frequency, bandwidth, and focal characteristics close to the desired parameters and generate 

a symmetrical beam pattern. 

3.9.5 Chord Block Measurements. 

Measurements on the chord block were performed with results for Transducer 1. The water path 

was selected to achieve the optimal zone balance. A water path of 2.5′′ produces the zone 

balance of 1 dB (response from the #2 FBH at 6.4′′ is equal to 50% FSH and from the #2 FBH 

response at 7.5′′ equals 56% FSH). The chord block was scanned with this water path, and 

figure 104(a) shows the resulting C-scan. The locations of different FBHs are marked at the top 

of the image. 

 (a) 

 (b) 

 (c) 

 (d) 

)

#2 FBH

@ 6.4”

#2 FBH

@ 7.0”

#2 FBH

@ 7.5”

#3 FBH

@ 7.5”

BW

2.5”

2.538”

2.675”

0 100

axial

hoop

)

#2 FBH

@ 6.4”

#2 FBH

@ 7.0”

#2 FBH

@ 7.5”

#3 FBH

@ 7.5”

BW

2.5”

2.538”

2.675”

0 100

axial

hoop

FIGURE 104. SCANS OF THE CHORD BLOCK WITH DIFFERENT WATER PATHS 

(a) WATER PATH = 2.5′′; (b) WATER PATH = 2.538′′, FOCUS AT 7.5′′ DEEP #2 FBH; 

(c) WATER PATH = 2.675′′, FOCUS AT 7.0′′ DEEP #2 FBH; AND (d) BACK-WALL SCAN 

To use the automatic SNR calculation feature of the GE-MZ software, the data was converted 

into MZ format. The results of SNR analysis using the GE-MZ software are shown in figure 

105. For all #2 FBHs at 6.4′′, 7.0′′, and 7.5′′, the SNR is greater than 2. Significant distortion of 

spatial response from the #2 FBH at 6.4′′ was observed and is likely due to the microstructure 

118
variations. The water path was adjusted by focusing at the 7.5′′ deep #2 FBH and then at the 7.0′′

deep #2 FBH. The resulting water paths, were 2.538′′ and 2.675′′, respectively. The chord block 

was rescanned with the new water paths, and figures 104(b) and 104(c) show the resulting 

C-scans for 2.538′′ and 2.675′′ water paths, respectively. From figure 104(c), one can see that 

focusing at the center of the zone (7.0′′) does not produce the acceptable 3 dB zone balance. The 

reason for this can be explained by looking at attenuation changes in the axial direction. The 

back wall of the chord blocks was monitored to assess attenuation variability, and figure 104(d) 

shows the back-wall C-scan. One can see that the attenuation increases from right to left of the 

image, i.e., from shallower to deeper hole positions. The same behavior can be observed by 

looking at the noise in the billet from which the chord block was cut, as shown in figure 88. 

Noise is higher towards the right end of the box. It was demonstrated before that in Ti billet 

material, high noise and low attenuation are correlated [29]. Since it appears that attenuation is 

higher at the location of deeper holes, one needs to focus deeper to achieve the zone balance. 

Essentially, the focus needs to be even deeper than the 7.5′′ deep hole to ensure the zone balance. 

#2 FBH @ 7.5” #2 FBH @ 7.0” #2 FBH @ 6.4”

SNR

(using GE-MZ automated 

SNR calculation)

Peak 

Amplitude

56% 64% 50%

2.71 4.20 2.39

Water path = 2.5”

0 100

axial

hoop

#2 FBH @ 7.5” #2 FBH @ 7.0” #2 FBH @ 6.4”

SNR

(using GE-MZ automated 

SNR calculation)

Peak 

Amplitude

56% 64% 50%

2.71 4.20 2.39

Water path = 2.5”

0 100

#2 FBH @ 7.5” #2 FBH @ 7.0” #2 FBH @ 6.4”

SNR

(using GE-MZ automated 

SNR calculation)

Peak 

Amplitude

56% 64% 50%

2.71 4.20 2.39

Water path = 2.5”

#2 FBH @ 7.5” #2 FBH @ 7.0” #2 FBH @ 6.4”

SNR

(using GE-MZ automated 

SNR calculation)

Peak 

Amplitude

56% 64% 50%

2.71 4.20 2.39

Water path = 2.5”

0 100

axial

hoop

FIGURE 105. ANALYSIS OF ONE OF THE CHORD BLOCK SCANS 

(WATER PATH = 2.5′′) USING AUTOMATIC SNR CALCULATION FEATURE OF 

GE-MULTIZONE SOFTWARE 

Transducer 2 was also evaluated on the chord block. Zone balance was achieved at the 2.7′′

water path. Figure 105 shows the C-scan from 6.4′′, 7.0′′, and 7.5′′ deep #2 FBHs. 

It was found that the amplitude of the signals from FBHs was extremely sensitive to alignment. 

The change in angular orientation of the probe in axial-radial plane of the billet of 0.2 degree 

119
resulted in a 6 dB drop in the signal. It was also determined that when one tries to perform the 

alignment on the back side of the chord block, the amplitudes from FBHs were not maximized. 

These effects made the scanning of the block with transducer 2 very challenging. Figures 106(a) 

and 106(b) show two attempts to scan the block with a 2.7′′ water path. Figures 106(c) shows 

the chord block scan with water path = 2.92′′. 

 (a) 

 (b) 

 (c) 

6.4”

7.0”

7.5”

FIGURE 106. WAVEFORMS WITH RESPONSES FROM #2 FBHs IN CHORD 

BLOCK WATER PATH = 2.7′′

(The only difference is axial position and gain. Gain is the same for 6.4′′ and 7.5′′ deep holes, 

and for a 7.0′′ deep hole, the gain is 5 dB lower.) 

With extreme care to arrive at alignment, it was possible to resolve #2 FBHs with SNR >2. For 

back-to-back comparison between transducers 1 and 2 performances, a more precise motion 

control system is required. If this transducer is to be used in actual inspections, there should be 

strict requirements on the geometry of the billet and the accuracy of alignment; otherwise, there 

is significant concern of creating blind zones since the beam size is very small. Based on these 

observations, transducer 2 does not seem to be practical for production inspections. However, it 

could be used to verify or achieve higher resolution in the area of a suspected indication, which 

was detected using the lower resolution inspection. 

3.10 SUMMARY OF 14′′ CHORD BLOCK EVALUATION. 

It was demonstrated that #2 FBH sensitivity was achieved in the MZ inspection of the central 

zone of this 14′′ diameter billet with at least 2:1 SNR, for this chord block material. Evaluation 

of a CV transducer indicated that a # 3 FBH sensitivity was achieved at 1.122 to 1 SNR. A 

considerable amount of effort was required to align the transducer for these measurements. The 

level of difficulty required to achieve the response indicates that reliable production inspection of 

120
14′′ diameter product would require additional assessment and potential development activities. 

Note that these measurements were made on a chord block, which limited the scanning motion. 

The results for a dynamic billet inspection, i.e., a billet rotation inspection, are not available and 

would require a full-round sample. While the results are encouraging, a considerable effort may 

be required prior to implementation of a new inspection. 

4. SUMMARY AND CONCLUSIONS. 

The focus of this program was on improvement to inspection sensitivity for Ti billet. 

Considerable effort went into the design and characterization of transducers, laboratory 

evaluation of transducer performance, understanding the effect of attenuation on detectability, 

and a limited feasibility study for large-diameter billet. The following are the primary 

conclusions: 

• Computer-based transducer design models were developed in the Engine Titanium 

Consortium (ETC) Phase I program and further validated in the Phase II program. The 

models were successfully used in design of transducers for 10′′ and 14′′ diameter billet. 

• Several transducer characterization methods were compared. Factors included the ability 

of the characterization method to adequately indicate performance of the probe, the 

measurement of parameters used in model calculations, and the ease of use in making the 

characterization measurement. Ease of use is important because it is desirable for the 

transducer vendors to be able to characterize a transducer prior to shipment and without 

the requirement for special samples. A method, known as V(z), was developed for use by 

vendors for future use in characterizing transducers. It was also demonstrated to provide 

necessary performance and model input information. 

• Evaluation of F/10 and F/8 transducers in two of the zones (zones 2 and 4) showed that 

the F/10 transducer did not have sufficient signal-to-noise performance for the desired #1 

flat-bottom hole (FBH) sensitivity. 

• A full set of F/8 transducers was used to measure sensitivity to #1 FBH throughout the 

depth of the 10′′ diameter billet. On average, the #1 FBH sensitivity target was met. 

However, it was necessary to divide zone 6 into two subzones to meet the desired 

sensitivity. 

• Concerns exist with the current manufacture of large element transducers, particularly the 

ability to consistently deliver desired bandwidth and frequency repeatably. 

• Measurements of back-wall echo variation were correlated with attenuation variations 

within a billet. It was found that back-wall amplitudes can be used to account for 

attenuation differences between the calibration standard and the test billet. 

• Attenuation is a significant factor in billet inspection. Using existing setup procedures, 

the response from identical FBHs was found to vary from 24% to 93% when the expected 

response for all holes was 80%. Three different attenuation compensation procedures 

were evaluated. Both local and global compensation were included with the global 

121
method deemed easiest to accomplish in a production inspection facility. Consideration 

should be given to false calls and accept/reject criteria when selecting the attenuation 

compensation procedure. 

• It is recommended that the attenuation properties of the billet be taken into account in the 

design of calibration standards, particularly placement of FBHs. 

• Two multizone (MZ) transducers, F/10 and F/7, were designed and fabricated for 

evaluation of a 14′′ diameter chord block. The two curved element, piezo-composite 

transducers were evaluated on #2 and #3 FBHs at depths of 6.4′′, 7.0′′, and 7.5′′. They 

were also characterized using the V(z) method. From the transducer characterization 

results, it was determined that the transducers had the desired frequency, bandwidth, and 

focal properties. 

• The evaluation of the MZ transducers indicated that #2 FBH could be detected with a 

signal-to-noise ratio (SNR) of (>2:1). Amplitude of the signals was sensitive alignment. 

Based on limited work with the chord block, reliable MZ production inspection requires a 

full-round 14′′ calibration standard. 

• The evaluation of conventional 5 MHz transducer on a 14′′ diameter chord block 

indicated that #3 FBH was achieved at a 1.122:1 SNR. 

5. REFERENCES. 

1. Keller, M., Patton, T., Degtyar, A., Umbach, J., Hassan, W., Kinney, A., Roberts, R., 

Margetan, F., and Brasche, L., “Inspection Development for Nickel Billet—Engine 

Titanium Consortium Phase II,” FAA report DOT/FAA/AR-05/29, 2005. 

2. Costa, J.R., Gonzalez, R.E., Guyotte, R.E., Salvano, D.P., Swift, T., and Koenig, R.J. 

(chair), “Titanium Rotating Components Review Team Report,” Federal Aviation 

Administration, Aircraft Certification Service, Engine and Propeller Directorate, Boston, 

MA, December 14, 1990. 

3. Adams, J., Murphy, T., and Provenzano, N., “Update on Rotor Integrity Subcommittee 

Initiatives,” 2nd Annual FAA/AF Workshop of the Application of Probabilistic Methods 

to Gas Turbine Engines, November 1996. 

4. FAA Advisory Circular 33.14-1, “Damage Tolerance for High Energy Turbine Engine 

Rotors.” 

5. Copley, D., “ETC Multizone Update,” Proceedings of the Open Forum, May 1996. 

6. Mucciardi, T. and Raulerson, D., “Phased Array Inspection of Titanium Billet,” 

Proceedings of the Open Forum, May 1996. 

122
7. Brasche, L., Chiou, T., Thompson, R.B., Smith, K., Meeker, B., Margetan, F., Panetta, P., 

Chenail, R., Galli, F., Umbach, J., Raulerson, D., Degtyar, A., Bartos, J., Copley, D., 

McElligott, B., Howard, P., and Bashyam, M., “Contaminated Billet Study,” FAA report 

DOT/FAA/AR-05/16, 2005. 

8. Thompson, R.B., Chiou, C.-P., Gray, T., Margetan, F., and Roberts, R., “Use of UT 

Models in Titanium Inspection Development,” Proceedings of the Open Forum, May 

1996. 

9. Margetan, F.J., Han, K.Y., Yalda, I., Goettsch, S., and Thompson, R.B., “The Practical 

Application of Grain Noise Models in Titanium Billets and Forgings,” Review of 

Progress in QNDE, 14B, D.O. Thompson and D.E. Chimenti, eds., Plenum Press, NY, 

1995, pp. 2129-2136. 

10. Howard, P., Copley, D., and Gilmore, R., “Transducer Design for High Sensitivity 

Ultrasonic Inspection of Titanium,” Review of Progress in QNDE, Vol. 16, 1996, p. 893. 

11. Roberts, R., “Phased Array Beam Modeling Using a Discretely Orthogonal Gaussian 

Aperture Basis,” in Review of Progress in QNDE, Vol. 23, D.O. Thompson and D.E. 

Chimenti, eds., 2004. 

12. Howard, P., Copley, D., Young, J., Neiters, E., and Gilmore, R., “An Improved 

Methodology for the Inspection of Titanium Alloys,” Proceedings of the 1996 IEEE 

Ultrasonics Symposium, Vol. 1, M. Levy, S. Schneider, and B. McAvoy, eds., IEEE 

Press, 1996, pp. 727-730. 

13. Dumas, P., Poguet, J., and Fleury, G., “New Piezocomposite Transducers for 

Improvement of Ultrasonic Inspections,” in Review of Progress in QNDE, Vol. 22A, 

D.O. Thompson and D.E. Chimenti, eds., American Institute of Physics, Melville, NY, 

2003. 

14. Margetan, F.J., Roberts, R., Chiou, C.-P., and Thompson, R.B., “Determination of the 

Effective Focal Characteristics of Bicylindrically-Focused Ultrasonic Transducers,” 

Review of Progress in QNDE, Vol. 21A, D.O. Thompson and D.E. Chimenti, eds., 

American Institute of Physics, Melville, NY, 2002, pp. 791-798. 

15. Margetan, F.J., Brasche, L., Chiou, T., Keller, M., Degtyar, A., Umbach, J., Hassan, W., 

Patton, T., and Smith, K., “Fundamental Studies of Nickel Billet Materials—Engine 

Titanium Consortium Phase II,” FAA report DOT/FAA/AR-05/17, June 2005. 

16. Minachi, A., Margetan, F.J., and Thompson, R.B., “Reconstruction of a Piston 

Transducer Beam Using Multi-Gaussian Beams (MGB) and its Applications,” in Review 

of Progress in QNDE, Vol. 17A, D.O. Thompson and D.E. Chimenti, eds., Plenum Press, 

NY, 1996, pp. 907-914. 

123
17. Copley, D. and Howard, P., “Operational Performance of a Multizone Billet Inspection 

System,” in Review of Progress in QNDE, Vol. 14, D.O. Thompson and D.E. Chimenti, 

eds., Plenum Press, NY, 1995, pp. 2145-2151. 

18. Nieters, E., Gilmore, R., Copley, D., and Howard, P. in Review of Progress in QNDE, 

Vol. 14, D.O. Thompson and D.E. Chimenti, eds., Plenum Press, NY, 1996, pp. 1495-

1502. 

19. Margetan, F.J., Thompson, R.B., and Yalda-Mooshabad, I., “Backscattered 

Microstructural Noise in Ultrasonic Toneburst Inspections,” Journal of Nondestructive 

Evaluation, Vol. 13, 1994, pp. 111-136. 

20. Margetan, F.J., Thompson, R.B., Yalda-Mooshabad, I., and Han, Y.K., “Detectability of 

Small Flaws in Advanced Engine Alloys,” ISU Center for NDE, July 1993. 

21. Margetan, F.J., Thompson, R.B., and Yalda-Mooshabad, I., “Modeling Ultrasonic 

Microstructural Noise in Titanium Alloys,” in Review of Progress in QNDE, Vol. 12B, 

D.O. Thompson and D.E. Chimenti, eds., Plenum Press, NY, 1993, pp. 1735-1742. 

22. Panetta, P.D., Thompson, R.B., and Margetan, F.J., “Use of Electron Backscatter 

Diffraction in Understanding Texture and the Mechanisms of Backscattered Noise 

Generation in Titanium Alloys,” in Review of Progress in QNDE, Vol. 17A, D.O. 

Thompson and D.E. Chimenti, eds., Plenum Press, NY, 1998, pp. 89-96. 

23. Brasche, L., Duffy, T., Bartos, J., and Smith, K., “Engine Titanium Consortium Phase II 

Technical Proposal Volume I,” July 1998. 

24. Margetan, F.J., Gigliotti, M., Brasche, L., and Leach, W., “Fundamental Studies: 

Inspection Properties for Engine Titanium Alloys,” FAA Report DOT/FAA/AR-02/114, 

December 2002. 

25. Panetta, P.D., Margetan, F.J., Yalda, I., and Thompson, R.B., “Observation and 

Interpretation of Microstructurally Induced Fluctuations of Back-Surface Signals and 

Ultrasonic Attenuation in Titanium Alloys,” in Review of Progress in QNDE, Vol. 16B, 

D.O. Thompson and D.E. Chimenti, eds., Plenum Press, NY, 1997, pp. 1547-1554. 

26. Margetan, F.J., Panetta, P.D., and Thompson, R.B., “Ultrasonic Signal Attenuation in 

Engine Titanium Alloys,” in Review of Progress in QNDE, Vol. 17B, D.O. Thompson 

and D.E. Chimenti, eds., Plenum Press, NY, 1998, pp. 1469-1476. 

27. Margetan, F.J., Hassan, W., and Thompson, R.B., “An Experimental Study of 

Microstructure-Induced Ultrasonic Signal Fluctuations in Jet-Engine Titanium Alloys,” in 

Review of Progress in QNDE, Vol. 19, D.O. Thompson and D.E. Chimenti, eds., Plenum 

Press, NY, 2000, pp. 1433-1440. 

124
28. Gill, P.E., Murray, W., and Wright, M., “Practical Optimization,” Academic Press, NY, 

1981. 

29. Margetan, F.J., Gigliotti, M., Brasche, L., and Leach, W., “Fundamental Studies: 

Inspection Properties for Engine Titanium Alloys,” FAA Report DOT/FAA/AR-02/114, 

December 2002. 

125/126
APPENDIX A—MAJOR ACCOMPLISHMENTS OF THE ETC PHASE I PROGRAM

• Fabrication of samples necessary for fundamental studies and for development and 

validation of inspection techniques. Samples included various flat-bottom hole (FBH) 

samples, synthetic hard alpha samples, chord blocks, development standards, and actual 

notched or cracked hardware, all of which will be catalogued in the Airworthiness 

Assurance Nondestructive Inspection Validation Center Sample Database. 

• Transducer design models for cylindrical geometry, fixed-focus and phased array 

transducers, which allow the user to optimize a transducer for a given inspection 

scenario. Software was delivered to the original equipment manufacturers (OEM) for use 

in transducer design and procurement. The models were used in selection of a series of 

transducers for both flat part and curved surface inspection as part of optimization studies 

completed in Engine Titanium Consortium (ETC) Phase I. Those results will guide ETC 

Phase II inspection efforts. 

• Demonstrations of multizone and phased array systems to OEMs, the Federal Aviation 

Administration, and billet and forging suppliers were held as part of the Phase I program 

in cooperation with RMI Titanium of Niles, Ohio. The purpose of the demonstrations 

was to demonstrate the capabilities of zoned inspection systems in a factory environment 

and determine the users perspective of ETC technologies. Updates were also provided at 

each of the three open forums, including a summary of hard alpha detections. 

• Using the contaminated billet, the detectability of multizone inspection using discrete 

fixed-focus transducers and of the phased array annular transducer were demonstrated to 

be similar. 

• An industrywide specification for titanium (Ti) billet inspection was issued as Aerospace 

Material Specification (AMS) 2628 through the auspices of SAE Committee K. 

• Fundamental changes are now being realized in the approach taken to in-service eddycurrent inspection through the development of a suite of ETC tools that enable 

disciplined, reproducible digital data using equipment that is low in acquisition cost and 

generic across all engine models such that every engine shop in the world can afford to 

acquire tools. The tools are also robust and intuitive for the community to use improving 

the reliability and sensitivity of inspections performed during the service life of aircraft 

engines. 

• Development and commercialization of the portable eddy-current scanner and data 

acquisition system that enables acquisition of disciplined digital data and provides a 

platform for processing that data. The system was designed to be marketed for $50 

thousand or less, weigh less than 40 lb to enable use by a single inspector, and provide 

the majority of the capabilities of the large, expensive fixed-base scanners typical of 

military applications, meeting all of the original program goals. The development 

process was accomplished with input from the OEMs and airlines through various visits, 

design reviews, and Air Transport Association (ATA) demonstrations and meetings. 

A-1
• Initial application of the scanner was in the Hamilton Standard regional propeller 

recovery effort in the wake of the Carrolton, Georgia, crash. Initial use of the ETC 

scanner led to detection of a crack in a blade shank that had previously been accepted 

with another technique. This led Hamilton Standard to invest in a scanner of their own 

for use in the recovery effort. The scanner was successfully used during the 

refurbishment of propeller shanks with 100 propeller shanks inspected, four shanks 

rejected, and one additional crack found. By providing a more sensitive inspection, 

additional existing shanks could be used in the refurbishment effort, thereby allowing the 

closing action to be achieved on schedule without the mandatory grounding of aircraft. 

By avoiding groundings, not only was inconvenience to the traveling public avoided, but 

several regional carriers were able to stay in business with an estimated $15 million 

savings to the industry. 

• Beta site testing of the portable scanner allowed introduction of the technology to the air 

carriers and encouraged end user feedback in future developments. The first beta site 

was at United Airlines and involved the assessment of JT9D-7R4 14/15 stage HPC bores. 

This inspection is performed primarily with the engine in a semiassembled condition 

using fabricated tooling such that the scanner could be mounted to the engine while in the 

horizontal build fixture and the inspection performed through the HPC shaft with the high 

pressure turbine (HPT) installed if desired. The scanner was in place at United Airlines 

for 3 months, enabling inspection of four engines. Data was acquired during the beta site 

tests that allowed for comparison of the system to existing manual requirements in the 

Alert Service Bulletin for this hardware. Cracking was successfully found with both 

techniques. However, the portable scanner system allowed additional margin in detecting 

smaller cracks. 

• Additional applications were defined and developed by AlliedSignal, including the TFE 

731 fan disk blade slots, a significant accomplishment given the extreme slash angle and 

depth of the slot. AlliedSignal invested significant internal funds to address probe and 

technique development as part of the introduction process. AlliedSignal has made a 

significant commitment to the use of semiautomated inspection to address overhaul 

inspection with the purchase of two production units and further utilization expected 

through third-party overhaul shop implementation. 

• American Airlines requested beta site test application of the portable scanner to the fan 

hub of the small fan versions of the JT8D, i.e., straight 8s or baby 8s. The inspection 

areas included the blade slots, bore (face, corner, and inner diameter), and the web to rim 

transition radius. At this time, three engines have been inspected with the results 

matching those required by the Airworthiness Directive (AD). An American Airlines 

nondestructive testing (NDT) engineer was provided training, which enabled him to train 

other inspectors on the use of scanner. Data is being collected such that a request for an 

alternate means of compliance with the existing AD will be granted. As the existing 

technique is manual, no difficulty is anticipated in achieving an alternate means of 

compliance. Plans are to pursue the alternate means in 1998. A full demonstration of the 

scanner was provided to the airlines in April 1998. 

A-2
• A specification was generated by the ETC to encompass the needs of the industry for 

future generation eddy-current instruments. The specification included input from the 

OEM development and applications groups and major air carrier engine shops as well as 

the instrument manufacturers. The intent of the specification is to create a basis for and 

direction to the industry in the development of future instrumentation with elements of 

the specification already appearing in newer eddy-current instruments. 

• Flexible substrate technology used in array probes developed by General Electric (GE) 

was adapted to form a single element probe and applied to engine applications. The 

technology is now available commercially. 

• A boundary element model formulation was used to develop a tool for predicting field 

vectors emanating from eddy-current probes. This parameter is key in predicting the 

sensitivity of the probes to particular crack orientations in various geometries and was 

used extensively in the development of wire wound wide area probes. Using the model 

enabled a larger number of design iterations to be explored than were possible if probes 

were built to assess each design. The tool and further derivations of the tool are being 

used at Pratt & Whitney on an ongoing basis to design and build probes to address engine 

applications. One of the probes developed by Pratt & Whitney is being evaluated for 

application to JT8D fan hub inspection. 

• A low-pressure (LP) rotor rotator was designed and built to provide controlled rotation of 

the LP rotor while installed in the engine. It is generic in that it will work on all high 

by-pass ratio engines (no inlet guide vanes) and facilitates the collection of disciplined 

eddy-current data for on-wing inspection of fan and LP compressor disks, and LP turbine 

hardware. A demonstration was performed on a JT9D engine. Borescope manufacturers 

have shown an interest in the device as a way of assisting in the inspection of LP turbine 

hardware. 

• A probability of detection (PoD) methodology was developed, which allows the 

determination of the PoD of internal flaws in Ti for use in the validation of inspection 

systems. The approach differs from previous work in two important ways. First, it is 

based on statistical detection theory, requiring the determination of the distributions of 

signal and noise. These distributions are assumed to be controlled by three sources of 

variability associated with material microstructure, details of the measurement process 

such as scan plan, and flaw morphology. In contrast to existent methodologies, the 

statistical detection approach allows one to make a direct determination of the probability 

of false alarms as well as PoD, information that is valuable in a variety of contexts, 

including assessing the economic penalties of achieving a desired PoD by lowering the 

threshold too low. These consequences are taken into account by the OEMs in setting 

threshold levels, and thus influence the PoD that can be realized for a given inspection. 

Second, the methodology makes explicit use of mathematical models, which describe the 

physics of the inspection process. Because of the limited number of naturally occurring 

hard alpha inclusions that occur in Ti and the fact that billet disposition time pressures 

limit the amount of metallurgical information that can be obtained regarding the true size, 

shape, and composition of the inclusions, the utility of traditional empirical approaches 

for PoD determination is limited. Using physical models of the inspection process allows 

A-3
the maximum amount of information to be extracted from limited experimental 

observations. In addition, it allows one to predict the effects of changes to the 

inspections setup (e.g., probe properties and scan increments). The methodology has 

been exercised to generate PoD curves for FBHs and synthetic hard alphas. Validation 

using Contaminated Billet Study (CBS) indications is under way with the initial results 

being quite positive. Phase II will complete further validation and transition the researchgrade code to a useable tool for the OEMs. 

• Quantitative PoD information has been made available to life management groups, 

including input to Advisory Circular 33.14-1, “Damage Tolerance for High Energy 

Turbine Engine Rotors.” At the request of the Rotor Integrity Steering Committee, 

default PoD curves have been provided using previous methodologies. Based on the data 

assembled by the Jet Engine Titanium Quality Committee, PoD estimates have been 

made for naturally occurring hard alpha inclusions. However, these are not as 

comprehensive as would be desired because of 

− a limited number of hard alpha inclusions actually found in production, 

− the size of many of these defects has not been properly determined 

metallographically due to the need to make a rapid disposition of the billet, and 

− much of the ultrasonic data was saturated. 

These determinations of the PoD of naturally occurring hard alpha inclusions will be 

improved in Phase II, based on the results of the CBS. The experimental data will be 

obtained in Phase I, with analysis to be completed in Phase II. Finally, members of the 

life management community have indicated that the ability to predict the effects of 

inspection and defect parameters on PoD, as discussed above, should play an important 

role in future risk and life management studies. 

A-4
APPENDIX B—DESCRIPTION OF MULTIZONE INSPECTION FOR BILLET

The multizone system is a real-time, personal computer (PC)-based platform that employs 

custom-built, analog, electronics using up to eight parallel (nonmultiplexed) channels, each with 

a remote pulser/receiver matched to the ultrasonic transducer. Scanned helically, the billet is 

divided into concentric zones with a focused transducer used to acquire peak-detected C-scan 

image data for each zone. The depth of each zone is established by the depth of focus of that 

transducer. C-scan image data from all channels are displayed simultaneously on a 1024 x 1280 

cathode-ray tube (CRT) and scroll as the inspection advances along the billet length. The data 

are written to optical storage upon completion of the inspection. 

Custom postscan analysis software was developed to detect flaws using signal-to-noise-based 

algorithms. This software provides more reproducible results than conventional systems and 

greatly reduces operator fatigue and the chance for error. 

B.1 SYSTEM. 

Prior to the multizone system, the conventional test methods were limited in sensitivity due to 

material noise and lack of transducer focusing. In addition, there was no digital data acquisition 

or storage. To correct this situation, a system was designed that would use focused transducers 

to overcome material noise and, therefore, improve detection. However, multiple compound 

focused transducers were required to inspect the billet volume to uniform sensitivity. The 

transducers are fitted with dual curvature lenses to produce a diffraction-limited, symmetrical 

focus. This configuration provides more uniform sensitivity versus depth for a given scan rate. 

Inspection zones are determined by the transducer focal zone parameters, i.e., diameter, d, and 

focal length, F. Each transducer inspects a depth of material roughly equal to the –3 dB depth of 

field, εz, and at a spatial sampling rate at least half the –6 dB beam diameter, εx. (See equations 

B-1 and B-2.) 

εz = 3.6λ[F/d]2c1/c2 (B-1) 

εx = 1.03 λ[F/d] (B-2) 

The value, c1, is the sound velocity in water (0.058 in./μsec), c2 is the sound velocity in titanium 

(Ti) (0.243 in/μsec) and λ is the wavelength (0.049 in. in Ti at 5 MHz). A typical multizone 

transducer produces 5-MHz, F/8.0 beams that are approximately 0.1′′ diameter and 0.8′′ depth of 

field for Ti. At deeper depths, larger F numbers are used due to the manufacturability limitation 

in element diameters produced by commercial vendors. By increasing the transducer size and 

depth in the material of its focal zone, overlapping foci are produced to uniformly inspect the 

billet volume, see figure B-1. 

The ultrasonic instrument uses custom analog electronics with up to eight parallel, 

nonmultiplexed channels based on the NIM architecture. The parallel, nonmultiplexed 

architecture was used to eliminate any unattenuated sound, which might interfere with a given 

channel’s signal. A multiplexed solution also introduces possible speed limitations. Each 

transducer has its own remote pulser/receiver, which is matched to that specific transducer. The 

B-1
user can set various parameters for each channel such as attenuation, signal gate delay and width, 

and triggering rate. The data generated for each channel or zone is an analog, peak-to-peak 

C-scan voltage ranging from 0 to 10 volts, which is sent to the computer for display and storage. 

FIGURE B-1. MULTIZONE INSPECTION VS TYPICAL INDUSTRIAL INSPECTION 

The computer is an Intel-based PC (e.g., 486, Pentium, etc.) with a large amount of randomaccess memory (RAM) (64 MB or larger is recommended) running MSDOS and Windows®. 

The RAM permits storage of data from all eight channels in memory as it is acquired. This 

eliminates time-consuming disk accesses during acquisition. Standard data acquisition boards 

are used for digital handshaking signals between the computer and the electronics and for 

digitization of the analog C-scan data. A 21′′ 1280 x 1024 resolution color monitor allows all 

eight channels to be displayed simultaneously during acquisition. The archival storage 

requirement was achieved using a write-once-read-many (WORM) optical disk for archiving the 

image data. Report generation is accomplished using a high-quality color PostScript printer. 

Figure B-2 shows the complete system diagram. 

FIGURE B-2. MULTIZONE SYSTEM BLOCK DIAGRAM 

B-2
B.2 SYSTEM OPERATION. 

For inspection, the billet is immersed in a water tank and rotated about its axis. The maximum 

billet rotation speed is approximately 30 RPM. The transducers, aligned normal to the billet 

surface, are mounted on a stage or follower that rides on a track over the tank. As the billet 

rotates, the follower moves axially along the billet length and the material is inspected in a 

helical pattern. 

An axial encoder is mounted on the follower and is used to report axial position of the 

transducers during the scan. A rotary encoder is coupled to an adapter, which is attached to the 

end of the billet using double-sided adhesive tape. This encoder generates a once-per-revolution 

index or home pulse, which is used by the computer acquisition software to determine the 0°

location on the billet. The rotary encoder also generates a 2048 pulse-per-revolution signal. 

This signal provides a position-dependent trigger for the ultrasonic transducers known as a pulseon-position or POP pulse. The POP signal is used by the electronics to synchronize the 

triggering of the transducers for all eight channels to the billet rotation. Each channel will divide 

down the 2048 master POP to the sample rate required to inspect the particular zone. 

Outer zones with larger radii require more samples than inner zones to fully inspect the material. 

The sample count for a given zone is a function of the zone circumference and the beam 

diameter, εx, of the transducer. While transducers covering the larger radii zones fire more often 

than their smaller radii counterparts, all transducers fire simultaneously when they do trigger. 

This greatly minimizes the possibility of any unattenuated sound from any channel interfering 

with the signal from any other channel. 

Prior to inspection, the user must start the acquisition software on the computer. Based on the 

billet material and diameter, the operator is instructed on how to configure the electronics. 

Parameters such as attenuation, signal gate delay and width, and POP rate are set using 

thumbwheel switches on the electronics for the required number of channels. Information such 

as operator ID, billet length, and identification numbers must be entered into the software as 

well. Once the electronics are configured and the acquisition software is running, the rotary 

encoder begins triggering the electronics. The electronics fire the transducers and produce 

analog C-scan data values. These analog signals, along with axial position information, are sent 

to the PC acquisition software for storage in RAM and display. Upon scan completion, the user 

is given the opportunity to enter any comments about the scan. Data is then transferred from 

computer memory to disk and later archived to a WORM optical disk for permanent storage. 

B.3 POSTSCAN ANALYSIS. 

The implementation of digital data acquisition allows postscan analysis, which is not possible 

with any current billet inspection systems. A typical multizone inspection on a single billet can 

generate 30 MB or more of ultrasonic image data. These data can include flaw signals obscured 

by an acoustic noise level that varies both circumferentially and axially along the billet. An 

image analysis software package was developed to aid the operators in finding indications. This 

package, which runs on the system PC, provides a Microsoft Windows-based graphical user 

interface for displaying and manipulating the inspection results. Using data analysis tools such 

B-3
as maximum value, axial cross sections, and image magnification, the operator is able to locate 

signals that are above the local acoustic noise and display them at high resolution The operator 

can then isolate potential rejectable signals and a surrounding area of homogeneous noise for 

each indication. Using those two regions of interest (ROI), the operator can calculate the signalto-noise ratio (SNR) for each indication where SNR is defined as 

nn

ns

P

P SNR

μ

μ

−

− =

where Ps is the maximum value of the signal ROI, Pn is the third highest (to exclude electrical 

noise spikes) value in the noise ROI, and mn is the mean of noise ROI. Material acceptance 

criterion based on both peak amplitude and SNR are applied to the indication to make the 

accept/reject decision. The PC can quickly review 30+ MB of data, allowing the task to be 

completed before the billet is removed from the tank. Since the scan positional information is 

stored along with the image data, the operator can return the transducer to the location on the 

billet where an indication exists to verify its location and mark the billet for sectioning to remove 

the material surrounding the indication. 

B.4 TRANSDUCERS. 

The transducers in use for the majority of production multizone testing are ceramic element 

designs focused with loaded epoxy lenses and of nominal 5 MHz frequency. Diameters vary 

from 0.625′′ to 2.35′′. The objective of the transducer design was to provide an equal response 

from flat-bottom hole targets at the near and far ends of the inspection zone (within a 3 dB 

tolerance) and to minimize the sound beam area throughout the focal zone. 

B-4
System specification for operation of multiple fixed focus transducers (multizone). 

MECHANICAL 

Capacity Billet diameter 4.5′′ to 15.0′′

Billet length 12.0′′ to 240.0′′

Maximum weight 6000 pounds 

Rotation speed 1 to 60 rpm 

Maintain constant rotation speed within ±10% of setting 

Indexing Helical (constant speed) or step indexing 

 Index range 0.010′′ to 0.100′′ per revolution 

Index distance accuracy ±0.003′′

Bar follower/manipulators Capacity: 8 transducers 

Maximum transducer diameter 3.0′′, case length 3.0′′

Transducer face to billet distance 2.0′′ to 5.0′′

Transducer alignment adjustment about axis x (parallel to billet 

axis), y (across bridge, perpendicular to billet axis) and c (about 

transducer axis). Locking pin to engage slot in transducer housing 

to fix c-axis alignment. 

Manipulator and follower to maintain transducer normal to billet 

surface within ±0.5 degrees. 

Transducers: 

Reference Reference AMS 2628 Annex B for details of transducers 

Frequency 5 MHz 

Bandwidth 45% to 65% 

Diameter 0.625′′ to 2.35′′

Construction Stainless steel housing with alignment slot to engage locking pin in 

manipulator 

Instrumentation: 

Encoders Submersible billet rotation encoder coupled to billet via adapter 

plate and flexible coupling, generating 2048 pulses per revolution 

Index axis encoded to 0.001′′ resolution 

Architecture 8-channel parallel architecture 

 Based on NIM standard 

Pulsers Remote pulsers matched to transducer capacitance (150 to 

3500 pF) 

Instrument Repetition rate synchronized to internal clock or rotary encoder 

 Pulse-on-position pulse rates of 128, 256, 512, 1024, 2048 pulses 

per revolution, selectable for each channel 

Linear amplifier, bandwidth 

Attenuator range 0-59.5 dB 

One gate per channel 

Gate synchronized to initial pulse or interface echo 

 Gate delay 

Gate width range 

Gate output one analog channel per gate, 0 to 10 volts, 

proportional to maximum peak-to-peak range in gated region 

B-5
Oscilloscope display (typical Tektronix TDS 420) 

Data Acquisition Hardware: 

CPU 133 MHz Pentium, Industrial rack mount 

64 MB RAM 

500 MB SCSI Hard Disk 

 3.5′′ Diskette Drive 

Monitor 21′′, 1280 x 1024 x 256 resolution 

Video Board Number Nine GXI-TC (DOS Software) 

120 x 1024 x 256 (Windows NT Software) 

Archival Storage Preferred: ISO 9660 CD-ROM 

Acceptable: Pioneer Type “A” WORM Magneto-Optical Disk with 

Instar Optical Disk Software 

Printer Postscript Color printer (typical Tektronix Phaser 200I) 

Analog-to-Digital Conv. 8 parallel channels using simultaneous sample-and-hold 

8-bits over the range of 0-10 volts (Data Translations DT2829) 

Encoder Input Quadrature Encoder Board (Technology 80 Model 5312) 

Data Acquisition Mode Pulse-on-position 

Software: 

Data Acquisition GE-developed software running under MSDOS v5.0 or higher 

(Currently undergoing upgrade to run under Windows NT 

v3.51)Real-time scrolling display of eight channels 

Data Analysis GE-developed software running under Windows 3.11, Windows 

95, or Windows NT with IDL v4.0.1b from Research Systems 

Incorporated 

Data File Format network Common Data Format (netCDF) 

B-6