CHARACTERIZATION OF WELD DEFECTS IN

SINGLE V-BUTT WELDED MILD STEEL

PLATES USING ULTRASONIC A-SCAN

TECHNIQUE

Dissertation submitted in partial fulfillment of the requirements for the degree of

MASTER OF PHILOSOPHY

IN

MATERIALS SCIENCE AND ENGINEERING

T.M.R. TENNAKOON B.SC (PHYSICS SPECIAL), M.SC (NUCLEAR SCIENCE.)

DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING

UNIVERSITY OF MORATUWA

SRI LANKA

March 2010

ii

DECLARATION

I hereby declare that this dissertation is my own work and that, to the best of my

knowledge and behalf, it contains no material previously published or written by

another person nor material which, to substantial extent, has been accepted for the

award of any other academic qualification of a university or other institute of

higher learning except where an acknowledgement is made in the text.

Authors name; T.M.R. Tennakoon Date; 01.03.2010

Signature ; -----------------------

This is to certify that this dissertation is based on the work carried-out by

Mr.T.M.R.Tennakoon under my supervision. This dissertation has been prepared

according to the format stipulated and is of acceptable standard.

Certified by; Dr.R.G.N.De.S.Munasinghe (Supervisor)

Department of Materials Science and Engineering

Faculty of Engineering

University of Moratuwa.

Sri Lanka.

Signature --------------------- Date ---------------

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ACKNOWLEGEMENT

I gratefully acknowledge Dr. R. G. N. De. S. Munasinghe, Senior Lecturer and my

Supervisor for his enthusiastic guidance and encouragement in my research. My special

thanks are due to his superior knowledge about the subject. His contribution to this thesis

is many-folded, thus inseparable and mainly responsible for its successful conclusion.

I wish to express my deepest gratitude to Dr. S. U. Adikari, Head of Materials Science

and Engineering Department for his inspired guidance and suggestions, proposals and

advice in successfully completing this research.

I am deeply grateful to Prof. M. P. Dias, Prof. R. Hewamanna, Prof. B. M. A. O. Perera,

Dr W. Abeyewickreme - former Chairmen of the Atomic Energy Authority for their

guidance, support, and advice with superior knowledge and allowing me to use

equipments and receive other benefits via Atomic Energy Authority (AEA).

I thank all the staff members of the AEA and Materials Science and Engineering

Department who greatly supported me by providing all necessary facilities such as

equipment, computers, stationeries etc. through out my whole research.

I should appreciate specially my collogues of the AEA for their good support and

encouragement provided to me to complete the research successfully.

My deepest gratitude to my wife and two sons for their support, understanding and

encouragement that they have provided to complete this degree successfully.

T.M.R. Tennakoon.

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Abstract

In Ultrasonic A-Scan technique the depth and the size of the defects in a material

can be determined by the position and amplitude of the reflected echo on the CRT

screen. However the main difficulty in ultrasonic testing is that the precise

recognition of the defect type.

In conventional ultrasonic A-scan methods recognition of the defect type

(porosity, slag, crack etc.) is ascertained by a series of movement of the probe as

rotational, orbital, lateral and transverse to observe the echo pattern. Here the

human eye perceives many facts simultaneously by moving a transducer in

infinite increments in 3D to seek out additional information-the mind sorts and

processes the accumulated real-time facts and combines them with empirical data

from experience and case history before making final decision on the defect.These

uultrasonic echo patterns are quite complex since those may contain many signals

other than defect echoes, same defect may display different echo patterns,

different defects may display similar echo patterns and the amplitude and shape of

the signal may change due to slight movement of the testing probe.

Therefore the interpretation of defects using this technique is very complicated

and totally depends on the experience of the operator who carries out the testing.

Hence only well-experienced NDT personnel can identify defect type using this

method. This is one of the main drawbacks related to Ultrasonic A-scan

technique.

The objective of this research is to study the relationship between type of defect

and echo amplitude in single-V butt welded steel plates and to propose a new

concept to identify defect type with the help of a self developed software which

will be cheep, portable and simple to understand by the operator.

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To achieve this objective initially a mathematical relationship between echo

amplitude and defect type was developed for common weld defects in single-V

butt welded steel plates (slag, porosity, crack etc.) using newly derived

mathematical equations for above defects.

Since the amplitude of the echo signal affects by the defect size this parameter

alone can not be used to identify the type of defect. As such the possibilities of

using few other features such as width of defect echo, position of defect and

change of probe angle also were considered.

Experimental results show that any individual defect in single-V butt welded mild

steel plates produces echo signals with unique pulse width and range of amplitude

levels. In addition the results show that lack of penetration can be identified using

a second probe angle in addition to single probe angle as use in conventional

methods. Lack of side-wall fusion can be identified by using position of probe

with respect to weld center line.

These individual characters of defects, which are inherent to those defects, were

used to predict the type of unknown defects using a self developed software

programme named “ULTRASL1”.

The significance of this work is that the introduction of a specialized procedure

with a software programme to identify type of defect, so that Non-Destructive

Testing personnel with any level of experience can share the expertise of the best

operators in the industry. Hence it will support to reduce one of the main

problems concerning ultrasonic testing i.e. the difficulties in recognition of defect

type.

The work was limited for defects like slag(volumetric), isolated pore, porosity,

lack of inter-run fusion, lack of side-wall fusion, crack and lack of penetration in

single-V butt welded mild steel plates.

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List of Tables

PAGE Table 1: Modelling of natural flaws in single-V butt welded plates 39 Table 2: Type of defects created artificially 74 Table 3: Echo amplitudes for different couplant thicknesses. 75 Table 4: Roughness of surfaces of test samples 77 Table 5: Variation of echo amplitude with defect depth and Probe angle for equipment USK7B and USK 7 78 Table 6: Average amplitudes for different depths and probe angles 79 Table 7: Echo amplitudes from defects containing air and Welding slag 80 Table 8: Flaw echo amplitude with and without welding slag 81 Table 9: Echo amplitude for different diameters-Test 1 82 Table 10: Echo amplitude for different diameters-Test 2 83 Table11: Echo amplitude for different diameters-Normal beam probe 84 Table12: Echo height with respect to each probe angle-artificial blocks 85 Table 13: Echo height with respect to each probe angle –natural defects 86 Table 14: Set of data obtained from defects in fabricated welded specimens 89 Table 15: Set of data obtained from defects in welded specimens in-situ 91 Table 16: Comparison of test results-Laboratory testing 97 Table 17: Comparison of test results- Measurements taken in-situ 98 Table 18: Set of data obtained from defects in fabricated welded 103 Specimens using Digital Ultrasonic Flaw Detector. Table 19: Comparison of data obtained using Digital Flaw Detector and Analogue Flaw Detector 104 Table 20: A summary of the results of the entire experiment 111

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List of Figures PAGE

Figure 1: Ultrasonic A-scan presentation 03 Figure 2: Echo patterns of various defects 07 Figure 3: Classification of defects based on the pulse echo technique 09 Figure 4: Flow chart to determine the type of defect as per JIS Z 3060 10 Figure5: Graphical correlation between defect type,echo amplitude and

angle of probe 13

Figure6: Sound pressure from an ultrasonic probe 17 Figure7: Reflection of ultrasound from a spherical reflector 20 Figure8: Variation of defect echo amplitude with beam path distance 24 Figure9: Reflection and transmission at air-steel interface 26 Figure10: Transmission of sound waves in to steel through a couplant 29 Figure11: Reflection of ultrasound at steel – air interface 28 Figure12: Reflection of ultrasound at steel – slag interface 29 Figure13: An enlarged profile of surface finishes 30 Figure14: Measurement of Roughness average 31 Figure15: Effects of surface roughness on echo amplitude 31 Figure16: Reflection of ultrasound at a defect surface 34 Figure17: Portion of sound pressure received by the probe 35 Figure18: Different probe positions to obtain maximum echo 35 Figure19: Comparison of amplitude values of common defects 40 Figure20: Width of an echo signal from a defect 41 Figure21: Echo signal from back wall or flat circular reflector 42 Figure22: Echo signal from isolated porosity 43 Figure23: Echo signal from a slag 44 Figure24: Position of the probe with respect to the weld center line 45 Figure25: Reflection, refraction and mode conversion 48 Figure26: Sound pressure of reflected waves vs. angle of incidence 48 Figure27: Reflected and transmitted sound pressure at an interface 49 Figure28: Reflection of shear waves at a right angled corner by a 70°

probe. 49

Figure29: Reflection of shear waves at a right-angled corner by 450 and 600 angle probes.

50

Figure30: Comparison of percentages of echo amplitude from probe angles 450, 600 and 700

51

Figure31: Schematic Diagram of an Ultrasonic Flaw Detector 53 Figure32: Cross section of typical contact transducer 54 Figure33: Cross section of an angle beam probe 56 Figure34: Possible defects in single-V butt welded plates 61 Figure35: Defects considered in the research work 61 Figure36: Plate to plate butt joint and pipe to pipe butt joint 62

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Figure37: Measurement of echo amplitude vs. thickness of couplant 64 Figure38: Measurements of echo amplitudes with respect to 20% DAC 65 Figure.39 Variation of echo amplitude with defect area 67 Figure40: Mild steel stepped test block 68 Figure41: 15 mm thick single-V butt steel plate 68 Figure42: Echo amplitudes for probe angles 460, 610 and 730 62 Figure43: Variation of echo amplitude with couplant thickness-450 76 Figure44: Variation of echo amplitude with couplant thickness-600 77 Figure45: Variation of echo amplitude with couplant thickness- 700 78 Figure46: Graph of depth of hole vs. echo amplitude 79 Figure47: Echo amplitudes from defects containing air and slag 80 Figure48: Correlation graph of amplitude with and without welding slag. 81 Figure49: Graph of echo amplitude verses diameter-Test 1 82 Figure50: Graph of echo amplitude verses diameter-Test 2 83 Figure51: Graph of echo amplitude verses diameter- Normal beam probe 84 Figure52: Correlation graph between experimental and theoretical values

of stand-off distances 87

Figure53: Radiographic Testing Technique 93 Figure54: A radiograph of the fabricated test specimen-T1 94 Figure55: Experimental set up of an ultrasonic test 100 Figure56: Measurement of defect parameters 100 Figure57: Echo Pattern of a Slag Inclusion 101 Figure58: Echo Pattern of a Crack 101 Figure59: Echo Pattern of a Lack of Penetration 102 Figure60: Echo Pattern of an Isolated Pore 102 Figure61: Echo Pattern of a Lack of Fusion 103 Figure62: Correlation graph between data obtained using Digital Flaw

Detector and Analogue Flaw Detector 104

Figure63: Comparison of theoretical and experimental echo heights 109 Figure64: Identification of lack of fusion in double-V butt plates 120 Figure65: Identification of lack of penetration in double-V butt plates 121 Figure66: Reflection of shear waves at the inside surface of a curved

specimen 122

Figure67: Skip distances multiplying factor ‘fp’ for curved surfaces 123 Figure68: Beam path multiplying factor ‘fs’ for curved surfaces 123

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Abbreviations

NDT -Non Destructive Testing

UT -Ultrasonic Testing

RT -Radiographic Testing

MT -Magnetic Particle Testing

PT -Liquid Penetrant Testing

ET -Eddy Current Testing

CRT -Cathode Ray Tube

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LIST OF CONTENTS PAGE Declaration ii Acknowledgement iii Abstract iv List of Tables vi List of Figures vii Abbreviations ix 1.0 Introduction 1.1 Background 1.1.1 Non-Destructive testing of materials 1

1.1.2 Ultrasonic testing of materials 2 1.1.3 Advantages and limitations of UT 3

1.2 Objectives of the research 5 1.3 Methodology 5

2.0 Literature Review

2.1 Conventional method of defect identification 6 2.2 Recent development of defect identification methods 8 2.3 Recognition of defect type using echo height 12

3.0 Theoretical Basis

3.1 Sound pressure at a finite distance from a probe 14 3.2 Sound pressure received by the probe from a reflector 18 3.3 Factors which influence the received sound pressure 25 3.3.1 Effect of couplant type and thickness on

received sound pressure 26 3.3.2 Effect of impurities within the defect on

received sound pressure 28 3.3.3 Effect of roughness of the test surface on

received sound pressure 30 3.3.4 Effect of probe parameters on received sound

Pressure 32 3.3.4.1 Probe angle 32

3.3.4.2 Frequency of wave 32 3.3.4.3 Size of probe 32

3.3.5 Effect of defect orientation on received sound pressure 34 3.3.6 Effect of roughness of defect surface on received sound pressure 36

3.3.7 Affect of depth of defect on received sound Pressure 37

3.3.8 Affect of defect area on received sound Pressure 37

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PAGE 3.4 Acoustic energy equations of spherical, cylindrical and flat circular reflectors 38 3.5 Comparison of echo amplitudes from defects in single-V butt welded plates 39 3.6 Relationship of defect type to width of echo signal 40

3.6.1 Back wall or flat circular reflector 42 3.6.2 Crack and lack of sidewall fusion 42 3.6.3 Isolated pore and porosity 43 3.6.4 Slag 43

3.7 Relationship of defect type to its position 44 3.7.1 Identification of lack of sidewall fusion 44 3.7.2 Identification of defects in weld root 47 3.8 Usage of different probe angles to identify defect type 47

4.0 Equipment and Accessories used

4.1 Ultrasonic Flaw Detector 52 4.2 Ultrasonic Probe 53 4.2,1 Contact Transducers 55 4.2.2 Dual Element Transducers 55 4.2.3 Angle Beam Transducers 55

5.0 Research Methodology

5.1 Fabrication of test specimens 57

5.2 Effect of couplant thickness on defect echo amplitude 63 5.3 Effect of test surface roughness on defect echo amplitude 64 5.4 Effect of depth of defect on echo amplitude 65 5.5 Effect of impurities within the defect on echo amplitude 66 5.6 Effect of defect area on echo amplitude 66

5.7 Identification of lack of penetration by change of probe angle. 67 5.8 Identification of lack of sidewall fusion 70 5.9 Development of a relationship between defect type, echo amplitude, echo width, defect position and probe angle 70

5.9.1 Experimental Procedure 71 5.9.2 Development of a Computer Programme 73

6.0 Results `

6.1 Fabrication of test specimens 74 6.2 Effect of couplant thickness on defect echo amplitude 75

6.3 Effect of test surface roughness on defect echo amplitude 77 6.4 Effect of depth of defect on echo amplitude 78 6.5 Effect of impurities within the defect on echo amplitude 79 6.6 Effect of defect area on echo amplitude 82

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PAGE 6.7 Identification of lack of penetration by change

of probe angle. 85 6.8 Identification of lack of sidewall fusion 87

6.9 Development of a relationship between defect type, echo amplitude, echo width, defect position and Probe angle 88

6.9.1 Set of data obtained from identified defects in fabricated welded specimens 88

6.9.2 Set of data obtained from defects in-situ 90 6.10 Verification of test results

6.10.1 Verification of results by RT 92 6.10.2 Verification of results by conventional methods 94

6.10.3 Comparison of test results 96 6.10.4 Set of data obtained using Digital Ultrasonic

Flaw Detector 99 7.0 Discussion

7.1 Fabrication of test specimens 106

7.2 Effect of couplant thickness on defect echo amplitude 106 7.3 Effect of test surface roughness on defect echo amplitude 107 7.4 Effect of depth of defect on echo amplitude 108 7.5 Effect of impurities within the defect on echo amplitude 108 7.6. Effect of defect area on echo amplitude 108

7.7 Identification of lack of penetration by change of probe angle. 109 7.8 Identification of lack of sidewall fusion 110 7.9 Development of a relationship between defect type, echo amplitude, echo width, defect position and probe angle 117

8.0 Conclusions 110 9.0 Constrains 119 10.0 Future work 121 11.0 References 125 12.0 Annexure 1 (Computer programme) 130

12.1 Input and output data module 12.2 Flow chart of the computer programme

12.3 Software of the computer programme