Defect Characterization and

Dimensioning of Cracks in

Welds by the Ultrasonic

Diffraction Method

S. Golan

Office of Nondestructive Evaluation

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

March 1 980

Qc

100

U56

NO.80-1983

1980

Prepared for; National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

NBSIR 80-1983

DEFECT CHARACTERIZATION AND

DIMENSIONING OF CRACKS IN

WELDS BY THE ULTRASONIC

DIFFRACTION METHOD

S. Golan

Office of Nondestructive Evaluation

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

March 1980

Prepared for: National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technoiogy

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

Abstract . 1

Introduction 1

Basics of the Diffraction Technique 2

Optimum Conditions for Dimensioning of Cracks 3

Sizing of Simulated Cracks in Welds 4

Discussion 4

Table 1, Ultrasonic Measurements 6

Figures 1-14 9

References 22

DEFECT CHARACTERIZATION AND DIMENSIONING OF CRACKS IN

WELDS BY THE ULTRASONIC DIFFRACTION METHOD

by

S. Golan*

ABSTRACT

The possibility of applying an ultrasonic diffraction method for

dimensioning of crack-like defects in welds was investigated. A feasibility study was carried out and optimum test conditions were established

using a series of test specimens with narrow slits. A series of welded

specimens with in-welded cracks were tested. The possibility of using a quantitative diffraction technique for nondestructive examination of

pipeline welds in field conditions is discussed.

INTRODUCTION

With the conventional ultrasonic pulse-echo technique, flaw size is

related to echo-amplitude. Amplitude, however, being strongly affected

by many factors^, is rather insensitive to size. Consequently, the pulseecho technique is not suitable for quantitative measurements. In welds,

quantitative evaluation becomes even more complicated by the anisotropy

and inhomogeneity inherent in welded materials and by geometric factors.

The inability of the pulse-echo technique, which is based on reflectivity,

to serve as a quantitative tool provided the impetus for development of

techniques based on other phenomena of interaction between the flaw and

the acoustic field, such as scattering, diffraction, interference, etc.

It seems that the most promising technique to develop into a practical

and economical quantitative testing tool for field work is the diffraction

technique. Figure 1 shows an example of ultrasonic sizing of slits.

More such curves are given in reference 1. It is especially useful for

dimensioning of crack-like defects. This technique has three main advantages over some other emerging techniques: flaw size is computed using

only time measurements, which can be made more accurate and more reliable

than amplitude measurements; conventional pulse-echo equipment can be used;

and operators familiar with the pulse-echo technique can easily adopt the

diffraction technique.

* Permanent address, Israel Institute of Metals, Israel. This report was prepared while the author was on a temporary assignment to the NBS Office

of Nondestructive Evaluation.

1
BASICS OF THE DIFFRACTION TECHNIQUE

When an ultrasonic wave impinges on the tip of a crack, waves of

shear (s) and compressional (P) modes are scattered from the tip in a circular form. Figure 2 shows a photoelastic pattern of the scattered

acoustic field from the tip of a narrow slit in a glass block. From the

time interval between the entering of the incident wave and emerging of

the tip-diffracted wave the depth of the tip can be computed.^" Most

reliable results are obtained when time delay is measured from a conve- niently chosen reference signal. Figure 3 shows a diffraction triangle

formed by the transmitted, the diffracted and a compressional surface wave which serves as reference.^ Equation 1 gives the relationship between time delay and depth of the tip when a P surface wave is used as

a reference.

2 2 2

C^-C^ + 4 cosa C^(A+1) - (sina) Y

/I 2 -4 cos a (a+1 )^-C2

_ _

C^

C = 1

26

2a

y

= 0 ( 1 )

- Angle of incident beam

- Mode coefficients for impinging and diffracted beams

- For a compressional beam

- For a shear beam

- Shear wave velocity

- Compressional wave velocity

- Depth of source of diffraction; nondimensional

- Depth of source of diffraction; nondimensional

- Time delay expressed in units of length

- Distance between transducers

- Depth of source of diffraction.

From this general equation specific equations can be derived for

different modes of transmitted and diffracted waves and for various

diffraction triangles.^ If the reference signal is a bottom reflection

and the diffracted and reference signals are maximized (see Figure 4),

the relationship between time-delay and depth for a single or double

transducer technique is given in Equation 2.

2
y

= T - 5cosa ( 2 )

T - thickness of material.

OPTIMUM CONDITIONS FOR DIMENSIONING OF CRACKS

The diffraction techniques can be used to dimension cracks open to the

front or back surfaces as well as internal cracks, provided a tip-diffracted

signal can be detected. This, however, is quite often very difficult. The

tip-diffracted wave usually has a low amplitude, and high amplification is

necessary to reveal the signal. This, in turn, forms a noisy background which

might blur the tip signal. The noise signals come from various sources;

scattering from grains and inclusions, mode conversion, scattering from the

crack surface, back wall reflection, surface waves, side loops, internal re- flections from the wedge, and electronic noise. Figure 5 shows a typical

oscil loscopic trace in the background of a signal diffracted from the tip of

a fatigue crack. ^ The crack depth can be sized with sufficient accuracy and

reliability only if the tip diffracted signal can be clearly detected and

unambiguously identified among the spurious signals. As has been shown in

this and some other works^“^ under certain conditions signal-to-noise ratio

can be considerably improved and the tip signal can be detected. By analyzing

the oscil loscopic trace, the tip signal can be in many cases identified.

Three parameters determine the detectability of a signal; amplitude,

resolution, and dynamic response. Dynamic response is defined as the drop

in amplitude caused by displacing the transducer from its maximum amplitude

position (see Figure 6). When the dynamic response is high, a sharp drop

in amplitude will be observed when the transducer is slightly displaced.

This parameter is a very strong crack indicator.

All three parameters are mode and angle dependent. On a goniometer

the angle-amplitude dependence was determined for the PP mode (incident

and diffracted waves of the compressional mode)^. It was found that maximum amplitude is obtained when the angle between the transducers approaches

180° (a-^90°).

In former works^"^’^ the PP mode was primarily used in order to prevent

interfering signals of the P mode. In later work^ it was found that, by

using the SS mode and angles a in the range of 40° to 50°, higher amplitudes

are obtained. The dynamic response is higher^ because of the smaller angles

and, because of the lower velocity of shear waves, resolution is almost

doubled. By choosing an angle between 40° and 50° interfering compressional

mode signals do not appear. An additional improvement in amplitude is

achieved by using one instead of two transducers. This has additional advantages: it is more convenient to handle one transducer; no alignment

between the transducers is necessary; and the dead zone is smaller, as it

is not limited by wedge geometry.^ The high amplitude noise which appears

when one transducer is used can be minimized by chosing a wedge which filters

out the internal reflections.^

3
SIZING OF SIMULATED CRACKS IN WELDS

For the feasibility study a series of specimens with a simulated internal

crack in welds was prepared. The "inwelded crack" was formed by incomplete

penetration of a butt weld. The location, size and orientation of the "crack" was controlled by the bevel geometry of the welded plates (see Figure 7, 8 and

9). In order to achieve minimum crack width (0.00 to 0.06 mm), minimum deformation and, as closely as possible, a straight line of the tip, the plates were welded and cooled to room temperature while clamped in a fixture. The

material used was 25.4-mm low-carbon steel plates. Specimens of 22-mm width were cut from the welded plates. The specimens were machined, polished and

etched to reveal the "crack" and the weld cross section. In order to check the

ability of the diffraction method to dimension normal as well as inclined internal cracks in the as-welded condition, four types of specimens were prepared:

as-welded with normal "cracks"; machined to remove the excess of the welding,

with normal "cracks"; and as-welded and machined with 45° "cracks" (see

Figures 8 and 9). Measurements were made with the two-transducer technique,

45° 5-MHz shear waves, equilateral diffraction triangle, and a back-wall

reflection signal as reference. (The 45° angle is within the range of good

detectability). Because of the many advantages of the one-transducer technique

an initial attempt was made to employ it. This technique was, however, abandoned because of high-amplitude interfering signals reflected from the "crack"

surface (see discussion below).

The setup and the transducer arrangement are shown in Figures 10 and 11.

Measurements were taken after the amplitude was maximized by slight lateral

movements of the transducers. Results were verified by measurements from four

different transducer locations, with direct and double-skip beams (see

Figure 12), and from both sides of the specimen. In Table 1 a schematic presentation of the different beam paths and direct and ultrasonic measurement

results are given in x,y coordinates. The coordinate system is shown in

Figure 8. The x coordinate was determined from the positions of the transducers. The scatter of the x measurements was within ±2mm. The x data are arithmetic averages of five measurements. In Figure 13 values of depth (y

coordinate) made by direct and by ultrasonic measurements are compared. The

differences are not necessarily deviations but, perhaps, true differences in

the depth of the tip at the outer surface (measured directly) and at the

middle of the specimen (measured ul trasonical ly)

.

DISCUSSION

Although it was shown in several works^"^ that accurate and reliable

sizing of cracks with the diffraction technique is feasible, testing of welds

in materials less than 25 mm in thickness under field conditions presents

special difficulties which have not yet been completely .resol ved. This can

be seen in Table 1 where, in many places, measurements were not possible.

Some of the difficulties are described below and schematically presented in

Figure 14. Because of the coarser grains, the directional structure and the

microimperfections usually present in weld material, the ultrasonic noise from

the weld is more severe than in the parent material. This might be improved

by using a lower frequency.

4
The advantage of using the one- transducer technique is obvious. This

technique, however, might cause misleading results because of multiple re- flections from the crack surface. Because of the crown of the weld, scanning

of the weld with a direct beam is limited and often not possible and doubleskip beam paths must be employed. This increases noise and attenuation.

The high-amplitude back-wall reflection often masks the low-amplitude

tip-diffracted signal and resolution becomes difficult and often impossible,

especially in thinner material.

Some of the difficulties mentioned above might be diminished by changing

testing parameters such as frequency, diffraction triangle, mode combination,

transducer and wedges. Further investigation to diminish interfering factors

is recommended.

5
Table 1

Ultrasonic Measurements

B,C

1,2

3

W

N

- As welded specimens

- Machined specimens

- Normal "cracks"

- 45° "cracks"

- Weld imperfection

- Measurement not possible

(explained in text)

Specimen Coordinate Di rect Ultrasonic Measurements (mm)

Number Measurement

(mm)

I II III IV Average

lA y 9.1 M N 9.0

N

9

X 0.5 IN 1.0 1.0

y

X

9.4

0

95

0

N N N 9.5

0

y 20.2

N N N 20.0 20.0

X 0 0.5 0.5

w-y 7.0

N 7.0N 7.0 7.0

X -2.5 1.0 -1.0 0

IB y 9.4

N 10.5 9.5

N 10.0

X 0 -1.0 -1.5 -1.2

y 20.1 20.8 20.4

N 20.5 20.6

X 0 -1.5 0 0 -0.8

6
Table 1

I

Ultrasonic Measurements

(Continued)

Specimen Coordinate Direct Ultrasonic Measurements (mm)

N umber Measurement

(mm)

I II III IV Average

1C y 9.6

N

8.8 9.1 9.5 9.1

X 0 2.0 1.0 0 1.5

y 18.8 19.7

N N 19.0 19.4

X 0 0.5 0.0 0.3

2A y 12.1

N

12.0

N

11.5 11.7

X 0 0 -0.5 -0.3

y

X

18.4

0

N N N N N

2B y 11.7

N 11.2 11.0 11.5 11.2

X 0 0 0 0.5 0.2

y 18.1 18.3 17.4

N

18.5 18.1

X 0 -1.0 1.5 -0.5 0

2C y 11.0

N 9.0N 9.8 9.4

X 0 1.0 1.5 1.2

y

X

18.0

0

N

17.9

0

N N

17.9

0

w-y

N N

7.0 7.0 7.0 7.0

X 3.0 2.5 3.5 3.0

7
Table 1

Ultrasonic Measurements

(Continued)

Specimen

Number

Coordinate Direct

Measurement

Ultrasonic Measurements (mm)

(mm)

I II III IV Average

3A y

X

11.1

0

N N N 11.0

0

11.0

0

y

X

19.6

-8.4 N

19.4

-8.5 N 20.0-8.0

19.7

-8.3

w-y

X

Several points from weld imperfections

3C y

X

11.4

0

10.6

0

10.7

1.5

11.5

1.0

10.7

0

10.9

0.6

y

X

18.8

18.4

18.7

8.0 N N 18.5

8.0

18.6

8.0

8
Figure 1. Sizing of Slits With The Double Transducer Technique - PP array ( compress ional impinging, compressional

diffracted waves).

9
Figure

2.

Photoelastic

Visualization

of

P

Waves

(outer

circles)

and S

Waves

(inner

circles)

Diffracted

From

A

Tip

of

a

Slit

In

A

Glass

Block.
Figure

3.

Triangle

of

Diffraction

With A

P

Surface

Wave

As

Reference.
Figure

4.

Sizing

Of

Cracks

With

Tip

Diffracted

Waves

(A)

Single

Transducer

Technique

(B)

Double

Transducer

Technique.
DAB C

I Figure 5. Noisy Background of the Oscilloscope Trace. The expanded

I Section Contains: (A) A Signal Diffracted From The Tip

Of The Crack (B) Signals From Protrusions On The Crack

Surface (C) A Signal Diffracted From The Tip Of The Notch

and (D) Noise Signals From The Material. (C) Is The ZeroReference Point. Time AT Is Measured From (C) To The Left.

I

i

13
o c 0o o c 0

1) 0+-• ”5)

a

CO

O (/> c 0 > “O 0 "O

0)

w

«4- 0 0 0

"O 3 3 3

(/) £0 0 0 0 3 0 0 0

(/) 0 > > > 0> "co 0

"O 0

“O 0

“O

® I?- bB

B 3 0).. Q. a Q < CO CD .D 6

0pn).!|duje EE>oa
Figure

7.

Specimens

With

Inwelded

Cracks
Figure

8.

Cross

Section

of

As

Welded

Specimens

With

Normal

and

45°

Inwelded

Cracks
17

Figure

9.

Cross

Section

of

Machined

Specimens

With

Normal

and

45°

Inwelded

Cracks
Figure 10. Experimental Set-Up

Figure 11. Transducer Arrangement

1

18
Figure

12.

Oscil

loscopic

Trace

of

(a)

Direct

Beam,

(b)

Reference

Signals,

and

(c)

Double-skip

Beam.

The

signal

to

the

Left

and

to

the

right

of

the

tip-diffracted

signal

is

the

back-wall

reflection

with

transducers

shifted

from

their

maximum

amplitude

positions.
Figure 13. Ultrasonic depth measurements compared with direct depth

measurements of "inwelded cracks". Direct measurements

were taken at the outside surface and ultrasonic

measurements in the middle of the specimen.

20

I'l
Noise from grains and

small imperfections.

Multiple reflections from crack

surface when one transducer is

used.

Large dead zone because of weld

crown when a direct beam is used.

Poor resolution. The strong

backward signal mask the weak

diffracted signal.

gure 14. Schematic Presentation of Some Problems In Sizing

Cracks in Welds.

21
References

1. Golan, S. , "Sizing of Cracks with Scattered Ultrasonic Waves,"

Proc. of First International Symposium on Ultrasonic Materials

Characterization, National Bureau of Standards, Gaithersburg,

MD, June 1978 (in press).

2. Silk, M. G. and Lidington, B. N., "Defect Sizing Using an Ultrasonic Time Delay Approach," British J. NDT, 17, 33-36

(1975).

3. Silk, M. G. , "Sizing Crack-like Defects by Ultrasonic Means,"

Chapter 2 in Research Techniques in Nondestructive Testing , 3,

R. S. Sharpe, Ed., Academic Press, New York (1977).

4. Golan, S. , Baruch, J. and Yaroh, S. , "Sizing of Cracks with

Time Delay of Scattered Ultrasonic Waves," Israel Institute of

Metals - Technion, Internal Report 014-146 (1977).

5. Golan, S. , Adler, L. and Cook, V., "Ultrasonic Diffraction

Technique for Characterization of Fatigue Cracks," to be

publ ished.

6. Y. R. Birchack and C. G. Gardner, "Comparative Ultrasonic

Response of Machined Slots and Fatigue Cracks in 7075 Aluminum,

Materials Evaluation , 34, 275-280 (1976).

7. Krautkramer, Ultrasonic Testing of Materials , Second Edition,

Spri nger-Verlag, 186-189 (Berlin 1969).

22
NBS*114A (REV. 9-78)

u.s. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. BIBLIOGRAPHIC DATA

SHEET NBSIR 80-1983

4. TITLE AND SUBTITLE

liB.. i. No.

5. Publication Date Defect Characterization And Dimensioning Of Cracks In Wei

By The Ultrasonic Diffraction Methods

7. AUTHOR(S)

S. Golan

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, state, ZIP)

8. Performing Organ. Report No.

10^ Pn>tect/T^/Wofk itoit No.

11. Contract/Grant No.

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here,)

The possibility of applying an ultrasonic diffraction method for dimensioning

of crack-like defects in welds was investigated. A feasibility study was carried

ouit and optimum test conditions were established using a series of test specimens

with narrow slits. A series of welded specimens with in-weld cracks were tested.

The possibility of using a quantitative diffraction technique for nondestructive

examination of pipeline welds in field conditions is discussed.

17, KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Compressional (P) modes; cracks; defect characterization; dimensioning;

one-transducer technique; shear (S) mode; ultrasonic diffraction; and welds.

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