Development of an Ultrasonic Time of Flight Diffraction (TOFD)-

based Inspection Technique for Sizing Crack in a Complex Geometry

Component with Grooved Inspection Surface

Samir K.NATH

1 *, Krishnan BALASUBRAMANIAM

2

1 Central Power Research Institute, Thermal Research Centre, Nagpur-441 111,

Maharastra, India 2 Centre for Nondestructive Evaluation, Indian Institute Technology, Madras, Chennai-

600 036, Tamil Nadu, India

Corresponding authors: * Tel: (91) 7109-262553 (O), (91)712-2225914 (R), fax:

(91) 7109-262170, E-mail address: sknath2000@yahoo.com; sknath@cpri.in

Abstract: This paper presents the development of a manual TOFD based inspection technique for sizing a

surface-breaking crack in a complex geometry component with uneven/grooved inspection surface. The

simplified geometrical model has been developed for sizing the crack. Suitable couplant and probe

frequency for such inspection surface have been identified by experimental trials. Crack sizing was

successfully carried out on a real life complex geometry component, namely steam turbine rotor.

Identification and significance of various signals during laboratory experiment and field inspection have

been presented and discussed. The technique so developed here may be used in in-service inspection of

turbine rotor or similar industrial structures.

Keywords: ultrasonic time of flight diffraction (TOFD), simplified geometrical model

1. Introduction

The surface unevenness of an object under non-destructive evaluation poses as

one of the main limitations of ultrasonic approach regarding achieving improvements to

the probability of detection and flaw size estimation accuracy [1]. The most serious

source of error in sizing a defect would appear to be provided by unsuspected variation

in the surface of the material under test [2]. The surface unevenness may be due to the

service condition e.g. corrosion, erosion, pitting etc. in the boiler tube or due to the

design requirement e.g. gland seal area of a steam turbine rotor.

Suitable models with respect to a specific geometry need to be developed for

sizing of a detected defect. Several modeling softwares have been reported for TOFD

inspection of nozzle attachment weld, off shore structures [3-6]. In the thermal power

station, the most critical and complex structure is the turbine rotor. The rotor is

susceptible to cracking during its normal operation due to low cycle thermal fatigue

(TF) stresses. Sizing of such a surface-breaking crack by manual TOFD has been

reported [7]. In the said investigation the inspection surface used was even, and in fact

relatively smooth. However, at the gland seal area of a real turbine rotor the inspection

surface is made of machined grooves as shown in Figure 1 (a and b). Detection and

sizing of any crack in such complex structure requires much more detailed investigation

with respect to tracing the diffracted beam, variation in ultrasonic velocity under the

influence of such inspection surfaces.

(a)

(b)

Location of

crack

Inspection surface

with mixture of oil

and grease as

couplant

(c)

Fig.1 Photographs of the (a) rotor shaft (b) cracked portion (c) schematic illustration of

rotor.

In the present study, an aluminum specimen simulating the complex geometry at

gland seal area of a steam turbine rotor has been fabricated. Schematic diagram and

photograph of the specimen are shown in Figure 2 (a and b). The specimen contains

seven vertical and one inclined notch.

(a)

(b)

Fig.2 Complex geometry aluminum specimen containing vertical and inclined

notches/slits of various locations and sizes: X (distance between two notches) =

15 mm (a) schematic diagram (b) photograph of the specimen with EDM notches

The dimensional details of the vertical notches are given in Table 1 and the

present study is confined to these notches only.

Table 1 Dimensions of the notches in the complex geometry aluminum specimen

Slit No. Depth/Height (h), mm Length (l), mm Orientation

A 5.1 15 Vertical

B 7.5 15 Vertical

C 9 15 Vertical

D 12 15 Vertical

E 15.5 15 Vertical

F 16.2 15 Vertical

G 18.5 15 Vertical

The manual TOFD method used in the experiment is of contact type. Hence its

capability in detecting the crack largely depends on the ultrasonic energy loss within a

contact layer between the probe and the inspection surface of the object being

investigated. The variation in signal amplitude and transit time in comparison to a

plane/smooth inspection surface of similar complex geometry specimen has been

discussed.

Experiments have been conducted with both 2 MHz and 4 MHz probes to study

the variation in signal amplitude with ultrasonic frequency. Keeping the probe

frequency and the equipment gain setting constant, the scanning was carried out on a

similar specimen having plane i.e. ungrooved inspection surface also. The objective was

to study the effect of grooved inspection surface on signal amplitude and transit time.

Another objective of this study is to identify a suitable couplant for such inspection

surface which ensures maximum detectability of defects and is convenient to use.

Various types of couplants namely water, oil, grease, M-seal (a commercial sealant used

in metals) were used during experiments to investigate their resulting effects on the

signal amplitude and transit time. M-seal regular epoxy compound is a two component

room temperature curing, easy to use multi-purpose epoxy putty. It is useful in sealing,

gripping, joining and insulating variety of surfaces like ferrous and non-ferrous metals,

porcelin, ceramic, marble, granite, ivory, asbestos, glass, wood, leather, certain plastic

etc.

Finally, a case study for detection and sizing of surface breaking cracks by

manual TOFD technique in an intermediate pressure (I.P.) steam turbine rotor shaft of a

110 MW unit has been discussed.

2. Simplified geometrical model

The simplified geometrical model as shown in Figure 3 was used in previous

investigation [7] for similar specimen with plane inspection surface.

Fig.3 Simplified geometrical model for the inspection of complex geometry

components. Schematic illustration for (a) transducer arrangement and interaction of the

incident beam with the crack (b) A-scan image

For this probe arrangement, the transit times of different signals can be

calculated assuming ray-tracing behaviour as follows.

(a) The transit time of the lateral wave (LW) signal to the receiver

C

rSrHrrStLW

2'2'22 )()()( +++++−= ……………………(1)

(b) The transit time of the bottom-tip diffracted signal to the receiver

C

rShrHhrrSt

2'2'22

1

)()()()( +++++++−= ………………(2)

(c) The transit time of the back wall (BW) echo to the receiver

C

rSHHHrStBW

2'2'22 )()()( +++++−= …………………. (3)

(d) The depth of the tip of the crack (h) from the inspection surface can be

determined by solving Eq. (2)

(e) The probe delay ( 0t ) can be determined by any standard procedure.

Where,

C = Velocity of the longitudinal wave

S = Distance of the transmitter from the edge of the geometric step

S´ = Distance of the receiver from the edge of the geometric step

H = Thickness of the specimen

H´ = Height of the geometric step

r = Radius of curvature of the groove area

h = Depth of the crack

0t = Time delay in probe shoe, couplant and machined slot

In order for contact probes to act as efficient transmitters and receivers of

ultrasound on a work piece of typical surface finish, there must be a thin film of some

coupling medium, usually a fluid or gel, between the probe face and the work piece.

Normally this coupling film is so thin that its influence on the timing of the ultrasonic

signal is negligible. However, there may be circumstances where a thicker film is

necessary [8]. In the present study the variation in velocity and thus in transit time of

different signals is caused by the thick layer of couplant filled in the grooves. The

simplified geometrical model in the present study for the grooved inspection surface is

similar to the above one. However, due to the additional complexities because of the

grooved surface, certain correction factor needs to be applied to the velocity of the

interrogating ultrasonic wave. The correction factor is derived in the following manner.

(i) The positions of the transmitter and receiver probes i.e. S and S´ are fixed

with respect to certain criteria e.g. the central axes of both the transmitter

and receiver probes meet at the bottom tip of one vertical notch.

(ii) The longitudinal wave velocity (C) in the specimen with grooved surface is

measured by normal beam probe. Subsequently, the transit time of lateral

wave (tLWE) and back wall wave (tBWE) with respect to the assumed S and S´

in a complex structure with plane inspection surface are calculated from Eq.

(1) and (3).

(iii) Number of actual experimental trials with the assumed S and S´ values and

measured C are carried out on the specimen having grooved surface. The

respective transit time for lateral wave (tLWE) and back-wall wave (tBWE) in

each trial are measured. Finally average or mean values of transit time in n

trials i.e. tLWE (av) and tBWE (av) are calculated.

(iv) Now, the velocity correction factors (KLWE and KBWE) are calculated as

follows

( )

LWE

LWELWE

t

avtK = ………………………………………(4)

and BWE

BWEBWE

t

avtK

)(= ………………………………………..(5)

(v) Now in the actual experiment, the transit times of various diffracted signals

(tDE) are measured. The geometric step size i.e. ‘H´’ and the radius of

curvature i.e. ‘r’ of the groove are known constants. The values of S and S´

are determined in (i) above. The height (h) of the notches is evaluated by

solving the following Eq.6.

( ) ( ) ( ) ( ) DEBWE

tC

KrShrHhrrS =

+′+++′+++−2222

……(6)

3. Laboratory Experiment

Experiment on the aluminum specimen (Fig. 2 (b)) was conducted by manual

TOFD technique. The ultrasonic equipment (Make: M/s Sonotron NDT, Israel, Model:

ISONIC-2001) along with manual scanner was used. The transducers used were 60º L-2

MHz (Krautkramer, WSY 60-2, 56525-02195, 56525-02196) and 60º L-4 MHz

(Krautkramer, WSY 60-4, 54575-2212, 54575-2210).

The main challenge in the present study is to transmit sufficient ultrasound

energy in the material through the grooved inspection surface due to its improper

contact with the probe surface. The experiment was conducted separately with two

transducer frequencies with the objective of finding the optimum one. The central

frequency of one transducer is 2 MHz and the other is 4 MHz. As expected the

performance of the 2 MHz transducer was better here due to its higher acceptance angle

compared to 4 MHz one. Thus the 2 MHz transducer was chosen in the subsequent field

trial on a steam turbine rotor.

The grooves of the inspection surface were filled with different types of

couplants namely water, oil, grease and M-seal. Unlike grease and M- seal, water and

oil could not be retained on the surface and hence by blocking the both-ends of the

grooved surface these low viscosity couplants were temporarily retained within the

groves during experiments. Application and complete removal of the M-seal from the

surface after the experiments was very difficult. Mixture of grease and oil was found to

be the most suitable couplant due to the convenience in its application and removal.

Moreover, there was no significant variation in the experimental results with the

application of different couplants. Hence, the subsequent experiments were conducted

with the mixture of oil and grease as couplant.

The optimally chosen positions of transmitter (S) and receiver (S´) are fixed with

the help of a probe holding device as shown in Figure 4(a). Apart from keeping the

probes at the same relative positions, this device ensures that both the probes lie in the

same plane too.

(a)

(b)

Fig.4 (a) Schematic diagram of a probe holding device for complex geometry

component (b) Portion of the IP rotor and illustration of the inspection plan

Now with a particular probe set-up (with fixed S and S´) scanning is performed

by moving the probe pair along the length of the notch and the transit time of the lateral

wave, back wall wave and the diffracted signal from the tip of the notch are measured

from the corresponding A-scan image. The authors used the first “zero crossing” as

reference of each signal irrespective of its phases for measuring the transit time [9].

Total nine trials for each notch were conducted with the following two experimental

settings.

Case (A): 60º L-2 MHz probe with S = 31 mm and S´ = 56 mm

Case (B): 60º L-2 MHz probe with S = 40 mm and S´ = 64 mm

S and S´ setting was chosen in such a way that the central axes of ultrasonic

beam for both transmitter and receiver probes meet at the bottom tip of 10 mm deep slit

(case-A) and 15 mm deep slit (case-B) to have better diffraction there due to the

optimum included angle [10]. The entire range of depth/height of the slits was covered

with these settings. Two settings (case-A and B) were chosen with an objective to study

the effect of probe spacing in sizing of cracks.

Multiple trials on a specific notch/slit may give rise to multiple signal responses.

Actually in the experiment total nine trials were conducted. These multiple responses

did not vary widely to affect the inspection result much. Thus the number of the trials

was restricted to nine.

The heights/depths of the notches were determined by solving Eq. (6) as

explained in 2.0 above. The correction factor (KBWE) based on the transit time of the

back wall echo (tBWE) has been used in the equation. This is because both in case of

crack-bottom tip diffraction and back wall reflection, transmitting and receiving the

signals at the inspection surface will be approximately similar.

The transit time of the back-wall echo (tBWE) required in Eq. (5) was measured in

the experiment conducted on the similar complex geometry specimen with plane

inspection surface. The same was determined by solving Eq. (3) in Matlab®

also. Values

of S and S´ in case-A were used in both the above experiment and Eq. (3) to determine

(tBWE). As expected both values of tBWE are almost same [7]. Similarly for another set of

S and S´ in case-B tBWE was evaluated.

The transit time of the lateral wave echo (tLWE) for both case-A and case-B was

also determined in a similar manner. It was observed that the transit time of the lateral

wave as computed from Eq. (1) resulted in a shorter time than observed in the

experiments. This is due to the fact that , the lateral wave, in the experiments, travels

around the curved surface of the groove, which is longer than the path assumed in the

equation [7].

4. Field Trials

One Intermediate Pressure (IP) steam turbine rotor of a 110 MW unit of a

thermal power plant had developed crack near the gland seal area. The gland seal area is

grooved which is periodic in nature. The rotor has thirteen stages with a hollow shaft

and integral discs. The diameter of the rotor is different at different locations along its

length. Photographs of the rotor and its cracked location are shown in Figure 1 (a and

b). Schematic illustration with certain important dimensions is given in Figure 1 (c).

The crack was surface breaking covering almost all along the circumference as

confirmed by visual examination (VE) and other Non-destructive Evaluation (NDE)

technique namely Dye Penetrant Test (DPT) and Magnetic Particle Test (MPT). The

depth of the crack was required to be ascertained for assessing the fitness for purpose.

The conventional NDE techniques were not capable of measuring the depth of such

crack. Assuming the crack to be vertical i.e. lying in the radial-circumferential plane, 20

dB drop method in normal beam ultrasonic pulse-echo (PE) could be used for

measuring the depth [7]. However, there was insufficient space in the sidewall to place

the normal beam ultrasonic probe.

Ultrasonic TOFD method was employed for measuring the depth of the crack.

Initially the geometric dimensions e.g. ‘H´’ and ‘r’ at the inspection area of the rotor

were measured. The ultrasonic longitudinal wave velocity in the rotor material was

determined independently. The method used for the rotor was same as used in the

laboratory experiments. The inspection plan is illustrated in Figure 4 (b). The probe

spacing (S and S') was determined in such a way that the central axes of both transmitter

and receiver probes meet at the tip of 5 mm deep crack. The probes were held firmly at

these spacing in the customised probe holding device. The grooved surface on either

side of the cracked location was filled with a mixture of oil and grease. Then scanning

was carried out by moving the probe pair fixed in the holding device all along the

circumference applying a uniform manual pressure. Figure 5 shows the actual

inspection of the rotor.

Fig.5 Photograph of the TOFD inspection of rotor using the special fixture

The transit time of lateral and diffracted wave signals were measured. The depth

of the crack was determined by solving Eq. (6). The parameters used and the findings

during the inspection are given in Table 6 (a and b). The maximum depth of the crack

was found to be 5.4 mm.

Table-6 (a) Scan parameters (b) Findings in the field trial

Scan parameters

Velocity

(m/sec)

S (mm) S´ (mm) H´ (mm) r (mm)

5880 46.9 67.4 28 14

(a)

Findings

Measured Transit Time

(µs) of lateral wave

Transit Time (µs)

of diffracted wave

Correction factor

used in Eq. (8)

Measured height

(mm)

21.66 23.2 1.03 5.4

(b)

5. Observation

Similar experimental settings e.g. gain, probe center spacing etc. for both plane

and grooved surface cases were maintained. However, appreciable fall in the signal

amplitude and rise in the transit time were observed in case of grooved inspection

surface as compared to the plane surface. The comparative A-scan images are shown in

Figure 6 (a and b).

(a)

1

2

2 1

13.2 14.5 15.8 17.1 18.4 19.7

Time (µs)

Am

pli

tud

e (%

FS

H)

0

2

0

40

60

8

0

10

0

1

2

2 1

12.8 14.3 15.8 17.3 18.8 20.3

Time (µs)

A

mp

litu

de

(%F

SH

)

0

2

0

40

6

0

80

100

(b)

Fig.6 TOFD A-scan images at the defect free area with 60º L-4 MHz probe with

S = 31 mm and S´ = 56 mm for (a) smooth/plane inspection surface (b) grooved

inspection surface; FSH : Full Screen Height

In the experiment both 2 MHz and 4 MHz probes were used and it was observed

that the signal amplitude thus the detectability of the notch was less in case of 4 MHz

probe in comparison to 2 MHz probe. This is due to less acceptance angle in 4 MHz

case compared to 2 MHz case. In case of 4 MHz probe, the diffracted echo for deeper

notches could not be detected reliably. A-scan images for both 2 MHz and 4 MHz

probes with the similar experimental settings e.g. gain, probe center spacing etc. are

shown in Figure 7 (a and b).

(a)

(b)

Fig.7 TOFD A-scan images of notch-A in the complex block with grooved

inspection surface with S = 31 mm and S´ = 56 mm (a) 60º L-2 MHz

probe (b) 60º L-4 MHz probe; FSH : Full Screen Height

The results of all the trials in case-A are compiled in Table 2.

1

2

1 2

Am

pli

tud

e (%

FS

H)

0

20

4

0

60

80

1

00

15.0 15.7 16.4 17.1 17.8 18.5

Time (µs)

1

2

1 2

A

mp

litu

de

(%F

SH

)

0

20

4

0

60

8

0

100

12.8 14.3 15.8 17.3 18.8 20.3

Time (µs)

Table-2 Estimation of height of simulated crack in TOFD technique in Case-A in

different trials

Slit

No.

Parameters Trial-

1

Trial-

2

Trial-

3

Trial-

4

Trial-

5

Trial-

6

Trial-

7

Trial-

8

Trial-

9

A TransitTime (µs) 15.6 16.1 16 15.8 16.1 15.7 16 15.8 16

Height (mm) (h) 7.7 10.9 10.3 9 10.9 8.3 10.3 9 10.3

B TransitTime (µs) 15.8 16.4 16.2 16.1 16.3 16 16.2 16 16.2

Height (mm) (h) 9 12.7 11.5 10.9 12 10.3 11.5 10.3 11.5

C TransitTime (µs) 16.2 16.7 16.6 16.3 16.6 16.3 16.5 16.3 16.4

Height (mm) (h) 10.9 14.3 13.8 12 13.8 12 13.2 12 12.7

D TransitTime (µs) 16.4 17 16.9 16.7 16.9 16.6 16.8 16.6 16.7

Height (mm) (h) 12.7 16 15.4 14.3 15.4 13.8 15.2 13.8 14.3

E TransitTime (µs) 16.9 17.5 17.3 17.1 17.4 17 17.3 17.1 17.3

Height (mm) (h) 15.4 18 17.5 16.5 18 16 17.5 16.5 17.5

F TransitTime (µs) 17.2 17.7 17.6 17.5 17.7 17.4 17.6 17.5 17.5

Height (mm) (h) 17 19.5 19 18.5 19.5 18 19 18.5 18.5

G TransitTime (µs) 17.3 17.8 17.7 17.5 17.8 17.4 17.6 17.5 17.6

Height (mm) (h) 17.5 20 19.5 18.5 20 18 19 18.5 19

There is no significant variation in the estimation of notch height between case-

A and B as shown in Figure 8 (a and b) and Table 3. However there is more spread in

measured heights in case-B than in case-A as shown in Table 4.

(a)

(b)

Fig.8 Evaluation of height of notches (a) Case-A (b) Case-B

Table-3 Mean values and errors in measured heights in TOFD technique

Table-4 Spread in measured heights in TOFD technique

Notch

No.

Actual

height(mm)

Case-A Case-B

Max.height

(mm)

Min.height

(mm)

Spread

(mm)

Max.height

(mm)

Min.height

(mm)

Spread

(mm)

A 5.1 10.9 7.7 3.2 14.9 6.4 8.5

B 7.5 12.7 9 3.7 16.1 7.4 8.7

C 9 14.3 10.9 3.4 16.7 9.2 7.5

D 12 16 12.7 3.3 18.2 12 6.2

E 15.5 18 15.4 2.6 20.4 14.3 6.1

F 16.2 19.5 17 2.5 21.7 16.4 5.3

G 18.5 20 17.5 2.5 22.2 16.7 5.5

Slit

No.

Actual Height

(mm)

Case-A Case-B

Mean of Measured

Heights (mm)

Error

(mm)

Mean of Measured

Heights (mm)

Error

(mm)

A 5.1 9.7 +4.6 10.1 +5

B 7.5 11 +3.5 11.3 +3.8

C 9 12.8 +3.8 13.1 +4.1

D 12 14.6 +2.6 14.8 +2.8

E 15.5 17 +1.5 17.1 +1.6

F 16.2 18.6 +2.4 19 +2.8

G 18.5 19 +0.5 19.2 +0.7

Mean values and errors/velocity correction factor in transit time of Lateral Wave

and Back Wall Wave in case-A and case-B are shown in Table 5 (a and b). It was also

observed that there was no marked variation in signal amplitude and transit time while

using different couplants namely oil, water, grease etc.

Table-5 Mean values and errors/velocity correction factor in transit time of

Lateral Wave and Back Wall Wave (a) Case-A (b) Case-B

Trials Lateral Wave Back Wall Wave

Transit

Time

(µs)

Mean

(µs)

Transit

time

from

Eq. (1)

(µs)

Error

(µs)/Correction

factor

Transit

Time

(µs)

Mean

(µs)

Transit

time

from

Eq. (3)

(µs)

Error

(µs)/Correction

factor

1 15.2 15.54 14.44 1.10/1.08 18.4 18.65 18.34 0.31/1.02

2 15.8 18.9

3 15.7 18.8

4 15.5 18.6

5 15.7 18.8

6 15.4 18.5

7 15.6 18.7

8 15.4 18.5

9 15.6 18.7

(a)

Trials Lateral Wave Back Wall Wave

Transit

Time

(µs)

Mean

(µs)

Transit

time

from

Eq. (1)

(µs)

Error

(µs)/Correction

factor

Transit

Time

(µs)

Mean

(µs)

Transit

time

from

Eq. (3)

(µs)

Error

(µs)/Correction

factor

1 18 18.42 17.05 1.37/1.08 20.6 21.09 20.40 0.69/1.03

2 18.6 21.2

3 18.1 20.8

4 19 21.6

5 19 21.6

6 17.9 20.7

7 18.4 21.1

8 18.4 21.1

9 18.4 21.1

(b)

During the field trial, marked fall in the signal amplitude was observed at the

cracked location of the turbine rotor in comparison to its uncracked location. This is

shown in Figure 9 (a and b). However, there was minor shift of the diffracted signal to

the right as compared to the lateral wave signal.

(a)

(b)

Fig.9 TOFD A-scan images of the rotor with 60º L-2 MHz probe (a) uncracked

portion (b) cracked portion of the rotor

6. Results and Discussion

Good correlation has been observed between the estimated dimensions in the

experimental results and the actual ones regarding the height of the simulated cracks.

Probe frequency of 2 MHz along with mixture of oil and grease as couplant was found

to be more suitable in detecting and subsequently sizing almost all the notches.

Anisotropy of velocity in practical measurements on rolled plate samples has been

discussed by Silk [11]. Variation in velocity with respect to the rolling direction was

reported. In the present study too variation in bulk longitudinal wave has been observed

between un-grooved and oil/grease-filled grooved surface. The grooves of the

inspection surface dampen the amplitude and distort the shape of the signal. Moreover it

decreases the original longitudinal velocity in the materials. This has resulted in

increased transit time of the signal in comparison to the plane inspection surface under

similar test condition.

1

1

14.2 29.2 44.2 59.2 74.2 89.2

Time (µs)

A

mp

litu

de

(%F

SH

)

0

20

4

0

60

8

0

10

0

1 1

14.2 29.2 44.2 59.2 74.2 89.2

Time (µs)

A

mp

litu

de

(%F

SH

)

0

2

0

40

60

8

0

100

The grooved surface here is periodic in nature. And due to this topography with

repetitive structure the wave propagation will not be same as seen in case of plane

inspection surface. The specimen contains seven number of surface breaking vertical

notches of varying depths/heights. The bottom tips of all the notches are lying well

within the homogenous material of the specimen away from its grooved surface. The

diffraction is taking place at those bottom tips of the notches and the time of flight i.e.

the transit time of the diffracted signals are measured for determining the depths of the

notches.

In both the cases of the experimental settings, it observed that error in estimating

the notch height is more in case of the shallower notches as compared to the deeper ones

as shown in Figure 8 (a and b). This is compliant with the proven practical limitation in

accurately sizing shallower defects by TOFD technique. However Baskaran et al [12-

15] have reported the possibility of sizing of defects in thin section components by

TOFD. TOFD inspection in thin sections with similar inspection surfaces thus needs to

be further investigated.

The correction factor used in the Eq. (6) for the computation of the notch heights

is based on the transit times of the back-wall wave signal. In Table 5 it is observed that

the error in the transit times of the back wall wave in case-B is more compared to that of

in case-A. This has resulted in more spread in case-B than in case-A in the estimation of

notch height. However, there is little difference between the two cases in mean values

and errors in measured heights.

In the present study only vertical crack in the depth range of 5 mm to 20 mm can

be measured by a 60°, 2 MHz probes with a probe position described in the paper.

Probes with higher frequency e.g. 4 MHz will not be suitable as the detectability of the

diffracted signal is reduced here.

The present method is not capable of sizing inclined crack. The mathematical

expressions for sizing inclined crack are different from the ones meant for vertical crack

discussed here. The main challenge in sizing inclined crack in the present complex

geometry component is to generate the diffraction arc by B-scan (i.e. movement of the

probe pair across the crack) due to restriction in probe movement. Without getting the

diffraction arc it is not possible to size an inclined crack. Nath et al [16] has developed a

method for sizing surface-breaking inclined crack in simple geometry component with

smooth inspection surface.

The main challenge in the present study is to transmit sufficient ultrasound

energy in the material through the grooved inspection surface due to its improper

contact with the probe surface. We cannot smoothen it out by machining in a manner

sometimes carried out during weld inspection by grinding off the weld crown. Hence,

laboratory experiment on a grooved surface was carried out to develop a method which

can be used finally in a real life industrial component where grooved surface is a design

requirement e.g. gland sealing in steam turbine rotor. The industrial application is also

demonstrated in the paper.

The contact between grooved surface and the probe surface can be improved by

filling the grooves with suitable couplant for transmission of ultrasound energy. This

filling of the grooves should be temporary and not damage the surface. This means,

after the inspection is over, the original surface should be restored back for its intended

purpose (e.g. gland sealing). The challenge or limitation here is to fill the grooves

perfectly with suitable couplant. If the grooves are not filled completely i.e. certain gaps

(air pockets) are present, the transmission of the ultrasound will be highly affected.

Meeting of the central axes of both transmitter and receiver probes at the tip of

the crack enhances the diffraction coefficient which improves the detectability of the

crack. In case of narrow beam transducers, if the axes do not meet at the crack tip the

probability of detection will be very low. For an unknown depth of the crack, the

meeting of the axes at the crack-tip can be ensured either by first altering the probe

position for fixed probe angle or, secondly by varying the probe angles for a fixed probe

position. While inspecting a real component, the first option may not be always feasible

due to the space constraint of the inspection surface hindering altering the probe

position. Thus in a real setting various angles for both transducers should be used to

make sure that a crack is not missed.

The technique developed here has successfully been demonstrated in the field

trial.

7. Conclusions

It is possible to detect and size 5 mm to 20 mm deep vertical crack under an

uneven/grooved inspection surface of a complex geometry structure by the simplified

geometrical model developed in this study. Through number of experimental trials

2 MHz has been identified as suitable transducer frequency and mixture of oil and

grease as suitable couplant for such inspection surface. A velocity correction factor is

derived here. Incorporation of this velocity correction factor in the computation

improves the sizing accuracy to a large extent.

The technique developed here can be used in in-service inspection of turbine

rotor or similar industrial structures for ensuring fitness for purposes and improved

availability.

The work has been carried out for vertical crack only. However, the crack may

not be necessarily vertical; rather it could be inclined too. Hence further work needs to

be carried out for accurately sizing inclined cracks under similar surface condition.

Experiments with different probe angles may be conducted to study their effect

on crack detection and sizing. Moreover, reliability analysis of such inspection in terms

of probability of detection (POD) and probability of sizing (POS) of cracks should also

be carried out.

Acknowledgement:

The authors thank the management of Central Power Research Institute for

providing infrastructural support and according kind permission to publish this paper.

References:

1. V.G. Shcherbinskii ‘Influence of acoustic transparency of surface contact layer

on flaw detectability in ultrasonic testing’, Insight, Vol 40 No 6, pp 411-417,

1998.

2. B.H. Lidington, M.G. Silk, P. Montgomery and G. Hamond, ‘Ultrasonic

measurement of depth of fatigue crcaks’, British Journal of NDT, Nov. 1976.

3. T. Bloodworth, L. Johansson and A. Pettersson, ‘High-accuracy defect sizing for

nozzle attachment welds’, EU-JRC International conference, 20-22 Oct. 1998,

Amsterdam, Netherland.

4. P. Hansen, ‘3D inspection system for ultrasonic examination and evaluation of

complex geometry components’, EU-JRC International conference, 20-22 Oct.

1998, Amsterdam, Netherland

5. N. Gengembre, A. Lhemery and X. Tarrago, ‘A model to predict ultrasonic

fields radiated by water-coupled transducers into pieces of complex structure’,

EU-JRC International conference, 20-22 Oct. 1998, Amsterdam, Netherland

6. A.A.A. Scheerder, ‘Equipment condition monitoring using TOFD-experiences at

DSM’, 15 th. WCNDT, Roma 2000.

7. S. K. Nath, K. Balasubramaniam, C.V.Krishnamurthy and B. H. Narayana

‘Sizing of surface-breaking cracks in complex geometry components by

ultrasonic Time-of-flight Diffraction (TOFD) technique’, Insight, Vol 49 No 4,

pp 200-206, 2007.

8. J.P. Charlesworth and J.A.G. Temple, ‘Engineering Applications of Ultrasonic

Time of Flight Diffraction’, 2nd

. ed., Research Studies Press Ltd., England, 2001.

9. S.K. Nath, K. Balasubramaniam, C.V. Krishnamurthy and B.H. Narayana,

‘Detection and Sizing of Defects in Complex Geometry Weld by Ultrasonic

Time of Flight Diffraction (TOFD) Inspection’ Journal of Pressure Vessel

Technology, Vol 131, pp 051501-1-051501-9, 2009.

10. ‘Guide to Calibration and setting-up of the Ultrasonic Time of Flight Diffraction

(TOFD) technique for the detection, location and sizing of flaws’. British

Standard, BS 7706, 1993

11. M.G. Silk, ‘The transfer of ultrasonic energy in the diffraction technique for

crack sizing’, Ultrasonics, pp 113-121, May, 1979.

12. G. Baskaran, ‘Studies on Ultrasonic Time of Flight Diffraction Imaging of

Defects in Thin Sections’ Ph.D. Thesis, Indian Institute of Madras, Chennai-600

036, May, 2005.

13. G. Baskaran, K. Balasubramaniam and C.L. Rao, ‘Ultrasonic TOFD flaw sizing

and imaging in thin plates using Embedded Signal Identification Technique

(ESIT)’, Insight, Vol 46, pp 537-542, 2004.

14. G. Baskaran, K. Balasubramaniam, C.V.Krishnamurthy and C.L. Rao, ‘A Ray

Based Model for the Ultrasonic Time of Flight Diffraction Simulation of Thin

Walled Structure Inspection’, ASME Trans. Journal of Pressure Vessel

Technology, Vol 127 No 3, pp 262-268, 2005.

15. G. Baskaran, K. Balasubramaniam and C.L. Rao, ‘Shear-wave time of flight

diffraction (S-TOFD) technique’, NDT & E International, Vol 39, pp 458-467,

2006.

16. S.K. Nath, K. Balasubramaniam, C.V. Krishnamurthy, B.H. Narayana, ‘An

Ultrasonic Time of Flight Diffraction Technique for Characterisation of Surface-

Breaking Inclined Cracks’, Materials Evaluation, Vol 67, No 2, pp 141-148,

February 2009.