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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: [email protected]; [email protected]
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.
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