E-Journal of Advanced Maintenance Vol.13 No.2 (2022) 14-25
Japan Society of Maintenology

Evaluation of the Capability to Estimate a Crack Profile by 3D SAFT-UT
Yoshinori KAMIYAMA1, Toshihiro YAMAMOTO1,* and Takashi FURUKAWA1
1

Japan Power Engineering and Inspection Corporation, 14-1 Benten-cho, Tsurumi-ku, Yokohama
230-0044, Japan
ABSTRACTIn nuclear power plants, inservice inspection (ISI) is periodically performed to maintain the pressure boundary
integrity of components and piping. During ISI, a nondestructive testing method must find flaws and provide
information required to determine flaw sizes when flaws are found. The aim of this study is to show synthetic
aperture focusing technique for UT utilizing FMC/TFM and a matrix array transducer can more robustly provide
accurate crack sizing results than commonly used UT methods. This advanced UT method (referred to as 3D
SAFT-UT) and phased array UT using a linear array transducer were performed on nickel-base alloy test blocks
with a thermal fatigue crack to compare their results. Phased array UT can obtain tip echoes only at rather flat partof a crack profile, whereas 3D SAFT-UT can obtain tip echoes over a wide range of a crack and estimate its crack
profile. To evaluate the sizing stability against the variation in the azimuth angle of the incident direction, thecapabilities to estimate a crack depth with different azimuth angles were compared between phased array UT and
3D SAFT-UT. The results show 3D SAFT-UT can provide relatively good crack profile and crack depth
estimations even if the transducer is not right in front of the crack.
KEYWORDS

ARTICLE INFORMATION

Synthetic aperture focusing technique, SAFT, FMC/TFM, ultrasonic
testing, UT, matrix array transducer, crack profile, thermal fatigue crack

Article history:
Received 27 September 2021
Accepted 26 January 2022

1. Introduction
In nuclear power plants, inservice inspection (ISI) is periodically performed to provide assurance
that the structural integrity is maintained. The ASME Boiler and Pressure Vessel Code (BPVC) Section
XI (or the JSME code of rules on fitness-for-service for nuclear power plants in Japan) formulates a setof requirements for maintaining the pressure boundary integrity of components and piping in nuclear
power plants [1, 2]. This includes the rules for nondestructive testing (NDT) methods used to perform
ISI, the flaw acceptance criteria, and the procedures for flaw growth analysis. When flaws are
discovered during ISI, they must be evaluated. In accordance with the flaw characterization procedures,
flaw indications are converted into flaw size information. If the flaw sizes exceed the acceptancestandards, analytical evaluation is performed by employing fracture mechanics to predict future flaw
growth and to determine whether a component is acceptable for continued service without arepair/replacement activity [3]. Therefore, an NDT method must provide information required todetermine flaw sizes for maintaining the structural integrity of nuclear power plants.
In the year 2008, Ohi Nuclear Power Plant experienced an incident that although stress corrosion
cracking (SCC) was detected in a nickel-base alloy weld by eddy current testing (ECT), remote visual
testing (RVT), and ultrasonic testing (UT) from the crack opening surface, its crack tip was not
identified in the UT results [4]. Initially, the crack depth was presumed to be too low to evaluate.
However, by grinding the surface around the crack, the maximum depth of the crack was ultimately
determined to be about 20.3 mm while the wall thickness was 74.6 mm, and also it was found that thecrack had a steep profile. A regulatory report mentions this incident offered a new insight that SCC ofa high aspect ratio (deep and narrow) may occur in a nickel-base alloy weld [5]. This report also
indicates that such a narrow crack tip returns only weak echoes, which was considered to be the reason
why the above-mentioned UT depth sizing failed. To avoid missing a crack tip in UT, it is desired toestablish a UT method that can grasp the profile of a crack and then focus on locating its crack tip.
*

Corresponding author, E-mail: yamamoto-toshihiro@japeic.or.jp
ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

14

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT

Synthetic aperture focusing technique (SAFT) is an imaging technique which was developed toprovide high-resolution images with small-aperture receivers at different positions and has beensuccessfully applied in imaging applications such as radar, radio astronomy, and UT [6, 7]. This
technique synthetically simulates a virtual aperture from a series of received signals during postprocessing so as to improve the resolution of images that capture observed objects. These received
signals are obtained with several receivers placed at different positions or a single receiver passing
through several observation points.
SAFT for UT (SAFT-UT) utilizes the imaging algorithms used in SAFT to enhance conventional
UT and provide more detailed information of detected flaws. The use of an array transducer, which has
multiple piezoelectric elements in a single transducer, provides SAFT-UT with an efficient means tocollect extensive data. An array transducer consisting of the elements aligned in one line is called alinear array transducer in particular. When the elements of an array transducer are arranged in a twodimensional grid, this transducer is called a matrix array transducer. The reference [8] describes fullmatrix capture (FMC) and total focusing method (TFM) for realizing an advanced SAFT-UT by the useof an array transducer. FMC is an acquisition methodology in which the complete set of A-scans(amplitude versus time records) from all combinations of transmit and receive elements in an arraytransducer is preserved for post processing. TFM is a post-processing algorithm that proceeds by first
discretizing the target region into a grid. All the signals obtained by FMC are then summed to synthesize
a focus at every point in the grid. FMC/TFM can provide a higher resolution than conventional SAFT.
A UT system produced in the development introduced in [9] utilizes a matrix array transducer and
FMC/TFM to provide a three-dimensional image that shows more detailed features of observed objects.
Based on this UT system, a 3D UT instrument “Matrixeye” was developed by Toshiba Corporation [10,11]. SAFT-UT utilizing a matrix array transducer and FMC/TFM (hereinafter referred to as 3D SAFTUT) is expected to capture the profile of a crack rather than just find the tip of a crack.
In the past studies [12, 13], 3D SAFT-UT was compared with conventional angle beam UT andphased array UT in terms of depth sizing accuracy for SCC in nickel-base alloy welds, and 3D SAFTUT achieved superior results in crack tip detection and depth sizing for SCC. The reference [13] also
shows that 3D SAFT-UT was performed on a test block that had a slit formed by electrical dischargemachining (EDM), and the depth sizing accuracy was maintained even when the transducer was not
right in front of the slit.
In this study, 3D SAFT-UT was performed on nickel-base alloy test blocks with a thermal fatigue
crack. To confirm that 3D SAFT-UT can robustly provide accurate sizing results, the crack sizing
accuracy and the crack profile estimation were evaluated even when the azimuth direction of the central
axis of the transducer was inclined from the perpendicular to the crack. In addition, phased array UTwith a linear array transducer was performed as a commonly used UT method under the same conditions
to compare the results between phased array UT and 3D SAFT-UT.

2. Evaluation of crack sizing accuracy and crack profile estimation
2.1. Experimental setup
Two test blocks, each of which had one thermal fatigue crack at the center, were inspected byphased array UT and 3D SAFT-UT from the opposite side of the crack opening. Figure 1 shows these
two test blocks B1553 and B1554. The test blocks are circular plates of 200 mm in diameter and about
20 mm in thickness. A small portion of the perimeter of each test block was cut off to make a straightedge parallel to the crack for positioning the transducer. The material of the test blocks is nickel-basealloy 600. Nickel-base alloy is widely used as welding material for dissimilar metal welding in nuclear
power plants. Because it is troublesome to make a test block using weld metal, nickel-base alloy basematerial was used for these test blocks.
Phased array UT was performed with a phased array UT instrument MC-256, manufactured by
Eishin Kagaku Co., Ltd., and a 32-channel linear array transducer. To change the frequency of ultrasonicwaves, two transducers with the nominal frequencies of 3 MHz and 5 MHz were used. In thesetransducers, 32 piezoelectric elements of 20 mm in length are aligned at 0.5 mm intervals. One of thetransducers was mounted on a polystyrene wedge with an incident angle of 16.5° to generate refracted
longitudinal waves. Figure 2 (a) shows the appearance of the transducer and wedge. In this measurement,

15

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UTthe transducer and wedge were placed on the test block so that the azimuth angle of the incident direction
was set to a certain angle from the perpendicular to the longer direction of the crack, and this skew
angle was changed as shown in Fig. 3. The transducer and wedge were mechanically moved on the test
block along the perpendicular to the incident direction as shown in Fig. 4 (a) for each skew angle, andelectronic sector scanning was performed at 1 mm intervals.
3D SAFT-UT was performed with Matrixeye 256-ch and a matrix array transducer. To change thefrequency of ultrasonic waves, two transducers with the nominal frequencies of 2 MHz and 5 MHz wereused. These transducers consist of 16×16 piezoelectric elements arranged at 3 mm intervals in eachdirection. The nominal refracted beam angle of the wedge attached to the transducer is 45° forlongitudinal waves in austenitic stainless steel (the refracted beam angle in alloy 600 is almost the sameas that in austenitic stainless steel). Figure 2 (b) shows the appearance of the transducer and wedge. The
transducer was placed so as to make the central axis of the transducer pass through the center of thecrack. The azimuth angle of the central axis was set to a certain angle from the perpendicular to thelonger direction of the crack, and this skew angle was changed as shown in Fig. 3. The measurement
was carried out at one transducer position for each skew angle as shown in Fig. 4 (b).

B1553

B1554

Fig. 1. Test blocks with a thermal fatigue crack(a) Transducer for phased array UT(b) Transducer for 3D SAFT-UT

Fig. 2. Transducers used in the experiments

Fig. 3. Incident directions in the experiments

16

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT

Moved

Incident
direction

Incident
direction

y
X

y
X

(a) Phased array UT(b) 3D SAFT-UT

Fig. 4. Acquisition of echo signals

2.2. Crack sizing accuracy with incidence right in front of the crack
The crack sizing accuracies were evaluated based on the inspection results obtained by performing
phased array UT and 3D SAFT-UT on the two test blocks B1553 and B1554 with the skew angle 0°.
Figures 5 and 6 show the D-scan images (echo amplitude distributions on the cross-sectional view of
the cracks) created from the results of phased array UT and 3D SAFT-UT, respectively. In these D-scan
images, the color changes from blue to red as the signal amplitude increases. In Fig. 5, tip echoes (echoesfrom a crack tip) are observed only at rather flat part of the crack profiles. As a general trend, phasedarray UT has difficulty obtaining echoes from a crack tip where the crack depth varies significantly. In
contrast, Fig. 6 shows that 3D SAFT-UT can obtain tip echoes over a wide range of the cracks.
In Fig. 7, the crack profiles estimated from the above-described inspection results of phased array
UT and 3D SAFT-UT are compared with the actual crack profiles obtained by destructive examination.
For phased array UT, the maximum amplitude point of a tip indication in each B-scan image (an echo
amplitude distribution on a plane orthogonal to a D-scan image with the Z axis as the vertical axis) was
regarded as a crack tip. For 3D SAFT-UT, the upper boundary of a crack indication area that exceeded
the noise level was regarded as crack tips. The maximum amplitude point of an indication is sometimes
not appropriate to determine a crack tip in an echo amplitude distribution obtained by 3D SAFT-UT
because 3D SAFT-UT provides indications of not only a crack tip but also a crack face. The noise level
was determined based on the echo amplitudes obtained in an area without flaws. Because slant edges
of a crack cannot be detected by phased array UT as shown in Fig. 5, only rather flat part of a crack
profile was obtained by phased array UT. On the other hand, because 3D SAFT-UT presents a relatively
clear indication from slant edges of a crack, a crack profile was obtained over a much wider range by
3D SAFT-UT. This provides more detailed information of a crack shape and suggests a possibility forrobust crack sizing. In the crack profiles estimated from the results of 3D SAFT-UT, local drops areobserved at 𝑥 = 6 and 𝑥 = 11 in Fig. 7 (a) and at 𝑥 = −10 in Fig. 7 (b). Crack tips may be partly
missed even by 3D SAFT-UT due to the shape of the detailed structure of crack tips and so on. Although
it is ideal to obtain a complete crack profile, the purpose of this study is to provide stable crack size
estimation and to avoid a wrong estimation as made in the incident experienced in Ohi Nuclear Power
Plant. The problem was to estimate only the maximum crack depth. Estimating a crack profile and thena crack depth prevents a big mistake in crack sizing even if crack tips are partly missed.
In Tables 1 and 2, the crack depths and lengths estimated from the above-described inspection
results are compared with the actual crack depths and lengths obtained by destructive examination.
Appendix VIII of the ASME BPVC Section XI provides requirements for performance demonstration
for UT in terms of UT procedures, equipment, and personnel used to detect and size flaws. According
to this, the root mean square error of the flaw lengths and depths estimated by UT, as compared withthe true values, shall not exceed 19 mm and 3 mm respectively when UT is performed on piping welds[1]. Table 1 shows that both phased array UT and 3D SAFT-UT overestimated the crack depths by lessthan 3 mm. As mentioned above, for 3D SAFT-UT, crack tips were determined by the upper boundaryof a crack indication area. This is considered to be the cause of overestimating a crack depth because acrack indication tends to spread out of the actual crack profile especially when the wavelength is longer

17

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT(i.e., in the case of 2 MHz). The crack lengths were estimated from the corner indications instead ofindications around crack tips. Both ends of a crack were determined by the amplitude drop techniqueand assumed to be the points where the indication amplitude drops to 6 dB or 12 dB below the maximum
value of the corner indication for the first time along the longer direction of the crack from the maximum
point. Table 2 shows 3D SAFT-UT provided better estimations than phased array UT and theseestimations are almost the same in both cases of 2 MHz and 5 MHz. Because phased array UT was
performed with a linear array transducer that comprises the elements of 20 mm in length and then theaperture width was 20 mm, the resolution of a crack length estimation was not so good in the case ofphased array UT. Phased array UT provided relatively close estimations in the case of −12 dB drop
rather than −6 dB drop. Because the crack depth becomes much lower near both ends of the cracks, theindication amplitude greatly decreases at both ends. This is considered to allow −12 dB drop techniqueto provide better results for phased array UT.

3 MHz

5 MHz
Tip echoes

Corner echoes

Z (mm)Z (mm)

B1553

Corner echoes

Tip echoes

Z (mm)

Tip echoes

Z (mm)

B1554

Bottomsurface

X (mm)X (mm)

Corner echoes

Tip echoes

Corner echoes

Bottomsurface

Bottomsurface

X (mm)

Bottomsurface

X (mm)

Fig. 5. D-scope images by phased array UT with incidence right in front of the crack

2 MHz

5 MHz
Tip echoes
Corner echoes

Corner echoes

Tip echoes

B1553

Tip echoes

Tip echoes
Corner echoes

Corner echoes

B1554

Fig. 6. D-scope images by 3D SAFT-UT with incidence right in front of the crack18

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT
10

10ActualActual
PA 3MHzPA 5MHz

8

Crack depth (mm)Crack depth (mm)

PA 3MHz3D SAFT 2MHz
3D SAFT 5MHz

6
4
2
0

PA 5MHz8

3D SAFT 2MHz
3D SAFT 5MHz

6
4
2
0

-20

-10

0
X (mm)

10

20

-20

(a) B1553-100
X (mm)

1020(b) B1554

Fig.7. Estimated crack profiles with incidence right in front of the crack

Table 1 Crack depth sizing results with incidence right in front of the crack (unit: mm)
UT method
Phased array
3D SAFT
Actualfreq.
3 MHz 5 MHz 2 MHz 5 MHz
B1553
7.7
9.1
9.9
8.4
7.1
B1554
7.9
8.7
9.7
8.9
7.3

Table 2 Crack length sizing results with incidence right in front of the crack (unit: mm)
UT method
Phased array
3D SAFT
freq.
3 MHz
5 MHz
2 MHz 5 MHz Actual
Threshold −6 dB −12 dB −6 dB −12 dB −12 dB −12 dB
B1553
15
22
23
32
28
28
27.2
B1554
24
29
25
29
27
26
26.4

2.3. Crack sizing accuracy with different azimuth angles of incidence
To evaluate the sizing stability against the variation in the azimuth angle of the incident direction,
the capabilities to estimate a crack depth with different azimuth angles were compared between phasedarray UT and 3D SAFT-UT. As mentioned in 2.1, the azimuth angle of the incident direction was set to
not only the direction perpendicular to the longer direction of the crack but also several inclined
directions (Fig. 3). Figures 8–10 show the D-scan images obtained by phased array UT and 3D SAFTUT with different incident directions. In these images, the X axis represents the direction perpendicular
to the incident direction instead of the longer direction of the crack.
When phased array UT is performed with an inclined azimuth direction of incidence, a transducerof 3 MHz is better to detect a crack tip than a transducer of 5 MHz owing to its lower directivity.
Therefore, Fig. 8 shows only the D-scan images obtained by phased array UT with the transducer of 3MHz. The D-scan images for the cases where the skew angle is ±45° were omitted because echoes fromthe cracks were too weak to perform depth sizing. When the skew angle is ±30°, tip echoes are still
difficult to find. Figures 9 and 10 show the D-scan images obtained by performing 3D SAFT-UT on thetest blocks B1553 and B1554, respectively. In these images, tip echoes are observed over a wide rangeof the cracks even when the azimuth direction of incidence is inclined.
In Fig. 11, the crack profiles estimated from the above-described inspection results of 3D SAFTUT are compared with the actual crack profiles. The estimated crack profiles are gradually losing theirshapes as the magnitude of the skew angle increases. These results indicate that 3D SAFT-UT can keep
providing a relatively good crack profile when the skew angle is within ±30°. In Table 3, the estimatedcrack depths are compared with the actual crack depths. When the magnitude of the skew angle

19

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT

increases, the depth sizing accuracy of phased array UT is quickly deteriorated. On the other hand,almost the same depth sizing accuracy is maintained by 3D SAFT-UT with different skew angles.

B1554

Z (mm)

-30°

B1553

Z (mm)Angle

Z (mm)-15°

X (mm)Z (mm)X (mm)Z (mm)0°

X (mm)Z (mm)X (mm)X (mm)Z (mm)

15°

Z (mm)X (mm)Z (mm)30°

X (mm)Z (mm)X (mm)X (mm)X (mm)

Fig. 8. D-scope images by phased array UT with different azimuth angles of incidence (3 MHz)20

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT

Angle

2 MHz

5 MHz

-45°

-30°

-15°

0°

15°

30°

45°

Fig. 9. D-scope images by 3D SAFT-UT with different azimuth angles of incidence (B1553)

21

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT

Angle

2 MHz

5 MHz

-45°

-30°

-15°

0°

15°

30°

45°

Fig. 10. D-scope images by 3D SAFT-UT with different azimuth angles of incidence (B1554)

22

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UT
1212ActualActual

10

45°

-30°

Crack depth (mm)Crack depth (mm)

-45°

-15°

8

0°

6
4
2
0

10

30°

15°

8

0°

6
4
2
0

-20

-10

0
X (mm)

10

20

-20

(a) 2 MHz – negative skew angles (B1553)
12

-10

0
X (mm)

10(b) 2 MHz – positive skew angles (B1553)
12

ActualActual45°-30°

Crack depth (mm)Crack depth (mm)

-45°

10

-15°

8

0°

6
4
2
0

10

30°

15°

8

0°

6
4
2
0

-20

-10

0
X (mm)

10

20

-20

(c) 5 MHz – negative skew angles (B1553)
12

-10

0
X (mm)

10

12

ActualActual45°-30°

Crack depth (mm)Crack depth (mm)

10

-15°

8

20

(d) 5 MHz – positive skew angles (B1553)

-45°

0°

6
4
2
0

10

30°

15°

8

0°

6
4
2
0

-20

-10

0
X (mm)

10

20

-20

(e) 2 MHz – negative skew angles (B1554)
12

-10

0
X (mm)

10

12

Actual10

Actual
45°

Crack depth (mm)

-30°

-15°

8

20

(f) 2 MHz – positive skew angles (B1554)-45°

Crack depth (mm)

20

0°

6
4
2
0

10

30°

15°

8

0°

6
4
2
0

-20

-10

0
X (mm)

10

20

-20

(g) 5 MHz – negative skew angles (B1554)-100
X (mm)

10(h) 5 MHz – positive skew angles (B1554)

Fig. 11. Crack profiles estimated by 3D SAFT-UT with different azimuth angles of incidence

23

20

Y. Kamiyama, et al./ Evaluation of the Capability to Estimate a Crack Profileby 3D SAFT-UTTable 3 Crack depth sizing results with different azimuth angles of incidence (unit: mm)
UT method
Phased array
3D SAFT
Actualfreq.
3 MHz
2 MHz 5 MHz-45° Not measured
9.7
7.5
-30°
4.9
9.2
8.8
-15°
6.5
10.1
8.9
B1553
0°
7.7
9.9
8.4
7.1
15°
4.5
9.9
8.6
30° Not available
9.4
8.9
45° Not measured
10.0
7.6-45° Not measured
8.1
8.6
-30°
5.4
8.9
7.9
-15°
6.6
10.0
9.2
B1554
0°
7.9
9.7
8.9
7.3
15°
6.8
10.1
9.1
30°
3.6
10.1
8.6
45° Not measured
10.4
8.3

3. Conclusion
In this study, the capabilities to estimate crack sizes and a crack profile were evaluated based on
the inspection results obtained by performing phased array UT with a linear array transducer and 3D
SAFT-UT on nickel-base alloy test blocks with a thermal fatigue crack. The results indicate● In the case of the incidence right in front of the crack, both phased array UT and 3D SAFTUT overestimated the crack depths by less than 3 mm (the crack depth and length sizing errors
satisfy the criteria indicated in Appendix VIII of the ASME BPVC Section XI).
● When the magnitude of the skew angle of the incidence increases, phased array UT quickly
loses the depth sizing accuracy but 3D SAFT-UT maintains almost the same depth sizing
accuracy.
● Phased array UT can obtain tip echoes only at rather flat part of a crack profile, whereas 3D
SAFT-UT can obtain tip echoes over a wide range of a crack and estimate its crack profile.
● Even if the magnitude of the skew angle of the incidence increases, 3D SAFT-UT can provide
a relatively good crack profile within ±30° skew angles.
The purpose of this study is to provide accurate crack sizing results robustly by grasping the profile
of a crack and then focusing on locating its crack tip for maintaining the structural integrity of nuclear
power plants. The study concluded that 3D SAFT-UT can make more robust crack size estimations thancommonly used UT methods. In this study, phased array UT was performed with a linear arraytransducer to be regarded as a commonly used UT method. If a matrix array transducer is used, theresults of phased array UT are expected to be improved.

4. Acknowledgement
This work was conducted as part of the Japan Ageing Management Program on System Safety(JAMPSS) sponsored by Nuclear Regulation Authority (NRA).

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