2. Equipment, specimens and inspection procedures

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1 4th International CANDU In-service Inspection Workshop and NDT in Canada 2012 Conference, 2012 June 18-21, Toronto, Ontario Improved Flaw Characterization and Sizing in Pressure Tubes Alex KARPELSON Kinectrics Inc.; Toronto, Canada; Phone ; Abstract Results of the ultrasonic inspection of pressure tubes sometimes are not satisfactory due to inability to identify and characterize a flaw (i.e. determine its type, shape and orientation) and accurately size it. Therefore, it is worthwhile to develop some special approach for pressure tubes examination, which could significantly improve inspection capabilities. Modification of the existing inspection techniques, probes and software for data analysis provides very convenient and reliable information concerning flaw orientation, geometry, and dimensions, because new images are generated using various ultrasonic techniques. This means, one can look at the flaw from different directions and at various angles. As a result, the flaw type, orientation and shape can be identified, and flaw can be accurately sized. It leads to the significantly decreased number of the required replicas, improved efficiency and simplification of data analysis process, and decreased analysis time of the acquired inspection data. Keywords: Ultrasonic inspection, pressure tube, flaw characterization and sizing 1. Introduction Ultrasonic (UT) methods, which are currently in use for pressure tube (PT) inspection, are not always adequate for flaw characterization. Utilities occasionally experience problems trying to characterize a flaw (particularly, crack or off-axis flaw) and define its shape, orientation, and size. Modified or new-developed inspection techniques, UT probes, and software should be employed to improve flaw identification and characterization, increase sensitivity and accuracy of measurements, decrease analysis time and improve efficiency of analysis, and decrease number of required replicas. 2. Equipment, specimens and inspection procedures A computerized scanning rig with a calibrated UT system was employed for the experiments. The UT system includes the Winspect data acquisition software, a SONIX STR-8100 digitizer card, and a UTEX UT-340 pulser-receiver. Standard CIGAR techniques and probes [1] and novel techniques and probes with different center frequencies, various aperture diameters and different focal lengths were used for testing. The rationale was to determine techniques and probes, which could provide the best flaw characterization and sizing accuracy. A large number of PT specimens with various types of inside diameter (ID) notches were used for testing. These specimens contained rectangular and V-notches (symmetric and asymmetric) with various widths, depths, orientation angles (axial, circumferential and off-axis notches), root angles, inclination angles, and root radii. Rationale was to determine the ability and sensitivity of different techniques and probes to characterize various flaws (particularly cracks and off-axis flaws), to determine their orientations and shapes, to estimate their root radii and inclination angles, and to size these flaws. Special software, presenting 3D (isometric) images of flaws, was developed. These 3D-images are based on the time-of-flight normal beam (NB), angle pulseecho (PE), and angle pitch-catch (PC) 3D B-scans. The rationale was to have the ability to produce 3D-images of flaws, generated by using different techniques and probes.

3. Combined technique for flaw root radius assessment The direct measurement of root radius R cannot be performed using existing UT inspection systems, due

1mm, then transducer will not notice it. 2. The diameter of the UT beam is larger than typical root radius.

$Thus, only a small fraction of the reflected wave goes back in the direction of the receiving probe and impinges on it.$ The most reliable indirect method for R assessment is probably the combined technique [2], which employs PC, PE clock-wise (CW) and PE counter-clock-wise

The most reliable indirect method for R assessment is probably the combined technique [2], which employs PC, PE clock-wise (CW) and PE counter-clock-wise

Using this technique, the time interval t between the center of the notch PC response and the intersection point of the CW/CCW responses can serve as a

As a result, both transducers simultaneously transmit UT signals. Then each probe receives its own signals as well as signals transmitted by the other probe.

The obtained image contains responses typical for these three techniques.

2 3. Combined technique for flaw root radius assessment The direct measurement of root radius R cannot be performed using existing UT inspection systems, due to the following: 1. Typically the UT wavelength λ 0.2mm is much larger than R. Subsequently, if the flaw root radius R<0.1mm, then transducer will not notice it. 2. The diameter of the UT beam is larger than typical root radius. Therefore, only small portion of the transmitted UT wave impinges on the notch root and reflects from it. 3. The weak reflected wave, reflected from the small area of the flaw root, which usually has a complex shape, is very diverging. Thus, only a small fraction of the reflected wave goes back in the direction of the receiving probe and impinges on it. The most reliable indirect method for R assessment is probably the combined technique [2], which employs PC, PE clock-wise (CW) and PE counter-clock-wise (CCW) reflections simultaneously. Using this technique, the time interval t between the center of the notch PC response and the intersection point of the CW/CCW responses can serve as a criterion of R assessment. Combined technique can be realized by connecting e.g. CW and CCW probes simultaneously to a pulser-receiver PE jack. As a result, both transducers simultaneously transmit UT signals. Then each probe receives its own signals as well as signals transmitted by the other probe. Subsequently, three techniques can be realized simultaneously: CW PE, CCW PE, and circumferential PC. The obtained image contains responses typical for these three techniques. Note, that combined technique can also be realized by using three standard B-scans, CW, CCW and PC, and data fusion software. The results of combined B-scans of axial ID symmetric V-notches with various root radii are presented in Fig. 1. Root radius 40µm Root radius 100µm t=0.15µs Root radius 200µm t=0.09µs Root radius 400µm t=0.06µs t=0.001µs Figure 1. Circumferential 2D combined CW+CCW+PC B-scans of ID symmetric axial 60 0 /60 0 V-notches with depth d=0.3mm and root radii 0.04mm, 0.1mm, 0.2mm, and 0.4mm. Probes: CIGAR Head, CW & CCW, frequency f=15mhz, focal length FL=40mm, diameter D=9.5mm, water-path WP=20.6mm, incident angle α=25 0.

Images in Fig. 1 demonstrate that there is an obvious correlation between criterion t and R for a wide range of root radii.

flaw depth. Based on the obtained results, the correlation curve between criterion t and R was obtained. This correlation is almost linear one.

2-3, demonstrate that fatigue cracks and delay hydride cracks (DHC) can be clearly detected. CW response from DHC CW response from notch a CCW response from DHC b CCW response from notch Figure 2.

6mm, α=25 0. Color scale is shown in Fig. 1. CW response from DHC CW response from notch a CCW response from DHC b CCW response from notch Figure 3.

3 Images in Fig. 1 demonstrate that there is an obvious correlation between criterion t and R for a wide range of root radii. Parameter t does not depend on the flaw depth, because the position in time of both points (the CW/CCW responses intersection point and the center point of flaw PC response) equally depend on the flaw depth. Based on the obtained results, the correlation curve between criterion t and R was obtained. This correlation is almost linear one. Besides the ability to assess flaw root radius, the combined technique allows estimation of flaw shape, dimensions, and detection of cracks. For example, combined images, shown in Figs. 2-3, demonstrate that fatigue cracks and delay hydride cracks (DHC) can be clearly detected. CW response from DHC CW response from notch a CCW response from DHC b CCW response from notch Figure 2. Circumferential 2D combined B-scans of rectangular axial notch (3mm wide and 0.5mm deep) with DHC 1mm deep (a) and without DHC (b). Probes: CIGAR Head, CW & CCW, FL=40mm, f=20mhz, D=9.5mm, WP=20.6mm, α=25 0. Color scale is shown in Fig. 1. CW response from DHC CW response from notch a CCW response from DHC b CCW response from notch Figure 3. Circumferential 2D combined B-scans of axial V-notch (60 0 root angle and 0.8mm deep) with DHC 0.7mm deep (a) and without DHC (b). Probes: CIGAR Head, CW & CCW, FL=40mm, f=20mhz, D=9.5mm, WP=20.6mm, α=25 0. Color scale is shown in Fig Technique for depth measurement of sharp flaws Typically, the standard CIGAR NB technique and shear wave angle PE techniques allow estimating the flaw depth by using NB reflection from within the flaw or angle PE reflection from flaw root. However, if NB reflection from within the flaw or angle PE reflection from root of the flaw are very weak or cannot be detected at all, then flaw depth cannot be measured. Nevertheless, it is still possible to do it by applying the CW/CCW PE responses and using the method where flaw depth is proportional to the CW/CCW response length and/or duration of the response. At the same time, recall, that duration and length of the flaw response depend not only on the flaw depth, but also on the inclination angle of the flaw side. Therefore, this factor should be taken into account in order to determine a correct flaw depth. Fig. 4 shows CCW PE

B-scans of eight ID axial V-notches with various depths. Scans in Fig. 4 demonstrate an obvious correlation between the length/duration of the PE angle response and notch depth.

75mm Depth 1mm Depth 0.5mm Depth 0.25mm Depth 0.15mm Depth 0.1mm Depth 0.2mm Depth 0.05mm Figure 4. Circumferential 2D CCW PE B-scan of axial ID symmetric 75 0 /75 0 V-notches 0.05, 0.1, 0.15, 0.2, 0.

4 B-scans of eight ID axial V-notches with various depths. Scans in Fig. 4 demonstrate an obvious correlation between the length/duration of the PE angle response and notch depth. Based on the obtained results, two correlation curves, response length vs. notch depth and response duration vs. notch depth, were obtained. These correlations are similar to linear ones. Depth 0.75mm Depth 1mm Depth 0.5mm Depth 0.25mm Depth 0.15mm Depth 0.1mm Depth 0.2mm Depth 0.05mm Figure 4. Circumferential 2D CCW PE B-scan of axial ID symmetric 75 0 /75 0 V-notches 0.05, 0.1, 0.15, 0.2, 0.25, 0.5, 0.75, and 1mm deep. Probe: CIGAR Head, CCW, FL=33mm, f=10mhz, D=9.5mm, WP=20.6mm, α=25 0. Color scale is shown in Fig Technique for flaw side inclination angle assessment Angle PE B-images show that positions and dimensions of the CW and CCW PE responses depend on the inclination angles of the flaw sides; see for example Fig. 5 below. Long CW response from notch side inclined at 90 0 Short CCW response from notch side inclined at 60 0 Figure 5. Circumferential 2D combined B-scan of axial ID asymmetric 90 0 /60 0 V-notch 0.4mm deep. Probes: FL=40mm, f=15mhz, D=9.5mm, WP=20.6mm, α=25 0. Color scale is in Fig. 1. Obtained results show that, for example, ratio of time durations of the CW and CCW responses can be used for assessment of the inclination angles of the flaw sides. Based on the obtained results, the correlation curve response duration vs. inclination angle of the notch side was obtained. This correlation is close to linear one.

6. Large probe NB technique The large NB probe works not only as a NB transducer, but also, due

particularly after reflections from the inside and outside tube walls.

The central part of a large transducer works as a small NB probe, while the peripheral parts of

That is why transducer with large diameter can see flaw simultaneously at various angles and

2D circumferential B-scans performed using large probe and various ID notches are presented in

Circumferential 2D B-images of ID axial asymmetric V-notch 90 0 /15 0 0.

8mm deep (c), symmetric V-notch 45 0 /45 0 0.4mm deep (d), asymmetric V-notch 60 0 /45 0 0.

5, WP=15mm. Color scale is shown in Fig. 1. Crack Crack a b Figure 7.

5 6. Large probe NB technique The large NB probe works not only as a NB transducer, but also, due to its large diameter, this probe transmits and receives the UT waves at various angles, particularly after reflections from the inside and outside tube walls. Therefore, the large transducer can be represented as a number of small probes stuck together. The central part of a large transducer works as a small NB probe, while the peripheral parts of this transducer work as small angle probes in the PE and/or PC modes [2]. That is why transducer with large diameter can see flaw simultaneously at various angles and from different directions. 2D circumferential B-scans performed using large probe and various ID notches are presented in Figs a b c d e f Figure 6. Circumferential 2D B-images of ID axial asymmetric V-notch 90 0 / mm deep (a), trapezoidal notch 1mm deep and 0.5mm wide (b), undercut notch / mm deep (c), symmetric V-notch 45 0 / mm deep (d), asymmetric V-notch 60 0 / mm deep (e), and symmetric segment notch 0.5mm deep (f). Probe: FL=55mm, f=15mhz, D=0.5, WP=15mm. Color scale is shown in Fig. 1. Crack Crack a b Figure 7. Circumferential 2D B-images of ID axial rectangular notches 0.5mm deep and 2.54mm wide with fatigue cracks 0.9mm (a) and 1.2mm deep (b). Probe: FL=55mm, f=15mhz, D=0.5, WP=15mm. Color scale is in Fig. 1. One can clearly see that it is not only possible to evaluate notch width and depth, but also to approximately reproduce the shape of the notch.

7. Off-axis flaw characterization and sizing A standard CIGAR shear wave probe in the PE mode can hardly detect the off-axis flaws (particularly flaws oriented approximately at ±45 0 to the tube

Such a flaw can be detected in the NB and PC modes of operation, but only as a shadow in the main NB or PC response because the signal, reflected from the bottom of such an angle oriented flaw,

Four novel methods, such as quasi-nb technique, double angle PC technique, double angle PE technique, and technique employing cylindrically focused probe, allow distinguishing angle orientations of

These images clearly demonstrate that quasi-nb method allows detecting angle orientations of the off-axis flaws and measuring their depths. Similar results were obtained using double-angle PC method.

6 7. Off-axis flaw characterization and sizing A standard CIGAR shear wave probe in the PE mode can hardly detect the off-axis flaws (particularly flaws oriented approximately at ±45 0 to the tube axial axis) because the reflected signals do not return to the probe. Such a flaw can be detected in the NB and PC modes of operation, but only as a shadow in the main NB or PC response because the signal, reflected from the bottom of such an angle oriented flaw, usually does not return to the probe. Such a shadow does not allow measuring the flaw depth. Four novel methods, such as quasi-nb technique, double angle PC technique, double angle PE technique, and technique employing cylindrically focused probe, allow distinguishing angle orientations of the off-axis flaws and sizing their depths. Obtained quasi-nb 2D circumferential B-scans and 2D amplitude C-scans of various ID offaxis notches are presented in Fig. 8. These images clearly demonstrate that quasi-nb method allows detecting angle orientations of the off-axis flaws and measuring their depths. Similar results were obtained using double-angle PC method. Recall that standard CIGAR NB and PC techniques can usually generate only shadow C-images, similar to ones presented in Fig. 8b, but they cannot provide B-images defining depths of the off-axis flaws, like images in Fig. 8a. a Axial notch Angle oriented notches Circumferential notch b Figure 8. Circumferential 2D B-scan (a) and respective amplitude 2D C-scan (b) derived of 3D PE quasi-nb B- scan of seven ID off-axis V-notches (7mm long and 0.75mm deep with 45 0 root angle) oriented at various angles to the tube axial axis: 0 0, 15 0, 30 0, 45 0, 60 0, 75 0, and Probe: FL=10mm, f=20mhz, D=0.375, WP=10mm, α=5 0. Color scale is shown in Fig. 1. Double-angle PE 2D circumferential B-scans and 2D amplitude C-scans of various ID off-axis notches are presented in Fig. 9. Recall that standard CIGAR angle PE technique cannot at all generate either B-images or C-images of the off-axis flaws. Also note that both double-angle

techniques, PE and PC, were realized employing specially designed angle probe head shown in Fig.

Special angle probe head (a), circumferential 2D B-scan (b) and respective amplitude 2D C-scan (c) derived of 3D double-angle PE B-scan of seven ID

75mm deep with 45 0 root angle) oriented at various angles to the tube axial axis: 0 0, 15 0, 30 0, 45 0, 60 0, 75 0, and 90 0.

Technique, employing cylindrically focused probe, allows distinguishing angle orientations of the off-axis flaws and sizing their depths on a single

Circumferential angle 2D PE B-scan of seven ID 75mm deep with 45 0 root angle) oriented at the following angles to the tube axial axis: 90 0

7 techniques, PE and PC, were realized employing specially designed angle probe head shown in Fig. 9a, where probes are oriented at angles in both axial and circumferential directions. a b c Figure 9. Special angle probe head (a), circumferential 2D B-scan (b) and respective amplitude 2D C-scan (c) derived of 3D double-angle PE B-scan of seven ID off-axis V-notches (7mm long and 0.75mm deep with 45 0 root angle) oriented at various angles to the tube axial axis: 0 0, 15 0, 30 0, 45 0, 60 0, 75 0, and Probes: FL=40mm, f=20mhz, D=0.375, WP=20.6mm, both incident angles Color scale is shown in Fig. 1. Technique, employing cylindrically focused probe, allows distinguishing angle orientations of the off-axis flaws and sizing their depths on a single scan, see Fig. 10. Axial notch Angle oriented notches Circumferential notch Figure 10. Circumferential angle 2D PE B-scan of seven ID off-axis V-notches (7mm long and 0.75mm deep with 45 0 root angle) oriented at the following angles to the tube axial axis: 90 0 (circumferential notch), 75 0, 60 0, 45 0, 30 0, 15 0, and 0 0 (axial notch). Probe: cylindrically focused in the circumferential direction, FL=25mm, f=15mhz, D=9.5mm, WP=27mm, α=6 0. Color scale is shown in Fig. 1 Images in Figs show that quasi-nb technique, double angle PE technique, double angle PC technique, and technique using cylindrically focused probe, are capable of detecting various off-axis flaws, determining their orientation and measuring their depths.

8. 3D-visualization of NB, CW, CCW, and PC B-scans Using different 3D (isometric) time-of-flight B-scans, the orientation, shape, and dimensions of the flaw can be determined.

Obtained 3D images provide convenient and reliable information concerning flaw identification, characterization and sizing.

8 8. 3D-visualization of NB, CW, CCW, and PC B-scans Using different 3D (isometric) time-of-flight B-scans, the orientation, shape, and dimensions of the flaw can be determined. Special software, that can automatically measure parameters of various responses and generate 3D image of a flaw, was developed. Obtained 3D images provide convenient and reliable information concerning flaw identification, characterization and sizing. As a result, it would lead to simplification of the analysis process, increased realibility and improved efficiency of analysis, the decreased number of required replicas and the decreased analysis time of the acquired UT inspection data. Using this software, the stored field inspection data can be revisited, analyzed more carefully, and compared with the most recent inspection data. Typical 3D time-of-flight images of various flaws are presented in Figs Figure 11. Time-of-flight 3D image derived of 3D "quasi-nb" B-scan of seven V-notches 7mm long, 0.75mm deep, with root angle 45 0, and oriented at various angles to the tube axial axis: 90 0, 75 0, 60 0, 45 0, 30 0, 15 0, and 0 0. Probe: FL=10mm, f=20mhz, D=0.375, WP=10mm, α=5 0. Scale 10:1 in depth direction. Figure 12. Time-of-flight 3D image derived of 3D double angle PC B-scan of ID V-notch 10mm long and 0.75mm deep, with 20 0 root angle, and oriented at 45 0 to the tube axial axis. Probes: CIGAR Head, CW+CCW, f=10mhz, FL=33mm, D=9.5mm, WP=20.6mm, α=25 0. Scale 10:1 in depth direction. Figure 13. Time-of-flight 3D image derived of NB 3D B-scan of two-step axial ID rectangular notch (primary notch mm deep, 2.5mm wide, and 25mm long; secondary notch 0.076mm deep, 0.15mm wide, and 3mm long in the centre of primary notch). Probe: CIGAR Head, NB, FL=10mm, f=20mhz, D=6.35mm, WP=10mm. Scale 10:1 in depth direction.

Figure 14. Time-of-flight 3D image derived of 3D NB PE B-scan of axial rectangular notch (25mm long, 2.5mm wide and 0.

Figure 15. Time-of-flight 3D image of 2-step circumferential scrape derived of 3D NB PE B-scan.

Probe: NB, PE, flat, f=10mhz, D=4mm, WP=10mm. Scale in depth direction is 10:1. Figure 16.

9 Figure 14. Time-of-flight 3D image derived of 3D NB PE B-scan of axial rectangular notch (25mm long, 2.5mm wide and 0.5mm deep) with two fatigue cracks ~1.7mm deep. Probe: NB large, D=9.5mm, FL=45mm, f=15mhz, WP=12mm. Scale in depth direction is 5:1. Figure 15. Time-of-flight 3D image of 2-step circumferential scrape derived of 3D NB PE B-scan. Primary scrape is ~30mm long, ~5mm wide, and ~0.2mm deep. Secondary scrape is ~20mm long, ~3mm wide, and ~0.3mm deep. Probe: NB, PE, flat, f=10mhz, D=4mm, WP=10mm. Scale in depth direction is 10:1. Figure 16. Time-of-flight 3D image derived of CW, CCW and CPC fused scans of ID axial V-notch (60 0 root angle and 0.3mm deep with root radius 100µm). Probes: CIGAR Head, CW and CCW, FL=33mm, f=10mhz, D=9.5mm, WP=20.6mm, α=25 0. Scale in depth direction is 10:1.

Figure 17. Optical image of flaw replica (~5.5mm long, ~2.5mm wide with maximum depth ~0.083mm) and timeof-flight 3D image derived of 3D NB PE B-scan of flaw. Probe: NB, D=6.

16 to any point, the following flaw parameters in this point can be assessed: depth (circled in green at the bottom of the image), root radius (circled in red), and flaw sides inclination angles

10 Figure 17. Optical image of flaw replica (~5.5mm long, ~2.5mm wide with maximum depth ~0.083mm) and timeof-flight 3D image derived of 3D NB PE B-scan of flaw. Probe: NB, D=6.35mm, f=20mhz, FL=10mm, WP=10mm. Scale in depth direction is 10:1. Note that moving 3D-cursor in Fig. 16 to any point, the following flaw parameters in this point can be assessed: depth (circled in green at the bottom of the image), root radius (circled in red), and flaw sides inclination angles (circled in blue). Images in Figs demonstrate that novel software for data processing can definitely lead to a decrease in the number of required replicas, which should be taken and analyzed to determine flaw shape, orientation, and dimensions. This provides savings in terms of valuable outage time and worker dose uptake. This novel software also provides convenient and reliable information concerning flaw orientation, geometry, and dimensions. It will lead to the improved efficiency of analysis and the decreased analysis time of the acquired UT inspection data. It will also simplify and make more reliable the process of data analysis. Using this software, the old stored inspection data can be revisited and re-analysed. All these advantages will allow development of more accurate and reliable procedures, instructions and guidelines concerning flaw identification, characterization and sizing. 9. Conclusions Modified or new developed UT techniques and data analysis procedures were developed to improve flaw identification, characterization and sizing. Such techniques and software lead to more accurate determination of flaw orientation and shape, improved accuracy of flaw length, width and depth measurements, estimate of flaw root radius, decreased analysis time, and improved efficiency of analysis. All this, in turn, will lead to a decrease in the number of required replicas, and it will provide savings in terms of valuable outage time and worker dose uptake. References [1] M. Trelinski, Inspection of CANDU Reactor Pressure Tubes Using Ultrasonics, Proceedings of 17 th World Conference on Nondestructive Testing, Shanghai, China, October [2] A. Karpelson, Ultrasonic Detection of Cracks in Pressure Tubes, Proceedings of 32 nd Canadian Nuclear Society Conference, Niagara Falls, Canada, June 2011.

ULTRASONIC TESTING AND FLAW CHARACTERIZATION. Alex KARPELSON Kinectrics Inc., Toronto, Canada

ULTRASONIC TESTING AND FLAW CHARACTERIZATION Alex KARPELSON Kinectrics Inc., Toronto, Canada 1. Introduction Ultrasonic Testing (UT) is a commonly used inspection method. Various techniques are employed