Developments in Ultrasonic Phased Array Inspection II Complementary Benefits of 1D and 2D Phased Array and Single Element Transducers for Stainless Steel Weld Crack Inspections (part2) D. Braconnier, M. Takahashi, G. Dao, KJTD Co., Ltd., Japan ABSTRACT Inspecting cracks in stainless steel parts has increasingly become a popular issue. However, the large variety in the profile of parts, as well as the presence of scattering noise makes sizing a flaw difficult, and false indications pose an even greater challenge. This paper, a follow-up of the work presented at the 2007 6 th International Conference on NDE in Budapest, shows a complementary and gradual way to make the inspection using three kinds of UT probes. The purpose is to decrease the overall inspection time, and increase the confidence in the diagnosis and measurement of the flaw. The contribution of 1D phased arrays (Linear Array) and 2D arrays (Matrix Array) is emphasized, but the usage of UT single element probes also plays an important role. This paper includes both the UT theory behind the experiment as well as the actual results. Key words: Phased Array; Matrix; 2D; Ultrasonic; Inspection; Weld; Volume Focusing; Skew deflection; point focus; INTRODUCTION This paper is a follow up of the previous one presented at the 6 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components. The emphasis is put on the detailed characteristics between 1D phased array (Linear) and 2D phased arrays (Matrix). The Power Generation Industry offers a lot of opportunities to inspect circumferential welds on piping as well as on other parts. In addition to normal wear and tear, these parts are exposed to various adverse conditions like chemical corrosion or mechanical stress, and should therefore be inspected on a regular basis. KJTD designs and manufactures advanced high-tech solutions with phased arrays and continues to improve the evaluation methods for depth measuring from using A-scans to other phased array technologies. KJTD is both knowledgeable and experienced in phased array technology, and has been a pioneer in NDE technology for many years. The targeted method is actually simple. The creeping wave probe provides an overview of the probability of flaws existing or not, and if they exist, their approximate location. The linear phased array can confirm the presence of flaws, with certainty and their precise location. At last, the matrix array can identify the flaw size with outstanding accuracy. Both kinds of phased array, linear and matrix, provide results that make distinguishing real flaws from just echoes coming back from the weld boundary a lot easier, which is one of the most difficult challenges in this application. DESCRIPTION OF THE METHOD AND TOOLS USED FOR THE EXPERIMENT Phased array technology is based on sampling the surface of the probe in small elements that act as punctual probes transmitting to and receiving from any direction, and whose signals are phased so that the UT beam has the characteristic the operator wishes. Symmetrical electronic lenses allow focusing at the desired depth, taking in account the wedge and part refracting interface. Dissymmetrical lenses allow deflecting the beam along a different axis of propagation from the natural axis of the probe. Phased array does have limitations. If elements are too large versus the wavelength, the ability to focus or deflect will be limited, because the element will have sensitivity only in front of the transducer. However, the focusing and deflection ability of phased array is usually very profitable to provide images without moving the probe, with very good accuracy and clarity. Besides, scanning
with a phased array can capture overlaps between images with different positions, so that the immunity of the analysis to the speckle noise improves by a significant factor. 1D arrays, which are usually called linear phased arrays, generally have a linear sampling of their surface in small elements. They can adjust their beam only within the electronic plane (defined by the sampling axis and the depth axis). This type of array has been widely used for 10 years in various applications all around the world. 2D arrays (see Figure), also called matrix phased array are constituted of 2-dimentional sampling of the surface of the probe. They are still very rare, because the 2 dimensional sampling leads to a number of elements proportional to the square of what would be necessary for a linear (1D) phased array to sample the surface of the probe. Furthermore, it is still hard to find instruments that can drive enough elements so that it is meaningful to even use matrix probes. However, when a matrix is well designed there are several advantages, like point focused beams, 3-D parameters, and deflection along the axis with any tilt and skew angle, possibly combined. Figure1: 2D phased array Rho Theta definition example THE TEST PIECE The test piece is a weld of 2 plates of carbon steel of 40mm thickness and is shown in Figure2. There are actual flaws in the middle of the weld 1 as well as in the root 3 (crack) and spread along the depth of the weld 2, in 3 locations along the axis of the weld. Flaws are not specifically small, but are like flaws that we can encounter in the field. They have irregular shapes so that their reflectivity varies from angle, and they are not positioned necessarily perpendicular to the axis of the weld. For the most part, they can be accessed either from side (A) or (B). Figure 2: Test piece used for the experiments
PHASED ARRAY USED FOR EXPERIMENTS The usage of single element probes has already been described enough so that the emphasis is put on phased arrays. 1D phased array (linear array) and 2D phased array (matrix array) from IMASONIC have been used. In both cases, we selected a frequency of 5MHz that is fitting with most inspection requirements. Concerning the linear phased array, we found the best all around solution, which we will refer to as the Universal Linear Array. The idea is to use a very small pitch, but with a lot of elements so that no matter the inspection range or angle, the probe will provide a very good sensitivity and beam spot size in a large range of use. A very interesting point lies in the fact no wedge is needed, the probe is directly in contact with the part. Later it is reported that with proper calibration, sector scans are possible from 20 to 70 degrees in longitudinal mode, and even shear wave are well managed from 30 to 60 degrees in the same conditions, without wedges. Concerning the matrix array, the limitation of element number and the constraints from the cost implied to use a 128 Rho Theta matrix with a wedge. The wedge provides a few handicaps and limitations are described later. The Phased Array Instrument: FlashFocus, from KJTD inc., shown in Figure3, is a massive parallel 128/128ch acquisition system equipped with both Conventional Focusing and Volume Focusing, and has the ability to drive complex matrices in addition to just linear phased arrays. In the case of a matrix, it allows a combination of deflection along tilt and skew angles at the same time. Figure3: FlashFocus from KJTD EXPERIMENTAL RESULTS: LINEAR PHASED ARRAY Calibration is performed simply with the corner between the back-wall and the side wall of the test piece.
Figure 4: Calibration results on the corner of the test piece. As shown in Figure 4, the position of the corner echo is always at the correct depth and although the probe is directly in contact with the part (no wedge) the gain varies only 9dB in an almost linear progression from 20 to 55 degrees. After the calibration was performed, the experiments were done successively on different locations. One important advantage of not using a wedge is that the distance between the center of the virtual probe and the center of the weld will be minimal, and will not vary as a function of the incident angle. In addition, fears of wedge ghost echoes are non-existent. Figure 5: Results from location A. Results for location : ① The probe is perpendicular to the weld The maximum peak of the amplitude is at 45 The probe position is angled so that the amplitude detection is at its maximum. We can see that there is a significant angle between the probe orientation and the weld.
The angle of the probe is increase until the signal is at its maximum amplitude. Angling the probe was the only way to detect the flaw. A higher gain of 23dB was also needed. Figure 6: Results from location B1. The inspection from side A shows that there is a significant difference in amplitude, about 6dB, when the probe position is angled from the perpendicular axis to the weld. In both cases, the detection of the flaw is clear and the signal to noise ratio is excellent and with a quite small gain of 10dB. The flaw is located exactly in the middle of the weld. Inspecting from side B definitively leads to the conclusion that the flaw is oriented. Indeed, from this side, the detection was not possible from a position of the probe perpendicular to the weld (no angle). Despite the orientation, detection from side B required a higher gain of 23dB. Finally, we can see that the signal which is located after the flaw on the Ascan corresponds to the back-wall echo (or the root). We can see it on the Sscan, with a 20 angle. Results for location : ② The probe position is adjusted with a small angle to get the maximum response The maximum amplitude is reached at 45 degrees deflection Figure 7: Results from location A2. With a small angle between the probe and the weld, and increasing the gain to 23 db We can see a series of flaws along the depth until the root, which couldn t be visualized from side A.
When the probe is even angled more, the flaw indications decrease until a new indication appears with a high deflection angle. Figure 8: Results from location B2. The inspection from side A shows a clear detection with low gain, and low inclination of the probe position versus the weld. The flaw is located exactly in the middle of the weld. Accessing from side B with a small angle from the probe position to the weld, and increasing the gain to 23dB, more than 1 indication appears. Two signals appear under and towards the upper-middle of the weld, and one signal appears from the root of the weld, around the same axis. We can even see that the root provides a signal in both longitudinal waves as well as shear waves. With a more precise evaluation, we can consider that there are most likely different flaws at the same location along the depth of the weld. At last, we can see also that there are reflections from very big angles indicating a probable complex shaped flaw. Results on location : ③ Probe is perpendicular to the weld Flaw can be seen only with a small angle of 30. Probe position is angled by a little. Peak of both tip and root echo is obtained.
Probe is moved away from the weld by a few mm. Maximum amplitude detection at 45 Probe angle is increased. Root, tip and skip signals are detected Same view for the root echo on the Ascan Same view for the tip echo on the Ascan Figure 9: Results from location A3. Small angle of the probe orientation is needed to get the maximum amplitude from the tip echo. Detection occurs with a small deflection of 30
Big orientation angle in the opposite direction is needed to get the max amplitude for the root echo Figure 10: Results from location B3. The inspection from side A shows a strong detection from the tip, but on the contrary, the root seems weak. Therefore, there is a small chance that this flaw is a crack. Also, it is oriented like the previous flaws. The fact that the detection is not obtained with only 1 orientation angle of the probe, but that the sensitivity varies leads us to conclude that the flaw has a complex shape. The fact that we can clearly see tip, root and skip echoes indicates that the flaw is probably an irregular big slit just below the surface. Also, it was not possible to see it from the bottom of the test piece with a visual inspection. SUMMARY OF RESULTS USING SHEAR WAVES WITH A CONTACT LINEAR PHASED ARRAY The linear phased array (0.5mm pitch, 5MHz) used had a wavelength of 1.2mm for longitudinal waves, explaining that even for large deflection angles, why grating lobes were non-existent in the previous experiments. The wavelength must absolutely be more than twice the element pitch to get rid of grating lobes. However, inspecting with shear waves drops the wavelength to 0.6mm, so if a large deflection angle exists, the conditions for avoiding grating lobes are not met, and they appear. Error! Reference source not found.(g1) shows them, and even more as the gain reaches 32dB. Analyzing their Time-Of-Flight, we clearly see that it corresponds to the back-wall echo, with longitudinal waves. It then becomes clear that the existence and position of the grating lobes are managed by a criterion based on the wavelength of the wave in question, but the resulting signal concerns both modes (longitudinal and shear). gl ① ① Detection of flaw A by orienting the probe to Detection of flaw B between grating lobes in the middle of the weld. The gain reaches 32dB. get the maximum amplitude. Gain is low (18dB). Figure 121: Results from location 1.
Inspecting with shear waves confirms the results obtained with longitudinal waves and clearly shows more than 1 indication, which leads to the conclusion that there is not only 1 flaw, but probably several lack of fusions and inclusions. ② Flaw A, probe is perpendicular to the weld. Flaw B ②, probe is perpendicular to the weld. Figure 12: Results from location 2. Results are perfectly consistent with longitudinal wave inspections. Indications from the root as well as from the area near by the middle of the weld can be seen. Thanks to the accuracy provided by shear waves, there are several small indications that can also be interpreted as porosities, and not necessarily just from a lack of fusion. ③ with a 37 angle Detection of the flaw A Detection of the flaw B ③ with a 48 angle Detection of the flaw A Figure 13: Results from location 3. ③
ip echoes and several root indications can be seen, due to detection with longitudinal waves. The evaluation shows the high probability that a lack of fusion and inclusion near the back-wall exists. Some are very large as a 5dB gain only gives an 80% full screen amplitude level detection. EXPERIMANT RESULTS: MATRIX PHASED ARRAY Scan with a skew angle Scan with tilt and skew of 90 degrees Figure14: The 2 main different Scan patterns with Matrix phased array. There are several methods of scanning when inspecting with matrix arrays. The sector scan can be in the same plane as the linear probe, but can also be done along a plane that has a skew angle from this initial plane (14, left). We can combine a few different skew angled sector scans in the same inspection. The expected advantage is that from only 1 position, it is possible to see oriented flaws. It is also possible to do the sector scan along a plane that is oriented along the skew. In this case, the expectation is that we can not only detect oriented flaws, but see their length along the weld axis (14, right). In all the following experiments, the following scans were used: Pattern I: 3 Views are obtained simultaneously: 1) No Skew Angle with a Sector Scan from 20 to 70 (15, 1). It corresponds to a configuration similar to Figure 14(left). 2) 90 Skew Angle with a 60 Tilt Angle (Figure 15, 2). Similar to Figure 14(right). 3) 90 Skew Angle with a 41 Tilt Angle (Figure 15, 3). Similar to Figure 14 (right).
1 2 3 Figure 15: Pattern I- Case 1) [left] 2) [middle] 3) [right]. Pattern II: 3 Views are obtained simultaneously, (no tilt angle, they all correspond to a configuration similar to Figure 14, left): 4) 0 Skew Angle with a Sector Scan from 30 to 60 (Figure 16, 4). 5) 10 Skew Angle with a Sector Scan from 30 to 60 (Figure 16, 5). 6) -10 Skew with a Sector Scan from 30 to 60 (Figure 16, 6). 4 5 6 Figure 16: Pattern II - Case 4) [left] 5) [middle] 6) [right]. In both cases, pattern I and pattern II, we can see a lot of echoes that are coming from the wedge. As seen in the previous photos of the test piece, the excess weld on the seam creates an uneven surface on the plate, making access difficult when choosing a suitable wedge. The compromise was to use a smaller wedge, but at the expense of poor damping of ghost reflections inside the wedge. Results from location 1:
F F Figure 13: Results from location A with pattern I (left) and pattern II (right). The access from side A confirms the experimental results from the linear phased array. There is at least 1 flaw in the middle of the weld with an orientation from the axis of the weld. This is directly visible in Figure 17 for a skew of 90 degrees, a tilt of 60 degrees (left middle), and for a skew of 0 and 10 degrees with sector scan (right). Figure18: Results from location B ① with patter I (left) and pattern II (right). Accessing from B side confirms also the additional shear wave experiments that were done with linear phased array: there is actually at least 2 different flaws with a small difference in depth. This is directly visible in Figure18 for a skew of 90 degrees and a tilt of 60 degrees (left middle). Results from location ②:
Figure19: Results from location A with patter I (left) and pattern II (right) It is interesting to see all those different point of views. The linear phased array experiments showed that there was more than 1 indication, probably several flaws, but without certainty. On the other hand, the experiment with the matrix phased array shows clearly that there are at least 2 different flaws that are at different positions and orientations. The sector scans with different skew angles (Figure19, right) shows that there are 2 flaws with different depths near 0 degrees in front of the probe, and a strong reflection flaw from the plane angled at 10 degrees skew. The skew oriented scans (Figure19 left) shows the main flaw in the 0 degree skew sector scan, but the 60 degree tilt 90 degrees skew scan shows very clearly that there are 2 flaws with a small extension and different depths. Figure 20: Results from location B with patter I Results are also partially confirmed by inspections from side B. One flaw is visible on the sector scan and the 60 degree oriented skew scan. Results from location : ③ Figure 21:Results from location A with patter I (left) and pattern II (right)
The experiment with the matrix phased array confirms the results obtained from the linear phased array. There are indications from the root and near by the root with several flaw orientations. It appears in several skew angle sector scans (Figure 21 right) as well as in 40 degree skew oriented scan (Figure 21 left). The flaw actually has a quite long extension in length. CONCLUSION Using linear arrays such as universal concept gives quite interesting abilities to detect flaws with large ranges of incident angles. By reducing again the pitch, it seems possible to use shear waves in direct contact to get a higher accuracy inspection method. However, moving the position of the probe (its angle) allows in a certain extent to access oriented flaws, but this is out of the scope of an Automated UT inspection system if we can keep it simple and as easy to use as actual systems. Matrix phased arrays bring here a very important advantage: without rotating the probe, with conventional 1 dimension scan robots, it is possible to steer the beam anywhere in front of the probe with different scan patterns. Doing sector scans with a few different skew angle, at the same time, selecting a few tilt angles and setting the skew at 90 degrees to build a scan along skew directions provides a unique advantage. This advantage is to be able to have many different points of views of the inspected material. This advantage is even much enhanced by the fact that matrix phased array allows to have point focus, so that the beam does not integrate a large section of the inspected volume, but gives a very sharp inspection. All this allows the user to identify flaws that are usually considered as only 1 flaw with conventional inspections, and this provides much more ability to size as well as to conclude about the flaw nature and geometry. There are still improvements that are needed. Especially the design of the matrix probe with its wedge can be done so that the image becomes even more comfortable to evaluate. REFERENCES 1) Takeko Murakami, Dominique Braconnier, KJTD ltd. The new technology of high speed ultrasonic detection flaw by array, 2005. 13-13, Nishiikebukuro 5-Chome, Toshima-ku, Tokyo, Japan, 2005. 2. Junichiro Nishida, MITSUBISHI HEAVY INDUSTRIES LTD., Yoichi Iwahashi, Takanori Yamashita, E-TECHNO, Dominique Braconnier, KJTD, Inspection of thick part with Phased Array Volume Focusing technique, 5 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, USA May 2006. 3 Masaki Takahashi, Dominique Braconnier, KJTD co. Ltd, 2D arrays and Volume Focusing combined inspection technique. 6 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Budapest, 2007. 4. Hironori Kawashima, Koji Ooi, Todenkogyo co. Ltd, Masaki Takahashi, Dominique Braconnier, KJTD co. Ltd, Complimentary benefits of creeping wave single element transducers, 1D phased array and 2D phased array for stainless pipe welding crack inspections. 6 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Budapest, 2007. 5. Masaki Takahashi, Dominique Braconnier, KJTD co. Ltd, 2D arrays and Volume Focusing combined inspection technique. 6 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Budapest, 2007.
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