HUMAN FACTOR IMPROVEMENT IN NDE WITH FMC IMAGING METHODS

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1 Originally published July 8, 2018 at The Sixth Japan-US NDT Symposium Emerging NDE Capabilities for a Safer World. ABSTRACT NDE in industry has a vital role in assuring integrity of assets and safety for people and environment. A problem of concern is reducing incidents from crack-like flaws, in particular the difficulty in sizing and discriminating anomalies in the pipe and differentiating between critical defects near failure and benign manufacturing flaws which will remain stable though-out the life of pipeline. The new Full Matrix Capture (FMC) family of UT imaging measurements is showing great potential and is considered the natural evolution of PA since it allows for characterization of flaws with any orientation. Among FMC techniques, IWEX uses two array probes and acquires up to 13 simultaneous modes, and it has been applied to girth welds, ERW, FW and FSW samples. With this paper we describe the IWEX technique and share the results and benefits of using it over PA such as straightforward interpretation and automation of the identification and sizing process which will reduce the dependence on human interpretation of inspection results INTRODUCTION The ideal NDE process should be carried out using a fully automated system: the planning, execution, evaluation and reporting would be all tasks performed by automated technologies and artificial intelligence that don t need human intervention, thus taking the Human Factor out of the equation. Not yet reaching this level of automation, it is still possible to improve the overall quality of the NDE process to increase both speed and quality, while maintaining safety. Ultrasonic Testing (UT) NDE is considered a safe, portable and reliable inspection method. Detection and characterization though depend upon factors like technician s experience and skills, especially for shear wave UT where an amplitude vs time signal (A-Scan) is the only given information from the specimen to be examined. With this technique there is very little data saved as a digital record as only screenshots can be saved. Advanced sizing is based on using maximum amplitude method for multi-faceted flaws, and tip diffraction method for smooth reflectors. The evolution of shear UT was represented by the use of an array probe with the Phased Array (PA) technique from the medical industry: the new capabilities of simultaneously steering of the beam in multiple directions and assembly all the A-Scans in images, has made it easier to interpret any anomalies found during inspection. However, skill and experience are still very important factors to avoid false positive and false negative results (for example focusing distance from the array probe and false signals from mode conversion may lead to the wrong conclusions). With FMC techniques, in particular with the Inverse Wave field Extrapolation method (IWEX), it is possible to image the results in a way that resemble the actual configuration of the flaw with improved information about the orientation of the anomaly and fewer false signals. HUMAN FACTOR One way to model the Human factor is by means of sub-factors: Human error, people s behavior that lead to unwanted results. Human risk, actions/situations that put in danger the health of people. Human performance, how well a task is carried out in time, quality or efficiency. Human aid, tools that allow a task to be carried out.

2 Taken for granted their mutual interdependence, error and performance in particular are strictly related not only to the personality, the experience and the skills of the technician but also to the type of NDE technology and technique (Human aid) utilized. Implementing human redundancy (the four-eyes principle) is a common practice to improve the Human factor although a study on this topic show that it is not always as effective as desired and that organizations should consider carefully this type of solution. The same study says that in a critical environment, such as exposure to radioactivity, the data acquisition process should not to be conducted redundantly to reduce inspectors exposure to high radiation levels. According to NDT practitioners and experts from fields other than nuclear, e.g. chemical industry, human redundancy is rarely carried out by having two inspectors inspecting the same area twice, i.e. in a form of duplication 1. Human redundancy implies redundancy in experience and skills as using double man power doesn t mean that the task is going to be 50% less difficult but only that the probability to have an undetected error is going to be lower: the data still needs to be properly gathered and evaluated by both the human beings. A way to increase human redundancy is to make the inspection technique more reliable during the gathering of the data and easier to evaluate: one scan would be enough for the inspection and more people, with sufficient skill, would be able to evaluate the data. Ideally, the final goal is to have the same inspection results for each object no matter the technician s experience and skill, which assumes that above a specified qualification level using the same technology, different individual should give almost identical results. PHASED ARRAY PA scans are not straightforward to interpret, an experienced and skilled person is necessary to evaluate the data as the results most of the time do not resemble the real configuration of the flaw. PA limitations are the amount of computational power required to generate a beam focused at every depth as sound waves can be focused only at a limited number of points within a given period of time, and the difficulty of detecting flaws perpendicular to the surface of the part being inspected. Advanced PA techniques, such as DPA (Dual Phased Array) probes for mimicking TRL (Transmit Receive Longitudinal) probes, can help with imaging and sizing vertical flaws. 2 FULL MATRIX CAPTURE Techniques based on FMC are now sufficiently mature to reduce the Human Factor in NDE. FMC consists of time domain A-Scan signals generated by pulsing a single element of an array probe and simultaneously recording all elements in the array. The process is repeated by pulsing each element in the array. For a N- element array probe the FMC consists of N^2 A-Scan signals. The size in bytes of the FMC data per scan position (or cross-section) depends upon the number of elements of the array probe(s), the maximum A- scan length and the digitizing bit range. In contrast to conventional ultrasonic approaches, all the collected FMC dataset is not used as is in an A- scan representation, but processed to produce an image using methods like the Total Focusing Method 1 M. Bertović, Human Factors in Non-Destructive Testing (NDT): Risks and Challenges of Mechanised NDT, Berlin: Bundesanstalt für Materialforschung und -prüfung (BAM), A. Van Den Biggelaar, F. Dijkstra and K. Chougrani, MODIFIED PHASED ARRAY CONCEPT FOR AUT OF LNG STORAGE TANKS, Rotterdam, The Netherlands: APPLUS RTD, 2010.

3 (TFM) or Inverse Wave Field Extrapolation (IWEX). 3 These image processing methods combine the A-Scan data into several images all representing a cross-section of the specimen to be examined. In order to form the images, the travel times from all array elements to all points in the image have to be calculated and made available to the hardware. During the inspection, this information is used to look up samples in the recorded A-scans and add their contribution to the respective points in the image. The maximum number of images produced in real time per cross-section depends mainly upon the processing power and the sample rate of the A-Scans. For each sound path an image is produced and classified as mode-k, where k indicates the number of skips (reflection off the specimen surface) that the sound wave encounters as it travels through the pipe steel back and forth to the array probe. Each mode is a combination of the three principal paths (blue) as shown in Figure 1. Figure 1: 0 skips (a), 1 skip (b), 2 skips (c) INVERSE WAVE FIELD EXTRAPOLATION The IWEX technique allows the simultaneous usage of different acoustical paths for generating the image. By combining the three principal paths described above, several modes for interrogating the weld volume can be formed. For applications like ERW/FW weld, girth weld, or FSW weld inspection the number of applied modes is normally 10 and are shown in Figure 2. In the measurement software, the images obtained by using the different paths are presented in a single image. By combining these images into a single overlay, different directions for inspection can image defects of arbitrary orientation. Thereby, the overall chance of missing a defect is minimized. The modes with an odd number of skips are used to image flaws with vertical orientation: the sound wave path is similar to a tandem configuration where the sender probe and the receiver probe have a specific offset. 4 The modes with an even number of skips are typically used to image the corner traps and the tip diffractions of flaws, and reflections from flaws when they are inclined to the specimen surface and roughly perpendicular to the sound wave. The cross modes, with an even number of skips, have a pitch-catch configuration and can image flaws parallel to the OD and the ID surface of the specimen. 3 N. Pörtzgen, D. Gisof and G. Blacquière, Inverse wave field extrapolation: a different NDI approach to imaging defects, IEEE, L. Hörchens, X. Deleye and K. Chougrani, Ultrasonic imaging of welds using boundary reflections, Melville, NY: American Institute of Physics, 2013.

4 The FMC of one scan position or cross-section with 64 elements array probes consist of 4096 A-Scans and it takes 131 megabytes of space if stored in its raw format. However, once the FMC is processed into images of each active mode, the FMC is discarded and only the image data in bitmap format is stored images recorded every mm produced an image file that takes less than 1Gb of storage versus 131Mb x 1000 > 100Gb of FMC data! Current hardware configuration consists of the imaging unit, laptop, two 64 elements array probes and two 37⁰ plastic wedges shaped according the type of examination with focal lenses to reduce the beam spread to ~3mm. OBJECTIVE This paper describes how FMC techniques, in particular Inverse Wave Field Extrapolation, can improve the Human factor helping with the interpretation process. Imaging technology and redundancy with multiple modes are applied to seam welds, girth welds, SCC and FSW samples. Detection, characterization and sizing are more straightforward as the results resemble the actual flaw configuration. IMAGE INTERPRETATION IWEX mode-0 left and mode-0 right are used to image inclined reflectors, tip diffractions and corner traps of flaws near the ID surface. Mode-1 left and mode-1 right are used to imagine the vertical component of flaws near the ID surface. Mode-2 left and mode-2 right are used to image inclined reflectors, tip diffractions and corner traps of flaws near the OD surface. Mode-3 left and mode-3 right are used to imagine the vertical component of flaws near the OD surface. Cross mode-0 is used to image the ID surface and parallel flaws near the ID surface, and cross mode-2 is used to image the OD surface and parallel flaws near the OD surface. All the modes contribute to detection, characterization and sizing of a flaw with a certain level of redundancy: in Figure 3 we have an example of SCC. The picture shows the cross-section on top and the

5 left side B-Scan, the OD C-Scan, Mid-Wall C-Scan, ID C-Scan and right side B-Scan respectively from bottom left to bottom right of the picture. Figure 2: SCC on 20 pipe In Figure 4 we can see how SCC generates reflections, corner traps and tip diffractions with more than one mode. In (a) only cross mode-2 is active and the tip diffraction of the bottom of the flaw is visible. In (b) cross mode-2 and mode-3 left and right are active: the vertical component gives a better idea of the shape of the flaw. In (c) only mode-2 right is active: one corner trap and one tip diffraction are visible. In (d) mode-3 right is added to show how the corner trap and the tip diffraction align vertically. In (e) we have the combination of (a), (b), (c), (d): again it is possible to see how the different modes align together. Sizing with any of the active modes gives consistent values with very low variation.

6 Figure 3: SCC sizing. Cross mode-2 (a), cross mode-2 and mode-3 right (b), mode-2 right (c), mode-2 right and mode-3 left and right (d), mode-2 right, mode-3 right and cross mode-2 (e) Figure 5 shows the image of an ERW cross-section with an inclusion rolled in an upturned fiber imperfection shape (d). Picture (a) has cross mode-2 and all left modes active, picture (b) has cross mode-2 and all the right modes active. In (a) it is possible to see the tip diffraction on the bottom of the flaw and the corner trap where vertical upturned fiber turns to into a horizontal lamination. In (b) we can see the reflection from mode-0 left and the tip diffraction from mode-2 left. (c) has all the modes active. Figure 4: 8 ERW, right modes (a), left modes (b), all modes (c), met section with inclusion (d) Figure 6 show side by side the cross-section generated with the imaging technique and the sectorial scan generated with PA of a 24 V bevel seam weld with a crack in the Heat Affected Zone. Picture in (a) shows the OD and ID surfaces and gives a better idea of the location of the flaw than picture in (b).

7 Figure 5: 24" V bevel seam weld with crack. All modes (a), PA skew 270⁰ (b) Figure 7 shows how helpful are the capabilities of imaging the vertical and the horizontal components of the flaw. In (a) we have all modes on, in (b) the PA sectorial scan and in (c) the picture of the cross-section of the specimen. PA shows only the tip diffractions and the corner traps. Although the corner traps clearly indicate a lack of penetration, the information we have for the void is that the two parts are in contact, but not bonded. Figure 6: FSW on Al with partial penetration and void. All modes (a), PA skew 90⁰ (b), cross-section (c) Figure 8 (a) shows the cross-section of a girth weld: the cap of the weld is imaged using the cross mode-2 and the root using cross mode -0. The cross-section also shows a slight misalignment of the two welded parts. Picture (b) shows that it is possible to stack all the cross-section together in a 3D model.

8 Figure 7: 6" girth weld. All mode (a), 3D view (b) CONCLUSION Using two array probes for the examination gives the possibility of imaging with one scan ID and OD surfaces and therefore improves the readability of the results. Since the FMC images resemble the actual configuration of the flaw, it is easier to characterize the defect: non NDT personnel can understand the data with minimum effort thus increasing human redundancy in the evaluation of the results. FMC based techniques have intrinsic data redundancy as detection, characterization and sizing can be normally done using multiple modes. FMC imaging has been successfully applied on seam weld, girth welds, SCC colonies. FSW trials showed promising results. Sizing based on corner traps and tip diffractions has proven to be more effective than sizing based on amplitude attenuation with a standard deviation as low as ±0.2 mm. 5 AKNOWLEDGEMENTS The authors would like to thank Peter den Boer of Applus+ RTD in Edmonton for providing the data in Figure 6, and Kiefner for providing the met section in Figure 5 (d). 5 H. Haines, L. Hörchens and P. Tomar, IDENTIFYING AND SIZING AXIAL SEAM WELD FLAWS IN ERW PIPE SEAMS AND SCC IN THE PIPE BODY USING ULTRASONIC IMAGING, Calgary: ASME, 2016.