Acoustic Transducer*

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, 29[4], pp. 400~407 (April 1992). SUMMARY REPORT Development of Ultrasonic Testing Equipment Incorporating Electromagnetic Acoustic Transducer* Michio SATO, Hideharu OKANO, Nuclear Engineering Laboratory, Toshiba Corp.** Masaaki ISHIKAWA System and Equipment Department, Toshiba Corp.** Received November 26, 1991 This paper describes an automatic flaw detection equipment and heat-resistant ultrasonic transducer for plate thickness measurement. The automatic flaw detection equipment is used during in-service inspection. It comprises an angle-beam electromagnetic acoustic transducer (EMAT), mounted on a vehicle. for scanning the pipe surface to be inspected. The EMAT functions without direct contact with the pipe surface through a coupling liquid, the vehicle does not require a guide track installed on the pipe surface, since it is equipped with magnetic wheels that adhere to the pipe, permitting it to travel along the circumferential weld joint of a carbon steel pipe. Another heat-resistant ultrasonic transducer is a normal beam EMAT and is used during plant operation, As a result, the automatic flaw detection equipment could detect a 1 mm deep notch cut on a test piece of 25 mm thick carbon steel plate. The vehicle location accuracy on the piping was +-2 mm. The normal beam EMAT could measure the plate thickness, within mm accuracy for the range of plate thickness 4 mm to 12 mm at 300dc. KEYWORDS: in-service inspection, ultrasonic testing, measuring instruments, carbon steel, piping, angle-beam EMAT, crack detection, vehicles, trackless, normal beam EMAT, high temperature, plate thickness measurement I. INTRODUCTION During a scheduled outage of nuclear power station, the integrity of piping is verified by in-service inspection(1). In this inspection, wide use is made of ultrasonic testing (UT), in which flaw detection in the weldment and gauging of piping thickness are main inspection items. To enhance the reliability of this testing technique, the subject to be tested must be prepared by such operations as pipe coating removal and surface polishment(2). The temperature range permittable for the ultrasonic transducer would be limited by the temperature limit for the coupling medium(3). Also, the ultrasonic equipment needs to be mounted on the test subject. Since all these operations have to be carried out by personnel working in a radioactive environment, it is imperative to reduce the time for inspection. To this end, the authors have developed an automatic flaw detection equipment(4)~(7) and a heat-resistant ultrasonic transducer(8). The automatic flaw detection system developed comprises an angle-beam electromagnetic acoustic transducer (EMAT) mounted on a vehicle for scanning the piping surface to * This article is a summary of the 23rd AESJ Technical Award, No. 2306, (Mar. 28, 1991). ** Ukishima-cho, Kawasaki-ku, Kawasaki 210. ** * Shinsugita-cho, I sogo-ku, Yokohama

2 Vol. 29, No. 4 (Apr. 1992) Summary Report (M. Sato et al.) 401 be inspected. The heat-resistant ultrasonic transducer is a normal beam EMAT. It would be attached to the pipe surface during the plant operation and would be used to measure the pipewall thickness. This study was planned for diminishing the drawbacks involved in low flaw-detecting sensitivity, due to non-contact ultrasonic generation and reception, and lowered the accuracy of detecting flaw location, because of the elimination vehicle track. This report reviews the authors' study on the development of the EMAT and its application to UT. II. AUTOMATIC FLAW DETECTION EQUIPMENT(4)~(7) The equipment configuration is shown in Fig. I. The angle-beam EMAT, together with the equipment for the position sensing scanner, the rotary encoders and the optical sensor are mounted on the vehicle. The electronic equipment controls both the EMAT and the vehicle. Fig. 1 Automatic flaw detection equipment blockdiagram The vehicle is designed to travel solely in the circumferential (Y-) direction of the pipe, without traveling in the axial (X-) direction. Scanning over a bandwise area of prescribed width in the axial direction is ensured by the scanner mounted at the center of the vehicle which shifts the EMAT to and fro across the path of the vehicle travel. The electronic equipment comprises (a) instrumentation devices for controlling vehicle movement e.g. driving motor, X- and Y-direction counters, reference landmark sensor, and (b) ultrasonic instrumentation devices e.g. transmitter, receiver, signal averager and 99

3 402 Ultrasonic Testing Equipment EMAT J. Nucl. Sci, Technol., oscilloscope. The X- and Y-counters, as well as the optical sensor which detects the landmark pasted on the pipe are connected to the controller, which processes the incoming information to calculate the position of the vehicle and controls its movement. The ultrasonic wave signals, issued from transmitter and detected by receiver, are passed through the signal averager for improving the S/N ratio of the reflected echoes by summing up large numbers of signals received sequentially. The oscilloscope serves to monitor the wave form. ly. High-frequency current is passed through the conductor bars, which inducing an eddy current on the test surface. This generates the Lorentz force that, in turn, produces the ultrasonic wave. The propagation direction of the ultrasonic wave t is defined by(11) (1) The prototype EMAT, used in the present test, is shown in Photo. 1. III. EMAT Photo. 1 Angle-beam EMAT manufactured in trial The EMAT emits ultrasonic wave by Lorentz force(9)~(11), which is generated by interaction between the static magnetic field, created by a permanent magnet, and the eddy current induced on the surface of the pipe during the inspection process using a highfrequency electrical current supplied to the magnetic coil from an oscillator. The principle of EMAT angle beam generation is shown in Fig. 2(11). The magnetic flux, generated by the permanent magnet, enters the test material surface perpendicular- Fig. 2 Mechanism of generation of angle beam ultrasonic wave The EMAT was designed to emit 2 MHz shear waves at 45d to the normal direction of the test material surface. The permanent magnet is Sm-Co and the insulating plates is a glass-epoxy substrate. The EMAT is 35 mm long, 30 mm wide, and 36 mm high. Trial operation of the prototype EMAT unit was carried out using man-made cracks which were constituted of notches cut on a carbon steel test piece, 25 mm thick with an opening 0.5 x50 mm, and the depth changed step by step from 6.25, 2.5 and 1.0 mm. This represented, respectively, 25, 10 and 4 % of the plate thickness. The test using the foregoing man-made cracks resulted in the performance reproduced in Fig. 3(6)(7). Reflection coefficient e, obtained from the relationship between notch height and EMAT aperture size, was derived using the conventional formula 100

4 Vol. 29, No. 4 (Apr. 1992) Summary Report (M. Sato et al.) 403 (2) Fig. 3 Relationship between notch height and notch echo amplitude This coefficient e represents the ratio of echoes between those from the notch and from the test piece corner. Equation (2) does not take into account any experimental error caused by ultrasonic beam divergence or associated with the EMAT positioning and so on. The benefit obtained from using the signal averager is well depicted in Photo. 2, where the upper traces, marked CH1, represent the raw pattern of the output wave, and the lower traces, marked CH2, are those obtained through the signal averager. In the present instance, the signals were superimposed 32 times, resulting in reducing the noise by 15 db. The number of superimpositions was determined from consideration of the limitation in the scanner speed due to the phase shift caused, which would impair the phase coincidence between pulses. Photo. 2 Reflected wave form from 1.0 and 6.25 mm high notch Figure 4 shows the method to generate a normal beam shear wave(9). In Fig. 4, the Lorentz forces are directed in the same direction and the resultant ultrasonic wave propagates perpendicular to the test material surface. Photograph 3 shows the prototype normal beam EMAT. Its size is 30 mm x 30 mm x 35 mm high. The permanent magnet was Sm-Co and the magnetic yoke was permendure. A poly-imid flexible substrate was used as the insulator. These materials are available up to 300dc(8). 101

5 404 Ultrasonic Testing Equipment EMAT J. Nucl. Sci. Technol., Figure 5 shows the accuracy of thickness measurement(8). The abscissa is the plate thickness measured by calipers and the ordi- Fig. 4 Mechanism of generation of normal beam ultrasonic wave Fig. 5 Result of measurement accuracy Photo. 3 Normal beam EMAT nate is the value measured using EMAT at 300dc. The experimental arrangement is almost the same as those for Fig. 1. In this experiment, the measurement signals are superimposed 128 times by the signal averager. The broken line indicates the agreement in both measurements, leading to the accuracy within mm. The measurement accuracy obtained would be sufficient for practical application. IV. VEHICLE(4)~(6) The vehicle, illustrated in Photo. 4, has a type 304 stainless steel chassis 220 mm x200 mm wide x 140 mm high. The other specifications are indicated in Table 1. The vehicle carries the scanner and moves on the stainless steel guide rails permitting the EMAT movement in the transverse direction normal to vehicle travel, driven by a 102

6 Vol. 29, No. 4 (Apr. 1992) Summary Report (M. Sato et al.) 405 Table 1 Vehicle specifications Photo. 4 Vehicle shown set on 500 A pipe ball screw. Note here that trackless vehicles inherently present the following incommodities : (1) Difficulty in precisely determining the crack location on a subject pipe due to the absence of a reference point. (2) Deviation of the vehicle travel path Fig. 6 Method of measuring vehicle trace deviation from design path 103

7 406 Ultrasonic Testing Equipment EMAT J, Nucl. Sci. Technol., from the true circumferential direction ue to inaccuracies in structural d geometry that results in chassis distortion and slight disparities in wheel size. The path actually traced by the EMAT detection head is indicated in Fig. 6 by the dashed line deviated from the solid line (design path) due to vehicle drift. In order to correct the deviation, a reference landmark was pasted on the subject pipe surface, and was detected by the optical sensor. On the vehicle, the distance traveled X and the transverse scanning displacement Y are recorded by rotary encoders, respectively. The operational flow diagram for the system is depicted in Fig. 7. After clearing the memory, the needed parameters are input and the vehicle proceeds forwards to detect the reference landmark and records the reference point. Then, the vehicle further travels on to the prescribed inspection area. Upon completing the inspection, the vehicle again travels forward to seek the other reference landmarks after completely encircling the pipe. Upon detecting the landmark, the vehicle stops to let the controller compute the deviation of the vehicle from design path, and corrects the record of actual crack position(6). Fig. 7 Operational flow chart After the demonstration of the system performance, the agreement between the design path and the actual trace was proved to be within +-2 mm, even when the vehicle had deviated by about 10 mm during encircling the pipe. This accuracy of crack position determination can be considered satisfactory for practical applications. V. CONCLUSION The foregoing performance of the ultrasonic testing equipment assures successful practical application to in-service inspection on nuclear plants, with further effort directed toward enhancing the crack location accuracy. Efforts will also be spent for improving the S/N ratio of the detection signals. Following to the practical application of the system, development and research works will be undertaken to permit obtaining B-scope and C-scope records of the subject piece. ACKNOWLEDGMENT This research was supported by the gov- 104

8 Vol. 29, No. 4 (Apr. 1992) Summary Report (M. Sato at al.) 407 ernment (Science and Technology Agent) and Japan Power Engineering and Inspection Corporation (JAPEIC). The authors are also much indebted to all the personnel in Toshiba who have contributed their efforts for the success of this work. REFERENCES (1) SENDA, Y. : Nucl. Eng., (in Japanese), 24[12], 9~14 (1978). (2) JIS Z 3060, (3) ARAKAWA, T., at al.: Proc. 9th Int. Conf. on NDE in the Nuclear Industry, 315~319 (1987). (4) SATO, M., et al.: Preprint 1987 Fall Mtg. At. Energy Soc. Jpn., (in Japanese), p. 3. (5) SATO, M., et al.: Trans. ANS/ ENS Winter Mtg., p.333~334 (1988). (6) SATO, M., et al.: J. Nucl. Sci. Technol., 26 5], 536~543 (1989). [ (7) SATO, M., et al.: Proc. 9th Int. Conf. on NDE in the Nuclear Industry, p. 327~331 (1988). (8) SATO, M., et al.: Proc. 10th Int. Conf. on NDE in the Nuclear Industry, p.737~741 (1990). (9) SHIMIZU, H., et al. : Ultrasonic Symp. Proc., p. 89~93 (1977). (10) THOMPSON, R. B., et al.: ibid., p. 74~84 (1977). (11) MORAN, T. J., et al.: J, Appl. Phys., 47[5], 2225~2227 (1976). 105