Experiences with a New Real-Time-Scanner System

Introduction

Ultrasonic weld inspection at joint components or constructions during fabrication and for inservice are required in a lot of standards around the world, to avoid catastrophes for the humans and the environment. Due to these facts inspection techniques were developed in the past and in the present to increase the safety during operation. On the other hand also economical requirements coming out more and more to make the used techniques cheaper and faster. The application of phased array techniques e.g. for the inspection of the ligaments of the dome or bottom of a pressure vessel in a nuclear power plant, has reduced the time for the examination to the half of the time needed with conventional ultrasonic techniques. For the inspection of welds in heavy components with wall thicknesses up to 180 mm, the application of phased array techniques delivers also a big advantage due to the reducing of the number of probes. For weld inspection by wall thicknesses of 15 mm up to 40 mm the examination is carried out using conventional ultrasonic probes moving parallel and perpendicular to the weld manual or mechanical. With a Real-Time- Scanner (RTS) system the perpendicular moving can be carried out electronically controlled. If this scanning is carried out together with the angular scanning of the beam, as known from the phased- array technique, an examination of the weld is possible. The present paper will give information about the RTS system and some information for practical applications.

Equipment and Principle of the Real-Time-Scanner

The features for an RTS system are, steering the delay time of the transmitter pulses of 8 elements so that an angular scan and focusing of the sound field is possible as well as steering the linear scanning direction. An equipment in which these requirements were realized was developed at BAM. Characteristic data of this equipment are listed in which also the experimental Arrangement is schematically shown. First experiments with the linear array probe consisting of 32 copolymer elements, each 0.7 mm width, 0.05 mm gap between the elements and a frequency of 17 MHz. Principle sound field steering like angular scan and focusing is carried out using a group of 8 elements. The electronically controlled scanning of the sound field is done by moving this group in the linear direction. Application areas of such a system could be the detection of corrosion phenomena's in thin metal sheets, the detection of delamination produced through impacts at carbon and glass fibre composites and the inspection of welds. Regarding these possible application areas, experiments were carried out in the lab. Some of the results will be presented in the next chapter.

Experimental Results

For the characterization of the linear transducer, experiments were carried out at a 5 mm thick metal sheet with 0.7 mm flat bottom holes in different depth, starting by 1 mm and going up to 3.5 mm in a depthwidth of 0.5 mm. The second aim of this investigations were to show how the system can be used for Korrosion detection. An example of the results is shown was shown. In the C-scan the indications of the flat bottom holes are clearly recognizable, including the dependence of the echohight from the time of flight, i.e. the attenuation of the sound with the increasing of the distance from the surface. This behaviour is also shown due to the echodynamic pattern. Further the differences of the holes are clearly visible. From the distance between the center lines of the holes of 1.5 mm the gap between two holes is calculated to 0.8 mm. That is nearly equivalent to the size of one element. From this result we can estimate the resolution in the scanning direction of the finear transducer array which is approximately 0.3 mm if the 6 dB drop is considered and the sound field is focused in a certain distance from the surface, i.e. without dynamic focusing. At the right hand side of the figure the principle experimental Arrangement is shown again. Another example for weld inspection was presented. The linear transducer array is divided in two systems, each of it with 32 elements, whereas the physical behaviour of 7 elements are over- lapping. With this special design of a linear transducer array with two wedges, the scanning for the detection of defects in the weld and the adjacent base material (heat effected zone), is electronically controlled for the perpendicular direction to the weld line. The movement along the weld line of the probe is mechanically done. The design with two wedges and therefore a reducing of the wedge sound path is necessary due to the high attenuation in the wedge material. Further advantages are the easy adaptation of such probes to the surface of the component to be inspected and the limitation of the needed electronically realized delay time for the transmitter pulses. Pipeline weld inspection can be now canied out with a fast ultrasonic- and imaging system, because all of the image presentations, weil known from the application of phased array systems, are available too. Experiments will be carried out in the near future.

Conclusion

The contribution showed the possibility of ultrasonic inspection by using modern and advanced real- time-scanner systems. With the developed equipment some of well known inspection problems can be solved with advantage i.e. faster inspection and therefore cheaper. The detection of damaged areas due to Korrosion and the image presentation of the results is also possible as well as the inspection of delamination at carbon or glass fibre composites. Weld inspection, especially at welds with a thickness between 15 and 40 mm, using a linear array with approximately 3 MHz, can be carried out with a reduced mechanical movement of the probe. First experiments with the new system were carried out in the lab and in the near future, practical applications e.g. corrosion- and weld inspection are foreseen.

References

• This paper was introduced during the ASNT fall Conference 96 (Report) in Seattle WA.
• Sasaki et al. "High Speed C-Scan Imaging System with Electronic Scanning 25 MHz Ultrasonic Beam", Acoustic Imaging, Vol 19 (1992) page 251-261
• Schenk et. al. A Real -Time-Scanner-System for Linear Multielment-Probes DGZfP 1996
• Lloyed " Ultrasonic System for Imaging Delaminations in Composite Materials", Ultrasonic Vol. 27 (1989) page 8-18

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