A new approach of non-destructive testing for thick welded structural materials based on laser-ultrasonic technique is investigated. In this study, weld part of structural materials, which should be conventionally inspected after welding, is inspected during welding process in order to save time and cost of manufacturing. The laser-ultrasonic is a method to generate and detect ultrasonic signals by laser beams and has potential to be applied to remote inspection/monitoring of materials under welding at elevated temperature. Bulk longitudinal acoustic wave generated by a Q-switched Nd:YAG laser irradiation and detected as surface vibration by laser interferometer coupled with a long pulse detection laser is used to detect defects around the weld. To overcome the lack of sensitivity of laser-ultrasonic testing on thick welded part having a thickness of more than 100 mm at higher temperature, we have originally developed a modified synthesis aperture focus signal processing technique (m-SAFT). The in-process testing with actual piping weld having a thickness of 150 mm with high temperature more than 200 degrees C. was demonstrated. By using m-SAFT, an actual weld defect of 1.5 mm in diameter at 106 mm depth in the specimen was clearly observed. The measurement result well agreed with the result of conventional ultrasonic testing conducted after weld process and also the cross-sectional observation of the specimen.

For safety operation of nuclear power plants, soundness assurance of structures has been strongly required. In order to evaluate properties of inner defects at plant structures quantitatively, non-destructive inspection using ultrasonic testing (UT) has performed an important role for plant maintenances. At nuclear power plants, there are many structures made of cast austenitic stainless steel (e.g. casings, valve gages, pipes and so on). However, UT has not achieved enough accuracy measurement at cast stainless steels due to the noise from large grains. In order to overcome the problem, we have developed comprehensively analyzable phased array ultrasonic testing (PAUT) system. We have been noticing that dependency of echo intensity from defect is different from grain noises when PAUT conditions (for example, ultrasonic incident angles and focal depths) were continuously changed. Analyzing the tendency of echoes from comprehensive PAUT conditions, defect echoes could be distinguished from the noises. Meanwhile, in order to minimize the inspection time on-site, we have developed the algorithms and the full matrix capture (FMC) data acquisition system. In this paper, the authors confirmed the detectability of the PAUT system applying cast austenitic stainless steel (316 stainless steel) specimens which have sand-blasted surface and 3 slits which made by electric discharge machining (EDM).

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