The laser ultrasonic technique is a non-contact measurement method for generating ultrasonic signals by the use of high-power pulse laser and receiving ultrasonic signals by the use of stabilized laser. When a pulse laser beam is irradiated on the surface of a specimen, ultrasonic waves are generated due to the thermo-elastic effect of the material. The ultrasonic signals propagating back into the specimen can be received by a laser interferometer. The ultrasonic signals generated by the pulse laser beam have a broadband spectrum, and the laser beam that receives the ultrasonic signals has a high resolution because it is measured within a small focus area. Since the technique is a non-contact type measurement method using lasers, it is an advanced technology that can overcome limitations of the conventional contact type ultrasonic technique.
In this study, various types of defects were measured by the use of the laser ultrasonic technique and the performance of the scanning device and laser ultrasonic device were evaluated by an authorized testing institution. The test results showed that the scanning device has a scanning range of 301 mm and a scanning control precision of 7 ㎛; while the laser ultrasonic device has the maximum receiving area of 30 MHz and the maximum generating area of 250 MHz. Ultrasonic signals generated from an ultrasonic transducer for immersion were received by a laser interferometer.
Through ultrasonic experiments, this study established a laser ultrasound technique that can measure backside defects, surface defects and internal defects. Backside defects were measured with transverse waves, surface defects with surface waves and internal defects with longitudinal waves. The depth of the backside defects was 5 mm, the depth of the surface defects was 0.1 to 0.5 mm and the depth of the internal defects was 0.2 to 0.5 mm. Finally, piping test specimens were fabricated with the electric discharge machining to measure the internal defects caused by wall-thinning damage. Internal defects of a 20 to 50 percent depth compared to the thickness of the pipes were made. In the contact-type ultrasonic method, it is difficult to measure internal defects of curved pipes, but it was possible with the non-contact type ultrasonic method using laser. When the experimental values and theoretical values were compared, it turned out that the reliability of the experimental results is high with an error rate within 3 percent.
This study demonstrated the applicability of the laser ultrasonic technique through various experiments and also conducted experiments to overcome the limitations of the existing method by taking advantage of the non-contact method. It is necessary to further improve the objectivity of ultrasonic signal analyses by enhancing the interference efficiency and S/N ratio of laser interferometer in the future.