Study on Blowhole Detection in Fillet Welded Sheet of Lap Joint by Laser Ultrasonic Technique

Abstract

We developed a blowhole detection method using laser ultrasonic in this study. The blowholes are a problem in fillet welded lap joints using galvanized steel sheets often used in automobile manufacturing. Fillet welded lap joint composed of 2.3 mm or 1.6 mm thickness plates were used as a specimen, and measurement was performed on the sections without or with defects whose positions were identified in advance by RT (radiographical test). It was difficult to detect the directly reflected ultrasonic from the defect by setting the detection position on the upper plate side. However, measuring the attenuation of transmitted waves due to defects on the lower plate side was usable in determining the presence or absence of defects. In addition, an automatic judgment algorithm was developed and evaluated. The algorithm uses the normalized RMS slope as a judgment value after extracting the ultrasonic part where the attenuation was seen. The blowhole positions at RT and the position where the judgment value is －0.05 or less corresponded partially. They were in good agreement, especially under the conditions of 1.6 mm thickness and 0.5 m/min scanning speed. Although it is necessary to study further the adjustment of measurement conditions and the clarification of the applicable range, it can be said that high-speed non-contact measurement of blowholes was possible by the proposed method using laser ultrasonic.