Study on Detecting Method of Internal Defects by Laser Ultrasonics in Lap Joint Welding of Galvanized Steel Sheet and Finite Element Analysis of Its Detectability

Blowholes caused by vaporization of the galvanized layer are a problem with galvanized steel sheets, which use lap joint welding. The laser ultrasonic method is the possible solution to realize the desirable 100% inspection instead of the conventional sampling inspection. We have previously proposed a method to detect blowholes by capturing the reduction in ultrasonic intensity when it passes through internal defects through signal processing. However, there was a problem that the detection indicator devised varied. In this study, we investigated the causes and trends of detectability using finite element analysis. To efficiently calculate the results obtained by scanning measurement, we proposed and established a method to reproduce the results by taking the results from static measurements, which were shifted in the direction of the weld line little by little. As a result, it was found that one of the reasons for the detection indicator variation with scanning by the lasers is the three-dimensional positional relationship between the blowhole and the scanning measurement line. In addition, it was possible to propose the performance required for the ultrasonic generation laser such as the traveling speed and the repetition frequency by back-calculating the spatial resolution from the rate of detection needed.