DYNAMIC WELD DEFECT DETECTION

A computer-controlled weld defect detection system has been developed which can detect defects during the welding process by ultrasonic means. This system includes a flash analogue-to-digital converter, built-in memory (for storing sampled data), a peak characteristics extractor, and a welding process controller. Using this system, common weld defects can be successfully detected concurrently with the welding process. By this method, weld quality can be assured if no defects were detected outside the threshold values laid down. Welding costs could be minimised, either by reducing repair time or by halting the weld process as soon as the defects outside the threshold were detected. Experimental results demonstrated that high temperature areas around the weld pool were the major source of difficulty and unreliability in ultrasonic defect detection during welding, this had to be allowed for. Not only did ultrasonic signal velocity change (therefore time-of-flight) but so did the signal amplitude (causing problems in accurately locating or sizing defects). Another problem created by the high weld pool temperature field was that of 'noise' in the ultrasonic signals. In order to successfully detect these signals reliably, it was necessary to employ methods of reducing noise peak and noise areas. Both data-averaging and B-scan data analysis were shown to reduce 'white noise' and 'grain noise' effectively. This work has demonstrated that avoiding 'grain noise' dominated areas is possible in many cases.