Abstract
Welding processes have been widely used for the construction of metal structures. Their reliabilities depend upon the weld quality as well as the weld size. Therefore, various nondestructive inspections have been applied to the welded region, in which the methods with X-ray and ultrasound are widely used for the detection of defects and for the measurement of weld size. However, these methods are too difficult to detect the fusion zone nondestructively, in spite of large demands of its evaluation. Along with the recent application of high strength steel sheet to the automobiles, this problem becomes a critical subject in the spot welds, in which the discrimination of fusion zone called "nugget" from the solid-state welded zone called "corona bond"is strongly demanded. This study focused on the spot welds and aimed to discriminate the nugget from corona bond nondestructively. A new ultrasonic measurement with line focused probe was proposed for the evaluation of the nugget. In the experiment, two types of thin sheet specimens were prepared for the clarification of the acoustic phenomena in the fusion zone. One was the resistance spot welded specimen and another was the gas tungsten arc welded specimen. It was shown by the measurement of these specimens that the sound velocity in the fusion zone is different from one in the base metal and it depends on the inhomogeneous micro structures. This different sound velocity causes different amplitude of reflected waves in the nugget, which also depends upon the measuring condition. From these results, it was shown that the measurement based on the different amplitude and the dependence on the measuring condition enables the nondestructive discrimination of the nugget from the corona bond.
