Lawrence Berkeley National Laboratory

In automobile manufacturing, the quality of spot welds joining thin mild steel sheets is assessed primarily by the diameter of the button remaining after destructive teardown, relative to the thickness of the sheets. To facilitate a comparison with destructive testing, several features of ultrasonic images of spot welds were assessed for their ability to predict weld button area. An experiment was performed in which representative weld test coupons were imaged with a prototype portable, hand-held phased array system, and then torn down destructively and their buttons measured. Features of the ultrasonic images were examined for their ability to predict the corresponding button areas. These features included: transmissive area; a frequency ratio proxy for reflectivity; and dimensions of the surface indentation caused by the welding electrode. Regression models with these explanatory variables predicting button area all achieve 95% or better fits. Button diameters were predicted with 95% confidence to an accuracy of 2-4 times the pitch of the array. These results indicate that the ultrasonic measurement system is sufficiently accurate for weld quality assessment.