Detection of discontinuities in contact pressure between rough surfaces

A 10 MHz transducer with 80% bandwidth was used to detect changes in contact pressure over time for two rough solid surfaces. Changes in the reflected pulse frequency spectrum were analyzed using the second central moment (skewness) in order to identify changes in the non-linear interface properties, particularly decreased acoustic reflection for lower frequencies. This process was applied specifically to monitor the contact pressure between a copper welding electrode and a steel plate for resistance spot welding. Discontinuities in the expected contact pressure at this interface were used to assure process quality and detect process disruptions, such as expulsion.