Ultrasonic detection for tensile-shear strength of spot welding under different indentations based on wavelet analysis

Abstract

Traditionally, the tensile-shear strength is detected using destructive testing. A viable non-destructive testing means, such as an ultrasonic technique, is necessary in the production environment to assess the tensile-shear strength. In this paper, electrode tips that were formed into different sizes were used to obtain different spot weld indentations. The method, based on the echo amplitude of a faying surface, which is viewed as an effective method by previous studies, was proven to be limited when changing the size of the electrode tip, owing to the occurrence of a spot welding structure with particular characteristics, ie an indentation and corona bond. The nuggets calculated, based on the echo amplitudes, were at the borders of the indentation, which increase with an increase in the size of the electrode tip. To address this issue, a wavelet function was designed based on Chebyshev low- and high-pass filters, relating to the frequency and bandwidth parameters of an ultrasonic probe. The largest energy value from the approximate coefficient of five-level wavelet decomposition is used to distinguish the nugget, corona bond and indentation. The nugget areas obtained, based on the largest energy value from the approximate coefficient, decrease with an increase in the size of the electrode tip and increase linearly with the tensile-shear strength.