Ultrasonic Nondestructive Evaluation of Porosity Size and Location of Spot Welding Based on Wavelet Packet Analysis

Abstract

There is a lack of efficient methods for ultrasonic nondestructive quantitative evaluation of porosity in resistance spot welding, and the precise location of porosity in a nugget can be determined only when the sizes of porosity and nugget are calculated concurrently. The sizes of both porosity and nugget in a resistance spot welded joint made from stainless steel based on the decomposition and reconstruction technique of wavelet packet were investigated. The newly reconstructed sequences in some decomposed nodes make the porosity more salient than the original signal. The signals in different porosities reflected by a fixed wavelet packet node were found to be inadequate. Thus, the wavelet packet coefficients in different nodes were revised to reconstruct a new signal. This method is less affected by the conditions of workpiece surface than the original signal and application of a fast Fourier transform. The porosity size and location can be calculated more accurately, and the calculation precision is limited to 7%.