Ultrasonic nondestructive evaluation of spot welds for zinc-coated high strength steel sheet based on wavelet packet analysis

Abstract

In the present paper ultrasonic testing of spot welds is briefly reviewed for evaluating the size or quality of spot welds. In this work, a zinc-coated high strength steel sheet DP 600 is used for the spot welding. A wavelet packet transform (WPT) processing is adopted to analyze the ultrasonic signals acquired by the pulse echo from spot weld. For the purpose of nondestructive evaluation to the spot weld, the work is to establish the relation between ultrasonic signals and nugget diameter or relevant maximum shear load. The results indicate that the characteristic value in relation with the nugget diameter and maximum shear load of spot weld is found on a basis of decomposed signals in different scale in WPT. Moreover, the characteristic value is indicated by the power of signals decomposed in WPT. The nugget diameter and maximum shear load are decreased with increasing the characteristic value. Moreover, it is found that the tensile stress of the spot welds has no clear relation with the value of characteristic signal. Furthermore, the obtained characteristic value may be an important basis for an automatic recognizing of weld size and maximum shear load. In addition, spatial averaging was employed to decline the stochastic disturbance and effect of the human factor on the signal before WPT.