A general failure criterion for spot welds under combined loading conditions

Abstract

The circumferential failure mode of spot welds is investigated under combined loading conditions. Failure mechanisms of spot welds under different loading conditions are first examined by the experimental observations and a plane stress finite element analysis. An approximate limit load analysis for spot welds is then conducted to understand the failure loads of spot welds under combinations of resultant forces and resultant moments with consideration of the global equilibrium conditions only. The approximate limit load solution for circumferential failure is expressed in terms of sheet thickness, nugget diameter and combinations of loads. Failure contours are generated for spot welds under opening and shear loading conditions. The results indicate that failure contours become smaller when the ratio of the sheet thickness to the nugget diameter increases. Based on the approximate limit load solution, a general quadratic failure criterion for spot welds under combined three resultant forces and three resultant moments is proposed with correction factors determined by fitting to the experimental results of spot welds under combined loading conditions. The failure criterion can be used to characterize the failure loads of spot welds with consideration of the effects of sheet thickness, nugget diameter and combinations of loads. Experimental spot weld failure loads under combined opening and shear loading conditions and those under combined shear and twisting loading conditions are shown to be characterized well by the proposed failure criterion. Finally, a simplified general failure criterion for spot welds under three resultant forces and three resultant moments is proposed by neglecting the coupling terms of the resultant forces and moments for convenient use of the failure criterion for engineering applications.