Spot-welded joints subjected to shear-tension loading. Part 2: Mixed-mode fracture parameters

Abstract

Fracture parameters that can be used in the fatigue analysis of spot-welded joints subjected to shear-tension loading, with local restraints, are presented. Finite element analysis has been used to obtain stress intensities in the vicinity of three different types of fatigue crack which, from practical experience, are introduced into a spot-welded joint, i.e. assumed notch crack, through-thickness crack, and through-nugget crack. A combination of elastic-plastic finite element and linear elastic fracture mechanics finite element analysis has been used and the results are presented in the form of fatigue and fracture parameters (i.e. structure configuration factor, notch configuration factor, crack configuration factor, and J-integral value) for a wide range of specimen and crack geometric parameters. These parameters are chosen to be representative of typical practical situations and have been determined from evidence presented in the open literature. The extensive range of structure, notch, and crack configuration factors obtained from the finite element analyses are then used to obtain equivalent prediction equations using a statistical multiple non-linear regression model. The accuracy of this model is measured using a multiple coefficient of determination, R2, where 0 ≤ R2 ≤ 1. This coefficient is found to be greater than or equal to 0.98 for all cases considered in this study, demonstrating the quality of the model fit to the data. Predictive equations for stress intensity factors and J-integral values, which are based on the elastic stress concentration factor, are also developed. Some approximate equations for crack configuration factors have been published in the past, all based on analytical solutions. All equations were derived in terms of two sets of non-dimensional parameters (i.e. the bending stress-membrane stress ratio at the nugget and the geometric ratio). These equations have to be coupled with a shell-beam finite element analysis in order to predict bending and membrane stresses at the weld nugget. Therefore, the accuracy of the results depends on the accuracy of the shell-beam finite element models used. In the present study, all prediction equations are directly related to the geometric ratios, which provides a simple and independent calculation procedure. Discrepancies between the results from the present study and those from the literature are discussed.