Uncertainties in Finite Element Analysis of Yield Point Phenomena in Advanced High-Strength Steel Spot Welds

Abstract

Effects of local plastic instabilities in the form of yield point phenomena (YPP) were investigated on local and global deformation and failure behavior of resistance spot-welded tension–shear (TS) samples made from M1700 advanced high-strength steel (AHSS) by means of finite element (FE) simulations. Material models with and without YPP were used in FE simulations, and sensitivity of results to YPP material model characteristics and failure criteria was investigated. Results from FE simulations revealed several stages of load oscillations in predicted global load–displacement curves with different amplitudes depending on material model with YPP. Results also showed the existence of heterogeneous distribution of equivalent stress in heat-affected zone (HAZ) of TS samples with strong sensitivity to the upper and the lower yield stress values in YPP material model. A stress-based failure criterion was added to the damage model to account for this stress heterogeneity, and results from this model showed that the loading and failure behavior of TS sample can significantly change from ductile to premature semi-brittle failure with significant reduction in strength of TS sample. Results from FE simulations were also compared with those from TS experiments and indicated the need for development of proper material and damage models suitable for materials with YPP.