Enhancements of a Stress-Based Approach for Fatigue Life Estimation of Multi-Material Connections Joined by Self-Piercing Rivets and Adhesive

Abstract

Due to the continuously reduced emission targets from legislation, lightweight constructions are getting more and more important for car manufactures. One way to reduce weight is the use of multi-material design for the body in white where new joining technologies, such as self-piercing rivets (SPRs) need to be considered instead of spot welding. Since both spot welds and SPRs are local circular joints, it appears reasonable to adapt and modify existing methods for spot welds to estimate the fatigue life of SPRs. These established methods typically use calculated radial stresses at the joint from inner forces (derived from FEM) and compare them to an SN-curve. Instead of comparing to a regression of calculated stress values, the presented method uses measured material specific SN-curves for this purpose. These curves contain the stresses of the materials at a notch, oriented on the rivet diameters. A modification of parameters in the stress calculation leads to a significant reduction of scattering in the comparison of simulated and tested fatigue life values. In addition to this method, an approach for the estimation of fatigue life of adhesively bonded joints was adapted. This method considers an effective stress value, which is evaluated either on a critical distance to the notch or is calculated as average over a defined path. Additional requirements during the development of the new method were the reduction of tests and a small complexity of the required FE-models to enable fast and still accurate fatigue life estimation on full vehicle level.