Compatibility of Rupp’s Structural Stress Method for Fatigue Life Prediction of Self-Piercing Rivets

Abstract

<div class="section abstract"><div class="htmlview paragraph">The Self-Piercing Rivet (SPR) is an effective method for joining aluminum sheets and dissimilar materials. The durability assessment of SPR joints is essential for the optimum design of the automotive body-in-white structure. Fatigue analysis is required for any structural system subject to cyclic loading where durability assessment is required. While there is no established fatigue life prediction model for SPR joints, Rupp’s model is a well-established fatigue life prediction method intended for resistance spot welds. Rupp’s model has been the automotive industry’s choice for fatigue life estimation due to its computational efficiency and ability to capture various loading conditions. The purpose of this study is to investigate the compatibility of Rupp’s model with SPR joints. Load-control fatigue testing was conducted on cross-tension SPR joints of aluminum sheets (Al 6016) with dissimilar thicknesses and SPR joints of dissimilar materials (Al 6016 to DX54D steel). Fatigue tests were performed under normal tension, shear, and combined (multiaxial) load cases using a fixture that was developed in-house. Simple finite element (FE) models were generated for the various cross-tension specimens tested in this study, where the sheets and SPR joints were modeled using shell and beam elements, respectively. Components of force and bending moment at the SPR joint were extracted from the FE model. Structural stress values were calculated using Rupp’s equations and the existing set of coefficients that have been obtained for spot welds of aluminum sheets. A new set of coefficients for Rupp's model were extracted for the SPR joints through statistical analysis and the compatibility of Rupp’s model with SPR joints was discussed.</div></div>