Finite element based design of a new dogbone specimen for low cycle fatigue testing of highly filled epoxy-based adhesives for automotive applications

Highly filled epoxy-based adhesives are used as thermal and electrical interfaces in automotive electronics. The successful design for reliability of electronic packages depends on understanding and modeling the fatigue behavior of these materials. Both mechanical and thermal loadings are varying in a cyclic manner in test or field conditions. To establish an adequate constitutive model describing the thermo-mechanical behavior under cyclic mechanical and thermal loading, material testing is required on a dedicated test specimen. Current specimen geometries which are used to describe fatigue behavior of highly filled epoxy-based materials fail to describe the mechanical behavior. They contain stress concentrations near the clamping area, which leads to the failure within these areas and not in the reduced section of the beam. In this paper, a new test sample with dogbone geometry is designed by utilizing finite element analysis (FEA). A design space (DoE) is used to analyze the effect of important parameters (e.g. thickness, length and radius) that influence the stress distribution. The optimum design point is selected by FEA of stresses in the reduced section of the beam and near the clamping. In order to validate the simulations, experiments are done with samples, which are produced with the help of PTFE coated steel molds. This approach is shown to improve the existing dogbone designs and validity of the low cycle fatigue testing.