Material Characterization and Modeling for the Simulation of Adhesive Joints with Polyurethane Adhesives

Abstract

High-strength, toughness-modified epoxy adhesives (EP) combined with spot-welding is state of the art for the assembly of the metallic body-in-white (BiW) in automotive industry. To open up further potentials in lightweight engineering, multi-material design now is in the focus of the BiW development. Such structural parts motivate the development of innovative joining techniques, especially advanced adhesive joining. Through challenges in the production process and during vehicle operation, a trend towards more flexible adhesives can be observed in recent applications. Joining dissimilar materials, e.g. steel and carbon fiber-reinforced plastics, polyurethane adhesives (PU) offer many advantages over EPs. These include an increased flexibility when joining materials with different coefficients of thermal expansion, potentially higher energy absorption and the compensation of tolerances required in the assembly. However, no methods are established so far to simulate the local mechanical behavior of this softer type of adhesive, which exhibits a highly non-linear elastic behavior. As experiments indicate that PUs show characteristic features of rubber-like materials, they can be approached by hyperelastic material models for quasi-static loading, i.e. rate effects are neglected. Experiments describing the behavior of the adhesive bulk form the basis of the hyperelastic material characterization. For the typical joint load cases of shear and normal loading, it becomes evident that – beyond the established characterization tests (uniaxial tension and hydrostatic compression) for rubber materials – it is crucial to take hydrostatic tension into account. Exposed to equitriaxial loads, the change in volume of the material shows an asymmetric behavior in tension and compression that needs to be described in the material model. Validations for uniaxial compressive loading, so-called pure as well as simple shear tensile tests are conducted. For the prediction of a local damage initiation, and thus structural strength, a failure criterion for the adhesive material is proposed based on hydrostatic strength. The developed approach is considered to be the basis for the simulation of the structural response of PU-joints in a reliable, conservative design concerning stiffness and strength.