A micromechanical-based finite element simulation of process-induced residual stresses in metal-CFRP-hybrid structures

Automotive lightweight design is a considerable measure to meet the worldwide need for reducing CO2 emissions. However, the lightweight potential of common materials like high strength steels or aluminium is limited. Hybrid materials allow to combine metals and CFRP in a manner to offset the drawbacks of every single material and reach an optimum of mechanical properties (Wang et al., 2016). Nonetheless, an essential shortcoming of hybrids are thermally induced residual stresses after cooling down from moulding temperature, driven by varying coefficients of thermal expansion and chemical shrinkage of the FRP. For a reliable prediction of the residual stress evolution in hybrid-structures during curing and after cooling down, a numerical homogenization technique of representative unit cells is proposed to calculate effective cure-dependent properties. Starting from a heterogeneous microstructure, a thermo-chemical-mechanical constitutive model for the curing process of the epoxy resin is presented and applied to two representative volume elements (RVE). The transition between the micro- and macro-scale is done by using a numerical homogenization framework. The effective cure-dependent properties predicted by the homogenization are used to simulate the curing-process and analyses the residual stresses of a metal-CFRP plate. The results are compared with experimental data obtained by the incremental hole drilling measurement.