Automotive lightweight design is a considerable measure to meet the worldwide need for reducing CO2 emissions. However, the lightweight potential of common lightweight materials like high strength steels or aluminum is limited. Even carbon fiber reinforced plastics (CFRP) components, which have superior lightweight characteristics, show limitations for the car body design, as catastrophic failure or high production costs. Hybrid allow to combine metals and CFRP in a manner to offset the drawbacks of every single material and reach an optimum of mechanical properties and costs. Nonetheless, an essential shortcoming of hybrid materials are thermally induced residual stresses after cooling down from the molding temperature to ambient temperature, driven by contrary coefficients of thermal expansion and chemically induced shrinkage of the CFRP.
Conventional methods for determining residual stresses in thickness direction are not reliably applicable for CFRP-metal hybrids. Several specific hybrid characteristics, as anisotropic mechanical properties with sudden through-thickness changes and residual stress gradients at interface layers, pose a challenge for the determination of residual stresses with conventional methods. A reliable experimental approach to accurately determine and analyze the residual stresses is the hole-drilling method. In order to comprehend the experimentally measured residual stresses over the thickness direction, finite element simulations are carried out.
For a reliable prediction of the residual stresses on the macroscopic level during the curing process, a numerical homogenization technique of representative unit cells are used to calculate effective cure-dependent thermo-mechanical properties. The simulation of the cutting process, which is applied in the context of the hole-drilling method, will be presented with the goal of obtaining information about the sensitivity of fiber waviness and distribution of initial voids. Furthermore, it is not clear how inter- and intralaminar damage processes in the nearby process zone influence the measurement of residual stresses in thickness direction. In this context a multiscale technique is needed, which combine the microscale (fiber, matrix and interface damage) and the macroscale (released strains at the top of the surface).