Microscopic yielding of CF/epoxy composites and the effect on the formation of thermal residual stresses

A micromechanical finite element calculation was performed in order to investigate the influence of matrix plasticity and temperature dependent mechanical and thermal properties on the formation of thermal residual stresses in unidirectional carbon fiber epoxy composites. Several thermal and mechanical experiments were carried out on epoxy neat resin specimens in order to investigate the thermomechanical behavior. The results were incorporated into a numerical calculation, and the simulation of the material behavior showed good agreement with the experiments. Thermal residual stresses were determined by a micromechanical analysis using a hexagonal unit-cell. The results of our non-linear calculation were compared with results of a numerical and analytical calculation with a linear elastic matrix. It was found that the residual stresses cause local matrix yielding at positions of high shear stresses during cooling down. The matrix stress was redistributed after local yielding occurred and leads to a reduction of residual stresses in the entire matrix compared to a linear elastic matrix. The results explain how local matrix yielding reduces concentrations of high residual stresses.