Thermomechanical analysis of micromechanical formation of residual stresses and initial matrix failure in CFRP

Process induced thermal residual stresses and matrix failure of unidirectional CFRP has been investigated by finite element methods. Partial discrete model composites consisting of a microscopic area of fibers and matrix surrounded by a homogeneous area were chosen. Four cases have been investigated concerning the formation of residual stresses and initial matrix failure: A free UD-laminate, a constrained UD-laminate, a cross ply laminate and a thick laminate which is subjected to a temperature gradient during cooling down. On the basis of experimental results from thermo-mechanical tests of the neat resin, the temperature dependent matrix stress-strain behavior as well as the parabolic failure criterion were formulated and introduced into the finite element program. The actual stress state on the microscopic level depending on different boundary condition could be described. The authors showed that the approach of a partial discrete model is suitable to determine the initial matrix failure of different macroscopic specimens under consideration of micro-mechanical effects. The results showed that high tri-axial stresses occur in the constrained laminate and the cross ply laminate, which lead to initial matrix failure in the 90degrees-ply. The consideration of a temperature gradient affects the stress distribution in the matrix though the influence on the maximum residual stress values is small. In this case, initial matrix failure can be excluded.