Abstract
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 cool down. On the basis of experimental results from thermo-mechanical tests of the neat resin, the temperature dependent matrix stress/strain behavior was formulated and implemented into the finite element program. The actual stress state depending on different boundary condition could be described. The parabolic failure criterion was incorporated into the FE-Analysis. The failure criterion is based on the temperature dependent strength of the matrix, in order to take the competition of increase in residual stresses and increase of strength into account. 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, which lead to initial matrix failure. The interaction of the 0°-layer in a cross-ply laminate lead to a stress state which causes initial matrix failure in the 90°-layer. The consideration of a temperature gradient changes the stress distribution in the matrix but shows a small influence on the maximum residual stress values. In this case initial matrix failure can be excluded.
