Fatigue strengths of cross-ply CFRP laminates at room and high temperatures and its phenomenological modeling

Fatigue life prediction for a symmetric alternating cross-ply carbon/epoxy laminate subjected to cyclic loading in the fiber direction is attempted on a ply-by-ply basis. First, tension–tension fatigue tests are performed under constant amplitude cycling at room and high temperatures. The normalized S–N relationship for the cross-ply laminate using the normalized stress level agrees with that for the unidirectional laminate made of the same prepreg tape. This implies that the on-axis fatigue behavior of the cross-ply laminate is substantially governed by that of the constituent plies in the laminate. Then, to evaluate the in situ strength of the constituent plies in the cross-ply laminate, static tension and tension–tension fatigue tests are carried out on unidirectional laminates with different number of plies. While the tensile strength of unidirectional laminate significantly depends on the thickness of laminate, the relative fatigue strength is insensitive to the number of plies. Using a modified Tsai-Hill static failure criterion that considers the in situ strength of plies, we can adequately predict the static tensile strength of the cross-ply laminate. Second, based on that fact and the assumption that the final failure of the cross-ply laminate is determined by the failure of axial plies, a simple fatigue failure model for the cross-ply laminate is developed by means of the classical lamination theory and the ply fatigue model considering the in situ static strength of plies. Finally, validity of the proposed model for the fatigue failure of symmetric alternating cross-ply CFRP laminates is evaluated by comparing with experimental results. It is demonstrated that the fatigue strength of the cross-ply laminate in the fiber direction is successfully predicted by the proposed fatigue model with a consideration of the in situ strength of ply.