A multiaxial fatigue failure criterion based on the principal constant life diagrams for unidirectional carbon/epoxy laminates

A new fatigue failure criterion for unidirectional composites has been developed on the basis of the modified Tsai-Hill static failure criterion that can distinguish between the static strengths in tension and compression. A new idea is to calculate the principal fatigue strengths involved by the fatigue failure criterion using the associated constant fatigue life (CFL) diagrams. First, the in-plane principal CFL diagrams for a unidirectional carbon/epoxy laminate in longitudinal, transverse and in-plane shear loading conditions are identified by experiment. Then, the four-segment anisomorphic CFL diagram approach, which was developed in an earlier study, is shown to be applicable in accurate description of all of the principal CFL diagrams that are highly nonlinear and asymmetric in shape. Finally, it is demonstrated that the off-axis S–N relationship for the unidirectional carbon/epoxy laminate can accurately and efficiently be predicted for any stress ratio and any fiber orientation using the proposed fatigue life prediction methodology.