Fatigue failure model for composite laminates under multi-axial cyclic loading

Fatigue life prediction equation is proposed based on fatigue modulus concept. Fatigue modulus degradation rate at any fatigue cycle was assumed as a power function of number of fatigue cycles. New stress function describing the relation of initial fatigue modulus and elastic modulus was used to describe material non-linearity at the first cycle. Fatigue modulus at failure is assumed to be proportional to applied stress level. Material constants in the equation can be determined from static and uniaxial cyclic loading. Using cumulative damage model, fatigue life prediction on composite laminates under multi-axial stress conditions was derived as functions of applied stress level and number of fatigue cycles. The fatigue failure model was verified by fatigue tests using bi-directional (cross-ply) carbon fiber-epoxy composite tubes under combined axial and torsional loading. It was shown that the proposed equations have wide applicability and the presented prediction agreed well with experimental data.