A two-stage theory on fatigue damage and life prediction of composites

Fatigue tests on unnotched and notched composite laminates under compressive cyclic loading have been carried out. The damage mechanism is discussed, and it is seen that the process of composites fatigue damage under the compression cycles loading appears at two different stages. A dual fatigue cumulative damage rule to predict fatigue damage formation and propagation of notched composites is presented according to the traditional phenomenological fatigue methodology and a modern continuum damage mechanics theory. Then a three-dimensional damage constitutive equation for anisotropic composites is established. A new damage evolution equation and a damage propagation σ_a- σ_m-N^* surface is derived based on damage strain energy release rate criterion. Finally, the theoretically predicted results are compared with experimental results.