Complex stress state effect on fatigue life of GRP laminates. Part II, Theoretical formulation

The synergistic effect of in-plane stress tensor components on fatigue strength is not traditionally considered in the design of thin-wall box-beam structures, e.g. composite rotor blades in general. Fatigue life calculations account only for the normal stresses due to bending and centrifugal forces, neglecting the contribution of shear and transverse normal stresses. The theoretical formulation of a life prediction methodology accounting for all in-plane stress tensor components, through the use of a multiaxial fatigue strength criterion, is presented here. Comparison of theoretical predictions with experimental results from constant amplitude, uniaxial, off-axis tests demonstrates the drastic effect of shear and transverse normal stresses, besides that of axial normal stress, in reducing operational life of a GRP structural laminate.