Theoretical approach to the study of fatigue of composites under spectrum loading

A method is proposed to predict the fatigue life and residual strength of fiber reinforced composites subjected to variable amplitude (VA) loadings. Based on a recently developed two-parameter residual strength model, the damage accumulation rule is based on the "equivalent strength approach" allowing to account for the variable loading without any simplifying assumption. The cycle-by-cycle effects can be easily recovered on the basis of strength degradation kinetics and the approach reliability is checked on the basis of a series of constant amplitude (CA) literature data. The approach allows correcting the imperfections of Miner's rule and its unreliability when applied to fiber composites and to build on a solid foundation a numerical procedure for fatigue calculations under spectrum loadings. High-Low (H-L) and Low-High (L-H) three-block sequences of loadings have been simulated highlighting the basic concepts of the proposed damage rule. The analytical predictions reveal that the sequence and interaction of events has a strong influence both on the rate of damage accumulation and fatigue life.