A study of factors affecting life prediction of composites under spectrum loading

The influence of damage accumulation metric, constant life diagram formulation and cycle counting method on life prediction schemes for composite materials under variable amplitude (VA) loading is investigated in this paper. Special focus is put on residual strength based methodologies; different degradation functions are implemented and compared with the Palmgren–Miner (PM) damage accumulation rule. The predictions are validated by comparison with experimental data from a unidirectional glass/epoxy laminate cycled with three different loading spectra. The effect of constant life diagram (CLD) and cycle counting method used in life prediction is investigated through implementation of alternative existing formulations. Results indicate that a net improvement is achieved when linear strength degradation is implemented as damage metric in life prediction schemes, over the state-of-the-art PM summation. Depending on the spectrum pattern itself, CLD and counting method might be of predominant role; Rainflow counting and the simplistic concept according to which a single S–N curve describes fatigue response of the material under any loading condition (R ratio) are shown to yield consistently best life predictions.