Fatigue Response and Modelling of Variable Stress Amplitude and Frequency in AS-4/PEEK Composite Laminates, Part 2: Analysis and Formulation

Abstract

The tension-tension fatigue features at variable cyclic loadings in APC-2 composite laminates of three lay-ups, including cross-ply [0/90]4 s, quasi-isotropic [0/45/90/−45]2 s and angle-ply [±45]4 s, were investigated. The parameters involved are maximum stress, frequency, and stress ratio. First, the fatigue tests of two-step and multi-block loadings were accomplished. Next, based on Marco-Starkey cumulative damage theory, we proposed the nonlinear cumulative damage theory and nonlinear reduced undamage theory associated with damage parameters in Part 1 and developed the well-established fatigue failure criterion of multi-block loading to predict the life due to variable fatigue loadings. The difference of cycle fraction between two damage loading conditions was obtained, so that we could easily shift to different damage curve and predicted the life due to multi-block fatigue loading. The measured lives in glass/epoxy and carbon/PEEK composites are found well close to those predicted by our proposed models.