This paper aims to investigate the tension-tension (T-T) fatigue behavior at variable fatigue loadings in AS-4/PEEK composite laminates. Three lay-ups of specimens, such as cross-ply [0/90]4s, quasi-isotropic [0/45/90/-45]2s and angle-ply [±45]4s, were tested at room temperature and moisture. The parameters involved were maximum stress (low and high), frequency (5 Hz and 25 Hz), and stress ratio (R = 0 and 0.2). We received the fatigue life and found that the maximum stress and stress ratio are the two controlling factors on lives, whilst frequency affects significantly in [±45]4s laminates. Next, stiffness and strength degradation were defined to describe the laminate damage process due to cyclic loadings and these experimental data were correlated with suitable models proposed in power function form. From the damage curves we observe that three stages of damage growth can be clearly divided and different failure mechanisms exist among three lay-ups. All these valuable data will be used to predict fatigue life under multi-block cyclic loadings in association with Cumulative Damage Theory in Part 2: Analysis and Formulation.
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.
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