Nonlinear Constant Fatigue Life Diagrams for Symmetric Angle-Ply CFRP Laminates at Room Temperature

A fatigue life prediction method for composites which is based on the anisomorphic constant fatigue life (CFL) diagram is further tested on a class of composite laminates that exhibit matrix-dominated fatigue behavior. First, constant-amplitude fatigue tests are performed at room temperature on three kinds of symmetric angle-ply CFRP laminates [±θ]_ (θ=30,45,60°) at different stress ratios, respectively, in order to quantify the effect of stress ratio on the constant amplitude fatigue behavior of the angle-ply CFRP laminates. The experimental results demonstrate that the S-N curve significantly depends on the value of stress ratio. It is also observed that the stress amplitude for a given fatigue life becomes largest under a fatigue condition in which stress ratio is equal to the ratio of compressive strength to tensile one. The full shapes of the CFL diagrams for the angle-ply laminates are identified on the basis of the fatigue test results. The CFL diagrams for the [±30] and [±45] angle-ply lay-ups are favorably predicted using the anisomorphic CFL diagram approach, while for the [±60] lay-up the predictive accuracy was poor due to a significant distortion of the experimental CFL curves for a left neighborhood of the critical stress ratio. To accommodate such a distortion of CFL diagram, the anisomorphic CFL diagram is developed into a connected anisomorphic CFL diagram which consists of three segments: the T-T and C-C dominated segments and a transitional segment in between. It is demonstrated that the extended three-segments anisomorphic CFL diagram approach allows adequately predicting the S-N relationships for different stress ratios as well as accurately describing the CFL diagrams for the angle-ply laminates of different lay-ups.