Life Prediction Methodology for Composite Structures. Part I—Constant Amplitude and Two-Stress Level Fatigue

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Abstract
A strength-based wearout model is presented for predicting the residual strength and life of composite structures subjected to constant amplitude or two-stress level fatigue loadings. It is assumed that the structure undergoes proportional loading, that its residual strength is a monotonically decreasing function of the number of loading cycles, and that both the life distribution due to continuous constant amplitude cycling and the residual strength distribution after an arbitrary load history may be represented by two parameter Weibull functions. The model also incorporates a "cycle mix factor" to account for the degradation of strength and life that may be associated with frequent changes in the stress amplitude of the loading. Experimental results from two-stress level, uniaxially loaded laminates are used to evaluate the model's predictive capability for sequencing effects, as in a low-high or high-low test, and for cycle mix effects, as in a low-high-low-high repeating test. Good correlation between theory and experiment is obtained for all loadings and laminates studied.