LIFE PREDICTION METHODOLOGY FOR COMPOSITE STRUCTURES .1. CONSTANT AMPLITUDE AND 2-STRESS LEVEL FATIGUE

A strength-based wearout model is presented for predicting the residua l strength and life of composite structures subjected to constant ampl itude or two-stress level fatigue loadings. It is assumed that the str ucture undergoes proportional loading, that its residual strength is a monotonically decreasing function of the number of loading cycles, an d that both the life distribution due to continuous constant amplitude cycling and the residual strength distribution after an arbitrary loa d history may be represented by two parameter Weibull functions. The m odel also incorporates a ''cycle mix factor'' to account for the degra dation of strength and life that may be associated with frequent chang es in the stress amplitude of the loading. Experimental results from t wo-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-hig h repeating test. Good correlation between theory and experiment is ob tained for all loadings and laminates studied.