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An approach to construct the stress (or load) vs. cycles curve for composite structures under constant amplitude loading is presented. The approach is based on the assumption that the cycles to failure are a function of the probability of failure during any given cycle. In this first investigation, this probability of failure during any cycle is assumed to be constant and equal to the probability of failure obtained from static test results and the associated statistically quantified scatter. Expressions for the cycles to failure as a function of R ratio are derived. These expressions do not require any curve fitting and do not involve any experimentally determined parameters. The fatigue predictions do not require any fatigue tests for calibration. Comparisons to several test cases found in the literature show this first simple model to be very promising.

A simple and efficient method is presented to determine the interlaminar stresses in a symmetric composite laminate under uniaxial loading. Expressions for the interlaminar stresses are assumed in terms of exponentials based on shapes that the interlaminar stresses must take in order to assure overall (integral) force and moment equilibrium. The boundary conditions and the traction continuity between plies are satisfied exactly. The exponential terms in the stress expressions are determined by minimizing the laminate complementary energy. Typical results are presented and compared with previous results found in the literature. The current method is shown to efficiently deal with the problem including the ability to perform the analysis of thick laminates (100 plies or more) with relative ease and cost-effectiveness.

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