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Previous research has shown that boron/aluminum can develop significant internal matrix cracking when fatigued. These matrix cracks can result in a 40% secant modulus loss in some laminates even when fatigued below the fatigue limit. The present study shows that the same amount of fatigue damage will develop during stress- or strain-controlled tests. Stacking sequence has little influence on secant modulus loss. The secant modulus loss in unidirectional composites is small, whereas the losses are substantial in laminates containing off-axis plies. This paper presents a simple analysis that predicts unnotched laminate secant modulus loss due to fatigue. The analysis is based upon the elastic modulus and Poisson's ratio of the fiber and matrix, fiber volume fraction, fiber orientations, and the cyclic-hardened yield stress of the matrix material. Excellent agreement was achieved between model predictions and experimental results. With this model, designers can project the material stiffness loss for design load or strain levels and assess the feasibility of its use in stiffness critical parts.
boron/aluminum, metal matrix composites, fatigue, stiffness (secant modulus) loss, shakedown analysis, saturation damage state, composites
Research engineer, National Aeronautics & Space Administration, Langley Research Center, Hampton, VA