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The fatigue behavior of multidirectional graphite-epoxy laminates is examined and analyzed in terms of the single lamina behavior. The nonlinear viscoelastic moduli of the epoxy matrix are changed with temperature and thus the characteristics of the laminae are altered. As a result, the static and fatigue strengths of a laminate are changed due to the redistribution of the stress field in the laminate and the weakening effect of temperature on the matrix. The fatigue behavior of a single lamina is characterized by its static strength and its “fatigue function” that expresses the degradation in the strength of the lamina due to cyclic loading. The effect of temperature is introduced through the use of “shifting factors” for both the static strength and the “fatigue function.” The laminate strength is predicted by considering the cyclic stress field in each lamina, the interlaminar stresses, and the experimentally determined shifting factors. The first failure of a lamina in a laminate is examined first in terms of stress redistribution and second in terms of total failure and final laminate fracture. The analytically determined results are compared with the actual fatigue behavior of many T300/5208 graphite-epoxy composite laminates. The experimental results are shown to be in good agreement with the theoretical predictions. The temperature “shifting factors” enable one to predict long-term behavior at some temperature from short-time testing at elevated temperatures.
composite materials, graphite-epoxy, static strength, fatigue (materials), temperature dependence, shifting functions, failure criterion, life prediction
Senior Fellow, National Research Council,
Chief, Materials Science and Applications Office, Ames Research Center, National Aeronautics and Space Administration, Moffett Field, Calif.