Residual Strength of Composite Laminates Subjected to Tensile-Compressive Fatigue Loading

Abstract

The residual strengths of graphite epoxy (T300/934) laminates, in tension and in compression, were measured after exposure to tension-compression fatigue loading (R = −1). No direct correlation was found between remaining residual strengths and percentage of average fatigue life. However, a correlation scheme, based on a cumulative damage model and a stiffness change of the material, was developed for the individual specimen under test. This scheme predicts that strength will begin to degrade at about 60 to 80% of the fatigue life of the specimen, depending on the laminate structure. By monitoring stiffness change during fatigue, it was possible to predict the fatigue life of each individual specimen, and to terminate fatigue loading at some preset value of stiffness degradation before failure. It was found that only the strength of the final failure mode was degraded (that is, tension or compression), while the strength in the opposite direction did not change until almost the final failure. It was concluded that monitoring stiffness of a laminated structure will permit more effective use of the laminate and avoid premature failure.