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The stiffness of a flat specimen, made of graphite epoxy (T300/934), was measured during reverse fatigue loading. Laminates of basic layup, unidirectional cross-ply and angle-ply, as well as multidirectional (isotropic) layup, were tested. It was found that all the laminates exhibit rapid stiffness degradation on the last (third) fatigue stage. Except for the [0°] unidirectional laminate, the last stage starts at about 70 to 80% of the fatigue life, regardless of the load level and the final mode of failure (tension or compression). It is shown that for this material, stiffness degradation is a result of crack accumulation, mainly along the fibers, even for the fiber dominated laminates. This general phenomenon can serve to predict usable safe life of laminates under fatigue loading.
Professor, Technion-IIT, Haifa,
Stock #: CTR10153J