Experimental Investigation of Compressive Overloads on Delamination Growth Behavior in Fatigued Composites

The behavior of laminated graphite/epoxy composites under variable amplitude compressive loading is studied. Laminate composites in aerospace applications undergo bo± compressive and tensile fatigue stresses as well as spectrum loading. These materials are particularly susceptible to compressive loads and overloads, which cause the growth of any small pre-existing delaminations. Such delamination growth, and the resulting sublaminate buckling, are major causes of fatigue failure. This experimental program investigates the effects of overloads on laminate composite specimens in which small delaminations are introduced during fabrication. The delamination growth is monitored in real time during fatigue tests. Different variable amplitude load spectra are applied, and the delamination position through the depth of the specimens is varied. Up to now, research has focused on constant amplitude fatigue testing or simple two block testing. Some variable amplitude studies have been done but attention has mostly been given to the reduction in residual strength or stiffness of the specimen. Much of the prior work is done in tension instead of compression. This investigation will provide data specifically on the growth behavior of pre-existing delaminations in composites under compressive overloads.