Effect of Impact Damage and Holes on the Compressive Strength of a Graphite/Epoxy Laminate

The effect of low-velocity impact damage and circular holes on the compressive strength of a 48-ply orthotropic graphite/epoxy laminate has been studied experimentally. Unidirectional tapes made of Thornel 300 graphite fibers preimpregnated with Narmco 5208 epoxy resin were used to fabricate the [±45/0₂/±45/0₂/±45/0/90]_2s laminate. Aluminum spheres 1.27 cm in diameter were used as projectiles and propelled at speeds between 52 and 101 m/s. The extent of interior laminate damage caused by impact was determined by ultrasonic inspection and, in some cases, by cross-sectioning specimens for visual inspection. Some specimens were impacted and then loaded to failure in compression to determine their residual strength. Other specimens were loaded to a prescribed compressive strain and impacted at the applied load. Some of these loaded specimens failed catastrophically on impact. Specimens that did not fail catastrophically were subsequently loaded to failure in compression to determine their residual static strength. It was found that low-velocity impact damage can seriously degrade the laminate static compressive strength. Several impact-damaged specimens were subjected to low-strain compression-compression cyclic loading, which was found to degrade further the laminate compressive strength. Specimens with circular holes with diameters up to a third of the specimen width were loaded in static compression to failure, and it was found that holes can also degrade the compressive strength of the laminate. Impact at the higher speeds reduced the compressive strength of the laminate more than largest holes studied.