INVESTIGATION OF SUCCESSIVE FAILURE MODES IN GRAPHITE EPOXY LAMINATEDCOMPOSITE BEAMS

The determination of failure modes in composite materials is often dif ficult to characterize because of the composition and variety of these materials. Several theoretical failure theories have been proposed, b ut no single one is accurate for every composite material; most of the m predict only the first failure. Since most composites structures may still carry load after one or more plies have failed, the behavior of composite laminates is investigated beyond the first failure. This ar ticle presents a theoretical and experimental investigation of the suc cessive failure modes of graphite epoxy laminated beams. Two failure t heories are used: the maximum stress and the Tsai-Wu failure theories. Calculations are based on laminated plate theory. Once a ply fails in a laminate, it is assumed that it cannot carry any more load (a conse rvative assumption), and its elastic properties are set to zero. The f ailure analysis is then repeated with the modified laminae based on up dated [ABD] matrices until the next failure point is reached. Experime ntal results are obtained using the three point bend test on an Instro n machine. Five composite laminate types were used for testing. Two sp ecimens of each sample were used to establish repeatability of failure modes. A comparison of the theoretical and experimental results shows that failure based on the failure theories often occurs at substantia lly lower loads than for actual failure; moreover, while classical the ory predicts that layers will fail one by one in a laminate, plies in actual samples often fail as a group.