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A study was conducted to examine the damage tolerance of composite laminates with impact damage. A model is presented to predict the residual strength of such laminates. The in-plane strain distribution near the region of damage is determined using Lekhnitskii's solution for an anisotropic inclusion in an anisotropic medium with compromised constitutive properties of the impact region used for the properties of the inclusion. The residual strength is then predicted using an average criterion based on strain. Results from experiments of impacted specimens of [±45/0]5 Kevlar/epoxy and [±45/0]25 graphite/epoxy show good correlation with the analytical predictions. The model is applied over a wide range of impactor conditions with equally good results. Although the damage was inflicted with different impactor masses, velocities, and laminate boundary conditions, the data indicates that the residual strength is only a function of the damage present and not the manner in which the damage is introduced. The use of the measured damage in conjunction with the predicted properties of the compromised region thus shows good promise for its application over a wide range of structures in a practical fashion. An overall analytical philosophy to deal with the problem of impact in composite laminates is discussed.
composite materials, impact, residual strength, damage tolerance, damage resistance, graphite/epoxy, Kevlar/epoxy
Research associate, Hercules Aerospace Co., Magna, UT
Associate professor, Technology Laboratory for Advanced CompositesMassachusetts Institute of Technology, Cambridge, MA