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Experimental evidence has shown that significant stiffness loss occurs in graphite/epoxy laminates when matrix cracking and interply delaminations exist. Therefore, a cumulative damage model for predicting stiffness loss in graphite/epoxy laminates is proposed herein by applying a thermomechanical constitutive theory for elastic composites with distributed damage. The model proceeds from a continuum mechanics and thermodynamics approach wherein the distributed damage is characterized by a set of second-order tensor-valued internal state variables. The internal state variables represent locally averaged measures of matrix cracking and interply delaminations. The model formulation provides a set of damage dependent laminated plate equations. These are developed by modifying the classical Kirchhoff plate theory. The effect of the matrix cracking enters the formulation through alteration in the individual lamina constitution. The effect of interply delamination enters the formulation through modifications of the Kirchhoff displacements. The corresponding internal state variables are defined utilizing the kinematics of the interply delaminated region and the divergence theorem. These internal state variables depend on the components of the displacements created by the delamination.
laminated composites, damage, graphite/epoxy, continuum mechanics, plate theory, internal state variables, matrix cracking, delamination
Professor, Texas A&M University, College Station, TX
Research scientist, Lawrence Livermore National Laboratories, Livermore, CA
Head, Fatigue and Fracture Branch, NASA Langley Research Center, Hampton, VA