A New Material Model for the Nonlinear Biaxial Behavior of ATJ-S Graphite*

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Abstract
A new model for the deformation behavior of ATJ-S graphite, a non linear transversely isotropic material, under initial loading is described. This material model, which is based on a new deformation theory of ortho tropic plasticity, has excellent potential for description of the biaxial softening phenomenon found in ATJ-S graphite. Biaxial softening is char acterized by the development of larger strains in biaxial tension than in uniaxial tension, in contradiction to conventional Poisson effects. The new model is examined for sensitivity to (1) a nonsymmetric compliance matrix, (2) how the material model constants are obtained, and (3) varia tions in material properties. Comparisons are made between strain values obtained with the material model incorporated in a finite element com puter analysis and strain values from biaxial test results.

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