SOUTHERN METHODIST UNIV DALLAS TX DEPT OF CIVIL AND MECHANICAL ENGINEERING
Artificial graphites have been used for the past decade in reentry vehicle nosetips. Severe design requirements are imposed on nosetips because of their operational environment. A new model for the deformation behavior of this group of nonlinear transversely isotropic materials under initial loading is described. This material model, which is based on a new deformation theory of orthotropic plasticity, has excellent potential for description of the unusual phenomenon of biaxial multiaxial softening found in ATJ-S graphite. Biaxial softening is characterized by the development of larger strains in biaxial tension than in uniaxial tension, in contradiction to conventional Poisson effects. Moreover, another characteristic of graphite deformation behavior, different moduli in tension and compression, is accounted for. Comparisons are made between strain values obtained with the material model incorporated in a finite element computer analysis and strain values measured in off-axis uniaxial tests and biaxial tests.