A Non-Quadratic Plasticity Model Suitable for Metal Matrix Composites

Based on the assumption that uniform dilatation has negligible effect on the yield point, a high-order yield function of non-equal stress exponents in conjunction with a non-associated flow rule is proposed to formulate 3-D plastic constitutive equations for fibrous metal matrix composites (MMC). The involved nine plastic parameters, six for the yield criterion and three for the plastic potential, can be easily calibrated with the macro stress-strain data of simple tension, pure shear, and bi-axial tension tests. It is demonstrated numerically that this theory describes anisotropic plasticity of boron-aluminum (B/Al) composites very well. It is also shown that the uniform dilatation assumption is adequate for other fiber composites. By properly selecting the plastic parameters, the present model can reduce to the nine-parameter quadratic yield function developed for graphite fiber/organic matrix composites [1], Hill's quadratic yield model [2], and von Mises and Tresca isotropic yield criteria.