AN APPROXIMATE YIELD CRITERION FOR VOIDED NONLINEAR MATERIALS

An approximate yield criterion (Eq. (40)) for voided nonlinear materia ls is proposed in which the effect of material hardening is directly i ncluded, The matrix is assumed to be a power-law material and approxim ate velocity fields for the matrix are employed to derive the yield fu nction. Analysis indicates that the shape of yield surface in stress s pace expands along with increasing strain-hardening exponent N. For N = 0, the Gurson yield function is derived. And as N = 1, an analytical expression of the yield function in the form of ellipsoid is obtained . Furthermore, as 0 < N < 1, an approximate expression for the yield f unction is suggested. A comparison between the approximate yield funct ion and Cocks' model (1989) is carried out. The agreement of the two m odels is very good.