In a large class of porous elastic solids such as cement concrete, rocks, ceramics, porous metals, biological materials such as bone, etc., the material moduli depend on density. When such materials undergo sufficiently small deformations, the usual approach of appealing to a linearized elastic constitutive relation to describe their response will not allow us to capture the dependence of the material moduli on the density, as this would imply a nonlinear relationship between the stress and the linearized strain in virtue of the balance of mass as dependence on density implies dependence on the trace of the linearized strain. It is possible to capture the dependence of the material moduli on the density, when the body undergoes small deformations, within the context of implicit constitutive relations. We study the stress concentration due to a rigid elliptic inclusion within a new class of implicit constitutive relations in which the stress and the linearized strain appear linearly, that allows us to capture the dependence of the material moduli on the density. We find that the stress concentration that one obtains employing the constitutive relation wherein the material moduli depend on the density can be significantly different from that obtained by adopting the classical linearized elastic constitutive relation to which it reduces to when the density dependence of the material moduli are ignored.
