Effect of a soft interlayer with changing properties on the stress distribution around inclusions and yielding of composites

A model is presented that takes into account the presence of a soft interlayer with changing properties around an inclusion embedded into an infinite matrix. Property change in the interphase is expressed by a power law function of the form of f(r) = J(R/r)(p). The equilibrium equation is satisfied by a spherical and a dipole like solution. The continuity of stresses and displacements at all interfaces are used as boundary condition. Calculations are carried out for a matrix/elastomer/filler system to predict the effect of the soft interlayer on stress distribution. According to the model the deformation of this interlayer is very large compared to that of the matrix. Very soft interlayers lead almost exclusively to compressive deformations in the entire deformed volume. Radial stress decreases considerably and the stress maximum shifts to the surface of the inclusion. Although in a somewhat lesser extent than radial stress, shear stress also decreases in the presence of the soft interlayer and stress maxima become less localized. In such a case yielding is initiated at the equator of the inclusion. Particles covered with a very soft interphase behave like cavities. A soft interphase is advantageous because it changes stress distribution and decreases stress concentration, but particles loose their reinforcing effect already at a relatively thin layer thickness. (C) 2001 Elsevier Science Ltd. All rights reserved.