Assuming that the particles carry a load proportional to their volume fraction, Pukánszky and Vörös [24,25] proposed that:(4)σe=vfkσe+(1-vf)σmwhere the applied stress acting on the modified polymer, σe is a function of the volume fraction, vf, of particles, the proportionality constant, k, for stress transfer between the particles and the matrix, and the average stress in the matrix, σm.