Effect of compressible filler on the elastic properties of rubber

In some applications, notably as rail embedment polymers, it is customary to incorporate compressible filler into rubbery materials. This reduces the effect of “shape factor” so that a greater compression compliance is achieved than if the rubbery matrix were used, in spatially constrained situations, on its own. The effective bulk modulus of such materials is estimated from the shear modulus of the matrix and the volume fraction of voids and compared to the predictions of more elaborate theoretical models. Experimental measurements are also reported, both of the effective bulk and shear moduli and of the compression stiffness of pads of such material with a range of shape factors. The latter results are compared with the predictions from an equation available in the literature.