Experimental and computational aspects of cavitation in natural rubber

General constitutive equations for hyperelastic materials are obtained from a strain energy function expressed either in terms of strain invariants or principal stretches. For most applications the strain energy functional does not need to include dilatational components. However, the pressure-volume relation for nearly incompressible materials needs to be explicitly accounted for when rubber components are highly constrained. Thus, the hyperelastic response needs to be expressed in terms of dilatational and deviatoric components. Experimental evidence is reviewed to show that rubber is subjected to a loss of stiffness attributed to cavitation damage when subjected to a hydrostatic tensile stress state. The critical pressure is identified for which microscopic material imperfections will tear open to form internal bubbles and cracks. Cavitation damage in rubber is associated with a significant reduction in the bulk modulus. Thus, a variable bulk modulus can best be used to describe the behavior of rubber when cavitation damage occurs. The introduction of a cavitation-damage modulus does suggest a simple approach to realistically represent the mechanics of cavitation in rubber solids.