A stress softening, commonly known as the Mullins effect, is observed usually in filled rubberlike materials after the first load. Experimental observations have shown that the Mullins effect induces a permanent set and some anisotropy. In order to account for these two features, a strain energy density is proposed, which is based on material directions and coupled with a theory for network alteration. The constitutive law is based on a macromolecular approach, and the network alteration theory is motivated physically. The model has been compared to experimental data successfully.
