Experimental identification of fracture toughness of a carbon black-filled styrene butadiene rubber undergoing energy dissipation by Mullins softening

The impact of the loading history on the resistance to break of a carbon-black filled styrene butadiene rubber is explored experimentally. Carbon-black filled rubberlike materials soften significantly upon the first loading due to the well known Mullins effect. The impact of this effect on the critical energy release rate at break, G_c, of the considered material is quantitatively estimated. For this purpose, the classical notched pure shear geometry is considered and the seminal global analysis from Rivlin and Thomas (1953) is adopted. Moreover, the same analysis is extended to non-elastic materials in order to account for the Mullins softening and define the critical energy release rate, G_c^∗, characterizing the creation of new crack surfaces without including the energy dissipated by Mullins softening. Both global quantities, G_c and G_c^∗, appear decreasing with the increase of softening already undergone by the material, stressing the difficulty of proposing a predictive criterion for the material resistance to failure. Finally, thanks to the local measures of the strain fields on the free surface of the pure shear specimen just before the crack propagation, it has been possible to evaluate the amount of Mullins dissipation upon the crack propagation and to explore the possible existence of an intrinsic value, G₀, characterizing the crack propagation independently of any other source of dissipation.