Interface fracture and viscoelastic deformation in finite size specimens

In the small scale yielding limit, the viscoelastic energy dissipation and the predicted fracture toughness of the interface between two viscoelastic polymers are monotonically increasing functions of the crack growth rate. For a finite size specimen however such prediction are no longer strictly valid. We demonstrate here that there is a maximum viscoelastic energy dissipation (and thus fracture toughness) at a characteristic interface crack velocity which depends not only on the micromechanical properties of the interface and the viscoelastic properties of the bulk polymers but also upon the specimen dimensions. Our analytical results allow us to deduce G=Gad[1+φ(aTȧ,c)] which relates the experimental fracture toughness to the intrinsic fracture toughness of the interface Gad, the WLF shift factor aT and c, a dimensionless parameter which is a function of the micromechanical properties of the interface, the mechanical properties of the polymers and the specimen dimensions. This result is similar to an empirical relation developed to fit experimental results; this relation however was assumed to be independent of c (specimen geometry). We demonstrate that the dependence of φ on c is substantially weaker than its dependence on aTȧ, which accounts for the success of the empirical relation in correlating the experimental data.