Cohesive element modeling of viscoelastic fracture: application to peel testing of polymers

A computational modeling technique for fracture propagation in viscoelastic materials using cohesive elements for the zone ahead of the crack tip is p resented. The computational technique is used to study the problem of incre ase in fracture energy with peel velocity in peel testing of polymers. A ra te-independent phenomenological cohesive zone model is used to model the in trinsic fracture toughness of the interface between the polymer sheets. A d imensional analysis reveals that the macroscopic fracture energy scales wit h the intrinsic fracture toughness and is a function of peel velocity, and parameters such as the thickness, bulk properties of the polymer sheets, an d other cohesive zone properties. The growth of fracture energy as a functi on of the peel velocity has been studied for polymer sheets characterized b y a standard linear viscoelastic solid. Viscoelastic losses in the peel arm vanish in the limits of very slow and rapid peeling. Peak dissipation is o btained at an intermediate velocity, which is related to the characteristic relaxation time and thickness. This behavior is interpreted in terms of th e size of elastic and viscous zones near the crack tip. It is found that th e total energy dissipated is dependent upon both the intrinsic fracture tou ghness and the characteristic opening displacement of the cohesive zone mod el. The computational framework has been used to model experimental data on peeling of Butadiene rubbers. It is found that the usual interpretation of these data, that the macroscopic dissipation equals the rate-independent i ntrinsic toughness multiplied by a factor that depends on rate of loading, leads to a large quantitative discrepancy between theory and experiment. It is proposed that a model based on a rate-dependent cohesive law be used to model these peel tests. (C) 2000 Elsevier Science Ltd. All rights reserved .