Crack velocity dependent toughness in rate dependent materials

Mode I, quasi-static, steady-state crack growth is analyzed for rate depend ent materials under plane strain conditions in small scale yielding. The so lid is characterized by an elastic-viscoplastic constitutive law and the pl ane ahead of the crack tip is embedded with a rate dependent fracture proce ss zone. The macroscopic work of fracture of the material is computed as a function of the crack velocity and the parameters characterizing the fractu re process zone and the solid. With increasing crack velocity a competition exists between the strain rate hardening of the solid, which causes elevat ed tractions ahead of the crack tip that tend to drive crack propagation, a nd the rate strengthening of the fracture process zone which tends to resis t fracture. Results for material parameters characteristic of polymers show that the toughness of the material can either increase or decrease with in creasing crack velocity. To motivate the model, the cohesive zone parameter s are discussed in terms of failure mechanisms such as crazing and void gro wth ahead of the crack tip. The toughness of rubber modified epoxies is exp lained by employing the fracture model along with micromechanical void cell calculations. (C) 2000 Elsevier Science Ltd. All rights reserved.