Modelling of Fracture Processes During Fast Crack Growth

The role of the fracture process region during fast fracture is discussed. Reviewed experiments on modern pressure vessel steels show that already at crack tip speeds of one or two hundred mls fracture processes are dominantly brittle. Modelling of the process region is discussed in the light of asymptotic elastic plastic fields and process region autonomy. It is suggested that a fraction of the crack surface (0.1 to 0.2) is created by ductile tearing of ligaments remaining after most of the crack surface has failed through cleavage. Ligament like test specimens of sub mm size are used to detennine the constants of an elastic viscoplastic model assumed for both the surrounding continuum and the crack bridging ligaments, The latter are replaced with a cohesive zone model embedded by an elastic viscoplastic continuum. The process region model captures cleavage in a foremost patt with very high cohesive stresses and rupture of ligaments in a trailing part where stresses are assumed to be rate dependent. In the surrounding continua the plastic deformation work is calculated considering high strain rate effects. The energy release rate in the process region is estimated. A balance between elastic energy release rate, energy dissipated at plastic deformation work in the plastic zone and energy release rate in the process region is examined. The energy released in the process region is consumed a) at cleavage and b) during rupture of ligaments. A cohesive zone model is used for the process region. The conclusion is that the experimental results can be explained only if the ligaments are considered. Crack growth rates down to one to two hundred mls are predicted by the model. Energy balance is possible at lower speeds but the crack immediately switches to a higher speed or comes to instant arrest as has been observed in experiments.