A comparison of mode I fracture behavior of fcc and bcc metallic materials: a discrete dislocation analysis

Mode I fracture behavior in fcc and bcc materials is analyzed using the discrete dislocation approach. Dislocations are modeled as line discontinuities residing in an isotropic linear elastic medium and interacting with each other through long-range stress fields. In addition, the short-range reactions underlying dislocation nucleation and annihilation are introduced to enable simulation of the evolution of the dislocation structure during loading. The motion of dislocations and their nucleation is considered to be controlled by thermal activation at higher temperatures and lower stress levels and by phonon-drag at lower temperatures and higher stress levels. A mode I crack is introduced into the model using a cohesive surface formulation. The results obtained show that differences in the orientation of the slip planes, relative to the low-cohesive strength {1 0 0} fracture planes, cause dislocations to promote crack tip blunting and plastic shielding in the case of the fcc material while such effects are only marginal in the case of the bcc material. Furthermore, temperature is found to have a pronounced effect on fracture toughness of the bcc material while its role in the fcc material is very small.