Computer simulations of the temperature dependence of the fracture stress in notched polycrystals

Computer simulations were performed in order to investigate the influence of the geometry of macroscopic specimens and their microstructures on the brittle-to-ductile transition (BDT). The model proposed simulates the strength with conventional elastic-plastic finite elements and the toughness with dislocation emission and shielding from prescribed microcracks within the structure. The gap between the different length scales is bridged by using the stress on a finite element within the loaded structure as boundary conditions for the mesoscopic model. For this the complete stress field of a microcrack with dislocations on arbitrarily oriented slip planes was solved and used in a dynamic simulation in which dislocations emerge from sources near and at the crack tip and move self consistently. Cleavage fracture occurs when the local stress intensity factor which depends on loading rate, temperature, size and orientation of the microcrack exceeds the Griffith value before general yielding occurs according to the finite element model. NiAl was used as a model material since all necessary input data were available. The model yields predictions of the scatter range of toughness data due to the microcrack distribution and orientation as a function of temperature and strain rate.