Computation of the fracture stress in notched NiAl-polycrystals

Computer simulations were performed in order to investigate the influence of the geometry and the microstructure of macroscopic specimens on the brittle-to-ductile transition of NiAl. A model is proposed which permits the bridging of the gap between the length scales of mesoscopic and macroscopic modeling. The strength of the macroscopic specimen is described by conventional J2 flow theory with a power law hardening material, while the toughness is treated on the mesoscopic length scale assuming preexisting microcracks that are shielded by the emission of dislocations. The temperature dependence of the yield stress and the activation energy of the dislocation rate were obtained from measurements on NiAl poly- and single crystals, respectively. Cleavage fracture of the macroscopic specimen occurs when the simulation on the mesoscopic scale computes unstable crack advance. The considered microcrack of finite length is loaded in mixed mode I and II.