Influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress

The aim of this work is to determine the influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress σ_f*. Besides, the dependence of the initiation temperature for shear fracture T_i and the temperature for general yield T_gy on strain rate is investigated. Finally, the local values of stress triaxiality and equivalent plastic strain at the occurrence of cleavage fracture for several steels and specimen types are compared with the failure curve for ductile fracture to check the validity of the theory of stress controlled cleavage fracture and the strain/triaxiality controlled shear fracture in the transition region. Based on experimental tests, the results are obtained by finite element analyses. The investigation shows, that σ_f* is dependent on temperature and strain rate and increases with decreasing test temperature and increasing strain rate. The transition temperatures T_i and T_gy increase with increasing strain rate. The theories of stress controlled cleavage fracture and of shear fracture controlled by equivalent plastic strain and stress triaxiality seem to be valid. That fracture mechanism occurs for which the critical condition is reached first.