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 r ate and specimen geometry on the microscopic cleavage fracture stress sigma (f)*. Besides, the dependence of the initiation temperature for shear fract ure T-i and the temperature for general yield T-gy on strain rate is invest igated. Finally, the local values of stress triaxiality and equivalent plas tic strain at the occurrence of cleavage fracture for several steels and sp ecimen types are compared with the failure curve for ductile fracture to ch eck the validity of the theory of stress controlled cleavage fracture and t he strain/triaxiality controlled shear fracture in the transition region. B ased on experimental tests, the results are obtained by finite element anal yses. The investigation shows, that sigma(f)* is dependent on temperature a nd strain rate and increases with decreasing test temperature and increasin g strain rate. The transition temperatures T-i and T-gy increase with incre asing strain rate. The theories of stress controlled cleavage fracture and of shear fracture controlled by equivalent plastic strain and stress triaxi ality seem to be valid. That fracture mechanism occurs for which the critic al condition is reached first. (C) 1999 Elsevier Science S.A. All rights re served.