Abstract: FEM simulations of a short crack initiated, extended and blunted at a pre-crack tip and calculations of distributions of stress, strain and triaxiality are carried out to investigate their effects on the triggering of cleavage fracture of 3PB pre- cracked steel specimen tested at –130 ℃. Combined with the observations of configuration changes and fracture surfaces of pre-crack tips, the results reveal that as long as the fatigue pre-crack is only blunted, in its vicinity a region, where the accumulated strain is sufficient to nucleate a crack and a region, where the stress (triaxiality) is sufficient to propagate a crack nucleus is separated by a distance. The nucleated crack cannot be propagated and the cleavage fracture cannot be triggered. A short crack produced at the fully blunted fatigue pre-crack will change the distributions of strain, stress and triaxiality and specially their relative locations ahead of the pre-crack. While the strain retains, the stress (triaxiality) is rebuilt. The initiated, extended and then blunted short crack makes a tip configuration, which is much sharper than that of the fully blunted original pre-crack tip. The sharpened crack tip configuration re-builds a “sharper” distribution of stress (triaxiality) and make above two regions closer. Finally the two regions overlap each other and a cleavage crack can be initiated and propagated at a distance ahead of the blunted pre-crack.

Key words: Cleavage, Crack tip, FEM calculation, FEM simulation, Physical model