Fracture behavior at crack tip - a new framework for cleavage mechanism of steel

The fracture behavior at crack tip was analyzed based on: (1) observations of fracture surfaces and measurements of local critical parameters for cleavage of three point bending (3PB) precracked specimens of C-Mn steel, (2) detailed observations of configuration changes at precrack tips by metallographic cross sections in specimens unloaded at various applied loads, (3) sophisticated FEM calculations of distributions of stress, strain and triaxiality and simulations of short cracks initiated and extended at precrack tips. The results show that before a critical load (a critical COD) is reached, the crack tip is only blunted and in its vicinity three criteria for cleavage fracture (εp≥εpc for initiating a crack nucleus; σm/σe≥Tc for preventing the crack tip from blunting; and σyy≥σf for propagating the crack) are satisfied in different regions separated from each other, the nucleated cracks cannot be propagated and cleavage fracture cannot be triggered. As the applied load increases higher than this critical load, a short crack is initiated and extended at the precrack tip and then is blunted again. The plastic strain and the stress in front of the precrack are redistributed. While the plastic strain remains in front of the tip, the stress triaxiality is rebuilt. At a second critical load, the regions where the three criteria are satisfied overlap each other and a cleavage crack can be nucleated and propagated. The minimum distance for cleavage may be determined by the beginning of the overlapping of the mentioned regions. Combined with the three criteria previously suggested, the fracture behavior at crack tip and the corresponding changes of driving forces (εp, σm/σe,σyy) provide a complete physical model for cleavage of steels in the local scale