A study of crack tip blunting and the influence of blunting behavior on the fracture toughness of ultra high strength steels

Cross-sections of strained but not fractured compact tension J_IC speciments have been examined to investigate crack-tip blunting behavior as a function J level for four different microstructures. The microstructures were the as-quenched microstructures of HP9-4-20 and HO9-4-10 steels and the microstructures obtained by tempering these steels at 565°C. Smooth blunting was observed for the as-quenched microstructures while the fatigue cracks for tempered microstructures blunted to geometries characterized by two or three corners or vertices. The blunting geometries were clearly defined at J levels well below J_IC. For the case of smooth blunting voids tended to form directly ahead of the crack tip and crack extension by fracture occurred when the ligament between the blunting crack tip and the void directly ahead of the crack tip failed by shear fracture at an angle of about 45°C to the plane of the crack. Blunting to vertices was characterized by the growth of large voids very close to the corners or vertices of the blunting rack tip; it appears that the blunting geometry was maintained by the coalescence of these voids with the blunting crack tip. The results further suggest that if two microstructures have the same constrained ductility and identical inclusion distributions and one blunts smoothly and the other to vertices the microstructure which blunts to vertices can have substantially higher toughness.