Fracture topography of HSLA steels

The topography of the fracture surfaces of several high strength, low alloy (HSLA) steels was determined using scanning electron microscopy. The steels contained various additions of titanium, vanadium and niobium. Two types of specimens were investigated. They were either fractured tensile specimens which had undergone uniform strain followed by necking to fracture or fractured three-point bending specimens broken during determination of the J-integral fracture parameter. The distribution of the dimple size from both types of specimens was determined. It was found that the average dimple size for the tensile specimens is about 1.5 μm for all steels. For the three-point bending specimens, however, the average dimple size is about 15 μm. Also, the relative depth of the dimples is higher for the tensile specimens than it is for the three-point bending specimens. These results are interpreted in terms of the plastic strain in the specimens where, in the tensile specimens, the large uniform strain contributes to the formation of elongated dimples and, in the three-point bending specimens, the state of triaxity at the crack tip contributes to large shallow dimple formation.