Fracture toughness transition curve shape for ferritic structural steels

A critical issue in making failure assessments for brittle fracture is the temperature dependence of the fracture toughness transition curve. Normally, fracture mechanical tests are performed only at some predescribed temperature like -10°C. This is also the way the steel producers usually present their data. For actual failure assessment this information is seldom adequate. Some kind of extrapolation of the fracture toughness values to higher or lower temperatures are often required. This can be achieved only if the shape of the fracture toughness transition curve is known. Several presently applied cleavage fracture models yield that the fracture toughness should be inversely related to the materials yield stress. Thus these models indicate that the fracture toughness transition curve shape should be related to the temperature dependence of the yield stress. In order to clarify the issue, a micromechanism based statistical cleavage fracture model is applied to analyze existing fracture toughness data. It is shown that the fracture toughness transition curve shape is really insensitive to the yield stress. Furthermore it is shown that the majority of ferritic steels have similar fracture toughness transition curve shapes, thus making it possible to describe all the steels fracture toughness temperature dependence with a single curve. Finally a theoretical reasoning for the observed behavior is given, based on the cleavage fracture model.