Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C-Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of −110◦C for a C-Mn steel. An elastic-plastic finiteelement method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σf is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σf is closely related to the cleavage fracture mechanism (critical events) in steels. The σf values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σf is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σf cancharacterize the intrinsic toughness of steels andmaybe used in a “local approach” model for assessing integrity of flawed structures. The σf values could be measured by both 4PB and 3PB tests.