Effects of geometry of notched specimens on the local cleavage fracture stress σf of C–Mn steel

An elastic–plastic three-dimensional finite element method analysis is used to determine the stress and strain distributions ahead of notches of four-point bending (4PB) specimens with various sizes (W, B and a) and widths (B). By measuring the location of the cleavage initiation sites for a C–Mn steel, the local cleavage fracture stress σf is accurately determined. With increasing specimen sizes and widths the fracture load Pf increases considerably, but σf remains nearly constant. The reason that the σf of the specimen with minimum size is slightly larger than that of the other specimens is analyzed by an active zone model of cleavage fracture for notched specimens. The critical event for cleavage fracture is the propagation of a ferrite grain-sized crack into the neighboring matrix, and is independent of specimen sizes and widths. σf is mainly determined by the length of the critical microcrack, and the specimen sizes and widths have little effect on it.