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 σ_f is accurately determined. With increasing specimen sizes and widths the fracture load P_f increases considerably, but σ_f remains nearly constant. The reason that the σ_f 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. σ_f is mainly determined by the length of the critical microcrack, and the specimen sizes and widths have little effect on it.