By three-dimensional finite element calculation and measuring the location of the cleavage initiation sites, the local cleavage
fracture stress is accurately measured for 4-point bending (4PB) notched
specimens with various sizes (W, B and a) and widths (B) of a C-Mn steel. It is found that with increasing specimen sizes and widths the fracture load increases considerably, but the local cleavage fracture stress remains nearly constant. The cleavage fracture of the notched specimens with various sizes and widths is mainly controlled by the criterion of normal stress. The stable lower boundary value of the local cleavage fracture stress could be obtained by using notched specimens with larger sizes, and it may be used to assess the
fracture toughness of steel and safety of structure. The critical event for cleavage fracture in notched specimens is the propagation of a ferrite grain-sized crack into the matrix, and is independent of specimen sizes and widths. The local cleavage fracture stress is mainly determined by the length of the critical microcrack.