Effects of notch geometry on the local cleavage fracture stress σf

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Abstract
An elastic–plastic finite element method (FEM) is used to analyse the stress and strain distributions ahead of notches with various depths and flank angles in four‐point bending (4PB) specimens of a C–Mn steel. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σf is measured. By increasing the notch depth and notch flank angle from 2.25 to 8.25 mm and 10 to 90°, respectively, the distributions of high stress and strain at the moment of fracture show considerable variations. However, the value of σf stays relatively constant. The critical fracture event is thus shown to be identical, i.e. the propagation of a ferrite grain‐sized crack into the neighbouring matrix. It is concluded that σf is mainly determined by the length of the critical microcrack, while the notch geometry and its associated stress volume have little effect on the value of σf . The cleavage site ahead of a notch is determined by the stress distributions and the positions of the weakest grains.