Critical assessment of the local cleavage stress σ ƒ * in notch specimens of C-Mn steel

An elastic-plastic finite element method (FEM) was used to calculate the stress and strain distributions ahead of notches with various root radii in a bending specimen of C-Mn steel with grain sizes of 10 and 30 μm. By accurately measuring the distance of the cleavage initiation site from the root of the notch, the local cleavage stress σ _f{hook} ^* was measured. When the notch radius increased from 0.25 to 1.0 mm, the distribution of high stress had a definite variation but the σ _f{hook} ^* remained relatively constant. In notch specimens with different root radii, the critical fracture event is identical, i.e., propagation of a ferrite grain-sized crack into the neighboring matrix. Therefore, the σ _f{hook} ^* is mainly determined by the length of the critical microcrack, here, the size of ferrite grain instead of the high stress volume for finding an eligible brittle particle. The critical strain for initiating a crack was about 1 pct. The cleavage site ahead of a notch was related to the relative distributions of stress and strain and the random distribution of the weakest grains. The higher fracture load of the fine-grain material can be attributed to its higher value of σ _f{hook} ^* /σ_o as compared with the coarse-grain. The σ _f{hook} ^* /σ_o is a potential engineering parameter for toughness assessment in notch specimens.