Fracture initiation of geometrically scaled, notched three-point-bend bars of a low-alloy ferritic steel

The purpose of the experimental work reported in this contribution was to provide data that may serve for the development of scaling rules for ductile fracture at blunt notches. Bending experiments were performed with three sizes of geometrically scaled notched bend specimens of the ferritic pressure vessel material 20MnMoNi55, material number 1.6310 (heat number 69906) using carefully scaled supports, using geometrically similar U-notched bend bars with rectangular cross-sections and scaled blunt notches covering a scale range of 14. Fracture initiation was reliably detected by means of the dc potential drop technique. Comparison of the normalised load versus load-point-displacement curves revealed a significant size effect for the conditions of fracture initiation, with large specimens fracturing at smaller normalised displacements than smaller ones. The energy up to the initiation point normalised to the ligament can be linearly correlated with specimen size, while the energy normalised to the active volume (ligament size multiplied by notch width) consists of two terms: a hyperbolic decay term analogous to the initiation of crack extension in fracture mechanics and a constant term corresponding to the classical size independent plasticity theory. For the smallest specimens investigated the first term amounts to about 28% of the total volume specific work, for the largest specimens there is almost no contribution of the first term.