Study of mechanism of cleavage fracture at low temperature

Abstract

In this investigation, a series of crack opening displacement (COD) tests were carried out at several low temperatures for C-Mn weld steel. Some of the specimens were loaded until fracture, and the mechanical properties and microscopic parameters on fracture surfaces were measured. Other specimens were unloaded before fracture at different applied loads. The distributions of the elongated cavities and the cleavage microcracks ahead of fatigue crack tips were observed in detail. Based on the experimental results, the combined criterion of a critical strainε _p ≥ ε_c) for initiating a crack nucleus, a critical stress triaxiality(σ _m/σ ≥ t_c) for preventing it from blunting, and a critical normal stress(σ _yy/σ_f) for the cleavage extension was proposed again, and the critical values of ε_p and σ_m/−σ for the C-Mn weld steel were measured. The reason why the minimum COD value could not be zero is explained. The mechanism of generation of the lower limit COD value on the lower shelf of the toughness transition curve is proposed.