Stress triaxiality effect on cleavage fracture stress

In this work the effect of stress triaxiality on cleavage fracture stress was investigated by means of micro-mechanical modelling. A unit-cell model was used to determine macroscopic conditions for brittle fracture, in accordance with the slip-induced cleavage fracture model, because of included particle fracture and consequent crack initiation in the matrix. This approach allowed to correlate the critical fracture stress to the particle strength and the ability of the matrix to undergo plastic deformations. Numerical simulations results indicate that the critical fracture stress is strongly dependent on the stress triaxiality, which justify the variability observed in experiments on different specimen geometry. Additionally, the unit-cell modelling allowed to establish a simple relationship between the expected cleavage fracture stress and stress triaxiality, also considering temperature and strain rate. Finally, the use of the plastic constrain factor (defined as the ratio of the critical fracture stress and the yield stress) allows to obtain a master curve that can be used to predict the expected critical fracture stress at different stress triaxiality, temperature and strain rate in different steel grades and alloys.