This study discusses the brittle fracture controlling parameter for the steel components under mixed mode loading with focusing on the Weibull stress concept proposed as an original Beremin model. The Weibull stress based on the original Beremin model, which is calculated by integrating a maximum principal stress over the fracture process zone around crack-tip, is widely demonstrated to be a fracture controlling parameter independent of crack-tip plastic constraint for 3PB (3-point bending) specimen under mode I loading. Critical Weibull stress obtained by 3PB test founds to be different from that obtained by 4PS (4-point shear) specimen under mixed mode loading (ratio of loading mode I and mode II is 0.51). The reason why the Weibull stress can not be a unified fracture controlling parameter is interpreted from the crack-tip stress fields that shows the different principal stress ratio between the specimens subjected to mode I and mixed mode loading conditions. A new fracture model considering stress components other than maximum principal stress is necessary to propose a fracture controlling parameter independent of plastic constraint and loading mode mixture.
