The nucleation of an edge annular concentric crack on the surface of a smooth cylinder made of an elastoplastic material with ultimate strain is considered. To obtain critical parameters of quasi-brittle fracture, a coupled (sufficient) fracture criterion of an elastoplastic material is used. Simple relations are obtained for the critical stresses, the length of the prefecture zone and the critical stress intensity factor (SIF) for the mode III fracture. In the framework of the proposed model, the critical SIF obtained by the sufficient criterion is a variable, it depends on the characteristic linear size of the material structure, the parameters of the stress-strain diagram, and the length of the crack. The rule of selecting the parameters of the proposed fracture model for two laboratory experiments is indicated: according to the stress-strain diagram of material under simple shear and critical SIF. The obtained critical parameters are conservative estimates of critical stresses. It is advisable to use these estimates in the calculation of steel structures operating at temperatures below the cold brittleness threshold.

考虑了弹塑性材料制成的光滑圆柱体表面边缘环形同心裂纹的形核，该圆柱体具有极限应变。为了获得准脆性断裂的临界参数，使用弹塑性材料的耦合（充分）断裂准则。得到了Ⅲ型裂缝的临界应力、辖区长度和临界应力强度因子（SIF）的简单关系。在所提出模型的框架内，充分准则得到的临界 SIF 是一个变量，它取决于材料结构的特征线性尺寸、应力-应变图的参数以及裂纹的长度。两个实验室实验所提出的断裂模型的参数选择规则如下：根据材料在简单剪切和临界 SIF 下的应力-应变图。获得的临界参数是临界应力的保守估计。建议在计算温度低于冷脆性阈值的钢结构时使用这些估计值。