During the past couple of decades, modern fracture criteria have been developed based on a crack-tip material length scale so-called the critical distance, where the critical energy/stress/strain parameter required for the crack propagation is measured. However, in most cases, the critical distance has been considered as a material parameter measured under pure mode I crack tip displacements and independent of the mode mixity. In this paper, the concept of “effective critical distances” is introduced by considering the effect of mode mixity on the measured critical distances via a strain-based approach to construct a novel mixed-mode I/II/III brittle fracture criterion (ECD-SN criterion). Along with the traditional stress and energy-based criteria of MPS, and SED, the proposed criterion is examined by predicting the experimental data reported in the literature for the mixed-mode I/II/III fracture toughness and fracture angles. Results reveal superior estimates of the test data by the ECD-SN criterion as compared to the traditional criteria highlighting the necessity of adopting an “effective critical distance” in the three-dimensional brittle fracture theories.

在过去的几十年中，基于裂缝尖端材料的长度尺度（称为临界距离），开发了现代的断裂准则，其中测量了裂纹扩展所需的临界能量/应力/应变参数。但是，在大多数情况下，临界距离已被视为在纯模式I裂纹尖端位移下测量的材料参数，并且与模式混合无关。在本文中，通过基于应变的方法来构造混合I / II / III脆性断裂准则（ECD- SN标准）。连同MPS和SED的传统基于压力和能量的标准，通过预测文献中报道的混合模式I / II / III断裂韧性和断裂角度的实验数据来检验提出的标准。结果显示，与传统标准相比，ECD-SN标准对测试数据的估算更高，突出了在三维脆性断裂理论中采用“有效临界距离”的必要性。