Extension fractures are typical for the deformation under low or no confining pressure. They can be explained by a phenomenological extension strain failure criterion. In the past, a simple empirical criterion for fracture initiation in brittle rock has been developed. In this article, it is shown that the simple extension strain criterion makes unrealistic strength predictions in biaxial compression and tension. To overcome this major limitation, a new extension strain criterion is proposed by adding a weighted principal shear component to the simple criterion. The shear weight is chosen, such that the enriched extension strain criterion represents the same failure surface as the Mohr–Coulomb (MC) criterion. Thus, the MC criterion has been derived as an extension strain criterion predicting extension failure modes, which are unexpected in the classical understanding of the failure of cohesive-frictional materials. In progressive damage of rock, the most likely fracture direction is orthogonal to the maximum extension strain leading to dilatancy. The enriched extension strain criterion is proposed as a threshold surface for crack initiation CI and crack damage CD and as a failure surface at peak stress CP. Different from compressive loading, tensile loading requires only a limited number of critical cracks to cause failure. Therefore, for tensile stresses, the failure criteria must be modified somehow, possibly by a cut-off corresponding to the CI stress. Examples show that the enriched extension strain criterion predicts much lower volumes of damaged rock mass compared to the simple extension strain criterion.

延伸断裂是低围压或无围压下变形的典型特征。它们可以通过现象学扩展应变失效准则来解释。过去，已经开发了一种简单的脆性岩石起裂经验标准。在本文中，表明简单的拉伸应变准则在双轴压缩和拉伸中做出了不切实际的强度预测。为了克服这一主要限制，通过向简单准则添加加权主剪切分量，提出了一种新的拉伸应变准则。选择剪切重量，使得富集拉伸应变准则代表与 Mohr-Coulomb (MC) 准则相同的破坏面。因此，MC 准则已被推导出为预测拉伸破坏模式的拉伸应变准则，这在对内聚摩擦材料失效的经典理解中是出乎意料的。在岩石的渐进损伤中，最可能的断裂方向与导致剪胀的最大拉伸应变正交。富集拉伸应变准则被提议作为裂纹萌生 CI 和裂纹损伤 CD 的阈值表面以及峰值应力 CP 下的失效表面。与压缩加载不同，拉伸加载只需要有限数量的临界裂纹即可导致失效。因此，对于拉伸应力，必须以某种方式修改失效标准，可能通过与 CI 应力对应的截止值。示例表明，与简单的拉伸应变准则相比，富集拉伸应变准则预测的受损岩体体积要小得多。