In the classical fracture mechanics theory, it is generally believed that the crack surface is open under tension and closed under compression. However, under uniaxial tension at a small angle with the crack surface, especially when parallel, the stress field at the crack tip calculated by traditional theory is small. Although many scholars have proved that the stress field at the crack tip needs to consider T-stress, it is just a non-singular term. This is because under small angle uniaxial tension, the crack surface may also be closed. Similarly, it may be open under uniaxial compression. Therefore, aiming at the shortcomings of the existing theories, both the far-field stress and the possible stress acting on the crack surface are considered in this paper. Based on the stress boundary conditions of the far-field and the crack surface, the stress function of the infinite plate with a single crack under complex load is deduced and the stress field at the crack tip is obtained, that is, the stress intensity factors K_I, K_II and T-stress. Based on the generalized maximum tangential stress (GMTS) criterion, the crack initiation angles of open or closed cracks under uniaxial tension or compression are studied respectively, the relationships between crack initiation angle θ₀ and β (β: angle between the tensile/compressive direction and crack surface) are obtained. A parameter η is introduced to judge whether the crack surface is open or closed under different loading angles. Compared the above research results with the methods proposed by predecessors and experimental data, it is found that they are in good agreement. The results show that the stress intensity factors K_I, K_II and T-stress are related to not only the far-field stress, but also the possible stress acting on the crack surface; The stress on the crack surface changes with far-field stress; The crack surface under small angle tension regarded as closed while under compression as open is reasonable.

在经典断裂力学理论中，一般认为裂纹面受拉张开，受压闭合。然而，在与裂纹面成小角度的单轴拉伸下，尤其是平行时，传统理论计算的裂纹尖端处的应力场很小。虽然许多学者已经证明裂纹尖端的应力场需要考虑T-压力，它只是一个非单数的术语。这是因为在小角度单轴拉伸下，裂纹面也可能闭合。类似地，它可以在单轴压缩下打开。因此，针对现有理论的不足，本文既考虑了远场应力，也考虑了作用于裂纹表面的可能应力。基于远场和裂纹面的应力边界条件，推导了具有单个裂纹的无限板在复杂载荷作用下的应力函数，得到了裂纹尖端的应力场，即应力强度因子KI , K _II和T _{_}-压力。基于广义最大切向应力（GMTS）准则，分别研究了单轴拉伸或压缩作用下开放或闭合裂纹的裂纹起始角，裂纹起始角θ ₀与β之间的关系（β：拉伸/压缩方向之间的角度）和裂纹表面）得到。引入一个参数η来判断裂纹面在不同加载角度下是开放的还是封闭的。将上述研究结果与前人提出的方法和实验数据进行对比，发现它们具有很好的一致性。结果表明，应力强度因子K _I , K _II和T应力不仅与远场应力有关，而且与可能作用在裂纹表面的应力有关；裂纹表面应力随远场应力变化；小角度拉力下的裂纹面视作闭合，受压下的裂纹面视作开口是合理的。