Due to the existence of cracks and defects in composite materials during the manufacturing process, damage and crack growth in these materials must be anticipated before application. In the present research, an efficient theoretical mixed mode I/II fracture criterion is proposed for fracture investigation of orthotropic materials considering the effects of non-singular stress term in William’s series expansion. This criterion is developed combining the maximum shear stress (MSS) theory with reinforcement isotropic solid concept (RIS) as a superior material model. RIS is based on the fact that cracks in orthotropic materials generally propagate along the fibers in the isotropic matrix media regardless of initial crack-fiber orientation. According to this fact, RIS offers to employ an isotropic stress field around the crack tip with defined stress reduction coefficients as fiber effects. This criterion is applicable for two cases wherein crack is placed both along and across the fibers. A comparison between available experimental data for two wood species and the results of the proposed criterion shows that this criterion can accurately predict both crack initiation and crack propagation direction and is in full compliance with the fracture nature of composite materials.

由于复合材料在制造过程中存在裂纹和缺陷，因此在使用前必须预测这些材料的损坏和裂纹扩展。在目前的研究中，考虑到非奇异应力项在威廉级数展开中的影响，提出了一种有效的理论混合模式 I/II 断裂准则，用于正交各向异性材料的断裂研究。该准则是将最大剪应力 (MSS) 理论与钢筋各向同性实体概念 (RIS) 结合起来开发的，作为一种优越的材料模型。RIS 基于这样一个事实，即正交各向异性材料中的裂纹通常沿各向同性基体介质中的纤维传播，而不管初始裂纹-纤维取向如何。根据这个事实，RIS 提议在裂纹尖端周围采用各向同性应力场，并具有定义为纤维效应的应力降低系数。该标准适用于裂纹沿纤维和横跨纤维放置的两种情况。两种木材的可用实验数据与提出的准则结果的比较表明，该准则可以准确预测裂纹萌生和裂纹扩展方向，完全符合复合材料的断裂性质。