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当纤维表面的界面相（如涂层）较脆时，纤维发生断裂之前界面相一般会发生多重开裂的损伤，这种损伤裂纹垂直于纤维轴向，故能导致纤维强度下降。本文以四相（纤维，界面相，基体，复合材料）圆柱体模型为基础，并假设界面脱粘后不再传递剪应力。首先用剪切滞后理论求得了界面相发生多重开裂后，纤维、界面相中的应力集中系数，以及界面上的剪应力，并同时考虑了纤维与界面相间界面部分脱粘的影响；然后，假设纤维强度统计特性用Ｗｅｉｂｕｌｌ分布函数表示，从而根据界面相多重开裂在纤维中引起的应力集中系数Ｋ＿ｆ得到广纤维破坏概率的变化。最后利用界面脱粘区的大小，定性研究了界面剪切强度τ＿０对纤维强度的影响，结果表明：存在一个最佳的界面剪切强度τ＿０，使界面相多重开裂对纤维强度的影响最小。
When the interface phase (such as coating) on the surface of the fiber is brittle, the interface phase usually undergoes multiple cracking before the fiber breaks. The damage crack is perpendicular to the fiber axis and therefore can lead to a decrease in fiber strength. Based on the four-phase (fiber, interfacial phase, matrix, composite) cylinder model, we assume that the interfacial bond will no longer transfer shear stress. Firstly, the shear lag theory was used to determine the stress concentration factor in the fiber and interfacial phase and the shear stress on the interfacial surface after multiple cracks in the interfacial phase. At the same time, the influence of the debonding on the interface between the fiber and interfacial phase was considered. Then, The statistical properties of fiber strength are expressed by the Weibull distribution function, and the variation of the probability of failure of the wide fiber is obtained according to the stress concentration coefficient K_f caused by the multifractal cracking in the fiber interface. Finally, the influence of interfacial shear strength τ_0 on the fiber strength was qualitatively studied by the size of the interface debonding zone. The results show that there exists an optimum interfacial shear strength τ_0, and the interfacial multifracture has the least effect on the fiber strength.