Clarification of Characteristic Factors for Increasing the Strength and Reliability of Fibrous Composites

通过蒙特-卡罗模拟技术和随机模型阐明了提高纤维复合材料强度和可靠性的特征因素。模拟技术的基础上开发的有限元法，其中使用的两个节点的线元表示的纤维和四个节点的等参元素表示的矩阵。模拟结果表明，纤维与基体之间存在适当的界面结合强度，提高了复合材料的拉伸强度，减小了强度的分散性。其强度和可靠性与纤维断裂前后的损伤程度和损伤类型密切相关。换句话说，纤维与基体之间的小尺度脱粘促进了纤维的累积断裂，起到了提高强度和可靠性的作用。另一方面，Marko nu过程被应用于常用于单向纤维复合材料轴向强度分析的束链单层模型。假设一组纤维断裂点（所谓的簇）在时间上间歇地在簇周围经受局部载荷分担，并且如果簇达到临界尺寸，则复合材料断裂。使用时间相关的威布尔分布作为光纤的寿命分布函数。然后，分析得到了复合材料强度的累积概率。结果表明，团簇越大，分布宽度越小，形成一条主型分布曲线。这种分析被应用到一个3维六边形纤维阵列。分析的分布也显示出与在单层中获得的相同的行为。Marko nu过程分析进一步扩展到纤维断裂后界面脱粘和基体开裂的复合材料。

Characteristic factors for increasing the strength and reliability of fibrous composites were clarified by a Monte-Carlo simulation technique and stochastic models. The simulation technique was developed based on a finite element method, in which a two-node line element representing a fiber and a four-node isoparametric element representing a matrix were used. The simulated results show that an appropriate interfacial bond strength between the fiber and matrix exists, which increases the tensile strength of the composite and decreases the scatter in strength. The strength and reliability was closely related with the degree of damage and its type around a fiber breakage. In other words, small-scale debonding between the fiber and matrix promotes cumulative fiber breakages and plays a role in increasing the strength and reliability.On the other hand, Marko nu process was applied to a chain-of-bundles monolayer model which is often used for axial strength analysis of unidirectional fibrous composites. It was assumed to the process that a group of fiber breakage points, the so-called cluster, evolves in time intermittently subjected to local load shares around a cluster, and that the composite fractures if a cluster achieves a critical size. The time-dependent Weibull distribution was used as a lifetime distribution function of the fiber. Then, the cumulative probabilities in composite strength are analytically obtained. The results showed that larger clusters reduce the width of distribution and form a master-like distribution curve. This analysis was applied to a 3-dimensional hexagonal fiber array. The analyzed distribution shows also the same behavior as obtained in the monolayer. The Marko nu process analysis was further extended to a composite with interfacial debondings and matrix crackings following fiber breakages.