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

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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.