Fatigue of hybrid composites by a cohesive micromechanic model

Abstract

A cohesive micromechanic fatigue model (CMFM) which identifies a nonconservative bonding reaction between a broken molecular chain and its neighbor as the main microscale source of fatigue damage accumulation has been recently developed for a unidirectional material constructed from a parallel set of chain-like elements. The successive breakages in each cycle are controlled by the statistical strength distribution of the elements and the probability and amount of interference which a broken element causes to its neighbors. The model gave a physically sound explanation to the fatigue power law S— N curve and the endurance limit phenomena by direct interpretations of microstructure parameters. In this study the model is expanded by considering a material which have two components with different mechanical and statistical properties and are mixed to give a hybrid composite. The main target is to find the best combination for fatigue resistance, or more specifically, to explore the possibility of making a composite which is more fatigue resistant than each of its two components. It is found that mixing a brittle component (high modulus and low failure strain) with a soft one (low modulus and high failure strain) having a specific microstructure gives the desired effect if some requirements on the structural and mechanical properties are met. The two materials are mixed in a form of bundles, so that the composite microstructure and fatigue resistance are controlled by their relative volume faction (a macro property) as well as the number of elements in each bundle (a micro property).