Improving fracture toughness of brittle matrix composites using end-shaped ductile fibers: The effects of adhesion and matrix shrinkage

Ductile fibers are added to brittle matrix composites to increase the fracture toughness. To further improve fracture toughness, end shaped ductile fibers are added to act as anchors to utilize more of the fibers' plasticity. Previous research focused on optimizing the volume of the shaped end for a given end shape family. Results indicate that for a given end shape family there is an optimum volume; above or below this volume results in a lower fracture toughness contribution. This research investigates two additional factors, adhesion of the matrix to the fiber and matrix shrinkage, and determines their effects on the fracture toughening of brittle matrix composites. The fiber was an annealed copper and the matrices used were a low shrinkage epoxy, a high shrinkage epoxy, and polyester. Results indicate that controlling the surface chemistry of the fiber can give an additional degree of freedom to the utilization of the fiber plasticity, although the importance of this control depends on the particular system. The fiber surface chemistry affects the bond strength and the adhesion; if the fiber cannot debond from the matrix, then shaping the end does not permit use of the plastic potential. Depending on the system, the adhesion and bond strength of the matrix to the fiber significantly affects the amount of fiber plasticity utilized. To determine the effects of friction and matrix shrinkage on the utilization of the fiber plasticity, release agent was applied to the end shaped fibers to reduce the adhesion, bond strength, and friction during pull out. Results indicate that frictional work and adhesion has a large impact on the utilization of the fiber plasticity; with release agent, the end shaped fiber utilizes little of the fiber plasticity. Furthermore, this indicates that for the matrices investigated, matrix shrinkage has a minor influence on the utilization of the fiber plasticity.