Shaped ductile fibers to improve the toughness of epoxy-matrix composites

Pull-out tests on ductile fibers embedded in epoxy resin have shown an additional component of pull-out work from the plastic deformation of the fiber, which component is not available in traditional brittle fibers. This additional deformation energy is available to improve the toughness of composite materials without sacrificing composite stiffness. The plastic deformation energy in ductile copper and nickel fibers is exploited by anchoring the fibers in the matrix through modification of fiber ends. Shaped fiber ends were produced by end-impacting and knotting fibers to facilitate anchoring, similar to previous work with 'bone-shaped' short (BSS) fibers. Both shaped and straight fibers were embedded in epoxy to various depths and angles, and pull-out tests were performed to determine whether an increase in pull-out work was evident for shaped fibers. Cold-working the fibers or treatment with release agent prior to embedment served as a tool to isolate the frictional component from the plastic deformation component of pull-out work during pull-out tests. The effect of fiber yield stress and hardenability on the pull-out work was also investigated. Calculations based on the test results and a model for predicting fracture toughness increment showed a 5-109% higher toughness increment for shaped fibers compared to straight fibers, dependent on material conditions and fiber orientation.