Fiber push-in failure in carbon fiber epoxy composites

Micromechanical quasi-static and cyclic fiber push-in tests performed with an indenter in-situ in a scanning electron microscope on polished slices (thickness around 300 µm) of carbon-fiber epoxy composites provide information on the effect of fiber density on the fiber-matrix debonding and the fiber failure. Specifically, "close-packed", i.e., fibers surrounded by six nearest neighbor fibers (called "hexagonal" hereafter) versus "isolated" fibers, defined as fibers at least one fiber diameter distant from the next nearest fiber, were compared. In spite of the more compliant behavior of isolated fibers under quasi-static and cyclic indentation loads, initiation of fiber-matrix debonding and push-in failure (fiber splitting into two or three parts) occurred at roughly comparable loads for both, "hexagonal" and "isolated" fibers. Earlier tests on thinner slices of such composites (thickness around 30 µm) had resulted in full debonding and fiber push-out without fiber failure. The fibers in the thicker slices showed first indenter imprints on the surface and an increasing hysteretic behavior in the load-displacement curves before debonding. The analysis of the cyclic load-displacement curves yields the elastic-plastic and the hysteretic (debonding) energy contributions.