Multiscale considerations for interface engineering to improve fracture toughness of ductile fiber/thermoset matrix composites

Mechanical interlocking at both large-scale and fine-scale as well as continuous and intermittent chemical bonding were employed to engineer the friction during fiber pullout to improve toughness. Excellent toughening results have been achieved over a large range of typical fracture feature size in the large micro-/small meso-scale of 1-100 μm. Interphase engineered features for size scales below the microscale were smoothed over or subsumed during fracture. For mechanical interlocking at the large scale, the overall fiber shape is critical and there is little interfacial adhesion. For mechanical interlocking at the fine scale, a protruding "break-away" feature performs better than a concave, cavity-like feature. For chemical bonding, intermittent coating gives superior toughness. In all of the interface or interphase designs, it is found that there is an intermediate anchoring of the fiber to best utilize fiber plasticity to improve composite toughness.