Continuous fiber reinforced composite materials are susceptible to matrix cracking and delamination upon impact. Active and passive wires can be embedded within the composite material to support the healing behavior. Upon a local heating stimulus the wires, oriented mostly in the out-of-plane direction, can assist the buildup of compressive stresses to close matrix cracks and delaminations. Shape memory alloy wires, prestrained to contract upon heating, can actively close cracks in a damaged region during the heat treatment. A closing action can also be achieved in a passive sense, based on differences in thermal expansion of the composite material and the wires employed.
The effects of the wire type, fraction and distribution on the closing and healing behavior of composite materials were studied for a simplified model case. The requirements for optimal matrix healing have been determined. The results are strongly dependent on the thermo-mechanical properties of the composite material and the wires. Small amounts of shape memory alloy wires are already sufficient to generate adequate compressive stresses at the healing temperature. Prestraining of the wires is not a prerequisite. Passive wires, such as glass and aramid fibers, can be employed in a similar way. A stitch type of wire distribution is more effective than a woven type of distribution.
Experimental verification of the closing behavior of active and passive out-of-plane wires has been carried out through consolidation experiments of pre-impregnated composite layers. Active shape memory alloy wires were capable of generating sufficient compressive stresses in the out-of-plane direction to support closure and welding of separate composite layers. The fracture toughness obtained was comparable to that of a reference plate of the same material manufactured with a hot press. The active wires also coped better with size variations in the out-of-plane direction than passive wires based on glass and aramid fibers.