Debond induced by strain recovery of an embedded NiTi wire at a NiTi/epoxy interface: micro-scale observation

Rapid development in smart structures has enhanced the great efforts in understanding the mechanical and thermo-mechanical behaviour of shape-memory alloy (SMA) materials. SMAs, in the form of wires and strips, have been embedded into advanced composite structures to control the shape and residual stress of the structures. It is well recognised that the mechanical properties of embedded SMA composites are highly dependent on the integrity of the interface, particularly by the existence of high thermal-induced shear stress at the SMA wire/epoxy interface. This paper discusses the debonding failure mechanism of embedded pre-strained SMA wires in an epoxy matrix environment using the scanning electron microscopy (SEM) technique. It was found that debonds occurred at the SMA wire/matrix interface for a wire with a pre-strained level of 8% at a temperature above Af. Several cracks in the matrix were found around the wire-end region and arrayed along the circumferential direction. A sharp crack, which was propagated towards the radial direction of the wire, was induced in a matrix with high air-bubble content during the wire/matrix debonding process. The effect of high-temperature treatment of SMAs was also studied. It was found that the surface layer of the wire peeled off when the wire was heated at 773 K for 10 min.