Adaptive composites with embedded NiTiCu wires

Adaptive composites have been produced by embedding shape memory alloy (SMA) ternary NiTiCu wires into an epoxy matrix host, reinforced or not with aramid fibers. These materials demonstrate attractive effects such as shape change or a shift in the vibration frequency upon activation. The SMA composites are produced by prestraining SMA wires and embedding them in the composite. When heated above their transformation temperature, the wires' strain recovery is limited, and recovery stresses are generated. As a result, if the wires are placed along the neutral axis of a composite beam, a shift in resonance vibration frequency can be observed. To optimize the design of such composites, the matrix - SMA wire interfacial shear strength has been analyzed with the pull out testing technique. It is shown that the nature of the wire surface influences the interfacial shear strength, and that satisfactory results are obtained for SMA wires with a thin oxide layer. Composite samples consisting of two different types of pre-strained NiTiCu wires embedded in either pure epoxy matrix or Kevlar-epoxy matrix were produced. The recovery force and vibration response of composites were measured in a clamped-clamped configuration, to assess the effect of wire type and volume fraction. The results are highly reproducible in all cases with a narrow hysteresis loop, which makes NiTiCu wires good candidates for adaptive composites. The recovery forces increase with the volume fraction of the embedded wires, are higher when the wires are embedded in a low CTE matrix and, at a given temperature, are higher when the wire transformation temperature is lower.