Shape memory strain gauges

Reinforced and plain polymers are highly elastic. In order to measure strain of parts made of these, strain sensors have to be highly elastic too. Such sensors, certainly for cyclic loading conditions with high amplitudes, are not available. Pseudoelastic shape memory alloys are used to realise high performance strain sensors with high elasticity. These sensors can be elastically strained up to 80,000 µm/m. In cyclic loading conditions strain levels or amplitudes of about 20,000 µm/m are possible. The high elasticity stems from the stress-induced phase transformation of the SMA. This phase transformation involves a comparably strong specific electrical resistivity change together with long elastic strain variations. The determined gauge factor exceeds 5 and is higher than that of most of the conventional metallic strain gauges. Sensor structures can be made of wires. These can be embedded into plastics and fibre reinforced plastics. The integration of sensors is possible via injection moulding, laminating and infiltrating processes. The paper presents recent results of the development of shape memory alloy strain gauges.