Title: Temperature dependent calibration of shape-memory alloy strain sensors

Abstract

Regular metallic strain gauges are elastic up to 0.3 percent strain. They can be applied on metallic structures to monitor strain and loading to implement structural health monitoring (SHM). Composite materials comprise a higher elastic strain, e.g. about 0.5 percent strain for glass-fibre reinforced plastics (GFRP). SHM of GFRP therefore demands strain sensors with higher elasticity. Novel shape-memory alloy strain sensors (SMASS) are fatigue resistant up to 1.5 percent strain and are a simple solution for that requirement. These novel sensors show a higher temperature dependence in comparison to regular strain gauges (Mäder et al., 2021). Investigations are made to find an adequate solution for temperature compensation. Hence, different configurations of Wheatstone bridges are tested. A four-point bending test device, which can be used inside a climate chamber, was developed. Four-point bending tests guarantee a homogenous strain distribution over the strain sensor and are regularly used for calibration. The SMASS were applied on GFRP bars in longitudinal and transverse orientation to the center line. The single SMASS were electrically connected in different Wheatstone bridge configurations. Different temperature levels were approached. Each temperature step included a balancing time to guarantee an equal temperature distribution in the whole setup. Several bending cycles were done on each temperature level. Strain, sensor signals and temperature were monitored. All tested sensor configurations eliminate the temperature dependent strain signal drift. All configurations retain an almost stable absolute zero value of the sensor bridge signal over all tested temperatures. The different configurations comprise various gauge factors. All gauge factors are non-linear and temperature dependent. The best performance was achieved with the bending configuration with one sensor on the tension and the other on the compression side of the specimen bar. Tension and bending can be reliably monitored. The monitoring of compression is possible at temperatures above 20°C.