Abstract
Predicting the remaining lifetime of an operating shape memory actuator is a great goal to achieve to increase its reliability. The shape memory wires used in these actuators are mostly activated using Joule Heating. Therefore the electrical resistance during activation can easily be measured. Studies show an increase in electrical resistance with an increasing number of activations due to fatigue. Therefore monitoring the electrical resistance leads to a prediction regarding the remaining lifetime of the actuator. The electrical resistance depends on the ambient temperature and the load case (yielded stress, activation frequency, voltage, and current) resulting in different maximum activation temperatures of the wire. The increase in electrical resistance should lead to a higher wire temperature. Before the wire fails due to fracture cumulating cracks should appear. These cracks decrease the diameter of the wire that leads to locally increased electrical resistance and therefore a local higher temperature. This work investigates if those hotspots can be monitored using thermal imaging. Binary Ni50Ti50 wires with a diameter of 0.28 mm were investigated. Measuring the exact temperature of the wires is difficult since the specimens are round and the emission coefficient is unknown. Therefore only qualitative measurement of the temperature is performed.
10 experiments at different stresses and voltages were performed. The results show some indications, that the position and the moment of the fracture can be determined using this setup. Several models are matching the wire temperature after activation to its fracture strength and fatigue behavior. Further investigations must be performed combining the presented models to the results of this work.
