This paper deals with lifetime analyses of shape memory wires. The main focus is to contribute to actuator applications and to design them in an optimal way with regard to actuator lifetime. In the past, different actuators have already been developed by our research group. Since shape memory wires have a high energy density and are suitable for lightweight construction, small but powerful release mechanisms can be designed. The activation of the wires is performed electrically via the internal resistance. As a result the wire heats up and the contraction of the wires is induced.

Our research group is concerned with the condition monitoring of shape memory wires, which are thermo-mechanically cycled. One of the parameters used to determine the condition of the wire is its electrical resistance. The electrical resistance increases with rising number of cycles due to crack accumulation (structural fatigue) and dislocations in the grid (functional fatigue) until the structural fatigue is so far advanced that the wire breaks. Both phenomena lead to a local increase in electrical resistance, which is also expected to increase the temperature of the wire locally. In this work the authors want to investigate whether these local increases in resistance can be detected thermographically and whether the position of the wire crack can be predicted.

 

For this purpose, a thermal imaging camera (Optris OPTPI640MO44T900) was integrated into an existing test rig. A housing was built around the wire and effects caused by external heat radiation were minimized. Since the expected temperatures are in the range of 20°C to 200°C, the used thermal imaging camera was selected for this temperature range. The authors focused on 0.3 mm thick binary Ni50Ti50 wires. Since measuring the exact temperature of the wire is difficult due to the roundness and the solely approximately known values for the emission coefficient, the temperature was only determined qualitatively. The investigations were intended to show whether a hotspot forms along the length of the wire, which becomes hotter than the surrounding area as the number of cycles increases, and finally marks the location of the crack.

 

To this end, severel tests were carried out, 3 each at induced stress parameters of 150 MPa and 400 MPa and Voltage parameters of 3.3V and 3.8 V. In each test, a hot spot could be identified where the wire will later crack. The hotspot that indicated the crack could be detected at the latest after 85% of the overall lifetime of the wire. In addition, one test was stopped when the hot spot was detected and the corresponding location was examined in a scanning electron microscope.

This work contributes to a better condition monitoring of shape memory wires which are increasingly used in innovative actuators.


A Method of Crack Monitoring for Electrically Activated SMA Wires Using Thermography

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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