Session: 02-02: Performance of Shape Memory Alloys

Paper Number: 90483

90483 - Design of a Modular Lifespan Test Bench for Shape Memory Alloy Wires_x000B_ 

Design of a modular lifespan test bench for SMA wires 

Joshua Mayer 1 

Philipp Molitor, M.Sc. 2 

Yannik Goergen, M.Sc. 1,2 

Dr.-Ing. Paul Motzki 1,2 

1 Intelligent Materials Systems Lab, Department of Systems Engineering, Department of Materials Science and Engineering, Saarland University, Saarbrücken, Germany 

2 Intelligent Materials Systems Lab, Center for Mechatronics and Automation Technology, ZeMA gGmbH, Saarbrücken, Germany 

To determine the lifespan of a system, you need to know the lifespan of all parts as well as the interaction of the parts as a system. Accordingly, in systems where shape memory alloy (SMA) wires are used as actuator elements, their lifespan must be determined as well. Typically, the lifespan given by the manufacturer is measured only by mechanical fatigue testing. In actuator systems, SMA wires are heated by an electric current to undergo a thermally activated phase transformation under mechanical stress.  

To investigate the fatigue life of SMA wires under those conditions a test bench is realized. It investigates the change in mechanical stress, strain, and resistance over lifetime. The test bench is modular, so single wires and wire bundles of diameters from 76 µm up to 500 µm and lengths from 20 mm up to 200 mm can be measured. The test bench consists of a linear guide rail, a displacement sensor, a load cell as well as the clamps for the SMA wires. To be able to measure the lifespan under different conditions like maximum mechanical stress and strain, parameters can be adjusted individually either by software or by mechanical adaptations. The preload, for example, can be set with a trapezoidal thread and a spring. For control and data acquisition, an NI system using a CRIO with an FPGA as real-time processor in combination with LabVIEW is used. To be able to determine the electrical resistance of the SMA wire sample during heating and cooling, custom-built electronics is used. This electronics also allows for heating the wire, which is controlled by the FPGA. The setup enables the investigation of the lifespan of SMA wires considering various system-dependent requirements. This includes but is not limited to tests, that allow the wire to be repeatedly loaded to a specified mechanical stress or strain.  

The first test runs a 100 µm thick and 125 mm long wire on a preload of 350 MPa. The wire is heated with 300 mA until a stroke of 3 % is reached, but not longer than 5 s. for cooling the time is 2.2 s. In those first tests, the wires already reached 240,000 cycles before failure. The stress-strain behaviour stabilizes within the first 1000 cycles, after showing a decline in the minimum and the maximum force and strain per cycle. Afterwards a linear decline until failure is observed. 

Presenting Author: Joshua Mayer Intelligent Material Systems Lab, Center for Mechatronics and Automation Technology (ZeMA gGmbH), Saarbrücken, Germany