Investigations Regarding the Longterm Behaviour of Electrically Heated SMA Wires Using Alternating Current

Abstract

Abstract Shape Memory Alloys (SMA) exist in several forms, such as wires, tubes, springs and several more. Most commonly available SMA forms are wires, contracting when heated. There are several ways to activate the effect in wires, but mostly they are electrically heated (Joule Heating) using direct current (DC). There are examples of using alternating current (AC) for the activation of SMA wires, but rather for a highspeed activation that can be considered loss-free, since the wire is heated rapidly. This work investigates the long term behavior of SMA wires activated using AC. An experimental setup according to Design of Experiment (DoE) was proposed to investigate the different influence of the long term behavior between an activation using AC and DC. 15 different levels of the activation parameters voltage, activation time and stress — each for the AC and the DC — were chosen. The value of the DC voltage was the same as the effective voltage for the AC. For each set of parameters, according to the experimental design, two specimens were investigated. The different activation techniques differ in one important aspect: the source generating a DC is controlled, whereby the output voltage is independently kept constant from the change in electrical resistance induced by the phase transformation of the SMA wires when heated. The AC power does not offer such a function. Therefor the applied value of the effective Voltage is only an initial value at the beginning of the activation. During the activation the values of the voltage and the current change due to the phase transformation. For further comparison, the activation energy for both activation methods was calculated. During activation the current and the voltage were measured at 600 Hz for the AC and at 50 Hz for the DC. The energy per activation is calculateto increase comparability between AC and DC.