Abstract
Shape memory alloys (SMA) are often used to drive actuators, whereby SMA have a very specific fatigue behavior, which influences the lifetime of these actuators. The optimization of the actuator’s lifetime is an important topic to make these innovative actuators more interesting and economic for the sales market. Usually, an actuator is developed first, and a lifetime investigation of the complete actuator is carried out afterwards. The reason is the missing handling guidelines for the SMA wires which provide information on influences limiting the service life of SMA-based actuators before the actuator is fully developed and manufactured.
While investigating the influences on the SMA wire during production, storage, and use of an actuator, one frequently appearing case stands out and will therefore be examined in more detail in the further course. In many actuator applications the wire is deflected by a pulley system to generate a greater actuator stroke while keeping the assembly space small. Although a large stroke is required, a wire is used instead of a spring to additionally take advantage of the higher forces generated by the wire compared to the spring. Since the wire experiences inconsistent mechanical stress at the point of deflection and a modified heat conduction occurs due to surface contact, it is questionable if this damages the wire, leading to an earlier structural or functional fatigue.
The experiments will be carried out in two different ways. First, the SMA wire is tested regarding lifetime as a reference value. Afterwards, a pulley-system is designed and used deflecting the SMA wire by 90°. Subsequently, the respective lifetimes of the wires from both experiments can be compared to each other and conclusions can be drawn to what extent a deflection of the wire influences the wires lifetime. Furthermore, the damage caused by the deflection is put in relation to an influence of the wire by overheating — in case of too long electrical activation — and to the influence of the mechanical pretension of the wire.
Through investigating these impairments of the SMA wire by deflecting it, handling guidelines for these specific cases can be formulated to be considered in future before the actual development of an actuator takes place. Thus, a more effective development with fewer iteration steps can be achieved and the lifetime of the actuator can be determined more precisely in advance. Furthermore, the design can be optimized easier regarding lifetime, by having knowledge apropos of the insight of these influences.
