Behavior of NiTi Wires for Dampers and Actuators in Extreme Conditions

Isalgue, Antonio

Shape Memory Alloys are considered smart materials because of their singular thermomechanical properties, due to a thermoelastic martensitic transformation, enabling possible uses as actuators (because of mechanical recovery induced from temperature changes) and as dampers (because of hysteresis). Many applications of SMA need a careful evaluation of the fatigue life under thermal and mechanical actions. The failure of a given device made of SMA can be due to structural failure (fracture of the specimen after successive loading-unloading cycles) or to functional fatigue, incapacity to do the expected task, or perform the expected path in stress-strain-temperature, even if fracture does not occur.

We had characterized NiTi wires for dampers in Civil Engineering, and tested in facilities, where they showed good performance. The mechanical energy absorbed by the unit weight of wire might be very high during its lifetime if maximum stresses remain relatively low allowing high fatigue life. In this work, we explore the consequences of overstraining the material during installation, the possibilities of partial healing by moderate heating, and some effects of over-stressing the wires. We show some results on NiTi wire working as actuator. There is some performance as actuator at high stresses (near 900 MPa) and only moderate temperature changes (some 100 K of temperature increase), even with Clausius-Clapeyron coefficient of stress change whit temperature on transforming of about 6 MPa/K.

The reason for the observed characteristics seems to be that when external high stresses are applied to a NiTi wire, it undergoes some plastic deformation, leaving internal stresses that alter the shape and position of the macroscopic stress-strain transformation path. Then, the wire might recover some length (partial shape memory effect) by moderate heating even at high stresses. The change of the hysteresis cycle induced by the high stresses applied justifies the observed performance.