An Investigation of Thermo-Mechanical Fatigue of a Commercial Shape Memory Alloy Under Extreme Operating Conditions

Commercial shape memory alloy (SMA)-based actuators are typically used at gentle operating conditions (employing small contraction strains (<2%) and low pulling stresses (<200MPa) leading to a small work output) to ensure millions of usable actuation cycles. Under much more severe operating conditions (larger operating stresses and strains), actuator work output is greatly enhanced, but at the expense of a shortened fatigue life. This trade-off between cycle life and work output can be exploited for many actuator designs that require far fewer actuation cycles (10s-10,000s cycles) than the millions of cycles typically available under current operating conditions. Since, the behavior, evolution and fatigue failure of SMAs, including the commercial alloy, has largely been unexplored at extreme operating conditions, we conducted an initial systematic study of shape memory alloy actuator wires at various dead loads, heating times, peak voltages and duty cycles to examine the effects of stress, heating rate and over temperature on structural and functional fatigue of SMA wires. Never-before-seen trends of the SMA’s functional evolution and fatigue life are presented. The desensitization of over temperature on fatigue as found in this study potentially eliminates the need for feedback algorithms or hardware mechanisms that would mitigate against the effects accidental thermal overloads on fatigue for actuators that only require low thousands of operating cycles. These findings can therefore not only be used to augment current actuator design space but push for new design ideas such as the ratcheting mechanism to more fully access the capability of the alloy. This study was carried out in order to increase SMA actuator wire performance and more accurately define its functional limits. This was achieved by investigating the effects of thermo-mechanical loading parameters at extreme operating conditions on the wire’s functional and structural fatigue lives using pulse width modulated joule heating.