Effect of Low and Reverse Loading Paths on the Actuation Characteristics of Shape Memory Alloy Torsional Actuators

Shape memory alloy (SMA) actuator systems used in aerospace applications often require bi-directional actuation against external loads. Applications where actuation against bi-directional loading is required include flight control devices such as trailing edge wing-flap actuators where actuation above and below a faired position creates hinge line torques in two directions. SMA elements are trained for actuation and dimensional stability against loading in a single direction.

This study describes actuating torsional SMA devices under both forward and reverse bias loading relying solely on the two-way shape memory effect (TWSME) as a restoring force. Several load paths intended to simulate an SMA actuator application were applied to trained equiatomic NiTi torque tube actuators and the effects on SMA performance during extended cycling were assessed.

Load paths that included extended cycling at a negative or insufficiently positive component (spring or isobaric) showed degradation in TWSME. However, the dimensional instability and degradation of TWSME over extended cycling at these loads was significantly reduced when using partial thermal cycles. Load paths that maintained sufficiently positive components resulted in stable actuation over hundreds of cycles. Results in this study show cooling a NiTi torque tube against a mechanical stop generates a back-stress that decays with repeated cycling and leads to a small degradation in TWSME but does not significantly impact the operational performance of the SMA element. Recommendations for using an SMA element relying on TWSME to meet bi-directional loading requirements will be discussed.