Stress, strain, and resistance behavior of two opposing shape memory alloy actuator wires for resistance-based self-sensing applications

Shape memory alloy actuator wires undergo a significant (∼4%) contraction and a corresponding change in resistance because of a temperature- and load-induced phase transformation. When a restoring force such as a pre-stretched bias spring is placed in series with a shape memory alloy wire, the system becomes an actuator that can generate a repeatable force. Simultaneously, the resistance of the wire can be correlated to strain and enable self-sensing, eliminating the need for external feedback sensors. The self-sensing task, however, is complicated in applications requiring multiple coupled wires, for example, advanced two-dimensional or three-dimensional positioning. The presence of coupled (passive or active) actuator wires with nonlinear, hysteretic force–displacement characteristics has a strong impact on an individual wire’s resistance behavior that has not been systematically studied to date. This article expands upon previous work that studied a single-shape memory alloy–spring system by adding a second opposing shape memory alloy wire and focusing on the resistance to strain mapping that is crucial for self-sensing applications. Systematic stress–strain and resistance–strain experiments are presented alongside physics-based modeling results that help to identify several sources of hysteresis in the resistance–strain behavior and facilitate intelligent calibration schemes for multifunctional self-sensing and actuation applications.