Recently, a novel power controller has been presented that simultaneously controls electric power input and measures resistance of a commercially available Flexinol shape memory alloy wire. This work exploits the new power controller by plotting shape memory alloy stress, strain, and resistance versus Joule heating power instead of input voltage or current. Heating power is directly related to shape memory alloy temperature, whereas standard constant voltage or constant current inputs cause more or less heating as the resistance of the wire changes due to phase transformation. A simple experimental setup consisting of a 50-µm-diameter Flexinol wire mounted in series with the tip of a compliant cantilever beam is used to systematically study the shape memory alloy behavior. Actuator performance is reported for a range of prestress values and actuation frequencies. All the experimental data are compared with simulated behavior that is derived from a free energy–based numerical model for shape memory alloy material. Additionally, a new resistance model based on the simulated wire temperature and phase fractions is introduced and compared to experimental data. Exploiting a multifunctional power controller and an improved understanding of the resistance change during phase transformation will help enable the use of shape memory alloy wires in simultaneous sensing and actuating applications.

Across mammals many vocal sounds are produced by airflow induced vocal fold oscillation. We tested the hypothesis that stress-strain and stress-relaxation behavior of rat vocal folds can be used to predict the fundamental frequency range of the species’ vocal repertoire. In a first approximation vocal fold oscillation has been modeled by the string model but it is not known whether this concept equally applies to large and small species. The shorter the vocal fold, the more the ideal string law may underestimate normal mode frequencies. To accommodate the very small size of the tissue specimen, a custom-built miniaturized tensile test apparatus was developed. Tissue properties of 6 male rat vocal folds were measured. Rat vocal folds demonstrated the typical linear stress-strain behavior in the low strain region and an exponential stress response at strains larger than about 40%. Approximating the rat’s vocal fold oscillation with the string model suggests that fundamental frequencies up to about 6 kHz can be produced, which agrees with frequencies reported for audible rat vocalization. Individual differences and time-dependent changes in the tissue properties parallel findings in other species, and are interpreted as universal features of the laryngeal sound source.

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.

This work focuses on the development of a concept for a micro-air vehicle (MAV) based on a bio-inspired flapping motion that is generated from integrated smart materials. Since many smart materials have their own biomimetic characteristics and the potential to be highly efficient, lightweight, and streamlined, they are ideal candidates for use in structural or actuator components in MAVs. In this work, shape memory alloy (SMA) actuator wires are used as analogs for biological muscles, and super-elastic SMAs are implemented as flexible joints capable of large bending angles. While biological organisms have an intrinsic sensing array composed of nerves, the SMA wires also provide self-sensing by virtue of a phase-dependent resistance change. Study of the biology and flight characteristics of natural fliers concluded that the bat provides an ideal platform for SMA muscle wires because of its comparatively low wingbeat frequency and superb maneuverability. A first-generation prototype is built to further the understanding of fabricating Nature’s designs. The engineering design is then improved further in a second-generation prototype that combines 3D printing and new techniques for embedding SMA wires and shaping SMA joints for improved robustness, reproducibility, and lifetime. These prototypes are on display at the North Carolina Museum of Natural Science’s Nature Research Center, which has the goal of bridging the gaps between biology and engineering.