Spatial Phase Fraction Analysis from In-Situ XRD Experiments of SMA Knitted Actuators

The reversible phase transformation between a B2 austenite phase and a B19’ martensite phase is the governing mechanism for the large, three-dimensional actuation deformations of shape memory alloy (SMA) knitted actuators. SMA knitted actuators are manufactured from a monofilament of originally straight SMA wire that is bent into an interlocking network of adjacent loops. Depending on the thermo-mechanical load path, SMA knitted actuators can provide excellent isothermal energy-absorption using the superelastic effect (SE) or function as large-deformation actuators that respond to thermal inputs using the shape memory effect (SME). The magnitude of SMA knitted actuator characteristic performance metrics (e.g., deformations, forces) is dependent on the ability of the knitted actuator to undergo the reversible phase transformation, which is a function of the material stresses (σ), strains (ε), and temperature (T). This research quantifies the austenite phase fraction ξ(σ, ε, T) in x-ray diffraction experiments, where the material stresses and strains are dependent on the knit geometry, the measurement position within the knitted loop, and the external mechanical loads. The measured scattered intensities are compared to a fully-austenitic and fully-martensitic reference. This research establishes a multiscale dependency that enables the correlation of macroscopic performance metrics to the microstructural austenite phase distribution. By understanding this multiscale dependency, the macroscopic geometry can be altered to optimize the performance of SMA knitted actuators based on the fundamental governing mechanisms of the SMA material.