April_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 0 6 4 technology for understanding SMA knitted actuators in gen- eral. The contractile SMA knitted actuator pattern consists of interchanging rows of knit and purl loops. In the knit manufacturing process, an originally-straight NiTi wire is deformed into the network of knitted loops in its martensite phase. The partial recovery of large bending defor- mations in the knitted loops upon heating above the actuation temperature results inamacroscopic contractionof theknitted actuator. The force-extension profiles of SMA knitted actuators depend on the SMA knitted actuator geometry and the tem- perature (Fig. 1c). The fully-martensitic force-extension profile ( T < M f ) of the knitted actuator shows the strain-hardening be- havior characteristic to knitted textiles. The partially-austenitic force-extension profile ( T > A f ) unites knit architectural behav- ior and NiTi material behavior with its geometrically tunable plateau. The normalized difference between the two profiles is called the actuation contraction (ζ) and is a commonmetric of actuation performance. Typical SMA knitted actuators provide actuation contractions between 5 - 45%, as predictedby the di- mensionless knit index (i k = A l / d 2 ) [6] , while the actuation stroke and forces are scalable by changing the number of courses and wales. MECHANICS ENABLING PRODUCT DESIGN Recent developments in the derivation of design rules and definition of SMA knitted actuator specific properties have provided the grounds for proof-of-concept SMA textile prod- uct designs [7] . Such properties include the textile actuation and relaxation temperatures in distinction to the SMA phase transformation temperatures, initial architectural and mate- rial shakedown, performance repeatability, and knit scalabil- ity derived from macroscopic experiments. The connection of macroscopic SMA knitted actuator mechanics and microscale governing SMA material mechanisms further increases the comprehension of the knitted actuator performance. Classic micro-mechanical experimentation, such as diffractometry or microscopy can resolve the governingmaterial mechanisms of NiTi, including twinning, plasticity, or micro-cracking, but of- ten rely on the heavily preprocessed specimen for high-quality results. While such processing is unattainable for SMA knitted actuators, novel experimental techniques can resolve a re- duced set of in-operando SMA knitted actuator micromechan- ical properties. A Bruker D8 Discover 2Dmicro-diffractometer was equip- ped with a custom tensile straining- and temperature-control device for SMA knitted actuators to resolve the spatial distribu- tion of SMA material phases along the knitted loop (Fig. 2a) [8] . The elevated actuation and relaxation temperatures explored in the macroscopic tensile experiments were confirmed by x-ray micro-diffraction. Additionally, it could be shown that the phase transition is primarily affected by the combination of bending and contact stresses in the knitted loop and is dis- tributed according to the presence of those stresses along the knitted loop. Understanding the distribution of NiTi phase fractions along the knitted loop elucidates loop geometry op- timization strategies, targeted heat treatment, and filament design to further tailor and improve the shape memory tex- tile performance. Fig. 2 — (a) In-situ strain- and temperature-control x-ray microdiffraction experiments were enabled in a Bruker D8 Discover 2D experiment by adding a custom straining device, resistive heating attachments, and an IR camera. (b) A self-fitting sleeve in its oversized, relaxed state. (c) The operation procedure for the use of self-fitting sleeves. The garment starts in its initial, oversized, martensitic state (1) and is subject to forces upon donning (2). When completely-donned, the garment relaxes, but remains partially-deformed (3). Heating above the austenite finish temperature leads to the recovery of deformations, self-fitting, and exertion of pressures once the garment dimension equates the body dimension. (a) (b) (c) 1 0 FEATURE