Active Knit architectures for radical shape change

At LMSSC, Cnam, Paris, July 1st 2011, 15 p.m.

Diann Brei
Associate Professor, Mechanical Engineering Department, University of Michigan, Ann Arbor, USA

Nature builds an immense set of materials exhibiting a wide range of properties using only a small number of basic compounds. The range of materials comes about through architecture, giving functional structure to the basic compounds. Analogously, a new genre of actuators can be derived from existing smart materials through architecture.
This presentation introduces this concept through a novel cellular architecture, Active Knits which yield high strains (10-100%) with moderate forces (tens of Newtons or more) from contractile Shape Memory Alloy (SMA) fibers. Through combinations of the two primary knitted loops – purl and knit – a variety of complex, distributed behaviors can be achieved including contraction, rolling, spiraling, accordion, arching, and any permutation of these across the fabric. A variety of active knit behaviors will be illustrated and two examples will be presented in detail to demonstrate the potential of architecture: garter and rib. The garter knit stitch generates significant contraction against moderate to large loads when heated due to the continuous interlocked network of loops of active wire.
A two-dimensional analytical model will be summarized for the states of operation of the knit assuming different friction conditions (stick or slip) for adjacent interlocking loops during the operational transitions. Elastica Theory and Euler-Bernoulli beam bending were used to capture the deformations within a loop of wire based on the stress strain behavior of the SMA material. The operational model was experimentally validated with a garter knit textiles that demonstrated large strains (up to 250% recoverable, over 50% actuation strain) against moderate forces (order of tens of Newtons).
In another example of active knits, a three-dimensional experimental study will be described for rib stitch knitted actuators, which actuate normal to the surface to produce span-wise discrete periodic arrays. SMA rib stitch prototypes in both individual form and in stacked and nestled architectures were experimentally investigated for their ability to withstand aerodynamic forces while supplying the necessary displacement for aerodynamic flow control. These examples give a glimpse of the myriad of applications for this new genre of cellular active knit architectures that have potential to generate large, complex, three-dimensional motions with significant forces.

Laboratoire de Mécanique des Structures et des Systèmes Couplés - LMSSC