Multiscale tailoring of helical lattice systems for bespoke thermoelasticity

Abstract

Meta-)Materials, e.g. functional or architectured materials that change shape in response to external stimuli, often do so by exploiting solid-solid phase transitions or concerted elastic deformations. For the resulting system to be effective the (meta-)material needs to have desirable and tunable properties at length scales sufficiently small that desirable continuum behaviour of the resulting component is obtained. Developing such (meta-)materials has proven to be an endeavour which requires considerable expertise in science, engineering and mathematics. Here, we pursue an alternative approach where the design for functionality is integrated across multiple length scales in the system. We demonstrate this approach by designing and prototyping helical lattices that act as one-dimensional thermoelastic materials with unusual properties such as negative thermal expansivity-with magnitude far exceeding the most extreme values reported in the literature-and zero-hysteresis shape memory. Our strategy is independent of characteristic length scale, allowing us to design behaviour across a range of dimensions.

Exploring Adaptive Behavior of Non-linear Hexagonal Frameworks

“…An extension to the analytical model of Pirrera et al (2013) for lattices with strips of nominal width, thereby including their transverse curvature and in-plane strains, demonstrates a robust consistency with finite element simulation and experimental characterizations (McHale et al, 2020b). Furthermore, the helical lattice can be tuned to exploit external fields, such as thermal effects (O'Donnell et al, 2019; McHale et al, 2020a), to modulate the effective stiffness and stability landscape. The potential of helical lattices to obtain truly bespoke elastic responses was considered by Dixon et al (2019).…”

“…Instead, using a structural approach, instabilities can be integrated into the design and exploited to obtain a unique richness in system response (Reis, 2015;Bertoldi, 2017). By employing these structural response mechanisms at sufficiently small length scales, a desirable macroscopic continuum response can be achieved (O'Donnell et al, 2016;O'Donnell et al, 2019). Herein, we propose a new approach for creating novel material behavior from a hierarchy of non-linear elements; namely, rather than utilizing complex topologies to achieve the desired non-linear behavior, we can subsume this complexity into well understood non-linear "springs" acting as base-units of a hierarchical design.…”

Reconsidering Laminate Nonsymmetry

“…Such an enhanced design space for laminate selection provides greater scope for tailoring, especially in critical areas between regions where two laminates are blended (e.g across rib bays in aircraft wing skins), including effects of ply drop-offs. In particular, the induced coupling between in-and-out of plane behaviour could be utilised for beneficial purposes in geometrically non-symmetric structures [7,8], harnessed as a route to achieve novel structural responses [9,10] and offer greater design freedom for impact resistance [11].…”

“…early onset stringer debonding. It follows, that when trying to achieve optimal designs, particularly in those instances where an inherent non-symmetry exists, laminates of non-symmetric stacking sequence should also be considered [10]. After all, such an approach has shown potential increases in the load carrying capacity of a stringer termination [13].…”

“…The ASF provides a basic metric for capturing how the stiffness matrices deviate from an equivalent quasi-isotropic system composed of the same material. Changes in the magnitude of the ASF capture deviation in the stiffness matrices from Γ QI , as defined by equation(10). When the ASF is zero, by definition all lamination parameters are equivalently zero, corresponding to the quasi-isotropic laminate.…”