At the Intersection of Bistability and Elastic Instability: Switching and Locking Structures using Asymmetric Carbon Fiber Composites

Abstract

The next evolution of engineered structures would be to have an on-demand ability to become soft and foldable for packing in compact dimensions. This adaptive capability makes them convenient in transportation such as space structures where packaging of large deployable structures is crucial that help meet ever increasing energy demands of satellites. To render such abilities, smart materials would become necessary – materials that show adaptability to certain stimuli and change one or more characteristic properties. For example, shape memory alloys (SMAs) shrink in length upon heating with increased longitudinal stiffness. Thus, stiffness modulation and morphing ability is a crucial aspect of switchable systems. In our interest, Asymmetric Carbon-Fibre Reinforced Polymer (CFRP) laminates, have shown bistability, i.e., they have two stable equilibria or states that arise due to thermal imbalances during the curing process. This forces the laminate to exhibit two mutually perpendicular characteristic curvatures in two different stable states. The change from one state to the other is termed as a snap-through process. This study for the first time investigates the bistable laminates from a holistic perspective by understanding their quasi-static behaviors in mainly two important scenarios i.e. Out-of-plane and In-plane direction loadings. The authors in their first study uncover the different snap-through mechanics utilizing asymmetric boundary conditions. Three distinct snap-through characteristics are presented — two-step, one-step, and no-snap process — which depends on the load location and boundary conditions. For the second study, in-plane compression testing for bistable laminates reveal two drastically different responses – one very compliant and soft that behaves like a softening non-linear spring, and the other stiff response similar to thin columns which buckle under large loads. This material offers on-demand switching between these two responses by simply snapping their state from one to the other. Through extensive finite element simulation and experimentation, we present effective strategies to enhance their stiff response (buckling load) for improving the stiffness switching ratio. Learning through these behavioral characteristics of bistable laminates, a novel concept is implemented for morphing structures. A 'locking' feature is introduced by harnessing characteristic curvatures of these bistable laminates and strategically implementing them in morphing structures. We take inspiration from various origami folding patterns and incorporate a waterbomb geometry in these bistable laminates. This helps in changing the load-bearing capabilities of the bistable laminate, by switching from a very soft foldable state to a lockable stiff state. We present a case study on two origami designs, namely Kresling and Yoshimura, where using this bistability property delivers massively reconfigurable structures that show meta-stable load-bearing states.