A particular set of requirements is identified for the response of a passively actuated shape-adaptive structure. It is shown that neither conventional structures nor simple structural instabilities have suitable properties in order to meet these requirements. A novel solution is proposed which combines multiple structural instabilities to create a compound instability. This approach enables the shape-adaptive structure to achieve large output displacements only after a triggering input displacement is reached. First, this approach is investigated via an analytical bar-and-spring model, before a parametric sensitivity study demonstrates how the structural response of the model can be ‘tuned’ by varying its governing parameters. Next, a representative candidate structure with an analogous mechanical response is presented, and governing structural parameters are identified. It is found that there is an equivalent governing parameter across both models, and that this parameter can be used to tune the response of candidate structure in a way which mirrors the response of the bar-and-spring model. This analogy allows the simple bar-and-spring model to inform the design of its more complex counterpart, which can then achieve the desired functionality of the morphing structure.
Elastic instabilities have traditionally been considered a failure mechanism; however, recent years have seen numerous studies exploiting instabilities as a means to achieve structural functionality. By contrast, interacting instabilities and compound buckling are still largely viewed as a failure mechanism. In this paper, we show that interacting instabilities can also be exploited to achieve bespoke functionality. We focus on ‘sequential instabilities’, whose associated critical points cannot both lie on a fundamental equilibrium path. We obtain sequential instabilities by combining canonical bifurcations, (e.g. limit point, pitchfork) as building-blocks. Initially, this concept is explored through simple bar-and-spring models that are found to have several properties not exhibited by the building blocks from which they are constructed. Further, the utility of the building block approach, and that of sequential, interacting instabilities, is demonstrated through the development of a morphing structure which must rapidly deploy after a critical input displacement is attained, and meet specific post-deployment stiffness requirements. Two design concepts are proposed, each comprising building blocks to mirror the fundamental working principles identified through the bar-and-spring models. Finite-element models of the design solutions are presented, demonstrating how the designs positively use sequential, interacting instabilities in order to meet the challenging requirements of the application. This work extends the contextual framework of instabilities that can be used to create structures with novel functionality.
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