Multi-stability of fiber reinforced polymer frames with different geometries

Shape adaptable structures are desired in many fields of application such as aerospace, soft robotics, and architecture. Multi-stable structures can enable shape adaptation as they possess multiple stable morphologies that can be held without the need of external work. Many concepts to realize multi-stable systems have been proposed in the last decades. However, the vast majority of multi-stable structures exhibit minimal changes in shape or high coupling between stable states, which limits their applicability. Recently, a novel class of multi-stable composite structures has been investigated, showing that a periodic arrangement of square frames combined with pre-stretched membranes enables many stable states together with large shape transformations. To investigate how this new class can be employed in a broader range of applications, this study extends the design space to any N-sided regular polygon frame. The multi-stability is investigated through experiments and finite element (FE) analysis. The polygonal frames and their respective periodic arrangements possess a large number of stable states, with similar behavior to that of the square frames. Moreover, neutral stability is observed for the limit case of a circular polygonal frame. This study proves that the concept is highly flexible in terms of design, thus opening up new possibilities for applications of multi-stable composite structures.ISSN:0263-8223ISSN:1879-108

Quantifying the strength of stability of multi-stable structures: A new design perspective

In recent years the use of multi-stable components for designing adaptive structures and energy harvesting systems has been broadly investigated. Most of these studies have concentrated on analyzing the equilibrium shapes and the snap-through loads of the bi/multi-stable elements. However, there is minimal work on quantifying the strength of these stable equilibrium shapes. The strength of stable states is a measure of the ease or difficulty of the transition between the different stable states of any multi-stable system. Quantifying the strength of stability becomes paramount while designing adaptive structures which integrate multi-stable elements. Previous studies have argued that snap-through loads represent the goodness of stability; however, the magnitude of the snapthrough load highly depends on the boundary conditions of the structure and the loads’ application point. This work presents a mathematical framework that quantifies the strength of stability of the stable equilibrium shapes of multi-stable elements and identifies the minimum energy path (MEP) between two stable states. The method is based on finding, either with an analytical approach or finite element (FE) models, the stable and transition points of the energy landscape. Results prove that the method provides a quantitative measure of stability, a key parameter when designing adaptive structures. In addition, the minimum energy path between two stable states provides a tool to simplify the design of efficient actuation strategies for multi-stable systems. Four numerical examples are detailed to demonstrate the robustness of the approach.ISSN:0263-8231ISSN:1879-322

Instability-driven shape forming of fiber reinforced polymer frames

Thin fiber reinforced polymer (FRP) composites are widely implemented in adaptive and morphing structures. However, realization of the necessary complex 3‐dimensional FRP structures requires the use of expensive molds thereby limiting the design space and flexibility. Using the elastic strain energy of pre‐stretched membranes holds potential for addressing this challenge. In this work, a novel manufacturing technique for fabricating 3‐dimensional FRP structures moldlessly is presented where pre‐stretched membranes are used to drive out‐of‐plane buckling instabilities of FRP composite shells. To explore the potential of this approach, a simple square frame design is investigated. An analytical model based on high deformation beam buckling theory is developed for understanding the parameters driving the out‐of‐plane behavior of these structures. Experimental and finite element results are used for model validation and reveal excellent agreement, with errors less than 10% over a large portion of the design space. Analytical and finite element models demonstrate that the out‐of‐plane deformation can be tailored by varying the structure’s geometric and material parameters. A new design space for FRP composite laminates is characterized, enabling highly flexible design. The manufacturing and modeling techniques can be extended to other geometries for the realization and analysis of arbitrarily complex surfaces.ISSN:0263-8223ISSN:1879-108

A Highly Multi-Stable Meta-Structure via Anisotropy for Large and Reversible Shape Transformation

Shape transformation offers the possibility of realizing devices whose 3D shape can be altered to adapt to different environments. Many applications would profit from reversible and actively controllable shape transformation together with a self-locking capability. Solutions that combine such properties are rare. Here, a novel class of meta-structures that can tackle this challenge is presented thanks to multi-stability. Results demonstrate that the multi-stability of the meta-structure is strictly tied to the use of highly anisotropic materials. The design rules that enable large-shape transformation, programmability, and self-locking are derived, and it is proven that the shapes can be actively controlled and harnessed to realize inchworm-inspired locomotion by strategically actuating the meta-structure. This study provides routes toward novel shape adaptive lightweight structures where a metamaterial-inspired assembly of anisotropic components leads to an unforeseen combination of properties, with potential applications in reconfigurable space structures, building facades, antennas, lenses, and soft robots.ISSN:2198-384

Highly multi-stable FRP grids for shape adaptation

Multi-stability in lightweight structures has been the subject of intensive research. This property is advantageous for the realization of shape adaptable structures because it allows, for example, maintaining different configurations of the structure without the presence of a continuous power supply. However, the integration of multi-stable components into complex structures is not widely implemented yet due to high coupling between the stable modes, suppression of stable modes due to the influence of boundary conditions, and complex fabrication techniques. In this work, a novel class of highly multi-stable periodic structures is presented. The investigation is first conducted on a single cell structure that possesses eight stable modes and, second, expanded to grid structures with periodicity. The multi-stability of the unit cell is preserved in the periodic structures and, in fact, the grid possesses further stable configurations not observed in the unit cell. Prototypes are fabricated by combining flat thin fiber-reinforced polymer (FRP) composite frames with bi-axially pre-stretched membranes. Therefore, the proposed approach enables the fabrication of highly multi-stable FRP grids without the need of a mold. Finite element analysis and experimental results show that the multi-stability property depends on the level of anisotropy of the laminate employed. Highly anisotropic laminates strengthen the multi-stability while isotropic ones suppress it. The realization of such highly anisotropic components with additive manufacturing techniques such as 3D-printing has been proved to be feasible, enlarging the range of materials suitable for the proposed concept. The presented technique is expected to be advantageous for the realization of highly reconfigurable, yet foldable, space habitats and antennas

Tuning the Properties of Multi‐Stable Structures Post‐Fabrication Via the Two‐Way Shape Memory Polymer Effect

Abstract Multi‐stable elements are commonly employed to design reconfigurable and adaptive structures, because they enable large and reversible shape changes in response to changing loads, while simultaneously allowing self‐locking capabilities. However, existing multi‐stable structures have properties that depend on their initial design and cannot be tailored post‐fabrication. Here, a novel design approach is presented that combines multi‐stable structures with two‐way shape memory polymers. By leveraging both the one‐way and two‐way shape memory effect under bi‐axial strain conditions, the structures can re‐program their 3D shape, bear loads, and self‐actuate. Results demonstrate that the structures' shape and stiffness can be tuned post‐fabrication at the user's need and the multi‐stability can be suppressed or activated on command. The control of multi‐stability prevents undesired snapping of the structures and enables higher load‐bearing capability, compared to conventional multi‐stable systems. The proposed approach offers the possibility to augment the functionality of existing multi‐stable concepts, showing potential for the realization of highly adaptable mechanical structures that can reversibly switch between being mono and multi‐stable and that can undergo shape changes in response to a change in temperature

Tuning the Properties of Multi-Stable Structures Post-Fabrication Via the Two-Way Shape Memory Polymer Effect

Multi-stable elements are commonly employed to design reconfigurable and adaptive structures, because they enable large and reversible shape changes in response to changing loads, while simultaneously allowing self-locking capabilities. However, existing multi-stable structures have properties that depend on their initial design and cannot be tailored post-fabrication. Here, a novel design approach is presented that combines multi-stable structures with two-way shape memory polymers. By leveraging both the one-way and two-way shape memory effect under bi-axial strain conditions, the structures can re-program their 3D shape, bear loads, and self-actuate. Results demonstrate that the structures' shape and stiffness can be tuned post-fabrication at the user's need and the multi-stability can be suppressed or activated on command. The control of multi-stability prevents undesired snapping of the structures and enables higher load-bearing capability, compared to conventional multi-stable systems. The proposed approach offers the possibility to augment the functionality of existing multi-stable concepts, showing potential for the realization of highly adaptable mechanical structures that can reversibly switch between being mono and multi-stable and that can undergo shape changes in response to a change in temperature.ISSN:2198-384

Programmable FRP metamaterials for adaptive hinges with multiple 3D shapes

In many applications, structures need to reversibly change between different 3D shapes to adapt to different environmental conditions or operational requirements. To date, the majority of reconfigurable structures require continuous actuation to hold a selected configuration. Multi-stable elements could potentially be used for shape reconfiguration because they can maintain different 3D shapes without needing a continuous power supply. However, when integrating multi-stable elements with other components, boundary conditions are negatively affecting the multi-stability. A new concept that combines flat fiber-reinforced polymer (FRP) composite shells with bi-axially pre-stretched membranes to realize highly multi-stable structures was recently presented by the authors. In this study, the concept of FRP metamaterials is adapted to realize a hinge that possesses five stable angular configurations. Thanks to the low influence of the boundary conditions on the multi-stability and the large shape change that the structure undergoes, the proposed hinge design demonstrates that FRP metamaterials enable the realization of highly reconfigurable structures. The hinge can achieve angular positions larger than 180° in both directions. Two parametric studies show that the angular positions of the hinge can be tailored by changing either the bending stiffness of the FRP composite shells or the pre-stretch of the membrane. A reconfigurable array of stiff panels connected by adaptive hinges is presented, showing potentials for the realization of a solar array that can change its shape to track the sun or be reversibly stowed and deployed