Bistable laminated composites have a wide range of applications especially in light-weight structures that require switchable curvatures for holding large deformed shapes without any actuation force. Laminated composites are made from different layers of materials of different individual properties. The resulting material is a new one that has enhanced properties relative to the individual properties of the constituent layers. Hence, laminated composites may be favorable to applications that serve multiple functions including bistability.
Residual stress is commonly used on laminated composites to achieve multiple curved shapes. Thermally-cured bistable composites, for example, can exist in two curved shapes that are determined by the fiber orientations, curing temperature, and the size of lamination. Existing literature on bistable composites is based on thermally-cured fiber-reinforced polymeric materials whose performance is sensitive to variations in operating temperature and humidity. Further, there is little scope to tailor the shapes of these composites individually; the composites’ properties effect both shapes.
Postdoctoral researcher Venkata Siva Chillara and Professor Marcelo Dapino at the Ohio State University, Department of Mechanical and Aerospace Engineering in Columbus (OH) investigated actuation and stability design considerations for bistable laminated composites. Unlike prior literature on composites with thermally-induced bistability, the composites considered in this work were those with two sources of mechanically-induced residual stress. The salient features in mechanically-induced bistable composites are that their shapes can be tailored individually using two residual stress sources and that they can be invariant to operating temperature variations by virtue of their fabrication process. The study aimed at presenting a comparative analysis of the effects induced by various actuation modes on the composites’ response. Their work is published in the journal, Composite Structures.
The research team developed composites that include a pair of fiber-reinforced elastomeric strips laminated on either side of a sandwiched inextensible core. The composites were first mechanically-prestressed before being laminated to the core. A Lagrangian strain formulation together with classical laminate theory was used to describe the produced displacements and deflections observed in the composites. The authors performed uniaxial tensile testing to measure the deformation of bistable laminates. Taking advantage of the weak coupling between the composites’ shapes, measurement of the actuation forces and transitions in shape was made possible.
Calculations of the actuation forces through strain energy minimization involved the use of high-order displacement polynomials. The Rayleigh-Ritz method was used for computing the stable shapes of the composites as functions of the actuation forces. For model validation, the actuation force and deformation profiles were calculated from frictionless tensile experiments in conjunction with 3-D motion capture.
Chillara and Dapino observed close similarities between the actuation energies obtained from simulations and experimental measurements. The error range was 12%. They also explained the effects of the pre-strain ratios of the laminae on the bistability of the composites’ shapes. For example, a prestrain ratio higher than 0.2 was required for the bistability of a square laminate.
This is also the first study to research on bistability limits of two-source prestressed rectangular laminates. From the experiment, like thermally-cured fiber-reinforced polymeric (FRP) laminates, these types of composites experience a multi-stage snap-through kind of phenomenon. Bistability limits can be increased to the maximum by keeping the prestrains ratio constant at one and increasing the individual prestrains in the elastomeric matrix composite (EMC) strips. When the composites are not bistable either due to very low prestrain in one of the EMCs or due to very high aspect ratio, they have a single cylindrical shape.
When the effect of the various actuation modes on the behavior of the composites was analyzed, the authors concluded that minimal amount of energy is required for the moment application cases. A sensitivity study on the composites’ performance parameters revealed that the thickness of the core has a profound effect on the out-of-plane stiffness and less effect on the in-plane actuation energy.
Chillara, V., & Dapino, M. (2018). Stability considerations and actuation requirements in bistable laminated composites. Composite Structures, 184, 1062-1070.
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