skip to main content

Title: Defining relationships between geometry and behavior of bistable composite laminates
Bistability is exhibited by an object when it can be resting in two stable equilibrium states. Certain composite laminates exhibit bistability by having two stable curvatures of opposite sign with the two axes of curvature perpendicular to each other. These laminates can be actuated from one state to the other. The actuation from the original post-cure shape to the second shape is called as ‘snap-through’ and the reverse actuation is called as ‘snap-back’. This phenomenon can be used in applications for morphing structures, energy harvesting, and other applications where there is a conflicting requirement of a structure that is load-carrying, light, and shape-adaptable. MW Hyer first reported this phenomenon in his paper in 1981. He found that thin unsymmetric laminates do not behave according to the predictions of the Classical Lamination Theory (CLT). The CLT is a linear theory and predicts the post-cure shape of thin unsymmetric laminates to be a saddle. MW Hyer developed a non-linear method called the “Extended Classical Lamination Theory” which accurately predicted the laminate to have two cylindrical shapes. Since then, a number of researchers have tried to identify the key parameters affecting the behavior of such laminates. Geometric parameters such as stacking sequence, fibre more » orientation, cure cycle, boundary conditions, and force of actuation, have all been studied. The objective of this research is to define a relation between the length, width and thickness of square and rectangular laminates required to achieve bistability. Using these relations, a 36 in × 36 in bistable laminate is fabricated with a thickness of 30 CFRP layers. Also, it is proved that a laminate does not lose bistability with an increase in aspect ratio, as long as both sides of the rectangular laminate are above a certain ‘critical length’. A bistable laminate with dimensions of 2 in × 50 in is fabricated. Further, for laminates that are bistable, it is necessary to be able to predict the curvature and force required for actuation. Therefore, a method is developed which allows us to predict the curvature of both stable shapes, as well as the force of actuation of laminates for which the thickness and dimensions are known. Finite Element Analysis is used to carry out the numerical calculations, which are validated by fabricating laminates. The curvature of these laminates is measured using a profilometer and the force of actuation is recorded using a universal test set-up. « less
Authors:
; ; ;
Award ID(s):
1760943
Publication Date:
NSF-PAR ID:
10290956
Journal Name:
Journal of Composite Materials
Page Range or eLocation-ID:
002199832110058
ISSN:
0021-9983
Sponsoring Org:
National Science Foundation
More Like this
  1. It has been found that certain asymmetric composite laminates exhibit bistability, where the composite laminate exhibits multiple stable static equilibrium states. If the bistable composite is actuated, it will snap to its secondary equilibrium state and then remain there without further actuation. This study investigates how the amount of symmetry in a combined symmetric asymmetric rectangular laminate under an imposed clamped edge boundary condition affects the bistability and the curvature of the laminate. Laminates with varying amounts of asymmetry were fabricated and then measured using a profilometer to capture the curvatures of the equilibrium shapes. The results showed that upmore »to 20% symmetry can be introduced in the laminate without a substantial loss in snap through curvature, and that up to 83% symmetry can be introduced in the laminate before bistability is lost. Finite element simulations were conducted in Abaqus and showed good correlation with the experimental results.« less
  2. Transparent wood composites (TWCs) are a new class of light-transmitting wood-based materials composed of a delignified wood template that is infiltrated with a refractive- index-matched polymer resin. Recent research has focused primarily on the fabrication and characterization of single-ply TWCs. However, multi-ply composite laminates are of interest due to the mechanical advantages they impart compared to the single ply. In this work, 1- and 2-ply [0°/90°] TWC laminates were fabricated using a delignified wood template (C) and an acetylated delignified wood template (AC). The optical and mechanical properties of resultant C and AC TWC laminates were determined using ultraviolet-visible spectroscopymore »(UV-Vis) and tensile testing (5× replicates), respectively. In addition, the ability of classical lamination plate theory and simple rule of mixtures to predict multi-ply tensile modulus and strength, respectively, from ply-level mechanical properties were investigated and are reported herein. Experimental results highlight tradeoffs that exist between the mechanical and optical responses of both unmodified and chemically modified TWCs. Template acetylation reduced the stiffness and strength in the 0° fiber direction by 2.4 GPa and 58.9 MPa, respectively, compared to the unmodified samples. At high wavelengths of light (>515 nm), AC samples exhibited higher transmittance than the C samples. Above 687 nm, the 2-ply AC sample exhibited a higher transmittance than the 1-ply C sample, indicating that thickness-dependent optical constraints can be overcome with improved interfacial interactions. Finally, both predictive models were successful in predicting the elastic modulus and tensile strength response for the 2-ply C and AC samples.« less
  3. Soft actuators are typically designed to be inherently stress‐free and stable. Relaxing such a design constraint allows exploration of harnessing mechanical prestress and elastic instability to achieve potential high‐performance soft robots. Here, the strategy of prestrain relaxation is leveraged to design pre‐curved soft actuators in 2D and 3D with tunable monostability and bistability that can be implemented for multifunctional soft robotics. By bonding stress‐free active layer with embedded pneumatic channels to a uniaxially or biaxially pre‐stretched elastomeric strip or disk, pre‐curved 2D beam‐like bending actuators and 3D doming actuators are generated after prestrain release, respectively. Such pre‐curved soft actuators exhibitmore »tunable monostable and bistable behavior under actuation by simply manipulating the prestrain and the biased bilayer thickness ratio. Their implications in multifunctional soft robotics are demonstrated in achieving high performance in manipulation and locomotion, including energy‐efficient soft gripper to holding objects through prestress, fast‐speed larva‐like jumping soft crawler with average locomotion speed of 0.65 body‐length s−1 (51.4 mm s−1), and fast swimming bistable jellyfish‐like soft robot with an average speed of 53.3 mm s−1.« less
  4. An important idea underlying a plausible dynamical theory of circulation in three-dimensional turbulence is the so-called area rule, according to which the probability density function (PDF) of the circulation around closed loops depends only on the minimal area of the loop, not its shape. We assess the robustness of the area rule, for both planar and nonplanar loops, using high-resolution data from direct numerical simulations. For planar loops, the circulation moments for rectangular shapes match those for the square with only small differences, these differences being larger when the aspect ratio is farther from unity and when the moment ordermore »increases. The differences do not exceed about 5% for any condition examined here. The aspect ratio dependence observed for the second-order moment is indistinguishable from results for the Gaussian random field (GRF) with the same two-point correlation function (for which the results are order-independent by construction). When normalized by the SD of the PDF, the aspect ratio dependence is even smaller ( < 2%) but does not vanish unlike for the GRF. We obtain circulation statistics around minimal area loops in three dimensions and compare them to those of a planar loop circumscribing equivalent areas, and we find that circulation statistics match in the two cases only when normalized by an internal variable such as the SD. This work highlights the hitherto unknown connection between minimal surfaces and turbulence.

    « less
  5. Abstract

    Bistable elements are candidate structures for the evolving field of MEMS-based no-power event-driven sensors. In this paper, we present a strategy for producing bistable elements and investigate two compatible bilayer material systems for their realization using MEMS technology. Both bilayer systems leverage thermally-grown silicon dioxide as the principal stress-producing layer and a second material (either polyimide or aluminum) as the main structural layer. Arrays of buckled circular diaphragms, ranging in diameter from 100μm to 700μm in 50μm increments, were fabricated and their performances were compared to modeled and FEA-simulated results. In all cases, the diaphragms buckled when DRIE-released asmore »expected, and their buckled experimental heights were within 9.1% of the theory and 1.8% of the FEA prediction. Interestingly, the smaller diameter structures exhibited a directional bias which we investigate and forecast using FEA. These bistable mechanical elements have the ability to serve as building blocks for no-power threshold-driven smart switches. New contributions to the field include: (a) introduction of a new bistable material system made from aluminum and compressive oxide, (b) investigation of diaphragm diameter size as it related to the phenomena of bistability versus non-bistability, (c) FEA analysis of the critical transition between bistability and non-bistability, and (d) introduction of the ‘dome factor’ term to describe dome quality.

    « less