Deployable membranes designed from folding tree leaves.

A simple model of deploying tree leaves is assembled in different arrangements to produce polygonal foldable membranes for use as deployable structures. One family of folding patterns exhibits a small strain mechanism, which is investigated. Variations on the basic arrangements can be used to fold membranes with a discretized curvature.

Smooth thin film C/diamond membranes with controllable optical band gaps

Mixed phase carbon-diamond films which consist of small grain diamond in an a:C matrix were deposited on polished Si using a radio frequency CH4 Ar plasma CVD deposition process. Ellipsometry, surface profilometry, scanning electron microscopy (SEM) and spectrophotometry were used to analyse these films. Film thicknesses were typically 50-100 nm with a surface roughness of ± 30 A ̊ over centimetre length scans. SEM analysis showed the films were smooth and pinhole free. The Si substrate was etched using backside masking and a directional etch to give taut carbon-diamond membranes on a Si grid. Spectrophotometry was used to analyse the optical properties of these membranes. Band gap control was achieved by varying the dc bias of the deposition process. Band gaps of 1.2 eV to 4.0 eV were achieved in these membranes. A technique for controlling the compressive stress in the films, which can range from 0.02 to 7.5 GPa has been employed. This has allowed the fabrication of thin, low stress, high band gap membranes that are extremely tough and chemically inert. Such carbon-diamond membranes seem promising for applications as windows in analytical instruments. © 1992.

Fracture toughness of fibrous membranes

Random fibrous networks exist in both natural biological and engineering materials. While the nonlinear deformation of fibrous networks has been extensively studied, the understanding of their fracture behaviour is still incomplete. To study the fracture toughness of fibrous materials, the near-tip region is crucial because failure mechanisms such as fibril rupture occur in this region. The consideration of this region in fracture studies is, however, a difficult task because it involves microscopic mechanical responses at a small length scale. This paper extends our previous finite element analysis by incorporating the microscopic responses into a macroscopic domain by using a submodeling technique. The detailed study of microstructures at crack tips show a stochastic toughness of membranes due to the random nature of fibrous networks. Further, the sizes of crack tip region, which are sufficient to provide a reasonable prediction of fracture behaviour in a specific type of fibrous network, were presented. Future work includes improving the current linear assumption in the macroscopic models to become nonlinear.

Computational analysis of liquid crystalline elastomer membranes: Changing Gaussian curvature without stretch energy

Liquid crystalline elastomers (LCEs) can undergo extremely large reversible shape changes when exposed to external stimuli, such as mechanical deformations, heating or illumination. The deformation of LCEs result from a combination of directional reorientation of the nematic director and entropic elasticity. In this paper, we study the energetics of initially flat, thin LCE membranes by stress driven reorientation of the nematic director. The energy functional used in the variational formulation includes contributions depending on the deformation gradient and the second gradient of the deformation. The deformation gradient models the in-plane stretching of the membrane. The second gradient regularises the non-convex membrane energy functional so that infinitely fine in-plane microstructures and infinitely fine out-of-plane membrane wrinkling are penalised. For a specific example, our computational results show that a non-developable surface can be generated from an initially flat sheet at cost of only energy terms resulting from the second gradients. That is, Gaussian curvature can be generated in LCE membranes without the cost of stretch energy in contrast to conventional materials. © 2013 Elsevier Ltd. All rights reserved.