Fabrication and characterisation of short fibre reinforced elastomer composites for bending and twisting magnetic actuation

Polydimethylsiloxane (PDMS) films reinforced with short Nickel-coated Carbon Fibres (NiCF) were successfully fabricated, with the fibres aligned along different directions using an external magnetic field. The fibres were dispersed in the host matrix using sonication and mechanical mixing before being cured for 48 h in the magnetic field; thanks to the nickel functionalisation, the fibre orientation was achieved by a low intensity field (<0.2 T) which required an inexpensive experimental set-up. The main focus of this study was looking at the actuation potential of this magnetic composite material; successful actuation was achieved, showing its large displacement capability. The results confirm the presence of an instability controlled by the magnetic torque, as predicted by the introduced model. The composite films undergo a transition from a bending-only deformed configuration for the 0° fibre specimen, to a twisting-only configuration, achieved for fibres at 90°, whereas all the intermediate angles show both bending and twisting. This behaviour mirrors that which is used to propel a selection of marine mammals

Analytical bond model for general type of reinforcements of finite embedment length in cracked cement based materials

In this work, a computational model for simulating the relevant mechanisms governing the pull-out of a discrete reinforcement embedded into cement based materials is described. The model accounts for the material and geometric properties of the reinforcement, which can include an anchored end, the interface between reinforcement and surrounding medium, and the relative inclination of the reinforcement to the crack plane. The reinforcement is modelled as a Timoshenko beam resting on a cohesive-like foundation that allows all the failure modes seen in the experiments to be accounted for, namely: debonding at the interface between the reinforcement and the concrete, cracking and spalling of the concrete matrix, rupture of the reinforcement. A comprehensive comparison with the experimental data available in the literature highlights the good predicting capabilities of the proposed model in terms of both peak force and dissipated energy. Furthermore, since the model is capable of simulating a discrete reinforcement of any direction towards the crack plane, complex mechanisms like micro-spalling of the matrix at the exit point of the reinforcement can be captured conveniently. By carrying out parametric analysis is possible to optimize the geometry of the anchored ends for maximizing the peak force and/or the energy dissipation in the pull-out process. Therefore, the developed model constitutes a relevant numerical tool for the optimization of discrete and continuous reinforcements of concrete structures including Fibre Reinforced Polymer (FRP) systems and Steel Fibre Reinforced Concrete (SFRC).FEDER through the Operational Program for Competitiveness Factors – COMPETE and from the Portuguese Foundation for Science and Technology (FCT) under the project FCOMP-01-0124-FEDER-01484

Pull-out tests with CFRP laminates applied according to the ETS/NSM technique

The retrofitting technique of Near-Surface Mounted (NSM) fiber-reinforced polymer (FRP) strips is receiving more attention recently due to many advantages over the externally bonded technique (EBR). However, in some situation is necessary to increase flexural and shear simultaneous. Therefore, to overcome this drawback, a hybrid strengthening technique that combines the advantages of NSM technique and Embedded Through Section (ETS) technique, is proposed by using an innovative CFRP laminate with rectangular shape in the NSM part and circular shape in the ETS. In order to evaluate the efficiency of the strengthening system, this paper presents the results of a series of pull-out tests using the innovative laminate to quantify the influence of the angle and embedded length. Using the results of this experimental program and developing a numerical strategy, an analytical bond stress–slip relationship was obtained

A multiscale model for optimizing the flexural capacity of FRC structural elements

In the present work, a multiscale model for fibre reinforced concrete (FRC) beams failing in bending is presented. At the microstructural level, the fibre is modelled as a one-dimensional continuum with axial, shear and bending deformability, with cohesive-like interfaces to simulate the interaction with the surrounding concrete. At the macroscopic level, the response of the beam is simulated by discretising the cross-section into layers and by enforcing the proper compatibility conditions between the layers. In the post-cracking stage, the tensile capacity is assured by the fracture energy of the concrete and the fibre resisting mechanisms simulated by the fibre pullout constitutive laws determined at the microstructural level. The model can account for fibre distribution and orientation, controlled by the casting conditions and geometry of the mould. By using experimental data available from the open literature, it is proved that such an integrated approach is able to derive, by inverse analysis, the stress-crack width relationship of FRC, which is the fracture mode I information in the material nonlinear analysis of FRC structures with approaches based on the finite element method.J.O.A. Barros, T. dos Santos Valente and I. G. Costa wish to acknowledge the support by FEDER through the Operational Program for Competitiveness Factors - COMPETE and Internationalization Program (POCI), under the project NG TPfib POCI-01-0247-FEDER-03371

High Light Induced Disassembly of Photosystem II Supercomplexes in Arabidopsis Requires STN7-Dependent Phosphorylation of CP29

Photosynthetic oxidation of water and production of oxygen by photosystem II (PSII) in thylakoid membranes of plant chloroplasts is highly affected by changes in light intensities. To minimize damage imposed by excessive sunlight and sustain the photosynthetic activity PSII, organized in supercomplexes with its light harvesting antenna, undergoes conformational changes, disassembly and repair via not clearly understood mechanisms. We characterized the phosphoproteome of the thylakoid membranes from Arabidopsis thaliana wild type, stn7, stn8 and stn7stn8 mutant plants exposed to high light. The high light treatment of the wild type and stn8 caused specific increase in phosphorylation of Lhcb4.1 and Lhcb4.2 isoforms of the PSII linker protein CP29 at five different threonine residues. Phosphorylation of CP29 at four of these residues was not found in stn7 and stn7stn8 plants lacking the STN7 protein kinase. Blue native gel electrophoresis followed by immunological and mass spectrometric analyses of the membrane protein complexes revealed that the high light treatment of the wild type caused redistribution of CP29 from PSII supercomplexes to PSII dimers and monomers. A similar high-light-induced disassembly of the PSII supercomplexes occurred in stn8, but not in stn7 and stn7stn8. Transfer of the high-light-treated wild type plants to normal light relocated CP29 back to PSII supercomplexes. We postulate that disassembly of PSII supercomplexes in plants exposed to high light involves STN7-kinase-dependent phosphorylation of the linker protein CP29. Disruption of this adaptive mechanism can explain dramatically retarded growth of the stn7 and stn7stn8 mutants under fluctuating normal/high light conditions, as previously reported

Orientation effects in short fibre-reinforced elastomers

The large strain behaviour of a short fibre-reinforced composite is studied through numerical simulations. The reinforcing fibres yield the macroscopic response transversely isotropic which is indeed the case of many reinforcements currently used in composites: short carbon fibres, cellulose whiskers, carbon nanotubes. As a result of the analysis, it is shown that the reorientation of the fibres that takes place at large strain has a significant effect on the overall material response by changing the axis of isotropy. This behaviour can be adequately described by using a transversely isotropic model whose strain energy function depends on three invariants: two isotropic and one representing the stretch along the direction of the fibres. To assess its capabilities, the model is compared to the results of experiments carried out by the authors on nickel-coated chopped carbon fibres in a vulcanised natural rubber matrix for which the fibre orientation is achieved by controlling an external magnetic field prior to curing. Possible applications include micro-sized propulsion devices and actuators. Copyright © 2014 by ASME

Multiscale modelling nano-platelet reinforced composites at large strain

We study the behaviour of an incompressible particle-reinforced neo-Hookean (IPRNC) material when subjected to large plain strain deformation. The peculiarity of the model consists in the rectangular shape of the particle which yields the macroscopic response of the composites non isotropic. This is indeed the case for many reinforcements currently used in composites at all length scales: short-fibres, clays, graphene. The consequence of the anisotropic reinforcement in this model at short strain is evident in the stiffness that is observed to depend strongly on the platelet orientation; a transverse stiffening effect when the platelet is oriented perpendicular to the loading direction proves to be almost as significant as the longitudinal stiffness contribution usually considered for anisotropic reinforcements. The large strain effects of orientation are also significant and an understanding of them is relevant to a number of applications that can take advantage of the large strain non-linear response

Image-based operational modal analysis and damage detection validated in an instrumented small-scale steel frame structure

The use of image-based techniques in structural dynamics is constantly growing thanks to the decrease in the cost of high-speed cameras and the improvement in image processing algorithms. Compared to traditional sensors, such as accelerometers or velocimeters, the use of fast cameras for data acquisition allows the number of measurements points to be significantly increased. However, such an abundance of points, not always lead to an increased accuracy of damage detection algorithms. With this in mind, we compare different damage detection techniques by using displacement data and modal quantities. A small scale steel frame structure is used to validate the damage detection by using measurements acquired through a high-speed camera with different image processing techniques. The Hybrid Lagrangian Particle Tracking (HLPT) algorithm and Digital Image Correlation (DIC) are both used to extract displacement measurements from images. The results are compared with those obtained with seismic class accelerometers which are normally used in the lab for such an application. Damage localization and intensity have been determined through image-based measurements without losing the accuracy obtained with accelerometers