1,721,035 research outputs found
Combined squeeze-shear properties of magnetorheological fluids: Effect of pressure
No full textSeveral applications of magnetorheological (MR) fluids are nowadays present in the industrial world, however sometimes system requirements require better material properties. In technical literature an interesting previous work shows that MR fluids exhibit a pressure dependency called squeeze strengthen effect. Since a lot of MR fluid based devices are rotary devices, this paper investigates the behaviour of MR fluids under pressure when a rotation is applied to shear the fluid. The system is designed in order to apply both the magnetic field and the pressure following a Design of Experiment method. The experimental apparatus comprises a cylinder in which a piston is used both to apply the pressure and to apply a prescribed rotation. The magnetic circuit is designed to provide a nearly constant induction field in the MR fluid. The experimental apparatus measures torque as a function of the variables considered and the yield shear stress is computed. A statistical analysis of the results shows that there is a positive interaction between magnetic field and pressure, which enhance the MR fluid performances more than two times. Copyright © 2012 by ASME
Magnetorheological elastomers characterization under shear loading up to failure: A magneto-mechanical multivariate analysis
Full text linkThis work analyses the shear behavior of magnetorheological elastomers (MRE), a class of smart materials which presents interesting magneto-mechanical properties. In order to determine the effect of several variables at a time, a design of experiment approach is adopted. A set of several samples of MRE was manufactured, by varying the weight fraction of ferromagnetic material inside the viscoelastic matrix and the isotropicity of the material, by adding an external magnetic field while the elastomeric matrix was still liquid. The mechanical behavior of each sample was analyzed by conducting cyclic tests at several shear rates, both with and without an external magnetic field. Moreover, in order to estimate the maximum shear stress, the specimens were loaded monotonically up to failure. Shear stiffness, maximum shear stress and specific dissipated energy were calculated on the basis of the experimental data. The results were analyzed using an Analysis of Variance (ANOVA) to assess the statistical influence of each variable. The experimental results highlighted a strong correlation between the weight fraction of ferromagnetic material in each sample and its mechanical behavior. Moreover, the dissipated energy of the MRE drops down when the magnetic field stiffens the behavior or the shear rate increases. The ultimate failure shear stress is strongly affected by the external magnetic field, increasing it by nearly 50%. The ANOVA on the results provides a simple phenomenological model is built for each output variable and it is compared with the experimental tests. These models produce a fast and fairly accurate prediction of each analyzed response of the MRE under various shear rates and applied magnetic fields
Chiralisation of Euclidean polygonal tessellations for the design of new auxetic metamaterials
No full textChiral honeycombs are one of the main classes of mechanical metamaterials with the potential to exhibit auxetic behaviour. In this work, we propose a new class of chiral metamaterials based on uniform Euclidean tessellations and their dual counterparts. In total, ten new structures were designed and analysed using Finite Element analysis under periodic boundary conditions, with eight of these systems showing the capability of possessing a negative Poisson's ratio. The relationship between the various geometric parameters defining the systems and the resultant mechanical properties was also studied. We show that ‘chiralisation’, i.e. introduction of chirality and rotational elements within the system, has the ability to transform even complex geometries, which in their original state possess a high positive Poisson's ratio, into auxetic metamaterials and hope that this work can act as a blueprint for the design of auxetic structures with novel topologies
Novel chiral honeycombs based on octahedral and dodecahedral Euclidean polygonal tessellations
No full textIn this work, we explored the effect of ‘chiralisation’, i.e. the introduction of geometric chiral characteristics, on the mechanical properties of Euclidean polygonal tessellations containing octahedral or dodecahedral elements. This geometric transformation resulted in the design of three novel auxetic metamaterials which have the potential to exhibit large negative Poisson's ratios (ca. −1) coupled with high levels of in-plane isotropy. We have also examined the influence of the introduction of chiral nodes on the type of geometric arrangement (i.e. whether original or dual) of the base tessellation and also show how the extent of auxeticity may be controlled by tuning the geometric parameters of these systems. This work confirms the potential of Euclidean polygonal tessellations for the design of novel auxetic metamaterials and provides new insights into the deformation mechanisms and geometric conditions which impart this anomalous property
Magneto-mechanical characterization of magnetorheological elastomers
Full text linkThis work analyses the properties and the magneto-mechanical characteristics of magnetorheological elastomers, a class of smart materials not yet broadly investigated. First, set of several samples of this material was manufactured, each one characterized by a different percentage of ferromagnetic material inside the viscoelastic matrix. The specimens were manufactured in order to create isotropic and anisotropic configurations, respectively, with randomly dispersed ferromagnetic particles or with an aligned distribution, obtained through and external magnetic field. Then, the mechanical behaviour of each sample was analysed by conducting a compression test, both with and without an external magnetic field. Moreover, a three-point bending test was also performed on the same specimens. Stiffness, deformation at maximum stress and specific energy dissipated were calculated based on the experimental data. The results were analysed considering the mechanical responses, and an analysis of variance was carried out in order to assess the statistical influence of each variable. The experimental results highlighted a strong correlation between the percentage of ferromagnetic material in each sample and its mechanical behaviour. The anisotropicity of the material, aligned in columnar structures, also affects the stiffness measured in the compression test, while the external magnetic field’s main contribution is to reduce the samples’ maximum deformation. Using analysis of variance results as guidelines, we built a simple phenomenological model which produces quite reliable predictions regarding the mechanical response of the magnetorheological elastomers under compressive stress
Design and experimental validation of a novel magnetorheological damper with internal pressure control
No full textIn the present paper we investigated the behaviour of magnetorheological fluids (MRFs) under a hydrostatic pressure up to 40 bar. We designed, manufactured and tested a magnetorheological damper (MRD) with a novel architecture which provides the control of the internal pressure. The pressure was regulated by means of an additional apparatus connected to the damper that acts on the fluid volume. The MRD was tested under sinusoidal inputs and with several values of magnetic field and internal pressure. The results show that the new architecture is able to work without a volume compensator and bear high pressures. On the one hand, the influence of hydrostatic pressure on the yield stress of MRFs is not strong probably because the ferromagnetic particles cannot arrange themselves into thicker columns. On the other hand, the benefits of the pressure on the behaviour of the MRD are useful in terms of preventing cavitation
Analytical modelling of rolamite mechanism made of shape memory alloy for constant force actuators
No full textThis paper analyses the Rolamite architecture exploiting shape memory alloys as power element to obtain a solid state actuator. The Rolamite mechanism was discovered in the late sixties, initially as precision and low friction linear bearing. The most common Rolamite configuration consists of a flexible thin metal strip and two rollers mounted between two fixed parallel guide surfaces. The system can roll back and forth without slipping guided by the plates along its so called sensing axis. The system presents another relevant advantage in addition to low friction coefficient, which is the possibility to provide force generation in a quite simple way. In the original literature works the force was provided thanks to cutouts of various shape in the strip, though this method does not allow the Rolamite to be considered a proper actuator, but only a force generator. In this paper we developed the idea of exploiting the shape memory alloy as Rolamite power element and therefore to use the shape memory effect to change the elastic properties of the strip and to provide the actuation force. The mechanical analyses and the equations where the martensite-Austenite transition is modelled in a simplified way, show that this application is feasible, mainly thanks to the initial precurvature of the SMA strip. The discussion of the results highlights some important merits of this architecture such as long stroke, constant force and compactness
Mechanical strength of adhesively bonded joints using polymeric additive manufacturing
Full text linkThis paper investigates the combined use of one of the most widespread additive manufacturing techniques, fused deposition molding, with polymeric materials and structural adhesive. The aim is twofold: first, to enhance the adhesive performance exploiting the capability of the additive manufacturing to tailor the bonding surface of the adherend, and second to overcome one of the main limitations of 3D printing, i.e. the quite small printing volume, by means of adhesive bonding. Bonding multiple parts together without loss of performance could open new possibilities for this technology. The present research analyzes, by using a Design of Experiment technique, a wide set of single lap joints with two adhesives and seven different surface morphologies. The results highlight that the adhesive bonding does not undermine the load carrying capacity of the joints as well as their stiffness, and, in some cases, it causes a slight improvement of the peak force. The morphology of the surface plays only a small role in the performance of the system, since it cannot provide a strong mechanical interlocking of the parts due to peel stresses and because of the predominant effect of stress concentrations at the corners, which cause substrate failure
Shear behaviour of magnetorheological elastomers: Viscoelastic and magnetorheological properties
No full textMulti objective design optimization of self-expandable nitinol braided stents
No full textCardiovascular stents are indispensable medical devices used to treat vessel-related issues such as atherosclerotic plaque. In the past, stents were mainly made of materials like stainless steel or cobalt-chromium alloy. However, over the last two decades, research has focused on the use of Nitinol (NiTi) due to its superior properties such as super-elasticity, biocompatibility, and strength. The aim of this paper is to optimize the design of an open-ended braided stent, subjected to radial compression, with enhanced performance. The optimization process uses Multi-Objective Particle Swarm Optimization (MOPSO), which explores three design variables, namely wire diameter, number of coils, and braided angle, to determine the optimal shape that maximizes radial pressure stiffness and radial force exerted on the vessel walls while minimizing foreshortening. The analytical model developed is compared against literature findings, and the optimization results are implemented in a finite element analysis solver and compared with existing references. The results show that the optimized design using MOPSO enhances the stent’s average radial force and radial pressure stiffness by 28% and 40% respectively, while decreasing foreshortening by nearly 5%. The results demonstrate the feasibility of MOPSO for optimizing braided NiTi stents and the use of FEM for validating optimized designs
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