1,354,411 research outputs found
On the performance of a nonlinear vibration isolator consisting of axially loaded curved beams
No full textA desirable characteristic for nonlinear vibration isolators is a high static stiffness and a low dynamic stiffness. A curved beam is a possible candidate for this role provided that the amplitude of vibration about the static equilibrium position is sufficiently small. However, for large amplitude oscillations, the nonlinear dynamics may have a detrimental effect. This paper considers the force transmissibility of a single degree-of-freedom system where the stiffness element is a curved, axially loaded beam. The transmitted force is calculated by numerical time domain integration of the equations of motion. The exact force-deflection relation for the beam is used for the spring. By comparison, a frequency domain solution is sought using the Harmonic Balance (HB) method in which the system is modelled as a Duffing oscillator. It is shown that the HB and time domain solutions are in close agreement for small amplitudes of excitation and both predict advantageous performance of the nonlinear isolator compared with its equivalent linear counterpart. However, significant discrepancies occur between the two solutions for large excitation since the beam can no longer be approximated by a linear and a cubic stiffness. It is also strongly asymmetric – soft in compression but stiff in extreme extension– which gives rise to an impulse in the transmitted force in each fundamental period. This numerical problem is alleviated by inserting a linear spring in series with the beam isolator with a modest compromise in isolation performance at the excitation frequency
Nonlinear vibration isolators with asymmetric stiffness
No full textA vibration source is commonly coupled to a receiving structure by a vibration isolator. A key trade-off in the choice of vibration isolator is the requirement for a wide frequency range of isolation without excessive static deflection. This compromise can, in principle, be circumvented by employing a softening nonlinear isolator that presents a high stiffness to the weight of the isolated mass but a low tangent stiffness in the vicinity of the equilibrium position. The first part of this thesis is concerned with the static response of a number of elements that are expected to exhibit such a nonlinear stiffness characteristic. A mechanism with geometrical nonlinearity is studied first and found to offer some benefits compared with a similar one reported in the literature. Beams are commonly employed as linear springs and their suitability as nonlinear isolators is considered here. It is shown that the stiffness of a simply supported beam loaded transversely at its centre is of a hardening type in contrast to what is reported in the literature. Post-buckled beams are also investigated as candidates for nonlinear springs of vibration isolators although the sudden change in stiffness at the buckling point is unfavourable. Curved beams and beams with eccentric loading are investigated as alternatives to a straight axially loaded post-buckled beam. Static analyses are presented which show that curvature or eccentricity in loading can be incorporated to smooth the force-deflection curves. A commercially available rubber isolation mount is also studied as an example of an axially loaded curved element and its force-deflection characteristic measured. The hypothesis that it can be modelled by a curved beam is found not to hold. The inter-variability observed between samples is evaluated which illustrates the potential for mistune of nonlinear mounts in general.Nonlinear stiffness gives rise to the possibility of asymmetry about the equilibrium position, either as an inherent characteristic of the isolator or as a result of a mistuned added mass or static preload. A nonlinear isolator with asymmetric stiffness is modelled as a Duffing oscillator. The force transmissibility of the oscillator is obtained analytically using the Harmonic Balance Method from which the performance of the isolator is evaluated quantitatively as a function of both static load and mistuned mass. A study is presented for the case of a nonlinear isolator comprised of a curved beam. The high stiffness of the beam in extension causes impulsive behaviour in the transmitted force which is alleviated by the inclusion of a linear spring placed in series. It is shown that this combination significantly outperforms a linear isolator with the same static deflection
Large deflection of a simply supported beam
No full textThe large deflection of a simply-supported beam loaded in the middle is a classic problem in mechanics which has been studied by many people who have implemented different methods to determine the solution, such as analytical exact solutions and the finite element method. The problem is investigated again here but the Galerkin method is used to obtain an approximate force-deflection characteristic of the beam. It is shown that the beam can be modelled with a Duffing-type stiffness with hardening nonlinearity. The exact solution and that from the finite element method are used to validate the results. The accuracy of the results and the suitability of the Sine function to model the deflected shape of the beam in the Galerkin method are investigated.The large deflection of a simply-supported beam due to a pure bending moment is also investigated. The exact solution is obtained and the results are used to describe the behaviour of the beam
Experimental study on the high-velocity impact behavior of sandwich structures with an emphasis on the layering effects of foam core
No full textIn this study, the effects of the core layering of sandwich structures, as well as arrangements of these layers on the ballistic resistance of the structures under high-velocity impact, were investigated. Sandwich structures consist of aluminum face-sheets (AL-1050) and polyurethane foam core with different densities. Three sandwich structures with a single-layer core of different core densities and four sandwich structures with a four-layer core of different layers arrangements were constructed. Cylindrical steel projectiles with hemispherical nose, 8 mm diameter and 20 mm length were used. The projectile impact velocity range was chosen from 180 to 320 m/s. Considering constant mass and total thickness for the core, the results of the study showed that the core layering increases the ballistic limit velocity of the sandwich structures. The ballistic limit velocity of the panels with a four-layer core of different arrangements, compared to the panel with the single-layer core, is higher from 5% to 8%. Also, for the single-layer core structure, by increasing the core density, the ballistic limit velocity was increased. Different failure mechanisms such as plugging, petaling and dishing occurred for the back face-sheet. The dishing area diameter of back face-sheets was proportional to the ballistic resistance of each sandwich structure
Experimental and numerical investigation of the effect of the combined mechanism of circumferential expansion and folding on energy absorption parameters
No full textIn this research, in order to increase energy absorption of thin-walled tubes, a combined deformation mechanism is proposed which involves the simultaneous combination of circumferential expansion and folding. Such a combined mechanism was not concerned in the literature. The study is carried out both experimentally and numerically. A special device was designed and made to conduct experimental tests on tubes. The samples were made of aluminum, and quasi-static loading was applied at two different speeds of 10 and 200 mm/min. Energy absorption parameters including specific energy absorption (SEA), crushing mean force, initial peak force, the deformation mode and crush force efficiency (CFE) were studied. Experimental results showed that combined mechanism (without lubrication) could increase absorbed energy up to 123% compared to the folding mechanism. If the lubricant is used, the increase will be up to 97%. The combined deformation mechanism (without lubrication) increases absorbed energy up to 94% compared to the circumstantial expansion. This value will be 107% with lubrication. In addition, the initial peak force in the combined mechanism decreases between 8% and 36% relative to the folding mechanism. The circumstantial expansion in the proposed mechanism is complete and the expansion stroke length is 100%, while this stroke was less in the previous researches due to design restrictions. Numerical simulations were conducted using LS-Dyna software and there is good agreement between the numerical results and experimental data
Rethinking Competitive Positioning: Customer Value, Flexibility, and Generalist Advantage
No full textThis paper contributes to prior research on firm positioning by bringing in demand-side factors. In
particular, we develop a formal model to show how an important, yet ofen overlooked demand-side
contingency –customers switching costs- can afect the competitive advantage of generalists over
specialists. In this regard, we suggest that generalists have an advantage in comparison to specialists
since they provide ex-ante flexibility to customers to switch between multiple firm oferings without
changing service provider. Such flexibility is particularly valued in the settings where customer
switching costs are high. We hypothesize that generalists lose this advantage, and grow less in
comparison to specialists, once customer switching costs fall. We test our hypothesis using a sample of
Latin American mobile communications carriers from 2003 to 2015. In particular, we draw on an
exogenous policy change (mobile number portability) that suddenly decreases customer switching
costs. Using a diferences-in-diferences methodology allows us to estimate the causal efects of
competitive positioning on firm performance in diferent demand settings. Our results reveal that
generalists grow less in comparison to specialists afer the policy change
A device for investigating neuromuscular control in the human masticatory system
No full textCopyright © 2004 Elsevier B.V. All rights reserved.K. S. Türker, R. S. A. Brinkworth, P. Abolfathi, I. R. Linke and H. Nazera
Effect of Microscale Fabrication on Multi-Directional Mechanical Properties of Additively Manufactured Poly Lactic Acid With Grid Infills
No full textIn additive manufacturing, infill patterns have a significant impact on both printing time and mechanical performance, creating a necessary trade-off between the two from an industrial perspective. This study aims therefore to find an easy-to-handle procedure for rapid evaluation of the influence of infill density and raster angle on the elastic properties of 3D-printed components, from the perspective of their adoption in the industrial process of component design. In particular, the study's goal is to predict the elastic modulus in three directions. Tensile tests were carried out on bulk specimens according to ISO 527 to determine the elastic properties of 3D-printed PLA necessary for the numerical analysis. Cubic specimens were then manufactured with three densities (20%, 40%, and 60%) and two raster angles (-45 degrees/45 degrees and 0 degrees/90 degrees). Quasi-static compression tests were conducted on those specimens to assess their homogenized elastic behavior in three directions. One important result of the experimental phase was the relationship between Young's modulus (E) in the three directions. The average of E in directions 1 and 2 (build plate) is named E-1,E-2 and on the build-up directions is E-3, for alpha = 0 degrees/90 degrees was E-1,E-2 = 0.8E(3) and for alpha = -45 degrees/45 degrees was E-1,E-2 = 0.28E(3). Three finite element models were developed and run with the elastic properties determined by tensile tests, namely: (a) a shell model (SHL) where the internal and external walls of the specimens were modeled using shell elements with the nominal geometry; (b) a solid model (SLD) with the nominal geometry and (c) a nonuniform section model (NUS) in which the geometry was taken from microscope image to account for manufacturing imperfections. The difference between simulation and experiment for SHL was 19%, SLD was 15%, and NUS was 13%, indicating an overall good correspondence and, at the same time, that the real geometry resulting from the manufacturing process has a non-negligible impact on the homogenized value. Besides validating the values and relationships, FEM elucidated which sections of the cubes experienced stress and contributed to stiffness under various patterns and loading scenarios
Developing and analysing an electromechanical model of a bio-inspired flapping wing mini UAV
No full textThe purpose of this paper is to describe and analyse an electromechanical model of a mini flapping wing mechanism for the purpose of system optimisation. The system comprises of a small DC motor connected to a voltage source, a gearbox and a slider crank that drives two wings. The DC gearmotor is modelled considering its both mechanical and electrical components. An equivalent viscous damper is considered to model the mechanical losses of the gearmotor. The crank mechanism is assumed massless and the inertia of the wings only considered in the model. The aerodynamic drag and lift are modelled using an equivalent viscous damping model as an energy sink. The parameters of the system are estimated using published experimental data and manufacturer datasheets for the corresponding DC gearmotor. The energy efficiency as the ratio of aerodynamic power to the input electrical power of the system and also the aerodynamic power are used as two measures to evaluate the system performance
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