Página 1 dos resultados de 23 itens digitais encontrados em 0.039 segundos

Snap-Through Instability of Graphene on Substrates

Li, Teng; Zhang, Zhao
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
Publicado em 17/10/2009 EN
Relevância na Pesquisa
66.08%
We determine the graphene morphology regulated by substrates with herringbone and checkerboard surface corrugations. As the graphene–substrate interfacial bonding energy and the substrate surface roughness vary, the graphene morphology snaps between two distinct states: (1) closely conforming to the substrate and (2) remaining nearly flat on the substrate. Since the graphene morphology is strongly tied to the electronic properties of graphene, such a snap-through instability of graphene morphology can lead to desirable graphene electronic properties that could potentially enable graphene-based functional electronic components (e.g. nano-switches).

Torsional Kinematic Model for Concentric Tube Robots

Dupont, Pierre E.; Lock, Jesse; Butler, Evan
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 12/05/2009 EN
Relevância na Pesquisa
35.78%
A recent approach to steerable needle design is based on combining pre-curved tubes concentrically. By rotating and extending the tubes with respect to each other, the position and orientation of the needle tip, as well as the shape of the inserted length, can be controlled. Prior models neglected torsional twisting in the curved portions of the tubes. This paper presents a mechanics model that includes torsion, applies to any number of tubes and allows curvature and stiffness to vary with arc length. While the general model is comprised of differential equations, an analytic solution is given for two tubes of constant curvature. This solution enables analytic prediction of “snap through” instability based on a single dimensionless parameter. Simulation and experiments are used to illustrate the results.

Temporal evolution and instability in a viscoelastic dielectric elastomer

Kollosche, Matthias; Kofod, Guggi; Suo, Zhigang; Zhu, Jian
Fonte: Elsevier BV Publicador: Elsevier BV
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
76.22%
Dielectric elastomer transducers are being developed for applications in stretchable electronics, tunable optics, biomedical devices, and soft machines. These transducers exhibit highly nonlinear electromechanical behavior: a dielectric membrane under voltage can form wrinkles, undergo snap-through instability, and suffer electrical breakdown. We investigate temporal evolution and instability by conducting a large set of experiments under various prestretches and loading rates, and by developing a model that allows viscoelastic instability. We use the model to classify types of instability, and map the experimental observations according to prestreches and loading rates. The model describes the entire set of experimental observations. A new type of instability is discovered, which we call wrinkle-to-wrinkle transition. A flat membrane at a critical voltage forms wrinkles and then, at a second critical voltage, snaps into another state of winkles of a shorter wavelength. This study demonstrates that viscoelasticity is essential to the understanding of temporal evolution and instability of dielectric elastomers.; Engineering and Applied Sciences

Optomechanics of Soft Materials

Bai, Ruobing; Suo, Zhigang
Fonte: ASME International Publicador: ASME International
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
45.88%
Some molecules change shape upon receiving photons of certain frequencies, but here we study light-induced deformation in ordinary dielectrics with no special optical effects. All dielectrics deform in response to light of all frequencies. We derive a dimensionless number to estimate when light can induce large deformation. For a structure made of soft dielectrics, with feature size comparable to the wavelength of light, the structure shapes the light, and the light deforms the structure. We study this two-way interaction between light and structure by combining the electrodynamics of light and the nonlinear mechanics of elasticity. We show that optical forces vary nonlinearly with deformation and readily cause optomechanical snap-through instability. These theoretical ideas may help to create optomechanical devices of soft materials, complex shapes, and small features.; Engineering and Applied Sciences

Planar analysis of a quasi-zero stiffness mechanism using inclined linear springs

Robertson, W.; Cazzolato, B.; Zander, A.
Fonte: The Australian Acoustical Society; South Australia Publicador: The Australian Acoustical Society; South Australia
Tipo: Conference paper
Publicado em //2013 EN
Relevância na Pesquisa
25.88%
Negative stiffness mechanisms have seen renewed attention in recent years for their ability to reduce the resonance frequency of a structure without impeding their load-bearing ability. Such systems are often described as having quasizero stiffness when the negative stiffness is tuned to reduce the overall stiffness of the system as close to zero as possible without creating an instability. The system analysed in this work consists of a vertical spring for load bearing, and two symmetric inclined springs which behave with a snap-through effect to achieve negative stiffness. While this structure has been analysed extensively in the literature, generally only the stiffness in the vertical direction has been considered in the past. Here, the horizontal stiffness is assessed as well, and it is shown that it is possible to achieve quasi-zero stiffness in both directions simultaneously if the spring stiffnesses and pre-loads are chosen sufficiently. Attention is paid to the tuning required in order to set the equilibrium point at a position which is arbitrarily close to having quasi-zero stiffness while avoiding issues arising from mechanical instability.; http://www.acoustics.asn.au/conference_proceedings/AAS2013/; http://www.acoustics.asn.au/conference_proceedings/AAS2013/papers/AAS2013-Information.pdf; William S. P. Robertson...

A lower bound on snap-through instability of curved beams under thermomechanical loads

Stanciulescu, Ilinca; Mitchell, Toby; Chandra, Yenny; Eason, Thomas; Spottswood, Michael
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
56.23%
A non-linear finite element formulation (three dimensional continuum elements) is implemented and used for modeling dynamic snap-through in beams with initial curvature. We identify a non-trivial (non-flat) configuration of the beam at a critical temperature value below which the beam will no longer experience snap-through under any magnitude of applied quasi-static load for beams with various curvatures. The critical temperature is shown to successfully eliminate snap-through in dynamic simulations at quasistatic loading rates. Thermomechanical coupling is included in order to model a physically minimal amount of damping in the system, and the resulting post-snap vibrations are shown to be thermoelastically damped. We propose a test to determine the critical snap-free temperature for members of general geometry and loading pattern; the analogy between mechanical prestress and thermal strain that holds between the static and dynamic simulations is used to suggest a simple method for reducing the vulnerability of thin-walled structural members to dynamic snap-through in members of large initial curvature via the introduction of initial pretension.

Characterizing Dynamic Transitions Associated With Snap-Through: A Discrete System

Wiebe, R.; Virgin, L. N.; Stanciulescu, Ilinca; Spottswood, S.M.; Eason, T.G.
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
46.08%
Geometrically nonlinear structures often possess multiple equilibrium configurations. Under extreme conditions of excitation, it is possible for these structures to exhibit oscillations about and between these co-existing configurations. This behavior may have serious implications for fatigue in the context of aircraft surface panels. Snap-through is a name often given to sudden changes in dynamic behavior associated with mechanical instability (buckling). This is an often encountered problem in hypersonic vehicles in which severe thermal loading and acoustic excitation conspire to create an especially hostile environment for structural elements. In this paper, a simple link model is used, experimentally and numerically, to investigate the mechanisms of snap-through buckling from a phenomenological standpoint.

Transient behavior of curved structures

Chandra, Yenny
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Thesis; Text Formato: application/pdf
Relevância na Pesquisa
46.15%
Slender curved structures can often be found as components of complex structures in civil, mechanical, and aerospace systems. Under extreme loadings, the structure might undergo snap-through buckling, i.e., the structure is forced to its inverted configuration, inducing fatigue. The focus of this research is the development of a reliable and accurate model for simulating the nonlinear response of shallow arches under transient loading and characterizing these responses to assess the structure's ability to survive if the structure undergoes instabilities. Since no analytical solutions for general systems with snap-through exist, numerical models are needed in order to predict the response of the structure. The finite element method provides the most generality and can be applied to systems with arbitrarily complex geometries. Unfortunately there are barriers to the numerical prediction. First, the structures exhibit a very complex dynamic response. Coexisting responses are identified under different initial conditions. Chaotic responses are also observed. A framework for analyzing the dynamic responses of slender curved structures is proposed by identifying the relevant features useful in characterizing the transient behavior of shallow arches. State of the art time integrators are often unable to retrieve long time records of the response after a physical instability event. The performance of several time-stepping schemes is analyzed by identifying the important features that affect the numerical accuracy and robustness. We also identify the region where the schemes are stable for such simulations. The interactions between the time-stepping schemes and the spatial discretizations are examined. This investigation results in recommendations for finite elements and time integrators that give the best performance. A new time integrator that is robust and accurate for long-term simulations is proposed. The established numerical framework is validated against experimental data. Fabrication imperfections in the experimental arch and prestressing due to the applied boundary conditions are accounted for. A methodology to determine the boundaries of the stability regions in the parameter space under consideration is proposed. Finally...

Horizontal stability of a quasi-zero stiffness mechanism using inclined linear springs

Robertson, W.S.P.; Cazzolato, B.; Zander, A.
Fonte: Australian Accoustical Society Publicador: Australian Accoustical Society
Tipo: Artigo de Revista Científica
Publicado em //2014 EN
Relevância na Pesquisa
25.88%
Negative stiffness mechanisms have seen renewed attention in recent years for their ability to reduce the resonance frequency of a structure without impeding their load-bearing ability. Such systems are often described as having quasi-zero stiffness when the negative stiffness is tuned to reduce the overall stiffness of the system as close to zero as possible without creating an instability. The system analysed in this work consists of a vertical spring for load bearing, and two symmetric inclined springs which behave with a snap-through effect to achieve negative stiffness. While this structure has been analysed extensively in the literature, generally only the stiffness in the vertical direction has been considered in the past. Here, the horizontal stiffness is assessed as well, and it is shown that it is possible to achieve quasi-zero stiffness in both directions simultaneously if the spring stiffnesses and pre-loads are chosen appropriately. Attention is paid to the tuning required in order to set the equilibrium point at a position which is arbitrarily close to having quasi-zero stiffness while avoiding issues arising from mechanical instability.; William S. P. Robertson, Ben Cazzolato, and Anthony Zander

Understanding of the phase transformation from fullerite to amorphous carbon at the microscopic level

Moseler, M.; Riedel, H.; Gumbsch, P.; Staring, J.; Mehlig, B.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/09/2004
Relevância na Pesquisa
35.78%
We have studied the shock-induced phase transition from fullerite to a dense amorphous carbon phase by tight-binding molecular dynamics. For increasing hydrostatic pressures P, the C60-cages are found to polymerise at P<10 GPa, to break at P~40 GPa and to slowly collapse further at P>60 GPa. By contrast, in the presence of additional shear stresses, the cages are destroyed at much lower pressures (P<30 GPa). We explain this fact in terms of a continuum model, the snap-through instability of a spherical shell. Surprisingly, the relaxed high-density structures display no intermediate-range order.; Comment: 5 pages, 3 figures

Dynamics of Snapping Beams and Jumping Poppers

Pandey, Anupam; Moulton, Derek E.; Vella, Dominic; Holmes, Douglas P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.18%
We consider the dynamic snapping instability of elastic beams and shells. Using the Kirchhoff rod and F\"{o}ppl-von K\'{a}rm\'{a}n plate equations, we study the stability, deformation modes, and snap-through dynamics of an elastic arch with clamped boundaries and subject to a concentrated load. For parameters typical of everyday and technological applications of snapping, we show that the stretchability of the arch plays a critical role in determining not only the post-buckling mode of deformation but also the timescale of snapping and the frequency of the arch's vibrations about its final equilibrium state. We show that the growth rate of the snap-through instability and its subsequent ringing frequency can both be interpreted physically as the result of a sound wave in the material propagating over a distance comparable to the length of the arch. Finally, we extend our analysis of the ringing frequency of indented arches to understand the `pop' heard when everted shell structures snap-through to their stable state. Remarkably, we find that not only are the scaling laws for the ringing frequencies in these two scenarios identical but also the respective prefactors are numerically close; this allows us to develop a master curve for the frequency of ringing in snapping beams and shells.

Snap-through instability of graphene on substrates

Li, Teng; Zhang, Zhao
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/07/2009
Relevância na Pesquisa
45.88%
We determine the graphene morphology regulated by substrates with herringbone and checkerboard surface corrugations. As the graphene/substrate interfacial bonding energy and the substrate surface roughness vary, the graphene morphology snaps between two distinct states: 1) closely conforming to the substrate and 2) remaining nearly flat on the substrate. Such a snapthrough instability of graphene can potentially lead to desirable electronic properties to enable graphene-based devices.; Comment: 13 pages, 4 figures; Nanoscale Research Letters, in press, 2009

Endocytic proteins drive vesicle growth via instability in high membrane tension environment

Walani, Nikhil; Torres, Jennifer; Agrawal, Ashutosh
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.08%
Clathrin-mediated endocytosis (CME) is a key pathway for transporting cargo into cells via membrane vesicles. It plays an integral role in nutrient import, signal transduction, neurotransmission and cellular entry of pathogens and drug-carrying nanoparticles. As CME entails substantial local remodeling of the plasma membrane, the presence of membrane tension offers resistance to bending and hence, vesicle formation. Experiments show that in such high tension conditions, actin dynamics is required to carry out CME successfully. In this study, we build upon these pioneering experimental studies to provide fundamental mechanistic insights into the roles of two key endocytic proteins, namely, actin and BAR proteins in driving vesicle formation in high membrane tension environment. Our study reveals a new actin force induced `snap-through instability' that triggers a rapid shape transition from a shallow invagination to a highly invaginated tubular structure. We show that the association of BAR proteins stabilizes vesicles and induces a milder instability. In addition, we present a new counterintuitive role of BAR depolymerization in regulating the shape evolution of vesicles. We show that the dissociation of BAR proteins, supported by actin-BAR synergy...

Observation of a Snap-Through Instability in Graphene

Scharfenberg, Scott; Mansukhani, Nikhita; Chialvo, Cesar; Weaver, Richard L.; Mason, Nadya
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/09/2011
Relevância na Pesquisa
56.08%
We examine the competition between adhesive and bending energies for few-layered graphene samples placed on rigid, microscale-corrugated substrates. Using atomic force microscopy, we show that the graphene undergoes a sharp "snap-through" transition as a function of layer thickness, where the material transitions between conforming to the substrate and lying flat on top of the substrate. By utilizing the critical snap-through thickness in an elasticity model for the FLG's bending, we extract a value for graphene-surface adhesion energy that is larger than expected for van der Waals forces.; Comment: 8 pages, 3 figures

Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes

Lindahl, Niklas; Midtvedt, Daniel; Svensson, Johannes; Nerushev, Oleg A.; Lindvall, Niclas; Isacsson, Andreas; Campbell, Eleanor E. B.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/03/2012
Relevância na Pesquisa
35.78%
The small mass and atomic-scale thickness of graphene membranes make them highly suitable for nanoelectromechanical devices such as e.g. mass sensors, high frequency resonators or memory elements. Although only atomically thick, many of the mechanical properties of graphene membranes can be described by classical continuum mechanics. An important parameter for predicting the performance and linearity of graphene nanoelectromechanical devices as well as for describing ripple formation and other properties such as electron scattering mechanisms, is the bending rigidity, {\kappa}. In spite of the importance of this parameter it has so far only been estimated indirectly for monolayer graphene from the phonon spectrum of graphite, estimated from AFM measurements or predicted from ab initio calculations or bond-order potential models. Here, we employ a new approach to the experimental determination of {\kappa} by exploiting the snap-through instability in pre-buckled graphene membranes. We demonstrate the reproducible fabrication of convex buckled graphene membranes by controlling the thermal stress during the fabrication procedure and show the abrupt switching from convex to concave geometry that occurs when electrostatic pressure is applied via an underlying gate electrode. The bending rigidity of bilayer graphene membranes under ambient conditions was determined to be $35.5^{+20}_{-15}$ eV. Monolayers have significantly lower {\kappa} than bilayers.

Theory of Sorption Hysteresis in Nanoporous Solids: I. Snap-Through Instabilities

Bazant, Zdenek P.; Bazant, Martin Z.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/08/2011
Relevância na Pesquisa
45.98%
The sorption-desorption hysteresis observed in many nanoporous solids, at vapor pressures low enough for the the liquid (capillary) phase of the adsorbate to be absent, has long been vaguely attributed to changes in the nanopore structure, but no mathematically consistent explanation has been presented. The present work takes an analytical approach to account for discrete molecular forces in the nanopore fluid and proposes two related mechanisms that can explain the hysteresis at low vapor pressure without assuming any change in the nanopore structure. The first mechanism, presented in Part I, consists of a series of snap-through instabilities during the filling or emptying of non-uniform nanopores or nanoscale asperities. The instabilities are caused by non-uniqueness in the misfit disjoining pressures engendered by a difference between the nanopore width and an integer multiple of the thickness of a monomolecular adsorption layer. The second mechanism, presented in Part II, consists of molecular coalescence within a partially filled surface, nanopore or nanopore network. This general thermodynamic instability is driven by attractive intermolecular forces within the adsorbate and forms the basis to develop a unified theory of both mechanisms. The ultimate goals of the theory are to predict the fluid transport in nanoporous solids from microscopic first principles...

Elastocapillary Snapping: Capillarity Induces Snap-Through Instabilities in Small Elastic Beams

Fargette, Aurélie; Neukirch, Sébastien; Antkowiak, Arnaud
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.98%
We report on the capillary-induced snapping of elastic beams. We show that a millimeter-sized water drop gently deposited on a thin buckled polymer strip may trigger an elastocapillary snap-through instability. We investigate experimentally and theoretically the statics and dynamics of this phenomenon and we further demonstrate that snapping can act against gravity, or be induced by soap bubbles on centimeter-sized thin metal strips. We argue that this phenomenon is suitable to miniaturization and design a condensation-induced spin-off version of the experiment involving an hydrophilic strip placed in a steam flow.

Substrate-regulated morphology of graphene

Li, Teng; Zhang, Zhao
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
35.83%
We delineate a general theoretical framework to determine the substrate-regulated graphene morphology through energy minimization. We then apply such a framework to study the graphene morphology on a substrate with periodic surface grooves. Depending on the substrate surface roughness and the graphene-substrate interfacial bonding energy, the equilibrium morphology of graphene ranges from 1) closely conforming to the substrate, to 2) remaining flat on the substrate. Interestingly, in certain cases, the graphene morphology snaps between the above two limiting states. Our quantitative results envision a promising strategy to precisely control the graphene morphology over large areas. The rich features of the substrate-regulated graphene morphology (e.g., the snap-through instability) can potentially lead to new design concepts of functional graphene device components.; Comment: 23 pages, 5 figures

Dynamics of periodic mechanical structures containing bistable elastic elements: From elastic to solitary wave propagation

Nadkarni, Neel; Daraio, Chiara; Kochmann, Dennis M.
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 22/08/2014
Relevância na Pesquisa
35.78%
We investigate the nonlinear dynamics of a periodic chain of bistable elements consisting of masses connected by elastic springs whose constraint arrangement gives rise to a large-deformation snap-through instability. We show that the resulting negative-stiffness effect produces three different regimes of (linear and nonlinear) wave propagation in the periodic medium, depending on the wave amplitude. At small amplitudes, linear elastic waves experience dispersion that is controllable by the geometry and by the level of precompression. At moderate to large amplitudes, solitary waves arise in the weakly and strongly nonlinear regime. For each case, we present closed-form analytical solutions and we confirm our theoretical findings by specific numerical examples. The precompression reveals a class of wave propagation for a partially positive and negative potential. The presented results highlight opportunities in the design of mechanical metamaterials based on negative-stiffness elements, which go beyond current concepts primarily based on linear elastic wave propagation. Our findings shed light on the rich effective dynamics achievable by nonlinear small-scale instabilities in solids and structures.

Nonlinear Dynamics of Discrete and Continuous Mechanical Systems with Snap-through Instabilities

Wiebe, Richard
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Dissertação
Publicado em //2012
Relevância na Pesquisa
66.32%

The primary focus of this dissertation is the characterization of snap-through buckling of discrete and continuous systems. Snap-through buckling occurs as the consequence of two factors, first the destabilization, or more often the disappearance of, an equilibrium position under the change of a system parameter, and second the existence of another stable equilibrium configuration at a remote location in state space. In this sense snap-through buckling is a global dynamic transition as the result of a local static instability.

In order to better understand the static instabilities that lead to snap-through buckling, the behavior of mechanical systems in the vicinity of various local bifurcations is first investigated. Oscillators with saddle-node, pitchfork, and transcritical bifurcations are shown analytically to exhibit several interesting characteristics, particularly in relation to the system damping ratio. A simple mechanical oscillator with a transcritical bifurcation is used to experimentally verify the analytical results. The transcritical bifurcation was selected since it may be used to represent generic bifurcation behavior. It is shown that the damping ratio may be used to predict changes in stability with respect to changing system parameters.

Another useful indicator of snap-through is the presence of chaos in the dynamic response of a system. Chaos is usually associated snap-through...