Showing papers on "Flexural rigidity published in 2016"

Superflexibility of graphene oxide

Abstract: Graphene oxide (GO), the main precursor of graphene-based materials made by solution processing, is known to be very stiff. Indeed, it has a Young's modulus comparable to steel, on the order of 300 GPa. Despite its very high stiffness, we show here that GO is superflexible. We quantitatively measure the GO bending rigidity by characterizing the flattening of thermal undulations in response to shear forces in solution. Characterizations are performed by the combination of synchrotron X-ray diffraction at small angles and in situ rheology (rheo-SAXS) experiments using the high X-ray flux of a synchrotron source. The bending modulus is found to be 1 kT, which is about two orders of magnitude lower than the bending rigidity of neat graphene. This superflexibility compares with the fluidity of self-assembled liquid bilayers. This behavior is discussed by considering the mechanisms at play in bending and stretching deformations of atomic monolayers. The superflexibility of GO is a unique feature to develop bendable electronics after reduction, films, coatings, and fibers. This unique combination of properties of GO allows for flexibility in processing and fabrication coupled with a robustness in the fabricated structure.

Boson peak and Ioffe-Regel criterion in amorphous siliconlike materials: The effect of bond directionality

Abstract: The vibrational properties of model amorphous materials are studied by combining complete analysis of the vibration modes, dynamical structure factor, and energy diffusivity with exact diagonalization of the dynamical matrix and the kernel polynomial method, which allows a study of very large system sizes. Different materials are studied that differ only by the bending rigidity of the interactions in a Stillinger-Weber modelization used to describe amorphous silicon. The local bending rigidity can thus be used as a control parameter, to tune the sound velocity together with local bonds directionality. It is shown that for all the systems studied, the upper limit of the Boson peak corresponds to the Ioffe-Regel criterion for transverse waves, as well as to a minimum of the diffusivity. The Boson peak is followed by a diffusivity's increase supported by longitudinal phonons. The Ioffe-Regel criterion for transverse waves corresponds to a common characteristic mean-free path of 5-7 A (which is slightly bigger for longitudinal phonons), while the fine structure of the vibrational density of states is shown to be sensitive to the local bending rigidity.

Maize stalk lodging: Flexural stiffness predicts strength

Abstract: Late-season stalk lodging in maize (Zea mays L.) is a major agronomic problem that has farreaching economic ramifications. More rapid advances in lodging resistance could be achieved through development of selective breeding tools that are not confounded by environmental factors. It was hypothesized that measurements of stalk flexural stiffness (a mechanical measurement inspired by engineering beam theory) would be a stronger predictor of stalk strength than current technologies. Stalk flexural stiffness, rind penetration resistance and stalk bending strength measurements were acquired for five commercial varieties of dent corn grown at five planting densities and two locations. Correlation analyses revealed that stalk flexural stiffness predicted 81% of the variation in stalk strength, whereas rind penetration resistance only accounted for 18% of the variation in stalk strength. Strength predictions based on measurements of stalk flexural stiffness were not confounded by hybrid type, planting density, or planting location. Strength predictions based on rind penetration resistance were moderately to severely confounded by such factors. Results indicate that stalk flexural stiffness is a good predictor of stalk strength and that it may outperform rind penetration resistance as a selective breeding tool to improve lodging resistance of future varieties of maize. D.J. Robertson, S.Y. Lee, M. Julias, and D.D. Cook, Division of Engineering, New York Univ. Abu Dhabi, Abu Dhabi, United Arab Emirates. Received 1 Nov. 2015. Accepted 20 Jan. 2016. *Corresponding author (douglascook@nyu.edu). Published in Crop Sci. 56:1711–1718 (2016). doi: 10.2135/cropsci2015.11.0665 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA This is an open access article distributed under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Published May 27, 2016

Directional anisotropy, finite size effect and elastic properties of hexagonal boron nitride.

Abstract: Classical molecular dynamics simulations have been performed to analyze the elastic and mechanical properties of two-dimensional (2D) hexagonal boron nitride (h-BN) using a Tersoff-type interatomic empirical potential. We present a systematic study of h-BN for various system sizes. Young's modulus and Poisson's ratio are found to be anisotropic for finite sheets whereas they are isotropic for the infinite sheet. Both of them increase with system size in accordance with a power law. It is concluded from the computed values of elastic constants that h-BN sheets, finite or infinite, satisfy Born's criterion for mechanical stability. Due to the the strong in-plane sp2 bonds and the small mass of boron and nitrogen atoms, h-BN possesses high longitudinal and shear velocities. The variation of bending rigidity with system size is calculated using the Foppl-von Karman approach by coupling the in-plane bending and out-of-plane stretching modes of the 2D h-BN.

Bending rigidity of transition metal dichalcogenide monolayers from first-principles

Abstract: Due to the presence of a sizeable direct band gap, three-atom-thick transition metal dichalcogenide (TMDC) monolayers have been suggested as important candidates for flexible electronic and optoelectronic devices recently. The in-plane elasticity of TMDC monolayers has been investigated extensively, however, little is known about their bending rigidity. Here, we have determined bending rigidities of single-layer MX2 (M = Mo, W; X = S, Se) by fitting the energetics of single wall nanotubes from first-principles to the Helfrich Hamiltonian for the configurational energy of membranes. This parameter-free approach can avoid the controversy induced by ambiguous definition of the thickness of monolayers, which are required in the empirical determination of bending rigidity using classical shell theory. The obtained direction-dependent bending rigidities of single-layer MoS2 are 9.10 and 9.61 eV along the armchair and zigzag directions, which are larger than that estimated using shell theory but similar to the previous analytic formula based on an empirical potential. Moreover, the relative magnitude of bending rigidities for different TMDCs are found to be MoS2 < MoSe2 < WS2 < WSe2, which is in conflict with shell theory. Further analysis indicates that this inconsistence can be understood by a competition mechanism between two-dimensional elastic modulus of monolayers and the structural relaxation of nanotubes.

A strain-consistent elastic plate model with surface elasticity

Abstract: A strain-consistent elastic plate model is formulated in which both initial surface tension and the induced residual stress are treated as finite values, and the exactly same strain expressions are consistently employed for both the surface and the bulk plate. Different than most of previous related models which follow the original Gurtin–Murdoch model and include some non-strain displacement gradient terms (which cannot be expressed in terms of the surface infinitesimal strains or the von Karman-type strains) in the surface stress–strain relations, the present model does not include any non-strain displacement gradient terms in the surface stress–strain relations. For a free elastic plate with in-plane movable edges, the present model predicts that initial surface tension exactly cancels out the induced residual compressive stress. On the other hand, if all edges are in-plane immovable, residual stress cannot develop in the plate and the initial surface tension causes a tensile net membrane force. In addition, the present model predicts that initial surface tension reduces the effective bending rigidity of the plate, while this reduction does not depend on Poisson ratio. In particular, self-buckling of a free elastic plate under tensile surface tension cannot occur unless the effective bending rigidity of plate vanishes or becomes negative.

An analytical study on the static vertical stiffness of wire rope isolators

Abstract: The vibrations caused by earthquake ground motions or the operations of heavy machineries can affect the functionality of equipment and cause damages to the hosting structures and surrounding equipment. A Wire rope isolator (WRI), which is a type of passive isolator known to be effective in isolating shocks and vibrations, can be used for vibration isolation of lightweight structures and equipment. The primary advantage of the WRI is that it can provide isolation in all three planes and in any orientation. The load-supporting capability of the WRI is identified from the static stiffness in the loading direction. Static stiffness mainly depends on the geometrical and material properties of the WRI. This study develops an analytical model for the static stiffness in the vertical direction by using Castigliano’s second theorem. The model is validated by using the experimental results obtained from a series of monotonic loading tests. The flexural rigidity of the wire ropes required in the model is obtained from the transverse bending test. Then, the analytical model is used to conduct a parametric analysis on the effects of wire rope diameter, width, height, and number of turns (loops) on vertical stiffness. The wire rope diameter influences stiffness more than the other geometric parameters. The developed model can be accurately used for the evaluation and design of WRIs.

Flexural Stiffness of Thick Walled Composite Tubes

Abstract: Composite tubes have been used in many applications such as pipes, robot arms, drive shafts, electrical conduits, printing rollers, tube structures for sports equipment, rocket structures, satellite truss structures, landing gears for helicopters, and structural building members etc For thin wall tubes made of isotropic materials, the flexural stiffness is usually determined by using strength of material approach with the expression EI, where E is the material modulus and I the cross section inertia For thin walled composite tubes where many layers with different orientations are involved, one may tempt to use the same equation with E replaced by Ex to take care of the fiber orientations However this may not be correct Adding to this the increasing thickness of the cylinder, then the inaccuracy of the equation may be even more By using elasticity approach, an equation for the determination of the stiffness of thick composite cylinders was derived by previous researchers It was shown by experimental work that the equation based on elasticity provides more accurate results that that based on strength of materials approach This paper presents some behavior of the bending stiffness of thick walled composite tubes

A multi-layered variable stiffness device based on smart form closure actuators:

Abstract: This contribution describes the properties and limitations of multi-layered mechanical devices with variable flexural stiffness. Such structures are supposed to be components of new smart, self-sen...

Static lateral stiffness of wire rope isolators

Abstract: This paper presents an analytical model for the static lateral stiffness of Wire Rope Isolators (WRI). The wire rope isolator, which is a passive isolation device, has been widely adopted as a shock and vibration isolation for many types of equipment and lightweight structures. The major advantage of the WRI is its ability to provide isolation in all three planes and in any orientation. The WRI in the lateral roll mode, is required to possess the required lateral stiffness to support and isolate the equipment effectively. The static lateral stiffness of WRI depends mainly on the geometrical characteristics and wire rope properties. The model developed in this paper is validated experimentally using a series of monotonic loading tests. The flexural rigidity of the wire ropes, which is required in the model, was determined from the transverse bending test on several wire rope cables. It was observed that the lateral stiffness is significantly influenced by the wire rope diameter and height of the is...

On the influence of interfacial properties to the bending rigidity of layered structures

Abstract: Layered structures are ubiquitous, from one-atom thick layers in two-dimensional materials, to nanoscale lipid bi-layers, and to micro and millimeter thick layers in composites. The mechanical behavior of layered structures heavily depends on the interfacial properties and is of great interest in engineering practice. In this work, we give an analytical solution of the bending rigidity of bilayered structures as a function of the interfacial shear strength. Our results show that while the critical bending stiffness when the interface starts to slide plastically is proportional to the interfacial shear strength, there is a strong nonlinearity between the rigidity and the applied bending after interfacial plastic shearing. We further give semi-analytical solutions to the bending of bilayers when both interfacial shearing and pre-existing crack are present in the interface of rectangular and circular bilayers. The analytical solutions are validated by using finite element simulations. Our analysis suggests that interfacial shearing resistance, interfacial stiffness and preexisting cracks dramatically influence the bending rigidity of bilayers. The results can be utilized to understand the significant stiffness difference in typical biostructures and novel materials, and may also be used for non-destructive detection of interfacial crack in composites when stiffness can be probed through vibration techniques.

Shape deformation of lipid membranes by banana-shaped protein rods: Comparison with isotropic inclusions and membrane rupture.

Abstract: The assembly of curved protein rods on fluid membranes is studied using implicit-solvent meshless membrane simulations. As the rod curvature increases, the rods on a membrane tube assemble along the azimuthal direction first and subsequently along the longitudinal direction. Here, we show that both transition curvatures decrease with increasing rod stiffness. For comparison, curvature-inducing isotropic inclusions are also simulated. When the isotropic inclusions have the same bending rigidity as the other membrane regions, the inclusions are uniformly distributed on the membrane tubes and vesicles even for large spontaneous curvature of the inclusions. However, the isotropic inclusions with much larger bending rigidity induce shape deformation and are concentrated on the region of a preferred curvature. For high rod density, high rod stiffness, and/or low line tension of the membrane edge, the rod assembly induces vesicle rupture, resulting in the formation of a high-genus vesicle. A gradual change in the curvature suppresses this rupture. Hence, large stress, compared to the edge tension, induced by the rod assembly is the key factor determining rupture. For rod curvature with the opposite sign to the vesicle curvature, membrane rupture induces inversion of the membrane, leading to division into multiple vesicles as well as formation of a high-genus vesicle.

Analysis of the flexural stiffness of timber beams reinforced with carbon and basalt composite materials

Abstract: In this work, an experimental study on the bending behavior of pine wood beams reinforced with carbon and basalt fiber reinforced plastics, externally glued with epoxy resins has been performed. Different grammage, unidirectional and bi-directional fabrics have been used, and one or three layers applied. The results show good behavior of basalt fiber reinforcements in the stiffness increase of timber beams, achieving similar results to those obtained with carbon fiber. Moreover, reinforcements made with bi-directional fabrics produced significant stiffness increases compared to the unidirectional fabrics. Finally, this work includes also the results obtained using the transformed section to predict the stiffness increase that the different types of reinforcement analyzed produces.