Showing papers on "Torsion (mechanics) published in 2010"

Improved side‐chain torsion potentials for the Amber ff99SB protein force field

Abstract: Recent advances in hardware and software have enabled increasingly long molecular dynamics (MD) simulations of biomolecules, exposing certain limitations in the accuracy of the force fields used for such simulations and spurring efforts to refine these force fields. Recent modifications to the Amber and CHARMM protein force fields, for example, have improved the backbone torsion potentials, remedying deficiencies in earlier versions. Here, we further advance simulation accuracy by improving the amino acid side-chain torsion potentials of the Amber ff99SB force field. First, we used simulations of model alpha-helical systems to identify the four residue types whose rotamer distribution differed the most from expectations based on Protein Data Bank statistics. Second, we optimized the side-chain torsion potentials of these residues to match new, high-level quantum-mechanical calculations. Finally, we used microsecond-timescale MD simulations in explicit solvent to validate the resulting force field against a large set of experimental NMR measurements that directly probe side-chain conformations. The new force field, which we have termed Amber ff99SB-ILDN, exhibits considerably better agreement with the NMR data. Proteins 2010. © 2010 Wiley-Liss, Inc.

Limit Analysis and Concrete Plasticity

Abstract: Introduction The Theory of Plasticity Constitutive Equations Extremum Principles for Rigid-Plastic Materials The Solution of Plasticity Problems Reinforced Concrete Structures Yield Conditions Concrete Yield Conditions for Reinforced Disks Yield Conditions for Slabs Reinforcement Design The Theory of Plain Concrete Statical Conditions Geometrical Conditions Virtual Work Constitutive Equations The Theory of Plane Strain for Coulomb Materials Applications Disks Statical Conditions Geometrical Conditions Virtual Work Constitutive Equations Exact Solutions for Isotropic Disks The Effective Compressive Strength of Reinforced Disks General Theory of Lower Bound Solutions Strut and Tie Models Shear Walls Homogenous Reinforcement Solutions Design According to the Elastic Theory Beams Beams in Bending Beams in Shear Beams in Torsion Combined Bending, Shear, and Torsion Slabs Statical Conditions Geometrical Conditions Virtual Work, Boundary Conditions Constitutive Equations Exact Solutions for Isotropic Slabs Upper Bound Solutions for Isotropic Slabs Lower Bound Solutions Orthotropic Slabs Analytical Optimum Reinforcement Solutions Numerical Methods Membrane Action Punching Shear of Slabs Introduction Internal Loads or Columns Edge and Corner Loads Concluding Remarks Shear in Joints Introduction Analysis of Joints by Plastic Theory Strength of Different Types of Joints The Bond Strength of Reinforcing Bars Introduction The Local Failure Mechanism Failure Mechanisms Analysis of Failure Mechanisms Assessment of Anchor and Splice Strength Effect of Transverse Pressure and Support Reaction Effect of Transverse Reinforcement Concluding Remarks

Equilibrium Conformations of Concentric-tube Continuum Robots

Abstract: Robots consisting of several concentric, preshaped, elastic tubes can work dexterously in narrow, constrained, and/or winding spaces, as are commonly found in minimally invasive surgery. Previous models of these “active cannulas” assume piecewise constant precurvature of component tubes and neglect torsion in curved sections of the device. In this paper we develop a new coordinate-free energy formulation that accounts for general preshaping of an arbitrary number of component tubes, and which explicitly includes both bending and torsion throughout the device. We show that previously reported models are special cases of our formulation, and then explore in detail the implications of torsional flexibility for the special case of two tubes. Experiments demonstrate that this framework is more descriptive of physical prototype behavior than previous models1 it reduces model prediction error by 82% over the calibrated bending-only model, and 17% over the calibrated transmissional torsion model in a set of experiments.

Temperature-Insensitive Torsion Sensor With Enhanced Sensitivity by Use of a Highly Birefringent Photonic Crystal Fiber

Abstract: We report on enhanced torsion sensitivity by using a highly birefringent photonic crystal fiber (HB-PCF)-based Sagnac interferometer. In order to increase the torsion sensitivity, we introduced an anisotropic microstructure into the cross section of an HB-PCF by enlarging the size of air holes of one row. This can result in a high birefringence of the order of 10-3 and low sensitivities to bending and temperature. The torsion sensitivity was measured to be high with ~0.06 nm/°.

Strengthening RC T-Beams Subjected to Combined Torsion and Shear Using FRP Fabrics: Experimental Study

Abstract: In several cases of loading and geometrical configurations, flexure beams, and girders are subjected to combined shear and torsion. Failure of a structural element under combined shear and torsion is brittle in nature. Externally bonded fiber-reinforced polymer FRP fabrics are currently being studied and used for the rehabilitation, repair, and retrofit of concrete structure. The objective of this study is to investigate the strengthening techniques for T-beams subjected to combined shear and torsion. Six half-scale beams—two control specimen and four strengthened beams—were constructed and tested using a specially designed test setup that subjects the beam to combined shear and torsion with different ratios. Four strengthening techniques using carbon FRPs were tested. The experimental results were reported and analyzed to assess the effectiveness of the proposed strengthening techniques. An innovative strengthening technique namely the extended U-jacket showed promising results in terms of strength and ductility while being quite feasible for strengthening. Future areas of research are being outlined.

Transversely isotropic nonlinear magneto-active elastomers

Abstract: Magneto-active elastomers are smart materials composed of a rubber-like matrix material containing a distribution of magneto active particles. The large elastic deformations possible in the rubber-like matrix allow the mechanical properties of magneto-active elastomers to be changed significantly by the application of external magnetic fields. In this paper, we provide a theoretical basis for the description of the nonlinear properties of a particular class of these materials, namely transversely isotropic magneto-active elastomers. The transversely isotropic character of these materials is produced by the application of a magnetic field during the curing process, when the magneto active particles are distributed within the rubber. As a result the particles are aligned in chains that generated a preferred direction in the material. Available experimental data suggest that this enhances the stiffness of the material in the presence of an external magnetic field by comparison with the situation in which no external field is applied during curing, which leads to an essentially random (isotropic) distribution of particles. Herein, we develop a general form of the constitutive law for such magnetoelastic solids. This is then used in the solution of two simple problems involving homogeneous deformations, namely simple shear of a slab and simple tension of a cylinder. Using these results and the experimental available data we develop a prototype constitutive equation, which is used in order to solve two boundary-value problems involving non-homogeneous deformations—the extension and inflation of a circular cylindrical tube and the extension and torsion of a solid circular cylinder.

Universal plot for hardness variation in pure metals processed by high-pressure Torsion

Abstract: High purity Al (99%), Al (99.99%), Au (99.999%), Ag (99.99%), Cu (99.99%), Pt (99.9%), Ni (99.996%), Fe (99.96%), Co (99.99%), Ti (99.4%), Zr (99.9%), Hf (99.99%), Cu-30%Zn and commercial purity Al (99%) were received in the form of rods and/or plates. The as-received specimens were annealed for 1 hour under an argon atmosphere at a temperature of 773 K for Al, 793 K for Ag and Cu30%Zn, 873 K for Au and Cu, 973 K for Ni, 1073 K for Co, Ti and Zr, 1273 K for Fe and Hf, and 1323 for Pt. The annealed specimens were cut to discs with 4 and/or 10 mm diameters and 0.8 mm thickness and to rings with inner and outer diameters of 24 and 30 mm and 0.8 mm thickness using a wire-cutting electric discharge machine. HPT was carried out on the annealed discs using the facilities described earlier. 12,15) The disc samples with 4 and 10 mm diameters were processed under a selected pressure in the range of 14–40 GPa and 0.6–6 GPa, respectively, for N ¼ 1{20 revolutions with a rotation speed of ! ¼ 0:2 or 0.5 rpm. The ring samples were processed for one revolution under a pressure in the range of 0.6–2 GPa with ! ¼ 0:5 rpm. It should be noted that the pressure was calculated by dividing the applied load by the area of the central hole or circular groove on the anvil. The samples after HPT were polished to a mirror-like surface and evaluated in terms of Vickers microhardness and

Fingertip Moisture is Optimally Modulated during Object Manipulation.

Abstract: Coordination between the normal force exerted by fingers on a held object and the tangential constraints at the fingertips helps to successfully manipulate objects. It is well established that the ...

On characterization of timelike horizontal biharmonic curves in the lorentzian heisenberg group heis3

Abstract: In this paper, we study energy of time-like horizontal biharmonic curves in the Lorentzian Heisenberg group Heis 3 . We characterize the biharmonic curves in terms of their curvature and torsion. We prove that all of the biharmonic curves are helices. Finally, we study the mechanics of biharmonic curves and provide conditions for energy of horizontal biharmonic curves.

Nonlinear buckling of torsion-loaded functionally graded cylindrical shells in thermal environment

Abstract: Based on the nonlinear large deflection theory of cylindrical shells, this paper deals with the nonlinear buckling problem of functionally graded cylindrical shells under torsion load by using the energy method and the nonlinear strain–displacement relations of large deformation. The material properties of the functionally graded shells vary smoothly through the shell thickness according to a power law distribution of the volume fraction of the constituent materials. Meanwhile, on the base of taking the temperature-dependent material properties into account, various effects of external thermal environment on the critical state of the shell are also investigated. Numerical results show various effects of the inhomogeneous parameter, the dimensional parameters and external thermal environment on nonlinear buckling of functionally graded cylindrical shells under torsion. The present theoretical results are verified by those in literature.

Nanotorsional resonator torque magnetometry

Abstract: Magnetic torque is used to actuate nanotorsional resonators, which are fabricated by focused-ion-beam milling of permalloy coated silicon nitride membranes. Optical interferometry is used to measure the mechanical response of two torsion modes at resonance, which is proportional to the magnetization vector of the nanomagnetic volume. By varying the bias magnetic field, the magnetic behavior can be measured with excellent sensitivity (≈108μB) for single magnetic elements.

A visualization of shear strain in processing by high-pressure torsion

Abstract: Optical microscopy was used to examine the shear strain imposed in duplex stainless steel disks during processing by high-pressure torsion (HPT). The results show a double-swirl pattern emerges in the early stages of HPT and the two centres of the swirl move towards the centre of the disk with increasing revolutions. Local shear vortices also develop with increasing numbers of revolutions. At 20 revolutions, there is a uniform shear strain pattern throughout the disk and no local shear vortices.

On the local and global buckling behaviour of angle, T-section and cruciform thin-walled members

Abstract: This paper reports the results of an investigation aimed at providing fresh insight on the mechanics underlying the local and global buckling behaviour of angle, T-section and cruciform thin-walled steel members (columns, beams and beam-columns). Due to the lack of primary warping resistance, members displaying these cross-section shapes possess a minute torsional stiffness and, therefore, are highly susceptible to buckling phenomena involving torsion – moreover, it is often hard to distinguish between torsion and local deformations. Almost all the numerical results presented are obtained by means of Generalised Beam Theory (GBT) analyses and, taking advantage of its unique modal features, it is possible to shed some new light on how to characterise and/or distinguish the local and global buckling modes of the above thin-walled members. Finally, some comments are made concerning the development of a rational and efficient (safe and economic) approach for their design.

Orientation-dependent plasticity in metal nanowires under torsion: twist boundary formation and Eshelby twist.

Abstract: We show that the plastic deformation of nanowires under torsion can be either homogeneous or heterogeneous, regardless of size, depending on the wire orientation. Homogeneous deformation occurs when 110-oriented face-centered-cubic metal wires are twisted, leading to the nucleation of coaxial dislocations, analogous to the Eshelby twist mechanism. Heterogeneous deformation is predicted for 111 and 100 wires under torsion, localized at the twist boundaries. These simulations also reveal the detailed mechanisms of twist boundary formation from dislocation reactions.

Torsional stiffness and weight optimization of a real bus structure

Abstract: In this paper, the structural optimization of a real bus structure is proposed. The proposed optimization has been accomplished by means of genetic algorithms. The structural behavior of the bus structure when subjected to weight and torsion was also analyzed using the finite element method (FEM). The results demonstrate that improved weight and torsional stiffness are achieved with the optimized structure.

A comparison of torque expression between stainless steel, titanium molybdenum alloy, and copper nickel titanium wires in metallic self-ligating brackets

Abstract: Objective: The force moment providing rotation of the tooth around the x-axis (buccal-lingual) is referred to as torque expression in orthodontic literature. Many factors affect torque expression, including the wire material characteristics. This investigation aims to provide an experimental study into and comparison of the torque expression between wire types. Materials and Methods: With a worm-gear–driven torquing apparatus, wire was torqued while a bracket mounted on a six-axis load cell was engaged. Three 0.019 3 0.0195 inch wire (stainless steel, titanium molybdenum alloy [TMA], copper nickel titanium [CuNiTi]), and three 0.022 inch slot bracket combinations (Damon 3MX, In-Ovation-R, SPEED) were compared. Results: At low twist angles (,12 degrees), the differences in torque expression between wires were not statistically significant. At twist angles over 24 degrees, stainless steel wire yielded 1.5 to 2 times the torque expression of TMA and 2.5 to 3 times that of nickel titanium (NiTi). At high angles of torsion (over 40 degrees) with a stiff wire material, loss of linear torque expression sometimes occurred. Conclusions: Stainless steel has the largest torque expression, followed by TMA and then NiTi. (Angle Orthod. 2010;80:884–889.)

Symmetries of the Dirac operator with skew-symmetric torsion

Abstract: In this paper, we consider the symmetries of the Dirac operator derived from a connection with skew-symmetric torsion, ∇T. We find that the generalized conformal Killing–Yano tensors give rise to symmetry operators of the massless Dirac equation, provided an explicitly given anomaly vanishes. We show that this gives rise to symmetries of the Dirac operator in the case of strong Kahler with torsion (KT) and strong hyper-Kahler with torsion (HKT) manifolds.

Modeling and simulation of strain-induced phase transformations under compression and torsion in a rotational diamond anvil cell

Abstract: Strain-induced phase transformations PTs under compression and torsion in rotational diamond anvils are simulated using a finite-element approach. Results are obtained for three ratios of yield strengths of lowpressure and high-pressure phases and are compared with those for the compression without torsion from Levitas and Zarechnyy Phys. Rev. B 82, 174123 2010. Various experimental effects are reproduced, including a pressure self-multiplication effect, plateau at pressure distribution at the diffuse interface, simultaneous occurrence of direct and reverse PTs, and irregular stress distribution for PT to a weaker phase. The obtained results change the fundamental understanding of strain-induced PT in terms of interpretation of experimental measurements and the extracting of information on material processes from sample behavior. Intense radial plastic flow moves the high-pressure phase to the low-pressure region, which may lead to misinterpretation of measurements. Various interpretations based on a simplified equilibrium equation for example, about zero yield strength of phase mixture and hydrostatic conditions during PT appears to be wrong because of inapplicability of this equation for cases with large gradients of phase concentration and yield strength. The approach developed represents a tool for designing experiments for different purposes and for controlling PTs, and it opens unexpected ways to extract material information by combining simulation and experiment.