Topic

# Torsion (mechanics)

About: Torsion (mechanics) is a(n) research topic. Over the lifetime, 16511 publication(s) have been published within this topic receiving 189016 citation(s). The topic is also known as: mechanical torsion.

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TL;DR: A new force field, which is termed Amber ff99SB‐ILDN, exhibits considerably better agreement with the NMR data and is validated against a large set of experimental NMR measurements that directly probe side‐chain conformations.

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.

3,670 citations

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Abstract: In this paper we develop a new constitutive law for the description of the (passive) mechanical response of arterial tissue. The artery is modeled as a thick-walled nonlinearly elastic circular cylindrical tube consisting of two layers corresponding to the media and adventitia (the solid mechanically relevant layers in healthy tissue). Each layer is treated as a fiber-reinforced material with the fibers corresponding to the collagenous component of the material and symmetrically disposed with respect to the cylinder axis. The resulting constitutive law is orthotropic in each layer. Fiber orientations obtained from a statistical analysis of histological sections from each arterial layer are used. A specific form of the law, which requires only three material parameters for each layer, is used to study the response of an artery under combined axial extension, inflation and torsion. The characteristic and very important residual stress in an artery in vitro is accounted for by assuming that the natural (unstressed and unstrained) configuration of the material corresponds to an open sector of a tube, which is then closed by an initial bending to form a load-free, but stressed, circular cylindrical configuration prior to application of the extension, inflation and torsion. The effect of residual stress on the stress distribution through the deformed arterial wall in the physiological state is examined. The model is fitted to available data on arteries and its predictions are assessed for the considered combined loadings. It is explained how the new model is designed to avoid certain mechanical, mathematical and computational deficiencies evident in currently available phenomenological models. A critical review of these models is provided by way of background to the development of the new model.

2,617 citations

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01 Jun 1985

Abstract: 1. Orientation, Review of Elementary Mechanics of Materials. 2. Stress, Principal Stresses, Strain Energy. 3. Failure and Failure Criteria. 4. Applications of Energy Methods. 5. Beams on an Elastic Foundation. 6. Curved Beams. 7. Elements of Theory of Elasticity. 8. Pressurized Cylinders and Spinning Disks. 9. Torsion. 10. Unsymmetric Bending and Shear Center. 11. Plasticity in Structural Members. Collapse Analysis. 12. Plate Bending. 13. Shells of Revolution with Axisymmetric Loads. 14. Buckling and Instability. References. Index.

1,200 citations

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Abstract: The vacuum stress between closely spaced conducting surfaces, due to the modification of the zero-point fluctuations of the electromagnetic field, has been conclusively demonstrated. The measurement employed an electromechanical system based on a torsion pendulum. Agreement with theory at the level of 5% is obtained.

1,150 citations

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Abstract: The conditions for spacetime supersymmetry of the heterotic superstring in backgrounds with arbitrary metric, torsion, Yang-Mills and dilaton expectation values are determined using the sigma model approach. The resulting equations are explicitly solved for the torsion and dilaton fields, and the remaining equations cast in a simple form. Previously unnoticed topological obstructions to solving these equations are found. The equations are shown to agree to leading order in perturbation theory with those derived in a field theory approach, provided one considers a more general ansatz than in previous analyses by allowing for a warp factor for the metric. Exact solutions with non-zero torsion are found, indicating a new class of finite sigma models. These solutions break the Eχ ⊗ Eχ or SO(32) gauge group down to a large variety of subgroups. Orbifolds with torsion are constructed. A perturbative analysis of the equations indicates a class of solutions whose existence has been recently argued for on other grounds. Brief comments are made on the implications for phenomenology.

1,095 citations