Showing papers on "Torsion (mechanics) published in 2013"

Exact charged black-hole solutions in D-dimensional f (T) gravity: torsion vs curvature analysis

Abstract: We extract exact charged black-hole solutions with flat transverse sections in the framework of D-dimensional Maxwell-f (T) gravity, and we analyze the singularities and horizons based on both torsion and curvature invariants. Interestingly enough, we find that in some particular solution subclasses there appear more singularities in the curvature scalars than in the torsion ones. This difference disappears in the uncharged case, or in the case where f (T) gravity becomes the usual linear-in-T teleparallel gravity, that is General Relativity. Curvature and torsion invariants behave very differently when matter fields are present, and thus f (R) gravity and f (T) gravity exhibit different features and cannot be directly re-casted each other.

Anomalous plasticity in the cyclic torsion of micron scale metallic wires.

Abstract: The plasticity of micron scale Cu and Au wires under cyclic torsion is investigated for the first time by using a torsion balance technique. In addition to a size effect, a distinct Bauschinger effect and an anomalous plastic recovery, wherein reverse plasticity even occurs upon unloading, are unambiguously revealed. The Bauschinger effect and plastic recovery have been observed in molecular dynamics and discrete dislocation dynamics simulations of ideal single-crystal wires; the results here are an excellent confirmation that these effects also occur in experiment in nonideal polycrystalline wires. A physical model consistent with the simulations is described in which the geometrically necessary dislocations induced by the nonuniform deformation in torsion play the key role in these anomalous plastic behaviors.

Displacement Theories for In-Flight Deformed Shape Predictions of Aerospace Structures

Abstract: Displacement theories are developed for a variety of structures with the goal of providing real-time shape predictions for aerospace vehicles during flight. These theories are initially developed for a cantilever beam to predict the deformed shapes of the Helios flying wing. The main structural configuration of the Helios wing is a cantilever wing tubular spar subjected to bending, torsion, and combined bending and torsion loading. The displacement equations that are formulated are expressed in terms of strains measured at multiple sensing stations equally spaced on the surface of the wing spar. Displacement theories for other structures, such as tapered cantilever beams, two-point supported beams, wing boxes, and plates also are developed. The accuracy of the displacement theories is successfully validated by finite-element analysis and classical beam theory using input-strains generated by finite-element analysis. The displacement equations and associated strain-sensing system (such as fiber optic sensors) create a powerful means for in-flight deformation monitoring of aerospace structures. This method serves multiple purposes for structural shape sensing, loads monitoring, and structural health monitoring. Ultimately, the calculated displacement data can be visually displayed to the ground-based pilot or used as input to the control system to actively control the shape of structures during flight.

Experimental research on concrete filled steel tube columns under combined compression-bending-torsion cyclic load

Abstract: Based on the force–displacement mixed control quasi-static test on eight CFST columns subjected to combined compression, bending and torsion cyclic load, the mechanical behavior of CFST columns with various section types, bending moment to torsion moment ratios and axial load levels was studied. The test results showed that the hysteretic curves of CFST columns under combined compression, flexure and torsion are plump due to the good seismic behavior and the ductility was also good. But for rectangular CFST columns with high bending moment to torsion moment ratio, the strength degradation was observed due to the local buckling of the steel plate at the bottom. The torsion capacity of CFST columns would be reduced by the bending moment. The plane section assumption of axial strain of CFST columns could be satisfied. The shear strain of the steel tube has good linear relationship with the rotation angle of the section when CFST columns subjected to combined compression, flexure and torsion. Based on the test results and literatures available, the mechanism of CFST columns was qualitatively analyzed.

Torsion and curvature effects on fluid flow in a helical annulus

Abstract: In this article incompressible viscous flow in a helical annulus is studied numerically. A second order finite difference method based on the projection algorithm is used to solve the governing equations written in the helical coordinate system. Considering the hydrodynamically fully developed flow, the effects of different physical parameters such as aspect ratio, torsion, curvature and Reynolds number on the flow field are investigated in detail. The numerical results obtained indicate that a decrease in the aspect ratio and torsion number leads to the increase of the friction factor at a given Dean number.

Analogies between Kirchhoff plates and Saint-Venant beams under torsion

Abstract: Torsion of linearly elastic homogeneous and orthotropic Saint-Venant beams is based on the solution of a Neumann-like boundary value problem for the cross-sectional warping. Equivalence of differential conditions of elastic equilibrium for the beam twist stresses in terms of warping function and for the bending-twisting moments in a Kirchhoff plate is assessed. The analogy provides new exact solutions in the theory of thin plates. Examples are developed for composite plates with no kinematic boundary constraints. Elliptic and equilateral triangle shapes provide useful benchmarks for computational mechanics.

The peak stress method for fatigue strength assessment of tube-to-flange welded joints under torsion loading

Abstract: It was shown in previous papers that in plane problems the elastic tangential stress and the elastic shear stress evaluated at the weld toe or at the root of fillet-welded joints by means of a finite element analysis, with a well-defined pattern of elements, are proportional to the mode I and mode II Notch Stress Intensity Factors (NSIFs), respectively. On the basis of such properties, the so-called Peak Stress Method (PSM) is a simplified, finite element-oriented application of the N-SIF approach to fatigue analysis of fillet-welded joints with un-machined weld seams. In the present paper, the PSM is extended to torsional loading conditions, which induce mode III stresses at the weld toe and at the weld root. First, it is shown that the finite value of the elastic anti-plane shear stress evaluated at the weld toe by means of a finite element analysis is directly proportional to the mode III N-SIF. Afterwards, taking advantage of the strain energy density criterion, an equivalent local stress is derived. Finally, a synthesis of experimental results of fatigue tests on tube-to-flange fillet welded joints subject to torsion loading and failing either from the weld toe or from the weld root is presented.

Experimental microindentation of pure copper subjected to severe plastic deformation by combined tension-torsion

Abstract: The samples of commercial pure copper bars were severely plastic-deformed by different pretension and subsequently by different turns of torsion. Microstructure evolution of deformed samples was characterized using optical microscopy (OM). Longitudinal observations showed the formation of gradient microstructure with the maximum grain refinement in the surface layer of bars. Microindentation tests were conducted in the indenter load range of 100–450 mN and at a loading rate of 9.6841 mN/s. The hardness and Young's modulus were analyzed in the transversal section of samples using the method of Oliver and Pharr. Based on the strain gradient plasticity theory, the densities of statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) were calculated according to the Taylor dislocation model, respectively. Experimental results reveal that the hardness and Young's modulus decrease with the indenter load increasing. Indentation size effect on the hardness is relatively more significant than the counterpart of Young's modulus. In a certain pretension condition, the hardness values increase fast and subsequently slowly with the increase of applied torsion turns. The torsion process seems to effectively increase the material hardening due to the introduced simple shear strain. Compared to the hardness analysis, Young's modulus gently increases with the increase of torsion turns. That is due to the saturation of mechanical damage and/or crack introduced by severe deformation. In the combined tension–torsion loading, both densities of GNDs and SSDs increase with the increase of torsion turns. However, the SSDs density is higher than that of GNDs in spite of the same order magnitude. The torsional shear strain enhances the non-uniform deformation and stimulates the propagation of GNDs.

Multiplectoneme phase of double-stranded DNA under torsion

Abstract: We use the wormlike chain model to study supercoiling of DNA under tension and torque. The model reproduces experimental data for a broad range of forces, salt concentrations, and contour lengths. We find a plane of first-order phase transitions ending in a smeared-out line of critical points, the multiplectoneme phase, which is characterized by a fast twist-mediated diffusion of plectonemes and a torque that rises after plectoneme formation with increasing linking number. The discovery of this phase at the same time resolves the discrepancies between existing models and experiment.