Showing papers on "Torsion (mechanics) published in 2012"
TL;DR: The results indicate that the revised CHARMM 36 parameters represent an improved model for the modeling and simulation studies of proteins, including studies of protein folding, assembly and functionally relevant conformational changes.
Abstract: While the quality of the current CHARMM22/CMAP additive force field for proteins has been demonstrated in a large number of applications, limitations in the model with respect to the equilibrium between the sampling of helical and extended conformations in folding simulations have been noted. To overcome this, as well as make other improvements in the model, we present a combination of refinements that should result in enhanced accuracy in simulations of proteins. The common (non Gly, Pro) backbone CMAP potential has been refined against experimental solution NMR data for weakly structured peptides, resulting in a rebalancing of the energies of the α-helix and extended regions of the Ramachandran map, correcting the α-helical bias of CHARMM22/CMAP. The Gly and Pro CMAPs have been refitted to more accurate quantum-mechanical energy surfaces. Side-chain torsion parameters have been optimized by fitting to backbone-dependent quantum-mechanical energy surfaces, followed by additional empirical optimization targeting NMR scalar couplings for unfolded proteins. A comprehensive validation of the revised force field was then performed against data not used to guide parametrization: (i) comparison of simulations of eight proteins in their crystal environments with crystal structures; (ii) comparison with backbone scalar couplings for weakly structured peptides; (iii) comparison with NMR residual dipolar couplings and scalar couplings for both backbone and side-chains in folded proteins; (iv) equilibrium folding of mini-proteins. The results indicate that the revised CHARMM 36 parameters represent an improved model for the modeling and simulation studies of proteins, including studies of protein folding, assembly and functionally relevant conformational changes.
3,421 citations
TL;DR: In this article, the authors used finite element modeling with an analytical component to evaluate the thermal characteristics in quasi-constrained high pressure torsion (HPT) processing.
264 citations
TL;DR: In this article, the size dependence in the torsional response of micro-sized polycrystalline copper wires is investigated experimentally using a novel automated torsion balance.
174 citations
TL;DR: In this article, a geometric theory of nonlinear solids with distributed dislocations is presented, where the material manifold is a Weitzenbock manifold, that is, a manifold with a flat affine connection with torsion but vanishing non-metricity.
Abstract: We present a geometric theory of nonlinear solids with distributed dislocations. In this theory the material manifold—where the body is stress free—is a Weitzenbock manifold, that is, a manifold with a flat affine connection with torsion but vanishing non-metricity. Torsion of the material manifold is identified with the dislocation density tensor of nonlinear dislocation mechanics. Using Cartan’s moving frames we construct the material manifold for several examples of bodies with distributed dislocations. We also present non-trivial examples of zero-stress dislocation distributions. More importantly, in this geometric framework we are able to calculate the residual stress fields, assuming that the nonlinear elastic body is incompressible. We derive the governing equations of nonlinear dislocation mechanics covariantly using balance of energy and its covariance.
147 citations
TL;DR: In this paper, a postbuckling analysis of a bridge is presented, where simple analytical expressions are obtained for the critical load at the onset of buckling, and for the maximum bending, torsion (shear) and principal strains in the structure during post-buckling.
Abstract: A versatile strategy for fabricating stretchable electronics involves controlled buckling of bridge structures in circuits that are configured into open, mesh layouts (i.e. islands connected by bridges) and bonded to elastomeric substrates. Quantitative analytical mechanics treatments of the responses of these bridges can be challenging, due to the range and diversity of possible motions. Koiter (1945) pointed out that the postbuckling analysis needs to account for all terms up to the 4th power of displacements in the potential energy. Existing postbuckling analyses, however, are accurate only to the 2nd power of displacements in the potential energy since they assume a linear displacement–curvature relation. Here, a systematic method is established for accurate postbuckling analysis of beams. This framework enables straightforward study of the complex buckling modes under arbitrary loading, such as lateral buckling of the island-bridge, mesh structure subject to shear (or twist) or diagonal stretching observed in experiments. Simple, analytical expressions are obtained for the critical load at the onset of buckling, and for the maximum bending, torsion (shear) and principal strains in the structure during postbuckling.
127 citations
TL;DR: A theory for the simplest such structure: an annular circular strip that is folded along a central circular curve to form a three-dimensional buckled structure driven by geometrical frustration is presented.
Abstract: Folding a sheet of paper along a curve can lead to structures seen in decorative art and utilitarian packing boxes. Here we present a theory for the simplest such structure: an annular circular strip that is folded along a central circular curve to form a three-dimensional buckled structure driven by geometrical frustration. We quantify this shape in terms of the radius of the circle, the dihedral angle of the fold, and the mechanical properties of the sheet of paper and the fold itself. When the sheet is isometrically deformed everywhere except along the fold itself, stiff folds result in creases with constant curvature and oscillatory torsion. However, relatively softer folds inherit the broken symmetry of the buckled shape with oscillatory curvature and torsion. Our asymptotic analysis of the isometrically deformed state is corroborated by numerical simulations that allow us to generalize our analysis to study structures with multiple curved creases.
115 citations
TL;DR: This paper reviews current clinical applications and shows how torsion can give insights into LV mechanics and the influence of LV geometry and myocyte fiber architecture on cardiac function, and provides recommendations for CMR measurement protocols.
Abstract: Recently there has been considerable interest in LV torsion and its relationship with symptomatic and pre-symptomatic disease processes. Torsion gives useful additional information about myocardial tissue performance in both systolic and diastolic function. CMR assessment of LV torsion is simply and efficiently performed. However, there is currently a wide variation in the reporting of torsional motion and the procedures used for its calculation. For example, torsion has been presented as twist (degrees), twist per length (degrees/mm), shear angle (degrees), and shear strain (dimensionless). This paper reviews current clinical applications and shows how torsion can give insights into LV mechanics and the influence of LV geometry and myocyte fiber architecture on cardiac function. Finally, it provides recommendations for CMR measurement protocols, attempts to stimulate standardization of torsion calculation, and suggests areas of useful future research.
104 citations
TL;DR: In this paper, the traverse force and torque during friction stir welding are computed using a three-dimensional heat transfer and viscoplastic material flow model considering temperature and strain rate-dependent flow stress of the work-piece material.
Abstract: Although friction stir welding (FSW) is now widely used for the welding of aluminum and other soft alloys, premature tool failure limits its application to hard alloys such as steels and titanium alloys. The tool pin, the weakest component of the tool, experiences severe stresses at high temperatures due to both bending moment and torsion. It is shown that the optimum tool pin geometry can be determined from its load bearing capacity for a given set of welding variables and tool and work-piece materials. The traverse force and torque during friction stir welding are computed using a three-dimensional heat transfer and viscoplastic material flow model considering temperature and strain rate-dependent flow stress of the work-piece material. These computed values are used to determine the maximum shear stress experienced by the tool pin due to bending moment and torsion for various welding variables and tool pin dimensions. It is shown that a tool pin with smaller length and larger diameter will be able to sustain more stress than a longer pin with smaller diameter. The proposed methodology is used to explain the failure and deformation of the tool pin in independent experiments for the welding of both L80 steel and AA7075 alloy. The results demonstrate that the short tool life in a typical FSW of steels is contributed by low values of factor of safety in an environment of high temperature and severe stress.
95 citations
TL;DR: In this article, the authors studied brittle fracture of polycrystalline graphite under torsion loading using axisymmetric specimens weakened by sharp and rounded-tip V-notches.
91 citations
TL;DR: In this article, more than 70 new fracture tests on notched specimens made of polymethyl-methacrylate were carried out at room temperature under torsion loading conditions.
85 citations
TL;DR: In this paper, the beam models are obtained by assuming higher-order (up to fourth) expansions for the unknown displacement variables over the cross-section, which allows bending/torsion modes to be coupled and capture any other vibration modes that require in-plane and warping deformation of the beam sections to be detected.
Abstract: This work extends advanced beam models to carry out a more accurate free-vibration analysis of conventional (straight, or with sweep/dihedral angles) and joined wings. The beam models are obtained by assuming higher-order (up to fourth) expansions for the unknown displacement variables over the cross-section. Higher-order terms permit bending/torsion modes to be coupled and capture any other vibration modes that require in-plane and warping deformation of the beam sections to be detected. Classical beam analyses, based on the Euler- Bernoulli and on Timoshenko beam theories, are obtained as particular cases. Numerical solutions are obtained by using the finite element (FE) method, which permits various boundary conditions and different wing/section geometries to be handled with ease. A comparison with other shell/solid FE solutions is given to examine the beam model. The capability of the beam model to detect bending, torsion, mixed and other vibration modes is shown by considering conventional and joined wings with different beam axis geometries as well as with various sections (compact, plate-type, thin-walled airfoil-type). The accuracy and the limitations of classical beam theories have been highlighted for a number of problems. It has been concluded that the proposed beam model could lead to quasi-three-dimensional dynamic responses of classical and nonclassical beam geometries. It provides better results than classical beam approaches, and it is much more computationally efficient than shell/solid modeling approaches. DOI: 10.1061/(ASCE)AS.1943-5525.0000130. © 2012 American Society of Civil Engineers. CE Database subject headings: Beams; Finite element method; Vibration; Thin-wall structures; Aerospace engineering. Author keywords: Beams; Finite element method; Higher-order theories; Vibration; Thin-walled structures; Aerospace engineering.
TL;DR: An original application of covariance analysis of two-dimensional ion images is demonstrated to reveal strong correlations between specific ejected ionic fragments from Coulomb explosion.
Abstract: We study how the combination of long and short laser pulses can be used to induce torsion in an axially chiral biphenyl derivative (3,5-difluoro-3',5'-dibromo-4'-cyanobiphenyl). A long, with respect to the molecular rotational periods, elliptically polarized laser pulse produces 3D alignment of the molecules, and a linearly polarized short pulse initiates torsion about the stereogenic axis. The torsional motion is monitored in real-time by measuring the dihedral angle using femtosecond time-resolved Coulomb explosion imaging. Within the first 4 picoseconds (ps), torsion occurs with a period of 1.25 ps and an amplitude of 3° in excellent agreement with theoretical calculations. At larger times, the quantum states of the molecules describing the torsional motion dephase and an almost isotropic distribution of the dihedral angle is measured. We demonstrate an original application of covariance analysis of two-dimensional ion images to reveal strong correlations between specific ejected ionic fragments from Coulomb explosion. This technique strengthens our interpretation of the experimental data.
TL;DR: In this article, the authors apply the five-dimensional gravity with T +k{T}^{n} to brane scenario to explore the solutions under a given warp factor, and find that the analytic domain wall solution will be a double-kink solution when the geometric effect of spacetime torsion is strongly enhanced.
Abstract: In this paper we apply the five-dimensional $f(T)$ gravity with $f(T)=T+k{T}^{n}$ to brane scenario to explore the solutions under a given warp factor, and we find that the analytic domain wall solution will be a double-kink solution when the geometric effect of spacetime torsion is strongly enhanced. We also investigate the localization of fermion fields on the split branes corresponding to the double-kink solution.
TL;DR: In this paper, a twin bridge cyclic shear test with in-plane torsion is proposed for the identification of kinematic hardening parameters for metallic sheets, and the results show that the Armstrong-Frederick model fails to capture the cyclic response of the selected materials, especially advanced high strength steels DP600 and TRIP700.
TL;DR: In this article, the sliding-torsional compliance tensor of a Timoshenko beam is evaluated by an energy equivalence with Saint-Venant theory and the relative location of shear and twist centres is investigated for sections of any degree of connectedness.
Abstract: Torsion and shear stress fields of a Saint-Venant beam and the relative location of shear and twist centres are investigated for sections of any degree of connectedness. The sliding-torsional compliance tensor of a Timoshenko beam is evaluated by an energy equivalence with Saint-Venant theory. Accordingly, the mutual sliding-torsional term is shown to depend linearly on the relative position of shear and twist centres and the standard definition of shear centre in a Timoshenko beam is found to be coincident with Saint-Venant twist centre. Coincidence of shear and twist centres is assessed for sections with vanishing Poisson ratio and for open, closed and multi-cell thin-walled cross sections. The eigenvalues of the shear factors tensor and the torsion factor are shown to be greater than unity, with the principal directions of shearing and bending compliances non necessarily coincident for non-symmetric cross sections. Numerical examples are developed to provide evidences of the location of the centres and of the principal shearing directions, for non-symmetric L-shaped cross sections with various thickness ratios.
TL;DR: Based on the quasi-static test on eight CFST columns subjected to pure torsion and compression-torsion cyclic load, the authors of as discussed by the authors showed that the hysteretic curves of CFST column under pure torion and low compression-to-torion cyclical load are very plump, which indicates that CFST Columns have good seismic capacity.
TL;DR: In this paper, a hybrid explicit-implicit approach was used to describe the crack in a 3D XFEM model, where the crack update is realized based on an explicit crack surface mesh which allows an investigation of different propagation criteria.
Abstract: This paper studies propagation criteria in three-dimensional fracture mechanics within the extended finite element framework (XFEM). The crack in this paper is described by a hybrid explicit–implicit approach as proposed in Fries and Baydoun (Int J Numer Methods Eng, 2011). In this approach, the crack update is realized based on an explicit crack surface mesh which allows an investigation of different propagation criteria. In contrast, for the computation of the displacements, stresses and strains by means of the XFEM, an implicit description by level set functions is employed. The maximum circumferential stress criterion, the maximum strain energy release rate criterion, the minimal strain energy density criterion and the material forces criterion are realized. The propagation paths from different criteria are studied and compared for asymmetric bending, torsion, and combined bending and torsion test cases. It is found that the maximum strain energy release rate and maximum circumferential stress criterion show the most favorable results.
TL;DR: In this paper, the effect of the volumetric steel fiber content, fiber aspect ratio, and longitudinal reinforcement on the failure of steel fiber reinforced concrete (R/C) beams was investigated.
TL;DR: In this paper, a semi-infinite crack steadily propagates in an elastic solid with microstructures subject to antiplane loading applied on the crack surfaces, where the loading is moving with the same constant velocity as that of the crack tip.
TL;DR: In this article, a 3D constitutive model for shape memory polymers is presented, which is based on an additive decomposition of the strain into four p-means.
Abstract: In this article, satisfying the second law of thermodynamics, we present a 3D constitutive model for shape memory polymers. The model is based on an additive decomposition of the strain into four p...
TL;DR: In this paper, a finite element modeling (FEM) was used to simulate processing by high-pressure torsion under quasi-constrained conditions using three different material conditions: strain-hardening, perfect-plastic, and flow-softening.
Abstract: Finite-element modeling (FEM) was used to simulate processing by high-pressure torsion under quasi-constrained conditions using three different material conditions: strain-hardening, perfect-plastic, and flow-softening. The results show there is a tendency for flow localization during processing and this becomes more obvious during the processing of perfect-plastic, and flow-softening materials or when processing samples having high thickness to diameter ratios. The analysis demonstrates the effect of the material condition, the disk aspect ratio and the effect of friction between the disks and the anvil walls. It is demonstrated that the predictions from FEM correlate well with published experimental results.
TL;DR: In this article, a finite element method was used to analyze the deformation behavior of high-pressure torsion (HPT) in a disk and found that the effective strain and strain rate developed increase from the center to the edge of the HPT-processed disk; however, they decrease at the surface corner of the disk due to the vertical wall constraint under high pressure, resulting in a stagnant region termed the dead metal zone.
TL;DR: The method described here may be useful for non‐invasively assessing the degree of torsion in studies of the evolution and biomechanics of the shoulder and arm, and for testing hypotheses about the etiology of repetitive stress injuries among athletes and others who throw frequently.
Abstract: Several recent studies have found that throwing athletes typically have lower humeral torsion (retroversion) and a greater range of external rotation at the shoulder than non-athletes. How these two parameters are related is debated. This study uses data from a sample of both throwers and non-throwers to test a new model that predicts torsion values from a range of motion data. The model proposes a series of predicted regressions which can help provide new insight into the factors affecting rotational range of motion at the shoulder. Humeral torsion angles were measured from computed tomography scans collected from 25 male subjects. These values are compared to predicted torsion values for the same subjects calculated from both kinematic and goniometric range-of-motion data. Results show that humeral torsion is negatively correlated (goniometric: r = -0.409, P = 0.047; kinematic: r = -0.442, P = 0.035) with external rotational range of motion and positively correlated (goniometric: r = 0.741, P < 0.001; kinematic: r = 0.559, P = 0.006) with internal rotational range of motion. The predicted torsion values are highly correlated (goniometric: r = 0.815, P < 0.001; kinematic: r = 0.617, P = 0.006) with actual torsion values. Deviations in the data away from predicted equations highlight significant differences between high torsion and low torsion individuals that may have significant functional consequences. The method described here may be useful for non-invasively assessing the degree of torsion in studies of the evolution and biomechanics of the shoulder and arm, and for testing hypotheses about the etiology of repetitive stress injuries among athletes and others who throw frequently.
TL;DR: In this paper, the responses of shape memory polymer (SMP) helical springs under axial force are studied both analytically and numerically, and the 3D phenomenological constitutive model for SMPs is also reduced to the 1D shear case.
Abstract: In this paper, the responses of shape memory polymer (SMP) helical springs under axial force are studied both analytically and numerically. In the analytical solution, we first derive the response of a cylindrical tube under torsional loadings. This solution can be used for helical springs in which both the curvature and pitch effects are negligible. This is the case for helical springs with large ratios of the mean coil radius to the cross sectional radius (spring index) and also small pitch angles. Making use of this solution simplifies the analysis of the helical springs to that of the torsion of a straight bar with circular cross section. The 3D phenomenological constitutive model recently proposed for SMPs is also reduced to the 1D shear case. Thus, an analytical solution for the torsional response of SMP tubes in a full cycle of stress-free strain recovery is derived. In addition, the curvature effect is added to the formulation and the SMP helical spring is analyzed using the exact solution presented for torsion of curved SMP tubes. In this modified solution, the effect of the direct shear force is also considered. In the numerical analysis, the 3D constitutive equations are implemented in a finite element program and a full cycle of stress-free strain recovery of an SMP (extension or compression) helical spring is simulated. Analytical and numerical results are compared and it is shown that the analytical solution gives accurate stress distributions in the cross section of the helical SMP spring besides the global load–deflection response. Some case studies are presented to show the validity of the presented analytical method.
TL;DR: In this article, the power spectral density (PSD) function of the equivalent stress together with the known spectral method for estimating the probability density function of stress amplitudes included in random loading were used.
TL;DR: In this article, a three dimensional finite element model of PC box-girder with corrugated steel webs taking material nonlinearity into consideration is established to investigate the behavior under pure torsion.
Abstract: A three dimensional finite element model of PC box-girder with corrugated steel webs taking material nonlinearity into consideration is established to investigate the behavior under pure torsion. The torque–twist curves and ultimate torsional strength predicted by FEM show good agreement with test data. Results obtained from finite element model indicate that specimens go through three stages subjected to pure torsion and the shear flow in concrete top or bottom flange is not equal to that in corrugated steel webs. Parametric study is carried out including effect of corrugation, effect of thickness of web, effect of strength of concrete, and effect of prestressed tendons. It is shown that the ultimate torsional strength of specimens is in linear proportion to shear modulus and thickness of corrugated steel webs and to compressive strength of concrete.
TL;DR: In this paper, the use of simple kinematic models to simulate the torsion warping restraint and transmission at thin-walled frame joints in the context of beam finite element structural analysis is presented.
TL;DR: In this article, the authors investigate the behavior of a crack lying at the interface between dissimilar materials with microstructure undergoing antiplane deformations and show that the stress singularity at the crack tip is strongly influenced by the microstructural parameters and it may or may not show oscillatory behavior depending on the ratio between the characteristic lengths.
TL;DR: In this article, it was shown that the known elastic properties of helium in the torsion rod can explain the observed TO amplitude dependence (which has been interpreted as a critical velocity) and the TO dissipation peak.
Abstract: A number of recent experiments have used torsional oscillators to study the behavior of solid helium. The oscillator frequencies increased at temperatures below 200 mK, an effect attributed to decoupling of a fraction of the helium mass – the signature of a “supersolid” phase. However, helium’s shear modulus also increases below 200 mK and the frequency of a torsional oscillator depends on its elastic properties, as well as on its inertia. In many experiments helium is introduced via a hole in the torsion rod, where its shear modulus contributes to the stiffness of the rod. In oscillators with relatively large torsion rod holes, changes in the helium’s shear modulus could produce the entire low temperature frequency shifts that have been interpreted as mass decoupling. For these oscillators we also find that the known elastic properties of helium in the torsion rod can explain the observed TO amplitude dependence (which has been interpreted as a critical velocity) and the TO dissipation peak. However, in other oscillators these elastic effects are small and the observed frequency changes must have a different origin.
30 Aug 2012-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the importance of optimising processing temperature to enhance grain refinement in high-pressure torsion processing has been demonstrated, where a solution-treated Al-Mg-Si (6060) alloy was processed at ambient temperature, 100°C and 180°C.
Abstract: Ultrafine-grained structures were obtained in a solution-treated Al–Mg–Si (6060) alloy processed by high-pressure torsion at ambient temperature, 100 °C and 180 °C. Processing at 100 °C provided the highest hardness and the smallest average grain size because of the enhanced segregation of solutes at grain boundaries which hindered grain growth during the processing. This research demonstrated the importance of optimising processing temperature to enhance grain refinement in high-pressure torsion processing.