Showing papers on "Torsion (mechanics) published in 2014"

Torsion and Antero-Posterior Bending in the In Vivo Human Tibia Loading Regimes during Walking and Running

Abstract: Bending, in addition to compression, is recognized to be a common loading pattern in long bones in animals. However, due to the technical difficulty of measuring bone deformation in humans, our current understanding of bone loading patterns in humans is very limited. In the present study, we hypothesized that bending and torsion are important loading regimes in the human tibia. In vivo tibia segment deformation in humans was assessed during walking and running utilizing a novel optical approach. Results suggest that the proximal tibia primarily bends to the posterior (bending angle: 0.15°–1.30°) and medial aspect (bending angle: 0.38°–0.90°) and that it twists externally (torsion angle: 0.67°–1.66°) in relation to the distal tibia during the stance phase of overground walking at a speed between 2.5 and 6.1 km/h. Peak posterior bending and peak torsion occurred during the first and second half of stance phase, respectively. The peak-to-peak antero-posterior (AP) bending angles increased linearly with vertical ground reaction force and speed. Similarly, peak-to-peak torsion angles increased with the vertical free moment in four of the five test subjects and with the speed in three of the test subjects. There was no correlation between peak-to-peak medio-lateral (ML) bending angles and ground reaction force or speed. On the treadmill, peak-to-peak AP bending angles increased with walking and running speed, but peak-to-peak torsion angles and peak-to-peak ML bending angles remained constant during walking. Peak-to-peak AP bending angle during treadmill running was speed-dependent and larger than that observed during walking. In contrast, peak-to-peak tibia torsion angle was smaller during treadmill running than during walking. To conclude, bending and torsion of substantial magnitude were observed in the human tibia during walking and running. A systematic distribution of peak amplitude was found during the first and second parts of the stance phase.

ff14ipq: A Self-Consistent Force Field for Condensed-Phase Simulations of Proteins

Abstract: We present the ff14ipq force field, implementing the previously published IPolQ charge set for simulations of complete proteins. Minor modifications to the charge derivation scheme and van der Waals interactions between polar atoms are introduced. Torsion parameters are developed through a generational learning approach, based on gas-phase MP2/cc-pVTZ single-point energies computed of structures optimized by the force field itself rather than the quantum benchmark. In this manner, we sacrifice information about the true quantum minima in order to ensure that the force field maintains optimal agreement with the MP2/cc-pVTZ benchmark for the ensembles it will actually produce in simulations. A means of making the gas-phase torsion parameters compatible with solution-phase IPolQ charges is presented. The ff14ipq model is an alternative to ff99SB and other Amber force fields for protein simulations in programs that accommodate pair-specific Lennard–Jones combining rules. The force field gives strong performan...

Determination of the Effect of Stress State on the Onset of Ductile Fracture Through Tension-Torsion Experiments

Abstract: A tubular tension-torsion specimen is proposed to characterize the onset of ductile fracture in bulk materials at low stress triaxialities. The specimen features a stocky gage section of reduced thickness. The specimen geometry is optimized such that the stress and strain fields within the gage section are approximately uniform prior to necking. The stress state is plane stress while the circumferential strain is approximately zero. By applying different combinations of tension and torsion, the material response can be determined for stress triaxialities ranging from zero (pure shear) to about 0.58 (transverse plane strain tension), and Lode angle parameters ranging from 0 to 1. The relative displacement and rotation of the specimen shoulders as well as the surface strain fields within the gage section are determined through stereo digital image correlation. Multi-axial fracture experiments are performed on a 36NiCrMo16 high strength steel. A finite element model is built to determine the evolution of the local stress and strain fields all the way to fracture. Furthermore, the newly-proposed Hosford-Coulomb fracture initiation model is used to describe the effect of stress state on the onset of fracture.

On the shear deformation modes in the framework of Generalized Beam Theory

Abstract: This paper extends previous work concerning the determination of cross-section deformation modes in thin-walled members with arbitrary polygonal cross-section, in the framework of Generalized Beam Theory (Goncalves et al., 2010 [1] ). In particular, the paper addresses the so-called “natural shear deformation modes” (i.e. the deformation modes that involve non-null membrane shear strains and are independent of the cross-section discretization employed), which are relevant for capturing the behaviour of thin-walled members with complex multi-cell cross-sections undergoing torsion and/or distortion. The contributions of the paper are (i) the derivation of fundamental properties of the shear modes, (ii) the proposal of an efficient mode extraction procedure and (iii) the development of analytical results for several particular cases. In order to illustrate the application of the proposed mode extraction procedure and demonstrate the validity of the derived properties, several cross-sections are analyzed, including complex multi-cell tubes.

Modeling the temperature rise in high-pressure torsion

Abstract: Experiments and finite element modeling were used to estimate the temperature rise during high-pressure torsion The results show the temperature rise is dependent upon the material strength, the rotation rate, the sample radius, the heat capacity and the volume of the anvils A general relationship is derived which predicts the temperature rise in samples of different geometries processed using different anvil sizes A simplified version of the equation is presented for general use

Power-interrogated and simultaneous measurement of temperature and torsion using paired helical long-period fiber gratings with opposite helicities.

Abstract: A power-interrogated sensor which allows for simultaneous measurement of temperature and torsion is proposed and experimentally demonstrated, which is based on utilization of paired helical long-period fiber gratings (HLPG) with opposite helicities. Unlike most of the previous fiber grating-based sensing system, here the paired HLPGs are simultaneously used as both the sensing and the interrogating elements and thus the bulk and high-cost wavelength-interrogating device can be eliminated. Moreover not only the torsion but also the torsion direction can be determined simultaneously. The temperature sensitivity obtained is estimated to be ~41 pm/°C within a range of 20-150 °C, and the torsion responsivities obtained are ~-1.414 nm/rad/m and ~1.276 nm/rad/m, respectively within a rotation angle of −360°~360°.

Rolling, sliding and torsion of micron-sized silica particles: experimental, numerical and theoretical analysis

Abstract: The contact mechanics of individual, very small particles with other particles and walls is studied using a nanoindenter setup that allows normal and lateral displacement control and measurement of the respective forces. The sliding, rolling and torsional forces and torques are tested with borosilicate microspheres, featuring radii of about 10μm. The contacts are with flat silicon substrates of different roughness for pure sliding and rolling and with silicon based, ion-beam crafted rail systems for combined rolling and torsion. The experimental results are discussed and compared to various analytical predictions and contact models, allowing for two concurrent interpretations of the effects of surface roughness, plasticity and adhesion. This enables us to determine both rolling and torsion friction coefficients together with their associated length scales. Interestingly, even though normal contacts behave elastically (Hertzian), all other modes of motion display effects due to surface roughness and consequent plastic deformation. The influence of adhesion is interpreted in the framework of different models and is very different for different degrees of freedom, being largest for rolling.

Effects of Principal Stress Directions on 3D Failure Conditions in Cross-Anisotropic Sand

Abstract: An experimental program was carried out to study the variation of shear strength of cross-anisotropic deposits of fine Nevada sand under three-dimensional (3D) conditions using a recently developed torsion shear apparatus. A total of 44 drained torsion shear tests were performed at a constant mean confining stress, σm, constant intermediate principal stress ratios, as indicated by b=(σ2−σ3)/(σ1−σ3), and principal stress directions, α. The experiments were performed on large hollow cylinder specimens deposited by dry pluviation and tested in an automated torsion shear apparatus. The specimens had a height of 40 cm, an average diameter of 20 cm, and a wall thickness of 2 cm. The 3D failure surface of fine Nevada sand is presented with discrete combinations of α and intermediate principal stress. The effects of these two variables on the shape of the failure surface are presented. The friction angles are highest for α=0° for all b-values and show a dip in strength with the lowest values near α=67.5°,...

The response of the electronic structure to electronic excitation and double bond torsion in fulvene: a combined QTAIM, stress tensor and MO perspective.

Abstract: New insights into the double bond isomerization of fulvene in the ground and excited electronic states are provided by newly developed QTAIM and stress tensor tools. The S0 and S1 states follow the ‘biradical’ torsion model, but the double bond is stiffer in the S0 state; by contrast, the S2 state follows the ‘zwitterionic’ torsion. Differences are explained in terms of the ellipticity and bond critical point (BCP) stiffness for both QTAIM and the stress tensor. Overall, the wave-function based analysis is found to be in agreement with the work of Bonacic-Koutecký and Michl that the bond-twisted species can have biradical or zwitterionic character, depending on the state. Using QTAIM and the stress tensor a new understanding of bond torsion is revealed; the electronic charge density around the twisted bond is found not to rotate in concert with the nuclei of the rotated –CH2 methylene group. The ability to visualize how the bond stiffness varies between individual electronic states and how this correlates with the QTAIM and stress tensor bond stiffness is highlighted. In addition, the most and least preferred morphologies of bond-path torsion are visualized. Briefly we discuss the prospects for using this new QTAIM and stress tensor analysis for excited state chemistry.

The BIPM measurements of the Newtonian constant of gravitation, G.

Abstract: This paper is a complement to the two short papers published in 2001 and 2013 in which we presented the results of the two BIPM determinations of the Newtonian constant of gravitation G. While this review contains no new results, it includes more detailed descriptions of certain key parameters that enter into the determination of G. Following a description of the overall method and the two versions of the experiment, we discuss the properties of the torsion strip, including the effects of anelasticity, then the electrostatic torque transducer, the source and test masses, dimensional metrology, angle measurement, the calculation and measurement of the moment of inertia, calculation of the torque, possible magnetic interactions and finally we discuss uncertainties and correlations in the derivation of a value for G.

Stress analysis of multi-layered hollow anisotropic composite cylindrical structures using the homogenization method

Abstract: This paper presents a general and efficient stress analysis strategy for hollow composite cylindrical structures consisting of multiple layers of different anisotropic materials subjected to different loads. Cylindrical material anisotropy and various loading conditions are considered in the stress analysis. The general stress solutions for homogenized hollow anisotropic cylinders subjected to pressure, axial force, torsion, shear and bending are presented with explicit formulations under typical force and displacement boundary conditions. The stresses and strains in a layer of the composite cylindrical structures are obtained from the solutions of homogenized hollow cylinders with effective material properties and discontinuous layer material properties. Effective axial, torsional, bending and coupling stiffness coefficients taking into account material anisotropy are also determined from the strain solutions for the hollow composite cylindrical structures. Examples show that the material anisotropy may have significant effects on the effective stiffness coefficients in some cases. The stress analysis method is demonstrated with an example of stress analysis of a 22-layer composite riser, and the results are compared with numerical solutions. This method is efficient for stress analysis of thin-walled or moderately thick-walled hollow composite cylindrical structures with various multiple layers of different materials or arbitrary fiber angles because no explicit interfacial continuity parameters are required. It provides an efficient and easy-to-use analysis tool for assessing hollow composite cylindrical structures in engineering applications.

Torsion sensors of high sensitivity and wide dynamic range based on a graphene woven structure

Abstract: Due to its unique electromechanical properties, nanomaterial has become a promising material for use in the sensing elements of strain sensors. Tensile strain is the type of deformation most intensively studied. Torsion is another deformation occurring in everyday life, but is less well understood. In the present study a torsion sensor was prepared by wrapping woven graphene fabrics (GWFs) around a polymer rod at a specific winding angle. The GWF sensor showed an ultra-high sensitivity with a detection limit as low as 0.3 rad m−1, indicating its potential application in the precise measurement of low torsions. The GWFs were pre-strained before wrapping on polydimethylsiloxane (PDMS) to improve the tolerance of the sensor to high torsion. The microstructure of the GWFs at different torsion levels was monitored using an optical microscope. The results demonstrated the formation of GWF waves and cracks under high torsion, a critical factor in determining the electromechanical properties of a GWF sensor.

Sheathed Cold-Formed Steel Studs under Axial and Lateral Load

Abstract: This research aims to identify and characterize the behavior of dissimilarly sheathed, load bearing, cold-formed steel studs under axial and lateral load. A series of tests on single studs, set in track, and sheathed with either oriented strand board, gypsum board, or combinations thereof are completed. The tests approximate the behavior of a sheathed stud within a larger wall. In each test, a predetermined level of axial load (or displacement) is introduced into the stud and lateral displacement (load) is then applied and increased until failure. This configuration results in axial load, bending, and (potentially) torsion on the stud. Observed failure modes for studs sheathed on only one face include torsion and/or fastener pull-through. For studs sheathed on both faces, failure modes include torsion, local buckling, fastener pull-through, and bearing (particularly for gypsum-sheathed studs). Analysis of the torsional response indicates the important role of the sheathing in limiting torsion and ...

Characterization of Thermo-Mechanical and Fracture Behaviors of Thermoplastic Polymers

Abstract: In this paper the effects of the strain rate on the inelastic behavior and the self-heating under load conditions are presented for polymeric materials, such as polymethyl methacrylate (PMMA), polycarbonate (PC), and polyamide (PA66). By a torsion test, it was established that the shear yield stress behavior of PMMA, PC, and PA66 is well-described by the Ree-Eyring theory in the range of the considered strain rates. During the investigation, the surface temperature was monitored using an infrared camera. The heat release appeared at the early stage of the deformation and increased with the strain and strain rate. This suggested that the external work of deformation was dissipated into heat so the torsion tests could not be considered isothermal. Eventually, the effect of the strain rate on the failure modes was analyzed by scanning electron microscopy.

Constitutive modeling of tension-torsion coupling and tension-compression asymmetry in NiTi shape memory alloys

Abstract: A three-dimensional phenomenological model based on microplane theory is extended to capture the coupling effects between tension and torsion in complex multiaxial loadings. Inelastic strain in a microplane approach is a component of transformation strain and anisotropic strain. Since the anisotropy effect is induced during martensitic transformation, anisotropic strain is defined as a function of transformation strain. Out-of-plane strain is induced in simple tension and pure torsion in free-end conditions. Anisotropy tensor is experimentally extracted and is used in the proposed model to predict the behavior in multiaxial loading. The ability of this extended microplane model to predict the tension-torsion coupling effects as well as the induced transformation anisotropic behavior of NiTi shape memory alloys is demonstrated. In addition, the microplane model is modified to capture the asymmetric behavior in tension and compression during uniaxial and multiaxial loadings. To this end, numerical correlations between the results of the modified microplane model are compared with experimental results to demonstrate the validity of the extended model.