Critical Twist Research Articles

TWISTED MAGNETIC FLUX TUBES IN THE SOLAR WIND

Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70$^0$, but the angle can further decrease owing to the motion of the tube with regards to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably can not reach 1 AU.

CONTINUUM SHELL MODEL FOR BUCKLING OF ARMCHAIR CARBON NANOTUBES UNDER COMPRESSION OR TORSION

Molecular dynamics (MD) simulations are performed using adaptive intermolecular reactive bond order potential to analyze single-walled and double-walled carbon nanotubes. These carbon nanotubes were analyzed for buckling under compression and under torsion. The MD simulations create a comprehensive database for the critical buckling loads/strains and critical buckling torques/twist angles for armchair SWCNTs and DWCNTs of varying diameters and lengths. Using MD results as a computational benchmark, an equivalent thick shell model of CNT is proposed, which is amenable for analysis using a commercially available software ABAQUS. Based on our MD results, an empirical equation that describes the size-dependent Young's modulus for a single-walled carbon nanotube is established. Buckling analysis of CNT under compression and under torsion are performed with the equivalent shell model using size-dependent Young's modulus, Poisson's ratio = 0.19 and shell thickness h = 0.066 nm. We show that the equivalent shell model gives good estimate of critical buckling load/strain and critical buckling torque with respect to the MD results. Variation of critical twist angle with length of CNT, predicted by the shell model is in good qualitative agreement with MD simulation. However, the equivalent shell model underestimates the critical twist angle by 30% because the continuum shell model overestimates torsional stiffness of CNT compared to an atomistic model of CNT. The equivalent shell model is less computational intensive to implement as compared with MD. Its accuracy for predicting the buckling states for long carbon nanotubes allows it to be used for moderately long CNTs under compression/torsion, in-lieu of MD simulations.

Investigations of fiber twist on the mechanical properties of sisal fiber yarns and their composites

The effect of fiber twist on the tensile properties of single sisal fibers was investigated by single fiber tensile test. Four kinds of sisal yarns with four different levels of twist were made and the mechanical properties were investigated on both unimpregnated and resin-impregnated sisal yarns. A critical fiber twist level for unimpregnated sisal yarns was found beyond which the tensile properties started to decrease with the increasing of the level of fiber twist. However, for resin-impregnated sisal yarns, both tensile strength and modulus decreased with the increasing of the fiber twist level. In addition, the effect of fiber twist on both the tensile and flexural properties of unidirectional sisal yarn reinforced composites was studied. It was shown that lower twist level led to higher mechanical properties of its reinforced composites. Compared with the experimental results, Rao’s model proved to accurately calculate the Young’s modulus of fiber yarns with different levels of twist. Thus, the Young’s modulus of sisal yarn reinforced composites could be predicted by the rule of mixtures hereafter.

The Mechanical Property of Milk Protein Fiber Pure Yarn and Blended Yarn

The milk protein fiber is one of the green fibers. It is blended with cotton. The strength of the blended yarn is tested. The relation between the strength and the twist factor and the blended ratio is analyzed. The optimum blended ratio and the critical twist factor is determined.

Test and Analysis of SPF Pure Yarn or SPF Tencel Blending Yarn Mechanics Property

The soybean protein fiber is one of the green fibers. It is blended with Tencel. The strength of the blended yarn is tested. The relation between the strength and the twist factor and the blended ratio is analyzed. The optimum blended ratio and the critical twist factor is determined.

Graphene in Color

Materials Science![Figure][1] CREDIT: J. T. ROBINSON The properties of bilayer graphene films depend on the relative orientation or twist of the two layers. Robinson et al. grew single-layer graphene on copper surfaces and then performed two transfers of these films onto silica-coated silicon substrates to create bilayer regions. Because these films are polycrystalline, a variety of twist angles between the layers were created across the surface and resulted in a patchwork of colored regions that appeared red, yellow, or blue. Raman spectroscopy was used to characterize the twist angles; the enhancement of the G peak at ∼1600 cm−1 occurred at optical excitation wavelengths that differed for the red and yellow regions, and the extent of enhancement corresponded to the deviation of the orientation of the layers from a distinctive critical twist angle. The twist angles were also confirmed by low-energy electron diffraction studies. The coupling could be minimized by chemical functionalization: fluorination of the top graphene layers with XeF2 quenched the colors, which could be recovered by thermally desorbing the fluorine atoms. ACS Nano 10.1021/nn304834p (2012). [1]: pending:yes

Twist buckling behavior of arteries

Arteries are often subjected to torsion due to body movement and surgical procedures. While it is essential that arteries remain stable and patent under twisting loads, the stability of arteries under torsion is poorly understood. The goal of this work was to experimentally investigate the buckling behavior of arteries under torsion and to determine the critical buckling torque, the critical buckling twist angle, and the buckling shape. Porcine common carotid arteries were slowly twisted in vitro until buckling occurred while subjected to a constant axial stretch ratio (1.1, 1.3, 1.5 (in vivo level) and 1.7) and lumen pressure (20, 40, 70 and 100mmHg). Upon buckling, the arteries snapped to form a kink. For a group of six arteries, the axial stretch ratio significantly affected the critical buckling torque ([Formula: see text]) and the critical buckling twist angle ([Formula: see text]). Lumen pressure also significantly affected the critical buckling torque ([Formula: see text]) but had no significant effect on the critical twist angle ([Formula: see text]). Convex material constants for a Fung strain energy function were determined and fit well with the axial force, lumen pressure, and torque data measured pre-buckling. The material constants are valid for axial stretch ratios, lumen pressures, and rotation angles of 1.3-1.5, 20-100mmHg, and 0-270[Formula: see text], respectively. The current study elucidates the buckling behavior of arteries under torsion and provides new insight into mechanical instability of blood vessels.

Effects of die shape modifications on the geometrical and dimensional accuracies of cold forged AUV propeller blade

Geometrical and dimensional accuracies are two major concerns in precision forging and they become more critical with increasing part complexity. In this study, the geometrical error of an autonomous underwater vehicle (AUV) propeller blade is quantified by comparing the blade and punch profiles. The nominal geometry of the blade is compared to the blade profile measured using the optical technique Infinite Focus Alicona system to determine profile deviation. The current study aims to investigate the contributions of die shape modifications on the error formation of the two most critical geometries, namely, blade thickness and twist angle. The results show that die modification has a significant effect on geometrical and dimensional errors.

Affection of Twist Factor and Blended Ratio to the Strength of Soybean Protein Fiber Blended Yarn

The soybean protein fiber is one of the green fibers. It is blended with the fine polyester. The strength of the blended yarn is tested. The relation between the strength and the twist factor and the blended ratio is analyzed. The optimum blended ratio and the critical twist factor is determined.

The Strength and Elongation of PLA Fiber Yarn

The PLA fiber is one of the green fibers. It is blended with Tencel fibers. The strength of the PLA fiber/Tencel blended yarn is tested. The relationship between the strength and the twist factor and the blended ratio is analyzed. The optimum blended ratio and the critical twist factor of the blended yarn is determined.

Torsion in Strain-Gradient Plasticity: Energetic Scale Effects

We study elasto-plastic torsion in a thin wire within the framework of the strain-gradient plasticity theory elaborated by Gurtin and Anand in 2005. The theory in question envisages two material scales: an energetic length-scale, which takes into account the so-called “geometrically-necessary dislocations” through a dependence of the free energy on the Burgers tensor, and a dissipative length-scale. For the rate-independent case with null dissipative length-scale, we construct and characterize a special class of solutions to the evolution problem. With the aid of such characterization, we estimate the dependence on the energetic scale of the ratio between the torque and the twist. Our analysis confirms that the energetic scale is responsible for size-dependent strain-hardening, with thinner wires being stronger. We also detect, and quantify in terms of the energetic length-scale, both a critical twist, after which the wire becomes fully plastified, and two boundary layers near the external boundary of the...

The Flare-Energy Distributions Generated by Kink-Unstable Ensembles of Zero-Net-Current Coronal Loops

It has been proposed that the million degree temperature of the corona is due to the combined effect of barely-detectable energy releases, so called nanoflares, that occur throughout the solar atmosphere. Alas, the nanoflare density and brightness implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release and so we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field transitions to a lower energy state, which is modelled by relaxation theory, i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with a critical magnetic twist value for every point along the instability threshold.

AN INVESTIGATION INTO THE TORSIONAL BUCKLING OF CARBON NANOTUBES USING MOLECULAR AND STRUCTURAL MECHANICS

In this work, the critical twist angles of two types of CNTs, zigzag and armchair of different lengths are determined using molecular and structural mechanics. The results show that the torsional buckling modes predicted by the two methods are the same. It is also shown that critical twist angle decreases as CNTs length increases. Structural mechanics underpredicts the critical angle for both armchair and zigzag nonotubes. It is also shown that the critical angle for armchair nanotubes is higher than that for zigzag nanotubes.

Repulsive Normal Force by an Excited Magneto-Rheological Fluid Bounded by Parallel Plates in Stationary or Rotating Shear Mode

This article reports results from an experimental study undertaken to determine the magnitude of the repulsive force exerted by a magnetorheological fluid on two bounding parallel and concentric plates under different magnetic field strengths. The plates are stationary without circumferential external loading or in a forced rotational rate for a constant inter plate gap separation. The results show that with stationary plates, the repulsive force increases with the magnitude of the excitation. Upon the addition of a steady-relative rotational speed, the force increases further. The increase in the shear-enhanced repulsive force rose up to a peak and then decreased as the test running, i.e., further increase in the shear strain. The force eventually settled at a steady value which was higher than that due to magnetic field only (no rotation). The main characteristics of the repulsive force against the strain curve are well described by a mathematical model which is derived on the basis of the particle chain tilt phenomenon. The critical twist angles of the plate in the current experimental set up under different magnetic flux densities, which give the maximum repulsive forces, were found to approach a constant value as predicted by the tilt particle chain model.

Microwave Generation on Geometrically Constrained Magnetic Wall: Effect of Twist Angle

We studied the current-induced magnetization dynamics of geometrically constrained magnetic walls (GCMWs) with twist angles of 90–180°, in order to examine their possibility as microwave generators using the effects of the twist angles. We numerically solved the Landau–Lifshitz–Gilbert equation with a spin-transfer torque term and found that the magnetic structures of GCMWs with certain twist angles oscillate between the Bloch and Néel walls (OBN) with microwave frequency, even though the twist angle is not 180°. We also found the critical twist angle below which the oscillation of the magnetic structure is inhibited. We showed that the oscillation frequency shows a hysteresis when we vary the twist angle of a GCMW or the corresponding applied magnetic field. Then, we also discussed the microwave generation induced by the periodic-deformation dynamics of GCMWs in the OBN.

Determination of the critical twist for polypropylene yarn

An equation that reflects the effect of the basic properties of the fibre and the linear density of polypropylene yarn on its twist was obtained. Determination of more accurate values of the yarn twist with this equation is due to the nonlinear character of the dependence of the twist on the properties of the fibre and the linear density of the yarn.

Twisted Elastic Rings and the Rediscoveries of Michell's Instability

Elastic rings become unstable when sufficiently twisted. This fundamental instability plays an important role in the modeling of DNA mechanics and in cable engineering. In 1962, Zajac computed the value of the critical twist for the instability. This critical value was rediscovered in 1979 by Benham and independently by Le Bret in elastic models for DNA; unstable rings have since become an important example of elastic instabilities in rods both for the development of new methods and in applications. The purpose of this note is to show that the problem had been completely solved by John Henry Michell in 1889 in a rather elegant manner and to reflect on its history and modern developments.

Nanomechanics of order reconstruction in nematic liquid crystals

We employ a continuum model to compute both torque and force transmitted through a thin twist cell filled with a nematic liquid crystal and bounded by flat plates with anchorings at right angles. The transmitted torque vanishes at the order reconstruction threshold when the cell thickness is comparable with the biaxial coherence length. At the same point, the force diagram exhibits an angular point which disappears above a critical twist mismatch. Both torque and force diagrams against the cell's thickness fail to be monotonic when the total twist is near pi/2 .

Effect of photospheric twisting motions on a confined toroidal loop

In this paper we investigate the effect of twisting motions at the photosphere on a curved loop confined by overlying field lines. We find that the twisting motions form a twisted loop along which a current builds up. However, the toroidal curvature of the loop appears to have a stabilising effect as there is no sign of the kink instability for a twist of 3.4 π whereas in a straight cylinder for the same twist profile the critical twist is approximately 2.5 π. When we include resistivity in the simulation there are indications that reconnection occurs and releases a substantial proportion of the free magnetic energy.

Structure of twist-grain-boundary–Cphases

We study properties of the Renn-Lubensky twist-grain-boundary-C (TGB(C)) phase, with layer normal rotating in a plane perpendicular to the pitch axis, and the Bordeaux TGB(C) phase, with the layer normal rotating on a cone parallel to the pitch axis near the upper critical twist k(c2) marking the transition to the cholesteric phase. We introduce a generalized model free energy for the smectic-C phase that allows either TGB(C) phase to be stable, and we calculate k(c2) and the order-parameter profile, which shows only modest spatial variation, for both phases.