Showing papers on "Torsion (mechanics) published in 2002"

Frequency tunable resonant scanner and method of making

Abstract: A MEM s scanning device has a variable resonant frequency. In one embodiment, the MEMs device includes a torsion arm that supports an oscillatory body. In one embodiment, an array of removable masses are placed on an exposed portion of the oscillatory body and selectively removed to establish the resonant frequency. The material can be removed by laser ablation, etching, or other processing approaches. In another approach, a migratory material is placed on the torsion arm and selectively stimulated to migrate into the torsion arm, thereby changing the mechanical properties of the torsion arm. The changed mechanical properties in turn changes the resonant frequency of the torsion arm. In another approach, symmetrically distributed masses are removed or added in response to a measured resonant frequency to tune the resonant frequency to a desired resonant frequency. A display apparatus includes the scanning device and the scanning device scans about two or more axes, typically in a raster pattern. Various approaches to controlling the frequency responses of the scanning device are described, including active control of MEMs scanners and passive frequency tuning.

The Intrinsic torsion of SU(3) and G(2) structures

Abstract: We analyse the relationship between the components of the intrinsic torsion of an SU(3) structure on a 6-manifold and a G_2 structure on a 7-manifold. Various examples illustrate the type of SU(3) structure that can arise as a reduction of a metric with holonomy G_2.

Fixed angle solar collector arrangement

Abstract: A solar collector array (40) is formed of multiple parallel rows (10) of solar panels, each said row being made of one or more building blocks (11) with an east-west oriented torsion tube (12) defining an east-west axis for the row, an array of flat generally rectangular solar panels (14), and a set of panel rails (22) and rail clamps (30) attaching the panels onto the torsion tube. A row of piers (16) aligned on the east-west axis each have a footing (18) that is supported in the earth, and a pier cap (20) affixed onto a top end of the pier. The pier cap holds the torsion (12) tube non-rotationally such that the torsion tube and panels are held at a preset elevation angle. The torsion tube (12) may serve as a conduit for power conductors from the panels (14).

Failure of cellular foams under multiaxial loading

Abstract: A thorough investigation of the mechanical behavior of a closed-cell cellular foam (Divinycell) under multiaxial stress conditions was undertaken. Two types of Divinycell, H100 and H250, with densities of 100 and 250 kg/m3, respectively, were investigated. The uniaxial tensile, compressive and shear stress–strain curves along the in-plane and the through-the-thickness directions of both materials were obtained. The materials showed quite different stress–strain behavior in tension and compression. The H100 material showed a nearly isotropic behavior, while the H250 material showed orthotropic behavior with a higher stiffness along the through-the-thickness than the in-plane direction. A series of biaxial tests were conducted, including: (i) constrained strip specimens in tension and compression with the strip axis along the through-the-thickness and in-plane directions; (ii) constrained thin-wall ring specimens in compression and torsion; (iii) thin-wall tube specimens in tension and torsion; and (iv) thin-wall tube specimens under axial tension, torsion and internal pressure. From these tests, biaxial strength results in the stress plane of the through-the-thickness and in-plane directions for different values of applied shear were obtained. Failure envelopes were constructed by the Tsai–Wu failure criterion based on the strength values in uniaxial tension, compression and shear. The experimental results were described well by the Tsai–Wu failure criterion.

Refined Structural Model for Thin- and Thick-Walled Composite Rotor Blades

Abstract: A refined structural model based on a mixed force and displacement method is proposed for the analysis of composite rotor blades with elastic couplings. The present formulation allows the modeling of either open-section or closed-section blades of arbitrary section shape, stacking sequence, and end restraint effects. The theory accounts for the effect of elastic couplings, shell wall thickness, section warping, warping restraint, and transverse shear deformations. A semicomplementary energy functional is used to derive, in a variationally consistent manner, the beam force-displacement relations. Bending and torsion related warpings and shear correction factors are obtained in closed form as part of the analysis. The resulting first-order shear deformation theory (Timoshenko) describes the beam kinematics in terms of the axial, flap and lag bending, flap and lag shear, twist, and torsion-warping deformations. The theory is validated against experimental data and other finite element results for graphite-epoxy composite beams of various cross sections such as I sections, box sections, and two-cell airfoils. Good correlation is achieved for all of the test examples. The influence of wall thickness and transverse shear on the static beam response is also investigated. Wall thickness effects are shown to become significant when the thickness-to-depth ratio of the beam reaches around 20%. The slenderness ratio has a significant effect on the transverse shear behavior of the beam, especially for beams with low slenderness ratios. It is also shown that the layup angle has a nonnegligible effect on the transverse shear behavior of the beam.

Design considerations of rectangular electrostatic torsion actuators based on new analytical pull-in expressions

Abstract: An important design issue of electrostatic torsion actuator is the relative locations of the actuating electrodes, where the bias voltage is applied. These geometrical design parameters affect both the pull-in angle as well as the pull-in voltage. In this paper, a new approximated analytical solution for the pull-in equation of an electrostatic torsion actuator with rectangular plates is derived. The analytical expression is shown to be within 0.1% of the one degree of freedom (1DOF) lumped-element model numerical simulations. Moreover, the analytical expressions are compared with the full coupled-domain finite-elements/boundary-elements (FEM/BEM) simulations provided by MEMCAD4.8 Co-solve tool, showing excellent agreement. The approach presented here provides better physical insight, more rapid simulations and an improved design optimization tool for the actuator.

Effect of loading condition and stress state on damage evolution of silicon particles in an Al-Si-Mg-Base cast alloy

Abstract: Damage evolution of Si particles in a Sr modified cast A356(T6) Al alloy is quantitatively characterized as a function of strain under tension, compression, and torsion. The fraction of damaged Si particles, their size distributions, and orientation distribution of particle cracks are measured by image analysis and stereological techniques. Silicon particle cracking and debonding are the predominant damage modes. Particle debonding is observed only under externally applied tensile loads, whereas particle cracking is observed under all loading conditions. The relative contributions of Si particle debonding and fracture to the total damage strongly depend on stress state and temperature. For all loading conditions and stress states studied, the average size of damaged Si particles is considerably larger than the bulk average size. The rate of damage accumulation is different for different loading conditions. At a given strain level, Si particle damage is lowest under compression and highest under torsion. The anisotropy of the damage is highly dependent on the deformation path and stress state. Under uniaxial tension, the cracks in the broken Si particles are mostly perpendicular to the loading direction, whereas in the compression test specimens they are parallel to the loading direction. The Si particle cracks in the torsion and notch-tension test specimens do not exhibit preferred orientations. The quantitative microstructural data are used to test damage evolution models.

Does a randall-sundrum scenario create the illusion of a torsion-free universe?

Abstract: We consider spacetime with torsion in a Randall-Sundrum scenario where torsion, identified with the rank-2 Kalb-Ramond field, exists in the bulk together with gravity. While the interactions of both graviton and torsion in the bulk are controlled by the Planck mass, an additional exponential suppression comes for the torsion zero-mode on the visible brane. This may serve as a natural explanation of why the effect of torsion is so much weaker than that of curvature on the brane. The massive torsion modes, on the other hand, are correlated with the corresponding gravitonic modes and may be detectable in TeV-scale experiments.

An elastoplastic theory of dislocations as a physical field theory with torsion

Abstract: We consider a static theory of dislocations with moment stress in an anisotropic or isotropic elastoplastic material as a T(3) gauge theory. We obtain Yang-Mills-type field equations which express the force and the moment equilibrium. Additionally, we discuss several constitutive laws between the dislocation density and the moment stress. For a straight screw dislocation, we find the stress field which is modified near the dislocation core due to the appearance of moment stress. For the first time, we calculate the localized moment stress, the Nye tensor, the elastoplastic energy and the modified Peach-Koehler force of a screw dislocation in this framework. Moreover, we discuss the straightforward analogy between a screw dislocation and a magnetic vortex. The dislocation theory in solids is also considered as a three-dimensional effective theory of gravity.

Chi(1) rotamer populations and angles of mobile surface side chains are accurately predicted by a torsion angle database potential of mean force.

Abstract: The equilibrium angles and distributions of chi(1) rotamers for mobile surface side chains of the small, 63-residue, B1 domain of protein L have been calculated from the static crystal structure by rigid body/torsion angle simulated annealing using a torsion angle database potential of mean force and compared to those deduced by Monte Carlo analysis of side chain residual dipolar couplings measured in solution. Good agreement between theory and experiment is observed, indicating that for side chains undergoing rotamer averaging that is fast on the chemical shift time scale, the equilibrium angles and distribution of chi(1) rotamers are largely determined by the backbone phi/psi torsion angles.

Geometry and Mechanics of Uniform n-Plies: from Engineering Ropes to Biological Filaments

Abstract: We study the mechanics of uniform n-plies, correcting and extending previous work in the literature. An n-ply is the structure formed when n pretwisted strands coil around one another in helical fashion. Such structures are encountered widely in engineering (mooring ropes, power lines) and biology (DNA, proteins). We first show that the well-known lock-up phenomenon for n=2, described by a pitchfork bifurcation, gets unfolded for higher n. Geometrically, n-plies with n>2 are all found to behave qualitatively the same. Next, using elastic rod theory, we consider the mechanics of n-plies, allowing for axial end forces and end moments while ignoring friction. An exact expression for the interstrand pressure force is derived, which is used to investigate the onset of strand separation in plied structures. After defining suitable displacements we also give an alternative variational formulation and derive (nonlinear) constitutive relationships for torsion and extension (including their coupling) of the overall ply. For a realistic loading problem in which the ends are not free to rotate one needs to consider the topological conservation law, and we show how the concepts of link and writhe can be extended to n-plies.

Lateral post-buckling analysis of thin-walled open section beams

Abstract: Thin-walled beams with open sections are studied using a nonlinear model. This model is developed in the context of large displacements and small deformations, by accounting for bending-bending and bending-torsion couplings. The warping and shortening effects are considered in the torsion equilibrium equation. The governing coupled equilibrium equations obtained from Galerkin’s method are solved by a Newton–Raphson iterative process. It is established that the buckling loads are highly dependent on the pre-buckling deformations of the beam. The bifurcated branches are unstable and strongly influenced by shortening effects. Some comparisons are presented with the solutions commonly used in linear stability, like in the standard European steel code (Eurocode 3). The regular solutions appear to be very conservative, especially for I sections with large flanges.

Stability of elastic bending and torsion of uniform cantilever rotor blades in hover with variable structural coupling

Abstract: The stability of elastic flap bending, lead-lag bending, and torsion of uniform, untwisted, cantilever rotor blades without chordwise offsets between the elastic, mass, tension, and aero­dynamic center axes is investigated for the hovering flight condition. The equations of motion are obtained by simplifying the general, nonlinear, partial differential equations of motion of an elastic rotating cantilever blade. The equations are adapted for a 1inear:zed stability analysis in the hovering flight condition by prescribing aerodynamic forces, applying Galerkin's method, and linearizing the resulting ordinary differential equations about the equilibrium operating con­dition. The aerodynamic forces are obtained from strip theory based on a quasi-steady approxi­mation of two-dimensional unsteady airfoil theory. Six coupled mode shapes, calculated from free vibration about the equilibrium operating condition, are used in the linearized stability analysis. The study emphasizes the effects of two types of structural coupling that strongly influence the stability of hingeless rotor blades. The first structural coupling is the linear coupling between flap and lead-lag bending of the rotor blade. A structural coupling parameter 6? is introduced to simulate variations in flap-lag structural coupling that arise for blades having nonuniform stiffness distributions. The second structural coupling is a nonlinear coupling between flap bending, lead-lag bending, and torsion deflections. Results are obtained for a wide variety of hingeless rotor configurations and operating conditions in order to provide a reasonably complete picture of hingeless rotor blade stability characteristics. The stability of torsionally flexible blades is strongly influenced by the effects of the bending-torsion sturctural coupling. Without precone, typical configurations are usually stable except for low values of 6? or low torsion frequencies. Addition of precone is strongly destabilizing for a wide range of configurations. Except for very low torsion frequencies, the results also indicate that the structural terms in the torsion equation dominate the torsion inertia and damping terms which permits the use of an approximate, but simplified, system of equations with fewer degrees of freedom. Finally, the accu­racy of the results is sensitive to the number and type

A rotational comb-driven micromirror with a large deflection angle and low drive voltage

Abstract: We have designed, fabricated, and tested a V-shaped torsion bar for use with a comb-driven micromirror. This torsion bar suppressed undesired sticking of the comb teeth, and enabled a large rotational angle and a low drive voltage. We observed 5-degree rotation of a comb-driven micromirror with a drive voltage of 90 V.

Multiaxial high cycle fatigue test system

Abstract: A multiaxial high cycle fatigue test system for testing bending, torsion, and tension of a test unit, comprises servo-hydraulic components, including a hydraulic service manifold, two small high frequency actuators along a first axis, and one large main actuator along a second axis The large main actuator is used to apply a radial centrifugal force, and the two small actuators are used to apply vibratory loading; the two small side actuators being offset independently of each other, to enable the machine to apply both bending loads and torque to the test unit. The test unit is subjected to torsion loading when the traverse actuators move in phase, that is when both actuators move either in or out at the same time. The test unit is subjected to bending loading when the actuators move out-of-phase, that is one actuator moves in when the other moves out or vice-versa.

Measurement of through-thickness stresses using small holes

Abstract: Previously the deep-hole method was developed to measure residual stresses in thick-section engineering components. In this paper it is demonstrated that stresses arising from external applied loading can also be measured. A series of calibration studies using aluminium and steel samples, with thicknesses varying from 5 to 50 mm, are presented. The samples were subjected to tensions, bending and torsion. It is shown that the deep-hole method can measure linear and non-linear stress distributions.

Accuracy of mechanical torque-limiting devices for implants.

Abstract: Purpose: To examine the accuracy of 3 mechanical torque wrenches. Materials and Methods: The torque outputs of the Nobel Biocare, Straumann ITI, and DynaTorq ITL mechanical torque-limiting devices were determined using a special setup on an Instron test machine. The devices were held in the test setup and oriented so that activation of the drivers caused a pure torsion effect. Results: Significant differences generally existed between individual units and the target torque levels for the Nobel Biocare torque controller. Discussion: The mean torque values of the ITI and ITL devices were within 10% of their respective target torque levels. Knowledge of applied torque levels to the screws that retain implant abutments and their attached prostheses is necessary to achieve optimal preload. The ITI and ITL devices tested in this study were capable of providing consistent torque at or near their respective targets. Conclusion: The torque output of each individual device deviated in varying degrees from target torque values. (INT J ORAL MAXILLOFAC IMPLANTS 2002;17:220‐224)

Curvature Measures of 3D Vector Fields and their Applications

Abstract: Tangent curves are a powerful tool for analyzing and visualizing vector fields In this paper two of their most important properties are examined: their curvature and torsion Furthermore, the conc

Squeeze film damping effect on a MEMS torsion mirror

Abstract: In this paper we discuss the dynamical characteristics of a torsion mirror in microelectromechanical systems. The squeeze film is modeled using the so-called modified molecular gas film lubrication (MMGL) equation with the coupling effects of surface roughness and gas rarefaction. The MMGL equation is linearized and then a simple mapping method is utilized to obtain the analytical solution of the transformed two-dimensional diffusion equation. From the numerical analyses, it is shown that the surface roughness parameter (Peklenik number), the gas rarefaction parameter (inverse Knudsen number) and squeeze film damping frequencies significantly affect the dynamic characteristics (spring and damping coefficients) of the torsion mirror.