Showing papers on "Torsion (mechanics) published in 1991"

Passive material properties of intact ventricular myocardium determined from a cylindrical model.

Abstract: The equatorial region of the canine left ventricle was modeled as a thick-walled cylinder consisting of an incompressible hyperelastic material with homogeneous exponential properties. The anisotropic properties of the passive myocardium were assumed to be locally transversely isotropic with respect to a fiber axis whose orientation varied linearly across the wall. Simultaneous inflation, extension, and torsion were applied to the cylinder to produce epicardial strains that were measured previously in the potassium-arrested dog heart. Residual stress in the unloaded state was included by considering the stress-free configuration to be a warped cylindrical arc. In the special case of isotropic material properties, torsion and residual stress both significantly reduced the high circumferential stress peaks predicted at the endocardium by previous models. However, a resultant axial force and moment were necessary to cause the observed epicardial deformations. Therefore, the anisotropic material parameters were found that minimized these resultants and allowed the prescribed displacements to occur subject to the known ventricular pressure loads. The global minimum solution of this parameter optimization problem indicated that the stiffness of passive myocardium (defined for a 20 percent equibiaxial extension) would be 2.4 to 6.6 times greater in the fiber direction than in the transverse plane for a broad range of assumed fiber angle distributions and residual stresses. This agrees with the results of biaxial tissue testing. The predicted transmural distributions of fiber stress were relatively flat with slight peaks in the subepicardium, and the fiber strain profiles agreed closely with experimentally observed sarcomere length distributions. The results indicate that torsion, residual stress and material anisotropy associated with the fiber architecture all can act to reduce endocardial stress gradients in the passive left ventricle.

An introduction to expert systems : the development and implementation of rule-based expert systems

Abstract: Fundamental principles concept of stress, strain and material relations analysis of stress and strain axially loaded members torsion stresses in beams combined stresses deflection of beams by integration deflection of beams by special methods energy methods inelastic behaviour buckling of columns moments of areas.

Overview No. 92: Materials and shape

Abstract: A procedure is described for selecting materials and section shapes for load-bearing components. The choice depends on the mode of loading (tension, bending, torsion), on the available shapes (solid sections, tubes, I-sections and so on), on the properties of the material of which the shape is made (modulus, strength, density), and on some criterion of performance (minimising weight, for instance). A distinction is drawn between macroscopic and microscopic shape, both of which can increase performance in bending or twisting, and which can be combined to give very efficient structures. The approach gives insight into the efficiency of natural materials such as wood; and it suggests routes for developing new, efficient, material-shape combinations.

Collapse of axially compressed cylindrical shells with random imperfections

Abstract: Using the first-order, second-moment analysis, a stochastic method is presented, whereby the stability of isotropic, orthotropic, and anisotropic nominally circular cylindrical shells under axial compression, external pressure, and/or torsion possessing general nonsymmetric random initial imperfections can be evaluated

A joint torque sensing technique for robots with harmonic drives

Abstract: The authors propose a joint torque sensing technique making use of the existing structural elasticity of robots. The technique provides joint torque sensing without reducing the stiffness of the robot or changing the mechanical structure of the joints. The elasticity of the flexsplines of the harmonic drives is utilized to measure the joint torque. The flexsplines are flexible thin cups, made from steel, in the harmonic drives that are driven by the wave generators. A finite-element analysis of the flexsplines shows that a special configuration of strain gauges, mounted on the flexspline, has to be employed to eliminate errors in sensor information due to rotation of the wave generator. Characteristics of the torque sensor are examined experimentally. The linearity and the dynamic response are almost the same as those of a conventional sensing technique. For a one-link robot arm, both theoretical and experimental investigations support the validity of the sensing technique. >

Thin‐Walled Multicell Box‐Girder Finite Element

Abstract: A thin‐walled‐box‐girder finite element that can model extension, flexure, torsion, torsional warping, distortion, distortional warping, and shear lag effects was developed using an extended version of Vlasov's thin‐walled beam theory. The element has two end nodes, but it has besides the six nodal degrees of freedom of a conventional beam element, additional degrees of freedom to account for torsional warping, distortion, distortional warping, and shear lag. The governing differential equation pertaining to each action was used to derive the exact shape functions and the stiffness matrix and nodal load vector of the element. An orthogonalization procedure was employed to uncouple the various distortional and shear lag modes. A numerical example was solved that compared the proposed method with the facet‐shell finite element analysis, with good agreement between the two sets of results.

Torsion exercising device

Abstract: A torsion exercising device which includes a support frame for supporting the exerciser and one end of a pivot arm assembly in pivotally adjustable relationship with a power frame mounted on the opposite end of the pivot arm assembly. The power frame includes a mount bar for rigidly securing the power frame to the pivot arm assembly, a torque tube rotatably and adjustably mounted in the power frame parallel to the mount bar and a torsion bar disposed in sliding relationship inside the torque tube, wherein the exerciser may adjust the rotational position and linear extension of the torsion bar inside the torque tube to create a desired position and resistance for pulling and pushing exercises.

Spinning Test Particles in Spacetime with Torsion

Abstract: We use the Fock-Papapetrou method to derive equations of motion for a spinning test particle moving both in the background gravitational field and a torsion field. The resultant two equations, the equations of world line and spin can be written in a covariant manner (as is the same for Papapetrou's result in general relativity). The equation of spin reproduces all the known equations of spin precession due to torsion.

Capacitor and pressure transducer

Abstract: A dual capacitor device for utilizing dependent capacitances for measurement purposes where an upper capacitor base member and a lower capacitance base member are arranged with parallel capacitor surfaces in horizontal planes and where the parallel capacitor surfaces are located on opposite sides relative to a vertical axis to define first and second capacitors. The lower capacitor base member has a centrally located section connected to an outer section of the lower capacitor base member by a torsion beam and the centrally located section is attached to the upper capacitor base member by a vertical arm member. The torsion beam means has a displacement axis normal to the central axis so that angular displacement of the torsion beam and the centrally located section about the displacement axis produces a capacitance change in the first and second capacitors. A force moment applied to the centrally located section by a spirally wound bourdon tube in response to pressure develops a low force on the torsion beam which is less than the micro yield point of the torsion beam material and develops a micro dimensional angular displacement of the centrally located section and a capacitance change as a function of the force to measure high pressures with small deflections of the capacitor. The structure is arranged to compensate for changing temperature.

Distortion of line and surface elements in model turbulent flows

Abstract: Material lines and surfaces transported in a random velocity field undergo bending and stretching. In this paper we investigate the time evolution of curvature in line and surface elements both analytically and by numerical simulation for a simple model turbulence. Our analysis is close to that of Pope (1988) for the evolution of curvature in surface elements. We show that the equation governing the evolution of curvature in a line element is very similar to that governing the evolution of the principal curvature in a surface patch. We investigate the circumstances in which the effect of straining fluctuations is to cause the exponential rate of growth of curvature discovered by Pope et al. (1989). Our simulation confirms that the presence of helicity in the turbulent flow results in the development of a non-vanishing mean torsion in a line element. The results of the simulation also suggest that the generation of curvature tends to occur in regions different from those associated with rapid stretching. The generation of torsion, however, is found not to be correlated with either bending or stretching.

Apparatus for the correction of scoliosis

Abstract: Method for correcting the shape of a spinal column. A rod is attached to the vertebrae in the longitudinal direction of the spinal column, at the rear thereof, across the part which is to be corrected. The rod consists of a material, for instance of an Ti-Ni-alloy, with a shape-memory with a transition temperature, lower than the body-temperature. The rod is bent and possibly twisted, corresponding to the curve and/or torsion of the spinal column at a temperature, under the transition-temperature. The rod has a perpendicular cross-section with a linear size in one direction, at least so large, that above the transition-temperature, the rod wants to assume its original shape with a constant force.

On the mechanics of large inelastic deformations : noncoaxiality, axial effects in torsion and localization

Abstract: The macroscopic behavior of materials subjected to large deformations is investigated by considering the structure and mechanics of the single slip. Physically based constitutive relations for the plastic flow and material spin are then rigorously derived by resorting to the concept of non-coaxiality; yielding a class of vertex-type plasticity models incorporating features like noncoaxiality and large material rotation. It is shown that these phenomena are inherently coupled through the concept of plastic spin as it relates to the persistence of noncoaxiality. The implication of such phenomena to large deformations is investigated by examining their influence on the development of axial effects accompanying finite shear deformation, as well as on the onset of localized shear bands.

Computer simulation of the dynamics of the plastic phase of succinonitrile

Abstract: The complex dynamics of the plastic phase of crystalline succinonitrile CN–CH2–CH2–CN, involving trans–gauche isomerization as well as molecular reorientations along the diagonals of the cubic unit cell, is investigated by means of molecular dynamics simulation. A simple molecular model in which the end CN groups and the CH2 groups are represented by single pseudoatoms is proposed. The model includes, in addition to the external degrees of freedom, three degrees of internal freedom described by two bending and a torsion coordinate. The intermolecular interactions are treated in terms of Lennard‐Jones potentials. Force constants are used for the bending coordinates and the intramolecular torsion is described by a two minima potential function with three parameters. The dynamical properties of the crystal are investigated by calculation of the time dependence of single molecule autocorrelation functions. It is found that the most suitable autocorrelation functions are those involving the torsion and the ori...

Torsion beam accelerometer

Abstract: A semiconductor accelerometer having a sensor element supported by a pedestal coupled to a substrate. Coupling between the pedestal and the sensor element includes opposing torsion beams along the axis of flexure coupled through a spring to the pedestal. The spring includes a pair of beams extending in a direction perpendicular to the torsion beams so that thermal expansion applies a tensile force to the torsion beams.

Analysis of elastic-plastic torsion of circular bars at large strains

Abstract: Large elastic-plastic torsion of uniform circular bars is investigated numerically, using special finite elements. Two end conditions corresponding to fixed-end torsion and free-end torsion, resp., are considered, taking full account of the axial effects. The analysis is carried out using an isotropic hardening as well as a recently proposed large deformation kinematic hardening model. Either linear hardening or power law hardening uniaxial stress-strain curves are assumed in the analysis. Results are presented in terms of predicted torque vs. twist curves for all cases. Furthermore, we present axial force vs. twist curves for fixed-end torsion or elongation vs. twist curves for free-end torsion. Special attention is paid to the so-called plastic spin, a concept which has recently been identified to play a key role in the description of induced anisotropy. The differences between free-end and fixed-end torsion are emphasized.

Conformal invariance, extremals, and geodesics in two‐dimensional gravity with torsion

Abstract: The conformal symmetry transformations are elucidated for two‐dimensional gravity with torsion in conformal gauge. The model is proved to describe one parameter family of surfaces with torsion. On these surfaces equations for extremal and geodesic lines are completely integrated and analyzed.

Mid-beam jointed reconfigurable bearingless main rotor assembly

Abstract: A mid-beam jointed reconfigurable bearingless main rotor (BMR) assembly that includes reconfiguration joints that facilitate BMR assembly reconfiguration by main rotor blade removal or folding to reduce the structural envelope of a helicopter for rapid deployment, routine transport, and/or storage. The BMR assembly comprises a rotor hub structure that includes a plurality of radially extending flexbeams, combinations of torsion flexure members and main rotor blades disposed in combination with corresponding flexbeams, and torque tube subassemblies disposed in combination with corresponding flexbeams and torsion flexure member, main rotor blade combinations. Each reconfiguration joint is formed by a combination of an outboard joint segment of a flexbeam and an inboard joint segment of the corresponding torsion flexure member. The rotor hub structure is fabricated so that the configuration joints are purposefully located at radial stations that are subjected to minimal flapwise loading, which for aerodynamic efficiency and manufacturing considerations should not exceed about fifteen percent of the overall span of the BMR assembly. The torque tube subassemblies are fabricated to provide access to the respective reconfiguration joints. For reconfiguration by the removal technique, each torque tube subassembly includes one or more access panels. For reconfiguration by the folding technique, each torque tube subassembly includes a removable splice tube.

Motor control system for electric power steering apparatus

Abstract: A motor control system controls a steering apparatus with an electric motor. A command signal is generated in accordance with a torsion torque of the steering apparatus and the rotational direction and torque of the electric motor is controlled in accordance with the command signal. The system includes a vehicle speed sensor, a hysteresis circuit for changing a hysteresis width in accordance with an output from the vehicle speed sensor, and a phase compensation designating section responsive to an output from the torsion torque sensor supplied via the hysteresis circuit for outputting a command signal corresponding to a change rate of the torsion torque, so that the rotation direction and torque of the electric motor is controlled in accordance with the command signal. The hysteresis circuit makes large the hysteresis width during a high speed running of a vehicle to avoid influence of external disturbance due to vibrations from roads.