Showing papers on "Torsion (mechanics) published in 1995"

Multi-level deformable mirror device with torsion hinges placed in a layer different from the torsion beam layer

Abstract: A bistable deformable mirror device (DMD) pixel architecture is disclosed, wherein the torsion hinges are placed in a layer different from the torsion beam layer. This results in pixels which can be scaled to smaller dimensions while at the same time maintaining a large fractional active area, an important consideration for bright, high-density displays such as are used in high-definition television applications.

A multiaxial fatigue strain energy density parameter related to the critical fracture plane

Abstract: The cyclic strain energy density parameter W * for the critical or failure plane has been successfully applied in predicting the multiaxial fatigue life of an iron-base and a nickel-base alloy. This parameter has the advantage of being independent of loading condition, allowing a universal energy-life curve to be determined for a variety of torsion, tension and bending stress and strain states. The critical strain energy density parameter has been verified using experimental data obtained from tubular and notched specimens of SAE-1045 steel and Inconel 718.

Experimental Study on the Thermoelastic Martensitic Transformation in Shape Memory Alloy Polycrystal Induced by Combined External Forces

Abstract: Combined tension and torsion experiments with thin wall specimens of Cu-Al-Zn-Mn polycrystalline shape memory alloy (SMA) were performed at temperatureT =A f + 25 K. The general stress-strain behaviors due to the thermoelastic martensitic transformation, induced by a combination of external forces of axial load and torque, were studied. It is shown that the progress of martensitic transformation (MT) at general stress conditions can be well considered as triggered and controlled by the supplied mechanical work (a kind of equivalent stress) in the first approximation. Pseudoelastic strains in proportional as well as nonproportional combined tension-torsion loadings were found fully reversible, provided that uniaxial strains were reversible. The axial strain can be controlled by the change of torque andvice versa due to the coupling among tension and torsion under stress, not only in forward transformation, but also in reverse transformation on unloading. The pseudoelastic strains of SMA polycrystal are path dependent but well reproducible along the same stress path. The evolution of macroscopic strain response of SMA polycrystal, subjected to the nonproportional pseudoelastic loading cycles with imposed stress path, was systematically investigated. The results bring qualitatively new information about the progress of the MT in SMA polycrystal, subjected to the general variations of external stress.

A high‐cycle fatigue criterion applied in biaxial and triaxial out‐of‐phase stress conditions

Abstract: A high-cycle fatigue criterion suitable for multiaxial non-proportional stress loading is proposed in this paper. The criterion is based on some microscopic considerations related to the crystalline structure of metals. The purpose of the present paper is mainly the application of this criterion in two loading cases : (a) biaxial loads involving two normal stresses or one normal and one shear stress, and (b) triaxial load with two normal stresses and one shear stress. Stress states of these kinds are very common in piping assemblies. Application of the proposed criterion in the case of triaxial loading, where the three stress components are of the same frequency, but out-of-phase, leads to a simple analytical formula. This formula is the equation of a bounding surface that delimits in the space of the above three stresses the safety domain against fatigue crack initiation. A remarkable theoretical result concerns the phase difference of the shear stress, which does not appear in the derived formula. Consequently, according to our proposal the safety domain (i.e. the limiting fatigue endurance) under combined out-of-phase biaxial normal stress loading and torsion is independent of the phase difference of the torsion. Obviously this result holds also for the simpler case of axial load and torsion. On the contrary the phase difference between the two normal stresses has a strong detrimental effect on the fatigue endurance of a metal. As is shown these theoretical conclusions are in good agreement with fatigue limit test data found in the scientific literature.

Does the time-of-swing method give a correct value of the newtonian gravitational constant?

Abstract: A standard way of measuring the Newtonian gravitational constant has been the time-of-swing method using a torsion pendulum. A key assumption is that the spring constant of the torsion fiber is independent of frequency. This is likely to be true to a good approximation if any damping present is proportional to velocity. However, recent work on the elasticity of flexure hinges suggests that typically the damping at low frequency is best modeled by including a frequency-independent imaginary component in the spring constant. In this case, the real part of the spring constant must vary, leading to an upward bias in a measurement of $G$.

Analysis of sections subjected to combined shear and torsion--a theoretical model

Abstract: The authors present a three-dimensional behavioral truss model capable of analyzing rectangular reinforced and prestressed concrete sections subjected to combined loading. This model employs the principles of the modified compression field theory (MCFT), and is capable of analyzing sections subjected to combined biaxial bending, biaxial shear, torsion, and axial load. Compatibility of curvatures is introduced to enable the model to handle combined shear and torsion, and to explain nonlinearity in the shear-torsion interaction curve. The model provides a check on spalling of the concrete cover for sections subjected to combined shear and torsion. Calculated deformations and ultimate loads from the model are compared with experimental results and are in good agreement.

Monolithic silicon rate-gyro with integrated sensors

Abstract: A monolithic single crystal Si rate-gyro is disclosed, consisting in the preferred embodiment of an outer torsional frame (101), self-resonating with a substantial amplitude, as controlled by a four-terminal piezo torsion sensor (111), connected to an inner frame (105) by torsion hinges (103). The inner frame itself is connected to a fixed inner post (117), by a set of torsion hinges (107), defining an axis of rotation perpendicular to the first axis. Rotation of the axis of oscillation of the outer body causes the moving mass and the inner frame to tilt and oscillate at the outer frequency due to Coriolis forces, thereby periodically deforming the inner hinges in torsion. These inner hinges are likewise equipped with a four-terminal piezo voltage torsion sensor (115), giving an indication of the rate of rotation of the sensor. The design allows for good sensitivity, due to the substantial swing of the outer oscillator, its high moment of inertia, excellent Si spring characteristics, and excellent sensitivity of the torsional sensors.

Axi-symmetric bodies of equal material in contact under torsion or shift

Abstract: Two axi-symmetric bodies are pressed together, so that their axes of symmetry coincide with the contact normal and the normal force is held constant. A small torque about the contact normal or a small tangential force is applied. For bodies of equal material, the normal and tangential stress states are uncoupled, and can solved separately. The surfaces of the bodies are thought as a superposition of infinitesimal rigid flat-ended punches. Consequently, the normal stress distribution can be calculated as a summation of differential flat punch solutions. A formula results, which is identical with the solution of Green and Collins. After application of a torque an annular sliding area forms at the border of the contact area. For reasons of symmetry, the common displacement of the inner stick area must be a rigid body rotation. Similarly to the normal problem, the solution can be thought as a superposition of rigid punch rotations. The tangential solution can be derived analogically, in form of a superposition of rigid punch displacements. The present method also solves the problem of simultanous normal and torsional or tangential loading with complete adhesion. As an example, Steuermann's problem for polynomial surfaces of the formA2nr2nis solved. The solutions for constant normal forces can be used as basic functions for loading histories with varying normal and tangential forces.

Inelastic Torsion of Steel I-Beams

Abstract: A large twist rotation model for the nonlinear inelastic analysis of the nonuniform torsion of I-section beams is presented in this paper. A finite-element procedure has been developed based on the model. The nonlinear equilibrium equations of beams in nonuniform torsion including the effect of the large twist rotation have been derived. The elastic-plastic behavior of beams in nonuniform torsion is studied using the finite-element procedure and the results are compared with tests. It is found that I-section beams have much larger torsional capacities than can be predicted by linear plastic collapse analysis, and that torsional failure occurs not by the formation of a mechanism but by the tensile rupture of the flanges. A method is proposed for calculating the full plastic nonuniform torque for practical design purposes.

Lateral Force Modulation Atomic Force Microscope for Selective Imaging of Friction Forces

Abstract: An approach to imaging friction force distribution on the nanometer scale is presented, where a sample is laterally vibrated and both the amplitude and the phase of resultant torsion vibration of the cantilever are employed for imaging. It has an advantage over the conventional friction force microscope (FFM) in that the contrast due to local gradient can be significantly reduced. The topographic contrast was almost completely suppressed on a gold evaporated film sample. A method for characterizing friction, the friction force curve (FFC), is proposed where the friction force amplitude and phase are recorded simultaneously with the normal force, as a function of the tip-sample distance. The usefulness of the FFC was verified in characterizing slip and deformation in the tip-sample interaction.

Torsion in Reinforced Normal and High-Strength Concrete Beams Part 1: Experimental Test Series

Abstract: This work examines the behavior of reinforced normal concrete (NSC) and high-strength concrete (HSC) beams subjected to pure torsion. The test series consisted of 12 totally over-reinforced beams, with parameters that influence torsional capacity and concrete strength as the only variable. Therefore, the cross-sectional dimensions, and strength and dimensions of the reinforcement, were constant for all beams. The concrete strength varied between 36 and 110 MPa (5,220 and 15,959 psi). The test series illustrates the advantage in using HSC. In addition to a higher cracking load and higher ultimate torsional capacity, use of HSC for a given cross section and given torque results in higher torsional stiffness, lower crack width, and lower reinforcement stresses compared to NSC.

Structural Dynamic Loads in Response to Impulsive Excitation

Abstract: The modal approach is used to analyze the dynamic loads on a flexible structure due to local impulsive excitations such as that caused by store ejection from a flight vehicle. First-order, time-domain equations of motion in generalized coordinates are constructed for restrained and free-free structures, without and with unsteady aerodynamic effects. The dynamic loads associated with the structural response are expressed by the mode displacement (MD) and by the summation-of-forces methods. The MD approach is simpler and easier to apply, but requires the inclusion of more modes for obtaining results of acceptable accuracy. A rigorous comparison between the resulting loads shows that the performance of the MD method is especially poor when the excitation is local and impulsive. A dramatic improvement is obtained when the generalized coordinates are based on normal modes calculated with fictitious masses at the excitation points. Fictitious masses are also used to generate artificial load modes that yield simple and efficient expressions for integrated shear forces, bending moments, and torsion moments at various structural sections.

Apparatus for measuring viscoelastic properties over ten decades: Refinements

Abstract: This paper describes refinements to an instrument for determining the viscoelastic properties of a solid material isothermally, with a single apparatus, over 10 decades of time and frequency. Torque is applied electromagnetically to a specimen fixed at one end. Specimen deformation is determined via a laser beam reflected from the other end upon a split diode detector. Phase resolution is improved by the use of a lock-in amplifier at high frequency and by the use of Lissajous figures to measure phase, allowing the study of materials of moderate loss (0.008≤ tan δ≤0.2) in addition to materials with high loss (tan δ≊1). The rigidity of the instrument is increased by modifications in the specimen support geometry. The range of equivalent frequency for torsion is from less than 10−6 Hz to more than 104 Hz. Digital methods are incorporated in the creep measurements and in the phase measurements.

Silicon resonant angular rate sensor using electromagnetic excitation and capacitive detection

Abstract: A new silicon resonant angular rate sensor is presented. The sensor consists of a packaged glass-silicon-glass structure and is made by a batch-fabrication process. The sensor is a tuning fork with both sides suspended by torsion bars. Electromagnetic excitation and capacitive detection are used. The applied angular rate generates the Coriolis' force and the resonator starts torsional vibration around the torsion bars. The test device shows a sensitivity of 0.7 fF sec/deg. In this paper, the working principles, fabrication process, and simulated and measured outputs of the sensor are described. The scaling rule of the angular rate sensor are also considered.

Estimation of Accidental Torsion Effects for Seismic Design of Buildings

Abstract: A procedure is developed for including the effects of accidental torsion in seismic design of buildings. It has four steps. First, the ratio between the fundamental frequencies of uncoupled torsional and lateral motions of the building is computed. This ratio and plan dimensions are then used to estimate the increase in displacements at the edge of the building resulting from all sources of accidental torsion. Third, from these edge displacements the increase in displacements at the location of interior resisting planes are estimated. Finally, the design forces in structural members are computed by amplifying the forces ignoring accidental torsion by the increase in building displacements determined previously. This procedure has several advantages over the code-specified static and dynamic analysis procedures to include accidental torsion, such as: (1) The elimination of cumbersome static or three-dimensional dynamic analyses to account for accidental torsion effects in building design; and (2) the inclusion of the effects of all sources of accidental torsion. Two building examples are presented to illustrate these advantages as well as the computational steps required to implement the new procedure.

Upper limit on the torsion coupling constant.

Abstract: Known data are used to place an upper bound on new coupling constants that arise in gravitation with torsion that is derived as the exterior derivative of a potential. The form of the quantum-mechanical interaction is deduced from the classical theory and verified by the nonrelativistic reduction of the Dirac equation. Both minimal and nonminimal coupling is considered. \textcopyright{} 1995 The American Physical Society.

Design for Torsion

Abstract: In the 1995 ACI Building Code, the section on design for torsion will be completely revised based on a thin-walled tube, space-truss analogy. This paper explains the origin of the new procedure, derives the design equations, and compares the method to test results

Fatigue behavior of a plain-woven glass fabric laminate under tension/torsion biaxial loading

Abstract: Fatigue strength and stress-strain response of a plain-woven glass fabric laminate subjected to pulsating tension/pulsating torsion biaxial loading were investigated. Biaxial loads were proportionally applied. S-N curves at different biaxial stress ratios were obtained. For the S-N diagram, the normalized fatigue strength defined by the ratio between the cyclic biaxial stress and the static strength, was used. All data roughly locate on a slightly wide band on the normalized S-N diagram, although the tendency that the slope of the S-N curve becomes low with an increase of the shear stress component is distinguishable. Modulus decays both in tension and shear with respect to loading cycles under biaxial loading are almost the same as those under uniaxial tension and pure torsion loadings, respectively. The direction of matrix cracks is dependent on the principal stress direction under biaxial loading although it does not coincide with the direction of the principal plane.

Vibration of Thin-Walled Box-Girder Bridges Excited by Vehicles

Abstract: This paper presents a procedure for obtaining the dynamic response of thin-walled box-girder bridges due to truck loading. The box-girder bridge is divided into a number of thin-walled beam elements. Both warping torsion and distortion are considered in the study. The analytical vehicle is the American Association of State Highway Transportation Officials (AASHTO) HS20-44 truck simulated as a nonlinear vehicle model with 11 independent degrees of freedom. Four different classes of road-surface roughness generated from power spectral density function for very good, good, average, and poor roads are used in the analysis. The proposed procedure has been checked against the folded-plate method studied by former investigators. The comparison between these two methods is very good. The analytical results show that the dynamic response of vertical bending moment is caused mainly by several vibration modes, and that of torsion and distortion is greatly affected by the higher modes.