Showing papers on "Stiffness published in 1979"

The Fibre Composite Helicopter Blade: Part I: Stiffness Properties: Part II: Prospects for Aeroelastic Tailoring

Abstract: In Part 1 expressions are derived for the coupled torsional, extensional and flexural stiffnesses of a fibre composite tube, such as a helicopter blade, which is subjected to torsion, longitudinal tension, chordwise and flapping bending moments and shear. The theory is an extension of Batho-Bredt engineering analysis and is valid for a cylindrical tube of arbitrary cross-section with an arbitrary circumferential distribution of fibre composite plies. Particular attention is paid to the coupling effects in which an asymmetric fibre lay-up results in a twisting of the tube under bending and/or tension. Consideration is also given to the influence on the stiffness characteristics of an initial twist in the tube. In Part 2 the analysis of Part 1 is developed numerically for tubes representative of GFRP blades. Static and dynamic aspects are considered which pave the way for aeroelastic tailoring studies of such blades.

Weight functions from virtual crack extension

Abstract: The stiffness-derivative method of Parks1 for calculating the linear elastic crack tip stress intensity factor for any symmetric crack configuration and a particular loading is extended to calculate the weight function vector field2,3 which serves as a Green's function for the stress intensity factor. The method, which combines the observations of Rice3 on the weight function and of Zienkiewicz4 on the differential stiffness method, permits very efficient determination of the weight function, requiring only one additional back-substitution on the already-factored stiffness matrix. Thus, the stress intensity factor for arbitrary loading of this configuration can subsequently be determined by quadrature alone. The promising extension of the method to three-dimensional configurations is outlined. While this manuscript was under review, the authors became aware of the recent work of Vanderglas,21 in which the same approach as ours is used to extend the stiffness derivative method. The present work was then voluntarily revised in order to address further certain aspects of the topic of shape function perturbation, which Vanderglas noted.

Non‐linear analysis of coupled wall systems

Abstract: The non-linear response history and failure mechanism of coupled wall systems under dynamic loads and static loads are investigated through an analytical model. The walls and coupling beams are replaced by flexural elements. Axial and shear stiffnesses are included for the wall members. The stiffness characteristics of each member are determined by inelastic properties. The suitable hysteresis loops to each constituent member are established to include the specific characteristics of coupled wall systems. The computed results are compared with results obtained from tests using model structures statically and dynamically tested on the Illinois Earthquake Simulator.

On the Elastic Constants of Plane Orthotropic Elasticity

Abstract: The four independent material parameters of plane orthotropic elasti city are introduced as the effective stiffness, the effective Poisson ratio, the stiffness ratio and the shear parameter. It is proved that stress boundary value problems with zero resulting force on internal contours lead to stress fields that are independent of the effective stiffness and the effective Poisson ratio, and a general transformation is described which is equivalent to a change of the stiffness ratio. These properties suggest the importance of the remaining shear parameter, that has the interesting property of being invariant with respect to a 90 degree rotation of the principal axes of the material.

Fibers, yarns and fabrics of low modulus polymer

Abstract: Fibers, yarns and fabrics are produced from polymers, such as the copolymers of ethylene and vinyl acetate, having an elastic modulus of from 5,000 to 60,000 psi. The fibers are also characterized by an area moment of inertia of from 400×10 -14 to 7,000×10 -14 in 4 and a stiffness parameter of from 1×10 -5 to 1×10 -8 lb-in 2 . Multiple fibers are spun into yarn, preferably cross-linked either chemically or by irradiation and are formed into pile fabrics for carpeting and similar uses. The pile fabric preferably has a minimum of 4,000 fibers per in 2 of backing and a minimum pile height of 1/8 inch.

Matched stiffness rotor flexbeam and blade system

Abstract: A flexbeam having matched in-plane and out-of-plane stiffness is described extending at least partway between the tips of a pair of rotor blades bonded thereto. The flexbeam so constituted produces a blade system with a primary in-plane frequency less than the primary forcing frequency. The matched stiffness of the flexbeam is provided by admixing, either compositionally or by laminating, materials of differing flexibilities to produce a high damping material of symmetrical stiffness.

The Mechanical Stiffness of Articular Cartilage in Confined Oscillating Compression

Abstract: The paper describes an experimental investigation of the mechanical stiffness of bovine and human articular cartilage in a compression test which allows strain and fluid flow only in the direction ...

On the measurement and calculation of vertical track modulus

Abstract: This paper presents the results of a series of tests and analyses directed towards the characterization of the track structure under vertical loads. It also presents and evaluates different analytical techniques for the calculation of the vertical track modulus. In a series of tests at the Association of American Railroads's Track Structures Dynamic Test Facility, the response of the track was obtained by monitoring track deflection under increasing vertical loads. This load and deflection data was then used to calculate vertical track modulus, track stiffness and track compliance. Three widely used techniques were utilized to calculate the vertical modulus. The results of the tests indicate that the modulus of the track is related to the level of loading; thus identical track can give different modulus values for different load levels. Of the three different techniques used to calculate track modulus, the beam-on-elastic-foundation technique was found to be the most applicable to field measurements since it requires a minimum number of track deflection values.

Non-woven low modulus fiber fabrics

Abstract: Non-woven fabrics suitable for use as carpeting and the like are produced by needlepunching staple length monofilament fibers having an elastic modulus of from 5,000 to 60,000 psi. The fibers are also characterized by an area moment of inertia of from 400×10 14 to 7,000×10 14 in 4 and a stiffness parameter of from 1×10 -5 to 1×10 -8 lb-in 2 .

Approximate Expressions for the Elastic Constants of Cord-Rubber Laminates

Abstract: The relatively simple equations given herein for the four independent elastic constants of a single ply of cord and rubber and for the three independent elastic constants of a cord-rubber angle-ply laminate can be used to assess with sufficient accuracy for many purposes the effect of changes in constituent material properties and/or ply geometry on the stiffness characteristics of unidirectional, filamentary composites used in pneumatic tires.

Dynamic response of elastic plates on the surface of the half‐space

Abstract: A ‘mixed method’ for the dynamic calculation of foundations on soil is presented. The half-space is computed analytically, the foundation with finite elements. The method is very well suited for the solution of three-dimensional problems including interaction. Numerical results for a simple case involving a dynamically loaded elastic plate are presented and the influence of plate stiffness is studied.

55 Nitinol wire: force developed as a function of "elastic memory".

Abstract: Closing spaces is done either by translation of the edgewise brackets over an archwire or by the use of closing loops of some common design. This investigation suggests the concept of the further refinement of the "wire rubber band" as a means of closing spaces. Advantages of such a wire hypothetically would be to level the dentition, align the dentition, and condense spaces in the dentition with only one or two stretched nickel titanium wires having the proper temperature transition range (TTR). The temperature of the mouth would make such wires return to their original prestretched length. A correctly-sized wire would have sufficient stiffness at mouth temperature, and it would be kept at or above its TTR. The system eliminates friction from edgewise bracket translation and also the use of many auxiliary closing devices as well as reducing the number of archwire changes, thus saving chairside time.

Dynamic stiffness for rectangular rigid foundations on a semi‐infinite elastic medium

Abstract: The present paper deals with the dynamic steady-state force-displacement relationships (complex stiffness) for rectangular rigid foundations resting on a semi-infinite medium, consisting of homogenous, isotropic, linear elastic materials. The foundations are considered to be excited under harmonic vertical and rocking vibration. This gives mixed boundary value problems which cannot be easily solved by analytical approaches. Therefore, a numerical method is proposed here. The method is based on quite, simple equations, and is straightforward in computation, compared with other methods. Although the proposed method gives just approximate solutions, it is satisfactory for engineering practices, and the soluations become highly accurate for a small value of ωB/Vs. The results obtained by the proposed method are compared with those of other methods to evaluate the accuracy of the results. The effects of length/width ratio and the area of the contact plane of the foundations are also discussed.

Measurement of Bending Deformation for Small Diameter Orthodontic Wires

Abstract: The stiffness tester and torque meter were found to yield nearly the same measurements of bending deformation for orthodontic wires as small as .007 inch diameter, provided the different bending apparatus are calibrated to each other. Although frictional losses in the test apparatus did not appear to be significant, the test span length does significantly affect measured values of Young's modulus and flexural yield strength, in a manner similar to that previously found for larger-diameter wires.