Showing papers in "Strain in 1975"

The Finite Element Method—A Basic Introduction

Abstract: Basic concepts The concept of stiffness analysis Bar finite elements Finite elements of continua Triangular finite element for plane elasticity Rectangular finite element for plane elasticity Rectangular finite element for plate flexure Analysis of folded-plate, box-girder and shell structures using rectangular elements Axially symmetric continua Programming Triangular finite element for plate fixture A review of developments in the finite element field Appendices.

On the effect of residual birefringence in anisotropic photoelastic materials

Abstract: The effect of residual birefringence on isoclinic angle and fringe order is considered for mechanically and optically anisotropic materials. Toupin's4 theory of deformable dielectrics forms the basis of the investigation. Equations are derived for fringe order, stress fringe value and isoclinic angle in linear orthotopic photoelastic materials. The principal effect of residual birefringence relates to the determination of the isoclinic angle. This is particularly so for certain fibre–orientations. Unlike the isotropic case, the isoclinic angle depends on the stress at which it is measured.

A computer program for calculating post–yield stresses from strains measured on strain hardening materials

Abstract: Details are presented of a computer program developed to calculate the biaxial post–yield stress history from strain gauge readings on components made of strain hardening materials, without knowledge of the material's post–yield stress/strain behaviour. The accuracy of the program is illustrated by using it to predict the stresses in a tensile test on stain/ess steel.

Determination of stress concentration factors for grooves and fillets by the electro–plating technique

Abstract: The electro–plating technique of stress analysis involves the use of copper–plated specimens and measurements are based on the appearance of grains grown in the plated copper as a result of cyclic loading. A description is given of the technique and its application to the determination of stress concentration factors for grooves and fillets on circular shafts subjected to torsion. Results are compared with corresponding published values obtained by other methods.

A speckle interferometric method for the determination of time–dependent displacements and strains

Abstract: A simplified theory of speckle interferometry as a means of measuring localised displacement, is presented. In a simple experiment good correlation was obtained between strains determined by this method and strain gauge readings. A special automated camera capable of monitoring a continuously varying displacement is described.

Measurement of strains in plain and perforated epoxy models by means of embedded strain gauges

Abstract: In the embedded strain gauge technique, the use of miniature gauges with very thin copper leads minimises reinforcing effect on the model The technique in this form, was applied to the investigation of three–dimensional strain distributions Suitability of the method was assessed by determining the mechanical properties of epoxy resin from strains measured in cylindrical and rectangular models subjected to uniaxial tension and in a rectangular beam model subjected to pure bending The central portion of the beam was subsequently perforated with a triangular array of holes to simulate the geometry of a heat exchanger tube plate Strains in the ligaments were then measured for various ligament efficiencies, with the model under bending and under tension

The effect of a sliding load on strain gauges embedded under the contact surface

Abstract: Strain gauges embedded in plastic models can be used in three dimensional stress analysis. When gauges are located immediately below a surface subjected to the tangential traction of a sliding load, unexpected values are recorded. This phenomenon was investigated by suitably loading a semi–infinite solid with normal and shear stresses and comparing theoretical strain values with the experimental results. The comparison showed that strain gauges embedded in epoxy can be successfully used under sliding loads provided that the gauge depth below the contact surface is greater than a critical value.

The torsional stiffness of a shaft with an elongated slot

Abstract: An analysis is given for estimating the torsional stiffness of a circular shaft with an elongated slot symmetrical about its centre line. The solution for the stress function is by a finite difference approach and because of symmetry, only one quarter of the cross–section is considered. A non–dimensional graph is presented of the ratio of the stiffness of a slotted shaft to that of a similar solid shaft, for different values of slot width to shaft diameter ratio. The effects of the stress distribution around the ends of the slot have been neglected and the graph gives a lower bound value for the stiffness ratio. Experiments have been carried out to verify the computation and to investigate the end effects.

Correcting for the effects of temperature on strain gauges —-use of computer–logger on Milford Haven Bridge project

Abstract: The effects of temperature changes on strain gauge readings depend partly on the characteristics of the gauge and measuring equipment, and partly on the thermal properties of the structure being tested. The effect on the gauge can be numerically corrected, or compensated for by experimental techniques, and if required, the effect of temperature changes on the structure can be reduced by careful selection of the time of reading. Correction of Demec, vibrating wire and electrical resistance strain gauge outputs is discussed. Experience with a computer controlled data logger used to correct for thermal effects on a 1/4-bridge electrical resistance strain gauge installation is described. The use of this system on the Milford Haven Bridge is estimated to have reduced potential temperature errors from between ±100 to 200 μstrain to about ±20 to 30 μstrain.

Brush coating of resists for normal and high frequency moiré techniques

Abstract: A limitation on the use of normal moire technique may be the practical difficulties which can be encountered on coating the resist layers for the production of etched gratings. A description is given of a simple coating method which can be applied to any testpiece and which requires only a brush and some skill on the part of the operator. The method can also be used to coat resist layers suitable for the production of high frequency gratings of more than 1 000 lines/mm (25400 lines/in).

On the application of complex potentials and photoelastic data to the solution of plane problems

Abstract: This article demonstrates the accuracy and convenience of a recently developed analytical technique for determining the stress and displacement fields for plane problems from point-wise photoelastic data and finite order, polynomial approximations of complex potentials of the Kolosoff type. The order of the approximations is consistent with the assumption of a stress field which varies essentially in a quadratic manner within each region or subregion in which the approximations are fitted to the photoelastic data. The theoretical basis of the technique is reviewed briefly and the method is then applied to the problems of an end-loaded cantilever, a ring and a disc under diametral compression, and a vertical point load on a half-space. Uncertainties associated with inevitable errors in experimental data are avoided by generating isoclinic and isochromatic data from the theoretical solutions of the four problems, these ‘photoelastic results’ then forming the basic initial data upon which the technique operates. Computations of the stresses, rotations and displacements indicate that the complex potentials method is equally applicable along lines of symmetry, on contours around point loads, in regions where the stress is predominantly uniaxial in character, and also where the stress field is completely two-dimensional. The computed and theoretical values are virtually in exact agreement whenever the density of the randomly distributed data points is such that the stress field is accurately approximated by a quadratic function within the region spanned by the data points. The local approximations of the potentials are fitted to the experimental results at these data points.

Practical aspects of transducer application

Abstract: In selecting transducers to use in a given application, it is not enough merely to consult the catalogues and choose one having the right range—-even when this is known initially. It is important to have regard to the parameter to be read and the method of connecting the transducer (in as far as its place of sensing is concerned) and to consider how the interface with the read–out equipment is to be effected. This choice may also be affected by the method chosen for display and/or recording. Such is the field covered by this article, which does not deal with the design of transducers—-except where this conditions their suitability for the particular measurements—-nor is their manufacture or calibration a subject of comment. Because the writer has been mainly concerned with development testing, particularly of aeroengines, the article will deal primarily with such measurements as represent transient, pulsating or vibratory effects and less with steady state checks. Many of the precautions—-but not all—-that are mentioned, are relevant also to the latter cases. Measurements at frequencies over 5 Hz are thus implied, unless otherwise stated.

Protection of vibrating wire surface strain gauges

Abstract: To protect vibrating wire gauges against damage from construction work and weather, a cover has been designed which permits access to the gauge for manual and automatic readings and which does not interfere with gauge performance. The protected gauges have been used to measure strains on steel plates and stiffeners forming the diaphragm, webs and flanges of the box girder of the Milford Haven bridge. 95 of the 100 gauges installed were still operating satisfactorily after 16 months use in very adverse conditions. (A) /TRRL/

Tests on ring–stiffened cylindrical shells with external pressure

Abstract: External pressure tests were conducted on thin multi–bay cylindrical shells to investigate the significance of out-of-roundness in relation to elastic and collapse behaviour. Conclusions are drawn about the developing elastic deflection profiles of the shells during loading and suggestions are proposed for the prediction of the collapse mode and the collapse pressure.

Birefringence in inhomogeneous stress fields

Abstract: The propagation of polarised light through a birefringent medium subjected to an inhomogeneous stress field in the direction of propagation, is usually described in terms of Neumann's differential equations, the Poincare sphere, or 2times2 complex transformation matrices. A comparative treatment of these three methods is presented.

A new self–oscillating strain gauge

Abstract: This article describes the development, characteristics and limitations of an invention which enables a dual strain gauge, together with a solid state module, to produce an F.M. signal uniquely related to strain in the gauges. The frequency of this signal can be read by a crystal counter at the end of long leads and through ordinary switches, without error therefrom.

The use of strippable photographic film for moire grating production

Abstract: Practical details are presented concerning the use of strippable film for the production of moire gratings. Strippable film is produced primarily for application in graphic arts. The influence of the use of the film on the accuracy of measurement in the direct moire method is discussed.