Showing papers in "Strain in 1983"

Some aspects of clip gauge design

Abstract: Measurements show that simple beam theory accurately models the performance of clip gauge transducers. Assuming typical dimensions for the single and twin cantilever gauge it is found that the single cantilever gauge possesses about four times the sensitivity of the twin cantilever gauge, for the same stiffness. Design rules are specified by which gauges of high sensitivity and low stiffness may be constructed. A simple and reliable technique for fastening clip gauges to a test piece is introduced.

Use of the Southwell method to predict buckling strengths of stringer stiffened cylindrical shells

Abstract: This paper presents the results of some tests that were conducted on stringer stiffened epoxy cylindrical shells. The object of the investigation was to determine if the Southwell technique for estimating buckling loads could reasonably be used for the non-linear collapse of cylindrical shells. The results showed that the technique provides a useful estimate of the buckling load provided care is taken in interpreting the results. In some instances the estimate is likely to be greater than the actual load. Where two buckling modes are encountered at the same time the Southwell technique appears to predict the critical load associated with either mode depending on the deflection parameter being measured as well as on the interpretation of the results. (Author/TRRL)

The design of a multi‐purpose multi‐component strain gauge dynamometer

Abstract: The design, construction and calibration of a multi-purpose multi-component strain gauge dynamometer is described. The dynamometer was constructed for use with a free surface water channel, which can also be used as a water tunnel or a cavitation tunnel. In this experimental facility, the large speed range available (0·03 to 6·1 m/s) and the variety of projects encountered require the measurement of a wide range of loads in several different configurations. The dynamometer system has been based on a unit block concept so that the blocks can be assembled into a suitable configuration and complexity of components to suit each particular problem. The basic design concept is described together with the calibration results and the magnitude of the interactions. It is concluded that the overall accuracy of the dynamometer blocks and the absence of interaction coupled with the versatility of the system confirm the choice of the original design.

Least squares asymptotic analysis of photoelastic data from stress concentration regions in plane problems

Abstract: On realise une separation complete des contraintes dans toute la region de concentration de contraintes pres des entailles dans les plaques planes, isotropes, lineairement elastiques, en utilisant des donnees photoelastiques faciles a obtenir. Ces donnees sont analysees par une methode de moindres carres basee sur des equations qui decrivent le caractere asymptotique du champ de contrainte a variation rapide pres de la racine de l'entaille

Rayleigh wave interaction with inhomogeneities—Part 1: Near surface cavity and inclusion

Abstract: The interaction of a Rayleigh pulse with a circular cavity and circular inclusion has been examined employing dynamic photoelasticity. The explosively generated R-pulse travels along the free boundary and is trapped in the ligament between the cavity and the free boundary with little energy propagating around the cavity. For cavities and inclusions at shallow depth below the free surface failure of the ligament occurs and most of the energy associated with the incident R-wave is converted by diffraction into body and surface waves. The existence of a shadow zone on the downstream side of the circular cavity provides a means of identifying the location of the cavity which is nearly tangent to the surface by e.g. ultrasonic inspection.

Strain field analysis with the X‐ray integration method

Abstract: The so-called X-ray sin2 method for measuring internal stresses is not always acceptable, because it postulates a constant orthogonal stress distribution in the layer passed by the beam. It is demonstrated that the often measured non-linear distribution of deformations is numerically connected with stress distributions, which are variable and inclined against the surface. The calculation of the internal stresses and their parameters is based on the solution of trigonometric polynoms of the variable with integrals. The penetration depth of the X-ray beam must be known. For general numeric solutions a computer program can be given. Solutions step by step can be carried out with a programmable pocket calculator. The ‘sin2 method’ is a special case of the integration method, which is still developing.

Rayleigh wave interaction with inhomogeneities—Part II: The buried cavity and inclusion

Abstract: The elastodynamic interaction between an explosively generated Rayleigh pulse and a buried imperfection such as a cavity or an inclusion in a half plane is investigated. Dynamic photoelasticity was employed to obtain full field information for data analysis. Measurements of the stress distribution along the free boundary of the half plane and the cavity and along the cavity/inclusion interface have been made. Results show that fractures occur for shallow as well as for deeper burial depths initiating from the cavity boundary.

Further proposals for the application of liquid crystals to photoelastic instrumentation

Abstract: Liquid crystal displays (L.C.D.'s) are well known for their application to electronic watches, calculators, games and digital thermometers, the technology involved being derived from electro-optic effects observed in liquid crystals since about 1968. This paper describes proposals for a totally different application of one particular class of liquid crystal, as an optical element in a photoelastic transmission or reflective polariscope. The proposals are currently under development* and considerable work remains before this particular application of liquid crystals is perfected. Early results are promising and if successful they could well produce a revolution in the design of optical equipment with particular attraction to automatic systems incorporating microcomputer control.

Three methods to ascertain proper compensation in a general system of retarders

Abstract: One can regard a three dimensional photoelastic model as a general system of linear retarders. These linear retarders can he individually separated out using a compensator. In the general case, a compensator can he used to cancel the combined effect of the linear retarders up to a certain required point along the light path. The discrete linear retarders from the point of entry of light to the model to the point along the light path under consideration can be replaced by a resultant optically equivalent model which is characterised by three discrete quantities called the characteristic parameters.1 If the compensator cancels the characteristic retardation completely and is orientated along the axis of the linear retarder of the optically equivalent model for any general light path, such a configuration of the compensator would correspond to the ideal case of compensation. However there could exist, for any particular three dimensional model, particular values of characteristic parameters, which, along with the value of retardation introduced by the compensator would result in a resultant optically equivalent model having the same primary and secondary characteristic directions2 as in the ideal case of compensation. This can lead to a misinterpretation of the results. In this paper three very simple methods have been described to ascertain the correct positioning of the compensator.

Reduction of static strain for increased life of transmission line conductors

Abstract: Fatigue failures of transmission line conductors are known to occur close to the suspension clamps. The causes of such failures are known to be due to high static stresses combined with dynamic stresses resulting from vibration. The overall stress in the aluminium strands of a conductor could be limited by reducing the static bending stress near the clamps. Passive protective devices such as neoprene lining, damping shims introduced in the suspension clamps would bring down the stress level, thereby increasing the service life of conductors. Results of tests conducted on a laboratory span are brought out in this paper. The experimental results reveal that neoprene lining and damping shims introduced in the suspension clamp would be very effective in bringing down the static bending stress and thus the overall stress level.

A note on Moiré gratings by a stacked card method

Abstract: An unusual method is described in which a stack of alternating plain and painted cards forms gratings for Moire applications. It is quite simple and versatile so that almost anyone may adopt it with ease and a minimum of expertise.

A simple graphical method to determine the action of a system of retarders

Abstract: Graphical methods for the study of the transformation of polarised light are essentially pictorial representations of the changes in the polarisation of light as it passes through a system of polarisers and retarders. The analysis is common in both photoelasticity and crystallography. These graphical methods are useful: (i) In finding the changes in the light ellipse parameters as the polarised light passes through various elements of the polariscope, without cumbersome calculations. (ii) In finding the characteristic parameters of a particular light path in a three dimensional photoelastic model or for a series of linear retarders. (ii) In estimating various errors in the elements of the polariscope. In the proposed method, knowing the characteristic parameters along a particular light path, a simple graphical method for determining the linear retarders which cause the variation of polarisation of light along the light path is illustrated. In a general loaded three dimensional photoelastic model, the characteristic parameters can be found at intervals along the light path. This graphical method makes use of the characteristic parameters and splits the light path into individual retarders which cause the combined effect for the light path.

On the use of hardness tests to estimate the extent of plastic deformation

Abstract: The standard hardness test result is dependent to some extent on the surface condition of the material being assessed. In particular, since the measurement involves permanent deformation of the surface the state of stress or strain in the material will influence the result. Consequently the hardness test may be used as an indicator of prior plastic deformation. The present investigation was concerned with the phenomenon of buckle propagation in undersea pipelines and a theoretical model of the collapse process was developed. The model predicted the extent of the developing plastic zones around the cross-section of the collapsing pipe. Hardness measurements on the cross-sections of collapsed pipe have been used to estimate the extent of the plastic zones and comparisons with the theoretical analysis are given.