The calibration of CERL-Planer capacitance strain gauges

CEGB-Planer capacitance strain gauges are used extensively for monitoring the deformation of components operating in the creep range. However, there are significant differences between the way the gauges are installed for use and the way they are installed for calibration. This paper describes the various types of calibrations that have been carried out to identify the errors associated with these differences and the results show that, providing the manufacturers' recommendations are adhered to, the errors are acceptably small.     

Application of resistance strain gauges on a wet surface

The application of strain gauges to determine the stress on wet and porous media such as rock has been examined. Weldable strain gauges have not been considered. Special consideration has been. given to different methods of water protection. For unbonded strain gauges tested under water a logarithmic dependence between time and resistivity has been found. Several kinds of moisture protection have been tried on the material side as well as on the back side of selected strain gauges. The bonding and the stability of the gauges under load in water has been examined.     

Development of temperature compensated high elongation resistance strain gauges for use up to 250°C

The paper describes the development and evaluation of temperature compensated high elongation resistance strain gauges for use from room temperature to 250°C. These gauges could be temperature-compensated on steels with coefficient of thermal expansion α = 11 ppm/°C. Thermal output within 250°C was about 1 μm/m/°C and maximum standard deviation of 5 gauges was 34 μm/m. Strain limit was > 4% at room temperature and at 250°C. Gauge factors of different strain ranges at R.T. and at 250°C are tabulated. Some applications are stated.     

Apparent strains in unmounted strain gauges at high temperature

A detailed investigation into the source of the unmounted apparent strains in resistance strain gauges over a temperature range of 20°C to 550°C is presented. Different measuring systems give different values of unmounted apparent strain. Formulae for two different measuring systems relating the unmounted apparent strains to resistance values are derived. Values computed from these formulae using measured values of resistance are compared with values of unmounted apparent strains measured directly. In addition, formulae which are essential in order to relate the actual strain to the indicated strain are derived for each of the systems. It is shown using the derived expressions that the magnitude of the unmounted apparent strains can be reduced considerably by connecting a shunt resistor across the active element.     

Effect of temperature on vibrating wire strain gauges

Vibrating wire strain gauges attached to buried pipe can exhibit changes in strain readings when subjected to changes in ground temperature. It is essential that the correct interpretation is placed upon the readings. An analysis is given which, it is suggested, will give true pipe strain values from the indicated strain readings. Two solutions are outlined, a graphical analysis which tends to be intuitive and a theoretical analysis which derives a solution from first principles.
True strain in the pipe is influenced by thermal effects due to changes in ground temperature, and mechanical effects due to the constraint placed upon the pipe by the surrounding soil. Both analyses consider the thermal and mechanical components of true strain and the theoretical analysis enables these individual components to be determined from the indicated strain readings.     

The fatigue of strain gauges - a user's view

The information needed by the strain measurement engineering concerning the fatigue of strain gauge installations is discussed, and compared with that readily available. It is suggested that far more extensive and detailed information is needed and should be made readily available.     

A simple technique for installing strain gauges on cylindrical surfaces

In this paper a simple technique for the installation of strain gauges on circular and conical cylindrical surfaces has been described. This technique may be adopted in an experimental stress analysis laboratory where sophisticated equipment is not available.     

A low cost calibration conditioner for dynamic strain gauge measurements

A conditioner for dynamic strain measurements that allows for a series of calibration signals with periodic and predefined time occurrence, during the whole period in which the measurements have to be carried out, is presented in this work. The calibration signals, having suitable size ratios between them, allow one to evaluate the linearity of the entire measurement chain.     

Analysis of strain averaged data from finite length gauges and predictions of peak strain for planar notch and fillet problems

From a previously derived modal (i.e., series) representation of the stress and strain fields in stress concentration regions, exact analytical expressions are obtained for the averaging effects of finite length gauges located on the boundary of a notch fillet. A simple method of analysing such data is described. It is then applied to two problems to show i) how easily poor or faulty data can be detected and ii) how a very reliable estimate can be made of the magnitude and location of the peak stress (or strain), even when using data from gauges beyond the very highly stressed (strained) region.     

Comparison of dynamic initiation toughness measured by strain gauges and the shadow optical method

The deformation and fracture behaviour in dynamically loaded Charpy specimens of BS11 rail steel were studied by two different measuring techniques. The transient records obtained from straingauges were compared with results obtained simultaneously by the shadowoptical method. Good agreement was obtained when measuring and comparing the stress intensity factor K in the initial stages of deformation behaviour prior to crack initiation. During fast fracture, the shadow optical method indicated the influence of dynamic effects on crack propagation.     

Lubricant effects on strain fields of seven wire strands subjected to tensile load

As a part of an extensive analysis on strand behaviour the lubricant effect in strain fields was examined. The strains were measured by using electrical resistance strain gauges. The results obtained, by loading the unlubricated and lubricated sample both statically and quasi-statically, showed relevant effects caused by different contact loads developed among the wires. By examining the strains measured during the severing of one helical wire the inapplicability to the strands of one Italian regulation relative to wire ropes, was also confirmed.     

The use of strain gauges to determine the principal axes of composite beams of unsymmetrical section

Strain gauges were used to obtain experimentally the principal axes of two composite beams. The method shown is easy to use and can give results which agree closely with calculated values, and may be applied to complex composite section beams.     

The use of vibrating wire strain gauges for long term measurements on buried gas mains

A trial agreement concerned with the effect of adjacent deep excavations on the risk of failure of buried cast iron gas and water mains was implemented by British Gas and National Water Council on 1 January 1983. To complement this agreement, field experiments are being carried out at selected sites throughout the UK. The British Gas Engineering Research Station (ERS) is working in cooperation with the Water Research Centre to install instrumentation to measure pipe strains, joint movement, ground temperature and ground movement.
Field measurements impose environmental and operating conditions of much greater severity than those encountered in a laboratory. This note outlines the site techniques developed by ERS which allow reasonably quick and reliable installation of equipment capable of withstanding the adverse conditions typical of backfilled trenches. The principle of operation of the VW gauge, its method of fixing, the factors affecting its output and the interpretation of the readings themselves are described.     

Random errors caused by temperature in magnitude of principal strains evaluated ith 3-element strain gauge rosettes

The effects of the uncertainties associated ith the apparent strain and gauge factor data, given by the manufacturer, in the evaluation of principal strains are examined. Principal strains are obtained by a reduction of the strains measured ith electrical resistance strain gauge rectangular or delta rosettes. The theoretical analysis points out the relevance of the effects caused.     

Sensitivity calibration of post-yield strain gauges under cyclic plastic straining

A plastic sensitivity calibration procedure is outlined for strain gauges under cyclic, four point bending. Tests show that the gauge resistance changes in an approximately linear manner with longitudinal strain for the first quarter cycle of loading. Calibration curves for subsequent reversals to the direction of deformation display zero-shift and non-linearity. Under balanced strain cycling, there is evidence of a cyclically-stable, sensitivity calibration loop. Theoretical considerations are given in which it is proposed that separate sensitivity factors apply to the elastic and plastic components of strain. It is shown that the plastic sensitivity factor is a function of (i) plastic strain induced hardening and softening in the gauge foil and (ii) any apparent change to the gauge resistivity due to imperfect bonding. The elastic component sensitivity factor equals the manufacturer's value only in the absence of hardening. The two sensitivities may be combined to give a total sensitivity factor when a post-yield strain gauge suffers elastic-plastic straining.     

The nature of room temperature creep

This note is aimed at highlighting hat may be inaccurate thinking of the nature of cold (logarithmic) creep - i.e. creep hich may be observed in most steels at room temperature (20C). There is a dearth of information on this topic although engineers have come to regard cold creep as a 'time dependent plastic' deformation. As such it is assumed that the assumptions of plasticity are obeyed during the creep process.     

Use of commercial metallic strain gauges as low temperature heaters

The heat capacity of a resistive strain gauge has been measured. Such gauges may be used as electrical heaters in low-temperature applications. The units are small, inexpensive, durable and reusable.     

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

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.     

Use of displacement data to calculate strain gauge response in non-uniform strain fields

A simple and effective method is described for calculating the strain indicated by a practical strain gauge mounted in a nonuniform strain field. The method uses integration or summation of the displacement field along the perimeter of the strain gauge area. This procedure gives results which are equivalent to those obtained by much lengthier calculations involving double integration of the strain field within the strain gauge area. The displacement summation method can easily take into account significant strain gauge design details such as number, length, width and spacing of the grid lines. The method also demonstrates that a strain gauge can be used as a displacement sensor as well as a strain sensor.     

Measurement of surface strains in GRP products

Current work on fibre composite materials required the measurement of surface strains on glass reinforced plastics (GRP) products subjected to static loading. A study undertaken to account for anomalous results obtained with electrical resistance strain gauges showed that strains measured at a given load, increased with decreasing gauge length and foil gauge resistance. Measurement of localised strains using gauges of length up to 25 mm are subject to considerable discrepancy when compared with gauge lengths of less than 13 mm.