The behaviour of a cylindrical vessel loaded through a series of internal diaphragms—an experimental study

The model work presented in this paper was stimulated by investigations carried out on 3.7 m diameter horizontal cylindrical vessels used as oily water separators. In these, internal diaphragms were installed to provide support for the horizontal sand filter bed running the full length of the vessel. By means of four cylindrical plastic models two different designs for the diaphragm are considered. On the basis of strain gauge investigations, conclusions are drawn which should provide a design approach for these components whereby certain peak stresses can be avoided.     

The Reinforcement Effect of Strain Gauges Embedded in Low Modulus Materials

The reinforcement effect of electrical resistance strain gauges is well‐described in the literature, especially for strain gauges installed on surface. This paper considers the local reinforcement effect of strain gauges embedded within low Young modulus materials. In particular, by using a simple theoretical model, already used for strain gauges installed on the surface, it proposes a simple formula that allows the user to evaluate the local reinforcement effect of a generic strain gauge embedded on plastics, polymer composites, etc. The theoretical analysis has been integrated by numerical and experimental analyses, which confirmed the reliability of the proposed model.     

Recent applications of slip ring assemblies

Packaged slip ring and complete slip ring transducer assemblies are described with information about several practical applications especially to motor car testing. Typical electrical specifications are given with illustrations of different types of slip ring, lead connections and measuring equipment.     

Strain gauge performance during fatigue testing

Strain gauges are used extensively to measure the changes in magnitude and distribution of dynamic stresses during fatigue testing. Over 200 000 fatigue cycles at high strains (4000 ± 2000/u/m) have to be withstood under certain conditions such as when evaluating the performance of heavy vehicle springs. In this work, various types of commercial strain gauges have been evaluated to determine the most appropriate installation for severe applications.
The best results were obtained using a constantan metal foil element strain gauge with phenolic glass backing.
Tests on heavy vehicle, multi-leaf, Taperlite springs, manufactured by Tinsley Bridge Limited (formerly BSC Light Products), have demonstrated the suitability of the chosen system and showed that the dynamic stresses remained essentially constant during fatigue testing.     

A Comparison of Various Patterns of Three‐Dimensional Strain Rosettes

Abstract: The strain tensor, principal strains and precision of the estimates of these values are derived for a range of different layouts of three‐dimensional strain rosettes. These values are based on the Monte Carlo technique applied to experimental work which was carried out on transducers tested in different laboratories. The estimates of precision are determined theoretically and compared with results based on experimental findings. A new design of a miniature tri‐rectangular tetrahedral rosette was manufactured and tested. Results suggest that this transducer does not perform as well as the rectangular patterns.     

A temperature calibration of a temperature/strain transducer based on two strain gauges

The operation of a temperature/strain transducer, called “duplex gauge”, based on two electrical resistance strain gauges, that are temperature compensated for use on materials with different coefficients of linear expansion, was briefly recalled. Three steel specimens were equipped with “duplex gauges” and were subjected to temperature variations and no external loads were applied. The chosen experimental setup (i) to measure the outputs of the gauge temperature compensated for steel and titanium silicate and (ii) to impose known temperature, assured good accuracy and precision in evaluation of the temperature calibration curve. The obtained results showed that the strain differences could be considered, at least for the experimental arrangement examined, a linear function of temperature over the range of 20 to over 140°C. The prepared calibration chart, that allows the correction of temperature induced apparent strain, will be used in future research when generic temperature and strain fields are applied to specimens.     

Measuring the load eccentricity in helical compression springs

An experimental device able to detect the load eccentricity in helical compression springs is described. The theory underlying the resolution of measured bending strains of the apparatus into effective load eccentricity is discussed. Laboratory results are offered for heavy duty springs and compared to theoretical estimates retrieved from the literature.     

The role of strain measurement technology in structural evaluation

This paper describes the importance of structural evaluation to the process of design assessment and outlines the significant role played by strain measurement technology in extending the scope of strain history data collection and facilitating strain monitoring of full-scale structures in the laboratory. After some general discussion of the various models of structural behaviour, it then proceeds to give a brief historical account and outlines the principles of thermoelastic stress analysis (TSA). A general desciption is provided of a specific TSA technique namely Stress Pattern Analysis by Thermal Emission (SPATE).
Despite the level of innovation and development in strain measurement technology, the use of bonded strain gauge elements is still by far the most commonly used strain measurement and monitoring technique in structural evaluation work at NEL. The paper highlights several recent test programmes carried out at NEL in the form of case studies which demonstrate different aspects of measuring, monitoring and validation of this and other strain measurement techniques; and verification of analytical models.     

Strain gauge measurements within the school laboratory practice

A simple technical device has been proposed suitable for exercising strain gauge measurements in introductory courses in tensometry. Its conception has followed from a theoretical strain analysis of the circular thin steel plate loaded by a concentrated central force. The analysis has taken into account a non linearity of the studied strain field and has used a strain tensor for original expression of Poisson's ratio and Young's modulus of the material of the plate. The derivation of analytic expressions of both these constants enabled one to estimate their values by means of the strain gauge measurements. In this way the proposed device demonstrates further application possibilities of the strain gauge method.     

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.     

A Novel Strain Sensor for Reinforced Concrete Structures

Abstract:  Reinforced concrete (RC) is the most commonly used structural material in civil engineering applications. RC structures have long-term service lives under normal loading conditions; however, overload caused by misuse or statistically remote events such as earthquakes may create damages that, if not detected in time, may eventually cause failure. Hence, it is important to monitor RC structures to take necessary precautions and save human lives. A long-gauge strain (LGS) sensor has been developed to monitor these structures. While it has been developed mainly with concrete applications in mind, the new sensor can also be used in a variety of applications, including measuring strains in pipelines, steel structures, and the like. The proposed sensor system has a very low cost compared with the commercially available competing systems. Prototypes of the proposed strain sensors have been built and calibrated. Test results prove the accuracy, repeatability and reliability of the proposed strain sensor. When the LGS sensor was incorporated into a concrete beam there was very good agreement between the experimental measurement of strain using the LGS sensor when compared with two strain-gauged parallel steel rebars in the same concrete beam.     

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.     

Strain gauges in a nuclear environment

A short review is presented of the different degradation mechanisms of strain gauges under ionizing radiation, corresponding to the different types of application in nuclear environments. The available experimental results are summarized. Particular attention is given to the assessment work still needed for small bonded gauges to be used in accurate force sensors for remotely controlled manipulators used in maintenance and refurbishing operations.     

Reduction of static strain for increased life of transmission line conductors

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.     

Strain Measurement on Composites: Errors due to Rosette Misalignment

Abstract: Electrical resistance strain gauges are increasingly used for the determination of the strain field in composite components. The effect of the angular misalignment of a strain gauge rosette on the determination of the strains in a composite material is investigated in this paper. The theoretical analysis shows that the strain error along the principal material directions depends on the difference of principal strains, on the angular misalignment of the rosette and on the angle between the maximum principal strain and the fibre direction. The paper also shows experimental evidence for the theoretical analysis.     

An Examination of Various Methods of Minimising Strain Gradients in 3D Embedded Strain Gauge Analysis

This paper is a review of different methods used to reduce the effect of strain gradients in experimental models with 3D embedded strain gauged transducers. A detailed analysis of various methods used in relation to research carried out on a prosthetic hip implant is investigated. The methods to reduce these gradient effects detail the importance of the selection of the most suitable 3D transducer pattern and establishing the locations on the model where strain gradients are least problematic. It is also shown that the use of large‐scale models helps minimise the effect a gradient may have on data. Another way of avoiding strain gradients is to use axisymmetric models and embed the gauges at the same radius in the model but at a different angular orientation from the central axis. The strain gradient across the 3D embedded strain gauge transducer can be reduced using the methods described here so that significant errors in the final tensor do not materialise.     

Strain Measurement in a Shape Memory Alloy with Strain Gauges

Abstract:  The recovery of shape due to temperature-induced martensite to austenite phase transformation in shape memory alloys (SMA), such as nickel titanium alloys, has been investigated and utilised in applications for many years. One of the problems in incorporating SMA in host materials, such as composites, is the poor bonding characteristics of the SMA. A closely related problem in experiments is the measurement of strains in SMA specimens; as thin wires have been used in many of the experimental investigations, strains have been deduced from length measurements. In the current work, electrical resistance strain gauges have been bonded on SMA strips. Tensile strains up to 8% during tensile loading as well as compressive strains during strain recovery due to subsequent heating have been measured. Strains determined by other methods, such as extensometer and length measurements, are also reported and compared. The major contribution of this work is the successful bonding of strain gauges on SMA and the measurement of large strains (up to 8%).     

Interlaminar Stress Determination in Carbon Fibre Epoxy Composites with the Embedded Strain Gauge Technique

Abstract: In order to determine interlaminar stresses, strain gauges have been embedded in carbon fibre/epoxy composites. Insulation problems occurring because of the electrical conductivity of the carbon fibres were successfully resolved with the help of special foils. Measurements on unidirectional and angle‐ply laminates were in good agreement with calculations based on the classical laminate theory. Moreover, the laminate strength was not significantly affected by the embedded strain gauges. The technique may also be applied to residual stress determination in composite components.     

Strain Measurement on Composites: Effects due to Strain Gauge Misalignment

Abstract: The present work analyses the errors affecting the strains measured by misaligned strain gauges installed on orthotropic‐composite laminae. Various analytical relationships are derived showing that, besides the fibre and strain gauge orientations, the misalignment error in unidirectional off‐axis orthotropic composite samples depends also on the lamina stiffness properties (E₁, E₂, ν₁₂, G₁₂). If the fibres are aligned with the loading axis, it is found that the higher Poisson's ratio ν₁₂ is the only elastic property influencing the misalignment error. Experimental results are shown confirming the theoretical predictions.