Classical Strain Gauges Research Articles

Температурна характеристика електричного опору тензометра при вимірюванні статичних та термічних напружень деталей при температурі до 700 °С

An overview of the existing methods of measuring static stresses at elevated temperatures, at which it is problematic to use classical strain gauges, and methods of compensating the imaginary temperature deformation was performed. Namely: the use of multi-component alloys, the manufacture of active and compensatory sensitive elements from different alloys, which should theoretically compensate for each other's shortcomings. In practice, this can be sufficiently achieved only in a narrow temperature range. The described primary transducer is a rectangular strain-gauge rosette for measuring static and thermal stresses in structural parts operating under extreme conditions at temperatures up to 700°C. This sensor is a two-layer rosette consisting of two sensitive elements (SE), the main axes of which are rotated relative to each other by 90°. The lower SE perceives the main deformation of the part, and the upper one, located above the lower one, plays the role of a temperature-compensating electrical resistance of the element and simultaneously registers the transverse deformation of the part. SE rosettes were made of a wire with a diameter of 30 microns of the Х20Н80 alloy and fixed to each other and to the part using Ц-165-32А cement. The experimental study consisted in determining the temperature characteristics of the resistance of both the lower and the upper SE of the studied strain gauge associated with the temperature expansion of the part-strain gauge system as well as with the shunting of the insulator-connector and the change in the specific resistance of the SE material. Experimental determination of apparent strain gauge deformation at different temperatures was performed. To determine the influence of the coefficient of linear expansion of the material of the part on the change in the resistance of the SE, strain gauges were attached to samples of various materials. The change in the apparent deformation of the strain gauge in the temperature range from room temperature to 700 °C is shown. Its maximum value for the ceramic sample was less than 350 µm/m, for the 30ХГСА steel sample it was less than 1000 µm/m, and for the ВЖЛ14Н-ВІ alloy sample it was less than 750 µm/m. Dependencies were obtained that allow corrections to be made in the result of a real study of the stressed state of the part to achieve the maximum accuracy of measurements.

КОНИЧЕСКИЙ СПИРАЛЬНЫЙ ДАТЧИК ВИБРАЦИИ С УЛУЧШЕННЫМИ МЕТРОЛОГИЧЕСКИМИ ХАРАКТЕРИСТИКАМИ

Relevance Currently, vibration of parts of various machines is widely used to increase the intensity and efficiency of technological processes for extracting raw materials and semi-finished products (granulating, drying, dissolving and leaching, extracting, adsorption gas separation, etc.). In other cases, on the contrary, it is necessary to eliminate the harmful effect of vibration on production processes to slow down the movement of units and mechanisms, to damp vibrations in various ways (controlled shock absorption systems, electromagnetic dampers, vibration protection, etc.). For active control of oscillatory processes occurring in these machines, vibration sensors directly connected to them are used. The main element of various vibration sensors is a sensitive element that converts mechanical movement into an electromagnetic signal, which allows it to be further processed. As sensitive elements of electromechanical transducers of vibration sensors for measuring small vibrations of the surfaces of vibration objects, classical strain gauge, magnetoelectric, magnetostrictive, piezoelectric and frequency-pulse elements are used. The design and creation of new vibration sensors with improved metrological characteristics is an urgent task for electrical engineering and electromechanics. In this work, the basic formulas and graphs of the experimental output data of the metrological characteristics of the new design of the vibration sensor are obtained. Aim of research To investigate the metrological characteristics of a new design of a conical spiral vibration sensor. Research methods To determine the oscillatory processes, the methods of the theory of electrical circuits, magnetic fields, methods of mathematical analysis and the theory of electrical oscillatory processes in (beats, induction, etc.) folded circuits are used. Results A new design of a conical spiral vibration sensor has been developed and its metrological characteristics have been investigated.

Model analysis of complex structures – advantages of physical sub-scaled model testing and shortcomings in physical model production

The possibilities of testing real constructions strength are often limited, especially in case of large constructions. Testing on the model, instead of the actual construction, results in a great saving of money and time. Sub-scaled model of the construction of the substructure, the slewing platform and the lower part of the pylons of the bucket wheel excavator SchRs630 is made. Numerical calculations (using Finite Element Method) and experimental testing of the model were performed. Experimental testing is performed using classical strain gauge method and Digital Image Correlation Method (Aramis system). Once again, advantages of Digital Image Correlation method compared to classical measurement methods are confirmed. The negative impact of conventional manufacturing methods in some zones is noticed.

Monitoring of an experimental motorway section

A portion of the lane reserved for slow vehicles on a motorway was totally rebuild of the A10 Motorway in the West of Paris. Four sections have been instrumented using classical strain gage sensors, placed at different depths within the pavement (subgrade, cement-treated subbase, and bituminous base) and temperature sensors. A fifth section is equipped with a more innovative instrumentation including geophones and temperature sensors, coupled with a remote data acquisition system. The purpose of this paper is to present the monitoring of these experimental sections since 2011. Strain gage measurements are used to develop a mechanical model of the pavement structure, to evaluate the modulus of the different pavement layers, as well as their evolution with time. The results confirm the good performance of the in-place recycled subbase. Geophones are very sensitive sensors measuring vertical displacement velocity. Different methods for the processing of these measurements are proposed in order to monitor pavement deflections and identify and classify truck silhouettes.

Photographic strain monitoring during full-scale failure testing of Örnsköldsvik bridge

Full-scale failure tests are rarely performed on structures, primarily due to their high costs and the lack of suitable test objects. This article reports the results of a ‘test-to-failure’ performed using a real bridge. The results obtained in such tests are valuable for assessing analytical models, updating finite element models and investigating the real behaviour of structures. The specific intention in these experiments was to study the shear failure of the bridge, which is a less well-understood mode of failure than is bending. To this end, it was necessary to strengthen the bridge using near-surface-mounted reinforcements made of carbon fibre–reinforced polymer bars in order to prevent bending failure. The bridge was heavily monitored during the test, using both traditional sensors such as electrical strain gauges and linear variable differential transducers alongside new monitoring systems such as fibre-optic sensors, strain rosette linear variable differential transducers and a novel photographic monitoring system. This article presents the photographic strain measurements and describes the use of the photographic tools in monitoring and characterizing the behaviour of the failure zone during the full-scale test. The strains measured using the photographic method were found to agree well with those measured using classical strain gauges. In addition, the strain contour plots generated using the photographic method provided crucial insights into the strains within the bridge’s failure zone. This study was conducted under the remit of the EU ‘Sustainable Bridges’ Project.

Feasibility of integrated optical fibre sensors for condition monitoring of composite structures. Part I: Comparison of Bragg-sensors and strain gauges

This first paper discusses the possibility of continuously monitoring the mechanical behaviour of composite structures by means of optical fibre sensors. In the introduction, some inherent advantages of optical fibre sensors, which make them ideally suited for this purpose, are discussed. Optical fibres with an intracore Bragg-grating were chosen as an alternative for classical electrical-resistance strain gauges. The main principles of Bragg-sensors are briefly discussed and some previously obtained results are summarized. Filament-wound pressure vessels have been used as a representative composite structural element. These vessels have been equipped with surface-mounted Bragg-sensors and classical strain gauges. Comparative measurements between these two sensor types are discussed in this first of two papers. The results of these experiments lead to the conclusion that optical fibre Bragg-sensors are more reliable than classical strain gauges. In the second paper 'fully monitored' experiments will be discussed: strain is monitored by means of Bragg-sensors and damage is monitored through acoustic emission detection. With additional research effort, and provided that adequate tools for the simulation of a degrading fibre-reinforced structure become available, permanent condition monitoring of critical structures could become realistic in the near future. The Bragg-sensor type optical fibre sensor can be considered a key instrument for monitoring applications of composite structures providing that they become available at economically interesting prices.

Actual light deflections in regions of crack tips and their influence on measurements in photomechanics

Crack-tip photomechanics procedures are based on certain simplifying assumptions that are seldom discussed. In a recent paper the theoretical bases of the shadow optical methods of caustics have been analysed and tested using the results obtained by three analytical-experimental procedures, namely classical strain gage techniques, isodynes, and strain-gradient index method. It has been concluded that the straing-radient index method appears to be a suitable tool for analysis of stress states near crack tips and notches and, in particular, for testing the predictive power of the pertinent singular solutions of the linear elastic fracture mechanics and the ranges of their applicability. In the present paper, a more detailed analysis of all results obtained in light deflection experiments allows to quantify the contribution of both involved effects and to determine the distortion of the faces of the investigated plates along their crack planes. The ability of the strain-gradient light bending method to analyse some features of the three-dimensional stress-state is reported. Finally, the presented experimental evidence allows to draw conclusions related to limits of applicability of certain photomechanical measurements near crack tips. † † An extensive summary of this paper is published in the Proceedings of the Second International Conference on Photomechanics and Speckle Metrology, Vol. 1554A, part of SPIE's 1991 International Symposium on Optical Applied Science and Engineering, 22–26 July 1991, San Diego, CA, USA. 1

A new principle for a short-duration six component balance

The presented principle of a sting balance applying semiconductor strain gauges bears a number of advantages in comparison with classical strain gauge balances. Simultaneous, highly time resolved measurement of all six aerodynamic force components is possible. The manufacturing is comparatively easy, a milling machine is sufficient to achieve reasonable accuracy. The balance needs only little space, which makes it suitable for small and especially for short models. The disadvantages of semiconductor strain gauges like zero drift and temperature effects do not play a significant role during the test time of five milliseconds. However, they have to be taken into account during the long lasting calibration process. The attachment of the model to the cross is critical with regard to preloadings and hysteresis. Therefore a connecting part is used between balance and model, which should not be removed once the balance is calibrated. The model should be attached to the balance so that the centre of mass is located at the origin of the coordinate system shown in Fig. 2. Then, the moment of inertia with respect to the balance fixed coordinate system is minimal, whereas the natural frequencies of the rotating degrees of freedom are maximized. If the flow over a model is studied the coordinate system only has to be rotated to obtain the aerodynamic forces. Furthermore, gauging and wiring of the strain gauges is uncomplicated, all locations of the gauges are easy to reach. As the construction is compact and of high stiffness it may withstand high loads. The existing balance has a design load of 1,000 N. Strain gauges with a gauge factor of k=130 were used. With an amplification factor of 10,000 even forces below 1 N can be observed, using an appropriate amplifier (e.g. “2311 Signal Conditioning Amplifier”, Vishay Measurements Group). The accuracy of the measurement mainly depends on the efforts invested in the calibration and data processing. With a suitable algorithm as proposed by Schnabl (1987), an accuracy in the order of one percent should be possible. So far the balance has not been applied for measurements on a model, but a number of tests is being made to investigate the natural frequencies and short time response. The results of these tests will lead to the final geometry of the balance and its attachment to the model. It is hoped to have the next prototype manufactured, gauged and calibrated by the end of this year. First results of the investigation of a model with the balance might be presented in 1990.