Technical Learning Home W hen external forces are applied to a stationary object, stress and strain are the result. Stress is defined as the object's internal resisting forces, and strain is defined as the displacement and deformation that occur. For a uniform distribution of internal resisting forces, stress can be calculated Figure by dividing the force F applied by the unit area A: Strain is defined as the amount of deformation per unit length of an object when a load is applied.
Strain gauges are usually based on a metallic foil pattern. The gauge is attached to the test piece with a special adhesive.
As the test piece is deformed, so the adhesive deforms equally and thus the strain gauge deforms at the same rate and amount as the test piece. If the adhesive cracks or becomes detached from the test piece any test results will be useless. Strain gauges are used not just for metals; they have been connected to the retina of the human eye, insects, plastics, concrete and indeed any material where strain is under investigation.
Modern composite materials like carbon fibre when under development are often constructed with strain gauges between the layers of the material.
The strain gauge is effectively a resistor. As the strain increases so the resistance increases. In a basic sense a strain gauge is simply a long piece of wire.
Gauges are mostly made from copper or aluminium Figure 1. As the wire in the strain gauge is mostly laid from end to end, the strain gauge is only sensitive in that direction. When an electrical conductor is stretched within the limits of its elasticity it will become thinner and longer.
It is important to understand that strain gauges actually deform only a very small amount, the wire is not stretched anywhere near its breaking point. This is because resistance is a function of both cable length and cable diameter.
The formula for resistance in a wire is where is resistivity in ohm metres, is length in metres and in m2. For example, the resistance of a copper wire which has a resistivity of 1.
|ME -Strain Gage Experiment||Connecting a Quarter-Bridge strain gage to channel 0.|
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|Signal conditioning for strain gages 1.|
It is measured in ohm metres. In this example to 0. This example shows only the difference when the characteristics of the copper wire have changed. It is not practically possible to stretch and extend a piece of copper wire by these amounts.
The example merely shows how resistance changes with respect to length and cross sectional area and demonstrates that strain gauges, by their very nature, exhibit small resistance changes with respect to strain upon them. These small resistance changes are very difficult to measure.
So, in a practical sense, it is difficult to measure a strain gauge, which is just a long wire. Whatever is used to measure the strain gauges resistance will itself have its own resistance.
The resistance of the measuring device would almost certainly obscure the resistance change of the strain gauge. A Wheatstone bridge Figure 3: With shunt resistor The solution to this problem is to use a Wheatstone bridge to measure the resistance change.
A Wheatstone bridge is a device used to measure an unknown electrical resistance. It works by balancing two halves of a circuit, where one half of the circuit includes the unknown resistance. Figure 2 shows a classical Wheatstone bridge, Rx represents the strain gauge.
Resistors R2, R3 and R4 are known resistances. Normally,or are used depending on the application.
For example if R2, R3 and R4 are and if the measured signal voltage between measurement points A and B was 0 Volts then the resistance of Rx is For our example we get This implies a perfectly balanced bridge.
In practice, because the strain gauge goes through different strain levels its resistance changes, the measured signal level between measurement points A and B is not zero. This is not a problem when using a system like the Prosig P as it can accurately measure the voltage between measurement points A and B.
It is necessary to know the relationship between resistance and voltage. Only then can the measured voltage be related to a resistance and, hence, a strain value. Figure 3 shows the addition of another resistor RS, called the shunt resistor.
The shunt resistor is a known fixed value, normally.
The Shunt resistor is added for calibration purposes and is a very high precision resistor. By measuring the voltage between measurement points A and B with the shunt resistor across Rx, a voltage with the shunt resistor in place is known.It describes the absolute physical limits for the resolution of strain gage transducer signals and how carrier frequency technique can get very close to the boundary of .
Nov 16, · The calculations allow you to design a gage installation to get a predetermined sensitivity and accuracy, but do not take into account certain practical realities, such as the exact effect of nearby stress concentrations. validating a math model is to use a strain gage measurement system to measure the strain at the surface of a structure.
The sensor used to transfer a mechanical strain to a quantifiable output is the strain gage. The strain gage has a resistance which changes as a function of mechanical strain. During this laboratory experiment, the strain in a loaded cantilever beam will be measured in both the axial and transverse directions using strain gages and a Wheatstone Bridge Circuit.
From the measured strain the strain gauge measure the tension and compression strain on the beam.
Strain gauge transducers: an evaluation of accuracy limits D. Marioli, P. Rolla and A. Taroni Dipartimento di Automazione Industriale, Universita degli Studi di Brescia, Via D. Valotti, Brescia, Italy. The detection and the conditioning of a strain gauge bridge low-level signal is analysed.
coatings, and electrical resistance strain gages. In this experiment the strain gage will be utilized. There are three steps in obtaining experimental strain measurements using a strain gage: 1.
Selecting a strain gage 2. Mounting the gage on the test structure and 3. Measuring strains corresponding to specific loads.