Strain gauges have been used for many years for strain measurements; and as the fundamental sensing elements of different transducers to measure pressure, force, temperature, and torque. While there are various monitoring techniques available to measure deformation in a structure, including ultrasonic wave propagation technique, acoustic emission, magnetic field analysis, X-ray radiography, and sensors, they are sometimes found unsuitable for every type of measurements and environments. This is due to their high susceptibility to ambient noise frequency, high-cost, inaccessibility in remote areas or fragile nature, which affect the reliability and accuracy of the measurement. However, a wide range of strain gauge sensors are now available to address these setbacks effectively with their design and features. In this article, we are going to look into some of these key characteristics of strain gauges.
Compact Size
In applications, where installation space is limited, strain gauges are ideal for strain measurement. For example, for in-vehicle measurement for providing running performance evaluation tests, operability, ride comfort, and safety, strain gauges are widely employed.
They are also available in the form of miniature strain gauges for use in printed circuit boards, computers, and industrial machinery. Such machineries or equipment are exposed to mechanical and thermal impacts not only during their manufacturing process, but also during their transport and in action. Therefore, the reliability of these systems is a result of experienced development and intensive testing, which is possible with the help of strain gauges.
Variety of Gauge Lengths
Strain gauges are available in different variants of gauge length, which is selected depending on the specimen to be tested. A strain gauge with short gauge length is generally ideal for measuring local strain within a short measuring range, whereas a gauge with longer lengths are preferred when strain is to be measured over a larger area.
The choice of gauges of longer gauge length also depends on the composition of the material. For example, if the structure is made from a heterogeneous material such as concrete, which is composed of cement and aggregate, with gauges of long gauge lengths, the measurement of irregular strain of such materials would yield better results.
High Accuracy
A strain gauge is designed to measure strains up to 10^-4 to 10^-2 Ω/Ω with great accuracy. This is due to the fact that a special circuit called Wheatstone bridge is configured with the strain gauge to measure the change in the resistance of the gauge, as a result of the change in the dimensions of the object due to the applied force. This bridge circuit allows the measurement of resistance changes in the strain gauge to be measured with great accuracy as part of the strain measurement. Generally, the level of accuracy differs with the form of Wheatstone bridge circuit the strain gauge is configured with. While all of these circuit forms offer high accuracy, but for very stringent accuracy requirements, a full bridge arrangement is preferred.
The accuracy of the strain gauge is sometimes deterred by increasing temperature with the expansion of test specimens. For this reason, self-compensated strain gauges are employed for operations with inconsistent temperatures. They help by compensating for the apparent strain due to increased temperatures.
High-Frequency Response
In response to the stress applied at rapid rates, strain gauges are capable of quickly measuring outputs over a wide frequency range. For example, in an engine mount of an automobile driven over various road surfaces, a strain gauge is able to perform strain measurements with minimum distortions and errors. Generally, the frequency response of a strain gauge differs for every gauge length and the material to which the gauge is bonded. The table below contains the data for the flat frequency, i.e. the maximum frequency at which the strain gauge can measure output without distortion, with respect to gauge length and material types.
Electrical Output
A strain gauge generally determines strain by measuring the change in resistance of the gauge as a result of the deformation caused by applied stress. This change in resistance is further used to calculate the voltage drop across the Wheatstone bridge, which is directly proportional to the strain induced due to applied stress. With any change in the strain gauge resistance, the strain gauge produces a non-zero output voltage. The full-bridge circuit configuration of a strain gauge provides the highest sensitivity and the fewest error components. Therefore, it significantly provides the highest output as noise becomes a less significant factor in the measurement.
A strain gauge signal conditioner typically provides a constant voltage source to power the bridge. The remote sensing feature in some of these conditioners allows to compensate for the error by locating the strain gauge a distance away from the signal conditioner and excitation source. Therefore, it allows for long-distance communication of sensor and signal conditioner.
These are the major characteristics of a strain gauge that make them stand out in a typical test and measurement scenario. Check out our full range of strain gauges for any measurement requirements.
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Mohr’s
circle-measurement of strains-strain
gauges |
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 Rotated Strains using Strain Rotation Equations |
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Previously, the Strain transformation equations were developed to calculate the strain state at different orientations. These equations were
 Plotting these equations show that every 180 degrees rotation, the strain state repeats. In 1882, Otto Mohr noticed that these relationships could be graphically represented with a circle. This was a tremendous help in the days of slide rulers when using complex equations, like the strain transformation equations, was time consuming. |
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Mohr's Circle |
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Mohr's circle is not actually a new derived formula, but just a new way to visualize the relationships between normal strains and shear strains as the rotation angle changes. To determine the actual equation for Mohr's circle, the strain transformation equations can be rearranged to give,
 Each side of these equations can be squared and then added together to give  Grouping like terms and canceling other terms gives
 Using the trigonometry identity, cos22θ + sin22θ = 1, gives  |
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 Basic Mohr's Circle for Strain |
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This is basically an equation of a circle. The circle equation can be better visualized if it is simplified to
This circle equation is plotted at the left using r and εave. One advantage of Mohr's circle is that the principal strains, ε1, ε2 , and the maximum shear strain, (γmax/2), are easily identified on the circle without further calculations. |
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Rotating Strains with Mohr's Circle |
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 Strain Rotation with Mohr's Circle |
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In addition to identifying principal strain and maximum shear strain, Mohr's circle can be used to graphically rotate the strain state. This involves a number of steps.
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The angle, 2θp, for the principal strains is simply half the angle from the blue line to the horizontal axis. Remember, Mohr's circle is just another way to visualize the strain state. It does not give additional information. Both the strain transformation equations and Mohr's circle will give the exactly same values. |
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