Strain gauge
Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern.
This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
Simmons came up with an original way to measure the force introduced into the sample by equipping a dynamometer with fine resistance wires.
Professor Ruge (and his assistant J. Hanns Meier) had the epiphany of measuring the resistance change caused by strain in metallic wires cemented on the thin walls of the water tank model.
Edward E. Simmons and Professor Arthur C. Ruge developed a widely used and useful measurement tool due to the lack of an alternative at their times.
Additionally, they were simple to install by the scientists, did not cause any obstruction or property changes to the observed object and thus falsifying the measurement results.
[2] A strain gauge takes advantage of the physical property of electrical conductance and its dependence on the conductor's geometry.
Conversely, when a conductor is compressed such that it does not buckle, it will broaden and shorten, which decreases its electrical resistance end-to-end.
A typical strain gauge arranges a long, thin conductive strip in a zig-zag pattern of parallel lines.
This does not increase the sensitivity, since the percentage change in resistance for a given strain for the entire zig-zag is the same as for any single trace.
If these steps are not followed the strain gauge binding to the surface may be unreliable and unpredictable measurement errors may be generated.
[5] (Murphy's law was originally coined in response to a set of gauges being incorrectly wired into a Wheatstone bridge.
Temperature effects on the lead wires can be cancelled by using a "3-wire bridge" or a "4-wire ohm circuit"[7] (also called a "4-wire Kelvin connection").
In any case it is a good engineering practice to keep the Wheatstone bridge voltage drive low enough to avoid the self heating of the strain gauge.
[2] In aviation, strain gauges are the standard approach to measuring the structural load and calculating wing deflection.
[9] In some applications, strain gauges add mass and damping to the vibration profiles of the hardware they are intended to measure.
With this technique one or two cameras are used in conjunction with a DIC software to track features on the surface of components to detect small motion.
The full strain map of the tested sample can be calculated, providing similar display as a finite-element analysis.
This technique is used in many industries to replace traditional strain gauges or other sensors like extensometers, string pots, LVDT, accelerometers.[16]..
The accuracy of commercially available DIC software typically ranges around 1/100 to 1/30 of a pixels for displacements measurements which result in strain sensitivity between 20 and 100 μm/m.
Stefan Keil published the first edition of a detailed book about strain gauges and how to use them called “Dehnungsmessstreifen”.
Although this first edition was only published in German, it became popular outside of Germany because of the wide range of uses of strain gauges in different fields.
