Extensometer
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An extensometer is a device that is used to measure small/big changes in the length of an object. It is useful for stress-strain measurements. Its name comes from "extension-meter". It was invented by Dr. Charles Huston who described it in an article in the Journal of the Franklin Institute in 1879. Huston later gave the rights to Fairbanks & Ewing, a major manufacturer of testing machines and scales.
There are many types of extensometer but they are mainly split into two categories (Contact and non Contact). Contact extensometers have been used for many years and are also subdivided into two further categories.
1) These are clipped onto a specimen (hence the name "clip-on" extensometer) before carrying out a tensile test. These devices are used for applications where extremely high precision is required over a relatively small extension(a few mm). They have the disadvantage that they can damage the specimen at the contact point due to the clamping forces required and the weight of the device itself which can influence certain test specimens.
2) For automated testing clip-on devices have been largely replaced by digital "feeler arm" extensometers. These can be applied to the specimen automatically by a motorised system and produce much more repeatable results than the traditional clip-on devices. They are counter balanced and so have negligible effect on the specimen. Better linearity, reduced signal noise and synchronisation with the corresponding force data are big advantages due to the lack of analogue to digital converters and associated filters which add time lags and smooth the raw data. In addition these devices can remain on the specimen until failure and measure very high extensions (up to 1000 mm) without losing any accuracy. Theses devices typically have resolutions of 0.3µm or better (the highest quality devices can read values as low as 0.02µm) and have sufficient measurement accuracy to meet class 1 and 0.5 of ISO 9513.
For certain special applications, non contact extensometers are beginning to bring advantages where it is impractical to use a feeler arm or contact extensometer.
Source:
Huston, Charles. "The Effect of Continued and Progressively Increasing Strain upon Iron," Journal of the Franklin Institute, Vol. 107, No. 1, January 1879, pp. 41-44.
[edit] Mining
In the mining environment, extensometers are used to measure displacements on batters/highwalls. Plotting displacement vs time enables Geotechnical engineers to determine if wall failures are imminent. For complicated failures, further equipment such as radar or laser scans are used enabling 3-dimensional and ultimately 4-dimensional analysis.
