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'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".
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. The earliest types were clipped onto a specimen (hence the name "clip-on" extensometer) during a tensile test. These are largely obsolete as they can damage the specimen at the contact point due to the clamping forces required and the weight of the device itself can have an influence on certain test specimens.
Clip-on devices have largely been replaced by digital "feeler arm" extensometers. These can be applied to the specimen automatically by a motorised system and are capable of producing more repeatable results. Better linearity, reduced signal noise and synchronisation with the corresponding force data are big advantages due to the lack of analogue to digital convertors and associated filters which add time lags and smooth the raw data.
Even though digital feeler arm extensometers are widely used for mechanical testing, for certain applications, high end non contact extensometers are beginning to bring major advantages where it is impractical to use a contact extensometer.

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★ Video extensometer
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 equipement such as radar or laser scans are used enabling 3-dimensional and ultimately 4-dimensional analysis.