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Last updated: May 22, 2017

What Does Extensometer Mean?

An extensometer is an instrument that measures the elongation of a material under stress. The elongation of the material is a physical deformation of the sample and is a type of strain associated with tensile measurements. Along these lines, extensometers are one part of a variety of tensile tests including the determination of yield strength, tensile strength, yield point elongation, strain-hardening exponent and strain ratio.

There are two classes of extensometers, which vary in the method the measurement is performed. Contact extensometers test the deformation by directly placing a physical object onto the sample such as a knife-edge. Non-contact extensometers use cameras and light to determine the deformational parameters without physically touching the sample.

Extensometers are important for industries dealing with structural components such as structural steel or cables. While manufacturers need to ensure their products can withstand the intended amount of strain, extensometers are also used in fieldwork. Geotechnical engineers can use them to test if a mining structure is going to collapse.


Corrosionpedia Explains Extensometer

Contact extensometers include clip-on extensometers and digital sensor arm extensometers. These types of extensometers are capable of detecting small displacements. Typical disadvantages are related to the physical contact with the sample, which may cause premature sample failure. As such, thin wires and sheets typically are not suitable for contact extensometers. Although certain extensometers such as digital sensor arm extensometers are modified to eliminate the disadvantages associated with typical clip-on extensometers, all problems cannot be eliminated. In such cases, non-contact extensometers may be the better option for measurement.

Non-contact extensometers include laser and video extensometers. As the name implies, the measurement is performed without any physical contact with the sample, thus preventing physical influence on the measurement. Laser extensometers monitor the reflections caused by the laser on the sample surface during elongation and afterwards process the reflection data into material displacement using complex algorithms. Laser extensometers are particularly useful for samples that must be measured in very cold or very hot environments.

Video extensometers use a camera to monitor visual displacement between labeled points in the sample. Along with laser extensometers, video extensometers can achieve high accuracy in measurement. Both techniques are more sensitive to ambient conditions than contact extensometers. For example, ambient lighting effects can interfere with measurements performed on video extensometers.

The choice of extensometer to use for a particular tensile test depends on multiple factors such as the type of material, the testing environment, the required testing standards such as precision, and the budget of the project.


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