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.