The use of an extensometer is a valuable tool to characterize the strain and change in length of a specimen under stress. Measuring the movement of the crosshead on a tensile tester is very precise, given the use of highly accurate encoders, and is often more accurate than an extensometer. However, once you attempt to measure the deformation of a specimen, the accuracy of the crosshead movement becomes dependent upon the system as a whole having to account for the movement in the test frame, load cell, coupling and grips known as system compliance.
The use of the appropriate extensometer allows more precise measurement of the strain/deformation of a specimen under load, especially when the system compliance is unknown. When using extensometers in testing, higher or lower load values do not influence the strain measurement of an extensometer unlike instrument encoders that are subject to system compliance.
- Selection of the Appropriate Extensometer
The selection of the appropriate extensometer is largely dependent on the material being tested, the geometry of the specimen, anticipated deformation and required accuracy. High elongation materials such as rubber, semi-rigid plastics and plastic films are best suited for high elongation models such as the Model 890 or TA3800. For materials with lower deformation under load such a metal, wire, composites and rigid plastics, the use of an axial extensometer such as the TA3442 and TA3452 would be recommended.