An extensometer is an instrument that measures test specimen elongation to characterize strain. The range of applications where extensometers are used is diverse and the technical requirements for these devices are multifaceted. There is no single device that meets all needs for all applications.
The requirements for an extensometer are determined by the characteristics of the material to be tested including shape and dimensions, test requirements, and the test standards that must be followed.
It is also necessary to decide whether an extensometer can be connected directly to the specimen. Very thin specimens such as foils can be sensitive to clamping forces, while small diameter wire specimens, for example, lack sufficient visible area for reliable noncontact measurements.
A high stiffness in the initial extension range, followed by high plasticity requires more than one extensometer. The first measures small strains (typically up to 5 mm) very accurately in the elastic range, and the second measures very high extensions (typically 500 mm). Materials with very smooth surfaces, or those made of transparent materials are not suitable for noncontact measurements without first fixing measuring marks onto the surface of the specimen.
To qualify the integrity of a measured signal, standards use quantitative terms such as resolution, deviation, and uncertainty. Requirements for the accuracy of extension measurements are normally given in application-specific test requirements and international standards.
Devices that are easy to set up and sequences that can be automated reduce personnel time and effort. Simplified set up routines can improve the quality of the test results by minimizing subjective influences.