Extensometers
Strain measurement devices for materials testing – a ZwickRoell core competencyAn extensometer is a strain measurement device used to measure the extension of a specimen under load. Extension measurement is one of ZwickRoell’s core competencies.
Contact extensometers Optical extensometers Selection criteria
What is an extensometer?
An extensometer is a strain measurement device used to measure the extension of a material under load.
- The extension of a material is a physical deformation that occurs when it is subjected to a load such as the pulling force associated with tensile testing. In addition to strain caused by tensile loads, extensometers also help determine compressive deformation or deflection under different types of load applications, including cyclic tests (incl. fatigue tests), compression tests and flexure tests.
- Extensometers measure strain directly on the specimen. This eliminates measurement influences from other testing components and increases accuracy.
- Strain measurement is required in the determination of characteristic values of a material. The tensile modulus, Young's modulus, yield point, strain at break, r-value and Poisson’s ratio are typical values determined with an extensometer. This information is essential when comparing materials, and help manufacturers determine whether they are able to withstand the loads to which they are subjected when used for their intended purpose.
- Extensometers are used in a wide variety of industries and an even wider range of materials. Examples include metals, plastics, fiber-reinforced composites, elastomers, films, textiles, ropes, paper and wood.

Types / categories of extensometers
To understand how an extensometer works, it is important to know that there are essentially two types of extensometers: contact and non-contact or optical extensometers.
Contact extensometers can be further categorized into clip-on and sensor arm extensometers.
Non-contact optical extensometers include video- and laser-based instruments.
- Sensor arm extensometers are attached directly to the specimen via knife edges mounted on the sensor arms. Strain is measured through evaluation of the change in angle or travel distance of the sensor arms. Sensor arm extensometer technology is proven and easy to understand. They are known for providing a high level of modularity, offering flexibility for different test tasks and adaptability from a manual to a fully automated system.
- Clip-on extensometers are a cost-effective solution for standard test tasks with low specimen throughput. They are directly attached to the specimen. The measurement value transmission from the specimen to the sensor is short and stiff, by which a high level of accuracy is attained. These extensometers, however, lack in flexibility: From a design perspective, most of them have a set initial gauge length and a short travel distance.
- Optical extensometers are camera-based and therefore measure without making contact. Markings on the specimen identify the initial gauge length—either by being placed directly on the specimen or via virtual gauge marks applied with the use of software. The gauge marks are tracked by an image-to-image comparison throughout the test and the travel distance or strain measurement is recorded. Since the camera captures a large part of the specimen, additional evaluation options are available, including 2D DIC (digital image correlation), measurements on several measuring points or automatic determination of the break location, which prevents specimen rejection.
Optical / non-contact extensometers
Optical extensometers (video extensometers and laser extensometers) measure without contact and therefore have no influence on the determination of the characteristic values of a material. An additional advantage provided by strain measurement devices featuring non-contact measurement is that they can be used right up to break without risk of damage, even with specimens that are critical in this respect.
More information on our video extensometers and laser extensometers can be found under the following links:
Why choose an extensometer from ZwickRoell?
ZwickRoell extensometers are:
- One of our core competencies resulting from decades of application technology experience.
- Developed and manufactured in-house alongside our other testing components, guaranteeing full testing system compatibility.
- Designed to exceed standard requirements, since extensometer accuracy is essential for reproducible and reliable test results.

Selection criteria
Almost all tensile testing standards such as ASTM and ISO require strain measurement. The best suited extensometer for an application depends on the requirements set forth by the standard as well as the material properties of the specimen.
Determination of the ideal extensometer is based on six main criteria. These include properties that must be met, such as extensometer accuracy, resolution, measurement range, required measured values and the test temperature at which the extensometer will be used. But the key added value is provided by features such as easy handling, reduced learning curve, the scope of functionality, cost per test and further information provided by added options.