ExtensometersStrain measurement devices for materials testing – a ZwickRoell core competency
An 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:
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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.
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.
Material & specimen shape
The selection process for the optimal extensometer starts with the material and specimen shape criteria
- Maximum extension: important for calculation of the required measurement range. Also, materials with less extension usually require a higher level of accuracy.
- Contact sensitivity: when testing thin or contact-sensitive materials, influence on the specimen can be minimized by using sensor arm extensometers with special knife edges. Optical extensometers provide the ideal solution since they have no influence on the specimen at all.
- Fracture behavior: important for tests up to specimen break, to make sure that the extensometer is not damaged. For high fracture energies, you should use optical extensometers or sensor arm extensometers with corresponding safety mechanism.
- Specimen dimensions: some specimen dimensions limit the extensometer selection due to extreme specimen widths or thicknesses.
- Specimen shape: can present special challenges. For example, components with irregular shapes that limit accessibility to the specimen.
Test sequence & standard
Whether you are testing according to industry or company standard: the test sequence and the required measured values clearly specify critical extensometer features.
- Type of load: what the extensometer is being used for: tensile, compression, flexure or cyclic tests? Some extensometers can be used for all four types of load and are designed for quick changes between test types.
- Initial gauge length: usually specified by the standard. The measurement range to be covered by the extensometer is based on the initial gauge length and maximum extension of the specimen.
- Accuracy: when it comes to extensometer accuracy, the standards normally refer to accuracy classes or grades. These are defined in the calibration standards for extensometers on the base of measured deviations and resolutions (ISO 9513, ASTM E83).
- Required measured values: what measured values are to be determined with a particular test and what are your requirements? For example, modules are determined right at the beginning of the test, so a corresponding level of accuracy must already be set. This level of accuracy can be ensured with an appropriate calibration.
- Closed loop strain rate control to ISO 6892-1 Method A1: this type of strain control imposes special requirements on the extensometer. To ensure that the test speed is automatically adjusted, the extensometer continuously feeds back current strain values to the electronics (at ZwickRoell this is 2000 times per second).
What is the test environment and how does it influence the extensometer?
- Test temperature: you must use a suitable extensometer when testing under temperature conditions. There are extensometers specially designed for use in a temperature chamber or high-temperature furnace, which are able to provide a very high level of accuracy in this environment.
- Light influences or convection, e.g. from an air conditioning system, can limit the accuracy of a non-contact, optical extensometer.
- Dust, dirt and vibrations that affect testing in production environments require a robust, low-sensitivity extensometer.
Functionality goes hand in hand with added value, as an extensometer has a lot more to offer.
- Flexibility: an extensometer that is highly flexible in terms of different applications, specimen types or functions, eliminates the need for multiple extensometers.
- Operator influence: how important is the reduction or elimination of operator influences to obtain reliable test results? Operator influences can lead to deviations and scattered test results.
- Automated functions: through automated functions, operator influences can be reduced and even eliminated. This significantly increases the repeatability and reproducibility of test results. Automatic functions make interventions unnecessary—from automated measurement of the test area and centering of the measuring points to automatic setting of the initial gauge length and attachment and detachment of the sensor arms.
- Added value through options: optical extensometers capture a large section of the specimen via camera(s) and can therefore gather more information from the measurement. Measurements can be performed on multiple measuring points, full-field strain evaluations via 2D image correlation, or automatic determination of the break location, which prevents specimen rejection.
- Retrofitting options: these provide future investment security. Some extensometers cover a wide range of applications right from the start. Others can be easily adapted for additional applications through retrofitting at a future point in time.
Ease of handling positions the user at the forefront.
- User profile: who is working with the machine? Is it continuously changing production personnel who perform tests without much prior training and little, if any, modification to the test procedure? Or is it a specialist who wants to be able to control every stage of the test sequence in a highly flexible manner and with access to a wide range of functions? The extensometer and software can be adjusted according to the type of user.
- Training requirements: automated functions significantly reduce training requirements. This includes a software program with intuitive operation, clear structure and adaptability to your operating procedures.
- Modification efforts: if you are often changing between various applications, you must also consider the efforts required for a system modification— how long does it take, can it be done by a single person, and can errors be made during the process?
- Specimen marking: depending on the specimen, optical extensometers require the use of specimen markings—and in some cases they don’t. Next to a variety of marking options that can be adapted according to the specimen and test to be performed, optical systems can also measure without markings. In these cases, the roughness of the specimen surface is used, and virtual gauge marks are applied to the specimen via the software.
Budget and costs
When it comes to cost, it is important to focus on the years that follow acquisition.
- Acquisition costs: while these are of high importance initially, low costs incurred for the operation of an extensometer (and the system as a whole) can quickly compensate for higher acquisition costs.
- Costs for training depend on the amount of training required and the number of operators who will use the system.
- Costs per test and possible specimen throughput: the more time the operator spends handling the system, the more time is required to run the test.
- Time required for system conversions or modifications: modifying a system for application changes is time consuming. A good example is the conversion required when using a temperature chamber. In this case, a lot of time can be saved if a single person is able to make the changes.
- Costs for follow-up tests: additional costs are incurred due to a lack in system accuracy and reproducibility. If the scatter of the values is too great, elaborate follow-up tests are required. In addition to personnel costs, you also encounter new material costs. Therefore, reliable test results play an important part here.
- Maintenance costs: Last but not least, ongoing maintenance costs are of significance. These can be reduced through wear-free parts or a special arrangement for use in dusty production environments.