Fastener Testing

By Alan Thomas, marketing department, ZwickRoell Ltd

Strain measurement devices play a key role in the testing of a variety of materials, with one such item being the extensometer – a strain measuring device that can be used to measure the extension of a test specimen 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, 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 helps manufacturers determine whether they can withstand the loads to which they are subjected when used for their intended purpose.

Extensometers can be used in a wide variety of industries and an even wider range of materials. Examples include metals, plastics, fibre-reinforced composites, elastomers, films, textiles, ropes, paper and wood.

Types and 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 categorised 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 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.

Choosing the right extensometer

In materials and component testing the range of applications where extensometers are used is extremely diverse. As a result, the technical requirements for these devices are many and varied, which means that there is rarely a single device that satisfies all needs. The requirements for an extensometer are determined primarily by the characteristics of the material to be tested. This includes its shape and dimensions, test requirements and the formal standards that must be met. These define the gauge length, accuracy, test sequence, and environmental conditions – such as test temperature.

Having said this, the right choice of extensometer cannot be limited to the basic material characteristics such as specimen dimensions, stiffness, strength and plasticity alone. It is also necessary to decide whether an extensometer can be connected directly to the specimen without influencing the load measurement or mechanically damaging the specimen itself. Very thin specimens such as foils can be sensitive to clamping forces, whilst very small wire specimens do not provide enough visible area for reliable non-contact measurements. 

A high stiffness in the initial extension range, followed by high plasticity, traditionally requires more than one extensometer. The first measures small strains (typically up to 5mm) very accurately in the elastic range, and the second measures very high values (typically 500mm). Specimens with very smooth, reflective surfaces, or made of transparent materials are not suitable for non-contact measurement without first fixing measuring marks onto the surface of the specimen. One very important consideration is the behaviour when the specimen fails. Metals and hard plastics will slip through the knife edges of a contact extensometer without damaging them, while swivelling knife edges will further reduce the risk of damage, even if the surface of the specimen is particularly rough.

Almost all tensile testing standards such as ASTM and ISO require strain measurement. The best suited extensometer for an application depends on the requirements specified by the standard, as well as the material properties of the test 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. However, the key added value is provided by features such as easy handling, reduced learning curve, the scope of functionality and cost per test.