Choosing the right extensometer for materials testing

Despite many technical developments within the materials testing industry over the past twenty years, one point has often been at the root of test result inconsistency, particularly in production related tensile testing, and this is the measurement of strain.

The traditional way to measure strain on metals was to use strain gauges attached directly to the surface of the specimen. Ensuring that this was carried out correctly required skilled operators, and was a costly and time consuming process. This technology was later built into mechanical devices and the clip-on extensometer was born. Ideally suited for certain applications, such as the measurement of the modulus of elasticity, these devices also had their associated disadvantages especially where the weight of the device and the force necessary to clamp them to the specimen influenced the test itself. Although possessing high accuracy they suffered from a limited measuring range and poor robustness, and hence could not be used to measure high extensions, especially to specimen failure.

In the late 1970's Zwick introduced a device which was to become known as the 'Macro'. This was the first commercial digital extensometer and was permanently fixed to the rear of the testing machine. It used a simple motorised system to automatically apply the feeler arms of the extensometer to the specimen at the start of the test, and to detach them automatically after the specimen failure. In contrast to the analogue technology in use at that time the Macro used digital technology to measure strain.

For the first time it was not necessary to rely totally on the skills of the operator and laboratories could now measure the proof stress (offset yield) and extension at failure without damaging the extensometer. Macros continued to work well even after testing more than a million specimens, and they have continuously evolved into the most popular extensometer today for testing metals. Used in quality control laboratories all over the world they are the key to achieving accurate and consistent test results.

The next challenge was the strain measurement on materials such as composites, and flexible materials such as rubber, polymers, and textiles. For each of these tests a different solution was required. Metals and composites require relatively short measurement ranges but with extremely high accuracy, whereas elastomers, and textiles typically require long travel up to 1000mm. So laboratories measuring modulus and total extension values on polymeric materials had to use two separate extensometers. In addition to this, certain tests call for devices to measure deflection directly on the specimen in flexure tests, whilst for elevated and sub ambient testing the extensometer itself is subjected to the environmental conditions inside a temperature chamber or furnace.

The Zwick multiXtens extensometer addresses these challenges and is able to measure to the highest possible accuracy (Class 0.5 of ISO 9513). With a range of interchangeable feeler arms, all using smart plug and play sensor technology, this device has a resolution of 0.02µm and a travel of up to 700mm, and can be used for tensile, compression, and flexure tests, both for room temperature and tests carried out inside a temperature chamber. It is without doubt the most flexible and unique extensometer on the market today.

More recently Zwick has developed two new extensometers which have opened up new possibilities for Laboratory Managers. These devices can measure strain without touching the specimen.

The videoXtens makes no contact with the specimen and uses digital video technology to measure the distance between two marks attached to the specimen before the test. In addition, by using a special form of edge detection technology, it is able to measure transverse strain (r value) without requiring another set of marks to be attached to the specimen. Achieving class 1 of ISO 9513 and having a resolution of down to 0.25µm this device offers significant advantages where contact with the specimen is not recommended.

The videoXtens can be also used for elevated temperature testing as it is mounted outside the temperature chamber and 'looks' at the specimen through a side aperture, so is not subjected to the varying environmental conditions that a clip-on extensometer would have to endure. For laboratories which carry out strain rate controlled testing Zwick believes that it is the only company in the world to offer a commercial non contact extensometer with strain control (closed loop control) of the testing machine.

The laserXtens is a brand new and revolutionary product and takes strain measurement one step further by measuring without contacting the specimen and without requiring specimen marking. Having an extremely high resolution (down to 0.15µm), accuracy (class 1 / ISO 9513) and with a measuring range of 900mm, it offers a similar level of accuracy as traditional extensometers. By using innovative laser technology it determines two unique 'fingerprints' on the specimen surface and by setting these as the gauge length it is able to measure the strain directly as the specimen is subjected to stress. One unique benefit of the laserXtens compared to traditional extensometers is its ability to measure strain on miniature specimens and under certain conditions it can avoid the time and expense of attaching strain gauges.

In summary there is no single extensometer which can be used for every type of mechanical test. It is necessary to discuss not only each type of material, but also the mode of testing in order to choose the most suitable device. The needs of R&D laboratories are quite different to production or quality control environments. Ambient and elevated tests pose additional challenges for measuring devices, and of course cost is often a fundamental issue.

Zwick believes that as a global leader in the measurement of strain they can offer the widest range of technical advice, equipment, and total system support to enable each customer to choose the most suitable product for their individual requirement and budget.