Have you ever considered video extensometry to accurately measure displacements in your tensile test bench, but ran into challenges in alignment and the high cost? Then it is well worth taking a closer look at Tinius Olsen’s VectorExtensometer.
The cost is only slightly more than that of classical extensometers but it is worth the investment because of the advantages that come with non-contact measurement and because of the simplified positioning of the sample relative to the camera.
Why extensometry?
Why do you need an extensometer with a tensile test bench after all? When a sample is placed in this kind of system and the system begins to exert force – pulling or pushing – that force is not only applied to the sample, but to all components in the chain. The load cell, the clamps, the cross head, the spindles; at high forces, everything is under tension and therefore everything deforms. ‘In pressure tests, the system can still correct for this, but for tensile tests this is more difficult because the clamps also yield,’ says Maarten Debrouwere, sales manager at Benelux Scientific.
For accurate measurement, it is therefore very common to integrate an external displacement meter into the system. A classical extensometer makes contact with the sample via small arms and thus measures the actual displacement of the sample, leaving out all other deformations in the system.
Classical method falls short
Such a mechanical clip-on extensometer works very decently but it also has a few drawbacks. For the sake of accuracy, the contact points consist of razor-sharp blades. This is not a problem for hard metals, but if you want to measure plastics, for example, they can damage the material and cause premature fracture.
A second disadvantage is that a lot of energy is released during a fracture. Most of that disappears up and down into the clamps, but some of the recoil goes into the measuring arms and the sensitive measuring system of the extensometer. “Those don’t like such high impact,” tells Maarten Debrouwere.
Video extensometry, the solution
To meet the challenges of classical extensometers, there is video extensometry. Indeed, via markings on the sample and a good camera, the displacement can also be measured. That method is non-contact so there is no risk of premature breakage or damage from recoil. However, there is another problem.
The lens you choose for the setup determines the focus distance. So if the camera is not at the right distance from the sample, it will not see the marks sharply and the measurement will be inaccurate.
Big step forward
Tinius Olsen recently launched the VectorExtensometer, which overcomes the problem of previous video extensometers. The system has a volumetric focal distance which means it can automatically find the correct distance to the sample. So an operator does not have to place the sample in the exact same place every time. This is a huge advance, also for the reproducibility of video extensometry. Debrouwere: “Moreover, the VectorExtensometer maintains a handsome resolution of 0.5 micrometers.”
Benelux Scientific is also very enthusiastic about the price tag. In fact, earlier systems quickly cost double the price, which put the technology out of reach for many applications. The price of the VectorExtensometer is comparable to that of a classical extensometer, making the step to video extensometry really not that big anymore. “I would like to eliminate that misunderstanding,” said Debrouwere.
Benelux Scientific is convinced that many existing setups with a normal extensometer can now be equipped with a video extensometer. With that the user benefits from all the advantages of that technique. This certainly also applies to standardized tests.
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