Optical Extensometers: A Closer Look for a Better View

For many testing applications, non-contact measuring extensometers offer advantages over classic, contact-type extensometers. One of their key features is in the name itself: No contact is made with the specimen and due to the optical measurement there is no interaction between the specimen and the measuring instrument. Another, lesser known advantage is that optical extensometers allow you to have the entire specimen in view: The cameras display images that provide more information than just the strain. They allow, for example, the break position to be determined automatically or the initial measuring length to be changed retroactively, which in turn reduces the specimen material and test time required.
Optical extensometers are superior to contact-type extensometers in many respects. Since no physical contact is made with the specimen, its behavior is not influenced by test knife edges and drag forces. This eliminates the possibility of the test results being distorted by the testing instrument itself. As a result of decoupling, optical extensometers are well-protected from damage that could be caused by the specimens and are not negatively affected by brittle-fracturing specimens or specimens prone to whipping with high energy at break, such as elastomers, wires, and ropes. In addition, small or short specimens are available for optical systems if the contact-type systems do not have any more room between the fixtures. Measurements in temperature chambers are performed using a special glass pane and deliver better results than sensor arm extensometers since they can be taken in a closed system. videoXtens–The Versatile Extension Measurement Solution There are three different technologies within the optical, non-conact extensometer solutions from ZwickRoell. The most well-known technology is the camera-based videoXtens, which covers the widest range of applications. A camera with a lens is hidden in the extensometer. Depending on the height of rise of the lens, there is a small or large field of view which represents the area of the specimen from which the camera records. Gauge marks are applied to the specimen that mark the gauge length. The testXpert III software recognizes the gauge marks automatically and transfers the initial gauge length into the system. During the test, the marks are tracked in that they are displayed frame by frame. A special marking method and intelligent algorithms in the software produce excellent measurement accuracy. All measurement channels are synchronized exactly. The software for the optical extensometers is integrated into testXpert III, which allows all of the test data to be saved and evaluated together in a single software. A closer look with array technology Due to its modular design, the videoXtens system can be configured according to the application. The required measurement range for the test and the desired measurement values determine the specifications for the necessary field of view and the resolution of the lens. There is one correlation that applies here: A lens with a large field of view provides low resolution and a lens with a small field of view provides high resolution. In order to provide high resolution over a large measurement range, ZwickRoell developed its array technology, in which multiple cameras with high resolution lenses are connected in series. The overlapping fields of view are combined into a single large image via software, allowing the videoXtens to deliver very high resolution and have a large measurement range. For example, there is the videoXtens 2-120 HP, which provides a resolution of 0.15 µm in a measurement range of up to 140 mm. It also fulfills the strict calibration requirements of ISO 527-1 Annex C for determination of the tensile modulus, even for tests at elevated or lowered temperatures in ZwickRoell temperature chambers. A Closer Look, A Better View Since optical extensometers keep the entire specimen or a large portion of the specimen in view, there are various options for obtaining more information through a pure software option upgrade–no complex hardware upgrade necessary. For example, you can achieve simultaneous determination of transverse strain and biaxial measurement. Or multiple measuring points–up to 100–can be set on the specimen surface.
Automatic Determination of the Break Point–No More Discarding Specimens The added value becomes even more apparent during automatic determination of the break point, through which the initial gauge length can be automatically set to to the break point. How does it work? By using testXpert III and the strain distribution option, up to 16 evenly distributed measuring points are established on the specimen. During the test, the individual strains between these measuring points are recorded and tracked. The initial gauge length is set automatically at the area with the largest amount of strain since that is where the break is expected to occur. As a result, the break will automatically occur in the middle of the gauge length. The benefits to the customer are clear: It saves a significant amount of specimen material and test time since specimens no longer have to be discarded.
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Changing the Initial Gauge Length Retroactively The images from the test can also be saved. The Test re-run option uses this recorded image series to change or shift the initial gauge length retroactively. A recalculation is performed in testXpert III based on the new initial gauge length. The strain as well as the associated measured values are recalculated and saved clearly as a separate test in the software. This option is used, for example, in component testing, when it is necessary to evaluate local strain at different points, or in standard tensile tests when specimen necking has occurred outside the original initial gauge length.
laserXtens–An Even Closer Look Without Gauge Marks laserXtens from ZwickRoell is unique since it does not require the application of gauge marks. This eliminates the need for specimen marking and the options can be used even more efficiently. ZwickRoell array technology enables the system to provide high resolution (0.07µm) in a large field of view (220 mm). A unique and patented system. laserXtens is based on the laser speckle correlation method. Laser beams are dispersed into the specimen through natural surface roughness and generate a speckle pattern that provides unique, distinguishing features similar to a fingerprint. As a result, virtual gauge marks can be defined as desired in the testXpert III software. An advantage of video systems is that the speckle pattern of the laserXtens always maintains the same level of quality over the entire specimen surface and for every additional specimen and is not dependent upon the user that applies the pattern. A strong basis for reliable test results. laserXtens is not only unique due to the ability to test without using gauge marks. It is also the first non-contact extensometer for strain-rate controlled tests to ISO 6892-1 Method A1. laserXtens provides a closer look. If maximum flexibility and functionality are needed, laserXtens can be combined with videoXtens to create two extensometers in one that include all options, which is a perfect basis for research laboratories and universities. Conclusion Optical extensometers cover standard applications and offer added value through additional functions in comparison to contact-type extensometers. ZwickRoell offers a wide range of products and a suitable solution can always be found: The right system for every application. This is possible due to decades of application-related experience and ZwickRoell’s own in-house development and production. As a result, today ZwickRoell provides innovations that deliver reliable test results and help reduce test time and specimen material.
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