Vogel Grote, MTS Product Manager, exploring the capabilities of point-to-point video extensometer technology.
Do Not TouchMTS Product Manager Vogel Grote, a 20-year veteran in materials testing and accessory product design, explains how non-contacting extensometers are becoming an effective and increasingly practical choice for more and more materials testing applications.
Q: Can you provide a brief history of non-contacting extensometers?
Grote: Non-contacting extensometers were developed several decades ago for applications where it was difficult, if not impossible, to acquire data using contacting extensometers. Because they do not physically touch the specimen, non-contacting extensometers are ideal for specimens that are thin or brittle, have irregular surfaces, require high-temperature or submerged test environments, or tend to release large amounts of energy at failure. The primary technologies are laser and digital video.
Q: How do these two technologies compare?
Grote: Conceptually, they are similar. They both use an “edge-to-edge” approach, in which reflective tape targets are attached to the specimen. In one case, a scanning laser hits the targets and the extensometer measures the reflected light to calculate the distance between one edge of the tape and the next. With video extensometers, high-resolution digital video cameras record images to measure how much the targets move. Both technologies offer high accuracy with continuous data output and no risk of stress concentrations or weight error.
Q: Why would a test professional choose one or the other?
Grote: Laser extensometers are reliable because they have no moving parts and do not require a separate PC for data processing, and they can be used in tests up to 10 Hz. They are limited only by a fixed field of view and the ability to measure one axis at a time, so they are a cost-effective choice for most monotonic and some dynamic tests. Video extensometers perform uniaxial or biaxial measurements and offer a range of lenses, so test professionals can change the area of measurement quickly and easily. They are typically used for tests up to 2 Hz on homogeneous materials that require large displacements, such as rubber, plastics and soft metals.
Q: What is changing in the world of non-contacting extensometry?
Grote: The most important change is that the underlying technology is getting much, much faster. Computer chipsets, data processing algorithms and digital video cameras have all made quantum leaps in terms of speed in the last five years. With these changes, the technology of non-contacting extensometers is finally catching up to the concept.
Q: Can you explain what you mean by that?
Grote: Contacting extensometers are simple, familiar and they provide immediate results. The only downside is that you have to keep a lot of them on hand — each one specialized for a different specimen size, geometry, test type or environment. Test professionals have always understood the appeal of non-contacting extensometers. They don’t touch the test, which is ideal. More important, one non-contacting extensometer could, in theory, replace hundreds of contacting devices, which is very attractive from a cost perspective. The problem has been that non-contacting extensometers could not deliver the results test professionals wanted. Setup was too complex, accuracy was unreliable, and you had to wait while the data processed. Faster underlying technology is enabling non-contacting extensometers to overcome these obstacles.
Q: Are non-contacting extensometers themselves changing as well?
Grote: Yes. The most important development is the introduction of point-to-point measurement. Point-to-point video extensometers use spectral paint that is applied to the entire surface of the specimen. Software recognizes patterns in the paint, enabling much more flexible measurement.
Q: What are the advantages of point-to-point non-contacting extensometry?
Grote: Point-to-point video extensometers can perform diagonal and rotational measurements, in addition to uniaxial and biaxial. With a multitude of surface points to choose from, the software can measure strain in several specimen areas simultaneously. This is critical for non-homogenous specimens, such as composites, which do not react to strain uniformly. Point-to-point extensometry also enables the use of multiple cameras, capturing measurements from the front and back of the specimen. With a multi-camera setup, test professionals can use a lens with a smaller field of view — and higher accuracy — to measure small displacement, then use a second camera with a larger field of view to establish full spectrum coverage.
Q: What is the advantage of a multi-camera setup?
Grote: Again, this arrangement is ideal for precise studies of complex events that occur when testing non-homogeneous specimens, hard metals and large structural specimens. Typically, test professionals are studying these materials’ failure mechanisms. Point-to-point video technology makes it easier to capture all of the nuances of the event for detailed evaluation.
Q: What about data processing time?
Grote: Right now, point-to-point technology can analyze load, displacement and strain in real-time up to 2 Hz. These rates will certainly increase. When they do, look for non-contacting extensometers to offer a lower total cost of ownership than contacting extensometers, especially for labs that conduct a wide variety of tests and stock a large number of contacting extensometers.
Q: When can we expect MTS to offer a point-to-point solution?
Grote: MTS is committed to making sure our customers can find the right extensometer for any application. We also know that new extensometry options need to prove themselves before they become useful. We are excited by the promise that new non-contacting extensometers hold, and we are making strategic investments to support the evolution of the technology.
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