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Video extensometers (VEs) are non-contact strain measurement devices that have no influence on the specimen being tested. Lines, dots or other patterns of black-and-white marks are placed on specimens, letting sensors and software algorithms within the VE detect motion – and therefore changes in length or displacement – during tests.
The devices can have many advantages, giving users greater flexibility and the best available accuracy across a broad range of applications. “Brittle or thin materials benefit particularly from this non-contact solution, as well as samples that release energy at failure, such as cables, ropes or belts,” said Shawn Byrd, technical manager at Tinius Olsen. It is also suited for measurements inside thermal cabinets or environmental test chambers.
Versatile technology
Video devices are especially effective where contact extensometers might have an adverse impact on test results or accuracy. Some of the advantages include negating errors owing to worn or damaged parts, or inertia from moving parts. Breaks or slippage from knife-edge engagement, a potential problem with extensometers that introduce load to samples, is also not an issue.
The technology’s greatest advantage, said Tinius Olsen, is its versatility. One camera can make longitudinal and transverse strain measurements, with only a simple adjustment and alignment required with respect to the test axis. The technology permits multiple fields of view, as well as realtime viewing and analysis.
Tests using VEs are well-suited for rigid materials such as metals and composites, plus a wide range of other materials including plastics, textiles, paper, thin sheets, foils and wires. It is appropriate for a variety of test procedures such as the measurement of material properties and true strain controlled tensile tests. It also includes exploration of cracks, investigation of strain behaviour on dynamic tensile tests, dynamic and high-speed tests and vibration analysis.
“VE also solves many of the problems that can occur when a specimen has relatively soft edges like a thermoplastic or has features that can rupture or break, causing damage to a clip-on extensometer,” said Byrd.
The best choice?
The devices determine the position of the markers through changes in the brightness of their light-dark edges. The camera digitises the image, and algorithms measure the change in grayscale along one image line on the specimen surface. The algorithms allow the instrument to measure edge positions with sub-pixel accuracy.
Transverse deformation is calculated from the measured change in the width of the specimen. Gauge length is automatically measured at the beginning of each test and used for strain calculation, eliminating errors owing to inaccurate specimen marking.
Despite the technique’s versatility, users should always consider the best technology for the application, said Byrd. “The best choice for a particular material might not be right for another. Keeping that in mind, given improvements in hardware and software, extensometer users need to seriously consider VEs based on their flexibility, reliability and cost-effectiveness.”
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