The Development Of Video-extensometer

With the development of modern science and technology, the applied ranges and kinds of engineering materials have been increased greatly. The serviceability of conventional extensometers for deformation measurements is limited in the following:①flexible materials such as fabric, plastic film②micro-specimen③in the circumstance of high temperature or high pressure④large deformationThen, it's necessary to develop a technique, which can be used to detect deformation in non-contact way.Video-extensometer is a kind of the non-contact displacement sensor based on digital image technique. It can be both applied in the above conditions and measuring the general solid deformation. Compared with conventional extensometer, Video-extensometer also has some advantages, such as, it can automatically judges the necking position, sets the gauge length or measurement area freely, and makes no fray on specimens. This kind of instrument has been developed abroad, but no home-made products appeared.The principle of the video-extensometer developed in this paper is found on digital image processing. Taking the advantage of no deformation about the markers employed for marking the gauge length of specimen, the test precision is atμm level. This sensor can be used to test the stress-strain relationship as well as Yong's modulus of materials.In the first part, this paper reviewed the history of testing strain, and discussed the internal and international status in quo of non-contact strain measurements. Then, techniques of edge detection and image match were introduced. Finally, the arithmetic, configuration of software and hardware, and the experimental result are described in the following sequence.①Obtain the undeformed image on the specimen using CCD, first, and then take a sub-range on the markers for correlation operation. Next, match the corresponding pixel in the current image, timely, by correlation. Thirdly, to find the most suitable sub-pixels, bilinear interpolation and template moving method are used, with eliminating improper points by digital filtering. Finally, the strain of the specimen is obtained from the number of pixels and sub-pixels.②the size of necking can be found, by the criterion of real-time and sub-pixel edge detection technique which are developed by the author.③The stress signals are measured by a force sensor, and the stress-strain diagram are plotted on a computer. Then, the Yong's modulus is determined by least square method.The key problems have been solved in this paper are acquiring images in real-time and sub-pixels match technique. A piece of software has been programmed with C++ builder 5.0.