A development of computer vision elements for processing the results of tensile tests of specimens and building diagrams of ultimate plasticity

The use of specimens with a round notch allows expanding the application of tensile tests in order to study the ultimate plasticity of materials. However the main difficulty in processing the results of experiments is the need to measure the radius of curvature of the neck and the diameter in the minimum cross-section of the specimen. These dimensions determine a stress triaxiality value of the material in the center of the neck according to the Bridgman model or the other ones. This article is devoted to the development of elements of a computer vision system designed to measure the neck, which is formed both on cylindrical specimens and on specimens with a notch. To extract the contour of the specimen and to measure the forming neck, the video recording of the test process on a digital single-lens reflex (DSLR) camera is used. The paper proposes a method for recording the test process using the background, which allows to increase the accuracy of determining the contour of the specimen during subsequent image processing. The Sobel filter is used to extract the contour of the specimen, and a special neck profile equation allows to determine the neck dimensions.