I. Video Extensometry
Video extensometry is a non-contact system for measuring the deformation of certain materials by capturing continuous images of a sample during a test with a digital video camera connected to a computer. The basic topology of the video extensometry you can see in Fig. 1. The test material sample is usually cut to shape and marked with special marks, usually, special stickers or paint, that distinguish the mark from the sample's colour and texture in the captured image. While the test specimen is stretched or compressed, the distance between the pixels in the captured image is constantly monitored in the captured video. This pixel distance can be measured in real-time and mapped to a calibration value to achieve direct deformation measurement and, if necessary, to operate a test machine in stress control. A resolution of less than one micrometre can be achieved with the right calibration value and good image processing algorithms. The calibration sample, which is usually a specially applied material with high accuracy, also influences the correct calibration value. Calibration begins with images taken with a calibration sample under the same test conditions as the new samples [1],[2].