Developing Video Measurement of Strain for Polymers Using Labview

Abstract

The purpose of this study was to develop a computer assisted video extensometer to measure strain, primarily for transparent polymers such as PET. Two markers, which were attached to the specimen, were tracked by the video system. Different techniques were proposed to track these markers implemented in NI LabVIEW software. The accuracy of video extensometry depends on the template creation procedure, shapes of the markers, algorithm and camera resolution. Comparison of the different techniques was discussed in this study. The technique with two different templates in two separate regions provided 15% to 25% more accuracy compared to other techniques discussed in this study. The accuracy of the proposed video extensometer was also shown. The accuracy of the proposed technique was 99% using a camera with 640x480 resolution. Moreover, comparison with a Laser Extensometer and the proposed video extensometer were also shown. Fringes were also captured when the PET was in tensile stress and under polarized light. The pattern-changes of these fringes were recorded by NI LabVIEW software. The disappearance of fringes before the yield point in tension indicated permanent plastic deformation in PET.