Accuracy enhancement of a video extensometer by real-time error compensation

Video extensometer using regular lenses generally cannot ensure accurate strain measurements because of the detrimental influences of out-of-plane motions unavoidably occurred during mechanical loading. Here we propose an accuracy-enhanced video extensometer with real-time strain error compensation. To this end, a hollow, rectangular reference compensation sample is attached onto the test sample surface, which allows it to move rigidly without deformation during loading the specimen. Four reference points at the two ends of the axial and transversal reference lines, which are parallel to the gauge lengths defined on the test sample, are specified and tracked by real-time digital image correlation (DIC) to record the virtual strains (i.e., strain errors) caused by out-of-plane motions. Thus, accurate strain measurements can be realized by the established video extensometer by directly subtracting these virtual strains. Uniaxial tensile tests of aluminum specimens were carried out to examine the accuracy of the proposed technique, and the measured strain results show good agreements with those obtained by strain gauges and stereo-DIC with an average strain error less than 35 microstrains. Experimental results indicate that the proposed technique provides a low-cost, easy-to-implement and practical solution for accurate strain measurements in material testing.