High-precision video extensometer based on a simple dual field-of-view telecentric imaging system

We present a novel dual field-of-field (dual-FOV) video extensometer for high-precision tensile/compressive strain measurement in material testing. The present dual-FOV video extensometer, based on the combined use of a bilateral/object-space telecentric imaging system and a cube prism, can orthogonally capture two separated FOVs on the sample surface. For any two gauge points selected in the two separated FOVs, the gauge length can be theoretically computed from the systematic parameters of the imaging system. By tracking the movement of the gauge points using advanced subset-based digital image correlation (DIC) algorithm, temporal changes of the gauge length, and consequently the average strains of the test sample can be obtained. Due to the enlarged gauge length, logitudinal strains can be measured with higher sensitivity and precision compared with previously reported video extensometers that use only a telecentric lens. The precision, accuracy, and practicability of the proposed video extensometer are validated by real tensile tests.