Optical extensometer and elimination of the effect of out-of-plane motions

• An optical extensometer is developed to measure local uniform strain on planar object.
• The optical extensometer consists of two image sensors and a large format lens.
• Virtual strain due to out-of-plane motions is eliminated by proposed correction sheet.
• Adoption of two image sensors elongates the scale distance and improves the precision.
• The resolution of the optical extensometer achieves 2–3 με.

A novel optical extensometer is developed to estimate the local uniform strain on planar surface accurately. The proposed system consists of a shared large format lens and two image sensors, which acquire pairs of images of two isolated small regions on the object surface simultaneously. Digital image correlation (DIC) algorithm is applied to determine the relative displacement between gauge points designated on recorded pairs of images. Then local strain can be extracted after dividing the relative displacement by the scale distance. Moreover, a special experimental setup called “correction sheet” is used to eliminate the virtual strain induced by out-of-plane motions. Uni-axial tensile experiments are performed to validate the reliability and resolution of the optical extensometer, and the measurement results demonstrate that the resolution of the optical extensometer achieves 2–3 με.