Low-cost and high-accuracy optical extensometer using modified field-of-view splitting and its application

Abstract The current optical or video extensometers adopt either an expensive telecentric lens or a common lens combined with cumbersome compensation methods; hence, they lack sufficient versatility. This study develops a low-cost and high-accuracy optical extensometer based on a common lens that eliminates the use of any compensation method. The proposed extensometer adopts two parallel cameras combined with a previously established field-of-view (FOV)-splitting scheme to form a modified FOV-splitting scheme that can record the upper and lower FOVs onto the left and right halves of an image. The vertical displacements of gauge points selected at the horizontal middle line are less affected by slight out-of-plane motions of the specimen. Furthermore, the gauge length of the extensometer is considerably improved by using the modified FOV-splitting scheme, thus enhancing the strain resolution of the extensometer. The out-of-plane translation tests and uniaxial tensile tests verify the feasibility of the proposed extensometer. Furthermore, the strain results obtained using the proposed method are indeed not significantly affected by out-of-plane motions and are in good agreement with those obtained using the strain gauge. Finally, the tensile failure test results of a cast iron specimen demonstrate the extension of the proposed extensometer to the strain measurement of brittle materials.