High-accuracy biaxial optical extensometer based on 2D digital image correlation

In two-dimensional digital image correlation (2D-DIC), the optical axis of the camera must be exactly perpendicular to the object surface to avoid the generation of out-of-plane displacement, which leads to low strain accuracy. In this work, we first studied the strain accuracy of a common optical extensometer when the optical axis of the camera is not perpendicular to the specimen surface. Derivation reveals that even a very small oblique angle θ will introduce a considerable strain error and that an opposite oblique angle θ generates an opposite strain error of the same magnitude. Therefore, taking the strain average of these two optical extensometers can almost entirely eliminate such effect of non-perpendicularity, which has the same form as the optical extensometer realized by dual-reflector imaging. Because the transverse strain of a specimen is less than its axial strain, the measurement of transverse strain requires higher strain-measurement accuracy. With the aid of a self-designed right-angled apparatus, we conveniently extend this 1D extensometer to a 2D situation, which is called the biaxial optical extensometer. Uniaxial tensile tests of a stainless-steel specimen are conducted to evaluate the strain accuracy of the biaxial optical extensometer quantitatively. Experimental results show that the transverse and axial strains obtained using the proposed biaxial extensometer are in good agreement with those obtained using strain gauges and that the proposed extensometer achieves higher strain accuracy compared to the 3D-DIC extensometer.