Common optical methods are not suitable for accurate measurement of elastic modulus of specimens with initial bending because it is rather difficult to separate the axial strain caused by the axial load from the surface strain which consists of the axial strain and the bending strain caused by additional bending moment. Inspired by the strain-gauge technique, averaging the two strains on the opposite surfaces can eliminate the effect of bending strain of the specimen with initial bending, which is easily implemented by dual-reflector imaging and two-dimensional digital image correlation (2D-DIC). With dual-reflector imaging, the front and rear surfaces of the specimen are recorded by using a single digital camera. Consequently, the strains of two optical extensometers constructed on these two surfaces are obtained with common 2D-DIC, and averaging these strains can eliminate the effect of bending moment and out-of-plane motion of the specimen. Therefore, the elastic modulus can be determined with high measurement accuracy. The effectiveness of the proposed method was first verified using self-manufactured tensile equipment which leads to large out-of-plane motion during the test. Furthermore, uniaxial tensile tests of stainless-steel specimens, including static and continuous tensile tests, were conducted to evaluate the accuracy of the proposed method. The experimental results show good agreement between elastic modulus obtained using the proposed method and a strain gauge and the relative error between them is less than 0.5%, which shows excellent performance of the proposed method on accurate measurement of elastic modulus of specimens with initial bending.
