High-resolution and high-accuracy optical extensometer based on a reflective imaging technique

Abstract Optical extensometer is a popular device for strain measurement due to its merits of non-contact measurement over a wide range of strain values compared to conventional contact measurement devices. Telecentric lens is often implemented in optical extensometers to improve the measurement accuracy by alleviating the effect of out-of-plane motion. However, the accuracy of an optical extensometer is always limited by the gauge length, which is generally less than the resolution of a digital camera. In this study, we propose a reflective imaging technique to effectively enhance the gauge length by recording two isolated long-distance regions on the specimen using a telecentric lens. The corresponding reflective imaging device was integrated in a single apparatus in which the installation of telecentric lens was very convenient. Through precise calibration, the gauge length of proposed extensometer could be increased from 1700pixels to 7477.5pixels. The performance of the proposed extensometer is evaluated using self-heating and static tests. Furthermore, the feasibility, repeatability, and reliability of the extensometer are validated by uniaxial tensile tests on a stainless-steel specimen. Experimental results showed that the axial strains obtained using proposed method were in excellent agreement with those obtained using strain gauge, and the root mean square error of strain was below 5μe in most cases. Therefore, the proposed extensometer is highly accurate and exhibits immense potential to replace mechanical extensometer and strain gauge for determining the mechanical properties of materials.