An optimized design for a field-of-view splitting-based long-gauge-length optical extensometer using two rhombic prisms

Although the field-of-view (FOV) splitting strategy, based on the reflective image technique, can effectively enhance the gauge length of an optical extensometer, its elements, including a right-angled optical prism and two plane mirrors, require meticulous adjustment before a measurement. In this research, we propose an optimized FOV splitting scheme that uses two identical rhombic prisms. Each rhombic prism can displace a beam of light without changing its direction. Therefore, two symmetrically positioned rhombic prisms converge the light that traverse from two separate regions of an object, which realizes the FOV splitting and greatly simplifies the adjustment of the optical elements. Based on such a FOV splitting scheme, we develop an enhanced optical extensometer. Repeated tensile and cyclic loading-unloading tests are conducted to validate the feasibility and reliability of the proposed extensometer. The experimental results that are obtained using the proposed method are in excellent agreement with those using a strain gauge, which indicates that the optimized FOV splitting scheme is easy to implement and it has high assembly accuracy. Finally, a tensile failure test of a cast-iron specimen is conducted; the measured results indicate that the proposed method is appropriate for measuring the strain of brittle materials in practical applications. (paper)