Dual-Light-Path Optical Strain Gauge Using Diffraction Grating and Position-Sensitive Detectors for Deformation Measurement

In this paper, a dual-light-path optical strain gauge (DOSG) that measures surface deformation in real time using diffraction grating and position-sensitive detectors (PSDs) is proposed. In the DOSG, a beam splitter cube is utilized to divide an incident beam into two beams. One is applied in the main light path to measure specimen deformation, while the other is used in the assistant light path to eliminate disturbances from out-of-plane rotation and displacement of the specimen. Further, the disturbances as systemic errors are theoretically analyzed, and eliminated with the assistant light path during the experiment. Meanwhile, random errors, which are primarily due to ambient light, beam power, irradiation position and incidence angle, are studied to improve measurement accuracy. Benefiting from the utility of PSDs, the developed system achieves a strain resolution of 1 με. In experiments, uniaxial tensile tests of aluminum alloy and Ni-based alloy confirm that the relative error of its elastic moduli is less than 3.4%, and the stress-strain curve exhibits a R-square value greater than 0.9951. In addition, the DOSG is extended to determine the mechanical behavior of Ni-based alloy at high temperatures up to 800 °C by combining it with an induction-heating apparatus and a tensile testing machine. These results verify that the proposed DOSG is feasible and reliable, with good potential for high-precision deformation measurement in both room temperature and high-temperature environments.