The digital gradient sensing (DGS) method is a full-field optical technique to measure the in-plane stress gradients of transparent materials. Elasto-optic constant is an important material characteristic for the DGS method and is usually not given. On the basis of introducing the working principle of DGS method, a calibration method of elasto-optic constant based on the least-squares method is proposed, and the accuracy of iterative convergence is improved by introducing liner error. A theoretical model of a semi-infinite plane under concentrated normal load on boundary is chosen to verify the feasibility of the proposed method. The full-field experimental data of polymethyl methacrylate specimen is obtained from the digital image correlation method. The experimental results show that the least-squares calibration method can determine the elasto-optic constant of transparent materials effectively.
Nondestructive optical techniques have been widely used to satisfy engineering applicat ion. 2D digital image correlation (DIC) method has superiority of convenient manipulation and high accuracy in measuring planar displacement. Moreover, fringe pattern profilometry (FPP) method has become a highly developed technique to measure surface profile. Combined with 2D-DIC method, FPP method can be applied to measure three-dimensional displacements conveniently. As a semi-custom integrated circuit, Field-Programmable Gate Array (FPGA) has been popular for its powerful programming performance on controlling experimental instrument. Furthermore, Labview, an efficient graphical programming language which excels in instrument communication, can be used to program FPGA. In this paper, a corrected dynamic FPP method combined with 2D-DIC method has been presented and achieved by Labview programming to measure dynamic deformation. An experimental system including a projector and a camera is used to project fringe patterns and acquire images alternately at a high speed. By the referred method a series of spatial-discrete displacement data in equal intervals of time are obtained. Then a four-dimensional interpolation is adopted to get full-field and continuous-time displacement data. Thus, the planar and out-plane displacements can be simultaneously measured. Experiments were performed and verified the feasibility of proposed method.