Improvement And Application Of The Digital Gradient Sensing Method

Transparent polymer materials are widely used in fields of architecture, automobile and aviation due to their advantages of good transmittance, thermostability, high mechanical strength, good resistance to chemicals, etc. But transparent materials may contain inevitable initial damage and residual stress in producing process, and are faced with fatigue damage and fracture in service. To study their mechanical properties, some optical methods were developed for measuring stress and strain of transparent materials, such as photoelasticity, Moire and coherent gradient sensing, etc. Digital Gradient Sensing (DGS) method can gain the stress state of transparent materials using simple optical equipment and has ability to solve fracture mechanics problems.In this paper, a calibration method of elasto-optic constant based on least-square method is proposed, and the accuracy of iterative convergence is improved by introducing liner error to the function. The full-field experimental data of polymethyl methacrylate (PMMA) specimen is obtained from digital image correlation method. The experimental results show that the least-square calibration method can determine the elasto-optic constant effectively.The stress intensity factor (SIF) of I-mode crack of PMMA is measured by DGS method. By simplifying a multi regression problem to a curve fitting, the SIF can be estimated in an easier way. The comparison between experimental result and theoretical solution of boundary collocation method verifies the accuracy of DGS method.Finally, an innovative backlight testing system by introducing In-Plane Switching (IPS) screen into the DGS method is proposed, which not only simplifies the experimental equipment, also improves the quality of speckle images and the accuracy of experimental results. The convex lens experiments using backlight testing system are compared with the traditional system. The feasibility and effectiveness of the backlight testing system are verified. Therefore, the new backlight testing system based on IPS screen can be used in DGS experiment and can enhance the convenience and reduce the error of DGS method.