Research Of Digital Shearing Speckle Pattern Interferometry (DSSPI) Technique Based On Liquid Crystal

Digital Shearing Speckle Pattern Inerferometry (DSSPI) is a very promising non-contact measuring and testing method. To realize automatic measuring of this method and improve it's precision, in this thesis the liquid crystal (LC) cell is used as a phase-shifter for the traditional DSSPI successfully. The phase-shifting DSSPI system is designed based on the liquid crystal phase-shifter and Wollaston prism. The automatic, high-precision, full-field and real-time measuring is realized due to the applying of computer program. In this thesis, the various property of LC is studied. The feasibility of using the LC cell as phase-shifter in the DSSPI is analyzed by theory. The new phase-shifting and shearing theory based on LC and Wollaston prism is developed. The four-step phase-shifting measuring method is proved to be most decent for this system by comparing the methods in common use. The filter and unwrapping technology is also studied to guarantee the success of the LC phase-shifting measuring method, and the adaptive window sin-cosine filter method is put forward. This method is proved that it can filter the fierce noise in the speckle phase map at the same time the jump of the phase can be well kept. The mechanical design and software program of the LC shifting DSSPI system is finished by fully consider its configuration optimizing and easy operating. Many experiments are performed using this system. The system is proved to be very efficient by measure the organic glass that is fixed all around and loaded in center. The maximum gradient of the out-of-place displacement of one section in this specimen is also measured to be 1.2 × 10 -5, the maximum of out-of-place displacement is 2×10-4. Some experiments are also performed to measure the debonding defect of a certain composite material. The location and shape of the defect can be seen clearly in the wrapped or unwrapped phase map, which illustrates that this system provides a very effective nondestructive testing method to measure the subsurface defect of object.