Study Of Novel Michelson-type Digital Shearography

ABSTRACT:Digital shearography is a well-established tool for strain measurement, vibration analysis, and non-destructive testing, because it can measure spatial derivatives of displacement directly, and has advantages of high resolution, full-field measurement, non-contact testing, and high interference immunity. It has been a popular research topic and many kinds of shearing devices have been employed in it. Among those shearing devices, the most popular one is the modified Michelson interferometer, with which the digital shearography is called the Michelson-type digital shearography. Michelson-type digital shearography enjoys several advantages, such as being easy to adjust the shear direction and amount, and being easy to carry out the temporal phase shift technique. However, it should be improved in some areas. Firstly, the view angle is limited by the structure and cannot exceed28°. Secondly, the present in situ phase shift calibration techniques are all based on pattern analysis and statistical methods, which will lead to unreliable calibrations in some case. Finally, most digital stroboscopic shearographic devices are not suitable for measurement of large object.This thesis focuses on the studies of the Michelson-type digital shearography. A method to enlarge the view angle and an in situ phase shift calibration technique are proposed. A stroboscopic Michelson-type digital shearographic device for vibration measurement of large object is developed.The main innovations of this thesis consist of:1. A method that embeds a4f optical system into the shearographic system optical arrangement to enlarge the view angle is presented. The theoretical analysis and experimental research both reveal that the new Michelson-type digital shearography with4f optical system can break through the28°view angle limitation. The view angle will no longer be limited by the structure, but only depend on some parameters of the digital camera. Thus wide view angle in Michelson-type digital shearography is easy to achieve.2. An in situ phase shift calibration technique based on optical method is introduced. It is an on-line method that can calibrate the phase shift automatically. It uses optical method rather than statistical method, so the calibration results are more reliable and arbitrary phase shift angle can be calibrated.3. A stroboscopic digital shearographic prototype based on modified Michelson interferometer is developed. It can achive the vibration measurement on a large erea The measuring field can reach an area of1.2m×0.9m at1.5m work distance. The design details of the prototype, including optical design, electronical design, and programming, are given in the thesis. Some experiments are also conducted to show the performances of the prototype. The experimental results prove that it is suitable for the applications of vibration measurement, displacement derivatives measurement, and non-destructive testing.