전단간섭계를 이용한 면외변형의 정량계측법

Abstract

In general, deformation measurement methods are largely divided into a contact type and a non-contact type. The representative contact deformation measurement methods include a strain gauge and a accelerometer, while the representative non-contact methods are Laser Doppler Vibrometry(LDV), Electronic Speckle Pattern Interferometer(ESPI), and shearography. Especially, ESPI can measure and analyze precisely in a real-time the deformation occurring to the whole fields of the objects. On the other hand, as this method is so sensitive to external disturbances, the applicability is so low in the industries without a optical table. Another method using speckles is shearography, which is developed on the basis of detecting the derivative components of deformation when the interferometry composes appropriately. Like a strain gauge, shearography can directly acquire the strain information. Stress concentration will occur when external forces are applied to an object with a defect, and the deformation of rigid bodies will not change the strain. Thus, shearography is very efficient to measure the defects of an object. Also as shearography is very endurable against external disturbances, is frequently employed as a nondestructive inspection technique on industrial areas. Because shearography easily composes interferometer and can measure even though coherence length of laser is relatively short, it has been more widely employed and is used to vibrations analysis of objects. Also, this method is applied efficiently in the nondestructive inspection sector such as the quantification of defects and the analysis of composite material structures. Until now, researchers used shearography so as to measure just the slope of deformation, to quantify defect lengths by measuring the distance between the inflection points of deformation slope taking place on defected regions of pressure vessels. Also, they used in order to compare shearography with ESPI to vibration modes occurring in a certain object. Currently, ESPI is mainly used to measure the deformation of objects, while shearography is used to measure only the defects of objects. Stroboscopic phase shift ESPI(SPS-ESPI), which is applicable to vibration analysis only, can quantitatively measure the amplitude in a certain condition, but cannot quantitatively measure the amplitude of each vibration mode occurring in objects under the same condition. LDV can analyze the vibration of objects under the same external oscillation condition, but it takes very much time to measure it. This thesis uses shearography and numerical integration not only to quantitatively measure various types of out-of-plane deformation occurring in objects but also to solve the problems of the other methods mentioned above and shearography. In the past, the researchers arbitrarily applied the shearing amount, which is the most important variable of shearography, according to the sensitivity of detecting the defects. In contrast, this thesis proposes an basic idea considering that shearography is basically an image processing method. The procedures of this method are as follows; a) shearing amount can be converted into the number of pixels in the surface, b) applied to the results of measurement by shearography, c) and then numerically integrated in order to quantitatively measure the deformation of objects. To verify the effectiveness of the proposed method, experiment is performed on a circular plate with out-of-plane deformation, pressure vessel with internal defects, and a flat plate with vibration. Then, the correlation between ESPI and shearography is investigated. Also, the experimental results demonstrate that shearing amount on the image plane can be substituted by the number of pixels, and shearography can measure out-of-plane deformation. Furthermore, an experimental result for the deformation measurement in a circular plate shows an error rate of about 1.74%. It is very precise comparing to ESPI. In an experiment for the defects measurement in defective pressure vessel, the rate is about 8.3%. Also, this study shows that the proposed method in this thesis can consistently measure the deformation in any shear, while ESPI cannot measure deformed regions. In analyzing the vibration of a plate, the proposed method can quantitatively measure the amplitude of not only first mode but high modes. Thus, it is more useful than SPS-ESPI. Comparing results of techniques such as the finite element method, LDV, and SPS-ESPI, the reliability of the proposed method is much more higher as its mean deviation is 2.84%. Based on the above-mentioned results, the proposed method can overcome the limits of ESPI that is so sensitive to external disturbances, and quantitatively measure the out-of-plane deformation and the strain, and its application in industrials areas is very effective.