| Recently,with the development of the scientific and technical innovation, optical mechanics continues to evolve and has wide application in lots of fields. But there are only few of the researchers who analyze the images in the frequency domain, not even come up with a systematic, analyzing approach to gain the displacement. In this paper, digital image frequency spectrum(DIFS) method used for analyzing digital speckle patterns with deformations in the frequency is proposed.The paper contains three major aspects: the fundamental principle of DIFS, the accuracy and the influencing factor of DIFS and DIFS’s application in the engineering measure and test.In the digital image frequency spectrum method, two subimages in the same position as two digital speckle patterns before and after deformation are used to form a Young’s fringe. After Fourier transform we can find the displacement between the two subimages directly.By moving the subimage, the DIFS method can gain the accurate displacement even though the displacement is small. In the meantime, we suggest the appropriate size of the subimage to guarantee the accuracy and explore the relationship between the subimage’s and the precision. Even though the image noise indeed has an effect on the new method’s accuracy, but does not affect the application of the DIFS method. In addition, compared to digital image correlation method, we find that the results of the DIFS method has the same order of magnitude, but the DIFS method spends much less time than the DIC method in computing the integer-pixel displacement.To testify the effectiveness and stability of the DIFS method used in the engineering measurement, three experiments are designed and conducted, which are standard fiberglass specimen’s tensile test, composite sandwich joints tensile test and composite-to-steel joints tensile test. Actual measurement results demonstrate that the proposed DIFS method is a high accurate measurement method with preferable stability and practicability. |
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