Digital Image Correlation Method And Its Experimental Study

Digital image correlation method as a new type of optical measurement technology, has achieved rapid development with advantages of high precision and full measurement and non-contact in research field of structural deformation measurement. Combining with the characteristics of civil engineering, this paper begin with key issues of calculation accuracy and computation speed and stability of digital image correlation method, has proposed a new method of curved surface fitting on correlation coefficient——two step surface fitting method. Algorithm of the sub-pixel displacement measurement is improved with aspect of accuracy and stability, furthermore this paper expand and apply digital image correlation method to the complex strain measurement experiment study, and has verified the feasibility of the "two step surface fitting method" in this paper.Firstly, this paper introduces and summarizes the research status and basic principles and concepts of the digital image correlation method. In Integer pixel search algorithm introduced the cross search method and three step search method. In displacement field measurement method improved the computational efficiency by introducing the displacement prediction search algorithm. Moreover, this paper introduced measurement method of strain field and technology of simulation speckle pattern.Secondly, we compared the calculation error with conventional algorithm of the curved surface fitting and gradient and Newton-Rapshon. In the algorithm of curved surface fitting, compared the calculation precision with the quadric curved surface fitting and the cubic curved surface fitting and the gaussian curve surface fitting, and furthermore, analyzed and compared improved results after adding exponential to correlation coefficient in algorithm of the quadric curved surface fitting and the cubic curved surface fitting. Then compared the accuracy of gradient method with three different gradient operator. Finally compared the computational accuracy and efficiency and noise immunity between the three algorithm of cubic curved surface fitting of adding exponential and gradient and Newton-Rapshon, and summed up the performance and characteristics of each algorithm.In succession, we proposed a new method of curved surface fitting on correlation coefficient——two step surface fitting method. We discussed these factors affecting calculation accuracy in improved fitting method, and get the optimal choice with analysis the factors. Compared with the calculation error between method of the two step surface fitting and Newton-Rapshon, it is concluded that the calculation precision of two-step fitting is better. And then we promoted computing speed of improved fitting method by introducing pre-storage technology in interpolation area. Finally, in comparison with the gradient method of noise resistance, the two step surface fitting method has stronger ability to resist noise.Finally, we developed the digital image correlation computing software based on matlab. Then through two groups of experimental results of translation motion measurement and One-way tensile stress tensile, verifies the reliability of the calculation results with digital image correlation method based on "two-step surface fitting method" in this paper.