Digital Image Correlation And Its Applications In Some Engineering Measurements

Digital image correlation (DIC) technique is an optical measuring technique based on image processing and numerical analysis, which was originally developed in the 80’s of last century. Compared with other optical methods based on the interference of coherent light wave, the DIC technique possesses many advantages such as simple optical arrangements, better adaptability to the environment and wide measuring range, so it is applicable to in-situ field measurement. Through twenty years development and improvement, it has obtained numerous successful applications in engineering and scientific research. However, the DIC technique still exist many deficiencies in terms of displacement measurement accuracy, strain measurement accuracy as well as calculation speed. This paper focuses on the sub-pixel displacement measurement and strain measurement and high precision of DIC measurement is realized by software.The theoretical research of DIC is performed in this dissertation, the key results of this part of works are: (1) An intensity-change model used in the spatial-gradient method based on the least-square fitting for subpixel-accuracy displacement measurement is proposed. Scale parameters and migration parameters can be expressed in the form of gray value by optimization during calculation, and subpixel can be obtained by liner least square. The proposed method can coped well with lighting illumination variation or grayscale intensity change on the object surface during measurement by adopting a linear intensity change model, so it is more applicable to in-situ field measurement. (2) Least-square fitting of local displacement based on two-dimensional quadratic polynomial is proposed. Reliability of the algorithm is confirmed through simulated uniform picture、nonuniform picture and experimental picture.The other part of work in this dissertation is the application of DIC on measurement of engineering materials. The key results of this part of work includes: (1) Basic mechanical parameters including elastic modulus and Poisson of different engineering materials were obtained using digital image correlation. (2) Full-field displacement fields of cracked specimen were recorded using DIC, and opening mode stress intensity factor was calculated through compact tension test and with-limited plate test and mixed mode stress intensity factors were determined through compact tension-shear test. Some key issues such as selection of displacements and convergence values were discussed in the prosess of testing SIF through compact tension test and a cross-fine method is proposed for crack tip selection in the prosess of testing SIFs through compact tension-shear test.