Research On Digital Image Correlation Method For Large Deformation Measurement

The digital image correlation(DIC)method is one of the widely used deformation measurement techniques due to the advantages of non-contact and full-field measurement.Although many remarkable advances have been made,the DIC method still faces challenges in large deformation measurement.This challenge also arises in digital volume correlation(DVC)method,which is direct generalization of digital image correlation methods into threedimensional space.The main research content of this thesis is to improve the DIC/DVC method for the large deformation measurement problem,and achieve the goal of further improving the large deformation measurement ability.(1)A pre-deformation assistant strategy was introduced to describe more complex subset deformation,and then a pre-deformation assistant forward additive Gauss-Newton algorithm was proposed to improve the measurement accuracy of DIC methods for complex deformation.Numerical experiments show that the pre-deformation assistance strategy can nearly double the measurement accuracy in complex deformation.Tensile experiments with pre-cracks demonstrate the reliability of the method in real experiments.(2)The quasi-conformal mapping theory was introduced to provide a computational tool for estimating the initial deformation from boundary and feature points for large deformation measurement problem,and then a large deformation DIC method based on quasi-conformal mapping was proposed.Numerical experiments show that the introduction of quasiconformal mapping extends the measurement range of large deformations and maintains high measurement accuracy.Uniaxial tensile and three-point bending experiments demonstrate the reliability of the method in real experiments.(3)In order to further break through the limitation of the feature extraction algorithm,a checkerboard speckle pattern was proposed.The checkerboard corners are used to replace the feature points as constraints,thereby improving the stability and robustness of the DIC algorithm based on quasi-conformal mapping.Numerical experiments show that the checkerboard speckle pattern provides a stable and reliable calculation basis for initial estimation.(4)The DIC method based on quasi-conformal mapping was extended to the DVC method.This method realizes reliable initial value calculation of 3D full-field deformation,and expands the ability of the DVC algorithm to measure 3D large deformation inside solids.Numerical experiments show that the method performs well in the measurement of large tensile deformations.The research in this thesis improves the measurement ability and measurement accuracy of the DIC/DVC method for large deformations,which has strong engineering application value,and provides a direction worthy of discussion for subsequent improvement.