Research And Application Of The DIC Method Based On Combination Of Element Matching And Subset Matching For Localized Deformation Measurements

In many practical cases,non-localized deformations and localized deformations occurred simultaneously.Because of the low measurement accuracy of the subset-DIC method for the localized deformation and the low measurement efficiency of the global-DIC method,a DIC method was proposed to ensure high measurement accuracy and efficiency in deformation measurements beyond strain localization based on a combination of element matching and subset matching for localized deformation measurements.Firstly,a numerical experiment on the formation of a fictitious shear band was carried out,the measurement results of the global-DIC method and the subset-DIC method were compared,and the applicability of the global-DIC method for local deformation measurement was analyzed.Then physical experiments of clay specimens in biaxial compression were carried out,the measurement results of the subset-DIC,the nine-node global-DIC method with subpixel initial values and the proposed method were compared,and the effectiveness of the proposed method was verified.Finally,the deformation and failure processes of several clay specimens in biaxial compression were observed by using the proposed method.The main conclusions are listed as follows:(1)The localized measurement accuracy of the global-DIC method is higher than that of the subset-DIC method in the numerical experiment on the formation of a fictitious shear band.The measurement efficiency of the global-DIC method is only 30.23%-58.60% of that of the subset-DIC method while ensuring the measurement accuracy.With the increase of the measuring point interval,the difference between the measurement efficiencies of two kinds of methods increases.The localized measurement accuracy of the nine-node global-DIC method is higher than that of the four-node global-DIC method,the five-node global-DIC method and the eight-node global-DIC method for appropriate sizes of elements.With the increase of the sizes of elements,the non-localized measurement accuracy of the nine-node global-DIC method is almost invariant,but the localized measurement accuracy of the nine-node global-DIC method is reduced.(2)The high measurement accuracy and measurement efficiency of the proposed method for deformations beyond strain localization are verified by the numerical experiment on the formation of a fictitious shear band and physical experiments of clay specimens in biaxial compression.The proposed method can ensure the accuracy of localized deformation measurements with a few time increases for an appropriate size.The maximum value of the maximum shear strain of the proposed method is higher than that of the subset-DIC method at the center of the shear band.Therefore,the proposed method and the nine-node global-DIC method with subpixel initial values are more suitable than the subset-DIC method for deformation measurements beyond strain localization,but the proposed method has more advantages than the nine-node global-DIC method with subpixel initial values in measurement efficiency.(3)For several clay specimens in biaxial compression,widths of shear bands obtained by using the proposed method are smaller than those obtained by using the subset-DIC method.The former is only 44.44%-88.89% of the latter.Along the path of shear bands,the maximum values of the maximum shear strains are usually smaller and smaller.With the increase of the longitudinal strain,variation coefficients of the maximum shear strain and the maximum principal strain are firstly decreased and then increased,and variation coefficients of the minimum principal strain are firstly increased and then decreased.According to variation coefficients of the maximum shear strain,the onset of strain localization can be accurately identified.There are 53 figures,6 tables and 93 references in this thesis.