Study On Local Deformation Measurement Accuracy And Compensation Methods In Digital Image Correlation

As a high-accuracy,non-contact and full-field optical measurement method,Dig-ital Image Correlation(DIC)plays an increasingly important role in engineering mea-surement.However,the measurement accuracy of DIC method still limits its applica-tion in engineering measurements.The accuracy of DIC method for simple deformation(such as tensile or compres-sion deformations)of the simple curved surface(?20 ??)has basically met the engi-neering measurement requirements.However,in many practical cases,measurement results of local deformation fields(with concentration strains,such as the Portevin-Le Chatelier(PLC)bands,deformations near gaps and crack tips,etc.)are seriously af-fected by the calculation parameters(the relative errors can reach several tens per cent or more under different calculation parameters),which also makes it difficult for the user to determine the reliability of the measurement results.In this paper,the measurement accuracy of DIC method in local deformation fields is studied and extended to the measurement accuracy in digital volume correlation(DVC).In addition,compensation methods based on two-dimensional digital image correlation(Two-Dimensional DIC,2D DIC)are also studied.Chap.2 focuses on the measurement accuracy of DIC method in the local defor-mation fields.Firstly,the PLC bands have been taken as example to experimentally analyze the displacement and strain measurement errors of the DIC method in the lo-cal deformation fields.A theory that the results of second-order shape functions are reliable and accurate when the relative error between the results measured by first-and second-order shape functions is less than 10%,is proposed.Secondly,the displace-ment and strain system errors are deduced theoretically,and a method of estimating the displacement error by the displacement field under different subset sizes is proposed.Finally,the experimental and theoretical analysis are employed to guide experiments and optimize calculation parameters.In Chap.3,the analysis of local deformation measurements is extended to Digital Volume Correlation(DVC)method.Aiming at the cellular traction measurement on the hydrogel substrate,the inverse composition algorithm based on the second order func-tion(IC-GN2)is introduced into DVC method,which trends to obtain more accurate displacement results than the first-order functions.In addition,the theoretical solution of the elastic medium stress problem of the semi-infinite plane is associated with the displacement field measured by DVC.The expression of the traction forces is derived based on the known displacement field and the preset locations and sizes for the trac-tion forces,which is verified by simulation experiments and physical cellular traction measurement experiments.In Chap.4,compensation methods based on 2D DIC are studied.Firstly,with the existing compensation method(based on the hypothesis of small out-of-plane de-formations),a more stable improved compensation method is proposed.Secondly,a novel compensation method based on coordinate transformation is proposed,which is not affected by oblique axis.Thus,2D DIC method is extended to a larger range of application.