| Digital image correlation (DIC) is a modern photomechanical method based on imageanalysis. DIC records the surface speckle images of a loaded specimen using a digital cameraand obtains the surface deformation field of the specimen by calculation the correlation ofcaptured speckle images. DIC method has been widely accepted and used in various fieldsfor its advantages such as low requirement on test environment, simple implementation andeasy adjustment on measurement range, et al. Traditional DIC method can be regarded as aMotion Estimation (ME) model in video image processing and is essentially an imageprocessing problem without any mechanical constraint. Therefore, there will be difficultieswhen trying to improve the measurement accuracy of DIC or develop new DIC models tomeasure complicated deformation field.This research deeply analyses the common of DIC and ME, and points out the specialcharacteristic of DIC. The existed DIC methods are surveyed and it is proposed that if thedeformation continuity in time and space can be introduced into DIC, DIC with mechanicalcharacteristic is expected to be developed. Starting from the basic assumption of ME, i.e.,constant intensity assumption, a general DIC model is developed by introducing spatio-temporal continuity of solid deformation. Then the solving models of DIC are classified intosix categories, which contains all of the existed DIC models. This research will focus on thetwo most practical categories, i.e., only considering the spatial continuity or temporalcontinuity.Considering the spatial continuity of deformation, high-order isoparametric element, i.e.,eight-node isoparametric element, is introduced into DIC and Q8-DIC with spatial continuityis developed. The results of numerical experiments and real experiments show that Q8-DIChave higher measurement accuracy compared with Q4-DIC and can measure the deformationof second order curved boundary. Meanwhile, an automatic mesh refinement method forgenerating optimized non-uniform mesh based on the residual error of gray level is developed.The optimized non-uniform mesh can improve the accuracy of DIC in both numericalexperiments and real experiments when processing displacement fields with regions of both high and low gradient.Considering the temporal continuity of deformation, a short time series digital imagecorrelation (STS-DIC) method is developed based on subset-DIC. The solving model of STS-DIC is established and deduced in detail. The effectiveness and advantage of proposed STS-DIC method is verified for numerical and experimental cases. The results show that theproposed STS-DIC greatly improves the measurement accuracy and resolution of traditionalDIC.The developed two DIC models with spatial or temporal continuity are applied to threespecial measurements: deformation with complicated boundary, high heterogeneousdeformation and very small deformation. For the deformation with complicated boundarymeasuring, the cell rotation of micropolar material with periodic arranged microstructures ofholes or inclusions is measured by Q8-DIC and the size effect of the two micropolar materialsis studied. For high heterogeneous deformation measuring, the surface deformation evolutionof PBXs in the damage and failure process under uniaxial compression is measured by Q8-DIC with non-uniform mesh. An effective mechanical model is proposed based on damagelocalization to describe and predict the nonlinear mechanical behavior of brittle material indamage process. For very small deformation measuring, the tension modulus, compressionmodulus and damage evolution under tension of nuclear graphite are measured by STS-DICin a four-point bending beam experiment.This research improves the ability of DIC on processing special problems andcontributes to expand the application range of DIC. Furthermore, this research emphasizesand establishes the relation of DIC and spatio-temporal continuity of deformation inmechanics, which makes contributions to develop more effective DIC method. |
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