A Digital Image Correlation Method Based On The Optimization Of Positions Of Points And Statistical Analyses Of Maximum Shear Strain Fields For Sandy Soil Specimens In Uniaxial Compression

In traditional digital image correlation(DIC)methods,points are regularly distributed,that is,points are parallel in horizontal and vertical directions.Thus,a few points can be distributed in positions where speckle qualities are bad.In the present paper,a digital image correlation method based on the optimization of positions of points,Newton-Raphson(N-R)iteration and particle swarm optimization is proposed.Firstly,the positions of original points are optimized;secondly,the displacement field for the irregular optimal points are obtained by use of the N-R iteration and PSO;finally,the displacement field is interpolated using two-dimensional green’s function-based interpolator to obtain the displacement field at regularity distributed points,and then,the strain field is obtained by use of the central difference method.Sandy soil specimens in uniaxial compression are tested on a testing machine.A digital camera is used to record the images with speckles of the sandy soil specimens surface when testing.A part of images are analysed by a digital image correlation method program after the test.To explore the occurrence and development of shear bands,firstly,maximum shear strains at these lines are obtained using a bicubic spline interpolation function with good smoothness from results based on a digital image correlation method.Secondly,some monitored lines are arranged at specimens according to positions of apparent shear bands at higher longitudinal strains.Finally,The evolution of the mean and standard deviation of the maximum shear strains at these lines are obtained.The mean of the maximum shear strain corresponding to the ending of the linear stage of statistical quantities is regarded as the occurrence of shear bands.The evolution of the size of shear localization region is obtained.It is found that the present method has reasonable accuracy for speckle images with the strain of 0.01-0.05,the size of subsets of 21-41 pixels,and the mean intensity gradient of 10-20pixel-3,which is related to the point optimization.It is expected that high accurate results can be obtained using the present method if only the positions of points distributed at positions with bad speckle qualities are optimized.The distribution of maximum shear strains along shear bands transit from several peaks with lower values to a few main peaks with higher values with an increase of the longitudinal strain.These main peaks originate from positions where maximum shear strains increase rapidly,and from positions with local higher or lower values.Generally,the mean and standard deviation of maximum shear strains at monitored lines are convex upwards with an increase of the longitudinal strain.Onset of shear bands corresponds to the initial linear hardening stages of compressive stress-longitudinal strains curves.The size of shear localization region is increased with an increase of the longitudinal strain.For the same size of shear localization region,specimens with higher water contents require higher longitudinal strains.At higher longitudinal strains,the size of the shear localization region for specimens with higher water contents is larger than that for lower water contents,which is related to its longer stage of plastic deformation.