Fine Measurements Of Shear Bands Using The DIC Method Based On The Particle Swarm Optimization Algorithm And Newton-Raphson Method

The shear band is a kind of failure precursors of geotechnical materials,and accurate measurements of shear bands are very important for better understanding the deformation and failure mechanisms of geotechnical materials.The digital image correlation(DIC)method is often been used to measure shear bands.In order to improve measurements accuracy of shear bands based on the DIC method,the following investigations are carried out in this thesis:(1)A DIC method with integer pixel accuracy is proposed based on the particle swarm optimization(PSO)algorithm.Results of the method are common global optimum.In order to get higher accuracy,the coarse-fine method and the alternate iteration method are proposed based on the PSO algorithm and the Newton-Raphson(N-R)method.For a poor initial value,the alternate iteration method can obtain global optimum while the N-R method may obtain local optimum.(2)In order to improve the quality of speckle patterns,an improved method for generating simulated speckle patterns is proposed,and a parameter,the mean of the quotient of one-and two-order intensity gradients,is proposed to assess speckle patterns quality.The displacement of speckle patterns translated repeatedly is measured by the DIC method,results of displacement measurement show that the proposed parameter is common effective.(3)An improved method for generating simulated shear bands is proposed to analyze the measurement errors of shear bands based on the DIC method.A horizontal shear band with the constant strain,a horizontal shear band with strain gradients,an oblique shear band with the constant strain and an oblique shear band with strain gradients are generated by use of the proposed method.The formulas of displacements and strains of the two kinds of oblique shear bands are deduced to analyze the measurement errors of the DIC method.The displacement errors nearby the boundary of a shear band based on the two-order DIC method are much lower than errors based on the one-order DIC method.When the subset size is larger or the distance from the subset center to the boundary of a shear band is shorter,displacement errors are higher.(4)A method for measuring the shear band inclination angle based on the DIC method and least square fitting is proposed.The shear band inclination angles of low liquid-limit soil specimens under uniaxial compression are measured by use of the proposed method.With the increase of the longitudinal strain,the evolution of shear band inclination angles for soil specimens mainly contains five modes:the increasing mode,the decreasing mode,the decreasing mode followed by the increasing mode,the invariant mode,and the increasing mode followed by the decreasing mode.The maximum and minimum of variations of shear band inclination angles are 9° and 0.6°,respectively,and the range of shear band inclination angles for all soil specimens is 46.5°?72.3°.(5)An improved "subset splitting" method is proposed to improve the measurement accuracy of shear bands.The displacement and strain fields in three images including the crack,the shear band with the constant strain or the shear band with strain gradients,respectively,are calculated by use of the proposed method.Results show that displacement errors and strain errors in the proposed method are less than those in the traditional DIC method.The errors of shear band inclination angles in the images including shear bands with the constant strain and strain gradients are 1.7°and-0.1°,respectively.(6)The displacement and strain fields of low liquid-limit soil specimens are calculated based on the improved "subset splitting" method.With the increase of the longitudinal strain,the evolution of shear band inclination angles of soil specimens mainly contains four modes:the increasing mode,the decreasing mode followed by the increasing mode,the invariant mode,and the increasing mode followed by the decreasing mode.With the increase of the longitudinal strain,the evolution of shear band widths of soil specimens mainly contains five modes:the increasing mode,the increasing mode followed by the decreasing mode,the invariant mode,the decreasing mode,and the decreasing mode followed by the increasing mode.The ranges of inclination angles and widths of shear bands of all soil specimens are 48°?72° and 15?38 pixels(1.35?3.42mm),respectively.