| Soil tests in laboratory have become the essential method to explore the mechanical properties of geotechnical materials.Triaxial test,as the most important and commonly used soil tests for determining stress-strain property,constitutive relation and strength parameters of soil,has been widely used.However,the loading tracking accuracy of the triaxial test system is easily affected by the nonlinear mechanical properties of soil specimen and nonlinear friction.At the same time,the traditional method which use external sensor can only obtain approximately the overall deformation of the soil specimen,but it can’t reflect soil’s local deformation.The research work is carried out for realizing nonlinear compensation control of axial loading and the full surface deformation measurement of the specimen.On this basis,the reliability of measured results of the improved triaxial experiment system are verified by experiments and the application of local deformation measurement method is studied.The detailed contents are as follows:(1)A nonlinear control strategy has been applied to the triaxial loading platform to compensate various interference factors in system caused by nonlinear mechanical properties of soil specimen,nonlinear friction,and so on.The platform is based on electro-hydraulic servo control technology which can not only satisfy the static load in the static triaxial test but also satisfy the response speed in the dynamic triaxial test.However,the accuracy and stability of the platform has been affected by many nonlinear interference factors,such as the nonlinear friction in electro-hydraulic system,the complex nonlinear mechanical response of load.In this paper,the nonlinear friction has been expressed by LuGre friction model and the disturbances have been estimated by disturbance observers.The control strategy which combines dynamic surface backstepping control method and iterative learning control method has been proposed to improve the tracking accuracy.The stability of control strategy has been validated by Lyapunov theory.Through the static and dynamic loading tests,the control strategy proposed in this paper reduces the RMSE of tracking error by about 40%and 30%respectively compared with PID control.(2)A digital image measurement system for measuring soil’s specimen has been built based on digital image correlation(DIC)and the error correction method for rectifying the 3D surface of specimen has been proposed to improve the measurement accuracy of the 3D full-field deformation.In this paper,DIC technique has been applied to measuring the soil specimen’s deformation.According to the characteristics of the test,the hardware and layout of the image measurement system are improved and designed.Because the camera’s optical axis and its image plane are virtual,the relative position between testing platform and image measurement system can’t be obtained through assembling process.Therefore,a calibration process has been developed in this paper which includes images’undistortion,stereo rectification,stereo matching and posture correction of specimen.Through several groups of loading tests,the results show that the RMSE of the corrected measurement results is reduced by about 50%compared with the uncorrected.(3)The application research of 2D-DIC technique has been realized using the experiment platform.The deformation has been measured accurately and the characteristic parameters of shear band have been determined under plane strain testing.The deformation of the specimens of Fujian standard sand in plane strain tests has been measured and the relative characteristics of shear band are quantified by DIC technique.The results show that the deformation is homogeneous in early stage of the test.With the loading,the local failure would appear,then the failure would gradually expand until the shear band is formed throughout the specimen.Based on the bifurcation theory,a method is proposed to estimate the moment of shear band initiation and complete penetration during progressive failure of soil specimen.The microscopic parameters of sand’s discrete element model have been calibrated based on the results measured by DIC technique and the rationality of the parameters are verified.(4)The 3D image measurement technology has been applied to triaxial experiment for measuring the specimens’3D deformation in the triaxial experiment system.The local deformation characteristics of triaxial specimens under different loading conditions were investigated.In triaxial test,the deformation of the ends of soil specimen is affected by the boundary conditions.So,the deformation of the ends is smaller than the middle part.The conventional method only obtains the average radial and the average volumetric deformation using burette,which assumes that the specimen is uniform.So,the local deformation can’t be obtained by this method.The image edge detector and 3D-DIC technique have been applied to triaxial testing for measuring the specimens’ 3D deformation and analyzing their local deformation characteristics.According to the measurement results,conventional results overestimate the strength of the soil specimen in static triaxial tests.At the same time,the results in dynamic triaxial tests show the local tensile and compression state of the specimen under dynamic tests is not uniform.In addition,the middle part of the specimen is the weakest region,which is the most sensitive to the dynamic loading and liquefaction occurs first. |
