Numerical And Experimental Simulation Of Strain Localization For Granular Materials

The problem of strain localization for geotechnical materials which generally composed by broken and scattered particles, has always been of concern to academics. When the geotechnical materials under the external load, it will be deformation from local position at first, and then the deformation suddenly localizes into narrow bands (called the shear bands). The shear bands let the geotechnical materials instability and failure. Around a peak stress, however, the shear bands appeared, and the stress drops sharply down to a residual stress state. It must be a direct relationship between the soften features and the shear bands’formation. Understanding this behavior is of great importance to solve some engineering problems related to soil stability. Analysis of the laws of strain localization for geotechnical material in micro-scope, is the focus of this study. According to this, the main works and results are as follows:First, the process of strain localization for granular material and its influencing factors were comprehensively analyzed by using numerical simulation. The PFC has an unique of advantage for the study of granular material, but it cannot use velocity field and displacement field in showing the characteristics of localization during loading and subsequent failure intuitively. The definition of effective strain in macro-scope is introduced into the granular materials, which is easy to be understood and accepted for us. The numerical models including different confining pressures, granular diameter, frictional coefficients, and stiffness coefficients are established and then programmed with Fish language for granular materials under certain initial and boundary conditions. Combining the numerical tests of PFC with expression of the effective strain in macro-scope, the rules and influencing factors for the formation of shear bands during biaxial tests are analyzed. The numerical results indicate that the effective strain in macro-scope is able to exhibit the development process and relevant characteristics of strain localization for granular materials:the entire shear bands are formed after the peak value and during the soften stage of the stress-strain curve, and that bigger voids and more obvious rotation of granules in shear bands than in other parts. It also demonstrates that the normal and tangent stiffness coefficients of granules, the frictional coefficients between granules, the confining pressures around the two sides of the sample, and the diameters of granules affect the formation, types and thickness of shear bands for granular materials in some degree. Second, the phenomenon of very large voids and particles’ high rotation gradients in the shear bands can reproduce when rolling resistance is considered. The PFC is weak in discuss the parameter of rolling, however, the DEM by Fortran can describe the mechanical characters in micro-essence for granular materials. Finding the differences in three conditions: no rolling test, free rolling test, rolling resistance test. Analysis the effects of rolling mechanism in strain localization by comparing the results of other researchers, then, finding that the discrete element modeling with rolling mechanism can reflect the macroscopic properties of granular materials better in micro-level. The relative motion between neighbor particles can be divided into sliding component and rolling component, indeed the rolling resistance is indispensable because of it’s the reason of the contact moment. By compare the sample in biaxial compression test perfect or not, finding that the shear bands of granular materials are related to the initial imperfection. The ultimate shear band was always developed from initial imperfection and eventually throughout the granular material.Third, for further analysis of the laws in strain localization of granular materials, combing numerical simulation and experimental to study whose mechanical character. The comparison of direct shear test with glass beads and its numerical simulation by PFC3D can find that:the result of numerical simulation would be close to the physical tests by adjusting micro-parameters. The model test of bearing capacity has been analyzed by PIV, and which simulation by PFC3D also, from the similar result we can say that:combining PIV and numerical simulation can be as an effective way to study on engineering problems. The numerical simulation of triaxial test for preliminary study by two different ways of making membrane. And the results indicates as the following:the two different ways of making membrane lead to single shear band pattern and "X" shear band pattern, respectively; the entire shear bands are formed after the peak value and during the soften stage of the stress-strain curve; the mainly deformation of sample took place in shear bands after the peak value and that voids in shear bands were more bigger than in other parts.