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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/71113

    Title: The micromechanical behavior of granular samples in direct shear tests using 3D DEM
    Authors: 黎,慶;Khanh,Le Hoang
    Contributors: 土木工程學系
    Keywords: 直接剪力試驗;緊密顆粒材料;三維分離元素法;接觸力;應力路徑;direct shear test;dense granular material;3D DEM;contact forces;stress paths
    Date: 2016-07-20
    Issue Date: 2016-10-13 12:07:12 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 利用三維分離元素法分析顆粒試體在直接剪力試驗中的微觀力學行為
      二維分離元素模型與三維分離元素模型的微觀力學行為的比較中我們可以瞭解到在這樣的測試中,維度對微觀力學行為的影響。分析結果顯示三維分離元素模型有很大的優勢,因為我們可以直接模擬出真實的土壤顆粒行為。舉例來說,顆粒的摩擦角有顯著地增加(從二維模型的26o 增加到三維模型的45o),其中主要的原因是由於模型中顆粒的相互作用,特別是垂直於平面方向的作用。在二維模擬中的接觸力只存在於剪切方向,而不會出現在垂直於平面的方向。然而,在三維模擬中,接觸力可以出現在第三個維度,這有助於在剪切的過程中,增加材料間的剪力阻抗。額外的剪力可能來自於其他平面的阻力(平行二維剪切面),數值模型顯示在各種正向力作用下應力分布不均的明顯證據,大部分的接觸力會分布在直剪盒的左下牆和右上牆,因為直剪試驗期間是由直剪盒的下盒向右側剪切。而接觸力在剪切平面上的分佈比其他地方更均勻。

    ;The micromechanical behavior of granular samples in direct shear tests using 3D DEM


    Direct shear test is known as one of the most generally-performed laboratory tests in geotechnical engineering area. For granular material, the friction angle can be determined by obtaining the peak or residual shear strengths under different normal stresses from this test. However, our understanding of the microstructure and micromechanical behaviors of granular material under specified stress conditions in the direct shear tests is very limited because of the design of the direct shear box itself. In this study, a simulation of the direct shear test in the 3-dimensional discrete element method (3D-DEM) model is conducted to evaluate its micromechanical behaviors during direct shear simulation.

    Huang et al. (2015) [1] have conducted a series of studies for direct shear simulation through DEM under the two dimensional (2D) condition. It was shown that the stress paths in dense granular material are very different from those in loose granular material. The stress path variations in the dense material are quite uniform and less complicated compare to the loose material. By contrast, the stress states in the loose material during direct shear tests are really complex and erratic. The simulations of the direct shear test in 2-dimensional DEM have evidently pointed out the potential to apply DEM for providing an in-depth understanding of the micromechanical behaviors of granular materials. Considering that there are some restrictions in the 2D-DEM model because real granular material should be spheres instead of circular plates. In this study, 3D DEM model of the direct shear test was employed to evaluate and compare to what observed in 2D DEM model by Huang el al. 2015.
    A comparison of the micromechanical behaviors between 2D and 3D DEM models was also performed to understand the effect of dimension in the variations of micromechanical behaviors in such tests. The analysis result shows the substantial advantages of the 3D DEM because behavior of real soil particles is simulated directly. For example, there is a significant increase in friction angle of particle assembly (from 260 in 2D model to 450 in 3D model). The main reason was attributed to the interaction of particles in model, especially in the out-of-plane direction. The contact forces in 2D simulation can only exist in the shear direction, not in the out-of-plane direction. However, in the 3D simulation, the contact forces could exist in the third dimension, which contributes to much more shearing resistance during direct shear simulation. The additional shear forces might come from the resistance forces at other planes (parallel to the 2D shear plane). The numerical model showed an obvious evidence of non-uniformity of stress under various normal stresses. Most of contact forces were distributed between the lower-left and upper-right wall of the direct shear box because the shearing was performed with the lower part of the box moving to the right during the shear time. The distribution of contact forces at the shear plane was more uniform than other places.

    Key words: direct shear test, dense granular material, 3D DEM, contact forces, stress paths.
    Appears in Collections:[土木工程研究所] 博碩士論文

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