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姓名 黎慶(Le Hoang Khanh)  查詢紙本館藏   畢業系所 土木工程學系
論文名稱
(The micromechanical behavior of granular samples in direct shear tests using 3D DEM)
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摘要(中) 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
摘要
  在大地工程的領域中,直接剪力試驗是一種十分普遍的實驗室實驗。透過直剪試驗,可以在不同正向應力的情況下,利用實驗獲得的尖峰剪力強度與殘餘剪力強度來決定顆粒材料的摩擦角。然而,因為直剪試驗盒本身的設計,在指定的應力條件下,我們對顆粒材料的微觀結構與微觀力學行為的了解是有限的。在本研究中,我們利用三維分離元素法來模擬直剪試驗並探討試驗進行中的微觀力學行為。
  黃文昭(2015)等人已經透過二維分離元素法來進行一系列直剪試驗模擬的研究。其研究說明了,緊密排列的顆粒材料與鬆散排列的顆粒材料的應力路徑有很大的不同。相對於鬆散材料,緊密材料的應力路徑變化相當的均勻且較不複雜;相反地,在直剪試驗時鬆散材料的應力狀態是非常複雜和不穩定的。利用二維分離元素法的直剪試驗模擬中已經明顯地指出,利用分離元素法來深入了解顆粒材料的微觀力學行為是可行的。因為在真實顆粒材料是圓球狀,而不是圓板狀,所以我們考慮到二維分離元素模型是存在一些限制的。在本研究中,我們會將三維分離元素法的直剪試驗的模型結果與黃文昭(2015)等人的二維分離元素法的模型結果做比較。
  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。



關鍵字:直接剪力試驗、緊密排列的顆粒材料、三維分離元素法、接觸力、應力路徑。
摘要(英) The micromechanical behavior of granular samples in direct shear tests using 3D DEM


ABSTRACT

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.
關鍵字(中) ★ 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
關鍵字(英) ★ direct shear test
★ dense granular material
★ 3D DEM
★ contact forces
★ stress paths
論文目次 Chapter 1 Introduction 1
1. Background 1
2. Research process and aims 3
3. Thesis outline 4
Chapter 2 Research Background 5
2.1 Common soil mechanics experiments 5
2.1.1 Tri-axial test [2] 5
2.1.2 Direct shear test 19
2.2 Recent researches about DS test by DEM 24
Chapter 3 Research method 38
3.1 Discrete element method 38
3.2 The particle flow code model in the three dimensional space (PFC3D) 38
3.2.1 The wall definition 39
3.2.2 The ball definition 39
3.2.3 Definition of measurement circle 39
3.2.4 Definition of stress 40
3.2.5 Definition of porosity 41
3.2.6 Contact stiffness model 41
3.3 Formulation of direct shear sample by DEM used in this study 42
3.4 Quantities that are measured during the direct shear test simulation 45
3.4.1 Calculation of shear stress 45
3.4.2 The measurement of stress, porosity and contact forces 46
3.4.3 Stress paths and orientation of major principal stress plane 46
3.4.4 Sign conventions used in PFC and this study 49
Chapter 4 Simulation results and discussions 51
4.1 Discussions in simulated test results 51
4.2 Micromechanical behaviors of granular material along the shear plane 54
4.3 Variations in orientation of major principle plane 72
Chapter 5 Conclusions and future works 80
5.1 Conclusions 80
5.2 Difficulties 85
5.3 Future works 87
References 89
Appendix 1 90
Appendix 2 92
Appendix 3 93
Appendix 4 95
Appendix 5 97
Appendix 6 100
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指導教授 黃文昭(Wen-Chao Huang) 審核日期 2016-7-20
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