基于能量耗散機制的砂土微觀力學特性研究
發(fā)布時間：2018-09-07 16:10
【摘要】：砂土作為一種典型的散體顆粒集材料,其顆粒的幾何、物理性質以及微觀力學行為組成了砂土顆粒集在宏觀尺度上的力學特性,如應力應變行為、剪脹效應、剪切局部化現(xiàn)象以及誘發(fā)向性異性等。借助于二維離散元分析軟件PFC2D模擬了一系列砂土顆粒集樣本在平面應變下的準靜態(tài)剪切實驗,本文試圖通過砂土顆粒在剪切過程中的微觀力學行為的研究,包括平動、轉動、接觸本構、破碎以及自組織結構等,全面地了解砂土材料的多尺度力學特性,并構建其微觀力學行為與宏觀力學行為之間的聯(lián)系。 在微觀尺度上,顆粒形狀、表面紋理與破碎行為構成了砂土顆粒主要的幾何、物理特征。本文分別提出了采用顆粒重疊成簇的方法模擬不規(guī)則形狀砂土顆粒,并采用自定義的接觸本構模型模擬顆粒表面紋理的抗?jié)L動效應,以及采用改進型聚粒算法生成可破碎性砂土顆粒,全面地討論了不同的顆粒幾何、物理特征對砂土宏-微觀力學行為的影響,并根據顆粒集系統(tǒng)的能量分配機制對這種影響的內在機理進行了闡述。此處,從宏觀尺度上看,砂土作為一種類連續(xù)介質材料,為了描述其力學行為,本文提出采用自由網格法計算砂土顆粒集的局部應變張量,通過剪切過程中局部剪切應變空間分布發(fā)展來反映砂土剪切帶的形成演化過程。 顆粒集系統(tǒng)能量分配機制作為砂土微觀力學行為與宏觀力學行為的紐帶貫穿于全文中,其在微觀尺度上以顆粒的力學和運動參量為基礎,并通過統(tǒng)計分析方法可以深入地解釋了砂土宏觀力學行為的內在物理機制。通過對剪切過程中砂土的宏-微觀力學行為以及能量分配機制的研究,得到如下結論： 由形狀和表面抗?jié)L動紋理所引發(fā)的顆�？罐D動效應都能顯著地提高砂土的抗剪切強度。然而,這兩種顆粒抗轉動效應的微觀作用機制卻完全不同：不規(guī)則形狀顆粒間的內鎖力可以提高接觸點滑動摩擦力的發(fā)揮程度以及減緩剪切帶的發(fā)展,從而貢獻于抗剪強度;表面抗轉動紋理則大大地提高了顆粒接觸點的彈性轉動勢能儲存以消耗外界功,貢獻于抗剪強度。由于其微觀作用機制的不同,不同的顆�？罐D動效應在宏觀上表現(xiàn)出完全不同的力學行為,特別是在應力峰值下,抗?jié)L動模型定義的圓形顆粒集與非規(guī)則形狀顆粒集相比,其剪切誘發(fā)的局部化現(xiàn)象以及各向異性特征更為明顯。 砂土顆粒的破碎伴隨著整個剪切過程,但破碎增長速率則不斷下降,達到臨界狀態(tài)后開始趨于平緩；在臨界狀態(tài)下,由于剪切帶的形成,顆粒破碎主要集中在剪切帶中,且不斷地反復破碎生成更多、更小的碎片顆粒。由于顆粒的反復破碎有效地增加了剪切帶中顆粒摩擦的比表面積,從而強化了系統(tǒng)的彈性應變能儲存,表現(xiàn)為砂土在臨界狀態(tài)下的應變強化和體積剪縮過程。另一方面,通過剪切過程中顆粒級配演化過程,發(fā)現(xiàn)顆粒破碎過程實際上是一個顆粒級配朝著砂土終極級配優(yōu)化的過程,終極級配下顆粒粒徑服從良好的分形特征。在考慮顆粒破碎準則的前提下,得到砂土顆粒粒徑的分形維數(shù)約為1.3。 最后,基于接觸力網絡結構的拓撲識別,發(fā)現(xiàn)了砂土顆粒在剪切過程中的自組織結構行為。本文提出細觀尺度下的顆粒子域結構單元構建了顆粒集的結構體系以抵抗外界荷載與變形作用。其中,3-cycle單元是系統(tǒng)中最穩(wěn)定的結構單元,且對力鏈傳遞存在雙重支撐作用；隨著n-cycle單元階次的提高,其結構穩(wěn)定性逐漸降低。根據顆粒集體系結構穩(wěn)定性的演化過程,砂土的剪切帶發(fā)展可以理解為穩(wěn)定的低階結構單元向不穩(wěn)定的高階結構單元轉化的局部化發(fā)展過程,也正是高階結構單元在剪切帶中的聚集引起了砂土在臨界狀態(tài)中的剪脹效應。更為重要的是,根據不同類型結構單元物理力學性能的研究,結構單元間的協(xié)調作用機制使得顆粒集系統(tǒng)從微觀的離散化力學狀態(tài)向宏觀的均勻化力學狀態(tài)進行了重要地過渡。從而,更加全面和深入地了解了砂土的多尺度力學行為。 總體來講,根據砂土顆粒的微觀力學行為并通過能量分析手段建立其與砂土宏觀力學行為之間的聯(lián)系,旨在研究砂土在剪切過程中變形破壞的內在機理,為砂土的屈服準則以及本構模型研究提供一定的理論支持,并最終服務于巖土工程實踐與設計中。
[Abstract]:Sand is a typical granular aggregate material. Its geometry, physical properties and micro-mechanical behavior constitute the mechanical properties of granular aggregates at macro-scale, such as stress-strain behavior, dilatancy effect, shear localization and induced anisotropy. A series of quasi-static shear tests of sand particles under plane strain are carried out in this paper. The micro-mechanical behavior of sand particles in shear process is studied, including translation, rotation, contact constitutive, breakage and self-organization structure. The multi-scale mechanical properties of sand materials are comprehensively understood and their micro-mechanical behavior and microstructure are constructed. The relationship between macroscopic mechanical behavior.
At the micro-scale, particle shape, surface texture and crushing behavior constitute the main geometric and physical characteristics of sand particles. In this paper, a method of overlapping particles into clusters is proposed to simulate irregular sand particles, and a custom contact constitutive model is used to simulate the anti-rolling effect of grain surface texture, and an improved method is adopted. The influence of different particle geometry and physical characteristics on the macro-micro mechanical behavior of sands is discussed comprehensively, and the intrinsic mechanism of the influence is explained according to the energy distribution mechanism of the particle collection system. In order to describe its mechanical behavior, a free-mesh method is proposed to calculate the local strain tensor of sand particle set, which reflects the formation and evolution of sand shear band by the spatial distribution of local shear strain during shear process.
The energy distribution mechanism of granular aggregation system is the link between the micro-mechanical behavior and the macro-mechanical behavior of sand. It is based on the mechanical and kinematic parameters of particles at the micro-scale. The internal physical mechanism of the macro-mechanical behavior of sand can be explained by statistical analysis. The following conclusions are drawn from the study of macro micro mechanical behavior and energy distribution mechanism of sand.
The shear strength of sands can be significantly improved by the anti-rotation effect of particles caused by the shape and surface anti-rolling texture. However, the micro-mechanism of the anti-rotation effect of the two kinds of particles is completely different: the internal locking force between irregularly shaped particles can increase the exertion of sliding friction at the contact point and slow down the shear band. The development contributes to the shear strength; the surface anti-rotation texture greatly improves the elastic rotational potential energy stored at the particle contact point to consume external energy and contribute to the shear strength. The shear-induced localization and anisotropy of the circular particle set defined by the anti-rolling model are more obvious than that of the irregular particle set.
The breakage of sand particles is accompanied by the whole shear process, but the growth rate of breakage decreases continuously and becomes gentle after reaching the critical state. In the critical state, due to the formation of the shear band, the breakage of sand particles mainly concentrates in the shear band, and breaks repeatedly to produce more and smaller debris particles. The specific surface area of particle friction in the shear band is effectively increased, thus the elastic strain energy storage of the system is strengthened, which is represented by the strain strengthening and volume shearing process of sand in critical state. The fractal dimension of sand particle size is about 1.3 considering the criterion of particle breakage.
Finally, based on the topological identification of the contact force network structure, the self-organizing behavior of sand particles in shear process is discovered. The particle subdomain structure element is proposed to construct a particle-set structure system to resist external load and deformation. The shear band development of sandy soil can be understood as the local development process of the transformation from stable low-order structural elements to unstable high-order structural elements according to the evolution process of structural stability of granular system. More importantly, according to the study of physical and mechanical properties of different types of structural elements, the mechanism of coordination between structural elements makes the granular aggregation system proceed from microscopic discrete mechanical state to macroscopic homogeneous mechanical state. An important transition has been made, so that the multi-scale mechanical behavior of sand can be understood more comprehensively and thoroughly.
Generally speaking, according to the micro-mechanical behavior of sand particles and by means of energy analysis, the relationship between sand particles and macro-mechanical behavior of sand is established. The purpose is to study the internal mechanism of deformation and failure of sand in shear process, and to provide certain theoretical support for the study of yield criterion and constitutive model of sand, and ultimately serve geotechnical engineering. Process practice and design.
【學位授予單位】：華中科技大學
【學位級別】：博士
【學位授予年份】：2013
【分類號】：TU521
本文編號：2228752
[Abstract]:Sand is a typical granular aggregate material. Its geometry, physical properties and micro-mechanical behavior constitute the mechanical properties of granular aggregates at macro-scale, such as stress-strain behavior, dilatancy effect, shear localization and induced anisotropy. A series of quasi-static shear tests of sand particles under plane strain are carried out in this paper. The micro-mechanical behavior of sand particles in shear process is studied, including translation, rotation, contact constitutive, breakage and self-organization structure. The multi-scale mechanical properties of sand materials are comprehensively understood and their micro-mechanical behavior and microstructure are constructed. The relationship between macroscopic mechanical behavior.
At the micro-scale, particle shape, surface texture and crushing behavior constitute the main geometric and physical characteristics of sand particles. In this paper, a method of overlapping particles into clusters is proposed to simulate irregular sand particles, and a custom contact constitutive model is used to simulate the anti-rolling effect of grain surface texture, and an improved method is adopted. The influence of different particle geometry and physical characteristics on the macro-micro mechanical behavior of sands is discussed comprehensively, and the intrinsic mechanism of the influence is explained according to the energy distribution mechanism of the particle collection system. In order to describe its mechanical behavior, a free-mesh method is proposed to calculate the local strain tensor of sand particle set, which reflects the formation and evolution of sand shear band by the spatial distribution of local shear strain during shear process.
The energy distribution mechanism of granular aggregation system is the link between the micro-mechanical behavior and the macro-mechanical behavior of sand. It is based on the mechanical and kinematic parameters of particles at the micro-scale. The internal physical mechanism of the macro-mechanical behavior of sand can be explained by statistical analysis. The following conclusions are drawn from the study of macro micro mechanical behavior and energy distribution mechanism of sand.
The shear strength of sands can be significantly improved by the anti-rotation effect of particles caused by the shape and surface anti-rolling texture. However, the micro-mechanism of the anti-rotation effect of the two kinds of particles is completely different: the internal locking force between irregularly shaped particles can increase the exertion of sliding friction at the contact point and slow down the shear band. The development contributes to the shear strength; the surface anti-rotation texture greatly improves the elastic rotational potential energy stored at the particle contact point to consume external energy and contribute to the shear strength. The shear-induced localization and anisotropy of the circular particle set defined by the anti-rolling model are more obvious than that of the irregular particle set.
The breakage of sand particles is accompanied by the whole shear process, but the growth rate of breakage decreases continuously and becomes gentle after reaching the critical state. In the critical state, due to the formation of the shear band, the breakage of sand particles mainly concentrates in the shear band, and breaks repeatedly to produce more and smaller debris particles. The specific surface area of particle friction in the shear band is effectively increased, thus the elastic strain energy storage of the system is strengthened, which is represented by the strain strengthening and volume shearing process of sand in critical state. The fractal dimension of sand particle size is about 1.3 considering the criterion of particle breakage.
Finally, based on the topological identification of the contact force network structure, the self-organizing behavior of sand particles in shear process is discovered. The particle subdomain structure element is proposed to construct a particle-set structure system to resist external load and deformation. The shear band development of sandy soil can be understood as the local development process of the transformation from stable low-order structural elements to unstable high-order structural elements according to the evolution process of structural stability of granular system. More importantly, according to the study of physical and mechanical properties of different types of structural elements, the mechanism of coordination between structural elements makes the granular aggregation system proceed from microscopic discrete mechanical state to macroscopic homogeneous mechanical state. An important transition has been made, so that the multi-scale mechanical behavior of sand can be understood more comprehensively and thoroughly.
Generally speaking, according to the micro-mechanical behavior of sand particles and by means of energy analysis, the relationship between sand particles and macro-mechanical behavior of sand is established. The purpose is to study the internal mechanism of deformation and failure of sand in shear process, and to provide certain theoretical support for the study of yield criterion and constitutive model of sand, and ultimately serve geotechnical engineering. Process practice and design.
【學位授予單位】：華中科技大學
【學位級別】：博士
【學位授予年份】：2013
【分類號】：TU521
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