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无粘性土的减载弹塑性分析【作者】张其光 【导师】李广信

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【作者】张其光导师李广信水利论文fF2r4r8s2f%A
【作者基本信息】清华大学,土木工程,2006年,博士水利论文OIO0X.i/tH

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#DV K ltZ;X rF0【中文摘要】 土的本构关系研究是土力学的核心内容。普遍存在的减载体缩现象是导致砂土液化和破坏的重要因素,但尚难以用现有土的弹塑性本构模型合理解释。零应变方向的主应力对平面应变状态下土的强度和变形均有重要影响,认识土的弹塑性特性是正确确定零应变方向主应力的前提。研究土在减载和往复荷载下的弹塑性特性,建立反映土减载屈服的弹塑性本构模型,具有重要的学术价值和工程意义。本文以分析无粘性土的减载弹塑性特性为目标,开展系统的研究工作,主要取得了以下成果:1.通过机理分析和试验研究,探讨了无粘性土的减载弹塑性特性及其机理。阐明可恢复变形并不都是弹性变形,减载体缩是剪应力减小引起的塑性体积变形,是减载屈服的宏观表现。指出减载屈服与土体内部结构改变密切相关,是土的弹塑性的重要体现。提出由于反向摩擦作用,减载过程存在初始弹性区。分析试验中体应变的变化规律,确定减载初始弹性区范围,通过试验和计算证明其适用性。提出考虑减载屈服情况下确定弹性模量及分离弹塑性应变的方法。2.在土的清华模型基础上建立了反映土减载屈服的弹塑性本构模型,并对其有效性进行验证。模型考虑了应力引起的各向异性及变形历史的影响,在减载过程中令屈服面和等向硬...更多化轴旋转,采用以任意等效应力点为起点的硬化规律形式,根据试验结果分别确定初始加载-减载-再加载过程的屈服面和硬化参数,将清华弹塑性模型推广到往复加载情况。模型能合理反映应力应变曲线滞回圈,加载剪胀和减载体缩等变形现象,体现土往复加载的弹塑性特性。3.研究了平面应变减载的弹塑性特性以及零应变方向主应力的变化规律。阐明零应变方向的主应力完全取决于土的本构关系,在不同条件下可能为小主应力、中主应力或者大主应力。用反映减载弹塑性的本构模型合理预测了零应变方向的主应力。通过对平面应变减载的弹塑性分析,指出发生减载屈服是保证平面应变条件和极限平衡条件的需要,揭示了减载过程中零应变方向的主应力经历从小主应力变为中主应力直至大主应力的转换过程,最终趋向于极限平衡状态等重要变形规律,深化了对土的弹塑性特性以及平面应变条件的认识。  还原

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【英文摘要】 Constitutive relationship of soil plays a key role in soil mechanics. Ubiquitous volume-retraction during load-decreasing process is an important factor leading to liquefaction and failure of saturated sand, which could not be reasonably explained by existing elastic-plastic constitutive models of soil. Principal stress in the direction of zero principal strain has significant effect on strength and deformation of soil in plane strain state. In order to predict the principal stress in the direction of zero principal strain correctly, it is necessary to understand elastic-plastic behavior of soil during load-decreasing process. An elastic-plastic constitutive model proposed embodying yield characteristic during load-decreasing process is of special academic and practical value. Elastic-plastic behavior of cohensionless soil during load decreasing and reloading processes was studied and the main issues were summarized as follows.1. Based on experimental and theoretical studies, mechanism...更多 of elastic-plastic behavior of cohensionless soil has been explored. It was revealed that the reversible deformation is not elastic on occasion. Volume-retraction during load-decreasing process is a kind of plastic volume deformation due to shear stress decreasing, which represents yielding during load-decreasing process on a macro-scale. Yielding during load-decreasing process as characteristic of elastic-plastic behavior is introduced by the micro-structure change of cohensionless soil. Due to reverse friction, in load-decreasing process there is an initializing elastic region, which was decided by volume strain change in conventional triaxial compression tests and proved by both contrast tests and numerical simulations. A method has been proposed to obtain elastic moduli and device strain into elastic and plastic parts considering yield characteristic during load-decreasing process.2. Based on Tsinghua Elastic-Plastic Model of Soil, an elastic-plastic constitutive model embodying yield characteristic during load-decreasing process has been developed taking account of anisotropy induced by stress and effect of deformation history. In the model, yield surface and isotropic hardening axis rotate during load-decreasing process and the express of hardening mechanism adopts alterable equivalent stress point as initial state. In addition, yield surface and hardening parameters during primary loading, load-decreasing and reloading processes were obtained from experimental data respectively. The model is proved to capture elastic-plastic behavior which includes hysteresis loop of stress recycle and dilatancy during loading process and volume-retraction during load-decreasing process. It is an extension of the existing Tsinghua Elastic-Plastic Model of Soil to load decreasing and reloading processes.3. Elastic-plastic behavior and evolution of principal stress in the direction of zero principal strain during loading and load-decreasing process were studied in plane strain state. Principal stress evolution in the direction of zero principal strain is entirely determined by constitutive relationship of soil, which may be major, intermediate or minor principal stress in different conditions. The model embodying elastic-plastic behavior during load-decreasing process was applied to predict reasonably principal stress in the direction of zero principal strain compared with experiments. Through analyzing elastic-plastic behavior, some conclusions could be obtained as below. In order to ensure deformation and stress boundary conditions in plane strain state, yielding during load-decreasing process should be embodied. Principal stress in the direction of zero principal strain changes from minor via intermediate to major principal stress and tends to state of limit equilibrium eventually. The conclusions above provide a deep insight into elastic-plastic behavior of soil and plane strain condition.  还原水利论文$S Tk ?s7jK

v$P$j O'P$r0【中文关键词】 无粘性土; 弹塑性本构模型; 减载体缩; 减载屈服; 平面应变水利论文"b qsC ~)u
【英文关键词】 cohensionless soil; elastic-plastic constitutive model; volume-retraction during load-decreasing process; yield during load-decreasing process; plane strain水利论文3Ir+J:H"i1B

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