共查询到18条相似文献，搜索用时 171 毫秒
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传统的塑性位势理论隐含了应力主方向和塑性应变增量主方向共轴的假定,无法客观地描述主应力轴旋转过程中的非共轴现象。基于广义位势理论提出的拟弹性弹塑性本构模型,把总的塑性应变分解为满足弹性分解准则的拟弹性部分和符合传统塑性理论假设的纯塑性部分,分解后建立的模型更为合理和简便,同时又可以解决土的非共轴问题。通过单剪试验结果的验证表明,基于广义位势理论的拟弹性弹塑性模型的模拟效果较好,传统的弹塑性模型（共轴模型）模拟得到的主应力方向和塑性主应变增量方向保持共轴,而拟弹性弹塑性模型（非共轴模型）的模拟结果则能够合理地描述主应力轴旋转过程中的非共轴特性,结果更符合实际,从而为解决土的非共轴特性问题提供了一种有效的方法。 相似文献
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非共轴本构模型在地基承载力数值计算中若干影响因素的探讨 总被引：1，自引：0，他引：1
土体在剪切变形过程中产生主应力方向的旋转时,主应变增量方向与主应力方向之间存在着非共轴现象,然而传统的弹塑性本构模型未能考虑该现象的影响。通过在屈服面的切线方向增加一项非共轴塑性应变增量,即可实现对非共轴现象的反映。采用显式积分算法和自动分步方法,将非共轴本构模型运用到桶形基础地基承载力问题的有限元计算中,并讨论了流动法则、内摩擦角、膨胀角等因素与非共轴模型的联系。计算结果表明：采用有限元程序默认容许误差时,该本构模型可达到理想的收敛精度,并且,该模型对关联、非关联流动法则均适用。采用共轴模型进行数值计算时,不同流动法则对计算结果的影响可以忽略;采用非共轴模型时,不同流动法则的计算结果之间存在差异。非共轴现象对地基承载力-位移曲线具有软化作用,并且,该软化作用在采用非关联流动法则时变得更加明显 相似文献
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《岩土力学》2017,(7):1959-1966
基于屈服面角点非共轴理论中采用Gram-Schmit正交化方法,提出了一种新的非共轴本构模型。模型修正了原有的流动法则,其中非共轴流动方向被定义为将单位应力增量方向在参考主应力正交方向上的投影,同时与塑性标量因子相关联。另外,根据广义应力的状态下的剪胀方程推导了一种新的塑性函数形式。以状态相关砂土模型为基本模型,分别采用新的非共轴模型和未修正的模型模拟了Toyoura砂的空心圆柱单剪试验和空心圆柱扭剪试验,将模型模拟的结果与试验数据进行对比,结果表明,新非共轴模型能更为合理地反映试验中非共轴现象及其变化规律,特别是固定主应力轴方向的单调剪切试验。 相似文献
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《岩土力学》2017,(1):263-271
循环动载下土体变形呈现的复杂各向异性,本质上依赖于土体微观组构特征的演化。为了揭示非比例循环动载下土体变形的微观机制,采用离散元方法模拟砂土的循环单剪行为。应用等幅剪应变的往复加载实现循环单剪应力路径,模拟得到了砂土的循环弱化、剪胀性、非共轴性及微观组构的演化规律。微观模拟表明,在循环剪切过程,土体表现为循环弱化行为,并最终趋于塑性安定状态。而试样组构主方向倾角和组构各向异性系数也会不断增大直至一个稳定值,同时主应变率、主应力轴和组构主方向旋转角度会存在一定的差异,即非共轴现象,随着单调剪切的进行,三者会逐渐趋于一致。土体的剪胀行为表现为循环压密性,而非共轴性呈现逐渐增强的演化规律。循环剪切过程中微观组构的主方向与应力主方向逐渐趋于一致,而微观组构各向异性呈现减弱的演化趋势。 相似文献
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剑桥模型只适用于正常固结软黏土,不能描述不等向固结土的应力-应变行为的各向异性特性。基于剑桥模型,在其椭圆屈服面中引入各向异性张量和一个形状参数,建立了一个各向异性屈服面,提出了一个适用于等向和不等向固结软黏土的本构模型。各向异性张量的初始值由初始固结应力状态确定,其演化过程由一个与塑性剪应变和塑性体应变都有关的硬化法则描述。形状参数的引入保证了各向异性屈服面的灵活性和适应性。通过对Boston Blue黏土、高岭土和Otaniemi黏土的三轴试验结果的模拟,验证了模型的模拟能力。 相似文献
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针对传统本构理论无法描述土体单剪试验非共轴变形的不足,采用非共轴修正模型进行改进。模型基于材料状态相关临界状态理论,采用宏-细观结合的方法,将1个新的各向异性状态变量引入本构模型来描述砂土的各向异性。考虑细观组构张量和应力张量的几何关系的变化,模型可以描述砂土在主应力轴旋转条件下材料状态的变化,材料状态变化直接导致模型的硬化规律和剪胀性发生变化,因此,模型可以描述该条件下原生向异性对砂土变形的影响。引入非共轴理论对本构模型进行修正,建立了三维非共轴各向异性模型。单剪试验的加载条件会造成主应力轴相对土体沉积面发生旋转,修正模型不但能够描述砂土在主应力轴旋转条件下其原生各向异性对变形的影响,而且可以描述主应力轴旋转造成的应力诱发各向异性对土体变形的影响,因此,该模型能够对整个单剪试验的变形规律进行描述,而且物理意义清晰。通过铝棒堆积体和Toyoura砂单剪试验验证表明,非共轴修正各向异性模型能对单剪试验的整个变形过程进行较好的模拟。 相似文献
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为反映真实工程条件下主应力轴旋转应力路径引起土体性状的变化,对杭州地区正常固结原状软黏土在固结不排水的主应力轴定向剪切和主应力轴单调旋转条件下的应力-应变关系进行试验研究。研究发现,不同主应力方向的定向剪切路径下,随主应力方向变化,试样中各应变发挥程度显著不同,但破坏时的临界八面体应变变化较为稳定,且当八面体应变达到5%时,强度发挥程度已接近甚至超过90%。若剪切过程中增加了主应力幅值不变的不排水主应力轴单调旋转应力路径,只要破坏时主应力方向一致,经历与未经历主应力轴旋转试样的临界应变分量接近,但主应力轴旋转会影响加载阶段试样主应力、主应变增量方向所表现出的不共轴性,并且此影响随旋转时剪应力水平的提高而趋于显著,即使在临界破坏状态下依然明显。试验结果表明,由于土体原生各向异性、黏塑性等性质的存在,并不适宜用相关联流动法则来分析主应力轴旋转条件下土体的应力-应变关系特征。 相似文献
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《岩土力学》2017,(1):1-9
采用空心圆柱仪对重塑空心软黏土试样开展了一系列非破坏试验,研究了列车循环动荷载作用下的性状。试验中主要选取心形应力路径,以圆形应力路径为辅助,对比分析了两种应力路径下软土的非共轴应变特征,探究循环振次对非共轴特性的影响、荷载频率的大小对非共轴角与大主应力方向角关系曲线形态变化的影响,同时进行了机制分析,并与另一动应力水平下试验结果进行了对比论证。此外,在不考虑频率影响下建立了简化的非共轴角与大主应力方向角之间的关系模型。研究发现,在大主应力方向角旋转的任意周期内,心形与圆形两种应力路径下非共轴角随大主应力方向角变化趋势各自有明显特点,相同动应力水平下两种应力路径产生的偏应力增量引起的单位偏应变增量大小也有所区别,但在大主应力方向角旋转的局部角度内,两种应力路径下的偏应变增量相近;心形应力路径下的非共轴角以及偏应力增量引发的单位偏应变增量大小受循环振次的影响不明显,任意振次中大主应力方向角在[-30°,40°]弧度区间内对应的偏应变增量远大于该区间之外的偏应变增量;随着频率增大,非共轴角随大主应力方向角的变化呈现越来越明显的波动。 相似文献
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A simple method called anisotropic transformed stress (ATS) method is proposed to develop failure criteria and constitutive models for anisotropic soils. In this method, stress components in different directions are modified differently in order to reflect the effect of anisotropy. It includes two steps of mapping of stress. First, a modified stress tensor is introduced, which is a symmetric multiplication of stress tensor and fabric tensor. In the modified stress space, anisotropic soils can be treated to be isotropic. Second, a TS tensor is derived from the modified stress tensor for the convenience of developing anisotropic constitutive models to account for the effect of intermediate principal stress. By replacing the ordinary stress tensor with the TS tensor directly, the unified hardening model is extended to model the anisotropic deformation of soils. Anisotropic Lade's criterion is adopted for shear yield and shear failure in the model. The form of the original model formulations remains unchanged, and the model parameters are independent of the loading direction. Good agreement between the experimental results and predictions of the anisotropic unified hardening model is observed. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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In this paper, the Discrete Element Method (DEM) is employed to numerically explore the response of hollow cylinder specimens of granular soils under complex stress paths. Two series of numerical tests are conducted to clarify the effects of the principal stress direction α and the intermediate principal stress through the b-value on the mechanical response of granular materials. The effects of α and b-value on the non-coaxiality of the principal stress and the principal plastic strain increment directions are investigated. It is observed that b-value and α significantly affect the non-coaxial behavior of granular materials. Finally, the results are discussed and compared with those obtained from physical laboratory tests. 相似文献
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This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of Leighton Buzzard sand. Experiments have also been performed on glass beads to understand the effect of particle shape. This paper explains phenomenological observations based on recently acquired understanding in micromechanics, with attention focused on strength anisotropy and deformation non-coaxiality, i.e. non-coincidence between the principal stress direction and the principal strain rate direction. The test results demonstrate that the effects of initial anisotropy produced during sample preparation are significant. The stress–strain–strength behaviour of the specimen shows strong dependence on the principal stress direction. Preloading history, material density and particle shape are also found to be influential. In particular, it was found that non-coaxiality is more significant in presheared specimens. The observations on the strength anisotropy and deformation non-coaxiality were explained based on the stress–force–fabric relationship. It was observed that intermediate principal stress parameter b(b = (σ 2 ? σ 3)/(σ 1 ? σ 3)) has a significant effect on the non-coaxiality of sand. The lower the b-value, the higher the degree of non-coaxiality is induced. Visual inspection of shear band formed at the end of HCA testing has also been presented. The inclinations of the shear bands at different loading directions can be predicted well by taking account of the relative direction of the mobilized planes to the bedding plane. 相似文献
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In order to simulate the soil response during principal stress rotation, anisotropic unified hardening (UH) model is developed within the framework of elastoplastic theory. Without introducing any additional mechanism to display the role of stress rotation specifically, this model achieves the simulation by considering the material anisotropy. The effect of inherent anisotropy is reflected using the anisotropic transformed stress method, but a new formula for the stress mapping is adopted to keep the mean stress unchanged. Analysis indicates that from the view of the transformed stress tensor, the anisotropic soil is subjected to loading during pure rotation of principal stress axes, so that plastic strains can be calculated. To represent the induced anisotropy, a fabric evolution law is proposed based on laboratory and numerical test results. At the critical state, the fabric tensor reaches a stable value determined by the stress state, while the critical state line is unique in the plane of void ratio versus mean stress. The anisotropic UH model has concise formulation and explicit elastoplastic flexibility matrix and can provide reasonable predictions for the deformation of anisotropic soils when principal stresses rotate. 相似文献
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This paper presents a fabric tensor-based bounding surface model accounting for anisotropic behaviour (e.g. the dependency of peak strength on loading direction and non-coaxial deformation) of granular materials. This model is developed based on a well-calibrated isotropic bounding surface model. The yield surface is modified by incorporating the back stress which is proportional to a contact normal-based fabric tensor for characterising fabric anisotropy. The evolution law of the fabric tensor, which is dependent on both rates of the stress ratio and the plastic strain, rules that the material fabric tends to align with the loading direction and evolves towards a unique critical state fabric tensor under monotonic shearing. The incorporation of the evolution law leads to a rotational hardening of the yield surface. The anisotropic critical state is assumed to be independent of the initial values of void ratio and fabric tensor. The critical state fabric tensor has the same intermediate stress ratio (i.e. b value) and principal directions as the critical state stress tensor. A non-associated flow rule in the deviatoric plane is adopted, which is able to predict the non-coaxial flow naturally. The stress–strain relation and fabric evolution of model predictions show a satisfactory agreement with DEM simulation results under monotonic shearing with different loading directions. The model is also validated by comparing with laboratory test results of Leighton Buzzard sand and Toyoura sand under various loading paths. The comparison results demonstrate encouraging applicability of the model for predicting the anisotropic behaviour of granular materials.
相似文献16.
The restrictions imposed by the form-invariance principle on the structure of the elasto-plastic constitutive law for small strains are examined. It is shown that the yield and plastic potential functions may depend on the joint invariants of the stress and plastic strain tensors in addition to their dependence on the direct invariants of these tensors and additional scalar hardening parameters such as the plastic work; inclusion of the joint invariants as parameters in the constitutive functions allows for induced anisotropy during plastic deformation. It is demonstrated that the physical meaning of the joint invariants is related to the deviation of the principal directions of the stress tensor from those of the plastic strain tensor. It is also shown that only three cases of anisotropy (orthorhombic, transverse and isotropy) are possible in the types of material models discussed herein. The model proposed in this paper does not imply coaxiality between the principal directions of the stress and, plastic strain increment tensors; however, implications of the assumption of coaxiality on the structure of the constitutive equations is investigated. It is shown that coaxiality does not necessarily imply isotropy, but no deviations from coaxiality can be expected in isotropic materials. 相似文献
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采用空心圆柱扭剪仪对干燥TJ-1模拟月壤试样进行了应力主轴固定的定向剪切试验及不同偏应力比时主应力方向的纯旋转试验。从应力-应变关系角度出发,研究主应力方向、偏应力比对其各向异性的影响,并探讨了上述因素对TJ-1模拟月壤非共轴性的影响。试验结果表明：主应力方向和偏应力比对TJ-1模拟月壤的各向异性均有显著影响;应力主轴旋转引起的非共轴现象比定向剪切时明显,且偏应力比较小时非共轴角随应力主轴旋转呈先减小后增加的趋势,偏应力比较大时非共轴角一直减小直至试样破坏时非共轴现象消失。上述成果可弥补干砂试样非共轴领域的研究空白,并可望为将来月球上基础设施的修建提供技术支持。 相似文献
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利用空心圆柱扭剪仪对杭州软黏土进行了一系列不排水试验,包括对原状软黏土在不同主应力方向上的定向剪切试验和主应力轴旋转试验以及对重塑软黏土的主应力轴旋转试验,主要研究不同应力路径下软黏土非共轴角的发展特性以及中主应力系数b、初始剪应力水平和次生各向异性对其非共轴特性的影响。试验结果表明,软黏土的非共轴特性虽与砂土存在相似之处,但又不尽相同。原状软黏土在定向剪切条件下的非共轴角均较小,并且与加载方向有关,然而受剪应变发展的影响,试样接近破坏时的非共轴角并不为0°;主应力轴旋转条件下,无论原状还是重塑黏土其非共轴角均随主应力方向角? 增加而循环波动变化,且周期约为90°;非共轴角基本随中主应力系数b的增加而减小,但这种影响并不十分显著;剪应力水平对非共轴角的大小和发展趋势均存在一定的影响。对于重塑土的试验表明,软黏土的非共轴特性并不完全由土体的初始各向异性所决定,次生各向异性的影响也很大。 相似文献