3D DEM simulation of hard rock fracture in deep tunnel excavation induced by changes in principal stress magnitude and orientation

To achieve the loading of the stress path of hard rock,the spherical discrete element model(DEM)and the new flexible membrane technology were utilized to realize the transient loading of three principal stresses with arbitrary magnitudes and orientations.Furthermore,based on the deep tunnel of China Jinping Underground Laboratory II(CJPL-II),the deformation and fracture evolution characteristics of deep hard rock induced by excavation stress path were analyzed,and the mechanisms of transient loading-unloading and stress rotation-induced fractures were revealed from a mesoscopic perspective.The results indicated that the stressestrain curve exhibits different trends and degrees of sudden changes when subjected to transient changes in principal stress,accompanied by sudden changes in strain rate.Stress rotation induces spatially directional deformation,resulting in fractures of different degrees and orientations,and increasing the degree of deformation anisotropy.The correlation between the degree of induced fracture and the unloading magnitude of minimum principal stress,as well as its initial level is significant and positive.The process of mechanical response during transient unloading exhibits clear nonlinearity and directivity.After transient unloading,both the minimum principal stress and minimum principal strain rate decrease sharply and then tend to stabilize.This occurs from the edge to the interior and from the direction of the minimum principal stress to the direction of the maximum principal stress on theε1-ε3 plane.Transient unloading will induce a tensile stress wave.The ability to induce fractures due to changes in principal stress magnitude,orientation and rotation paths gradually increases.The analysis indicates a positive correlation between the abrupt change amplitude of strain rate and the maximum unloading magnitude,which is determined by the magnitude and rotation of principal stress.A high tensile strain rate is more likely to induce fractures under low minimum principal stress.

Intact soft clay’s critical response to dynamic stress paths on different combinations of principal stress orientation

Comprehensive tests on Hangzhou intact soft clay were performed, which were used to obtain the soils' critical response to undrained dynamic stress paths under different combinations of principal stress orientation. The different combinations included cyclic principal stress rotation (CPSR for short), cyclic shear with abrupt change of principal stress orientation (CAPSO for short) and cyclic shear with fixed principal stress orientation (CFPSO for short). On one side, under all these stress paths, samples have obvious strain inflection points and shear bands, and the excess pore water pressure is far from the level of initial effective confining pressure at failure. Stress paths of major principal stress orientation (α) alternating from negative and positive have quite different influence on soil's properties with those in which α is kept negative or positive. On the other side, due to the soil's strongly initial anisotropy, samples under double-amplitudes CPSR and CAPSO (or single-amplitude CPSR and CFPSO) have similar properties on dynamic shear strength and pore water pressure development tendency when α is kept within ±45°, while have quite different properties when α oversteps ±45°.

Analysis of strength characteristics of loess before and after freezing using a hollow cylinder torsional shear apparatus

This paper aims to comprehensively analyze the influence of the principal stress angle rotation and intermediate principal stress on loess's strength and deformation characteristics. A hollow cylinder torsional shear apparatus was utilized to conduct tests on remolded samples under both normal and frozen conditions to investigate the mechanical properties and deformation behavior of loess under complex stress conditions. The results indicate significant differences in the internal changes of soil particles, unfrozen water, and relative positions in soil samples under normal and frozen conditions, leading to noticeable variations in strength and strain development.In frozen state, loess experiences primarily compressive failure with a slow growth of cracks, while at normal temperature, it predominantly exhibits shear failure. With the increase in the principal stress angle, the deformation patterns of the soil samples under different conditions become essentially consistent, gradually transitioning from compression to extension, accompanied by a reduction in axial strength. The gradual increase in the principal stress axis angle(α) reduces the strength of the generalized shear stress and shear strain curves.Under an increasing α, frozen soil exhibits strain-hardening characteristics, with the maximum shear strength occurring at α = 45°. The intermediate principal stress coefficient(b) also significantly impacts the strength of frozen soil, with an increasing b resulting in a gradual decrease in generalized shear stress strength. This study provides a reference for comprehensively exploring the mechanical properties of soil under traffic load and a reliable theoretical basis for the design and maintenance of roadbeds.

Shear-wave splitting beneath Yunnan area of Southwest China

Systematic analyses of seismic data recorded by the Yunnan regional seismograph network reveal significant crustal and upper mantle anisotropy. Splitting of the S phase of local earthquakes and teleseismic SKS, PKS, and SKKS phases indicates time-delays from 1.60 ms/km to 2.30 ms/km in the crust, and from 0.55 s to 1.65 s in the upper mantle which corresponds to an The polarization orientations of fast shear waves in direction, and the mantle anisotropy has a nearly styles and mechanisms exist between the crust and anisotropic layer with a thickness about between 55 165 km. the crust are complicated with a predominantly north-south west-east direction. Our results show different deformation upper mantle.

Relationship between Creasing-Recovery Force and Crease-Recovery Angle

To find out the reason of resulting in the crease recovery of a fabric and provide theoretical guidance for designing a new material with good creasing-recovery property,the relationship between the creasing-recovery force and the crease-recovery angle of a woven fabric was investigated by self-setup experimental device.The results show that the crease-recovery angle of a woven fabric is correlated with the creasing-recovery force of the fabric in a linear relation.Furthermore,it is found that the internal stress is the principal force of affecting the creasing-recovery property of a woven fabric.In addition,the relationship between the tensile property of a woven fabric and the creasing-recovery property of the fabric has also been investigated,showing that the lower relaxation velocity of tensile stress of a fabric is,the better creasing-recovery property of the fabric has.

Effect of Resonance on the Motion of Two Cylindrical Rigid Bodies

The effect of resonance on the motion of two cylindrical rigid bodies has been studied in the light of Bhatnagar [1] [2] [3] and under some defined axiomatic restrictions. Here we have calculated variation in Eulerian angles due to resonance in terms of orbital elements and unperturbed Eulerian angles.

连续纤维增强复合材料结构多尺度拓扑优化设计

连续纤维增强复合材料具有重量轻、比强度和比模量高的特点,日益受到航空航天等高端装备制造领域的青睐,同时连续纤维3D打印技术的发展使具有复杂几何构型和纤维分布的复合材料结构制造成为可能。为发挥复合材料多尺度的可设计性及获得更好的结构性能,本文基于独立连续拓扑变量提出了一种连续纤维复合材料结构多尺度拓扑优化方法。该方法引入基于主应力的纤维方向插值策略,解决纤维取向优化过程中易陷入局部最优的问题,实现连续纤维复合材料结构宏观拓扑、微观纤维方向和纤维疏密的并行优化设计。采用移动渐近算法(MMA)进行优化求解,数值算例证明了提出方法的有效性和稳定性,对连续纤维复合材料结构设计和路径规划具有指导意义。

主应力方向对饱和松砂不排水单调剪切特性影响的试验研究

利用大连理工大学新开发的“土工静力–动力液压–三轴扭转多功能剪切仪”,针对相对密度为30%的福建标准砂,在不排水条件下控制试验过程中主应力方向、中主应力系数、平均主应力保持不变,进行了单调剪切试验。以此着重探讨了主应力方向对有效内摩擦角、有效应力路径及应力-应变关系的影响。研究表明,主应力方向对在不排水条件下饱和松砂的单调剪切特性具有显著影响。

中主应力对饱和松砂不排水单调剪切特性的影响

利用土工静力-动力液压-三轴扭转多功能剪切仪,针对相对密度为30%的福建标准砂,在不排水条件下控制主应力方向、中主应力系数、平均主应力保持不变,进行了单调剪切试验。以此着重探讨了中主应力系数对相变有效内摩擦角、峰值有效内摩擦角及有效应力路径的影响。研究表明,中主应力系数对在不排水单调剪切条件下饱和松砂的强度参数具有显著的影响,而对有效应力路径及应力-应变关系发展模式影响较小。基于广义双剪强度准则,从理论上探讨了土的强度参数对于中主应力的依赖性,并与试验结果进行了对比。

定轴排水剪切试验中各向异性砂土的力学响应

南京云母砂主要由片状颗粒组成,具有较为明显的各向异性特性。采用香港科技大学的自动控制空心圆柱扭剪仪进行了应力主轴方向角分别为10°、30°、60°、80°的密实云母砂定轴剪切试验,研究云母砂的初始各向异性规律。为研究中主应力的影响,又对同一密度的试样在中主应力系数b=0.1、0.5、1.0三种不同情况下进行定轴剪切试验。试验结果表明,在大主应力方向角从0°～90°的变化过程中,强度先减小后增大,且随着b值的增加强度增大;变形表现出明显的各向异性,随着b值的增加体变增大。

激光冲击波对5B05铝合金表面应力状态的调整

采用X射线衍射仪分别对光斑中心及其边界点冲击前后3个方向上的残余应力进行测量。根据主应力及其方向角的计算公式,计算出了各测试点冲击前后的主应力值和主应力方向角,分析了其分布特性。结果表明,激光冲击波不仅能调整5B05铝合金表面残余主应力值,还能调整其方向。最大主应力增幅最大点与最小主应力增幅最大点并不在同一点上。5B05铝合金经激光冲击波作用后,表面应力状态得到了改善,应力分布变得分散,应力域变大。

初始应力条件对压实黄土动强度影响的研究

建筑物地基或土坡中的土体在受到地震或其他动荷载作用前往往处于复杂的初始应力状态,对原有的DTC-199型周期扭转荷载三轴仪进行简单的改造后,可以进行压实黄土在这种固结应力状态下动强度的试验研究。试验结果表明,在其他初始固结应力条件不变时,初始主应力方向角α0对压实黄土的动强度具有显著的影响,初始中主应力系数b0对动强度的影响较小;随着α0的增大,动强度逐渐降低,并在α0=90o时达到最小值,最大降幅在40%左右;b0对压实黄土的动强度的影响具有放大效应和区间效应,b0从0增大至0.5时,动强度逐渐增大,b0从0.5增大至1时,动强度逐渐减小,增加的幅度小于降低的幅度约5%。

考虑中主应力的常用破坏准则适用性研究

针对岩土工程中常用的考虑中主应力效应的5种破坏准则,推导各准则考虑中主应力影响的内摩擦角表达式,建立真三轴应力下内摩擦角与中主应力系数的关系,探讨中主应力的影响规律。与砾石料、密砂和黄土3种材料的真三轴试验结果进行比较,并采用标准差量化方法,探讨各准则对中主应力的反映程度及其适用性。研究结果表明:5种破坏准则均不同程度地反映中主应力效应,Drucker-Prager准则计算拟合效果较差,修正Lade-Duncan准则拟合效果较好,其余3个准则拟合程度相当,对于不同的材料应选用不同的破坏准则。

复杂应力路径下杭州原状软黏土破坏标准研究

采用空心圆柱扭剪仪对主应力轴方向固定不变定向不排水剪切应力路径下杭州原状软黏土破坏标准进行试验研究,重点探讨了中主应力参数b、大主应力方向角α及孔压积累对剪切过程中黏土抗剪强度的影响,并提出原状软黏土剪切破坏的标准。试验结果表明,中主应力参数的变化没有对应力应变变化模式和试验应力路径产生影响,随着中主应力参数的增大,黏土抗剪强度的变化并没有呈现出一定的规律性;大主应力方向角对原状软黏土强度的影响反映出原生各向异性对黏土抗剪强度的控制作用;黏土样在剪切后期会产生显著的孔压积累,影响了黏土强度的发挥。考虑中主应力参数等影响因素,通过分析广义剪应力-应变曲线的变化情况,取广义剪应力qJ的峰值作为原状软黏土的剪切破坏标准。

土在复杂应力状态下的动力特性研究

利用空心园柱扭剪仪，对瀑布沟心墙土料系统地进行了复杂应力状态下初始主应力偏传角α０，中主应力参数ｂ对土动力特性影响的试验研究．研究表明，α０角对土的动强度有不可忽视的重要影响，随着α０增加动强度明显降低；ｂ值对动强度也有影响，影响程度与α０相比相对要小．文中给出了考虑α０、ｂ影响的土，动强度及孔压模式，可供工程参考．

应力历史对饱和砂土力学性状影响的试验研究

三向应力历史条件的影响是准确预测地基变形的重要参考因素之一。针对相对密度为30%的福建标准砂,利用土工静力-动力液压-三轴扭转多功能剪切仪,进行了复杂条件下应力路径变化的应力控制式单调排水剪切试验。通过控制试验过程的平均主应力保持不变,改变应力历史条件,着重探讨应力历史对饱和砂土剪切强度和应力-应变关系的影响。试验结果表明:当平均主应力一定时,在排水条件下应力历史的改变,对砂土的剪切强度几乎没有影响,但会影响应力-应变特性。弹性轨迹随着中主应力系数和主应力方向角历史的不同而发生变化。引入考虑应力历史同剪切过程之间条件变化的参数,结合试验数据,给出了三维空间中砂土受到应力历史作用后形成的弹性边界的关系表达式。

中主应力与大主应力方向角对软黏土排水变形特性影响

为研究中主应力系数和大主应力方向角对各向异性软黏土变形特性的影响,利用GDS空心圆柱扭剪仪对温州原状软黏土进行了一系列不同中主应力系数和大主应力方向角的排水定向剪切试验。试验过程中在大主应力方向角和中主应力系数不变的条件下,逐渐增加剪应力直至试样破坏。分析了中主应力系数和大主应力方向角对温州原状土偏应力与大主应变关系、体应变、中主应变和小主应变与大主应变关系的影响。试验结果表明:试样的应力-应变关系在中主应力系数和大主应力方向角不同时表现出明显的各向异性。当中主应力系数为0.00和1.00时,大主应力方向角对应力-应变关系的影响较小;而当中主应力系数为0.50时,应力-应变关系中的割线剪切模量随大主应力方向角变化明显。当大主应力方向角为30°时,随着中主应力系数从0.00增加到0.50,中主应变由压缩状态变为拉伸状态;当大主应力方向角为45°时,随着中主应力系数从0.00增加到1.00,中主应变由压缩状态变为拉伸状态。

一种散粒体材料破坏准则研究

在Lade-Duncan准则和Matsuoka-Nakai准则的基础上,提出了适用于各种散粒体材料的新型破坏准则。该准则能够依据试验结果,调节准则方程中的参数,使得理论计算更加符合试验结果,这与传统的岩土类材料破坏准则相比有着较大优越性。通过构造三角恒等式,推导了π平面上距原点的长度函数,发现该准则在π平面上的形状则为光滑曲边三角形,这和大量的岩土类材料试验结果相符合。研究了该准则在一般应力状态下的内摩擦角与中主应力参数的关系,发现该准则能够取遍Lade-Duncan准则和Matsuoka-Nakai准则之间所有内摩擦角值。将各准则的计算结果与文献中已有的砾石料和中细砂的真三轴试验结果进行对比分析,表明所提出的准则要比Lade-Duncan准则和Matsuoka-Nakai准则更加符合试验结果,这进一步说明了该破坏准则能够适用于包括堆石料、砾石料、砂等散粒体材料。

饱和砂土排水与不排水剪切特性的比较研究

针对相对密度为30%的福建标准砂,利用土工静力-动力液压-三轴扭转多功能剪切仪,进行了复杂初始固结条件下应力路径变化的应力控制式单调排水与不排水剪切试验。试验过程中,控制平均主应力保持不变,分别探讨在排水与不排水条件下中主应力系数和主应力方向角对饱和砂土剪切特性和强度的影响。通过对比表明:在排水与不排水试验中,与排水条件无关,中主应力系数对归一化的应力—应变关系具有影响,但对体变或孔压的影响并不明显;当其余初始条件相同时,偏应力比随中主应力系数的增大而降低。主应力方向角的影响同样显著,排水试验的主应力方向角不同时,应力—应变关系所表现出的变化规律取决于水平面与竖直面上受到的剪应力作用,相变及峰值偏应力比和内摩擦角与主应力方向角之间存在抛物线型关系。不排水试验的峰值有效偏应力比随着主应力方向角的增大而减小。同时,对于不同的主应力方向,排水条件改变会影响试验得到的抗剪强度指标。

中主应力对砾石料变形和强度的影响

对砾石料进行了σ3不变,σ1和σ2等比例同时加载的等b试验,分别研究了中主应力对大、中、小主应变及强度的影响规律。将Mohr-Coulomb准则、Lade-Duncan准则、Matsuoka-Nakai准则和方开泽准则与砾石料的真三轴试验结果进行比较,发现Matsuoka-Nakai准则和方开泽准则反映的内摩擦角与b的关系与试验得到的规律差别较大,Lade-Duncan准则反映的规律与试验结果最接近,但数值上高估了b对内摩擦角的影响。因此,建议了一个破坏准则,推导了该准则的内摩擦角与b的关系式,并用几组真三轴试验结果进行验证,表明本文建议的准则比Lade-Duncan准则、Matsuoka-Nakai准则更准确地反映砾石料的内摩擦角与b的关系。