Effect of Relative Stress on Post-Peak Uniaxial Compression Fracture Energy of Concrete

1　IntroductionItisrealizedthatthelinearelasticfracturemechan icsisnownotapplicabletofractureinordinaryconcretestructures .Todescribethesofteningbehaviorofconcrete ,differentmodelsbasedonnon linearfracturemechanicalideaswereproposed ,suchasthefictitiouscrackmodel[1](FCM )byHillerborgetal.(1976 ) ,thecrackbandmodel[2 ] byBazant (1983) ,andthetwo parameterfrac turemodel[3 ] byJenqandShah (1985 ) .ThedifficultywithFCMliesindeterminingtheval uesofthethreeparametersbyexperiments .Toobtaintheval…     

应变软化及剪胀性土体中考虑大应变的孔扩张问题解析

采用应力跌落模型模拟土体的应变软化,以Mohr-couloub屈服准则和不相关联的流动法则考虑土体屈服塑性流动和剪胀特性,引进对数应变考虑土体的大变形,给出了应变软化及剪胀性土体中考虑大应变的孔扩张问题的弹塑性解析解,为了对比,还给出了小应变情况下位移的解析表达式.分析了大、小应变理论、不同剪胀特性、不同软化程度和考虑塑性区弹性变形与否对结果的影响.结果表明:在孔扩张压力较大时,用小应变理论会引起很大误差,必须考虑大应变;在相同扩张力作用下,土体的软化程度越高,扩张率越大,土体的剪胀角越大则扩张率越小.土体的软化和剪胀特性还影响极限扩张压力,软化越严重,极限扩张压力越小;剪胀角越大,极限扩张压力越大.     

Shear stress in MR fluid with small shear deformation in bctlattic structure

A theoretical model based on BCT lattice structure was developed. Resultant force in the BCT lattice structure was deduced, following the interaction force of two kinds of magnetic particles. According to empirical FroHlich-Kermelly law, the relationship between the magnetic induction and the magnetic field was discussed, and a predictive formula of shear stresses of the BCT lattice structure model was established for the case of small shear deformation. Compared with the experimental data for different particle volume fractions, the theoretical results of the shear stress indicate the effects of the saturation magnetization and the external magnetic field on the shear stress.     

基于次加载面摩擦模型的接触面剪切特性研究

为了反映土与结构接触面的应变软化特性以及剪切速率对土与结构接触面力学特性的影响,基于ABAQUS软件提供的FRIC子程序,采用显式积分算法,对能够表现接触面上应变软化的次加载面摩擦模型进行二次开发. 利用该模型模拟土与结构接触面的直剪试验,研究直剪试验过程中次加载面摩擦模型接触面上的应力演变过程. 研究结果表明,利用次加载面摩擦本构模型,能够较好地模拟土-结构接触面直剪试验中的应变软化和密实状态的类砂土残余切应力随剪切速率的增大而下降的现象. 在剪切位移的发展过程中,次加载面摩擦模型接触面上的切应力先增加后减小,接触面远离加载端一侧到另一侧的应力发挥水平先后达到最大值. 该研究表明,次加载面摩擦模型可以用于土与结构接触面的应变软化特性和速率相关性的模拟.     

Adiabatic shear localization evolution for steel based on the Johnson-Cook model and gradient-dependent plasticity

Gradient-dependent plasticity is introduced into the phenomenological Johnson-Cook model to study the effects of strainhardening, strain rate sensitivity, thermal-softening, and microstructure. The microstructural effect （interactions and interplay among microstructures） due to heterogeneity of texture plays an important role in the process of development or evolution of an adiabatic shear band with a certain thickness depending on the grain diameter. The distributed plastic shear strain and deformation in the shear band are derived and depend on the critical plastic shear strain corresponding to the peak flow shear stress, the coordinate or position, the internal length parameter, and the average plastic shear strain or the flow shear stress. The critical plastic shear strain, the distributed plastic shear strain, and deformation in the shear band are numerically predicted for a kind of steel deformed at a constant shear strain rate. Beyond the peak shear stress, the local plastic shear strain in the shear band is highly nonuniform and the local plastic shear deformation in the band is highly nonlinear. Shear localization is more apparent with the increase of the average plastic shear strain. The calculated distributions of the local plastic shear strain and deformation agree with the previous numerical and experimental results.     

考虑滑移和剪切变形的单箱双室组合箱梁剪力滞效应

为准确分析单箱双室组合箱梁的剪力滞效应,考虑钢混凝土的界面滑移效应和钢腹板的剪切变形,针对顶底板和翼板定义不同的剪力滞翘曲位移函数,基于能量变分法推导出单箱双室组合箱梁剪滞效应的控制微分方程及其闭合解。以单箱双室组合箱梁算例为基础,利用该方法分析其剪力滞效应的规律,结果表明：在同时考虑滑移和剪切变形时,组合箱梁的挠度比初等梁理论解大,且其挠度随界面滑移刚度的增大而减小;组合箱梁在均布荷载作用下,滑移量与荷载值近似成正比关系;在相同条件下,钢箱梁底板的剪力滞效应较混凝土顶板显著。     

土工合成材料界面应变软化特性的一种本构新模型

在土工合成材料工程的数值研究中,根据土工合成材料界面的力学特性,构建合理有效的本构模型是提高数值计算可靠性的关键.针对部分土工合成材料界面存在的应变软化特性,提出了一种新的本构模型.以剪应力峰值为分界点,将剪应力剪切位移曲线前后两部分分别进行模拟.剪应力峰值前,采用Clough和Duncan双曲线模型进行模拟;剪应力峰值后的应变软化阶段,以剪应力峰值为原点,用另外一条倒置的双曲线进行模拟.通过系统的公式推导描述了模型的构建过程,给出了应变软化阶段界面剪切刚度计算表达式.通过对界面直剪试验结果的模拟初步验证了模型的拟合能力.     

考虑中间主应力的深埋圆形巷道弹塑性位移解

平面应变条件下的深埋圆形巷道一般忽略中间主应力的影响,但塑性区围岩的变形与实际情况会产生较大差异．为了充分考虑中间主应力对深埋圆形巷道的影响,基于平面应变假设与关联流动法则将Mohr-Coulomb准则精确匹配为Drucker-Prager准则,在此基础上推导了理想弹塑性材料在塑性阶段的中间主应力表达式;根据所得的中间主应力表达式结合关联流动法则．不引入任何假设,得出塑性区体积扩容的关系式;进一步推导了考虑中间主应力影响的深埋圆形巷道塑性区应力位移解析式,其中径向应力、切向应力及塑性区半径的表达式与卡斯特奈（Kastner）解完全一致,但卡斯特奈（Kastner）解无法得出中间主应力,而新的位移解析式则与以往的文献完全不同;经与以往文献的位移理论解比较分析知,新的位移解答更加合理．因此,考虑中间主应力的解答为深埋圆形巷道的计算与设计提供更为科学的理论基础．     

各向异性砂土剪切带角度的理论分析

针对平面应变条件下各向异性砂土剪切带角度的试验规律,采用传统的3种理论和分叉理论进行对比分析。将平面应变条件下剪切带角度的试验结果按照传统3种理论整理发现,尽管传统3种理论可以估算同种砂剪切带角度的极小、中间和极大值,但无法解释其各向异性规律。砂土在平面应变条件下破坏时会产生明显的剪切带,当剪切带方向和砂土沉积面方向接近时,会较早诱发剪切带的产生,使材料强度降低,造成了平面应变条件下各向异性强度规律明显不同于常规三轴条件下的试验规律,采用分叉理论结合各向异性模型则可以有效解释这个规律。随砂土沉积面角度的变化,模型可以从细观角度解释常规三轴条件下剪切带角度的单调变化的试验规律,结合分叉理论可以描述平面应变条件下其先减小然后增大的规律。通过几种理论对比分析表明,模型结合分叉理论不但能够描述多种应力状态下的平面应变和常规三轴应力条件下剪切带角度表现的不同规律,而且能够从细观角度解释其各向异性成因。     

Analysis of complete plasticity assumption for solid circular shaft under pure torsion and calculation of shear stress

The distribution of shear stress on the cross-section of plastic metal solid circular shaft under pure torsion yielding,the applicability of complete plastic model assumption and the shear stress formula were researched.Based on the shear stress formula of circular shaft under pure torsion in elastic stage,the formula of torque in elastic stage and the definition of yield,it is obtained that the yielding stage of plastic metal shaft under pure torsion is only a surface phenomenon of torque-torsion angle rel...     

不同温度下AZ31镁合金绝热剪切变形局域化

为了了解镁合金在不同温度高应变率载荷作用下发生变形局域化的特点,进而揭示镁合金在高速冲击载荷作用下发生绝热剪切的特殊规律,采用分离式Hopkinson压杆对挤压态AZ31镁合金进行了常温及高温的高速冲击压缩试验,而后对不同温度冲击后的试样通过光学显微镜、扫描电镜和透射电镜进行变形机制的分析.结果表明:常温下试样在受到剪切力后,在剪切区的裂纹周围产生了大量的孪晶;高温下试样的剪切区内产生了明显的绝热剪切带,并且在剪切带周围发现了大量平行的孪晶.在TEM下观察到剪切带内为等轴晶晶粒,在剪切区内发生了动态再结晶过程.     

Effect of strain hardening and strain softening on welding distortion and residual stress of A7N01-T4 aluminum alloy by simulation analysis

The effect of strain hardening and strain softening behavior of flow stress changing with temperature on welding residual stress,plastic strain and welding distortion of A7N01-T4 aluminum alloy was studied by finite simulation method.The simulation results show that the weld seam undergoes strain hardening in the temperature range of 180-250 ℃,however,it exhibits strain softening at temperature above 250 ℃ during welding heating and cooling process.As a result,the strain hardening and strain softening effec...     

韧性金属平面应变拉伸失稳与应变局部化的剪切效应

讨论了韧性金属平面应变拉伸失稳与应变局部化的剪切三轴效应。对常规平面应变大变形厚向应力表达式进行了修正,计入塑性域内剪应力的影响,采用文献[1]的有限元方法进行数值模拟,并与忽略这种剪切效应的常规方法计算结果进行比较。讨论了初始表面不均匀性对剪切效应的影响程度。     

规则锯齿型软弱结构面剪切特性的数值分析

为了探讨规则锯齿型软弱结构面的剪切特性,利用FLAC3D建立岩体软弱结构面的三维计算模型,分析结构面的剪切强度和变形特征.结果表明：剪切应力和剪切位移的关系从初期的线性特征转变为非线性特征;软弱结构面和岩体的耦合作用导致结构面的等效粘结力和等效内摩擦角均小于二者的平均值;迎剪切方向的结构面主要发生剪切破坏,而沿剪切方向的结构面主要发生拉伸破坏.结构面的剪切强度与剪切速率成线性关系;随着结构面厚度的增加,试样的剪切强度逐渐减小.研究得到的结果可为理论分析和工程实践提供参考.     

高等级公路沥青路面剪应力分析与应用

基于线弹性层状体系理论，采用路面结构有限元法，探讨高等级公路沥青路面剪应力的分布规律、影响因素及其评价方法。通过选取不同路面结构参数，包括各结构层厚度、模量和泊松比等，在不同的点位，利用BISAR程序进行力学计算和分析，提出了沥青路面抗剪强度的确定和评价方法。研究结果表明：在不考虑各结构层材料性能和厚度时，最大剪应力均分布在距路表3cm深度范围内；影响剪应力的最主要因素是沥青层模量、泊松比和基层模量；对于普通3层沥青层面层结构，上面层和中面层应进行剪应力验算，下面层可根据实际情况确定是否进行验算；验算时，需找到准确的计算点位才能计算出各层内最大剪应力。     

接触面等应力增量比路径单剪试验及模型研究

为研究等应力增量比路径下土与结构接触面力学特性,进行高塑性粘土与混凝土接触面等应力增量比路径单剪试验,获得不同应力路径下应力应变关系曲线。试验结果表明,高塑性粘土能够较好地适应大变形,接触面剪应力与切向应变关系呈剪切硬化型曲线,法向剪胀不明显;接触面剪切强度与应力路径无关,应力应变关系与应力路径密切相关;初始法向应力一定,应力增量比越小,剪应力增长越快,对应的破坏剪应力也越高;无剪胀发生情况下,法向应变与法向应力关系曲线与单向压缩试验具有一致性;应力比与切向应变呈良好的双曲线关系。在试验研究的基础上,基于广义位势理论,将土与结构接触面问题看着应力空间上的二维数学问题,采用塑性状态方程代替传统的屈服面,建立了双重势面接触面弹塑性模型。模型能够反映应力路径、初始法向应力对接触面力学特性的影响,具有一般性,参数能够通过试验完全确定,可以方便地应用于有限元分析,具有一定的应用推广价值。     

Unified analytical stress- strain curve for quasibrittle geomaterial in uniaxial tension, direct shear and uniaxial compression

Considering strain localization in the form of a narrow band initiated just at peak stress, three analytical expressions for stress - strain curves of quasibrittle geomaterial （such as rock and concrete） in uniaxial tension, direct shear and uniaxial compression were presented, respectively. The three derived stress - strain curves were generalized as a unified formula. Beyond the onset of strain localization, a linear strain-softening constitutive relation for localized band was assigned. The size of the band was controlled by internal or characteristic length according to gradient-dependent plasticity. Elastic strain within the entire specimen was assumed to be uniform and decreased with the increase of plastic strain in localized band. Total strain of the specimen was decomposed into elastic and plastic parts. Plastic strain of the specimen was the average value of plastic strains in localized band over the entire specimen. For different heights, the predicted softening branches of the relative stress- strain curves in uniaxial compression are consistent with the previously experimental results for normal concrete specimens. The present expressions for the post-peak stress - deformation curves in uniaxial tension and direct shear agree with the previously numerical results based on gradient-dependent plasticity.     

Effect of pore pressure on deformation and unstable snap-back for shear band and elastic rock system

Fast Lagrangian analysis of continua（FLAC） was used to study the influence of pore pressure on the mechanical behavior of rock specimen in plane strain direct shear, the distribution of yielded elements, the distribution of displacement and velocity across shear band as well as the snap-back （elastic rebound） instability. The effective stress law was used to represent the weakening of rock containing pore fluid under pressure. Numerical results show that rock specimen becomes soft （lower strength and hardening modulus） as pore pressure increases, leading to higher displacement skip across shear band. Higher pore pressure results in larger area of plastic zone, higher concentration of shear strain, more apparent precursor to snap-back （unstable failure） and slower snap-back. For higher pore pressure, the formation of shear band-elastic body system and the snap-back are earlier; the distance of snap-back decreases; the capacity of snap-back decreases, leading to lower elastic strain energy liberated beyond the instability and lower earthquake or rockburst magnitude. In the process of snap-back, the velocity skip across shear band is lower for rock specimen at higher pore pressure, showing the slower velocity of snap-back.     

Experimental study on rheological deformation and stress properties of limestone

The systematic experiment regarding the general uniaxial compression test and the creep deformations of the typical limestones from the surrounding rock of the highway tunnels were made.The relationship between the axial stress and the delayed deformation steady value was obtained from the creep tests under low loading stresses.By the least square method,the parameters of Nishihara creep model were calculated from the creep curves.The results indicate that the strain change always lags behind the increase o...