A combined continuous-discontinuous approach for failure process of quasi-brittle materials

Rock,concrete and other geo-materials,due to the presence of microstructural inhomogeneity,their fracture processes and damage characteristics are associated with the distribution of micro-cracks contained in the materials.In this study,by introducing a cohesive zone model based on fracture mechanics into the framework of deformable discrete element method,a continuous-discontinuous coupling analysis approach for simulating the fracture of quasi-brittle materials is proposed.The cohesive interface elements are inserted into certain engineering or research region.It is assumed that damage and fracture occur only in the interface elements,while bulk material is modeled to be elastic.The Mohr-Coulomb criterion with tension cut-off is adopted as the damage initiation criterion,and a scalar damage variable representing damage in the material is used to describe the rate at which the material stiffness is degraded.Cracks are simulated explicitly by the failure of the interface elements.Numerical simulations are performed in order to validate the suggested method.Partial applications are also listed.The results show that this method provides a simple but effective tool for the simulation of crack initiation and propagation,and it can reflect the whole process of quasi-brittle materials from small deformation to large deformation and failure.     

基于孔隙压力黏结单元的准脆性材料水力劈裂模拟

传统数值方法对水力劈裂的模拟一般采用预设裂缝扩展路径,且将水流作用等效为压力荷载,难以反映裂缝渗流-开裂耦合效应. 采用自编程Python脚本程序批量插入孔隙压力黏结单元,考虑裂缝渗流-开裂耦合作用模拟准脆性材料水力劈裂随机扩展全过程. 在对经典理论模型及试验结果模拟验证的基础上,开展含多裂缝均质模型的水力劈裂全过程分析,并进一步建立混凝土细观尺度水力劈裂模型,分析骨料、界面过渡区和基体渗透性对混凝土劈裂全过程的影响. 结果表明,本研究模型可以有效模拟准脆性材料水力劈裂失效过程,准脆性材料多缝开裂过程伴随微裂缝的分叉扩展而非光滑的裂缝扩展路径,骨料及界面过渡区影响劈裂扩展路径造成裂缝分叉出现,混凝土基体的渗透性对其抵抗水力劈裂不利并影响失效软化过程.     

Statistical damage constitutive model for concrete materials under uniaxial compression

According to the damage mechanism of concrete material during the uniaxial compressive failure process,this paper further establishes the statistical damage constitutive model of concrete subjected to uniaxial compressive stress based on the statistical damage model under uniaxial tension. The damage evolution law in the direction subjected to pressure is confirmed by the tensile damage evolution process of lateral deformation due to the Poisson effect,and then the compressive stress-strain relationship is defined. The peak nominal stress state and the critical state occurring in the macro longitudinal distributed splitting cracks are distinguished. The whole loading process can be divided into the even damage phase and the local breakage phase. The concrete specimen is divided into the failure process zone and the resting unloading zone. The size effects during the local breakage phase under the uniaxial monotonic compressive process and the hysteretic phenomenon under the cyclic compressive loading process are analyzed. Finally,the comparison between theoretical results and experimental results preliminarily verifies the rationality and feasibility of understanding the failure mechanism of concrete through the statistical damage constitutional law.     

Relationship between subsurface damage and surface roughness of ground optical materials

A theoretical model of relationship between subsurface damage and surface roughness was established to realize rapid and non-destructive measurement of subsurface damage of ground optical materials.Postulated condition of the model was that subsurface damage depth and peak-to-valley surface roughness are equal to depth of radial and lateral cracks in brittle surface induced by small-radius(radius≤200 μm)spherical indenter,respectively.And contribution of elastic stress field to the radial cracks propagation was also considered in the loading cycle.Subsurface damage depth of ground BK7 glasses was measured by magnetorheological finishing spot technique to validate theoretical ratio of subsurface damage to surface roughness.The results show that the ratio is directly proportional to load of abrasive grains and hardness of optical materials,while inversely proportional to granularity of abrasive grains and fracture toughness of optical materials.Moreover,the influence of the load and fracture toughness on the ratio is more significant than the granularity and hardness,respectively.The measured ratios of 80 grit and 120 grit fixed abrasive grinding of BK7 glasses are 5.8 and 5.4,respectively.     

材料大塑性变形损伤研究

综述了材料大塑性变形损伤的主要研究方法：基于位错的连续介质位错理论和分子动力学方法,基于宏细观损伤变量的损伤力学方法和基于裂纹及考虑损伤的断裂力学方法．评述了各尺度研究方法的研究概况、特点及其适用范围．重点介绍了材料变形过程损伤的模拟方法,以及运用有限元法进行损伤演化数值模拟的经验公式、统计模型和基于宏细观损伤变量的物理模型．     

岩石的统计损伤本构模型及临界损伤度研究

岩土工程中的岩体常处于空间应力状态下,因此,研究空间应力状态下岩石的损伤破坏机理有重要的实际工程意义。基于岩石内部缺陷分布的随机性,建立了围压和轴压共同作用下岩石的统计损伤本构模型。并通过砂岩的三轴压缩试验确定了模型里的参数,定性分析了围压对损伤度的影响。研究结果表明：（1）模型能反映围压效应,即可以体现岩石强度随围压变化的规律;并且模型参数确定方法揭示了模型参数的物理意义;（2）通过岩石的全应力-应变全曲线特征及峰值应力与应变确定模型参数,不同的围压对应着不同的参数;（3）临界损伤度与岩石所处的应力状态有关,同一种岩石在不同围压下临界损伤度是变化的;（4）由分布参数m的物理含义可知,m可以做为表征砂岩脆性的参数;1/m可以做为表征砂岩延性的参数。     

非线性横摇导致船舶倾覆的突变研究

针对规则波中船舶非线性横摇而产生的倾覆现象,提出了采用了突变理论进行分析的方法.在船舶横摇运动方程的建模过程中,将阻尼力矩取至三次项,恢复力矩取至七次项. 在此基础之上,经数学变换,将船舶非线性横摇模型转换成了尖点突变模型的形式,并对模型进行了求解,得到了系统突变时的临界函数关系式与关系曲线.分析结果表明,倾覆现象与船舶自然横摇频率和恢复力系数有关,其倾覆的临界条件是自然横摇频率和恢复力系数的函数.当自然横摇频率与恢复力系数满足突变临界函数的关系式与关系曲线时,会导致系统的突变,即船舶的倾覆.     

Numerical analysis of application for induction caving roof

New method for handling roof of the base successive mining is proposed, which is induction caving in the roof. The key is that it is made certain to the station of the space-time in the induction caving roof, as the stress is released with the mining process. And applying the catastrophe theory, the influencing factors of induction caving roof are studied in the emptied areas, such as the mechanical property of the surrounding rock, the area of the gob, the scope and dimension of tensile stress. The results show that the key factor is the area of the gob to the method of the induction caving roof. Then according to the geology and the ore characteristic, the three dimension FEM mechanical model is built in Tongkeng Mine, the laws of the tensile stress are analyzed to the space and the time in the roof with the mining, then it is rational design to the mine step and time of the handing the roof. Foundation item: Project(50490274) supported by the National Nature Science Foundation of China     

Numerical study on freezing-thawing phase change heat transfer in biological tissue embedded with two cryoprobes

A two-dimensional model for freezing and thawing phase change heat transfer in biological tissue embedded with two cryoprobes was established. In this model, the blood vessels were considered as tree-like branched fractal network, and the effective flow rate and effective thermal conductivity of blood were obtained by fractal method. The temperature distribution and ice crystal growth process in biological tissue embedded with two cryoprobes during freezing-thawing process were numerically simulated. The results show that the growth velocity of ice crystal in freezing process from 200 to 400 s is more rapid than that from 400 to 600 s. Thawing process of frozen tissue occurs in the regions around cryoprobes tips and tissue boundary simultaneously, and the phase interfaces are close to each other until ice crystal melts completely. The distance of two cryoprobes has a more profound effect on the temperature distribution in freezing process at 400 s than at 800 s. Foundation item: Project(50436030) supported by the National Natural Science Foundation of China     

基于相场法的水-力耦合作用下裂隙岩体变形破坏机理

探究水-力耦合作用下裂隙岩体的力学响应和破坏机制,对解决水-力耦合工况下岩体工程安全与稳定问题具有重要意义。基于Biot多孔介质理论及弹性理论,提出一种水-力耦合相场数值模拟方法,提出交错时间积分方案来获得流体压力和固体变形的稳定解,得到的控制方程式采用全饱和多孔介质弹性理论的体积应变分离和偏应变分离。通过模拟水力压裂与天然裂隙相互作用等数值算例验证了数值模型的有效性和正确性。此外,为探究实际工程开挖过程中的卸载破坏机理,结合钻孔注液试验数值实例,建立一个裂隙岩体水-力耦合卸载破坏模型,模拟水-力耦合条件下裂隙岩体卸荷破坏全过程。结果表明：在水-力耦合卸载破坏过程中,混合拉-剪裂纹的扩展和连接主导了最终破坏模式;初始围压、初始卸荷应力状态对剪切裂纹的发育具有促进作用,水压则反之。     

模拟准脆性材料热开裂的热力耦合格构离散单元法

针对准脆性材料的热传导与热开裂问题,基于格构离散单元法（LDEM）提出新的热力耦合数值模型. 通过格构离散体系与连续体传热过程的等效换算,结合格构单元的线膨胀公式,在LDEM中实现温度传导与裂纹扩展的数值模拟. 以平板的热传导与热弹性应力问题、由温度梯度与热失配引起的开裂问题为例,验证所提模型. 将所提模型应用于细观混凝土温度?应力试验的数值模拟中. 结果表明,LDEM热力耦合模型能够有效模拟准脆性材料的热传导过程、在温度影响下的裂纹萌生与扩展,是研究准脆性材料热开裂过程与机理的有力工具.     

Experimental Study on AE Characteristics of Granite under Uniaxial Tension at Different Strain Rates

To investigate the acoustic emission (AE) characteristics of quasi-brittle materials like rock and concrete,and to further analyze their damage and failure mechanism under seismic and other dynamic loads,the uniaxial tension test of granite cylinder specimens within the strain rate range of 10~(-7)-10~(-4) s~(-1) was monitored by AE technology,and the typical AE characteristic parameters were analyzed using statistical and correlation analysis.The experimental results show that,with the increase of strain rate,the peak of AE hit rate appears earlier and increases;the proportion of AE hits with higher duration or amplitude increases significantly,the b-value shows a decreasing trend,and the distribution of AE frequency-amplitude is increasingly discrete.In addition,the obvious characteristic of double dominant frequency bands was observed in AE waveforms by using spectrum analysis,with the increase of strain rate,the percentage of A-type waveforms corresponding to low dominant frequency band increases,while that of D-type waveforms corresponding to high ones decreases accordingly,which is significance for the further study of the damage and failure mechanism of quasi-brittle materials.     

混凝土尖点突变断裂模型

本文试图应用初等突变理论,分析混凝土的稳定与不稳定断裂、延性与脆性断裂的关联.文中建立了混凝土尖点突变断裂模型.并据此方便解释了混凝土的猝断现象及裂缝的突发然展.     

基坑土钉支护的突变分析

尝试着将系统科学中的突变理论引入到土钉支护的理论分析当中来,从传统的研究事物连续、光滑、不间断的变化转入到研究事物不连续变化的工作中来.研究中使用了突变理论中最常见的折叠型突变和尖点突变,分别对土钉支护体系中的土体和土钉与土体之间的粘结摩擦力进行分析,以势函数为响应变量,以基坑侧向位移为主要的控制变量,求解出了基坑发生突变时的临界位移,并通过临界位移,对开挖中的基坑进行安全性的评价.通过具体工程的分析,计算出了土钉支护体系发生突变的两种主要破坏方式的临界位移,对比两种临界位移,判断土钉支护体系的破坏形式.     

拱坝坝肩含断续节理岩体破坏机理研究

结合金沙江溪洛渡高拱坝坝肩岩体断续节理的特点,采用模型的试验研究方法,根据断裂力学理论模拟了含断续节理岩体抗剪强度的相似性,研究了坝肩岩体的破坏过程,破坏形态和破坏机理。     

Characteristics of magnetic memory signals for medium carbon steel under static tensile conditions

To test the magnetic signals leaked from the surface of specimens during loading, the experiments of the static tensile of medium carbon 45^# steel were carried out. The results show that the magnetic field strength values rapidly vary when the load began, and the curves of the magnetic field strength change from irregularity to regularity with the increase of the load. Furthermore, by comparing with the state of on-line testing, it is found that the magnetic signals of out-of-line testing has more practicability. In the course of loading, though the dots of passing zero of the magnetic field strength continually changed their positions and quantities, the last rupture places were always approached by the dots of passing zero since the elastic loading phase. Some certain relations should exist between external stress and changing of magnetic signals inside the material, and correlative explanation is made based on dislocation theory and the mechanism of magnetic domain action, which provides the basis for further research of magnetic memory. Foundation item: Project(50235030) supported by the National Natural Science Foundation of China     

Rock fracture under anti-plane shear (Mode III) loading

Anti-plane punch-through shear test and anti-plane four-point bending test are used to study the crack initiation and propagation under anti-plane shear (Mode III) loading. The tensile and shear stresses at the crack tip are calcualted by finite element method. The results show that under Mode III loading the maximum principal stress σ₁ at crack tip is smaller or a little larger than the maximum shear stress τ_max. Since the tensile strength of brittle rock is much lower than its shear strength, σ₁ is easy to reach its critical value before τ_max reaches its critical value and thus results in Mode I fracture. The fracture trajectory is helicoid and the normal direction of tangential plane with the fractured helicoid is along the predicted direction of the maximum principal stress at the notch tip. It is further proved that Mode I instead of Mode III fracture occurs in brittle rock under Mode III loading. The fracture mode depending on the fracture mechanism must be distinguished from the loading form. Foundation item: Project (50374073) supported by the National Natural Science Foundation of China; project (2002032256) supported by the Postdoctor Science Foundation of China     

Synthesis of Lithium-cobalt Oxides by Sol-gel Method and their Cathodical Discharge Behavior at 500

Three kinds of lithium-cobalt oxides with lithium to cobalt atomic ratios of 0.6, 0.8 and 1.0 respectively were prepared using nitrates and citric acid by sol-gel process. During the baking of the dry-gel powder, two exothermic peaks occur, followed by the formation of Li₂O and Co₃O₄ and then the combination of LiCoO₂, which is testified by DTA and XRD. The powder formed from an alkaline sol is finer than that from the acidic one. There are a single phase intercalation of lithium ion and an electrochemical reduction reaction for higher-valence cobalt ion when simulating the discharging process of Li-B/LiCl-KCl/lithium-cobalt oxides at 500 °C, and the voltage and specific capacity are not sensitive to the initial ratios of lithium to cobalt. Foundation item: The Doctoral Foundation of the Education Ministry of China(No. 96053311) Biography of the first author: LI Zhi-you, born in 1968, majoring in materials for thermal battery.     

Virtual multi-dimensional internal bonds model and its application in simulation of rock mass

VMIB (virtual multi-dimensional internal bonds) is a multiscale mechanical model developed from the VIB (virtual internal bond) theory. In VIB theory, the solid material is considered to consist of random-distributed material particles in microscale. These particles are connected with normal bonds. The macro constitutive relation is derived from the cohesive law between particles. However, in VMIB, the micro particles are connected with both normal and shear bonds. The macro constitutive relation is derived in terms of bond stiffness coefficients. It has been theoretically certified that there exists a corresponding relationship between the two bond stiffness coefficients and the two macro material constants, i.e. the Young's modulus and Poisson ratio. This corresponding relationship suggests that it should be necessary and sufficient to simultaneously account for the normal and shear interactions between particles. Due to the fact that the fracture criterion is directly incorporated into the constitutive relation, both VIB and VMIB present many advantages in simulating fractures of materials. In the damage model of rock mass, a damage tensor is usually defined to describe the distribution of cracks. The damage value in one direction determines the relative stiffness of rock mass in this direction. In VMIB solid, the relative distribution density of micro bonds in one direction determines the relative macro stiffness of the material in this direction. The effects of the damage value and the relative distribution density of bonds are consistent. To simulate the failure behavior of rock mass with VMIB, the presented paper sets up a quantitative relationship between the damage tensor and the relative distribution density of bonds. Comparison of the theoretical and the experimental results shows that VMIB model can represent the effect of distributed cracks on rock mass with this relationship. The presented work provides a foundation for further simulating fracture behavior of rock mass with VMIB model, and an alternative approach for modeling other multi-cracked body. Supported by the National Natural Science Foundation of China (Grant No. 50609013)     

脆性材料断裂韧性的单一尺寸试件测定方法

脆性或准脆性材料(混凝土,陶瓷,大理石,岩石等)的断裂韧性随试件的大小而改变,具有明显的尺 寸效应．通常情况下描述材料尺寸效应的Bazant尺寸效应律是建立在一系列相似试件的基础上通过实验方 法确定的．提出了一种新的用单一试件测定断裂韧性的实验方法和计算公式,即采用含不同裂缝长度的单一 尺寸试件测定材料的断裂韧性与相似试件测定方法相比,测定结果吻合很好,该方法具有很强的实用性、便 于在实验室环境下用小试件测量,计算方法也很简便．