基于离散元法的脆性岩石细观蠕变失稳研究EI北大核心CSCD

为从细观角度探究脆性岩石的蠕变失稳过程及失稳机理,该文基于三维颗粒流程序(PFC3D)考虑岩石的时效变形损伤过程,引入岩石细观单元时效损伤的应力腐蚀模型,建立了基于离散元方法的岩石时效变形损伤破裂模型,并通过单轴压缩及单轴蠕变的室内实验和数值模拟对比验证了所建立的时效变形损伤破裂模型的合理性。数值模拟再现了岩石的初始蠕变、稳态蠕变和加速蠕变三个蠕变阶段,同时模拟结果表明,在单级加载条件下,随着应力水平提高,稳态蠕变应变率显著增大,岩石蠕变失效时间逐渐缩短,初始轴向应变、初始侧向应变和初始体应变不断增大,且细观裂纹扩展形式与单轴压缩破坏形式基本相同,都是以拉伸裂纹为主,裂纹的增长速率随着时间增加而不断增大,尤其在第三蠕变阶段裂纹增长速率迅速增大;在分级加载试验过程中,模型的轴向应变、侧向应变和体应变以及裂纹最终扩展形态与单级加载基本相同;此外将三维蠕变模拟结果与二维模拟结果进行对比,结果显示三维模型拟合程度更高。     

基于温度-应力耦合作用的岩石时效蠕变模型

基于岩石变形与热力学基本理论,建立了温度-应力耦合作用下脆性岩石时效蠕变损伤模型。根据此模型,在有限元数值软件COMSOL的基础上进行二次开发,考虑岩石介质的非均匀性,并以最大拉应力准则以及摩尔库仑准则为岩石单元的破坏准则,给出了温度-应力耦合作用下岩石时效蠕变损伤模型的数值求解方法。并结合室内实验结果验证了该模型方法的可行性和合理性。数值模拟结果表明该模型能准确描述不同温度条件下花岗岩典型蠕变全过程三个阶段,即初始蠕变阶段、稳态蠕变阶段和加速蠕变阶段。数值模拟也表明声发射累计数和岩石轴向应变的演化趋势整体上具有一致性,且在初始蠕变阶段和加速蠕变阶段出现较多的声发射现象。     

基于流变试验的向家坝砂岩长期强度评价

基于向家坝砂岩的常规三轴压缩试验和三轴压缩流变试验成果,分别通过应变等时曲线簇、稳态蠕变速率与应力水平的关系、裂纹损伤应力、流变荷载与破坏时间关系确定砂岩的长期强度,并比较这4种方法的适用性。结果表明:砂岩体积应变等时曲线簇的拐点比轴向应变、侧向应变等时曲线的拐点更为明显,而且其对应力水平和时间的反应更敏感;轴向稳态蠕变速率和侧向稳态蠕变速率与应力的关系符合指数函数关系,采用给定稳态蠕变速率阈值确定岩石长期强度比利用稳态蠕变速率-应力曲线的拐点确定长期强度更为合理;岩石的裂纹体积应变等时曲线簇以及其稳态蠕变速率与应力的关系也可以作为确定岩石长期强度的一种方法;岩石的裂纹损伤应力反映长期强度所在应力水平,为蠕变试验荷载分级提供依据。     

基于细观单元等效化方法的混凝土动态破坏行为分析

混凝土材料具有明显的应变率效应,对其力学性质增强机理的认识还不统一。在细观随机骨料模型基础上,采用特征单元尺度划分试件网格,推导了考虑材料拉/压强度应变率效应的细观单元等效本构关系,建立了非均质混凝土材料的细观单元等效化数值模型。基于二维模型对Dilger等混凝土动态压缩试验进行了数值模拟,获得的数值结果与试验数据及随机骨料模型结果吻合良好,证明了细观单元等效化方法的准确性;进而对三维混凝土试件动态单轴拉伸和压缩破坏模式及宏观力学性质的加载速率效应进行了研究。数值结果表明:随着加载速率的增加,混凝土裂纹(损伤)数量增大,混凝土破坏将耗散更多的能量,是混凝土动态强度提高的主要原因。     

孔隙率变化规律及其对混凝土变形过程的影响

混凝土内部随机分布的微裂纹和孔洞等细观缺陷影响其破坏机理及宏观力学性能,且孔隙率随着外荷载的变化而不断变化。该文中将含缺陷(微裂纹、各种空隙等)混凝土复合材料简化为空心球力学模型,基于弹性力学理论推导并获得了混凝土当前孔隙率与材料初始孔隙率及体应变之间的定量关系;推导并得到了含孔隙混凝土的有效弹性模量、有效泊松比及峰值应变等与孔隙率的定量关系,进而得到了单轴加载条件下含孔隙混凝土细观单元的等效多折线本构关系模型。最后,采用细观单元等效化力学模型,研究了单轴加载(拉伸和压缩)情况下不同孔隙率混凝土材料的破坏过程及宏观力学性能,探讨了孔隙率变化规律及其对混凝土变形过程的影响。     

砂岩裂纹发育细胞观机理CT实时研究

利用最新研制成功的医用Ｘ射线ＣＴ（ｃｏｍｐｕｔｅｒｉｚｅｄ ｔｏｍｏｇｒａｐｂｙ）机配套的专用岩石破坏试验加载设备,完成了单轴压缩使用下砂岩细观损伤演化特征的ＣＴ检测试验。得到了岩石破坏全过程中从微孔洞被压密到微裂纹发生、发展、贯通破坏、卸荷等不同发展阶段的ＣＴ图像。基于试验行到和岩石损伤扩展的细观机理,将岩石全过程曲线分为六段。     

岩石材料动载下泛形裂纹扩展数值模拟

岩石材料性能的非均匀性导致断裂面呈现不规则的泛形特性,基于此应用ABAQUS软件建立细观有限元模型,由Weibull分布表征岩石材料性能的非均匀性,对动态拉伸载荷下岩石材料泛形裂纹扩展进行研究。计盒维数计算得到的泛形断裂面的复杂度与实验结果吻合。不同应变率下的泛形裂纹扩展路径及复杂度的计算结果表明,断裂面的复杂度随应变率的增加而减小。进一步分析不同应变率下裂纹扩展的泛形断裂能,发现裂纹扩展的能量释放率随应变率增大而增大。低加载率下,裂纹向断裂韧性较小的单元扩展,但随着加载率的提高,裂纹瞬间穿过断裂韧性相对较高的单元,沿自相似方向扩展。上述结果揭示了应变率对泛形裂纹扩展路径的影响与材料性能的细观非均匀性有关,加深了对岩石材料泛形断裂机制的理解。     

分级循环加卸载下砂岩损伤力学特性研究

通过选取陕西省略阳市某滑坡表层的中风化砂岩进行静态单轴压缩试验与三轴循环加卸载试验,研究低围压循环加卸载条件下岩石的损伤力学特性,得到力学参数、损伤参数、滞回环面积等随循环过程的演化规律及循环荷载作用下砂岩的变形破坏机制。结果表明：循环加卸载试验过程中,围压增强砂岩抵抗变形破坏能力,随着围压增大,动弹性模量明显增大而动泊松比逐渐降低,砂岩试样内凹现象逐渐“模糊”;滞回环面积受围压与轴向载荷影响明显,与围压变化呈负相关,与轴向载荷变化呈正相关,轴向载荷增大时,不可逆应变逐渐累积,砂岩损伤加剧;砂岩内部原始裂隙在初始较低循环荷载作用下压密,后过渡到弹性变形与裂纹稳定扩展阶段,随着试验进行,裂纹加速演化不稳定扩张,岩样逐渐变形失稳破坏。     

含共面裂隙群砂岩强度与破裂特征的试验研究

节理岩体力学行为是地下工程稳定分析及支护设计的基础。采用岩石力学伺服试验机对含充填共面裂隙群砂岩试样进行单轴加载试验,分析了连通率和充填方式对砂岩强度及变形破裂特征的影响规律。试验结果表明:裂隙砂岩的力学性能较完整砂岩呈现出显著的劣化,且随着裂隙连通率的增大,峰值强度、峰值应变及弹性模量均逐渐减小;试样破裂过程中预制裂隙尖端先后出现Ⅰ类张拉裂纹和Ⅱ类剪切裂纹,但最终的失稳破裂面由II类裂纹扩展形成,Ⅰ类裂纹和Ⅱ类裂纹与预制裂隙的夹角随着裂隙连通率的增大逐渐增大;受充填方式的影响,裂隙砂岩的强度及变形特征均呈现出显著增大的趋势,而弹性模量提升幅度相对较小。     

GH33A合金在蠕变-疲劳交互作用下的裂纹扩展特性

通过对 GH33A 合金在蠕变与疲劳复合加载条件下的系列试验,发现拉伸保时使蠕变与疲劳发生了交互作用,加快了疲劳裂纹扩展速率,加速裂纹早期进入失稳扩展,大大降低了疲劳寿命。GH33A 合金具有良好的抗蠕变裂纹扩展能力,但疲劳裂纹扩展阻力较低。由此讨论了拉伸保时对裂纹扩展的影响,并对在蠕变-疲劳交互作用下的裂纹扩展模型作了探讨。     

钢管混凝土轴心受压构件的徐变预测模型及其徐变性能分析

为研究混凝土徐变对钢管混凝土轴心受压构件长期受力性能的影响,考虑构件截面内力重分布,建立了钢管混凝土轴心受压构件截面应力和应变以徐变系数为参数的随混凝土龄期变化关系的理论模型,结合已有试验数据和国内外常用12种混凝土徐变预测模型对该模型进行验证,并找到了适用于钢管混凝土轴心受压构件的徐变预测模型--Huo模型;在此基础上,计算并分析了钢管混凝土轴心受压构件混凝土龄期为10000 d的截面应力和应变;通过对混凝土强度等级、环境年平均相对湿度、初始加载龄期、含钢率、构件长度、截面应力水平等因素的不同取值,分析了各因素对钢管混凝土轴心受压构件徐变性能的影响程度及规律。结果表明:当钢管混凝土轴心受压构件的轴力不大于其极限承载力的60%时,随着加载龄期的增长,钢管截面应力逐渐增大,最大变化量达61.4%,而混凝土截面应力逐渐减小,最大变化量达26.2%;加载初期构件应变增长迅速,1000 d以后应变增长速度减慢,构件最终应变是初始应变的1.61倍;在轴压比相同的条件下,钢管混凝土轴心受压构件的徐变应变终值随着混凝土强度等级的提高而逐渐增大,随着含钢率的增大显著减小,随着初始加载龄期、环境年平均相对湿度、构件长度的增大而逐渐减小,轴压比不大于0.6时,其徐变应变终值随轴压比增长。研究成果可为钢管混凝土轴心受压构件在正常使用阶段徐变计算以及徐变变形控制提供依据。     

基于离散元的钢筋混凝土梁极限承载力研究

通过PFC2D建立考虑骨料级配的钢筋混凝土梁离散元细观数值模型,开展钢筋混凝土梁极限承载力的研究。混凝土标准棱柱单轴压缩模拟验证了混凝土单元细观参数的准确性。通过修改平行黏结模型获得了适用于钢筋模拟的平行-强化黏结接触模型,钢筋单轴拉伸模拟验证了钢筋单元细观参数的准确性。建立了无缺口钢筋混凝土梁离散元数值模型,在此基础上生成预制缺口钢筋混凝土梁数值模型,根据缺口位置及倾角分为9种工况,分析了各工况下梁的裂缝扩展规律及极限承载力。结果表明：缺口位置对裂缝的扩展过程影响较大,一般起裂位置均位于缺口顶端;缺口倾角仅对裂缝扩展初期有影响,后期裂缝依然会沿竖向发展;缺口梁极限承载力约为无缺口梁极限承载力的95%～98%。     

混凝土裂纹扩展过程模拟的扩展有限元法研究

扩展有限元法(theextendedFiniteElementMethod,XFEM)为数值模拟结构裂纹扩展过程提供了一条有效途径。该文介绍了用扩展有限元法对混凝土结构裂纹扩展过程进行数值模拟的实现方法。采用虚拟裂缝模型模拟混凝土非线性断裂行为,针对二维四边形单元推导了详细的有限元列式。采用3种方案对非线性方程系统进行求解,分析了其求解思路并概括了其求解步骤。通过对带初始边缘裂纹的单向拉伸混凝土板的数值模拟,对3种求解方案的计算结果进行了分析和讨论。     

Mechanical behaviour and fracture evolution of coal specimens containing an over-excavated hole: Experimental study and numerical modelling

Utilising a series of mechanically over-excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure-relief around an over-excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over-excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over-excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post-peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over-excavated holes.     

Damage mechanics based predictions of creep crack growth in 316 stainless steel

This paper describes a novel modelling process for creep crack growth prediction of a 316 stainless steel using continuum damage mechanics, in conjunction with finite element (FE) analysis. A damage material behaviour model, proposed by Liu and Murakami [1], was used which is believed to have advantages in modelling components with cracks. The methods used to obtain the material properties in the multiaxial form of the creep damage and creep strain equations are described, based on uniaxial creep and creep crack growth test data obtained at 600 °C. Most of the material constants were obtained from uniaxial creep test data. However, a novel procedure was developed to determine the tri-axial stress state parameter in the damage model by use of creep crack growth data obtained from testing of compact tension (CT) specimens. The full set of material properties derived were then used to model the creep crack growth for a set of thumbnail crack specimen creep tests which were also tested at 600 °C. Excellent predictions have been achieved when comparing the predicted surface profiles to those obtained from experiments. The results obtained clearly show the validity and capability of the continuum damage modelling approach, which has been established, in modelling the creep crack growth for components with complex initial crack shapes.     

Elastic anisotropy and micro-damage processes in polycrystalline ice

This paper discusses numerical predictions of a microstructural damage model for polycrystalline ice which is presented in a companion paper [1]. The results are relevant for ice deforming at the high end of the quasi-static domain of loading. First, the fracture mechanics-based model of damage is investigated by comparing model predictions of the stresses to form (nucleate) the first microcracks with test data. This is followed by a detailed simulation of loading under uniaxial compression using the damage model and an internal variable creep model, also summarized in the companion paper [1]. This simulation allows the prediction of the evolving damaged elastic properties, and delineates the relative contribution of creep and microcracking to the total deformation.The importance of load history on the deformation response is then illustrated by studying the influence of load path in biaxial loading. In these simulations, the competition between the mechanisms of failure by shear faulting and axial splitting is discussed in terms of the development of compliance anisotropy in the damaged body. Finally, the critical crack density is used as a macroscopic failure criterion to predict compressive strengths in the ductile-to-brittle transitional domain of strain rates, and its validity in more general states of stress involving different failure modes is questioned.     

A state-based peridynamic model for quantitative fracture analysis

A new state-based peridynamic model is proposed to quantitatively analyze fracture behavior (crack initiation and propagation) of materials. In this model, the general relationship of the critical stretch and the critical energy release rate is for the first time obtained for the state-based peridynamic model of linear elastic brittle materials, and the released energy density is defined to quantitatively track the energy released during crack propagation. The three-dimensional (3D) and two-dimensional (2D) (for both plane stress and plane strain) cases are all considered. As illustrations, the compact tension and double cantilever beam tests are analyzed using the proposed model, which is capable of successfully capturing fracture behaviors (e.g., crack path and concentration of strain energy density) of the considered fracture tests. The characteristic parameters (i.e., critical load, critical energy release rate, etc.) are calculated and compared with available experimental and numerical data in the literature to demonstrate validity of the proposed model.     

基于广义Kelvin链钢管混凝土徐变研究

混凝土徐变是混凝土材料本身固有的一个时变特性,是结构响应中一个重要组成部分,其计算方法通常是建立在单轴试验和理论基础上。为探讨钢管混凝土徐变特性,该文采用自制的压力自平衡混凝土徐变试验装置对混凝土圆柱和圆钢管混凝土柱进行了徐变试验,结果表明:钢管混凝土柱徐变变形要比普通混凝土柱的徐变变形小,在该文试验中两者相差接近50%,这可能是密闭钢管内核心混凝土无法与外界发生水分交换而不发生干燥收缩和干燥徐变以及钢管围压所致。根据粘弹性理论,引入多参数Kelvin链粘弹性元件模型,建立了求解单轴应力状态下混凝土徐变的Volterra型积分方程,模型参数近似表示为连续粘滞谱。通过离散时间变量t和分步积分,进一步得到了单轴应力状态下混凝土徐变应力-应变增量本构模型。依据徐变叠加原理,考虑Poisson效应,进一步将单轴应力状态下混凝土徐变应力-应变增量本构模型拓展到三轴应力状态,用于钢管混凝土徐变分析。对有限元商用软件Ansys进行二次开发,将反映三轴应力状态下混凝土徐变性能的本构方程引入Ansys提供的用户子程序Usermat中,并采用Fortran语言编程,从而实现了钢管混凝土徐变长期性能的有限元分析计算。将有限元数值解与试验结果进行对比分析,发现该文提出的模型是科学的和有效的。该文提出的方法将为混凝土徐变计算提供了另一条有效途径。     

裂纹面动摩擦作用对脆性材料动力破坏的影响EI北大核心CSCD

基于岩石类材料的I型裂纹模型,提出了一种考虑裂纹密度、裂纹相互作用以及裂纹面动摩擦作用的脆性材料动力模型。以正方形阵列分布的裂纹为例,定量分析了不同裂纹密度及不同摩擦行为对试件的裂纹扩展过程、试件受力和破坏的影响。数值计算结果表明:随着裂纹密度增大,裂纹间的相互作用增强,试件破坏时的加载应力降低,惯性效应引起试件轴向附加应力增大。裂纹面的滑动会降低裂纹面的动摩擦系数,促进裂纹发展,并降低试件的强度。相对于常数摩擦系数,考虑速度及状态依赖型摩擦模型对裂纹面的滑动过程更为合理。动强度因子对比结果显示出试件明显的应变率效应和尺寸效应。