热损伤花岗岩内细观裂隙分布非均匀性的研究

为认识花岗岩的细观热损伤特性,首先基于CT(Computed Tomography,计算机层析扫描)技术获取高温热处理后花岗岩横截面上的细观裂隙分布图,然后在细观裂隙分布图上选择分析区域并设置参考线,随后计算不同参考线上的细观裂隙密度,并采用细观裂隙密度的变异系数对细观裂隙分布非均匀性进行定量描述,最后讨论变异系数随参考线方向改变的变化规律。结果表明：不同参考线上的细观裂隙密度不同,细观裂隙密度的变异系数大于0,细观裂隙呈现非均匀分布。随着参考线方向改变,细观裂隙密度的变异系数明显改变,变异系数最大值是40%,最小值是25%,热损伤花岗岩横截面上的细观裂隙分布非均匀性呈现各向异性。     

含裂隙材料的空洞化损伤

本文研究了含有微小裂隙的韧性材料中细观损伤的演化。随外加应力的增大,在裂隙周围的基体中微小空洞不断地萌生并扩展。在一些情况下,由此而形成的内部损伤与仅有大小空洞的损伤有显著的不同特点。结果还表明,具有细观尺度的短裂纹,其损伤作用不宜用裂纹长度作标志。文中最后提出一个材料韧性断裂的判据。     

基于孔与裂隙网络模型的平行微裂隙对驱油的影响规律研究

认识双重多孔介质中油水两相微观渗流机制是回答形成什么类型的裂隙网络可提高油藏采收率的关键. 微裂隙的分布可以提高多孔介质的绝对渗透率,但对于基质孔隙中的流体介质,微裂隙的存在会引起多孔介质中局部流体压力和流场的变化,导致局部流动以微裂隙流动为主,甚至出现窜流现象,降低驱油效率. 本文基于孔与裂隙双重网络模型,在网络进口设定两条平行等长且具有一定间隔的微裂隙,分析微裂隙的相对间隔(微裂隙之间距离/喉道长度)和微裂隙相对长度(微裂隙长度/喉道长度)对于微观渗流特征的影响. 结果表明：随微裂隙相对长度的增加,出现驱油效率逐渐降低,相对渗透率曲线中的油水共渗区水饱和度和等渗点增加,油水两相的共渗范围减小等现象;随着微裂隙之间相对间隔增大,周围越来越多的基质孔穴间的压力差减小,在毛管压力的限制下,驱替相绕过这些区域,而导致水窜现象.     

冷却方式对高温状态花岗岩裂隙分布特点的影响

火灾后地铁隧道围岩发生破坏,特别是在消防射水冷却后其性能劣化更为显著。为了深入研究冷却方式对高温状态岩石裂隙分布特点的影响,本文首先将花岗岩试样加热至800℃,并分别采用自然冷却和水冷却两种方式降温,然后采用CT(Computed Tomography Technique,计算机层析扫描技术)观察岩石内部裂隙的分布,并对裂隙进行重构,接着结合经纬分析法确定最优经纬线划分密度,最后研究冷却方式对岩石内部裂隙分布的非均匀性和各向异性的影响。结果表明:(1)网格间距为0.922mm时,试验结果不再随着网格间距的减小而变化;(2)花岗岩800℃时水冷却后,在经向和纬向上的裂隙分布比自然冷却后在经向和纬向上的裂隙分布更趋近于均匀性;(3)花岗岩800℃时水冷却后的裂隙分布比自然冷却后的裂隙分布更趋近于各向同性。     

片岩损伤裂隙系统生成及裂隙率间接定量表征方法研究

在岩石试验机上对标准片岩试件进行轴向预压,使试件内部微裂隙自由扩展、连通,以模拟工程扰动形成的损伤裂隙系统．采用纵波波速间接定量表征片岩试件的损伤,从而得到含有一定损伤裂隙系统的标准片岩试件,用于研究经受一定损伤作用后岩石的物理、力学等特性．对预压后的试样进行冻融循环试验,并对不同冻融损伤状态下的试件进行超声波测试和单轴压缩。经非线性回归分析发现,纵波波速和单轴抗压强度之间相关性显著．研究方法和成果对于岩石损伤的定量表征和含一定损伤岩石试件的制备具有指导意义．     

基于吸力量测确定膨胀土活动带和裂隙深度

采用M itchell公式和裂隙扩展深度方程两种吸力法确定安康地区膨胀土大气影响深度和裂隙开展深度。其一通过对安康地区两处天然边坡开挖观测井,利用张力计进行不同深度处吸力值的现场量测,根据M itchell提出公式计算大气影响深度;其二根据非饱和土抗拉强度公式,建立膨胀土裂隙扩展深度方程,利用基质吸力量测结果求其理论解。结果表明,安康地区膨胀土吸力变化曲线随深度增加变幅减小,呈“波浪式”推移。M itchell公式确定安康地区膨胀土的大气影响深度为3.35m以内,裂隙深度方程确定裂隙开展深度为3.06³.14m。利用M itchell公式计算大气影响深度与膨胀土断裂理论公式确定的裂隙开展深度结果接近。     

温度对灰砂岩物理特征及抗拉强度影响的试验研究

为研究温度对灰砂岩物理特征及抗拉强度的影响,利用对径压缩试验对经历不同温度的灰砂岩圆盘试件的力学特性进行分析,结合动态信号测试分析系统实时监测并记录试件中部侧向应变,采用非金属超声检测分析仪、SEM等手段对经历不同温度的灰砂岩纵波波速、微观结构等特征进行表征。试验结果表明:1高温导致灰砂岩质量与纵波波速分别降低了3.83%与50.03%,损伤程度逐渐增大,且在经历温度为600℃时发生突变;2灰砂岩抗拉强度随经历温度的升高而减小,近似服从负线性分布,峰值压缩变形量与峰值侧向应变均随经历温度的升高而增大,在经历温度为600℃时发生突变;3高温导致试件颜色由灰白色变为淡黄色,断面起伏度降低,裂隙数量与裂隙类型增加。上述研究成果可以为热作用下地下结构稳定性的研究提供参考。     

大理岩微裂隙与其力学性质间的关系

香港元朗地区发育有大量的大理岩, 它的力学性质直接影响该区的建筑安全。我们采用超声测量技术、单轴压缩试验和岩石薄片的镜下观测等方法了解其内部的结构变化及其力学性质。得出的结论是：该区的大理岩力学性质变化很大。一般的说, 致密的黑色大理岩超声波速度高, 抗压强度大且具有脆性特征, P波速度6.2～6.5km·s-1, S波速度3.0～3.60km·s-1, 抗压强度在88～166MPa;而那些颗粒较粗的白色或灰色大理岩, 超声波速度变化很大, 抗压强度小且具有韧性特征, P波速度在4.2～6.9km·s-1, S波速度2.4～3.20km·s-1, 抗压强度在52～63MPa之间。P波速度与裂隙密度成反比关系。抗压强度与裂隙密度成线性反比关系, 而且对脆性大理岩, 其曲线斜率要比韧性大理岩的曲线斜率大出许多倍。     

含夹杂和微裂纹复合材料的损伤演化和分析

利用细观力学的Ｅｓｈｅｌｂｙ和Ｍｏｒｉ－Ｔａｎａｋａ理论，考虑纤维和微裂纹之间的相互作用，研究了定向分布微裂纹的演化规律及其对材料力学性能的影响，分析了纤维体积份数，弹性系数、微裂纹密度，纤维不同取向与基体开裂强度之间的变化关系，并给出了许多有意义的结论。     

单轴循环冲击下弱风化花岗岩的损伤演化

为研究爆破应力波作用下弱风化花岗岩的力学特性和损伤演化机理,利用直径50 mm的改进分离式Hopkinson压杆装置,开展以不同速度对花岗岩进行单次和等速循环冲击下的实验研究。研究结果表明：单次冲击中,用能量法确定的损伤阈值,可用于循环冲击实验中;不同应变率下弱风化岩石裂纹扩展阶段存在应力松弛平台,且随应变率升高而愈发明显,峰值应力与应变率呈正相关。等速循环冲击中,最大应力、应变与冲击速度呈正相关,与岩样累积冲击总次数呈负相关;损伤演化具有3个阶段呈倒S形,由其构建的双参数损伤演化模型拟合效果理想,且具有物理意义;利用模型中的参数α和β可计算中值点处的损伤度和相对循环次数,且与冲击速度正相关;不同损伤变量计算的损伤演化模型不同,合理定义损伤变量是必要的。

     

含微裂纹材料的损伤理论

本文从含微裂纹材料的变形能出发引出了裂纹的方位张量。在考虑裂纹受压闭合与滑动摩擦的基础上,给出了损伤张量、损伤应变及有效弹性常数。文中给出了损伤机构离散化的方法,并对方位密度给出了演化方程。最后给出一个单向拉压的应力应变关系例子,并揭示了裂纹扩展时的应力突跌现象。     

Coupled modeling of damage growth and permeability variation in brittle rocks

This paper presents a coupled model for anisotropic damage and permeability evolution by using a micro–macro approach. The damage state is represented by a second order tensor. The evolution of damage is determined from a crack propagation criterion. The free enthalpy function of cracked material is obtained by using micromechanical considerations. It is assumed that cracks exhibit normal aperture which is coupled with the crack growth due to asperities of crack faces. By using Darcy’s law for macroscopic fluid flow and assuming laminar flow in microcracks, the overall permeability of the RVE is obtained by a volume averaging procedure taking into account crack aperture in each orientation.     

热开裂损伤对花岗岩变形及破坏特性的影响

本文研究了20℃到600℃不同温度热处理对Stripa花岗岩变形及破坏特性的影响。结果表明,力学性质特征量随热处理温度升高有些非寻常变化。文章还指出可以用声速比来评价岩石的质量。     

Mechanical behaviors and damage constitutive model of ceramics under shock compression

One-stage light gas gun was utilized to study the dynamic mechanical properties of AD90 alumina subjected to the shock loading. Manganin gauges were adopted to obtain the stress-time histories. The velocity interferometer system for any reflector （VISAR） was used to obtain the free surface velocity profile and determine the Hugoniot elastic limit. The Hugoniot curves were fitted with the experimental data. From Hugoniot curves the compressive behaviors of AD90 alumina were found to change typically from elastic to ＂plastic＂. The dynamic mechanical behaviors for alumina under impact loadings were analyzed by using the path line principle of Lagrange analysis, including the nonlinear characteristics, the strain rate dependence, the dispersion and declination of shock wave in the material. A damage model applicable to ceramics subjected to dynamic compressive loading has been developed. The model was based on the damage micromechanics and wing crack nucleation and growth. The effects of parameters of both the micro-cracks nucleation and the initial crack size on the dynamic fracture strength were discussed. The results of the dynamic damage evolution model were compared with the experimental results and a good agreement was found.     

Effective bending stiffness for plates with microcracks

Summary In this paper, micromechanics methods are applied to characterize the damage of plate structures, both Love-Kirchhoff and Reissner-Mindlin plates, due to microcrack distribution. Analytical expressions for effective stiffness of a damaged plate with distributed microcracks are derived for the first time. The results are compared with the results based on continuum damage theory, and it is found that there are significant differences between the two. It is well known that constitutive relations at the structural level, e.g. curvature/moment relation, and shear/transverse strain relation, are fundamentally different from the constitutive relation at the material level, i.e. stress/strain relations. This is because a priori kinematic assumptions in engineering structural theories pose additional constitutive constraints on the relationships between stress resultant/couple and strain measures. The newly derived effective stiffness formulae for various plates reflect such constitutive constraints, and therefore are consistent with engineering plate theories. They provide an alternative means in structure designs and structure damage evaluations.The work is supported by an Junior Faculty Research Fund (BURNL-07427-11503-EGSLI) to Professor Shaofan Li provided by the senate committee on research of University of California at Berkeley. The authors would like to thank the referees of AAM helpful comments and suggestions.     

Influence of lateral confinement on dynamic damage evolution during uniaxial compressive response of brittle solids

A dynamic damage growth model applicable to brittle solids subjected to biaxial compressive loading is developed. The model incorporates a dynamic fracture criterion based on wing-crack growth model with a damage evolution theory based on a distribution of pre-existing microcracks in a solid. Influences of lateral confinement pressure (dynamic or static) as well as frictional coefficient on the rate dependence of fracture strength of basalt-rock are investigated systematically. It is found that the failure strength, damage accumulation and wing-crack growth rate are strongly influenced by the nature and the magnitude of confinement pressure. It is also verified that the effect of strain rate on fracture strength of brittle solids is independent of confinement pressure in a certain range of strain rate.     

损伤复合材料结构可靠性分析模型的初步探讨

大量实验研究表明,复合材料损伤容限设计主要是一个静力问题,基于此,本文用履盖的方法对局部损伤进行工程化处理,研究了固有损伤和冲击损伤对复合材料性能的影响;然后将这一处理方法引入到复合材料的可靠性分析中去,得到了带损伤的复合材料结构的可靠性分析模型,最后用两个算例检验了本文提出的方法,得出的结论与定性分析相同,对工程设计可以提供定量的指导。     

Development and Evaluation of the Quadrant Ring Test Method

Testing of ring-shaped specimens often is required for determining the hoop-direction mechanical properties of cylindrical composite structures. A quadrant ring test method was developed in an effort to produce a relatively uniform stress distribution in the ring specimen using a conventional tensile testing machine. Finite element analysis results indicated that the four-sector quadrant test is capable of producing a more uniform stress distribution in the specimen than the split-disk test. However, similar tensile strengths were obtained using the quadrant and split-disk tests, both significantly below those obtained from tensile testing of flat specimens. The lack of improvement in tensile strength produced by the quadrant test was caused by small rotations of the fixture quadrants during loading, producing greater peak stresses than for the ideal case of radially displaced sectors. These results suggest that test fixturing that produces a true outward radial displacement of the fixture sectors is required to produce accurate tensile strengths.     

Measuring maximum tensile strength of liquids at low stressing rates

A technique is presented for determining the maximum tensile strength of a given fluid through the analysis of the break-up at the end of a capillary-thinning experiment. This technique allows the characterisation of the tensile strength, an important parameter for understanding cavitation, of a fluid at lower stressing rates than previous methods, such as bullet-piston apparatus. The method was validated by tests on a range of concentrations and molecular weights of polyethylene glycol, comparing the results with the values and behaviours observed from the bullet piston apparatus. Excellent agreement was observed between the two techniques, with quantitative differences corresponding to the differences in stressing rates. The results from the capillary break-up experiments were also used to investigate the effect of concentration and molecular-weight on tensile strength.     

冲击载荷下岩石的损伤特性分析

把岩石类脆性材料受冲击波加载造成的损伤与弹性纵波速度的变化联系起来,定义了损伤度Ｄ,并且以辉长岩（Ｇａｂｂｒｏ）和石灰岩（Ｌｉｒｎｅｓｔｏｎｅ）为例,测量了冲击波加载后靶体的损伤度分布。同时对冲击加载后含损伤的辉长岩,测量了一维准静压下弹性纵波速度的变化和抗压强度,分析了含损伤材料的静力学行为。