分区破裂相似材料分层观测试验研究

以沙土作为相似材料,观测了圆桶模型内部裂纹的分布规律及沿轴向的变化规律。研究表明:在若干沙土层间预制界面,然后将各层沙土依次从铁桶中压出的实验方法可以更深入地观测硐室围岩裂纹沿轴向的变化规律。巴西圆盘模型不能解释分区破裂现象,而用空间共轭剪切带解释则更加合理。支护对分区破裂有一定的影响。     

Effect of confining pressure on deformation and failure of rock at higher strain rate

Influence of confining pressure from 0 to 28 MPa, which acts on the two lateral edges of rock specimen in plane strain compression, on the shear failure processes and patterns as well as on the macroscopically mechanical responses were numerically modeled by use of FLAC. A material imperfection with lower strength in comparison with the intact rock, which is close to the lower-left corner of the specimen, was prescribed. In elastic stage, the adopted constitutive relation of rock was linear elastic; in strain-softening stage, a composite Mohr-Coulomb criterion with tension cut-off and a post-peak linear constitutive relation were adopted. The numerical results show that with an increase of confining pressure the peak strength of axial stress-axial strain curve and the corresponding axial strain linearly increase; the residual strength and the stress drop from the peak strength to the residual strength increase; the failure modes of rock transform form the multiple shear bands close to the loading end of the specimen （confining pressure=0-0.1 MPa）, to the conjugate shear bands （0.5-2.0 MPa）, and then to the single shear band （4-28 MPa）. Once the tip of the band reaches the loading end of the specimen, the direction of the band changes so that the reflection of the band occurs. At higher confining pressure, the new-formed shear band does not intersect the imperfection, bringing extreme difficulties in prediction of the failure of rock structure, such as rock burst. The present results enhance the understanding of the shear failure processes and patterns of rock specimen in higher confining pressure and higher loading strain rate.     

内摩擦角对逐步开挖拱形巷道围岩分区破裂化的影响

巷道围岩分区破裂化(或分区破碎化、分区碎裂化)是指在深部开挖巷道时围岩交替呈现破裂区和未破裂区的现象,该现象与对围岩破坏的传统认识不同。正确认识不同强度围岩分区破裂化的发育程度,可为其治理及应用奠定基础。巷道掘进会引起掘进工作面附近岩石涌向巷道,这将导致掘进工作面附近垂直于巷道轴线平面的应力状态并非平面应变,而是三维状态。采用以FLAC~(3D)应变软化模型为基础发展的非均质应变软化模型(通过引入Weibull分布函数实现),研究了内摩擦角对逐步开挖拱形巷道围岩分区破裂化的影响,考察了围岩中剪切应变增量、最小主应力及最大主应力在巷道即将贯穿模型时的分布规律。研究发现:①内摩擦角越小,围岩的破坏越严重,剪切带的发育越充分,分区破裂化越明显;②并非围岩的破坏就会导致分区破裂化,围岩破坏到一定程度后,才会出现分区破裂化;③分区破裂化的机理为:在载荷及开挖作用下,在过巷道轴线的平面上,随机分布的缺陷发展成(共轭)剪切带,从而在垂直于巷道轴线的平面上,形成多个相间隔的圆环形的破坏区。本文过巷道轴线平面上的结果与室内实验结果在定性上一致。根据本文的研究,在垂直于巷道轴线的不同平面上,分区破裂化不同,这意味着不必选择统一的巷道支护参数,例如,在分区破裂化显著的部位,延长锚杆的长度,反之则不然。     

辽宁工程技术大学，辽宁 阜新 123000

提出了利用梯度塑性理论计算Ti-6Al-4V绝热剪切带的局部剪切应变新方法.绝热剪切带的最大局部塑性剪切应变依赖于临界塑性剪切应变、试样的标定长度、绝热剪切带总厚度、绝热剪切带的平均塑性剪切应变.计算表明,随着绝热剪切带总厚度的增加,绝热剪切带的最大局部塑性剪切应变以非线性方式下降.当绝热剪切带总厚度的取值接近1 mm时,尽管确定临界塑性剪切应变的方法不同,但是,绝热剪切带的最大局部塑性剪切应变的计算值差别很小.当绝热剪切带总厚度取值在0.335～1 mm之间时,绝热剪切带的最大局部塑性剪切应变的计算值位于Liao及Duffy(1998)实验结果的下限(75%)和上限(350%)之间.     

钛合金绝热剪切带的局部剪切应变估算的新方法

提出了利用梯度塑性理论计算Ti-6Al-4V绝热剪切带的局部剪切应变新方法.绝热剪切带的最大局部塑性剪切应变依赖于临界塑性剪切应变、试样的标定长度、绝热剪切带总厚度、绝热剪切带的平均塑性剪切应变.计算表明,随着绝热剪切带总厚度的增加,绝热剪切带的最大局部塑性剪切应变以非线性方式下降.当绝热剪切带总厚度的取值接近1 mm时,尽管确定临界塑性剪切应变的方法不同,但是,绝热剪切带的最大局部塑性剪切应变的计算值差别很小.当绝热剪切带总厚度取值在0.335～1 mm之间时,绝热剪切带的最大局部塑性剪切应变的计算值位于Liao及Duffy(1998)实验结果的下限(75%)和上限(350%)之间.     

Analyses of axial,lateral and circumferential deformations of rock specimen in triaxial compression

The axial, lateral and circumferential strains were analyzed for a rock specimen subjected to shear failure in the form of a shear band bisecting the specimen in triaxial compression. Plastic deformation of the specimen stemmed from shear strain localization initiated at the peak shear stress. Beyond the onset of strain localization, the axial, lateral and circumferential strains were decomposed into two parts, respectively. One is the elastic strain described by general Hooke’s law. The other is attributable to the plastic shear slips along shear band with a certain thickness dependent on the internal length of rock. The post-peak circumferential strain-axial strain curve of longer specimen is steeper than that of shorter specimen, as is consistent with the previous experiments. In elastic stage, the circumferential strain-axial strain curve exhibits nonlinear characteristic, as is in agreement with the previous experiment since confining pressure is loaded progressively until a certain value is reached. When the confining pressure is loaded completely, the circumferential strain-axial strain curve is linear in elastic and strain-softening stages. The predicted circumferential strain-axial strain curve in elastic and strain-softening stages agrees with the previous experiment. Supported by National Natural Science Foundation of China(50309004)     

考虑应变梯度效应的三点弯梁模型解析研究--第一部分:局部化带的传播

为分析应变软化和由此带来的应变局部化问题,将梯度塑性理论引入裂纹带模型。以拉应变局部化区域代替裂纹带,在三点弯梁裂纹带(具有一定尺寸的带宽由特征长度确定)内部存在着不均匀分布的拉应变,这与实验结果相符。对拉应变进行积分,得到了拉应变局部化区域的张拉位移的理论表达式,结果表明:该位移与拉应力成线性规律,拉应变局部化区域的宽度越大,弹性模量越小或降模量越小,则该位移越大。此外,采用应力平衡条件、应变软化的本构关系及平截面假定,还得到了拉应变局部化区域的扩展规律,结果表明:下降模量越大、三点弯梁高度越小及弹性模量越小,则在相同的拉应力的情况下,拉应变局部化区扩展的长度越小;抗拉强度对拉应变局部化区扩展长度的最大值没有影响。此外,还研究了梁中部横截面内中性轴到具有最大承载能力的点的距离的变化规律。     

孔隙压力条件下圆形巷道围岩的应变局部化数值模拟

利用FLAC模拟了不同孔隙压力及围压条件下圆形巷道围岩的应变局部化过程。在计算中,采用了应变软化本构关系及"先加载、后挖洞"的方式。模拟结果表明,当围压增加到一定程度后,孔隙压力较低或等于零时,在围岩中出现了V形坑或"平底锅"形坑,这与开采深度达到一定后,无瓦斯或低瓦斯矿井中的岩爆现象类似;当围压增加到一定程度后,孔隙压力较高时,在围岩中出现了多条平行裂纹及多个不规则的狭长薄片,破坏区的外轮廓类似梨形,这与开采深度达到一定后,在高瓦斯矿井中煤和瓦斯突出后形成口小腔大的孔洞现象类似。     

三维岩样单轴压缩自由面剪应变率及位移分布数值模拟

以往FLAC对岩样变形、破坏进行数值模拟主要是针对平面应变二维问题,离三维岩样单轴压缩的试验条件还有不少差距。因此,本文采用FLAC-3D研究了三维岩样在单轴压缩及强烈端面约束条件下,自由面的剪切应变率、离面位移及面内位移的分布及演变规律,研究了自由面垂直对称轴上定点位移随时步的演变规律。在应力峰值之前及之后,本构模型分别取为线弹性及莫尔–库仑剪破坏与拉破坏复合的应变软化模型。计算表明:在应力–时步曲线的应力峰值之前,自由面上的剪切应变率由均匀向不均匀分布转变;在应变软化阶段,试样的变形由对称性向非对称性转变。通过分析各个自由面的剪切带图案发现,在试样内部形成了两个空间剪切带,其中一个更占优势。在试样变形的对称性丧失之前,三维离面及面内位移曲面是光滑的、平坦的;在对称性丧失之后,三维曲面已变得凹凸不平了。在剪切带的位置,面内位移有显著的改变。自由面垂直对称轴上的离面位移在应力–时步曲线应力峰值之前发生了分离,而水平及垂直位移–时步曲线转折于应力峰值稍后或应变软化阶段。从离面位移易于识别出试样破坏的前兆。此外,在应变软化阶段,随着时步的增加,还观测到了离面位移的三种不同的变化规律:基本保持不变、增加及降低(反弹)。     

基于梯度塑性理论的岩样单轴压缩扩容分析

采用梯度塑性理论，对岩样剪应变局部化引起的扩容进行了理论分析。假设岩石的剪切本构关系为弹性-应变软化双线性，局部化启动于应力峰值强度，利用局部塑性剪应变与局部塑性体积应变的线性关系，得到了局部塑性体积应变、局部塑性体积增量及剪胀引起的剪切带总塑性体积增量的解析式，这体现了该理论在研究剪胀问题时的优越性。另外，还得到了弹性阶段及应变软化阶段的轴向应力-体积应变曲线的理论关系。塑性体积应变是专指由剪切带剪胀而引起的，因而，轴向应力．体积应变不具有尺寸效应，与局部化带的尺寸无关，但扩容角、剪切降模量及泊松比却对该曲线有重要影响。在弹性阶段及应变软化阶段轴向应力-体积应变均呈线性。在相同的应力水平下，扩容角越大则剪胀程度越大；剪切降模量越大，剪胀程度越小。在应变软化阶段，泊松比不影响塑性体积应变。