准脆性材料试件应变软化尺度效应理论研究

研究了由于剪切局部化而引起的试件长度的尺寸效应，基于可以考虑微结构相互作用的非局部理论，得到了非局部塑性剪应变与局部塑性剪应变及其二阶应变梯度的关系，通过获得剪切带内部的塑性剪切应变，得到了岩样轴向的平均应变与位移的理论关系，研究结果表明，这一关系具有尺寸效应，随着试件高度的增加，应力-应变曲线变陡，当试件高度非常大时，发生Ⅱ类变形行为，而且，随着剪切带倾角的增加，应力-应变曲线也变陡，将理论结果与前人的试验结果进行了比较，结果表明吻合良好。     

Ti-6Al-4V绝热剪切带的厚度及应变率效应研究

通过引入与Batra及Kim类似的论点,将绝热剪切带宽度定义为绝热剪切带的中心区域的宽度(w5%),在该区域上温度比其峰值小5%,利用Johnson-Cook模型及梯度塑性理论分析Ti-6Al-4V绝热剪切带的厚度及应变率的影响.计算表明,在名义应变率的下限(800s-1)及上限(1400s-1)之间,当绝热剪切带的总厚度选取为0.3235及0.0705 mm时,计算结果非常接近于绝热剪切带宽度的上限(55 um)及下限(12 um).当应变率较低时,绝热剪切带较宽.随着应变率的增加,绝热剪切带宽度快速降低.在高应变率时,绝热剪切带宽度基本保持恒定.该理论结果与Dodd及Bai的理论结果有类似之处,与Weerasooriya及Beaulieu针对钨合金的实验结果非常一致,与Klepaczko及Rczaig的数值结果的前半部分相似.     

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.     

Entire deformational characteristics and strain localization of jointed rock specimen in plane strain compression

Shear band （SB）, axial, lateral and volumetric strains as well as Poisson＇s ratio of anisotropic jointed rock specimen （JRS） were modeled by Fast Lagrangian Analysis of Continua （FLAC）. Failure criterion of rock was a composited Mohr-Coulomb criterion with tension cut-off. An inclined joint was treated as square elements of ideal plastic material beyond the peak strength. Several FISH functions were written to automatically find the addresses of elements in the joint and to calculate the entire deformational characteristics of plane strain JRS. The results show that for moderate joint inclination （JI） , strain is only concentrated into the joint governing the behavior of JRS, leading to ideal plastic responses in axial and lateral directions. For higher JI, the post-peak stress-axial and lateral strain curves become steeper as JI increases owing to the increase of new SB＇s length. Lateral expansion and precursor to the unstable failure are the most apparent, resulting in the highest Poisson＇s ratio and even negative volumetric strain. For lower JI, the entire post-peak deformational characteristics are independent of JI. The lowest lateral expansion occurs, leading to the lowest Poisson＇s ratio and positive volumetric strain all along. The present prediction on anisotropic strength in plane strain compression qualitatively agrees with the results in triaxial tests of rocks. The JI calculated by Jaeger＇s formula overestimates that related to the minimum strength. Advantages of the present numerical model over the Jaeger＇s model are pointed out.     

井眼剪切带分叉研究

井眼基本破坏形式是由于不稳定性造成的井眼岩石崩落坍塌和剪切带。井眼破坏的非均匀性采用传统的理论和方法不能得到较好的解释和预测。采用材料的分叉分析方法和变形局部化的数值模拟研究 ,可望对井眼失效的机理分析、预测和控制提供新的思路     

Localized shear deformation during shear band propagation in titanium considering interactions among microstructures

Closed-form analytical solutions of plastic shear strain and relative plastic shear displacement during shear band propagation are proposed under dynamic loadings based on gradient-dependent plasticity considering the effect of microstructures due to heterogeneous texture of Ti. According to the differences in shear stress levels, Ti specimen is divided into three regions: residual region, strain-softening region and elastic region. Well-developed shear band is formed in the residual region and the relative plastic shear displacement no longer increases. In the normal and tangential directions, the plastic strain and the displacement are nonuniform in the strain-softening region.At the tip of shear band, the shear stress acting on the band is increased to shear strength from the elastic state and the shear localization just occurs. Prior to the tip, Ti remains elastic. At higher strain rates, the extent of plastic strain concentration is greater than that under static loading. Higher strain rate increases the relative plastic shear displacement. The present analytical solution for evolution or propagation of shear localization under nonuniform shear stress can better reproduce the observed localized characteristics for many kinds of ductile metals.     

单向压缩条件下圆形颗粒体含孔洞试样的力学模拟

通过编程,用连续介质快速拉格朗日分析(FLAC)实现了圆形颗粒体在单向压缩条件下的力学行为模拟.颗粒体试样中间含一圆形孔洞,颗粒体被离散成正方形网格,若2个颗粒体发生接触,则需要在颗粒体之间预置界面.研究了施加的荷载对试样应力线、应变线及受拉破坏区域等的影响.结果表明,试样内部的最大压应力约为所施加压应力值的5倍,这有别于连续介质理论的结果.孔洞顶部及底部各有1个三角形受拉区,内部可见明显的拉裂纹,裂纹的长度随试样端部所施加应力的增加而增大.     

初始内聚力及摩擦角对岩样全部变形特征的影响

目的研究初始内聚力及摩擦角对剪切带图案及岩样全部变形特征的影响.方法采用FLAC内嵌语言FISH编制了计算平面应变压缩岩样轴向、侧向、体积应变及泊松比的FISH函数.在峰值强度之前及之后,岩石的本构模型分别取为线弹性模型及莫尔库仑剪破坏与拉破坏复合的应变软化模型.结果随着初始内聚力及摩擦角的增加,破坏模式由复杂向简单剪切破坏转变;发生破坏的单元数目降低;剪切带倾角增加;轴向应力的峰值强度及所对应的轴向、侧向应变的绝对值降低,岩样可以达到的最小体积增加.峰后的轴向应力-轴向应变曲线、轴向应力-侧向应变曲线、侧向应变-轴向应变曲线、泊松比-轴向应变曲线及体积应变-轴向应变曲线的斜率几乎不受初始内聚力及摩擦角的影响.但是,当初始内聚力及摩擦角最小时,5种峰后曲线略显平缓,这是由于发生破坏的单元数目最多及剪切带的倾角最低.结论剪切带倾角更接近Arthur倾角,说明了数值结果是可信的.岩样失稳破坏的前兆不随着初始内聚力及摩擦角的降低或增加而改变.     

断层岩爆是应变局部化导致的系统失稳回跳

讨论了应变局部化、岩爆及Ⅱ类变形行为的关系，并利用梯度塑性理论得到了4种等效的回跳准则。局部化是岩爆的前兆之一，也是出现Ⅱ类变形行为的原因。利用最小势能原理及梯度塑性理论，可得到系统的失稳判据；利用位移法，可以得到系统的Ⅱ类变形行为。若非弹性剪切位移(或平均塑性剪切应变)的增加快于弹性剪切位移(或平均弹性剪切应变)的降低，将出现Ⅰ类变形行为；反之，将出现Ⅱ类变形行为。弹性区与剪切带宽度的比率越大，或剪切弹性模量与剪切降模量的比率越小，系统越容易失稳。     

剪切带内部应变(率)分析及基于能量准则的失稳判据

应用应变梯度塑性理论对局部化剪切带内部(塑性)剪应变(率)规律进行了理论分析.研究了剪切降模量及岩石材料内部长度等参数对剪切带内部应变(率)的影响.推导了剪应力(率)与剪切带相对错距(速度)的本构关系.研究了剪切降模量和岩石材料内部长度对剪切带稳定性的影响.将岩石试件直剪试验试验机简化为钢块,采用能量准则对岩石试件(剪切带)及钢块系统的稳定性进行了理论研究,提出了系统失稳判据.研究表明;岩石材料的剪切降模量越大,岩石材料的内部长度越小,试验机的剪切刚度越小及试验机的等效高度越大剪切带--钢块系统越容易失稳.