三维应力状态下砂的临界状态本构模型分叉分析

就服从非关联流动法则软化型砂的临界状态本构模型,在不同应力路径下的分叉进行了理论和数值分析。理论分析表明：分叉现象强烈依赖于应力路径,即当平均应力p一定时,应力路径在洛德角在-25o～15o之间时,均会在应力-应变关系硬化阶段出现分叉现象,而其他应力路径下不会产生分叉。并且分叉时对应的应力比、主应变及剪切带倾角都随洛德角的增加而变化,其值是先增加而后减小。采用回映应力更新算法,编写了该本构模型材料子程序,借助有限元软件ABAQUS及材料子程序,通过数值计算方法预测到了分叉点所对应的应力状态,表明了分叉现象在数值计算中的存在性,并通过数值预测值和理论解的对比,两者结果基本一致。     

Numerical study on the bifurcation of the North Equatorial Current

A 1.5-layer reduced-gravity model forced by wind stress is used to study the bifurcations of the North Equatorial Current(NEC).The authors found that after removing the Ekman drift,the modelled circulations can serve well as a proxy of the SODA circulations on the σθ=25.0 kg m～-3 potential density surface based on available long-term reanalysis wind stress data.The modelled results show that the location of the western boundary bifurcation of the NEC depends on both zonal averaged and local zero wind stress curl latitude.The effects of the anomalous wind stress curl added in different areas are also investigated and it is found that they can change the strength of the Mindanao Eddy(ME),and then influence the interior pathway.     

Low-frequency variability of the North Equatorial Current bifurcation in the past 40 years from SODA

The low-frequency variability of the North Equatorial Current (NEC) bifurcation during 1958 to 2001 was investigated with the Simple Ocean Data Assimilation (SODA) 2.0.2 dataset.In agreement with recent observations,the NEC bifurcation latitude (NBL) shifted northward as depth increases, from about 12.7°N near the surface to about 17.1°N at depths around 500 m for the annual average. This study reveals that the interannual variations of NBL,with five years period,mainly focused on the upper 500 m with amplitude increasing as depth increased.The NBL shifted southward in the past 40 years,which was more significant in the subsurface at more than -0.02°/a.The NBL manifests itself in the transports of NMK (NEC-Mindanao Current (MC)-Kuroshio) system in strong relationship with MC (0.7) and Kuroshio (-0.7).The EOF analysis of meridional velocity off the Philippine coast shows that the first mode,explaining 62% of variance and 5 years period,was highly correlated with the southward shift of NBL with coefficient at about 0.75.The southward shift of NBL consists with the weakening of MC and strengthening of Kuroshio,which exhibited linear trends at -0.24Sv/a and 0.11Sv/a.Both interannual variation and trend of NBL were closely related to the variation of NMK system.     

含液各向异性多孔介质应变局部化分析

工程中的含液多孔介质如饱和或非饱和岩土材料往往具有各向异性特性。采用Rudnicki建立的针对岩土材料的各向异性本构模型,对轴对称压缩试验中的含液多孔介质骨架的各向异性力学行为进行了分析;基于不连续分叉理论,导出了静态非渗流条件下处于轴对称应力状态的含液多孔介质应变局部化发生的临界模量、剪切带方向以及不连续速度矢量的显式表达式,在此基础上计算并讨论了材料参数变化和孔隙液体存在对各向异性多孔介质应变局部化的影响。     

Transient bifurcation condition of partially saturated porous media at finite strain

Bifurcation of unsaturated soils into a localized shear band is a ubiquitous failure mode of partially saturated soils. The density and degree of saturation have major impacts on the inception of localized deformations in unsaturated soils. Unsaturated fluid flow may dramatically change the density and degree of fluid saturation of unsaturated soils. Therefore, the unsaturated fluid flow is a potential trigger for shear banding in such materials. In this paper, we derive a simplified bifurcation condition of localized deformation in unsaturated soils under the local transient condition at finite strain. This transient bifurcation condition is implemented into a nonlinear finite element code to study the inception of localized deformation in unsaturated soil specimens. Numerical simulations are conducted to study the impact of soil fabrics of density, a ‘bonding’ variable, and intrinsic permeability on the inception of localized failures via the transient bifurcation criterion. Mesh sensitivity analysis is performed to demonstrate the viscosity effect of unsaturated fluid flow on the localized deformation. Numerical simulations demonstrate that the transient bifurcation condition can detect the localized deformation triggered by the internal unsaturated fluid flow process in unsaturated soils. Copyright © 2016 John Wiley & Sons, Ltd.     

On the strength of transversely isotropic rocks

Accurate prediction of strength in rocks with distinct bedding planes requires knowledge of the bedding plane orientation relative to the load direction. Thermal softening adds complexity to the problem since it is known to have significant influence on the strength and strain localization properties of rocks. In this paper, we use a recently proposed thermoplastic constitutive model appropriate for rocks exhibiting transverse isotropy in both the elastic and plastic responses to predict their strength and strain localization properties. Recognizing that laboratory‐derived strengths can be influenced by material and geometric inhomogeneities of the rock samples, we consider both stress‐point and boundary‐value problem simulations of rock strength behavior. Both plane strain and 3D loading conditions are considered. Results of the simulations of the strength of a natural Tournemire shale and a synthetic transversely isotropic rock suggest that the mechanical model can reproduce the general U‐shaped variation of rock strength with bedding plane orientation quite well. We show that this variation could depend on many factors, including the stress loading condition (plane strain versus 3D), degree of anisotropy, temperature, shear‐induced dilation versus shear‐induced compaction, specimen imperfections, and boundary restraints.     

运用双源CTA探讨左冠状动脉分支夹角与左前降支斑块的关系

目的 运用双源CT血管成像（CTA）探讨左冠状动脉分支夹角与左前降支斑块间的潜在关系。方法 从笔者医院2016年1~5月进行冠状动脉CTA检查患者中,随机选取100例冠状动脉正常组和100例左前降支斑块组（支架置入术后复查、搭桥术后复查、心肌桥、冠状动脉起源异常患者除外）进行回顾性分析,分析结果由2名医生共同确定。100例冠状动脉正常组中,男性57例,女性43例,患者年龄为48~72岁,平均年龄57.54±8.38岁。100例左前降支斑块患者,其中男性64例,女性36例,患者年龄为41~86岁,平均年龄62.08±13.94岁。采用多平面重建（MPR）、最大密度投影（MIP）等技术测量左冠状动脉分支夹角,探讨其与左前降支斑块的关系。结果 冠状动脉正常组左冠状动脉分支夹角范围41.25°~112.14°,平均夹角69.45°±18.71°,左前降支斑块组左冠状动脉分支夹角范围54.14°~128.12°,平均夹角85.65°±15.96°,两组比较,左前降支斑块组左冠状动脉分支夹角大于冠状动脉正常组（P<0.05）。结论 双源CTA可以无创、客观评价左冠状动脉分支夹角及左前降支斑块病变,左冠状动脉分支夹角对左前降支斑块的形成具有重要影响。     

7个月周期极移的非线性动力机制

从滞弹性阻尼形变摄动造成CW频率调制假设出发，对CW的共振激发模型加上参数的时变调制，变成了参数共振模型。经正演计算发现，参数共振模型完全符合CW的实际，表明滞弹性阻尼形变摄动造成频率的3％调制，进一步使得CW振幅调制可达70％以上。这一参数共振模型是一个非线性动力系统，在非线性情况下，运动将发生分岔，即多解。     

Drained instability analysis of sand under biaxial loading using a 3D material model

The diffused and localized instabilities in sand under drained biaxial loading have been analyzed here following a plane strain bifurcation framework, where the rate independent material is defined using a generalized 3D non-associative constitutive model. This study is focused on how various instability modes emerge with respect to initial density, confining pressure, and the applied boundary conditions. Results from large deformation framework have been compared with those from small deformation approximation and the later is noticed to fail in capturing the emergence of diffused modes and predicts delayed onset of localization. The theoretical predictions compares well with existing experimental observations.