考虑剪切方向的改进Barton模型

为研究节理在剪切过程中的方向性和剪切强度规律,利用直剪仪对复制岩石节理试样进行了爬坡和顺坡两个方向的直剪试验。试验结果表明:剪切方向对岩石节理的剪切强度有明显影响,沿爬坡方向的剪切强度大于沿顺坡方向,但粗糙度系数(joint roughness coefficient,JRC)的减小和法向应力的增大可在一定程度上削弱剪切方向的影响。节理剪切破坏过程具有显著的阶段性特征,中低法向应力条件下以爬坡滑移为主要机制,而高法向应力下以相互啃食和凸起啃断为主要机制。采用平均有效倾角差指标参数描述剪切强度的各向异性,并在此基础上提出了考虑剪切方向的改进剪切强度模型,更好地预测节理正、反方向的剪切强度。通过百分位数参数建立了三维形貌指标与端部轮廓线二维高差指标的联系,大大简化了节理形貌指标的获取难度。该模型参数较少容易获取,物理意义明确,且把岩石节理剪切强度的各向异性考虑在内,具有广阔的潜在工程应用前景。

材料特性数据对成形仿真精度的影响(下)——实验方法的完善

仿真模型的预测能力与精度很大程度上取决于材料模型中所使用的实验数据的力学特性。不同的实验数据描述了材料不同的强化与塑性变形特性,也由此决定了仿真模型不同的预测特性。由于冲压件几何形状不同,产生的成形缺陷与成形极限判据也会有所不同,因此,需要针对成形件的受力特点选择最适合的仿真模型实验数据。但是,目前的实验方法不够完善,不能准确获得所需要的实验数据,或者所得的实验数据并不适用于仿真模型,也就是说,实验数据的匮乏制约了仿真模型的预测精度。根据冲压件的应力分布特点,讨论了可应用于仿真模型并能综合反映材料塑性变形特性的实验方法,在此基础上,进一步探讨了常用的简单加载实验方法的现状以及亟待解决与完善的问题,包括如何更好地获得完整的实验数据,进而通过仿真模型能够充分反映出材料在实验应力状态下与塑性变形相关的力学特性。

Cracking process of rock mass models under uniaxial compression

Anisotropic strength and deformability of the rock mass with non-persistent joints are governed by cracking process of the rock bridges. The dependence of cracking process of jointed rock masses on the two important geometrical parameters, joint orientation and joint persistence, was studied systematically by carrying out a series of uniaxial compression tests on gypsum specimens with regularly arranged multiple parallel pre-existing joints. According to crack position, mechanism and temporal sequence, seven types of crack initiations and sixteen types of crack coalescences, were identified. It was observed that both tensile cracks and shear cracks can emanate from the pre-existing joints as well as the matrix. Vertical joints were included and coplanar tensile cracks initiation and coalescence were observed accordingly. For specimen with joint inclination angle ,8=75~, it was found that collinear joints can be linked not only by coplanar shear cracks but also by mixed tensile-shear cracks, and that a pair of them can form a small rotation block. Seven failure modes, including axial cleavage, crushing, crushing and rotation of new blocks, stepped failure, stepped failure and rotation of new blocks, shear failure along a single plane and shear failure along multiple planes, were observed. These modes shift gradually in accordance with the combined variation of joint orientation and joint persistence. It is concluded that cracking process and failure modes are more strongly affected by joint orientation than by joint persistence, especially when joint inclination angle is larger than 45~. Finally, variations of macroscopic mechanical behaviors with the two geometrical parameters, such as patterns of the complete axial stress-axial strain curves, peak strength and elastic modulus, are summarized and their mechanisms are successfully explained according to their different cracking process.

A Study on Shear Wave Splitting for the Sequence of the Aftershocks of the Yao'an M_S6.5 Earthquake in Yunnan Province

The shear wave splitting study is based on data of the 3 component digital seismograms. This was recorded at 3 sets of stations, which were set up after the Yaoan M S6 5 earthquake, near its epicenter. The results indicate the following:①Shear wave splitting has been observed through analyzing 236 aftershock recordings within the shear wave window. ②The time delay was mostly in the range of 3 5～10 5ms/km and the average was 7 0ms/km.③The polarization direction of the fast split S wave was mostly in the range of N140°E～N164°E and the average was N152 4°E. ④The preferred polarization direction for the fast shear wave was different from the direction of the seismogenic structure of the mainshock (Maweijing fault) and the direction of the rupture of the aftershocks, but similar to the principal compressional axis of the regional stress field. ⑤Shear wave splitting for sequence of the aftershocks of the Yaoan earthquake was the result of anisotropy of EDA cracks controlled by stress field.

The Triggering Mechanism for the Largest Aftershock (M_S=6.3) of the Great Luhuo Event (M_S=7.6) in 1973

The Great Luhuo Event ( M S=7 6) occurred on February 6, 1973 in Sichuan Province. There were a lot of aftershocks in the source area. The largest aftershock ( M S=6 3) occurred along a normal fault zone between the Xianshuihe Fault and the Ganzi Yushu fault. Based on the focal mechanism solution, surface rupture, coseismic dislocation and seismo\|geological tectonics of the event, an elastic dislocation model of the Great Luhuo Event with a strike slip mechanism was designed and the Coulomb Failure Stress Change ( ΔCFS) in the slip direction on normal faults along and surrounding the source zone due to the event was calculated. The results showed that the largest aftershock occurred in an area with ΔCFS >0, and that ΔCFS =4 5MPa. Therefore, the largest aftershock was probably triggered by the Coulomb Failure Stress Change.

Upper mantle anisotropy of the eastern Himalayan syntaxis and surrounding regions from shear wave splitting analysis

Polarization analysis of teleseismic data has been used to determine the XKS(SKS,SKKS,and PKS)fast polarization directions and delay times between fast and slow shear waves for 59 seismic stations of both temporary and permanent broadband seismograph networks deployed in the eastern Himalayan syntaxis(EHS)and surrounding regions.The analysis employed both the grid searching method of the minimum tangential energy and stacking analysis methods to develop an image of upper mantle anisotropy in the EHS and surrounding regions using the newly obtained shear wave splitting parameters and previously published results.The fast polarization directions are oriented along a NE-SW azimuth in the EHS.However,within the surrounding regions,the fast directions show a clockwise rotation pattern around the EHS from NE-SW,to E-W,to NW-SE,and then to N-S.In the EHS and surrounding regions,the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with surficial geological features including major sutures and faults and with the surface deformation fields derived from global positioning system(GPS)data.The coincidence between structural features in the crust,surface deformation fields and mantle anisotropy suggests that the deformation in the crust and lithospheric mantle is mechanically coupled.In the EHS,the coherence between the fast directions and the NE direction of the subduction of the Indian Plate beneath the Tibetan Plateau suggests that the lithospheric deformation is caused mainly by subduction.In the regions surrounding the EHS,we speculate that a westward retreat of the Burma slab could contribute to the curved anisotropy pattern.The Tibetan Plateau is acted upon by a NE-trending force due to the subduction of the Indian Plate,and also affected by a westward drag force due to the westward retreat produced by the eastward subduction of the Burma slab.The two forces contribute to a curved lithospheric deformation that results in the alignment of the upper mantle peridotite lattice parallel to the deformation direction,and thus generates a curved pattern of fast directions around the EHS.

Notes on shear viscosity bound violation in anisotropic models

The shear viscosity bound violation in Einstein gravity for anisotropic black branes is discussed, with the aim of constraining the deviation of the shear viscosity-entropy density ratio from the shear viscosity bound using causality and thermodynamics analysis.The results show that no stringent constraints can be imposed. The diffusion bound in anisotropic plaases is also studied. Ultimately, it is concluded that shear viscosity violation always occurs in cases where the equation of motion of the metric fluctuations cannot be written in a form identical to that of the minimally coupled massless scalar fields.