Comparison between Empirical Estimation by JRC-JCS Model and Direct Shear Test for Joint Shear Strength

In order to study the reliability of the empirical estimation of joint shear strength by the JRC（joint roughness coefficient）-JCS（joint compressive strength） model,natural rock joints of dif-ferent lithologic characteristics and different sizes were selected as samples,and their shear strengths under dry and saturated conditions were measured by direct shear test and compared to those esti-mated by the JRC-JCS model.Comparison results show that for natural rock joints with joint surfaces closely matched,the average relative error of joint shear strength between empirical estimation and direct shear test is 9.9%;the reliability of the empirical estimation of joint shear strength by the JRC-JCS model is good under both dry and saturated conditions if the JRC is determined accounting for directional statistical measurements,scale effect and surface smoothing during shearing.However,for natural rock joints with joint surfaces mismatched,the average relative error of joint shear strength between empirical estimation and direct shear test is 39.9%;the reliability of empirical estimation of joint shear strength by the JRC-JCS model is questionable under both dry and saturated conditions.

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb（MC） has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown（H-B） standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion ＇c then σn tan φ＇ should replace ＇c plus σn tan φ＇ for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.

Interfacial intermetallic compound growth and shear strength of low-silver SnAgCuBiNi/Cu lead-free solder joints

The growth rule of the interfacial intermetallic compound （IMC） and the degradation of shear strength of Sn-0.SAg-0.5Cu-2.0Bi-0.05Ni （SACBN）/Cu solder joints were investigated in comparison with Sn-3.0Ag-0.5Cu （SAC305）/ Cu solder joints aging at 373, 403, and 438 K. The results show that （Cul-x,Nix）6Sn5 phase forms between the SACBN solder and Cu substrate during soldering. The interracial IMC thickens constantly with the aging time increasing, and the higher the aging temperature, the faster the IMC layer grows. Compared with the SAC305/Cu couple, the SACBN/Cu couple exhibits a lower layer growth coefficient. The activation energies of IMC growth for SACBN/Cu and SAC305/Cu couples are 111.70 and 82.35 kJ/mol, respectively. In general, the shear strength of aged solder joints declines continuously. However, SACBN/Cu solder joints exhibit a better shear strength than SAC305/Cu solder joints.

Investigation on jointed rock strength based on fractal theory

Strength of discontinuities with complex structure is an important topic in rock engineering.A large number of studies have shown that fractal is applicable in the description of this discontinuity.Using fractal interpolation method for the generation of rock joints,numerical experiments of shear tests of the jointed rock mass model were carried out using FLAC^(3D).The test results show that the real rock joints can be simulated by fractal curves obtained by fractal interpolation.The fractal dimension is an important factor for the characterization of jointed rock mass;test results show that the fractal dimension of rock joints can be related to the equivalent cohesion strength and shear strength of the rock mass.When the fractal dimension of the joint surface is less than critical dimension Dc 1.404,the cohesion strength and shear strength of the rock mass increase as the fractal dimension increases;for larger fractal dimensions,all mechanical parameters decrease as the fractal dimension increases.Joint surfaces with different degrees of roughness were obtained by the fractal interpolation method.Three types of failure modes were observed in the tests:climbing slip failure,climbing gnawing fracture,and non-climbing gnawing fracture.

Effect of rock joint roughness on its cyclic shear behavior

Rock joints are often subjected to dynamic loads induced by earthquake and blasting during mining and rock cutting. Hence, cyclic shear load can be induced along the joints and it is important to evaluate the shear behavior of rock joint under this condition. In the present study, synthetic rock joints were prepared with plaster of Paris(Po P). Regular joints were simulated by keeping regular asperity with asperity angles of 15°-15° and 30°-30°, and irregular rock joints which are closer to natural joints were replicated by keeping the asperity angles of 15°-30° and 15°-45°. The sample size and amplitude of roughness were kept the same for both regular and irregular joints which were 298 mm×298 mm×125 mm and 5 mm, respectively. Shear test was performed on these joints using a large-scale direct shear testing machine by keeping the frequency and amplitude of shear load under constant cyclic condition with different normal stress values. As expected, the shear strength of rock joints increased with the increases in the asperity angle and normal load during the first cycle of shearing or static load. With the increase of the number of shear cycles, the shear strength decreased for all the asperity angles but the rate of reduction was more in case of high asperity angles. Test results indicated that shear strength of irregular joints was higher than that of regular joints at different cycles of shearing at low normal stress. Shearing and degradation of joint asperities on regular joints were the same between loading and unloading, but different for irregular joints. Shear strength and joint degradation were more significant on the slope of asperity with higher angles on the irregular joint until two angles of asperities became equal during the cycle of shearing and it started behaving like regular joints for subsequent cycles.

Seismic behavior and mechanism analysis of innovative precast shear wall involving vertical joints

To study the seismic performance and load-transferring mechanism of an innovative precast shear wall(IPSW) involving vertical joints, an experimental investigation and theoretical analysis were successively conducted on two test walls. The test results confirm the feasibility of the novel joints as well as the favorable seismic performance of the walls, even though certain optimization measures should be taken to improve the ductility. The load-transferring mechanism subsequently is theoretically investigated based on the experimental study. The theoretical results show the load-transferring route of the novel joints is concise and definite. During the elastic stage, the vertical shear stress in the connecting steel frame(CSF) distributes uniformly; and each high-strength bolt(HSB)primarily delivers vertical shear force. However, the stress in the CSF redistributes when the walls develop into the elastic-plastic stage. At the ultimate state, the vertical shear stress and horizontal normal stress in the CSF distribute linearly; and the HSBs at both ends of the CSF transfer the maximum shear forces.

Influence of electroless Sn/Bi on Sn/Pb soldered joint strength

The solder joint strength of Pb/Sn soldering aluminum with electroless layer Sn/Bi and Cu was studied. The results show that the joint shear strength of electroless Sn/Bi on aluminum surface is lower than that of Cu. A Pb-riched region with porosity is formed in region of soldering fillet with electroless Sn/Bi. Both the electroless Sn/Bi layer and Pb-riched layer become thicker, which are the reasons why the shear strength of the solder joint with electroless Sn/Bi on aluminum surface is lower than that of electroless Cu, and the higher the thickness of the electroless Sn/Bi layer is, the lower the shear strength of solder joint is.

An Improved Formulation for the Ultimate Static Strength of Tubular Joints subjected to Combined Loads

-'The effect of interaction of loads on the ultimate static strength of tubular joints of offshore fixed platforms, is a practical problem. But there is still absence of rigorous theory to explain available experimental data and empirical criteria for the static strength of tubular joints. The idea of yield at hot spot of tubular joints is introduced in this paper. The interaction equations of plastic capacity for the tubular joints under combined loads (two and three different kinds) are derived. Thereafter the Yura's test data and empirical criteria of ultimate static strength for the tubular joints can be explained. The idea of classification of category of loads in accordance with experimental data and the present theory is suggested. Finally, the improved ultimate capacity equations for tubular joints are recommended. The physical significance of the coefficient of plastic reservation Qp is discussed.

新型承插式螺栓连接柱-柱节点抗拉性能研究

为实现模块化钢结构单元房间的上下连接,提出一种新型承插式螺栓连接柱-柱节点,以有无注浆、承插深度为参数设计并制作3个足尺节点试件,并对其进行抗拉试验,分析了各节点的破坏形态、应变分布以及承载能力等,探讨了该新型节点的抗拉性能.建立了数值分析模型,进行了轴拉荷载作用下的受力性能参数化分析,研究了承插深度、螺栓直径及内套筒厚度对节点抗拉承载力的影响.基于高强螺栓的抗剪承载力,提出了适用于该新型节点的抗拉承载力计算公式.研究结果表明:该新型节点可将轴向拉力有效传递至高强螺栓,试件破坏时均出现高强螺栓群被剪断,灌浆节点试件发生破坏时,出现钢材与灌浆料界面的黏结破坏及螺栓周围局部灌浆料的压碎;高强螺栓群在拉力荷载作用下呈端部螺栓受剪较大、中心螺栓受剪较小的分布,试件破坏时,各螺栓承受的剪力趋于相等;灌浆节点与无浆节点相比,灌浆料与高强螺栓协同工作性能良好,弹性阶段最大摩擦力平均值提高64.4%,极限抗拉承载力提高14.1%;承插深度由300 mm增至500 mm,节点极限抗拉承载力提高80.9%;承插深度和螺栓直径对节点抗拉承载力影响较大,内套筒厚度对节点抗拉承载力的影响较小;根据提出的节点抗拉承载力计算公式得到的计算值与有限元模拟值吻合良好.

预应力混凝土梁-型钢混凝土柱新型框架节点受剪承载力计算方法

梁柱节点受剪承载力计算是混凝土框架结构节点抗震设计的关键点之一,由于节点剪力传递机理与构造的复杂性,目前尚未形成完善的节点受剪承载力计算理论。该文阐述了预应力混凝土梁-型钢混凝土柱新型框架节点的受力机理,并基于桁架模型和斜压杆模型,提出了一种同时考虑预应力筋与型钢贡献的节点受剪承载力计算方法。开展了新型节点受力性能拟静力试验,分别选取了梁端破坏与节点剪切破坏两种破坏形式的节点试验数据,对所建立的新型节点受剪承载力计算方法进行了验证评估,结果表明,该文提出的预应力混凝土梁-型钢混凝土柱框架节点承载力计算方法具有较好的精度。

基于Ubiquitous-Joint模型的层状岩坡稳定性分析

在应力的作用下,层状岩土体表现出来的力学行为比较复杂,目前广泛使用的各向同性模型不再适用,因此,有必要建立合理描述层状岩质边坡的力学模型。以某公路边坡为工程背景,首先,探讨了层状岩质边坡的失稳机制;然后,利用快速拉格朗日差分法(FLAC3D),建立该边坡的数值分析模型,探讨了强度折减技术在U-biquitous-Joint模型中的实施方法,即对岩体和层理面参数同时进行折减,采用计算不收敛作为边坡临界失稳判据;最后,通过数值计算,分析层状岩质边坡稳定性的影响因素,得到边坡整体安全系数F随节理倾角β先减小后增大,呈现两头高中间低的形态。

回流焊温度对SnBi-SAC/ENIG焊点剪切强度的影响

针对有机电路板在回流焊接时的热变形问题,采用机械混合方法制备SnBi-SAC/ENIG复合焊点,研究回流焊温度对焊点微观组织和力学性能的影响。结果表明复合焊点在180℃下的剪切强度约为25 MPa,可满足芯片与基板预连接强度需求。随着回流焊温度的升高,复合焊点界面微观组织细化,存在弥散分布的小尺寸富Bi相与Ag_(3)Sn相。在250℃回流焊时,复合焊点强度提升至约55 MPa。该材料可适用于低温预连接-结构加固-高温回流的封装工艺,对抑制封装结构热变形具有显著效果。

岩石结构面Grasselli二维粗糙度指标优化及其JRC估算

采用只考虑迎剪侧表观倾角而忽略内部起伏角的Grasselli二维形貌参数θ_(G)来估算结构面粗糙度系数JRC仍有待改进。从θ_(G)的物理意义出发,发现爬坡区内部凸起高度对粗糙度的贡献可表示为其在总爬坡水平距离上的坡角,所有爬坡区提供的坡角之和定义为结构面内部起伏体粗糙贡献,将结构面迎剪侧表观平均倾角θ_(G)和爬坡区内部平均坡角θ_(H)叠加,提出岩石结构面粗糙度新指标θ_(C)。通过获取十条标准JRC剖面线的θ_(C),分析其与JRC之间的关系,建立基于θ_(C)的JRC估算式。计算不同采样间距和不同采样方向下的新指标θ_(C),结果显示θ_(C)具有分形特征且能够反映结构面形貌各向异性。进一步将JRC估算式应用于JRC-JCS模型,通过对比已有研究得到的试验结果以及不同抗剪强度模型的预测效果,验证基于新指标θ_(C)估算的JRC可以准确预测结构面峰值抗剪强度。最后,对二维指标θ_(C)进行三维拓展,提出三维粗糙度新指标(θ_(C))_(3D),并通过结构面形貌各向异性表征验证了其合理性。

节点柔性理论对导管架平台强度分析的影响研究

本文研究了海上导管架平台的管节点柔性模拟方法,分析其对结构内力及位移的影响,及其导致的不同构件刚度和强度变化。通过南海西部典型导管架平台的分析,分别对比3种理论的差别;与刚性假定的结果对比,总结出节点柔性技术对结构强度、疲劳寿命以及极限强度的影响的一般结论。

Investigation on the physical mechanism of cavity percentage dependent shear strength for rock joints considering the real contact joint surface

To explain the effect of joint roughness on joint peak shear strength(JPSS)and investigate the effect of different contact states of joint surface on JPSS,we try to clarify the physical mechanism of the effect of joint cavity percentage(JCP)on JPSS from the perspective of the three-dimensional(3D)distribution characteristics of the actual contact joint surface,and propose a JPSS model considering the JCP.Shear tests for red sandstone joints with three different surface morphologies and three different JCPs were performed under constant normal load condition.Based on test fitting results,the reduction effect of the JCP on JPSS is investigated,and a JPSS model for cavity-containing joints is obtained.However,the above model only considers the influence of JCP by fitting test data,and does not reveal the physical mechanism of JCP affecting the JPSS.Based on the peak dilation angle model for consideration of the actual contact joint morphology,and the influence of JCP on the roughness of the actual contact joint surface,a theoretical model of the JPSS considering the JCP is proposed.The derivation process does not depend on the test fitting,but is entirely based on the joint mechanical law,and its physical significance is clear.It is proposed that the essence of the influence of the JCP on JPSS is that the JCP first affects the normal stress of the actual contact joints,further affects the roughness of actual contact joints,and then affects the shear strength.

Discrete modeling of rock joints with a smooth-joint contact model

Structural defects such as joints or faults are inherent to almost any rock mass.In many situations those defects have a major impact on slope stability as they can control the possible failure mechanisms.Having a good estimate of their strength then becomes crucial.The roughness of a structure is a major contributor to its strength through two different aspects,i.e.the morphology of the surface(or the shape)and the strength of the asperities(related to the strength of the rock).In the current state of practice,roughness is assessed through idealized descriptions(Patton strength criterion)or through empirical parameters(Barton JRC).In both cases,the multi-dimensionality of the roughness is ignored.In this study,we propose to take advantage of the latest developments in numerical techniques.With3D photogrammetry and/or laser mapping,practitioners have access to the real morphology of an exposed structure.The derived triangulated surface was introduced into the DEM(discrete element method)code PFC3D to create a synthetic rock joint.The interaction between particles on either side of the discontinuity was described by a smooth-joint model(SJM),hence suppressing the artificial roughness introduced by the particle discretization.Shear tests were then performed on the synthetic rock joint.A good correspondence between strengths predicted by the model and strengths derived from well-established techniques was obtained for thefirst time.Amongst the benefits of the methodology is the possibility offered by the model to be used in a quantitative way for shear strength estimates,to reproduce the progressive degradation of the asperities upon shearing and to analyze structures of different scales without introducing any empirical relation.

Some remarks on the dynamical conformity of rock joints

A recently developed computerized method for assessing the rock joint coefficients is discussed. The performances of formerly introduced relative similarity indicators, along with the correlation coefficient, are subjected to critical analysis. These relative numerical indicators are replaced by two absolute indicators whose properties better describe surface textures of rock joints. The first absolute indicator results from the Fourier Matrix and evaluates wavy shapes of surfaces. The second absolute indicator quantifies the heights of surface reliefs, and is defined as the root mean square height of the surface outline. The behavior of the newly introduced numerical indicators are investigated by means of the deterministic periodic surface reliefs. The practical application of the new indicators is presented and the convenient performances of both the indicators are documented.

New criterion for rock joints based on three-dimensional roughness parameters

The shear behavior of rock joints is important in solving practical problems of rock mechanics. Three group rock joints with different morphologies are made by cement mortar material and a series of CNL(constant normal loading) shear tests are performed. The influences of the applied normal stress and joint morphology to its shear strength are analyzed. According to the experimental results, the peak dilatancy angle of rock joint decreases with increasing normal stress, but increases with increasing roughness. The shear strength increases with the increasing normal stress and the roughness of rock joint. It is observed that the modes of failure of asperities are tensile, pure shear, or a combination of both. It is suggested that the three-dimensional roughness parameters and the tensile strength are the appropriate parameter for describing the shear strength criterion. A new peak shear criterion is proposed which can be used to predict peak shear strength of rock joints. All the used parameters can be easily obtained by performing tests.

结构面抗剪强度各向异性研究中试样获取方法有效性分析

结构面抗剪强度的各向异性特征对工程岩体的力学性质、变形特性和稳定性分析都具有重要意义。结构面试样获取方法对抗剪强度各向异性特征的研究具有重要影响。为了对比不同试样获取方法在结构面抗剪强度各向异性研究中的有效性,分别对直接截取的圆形、方形和旋转截取的方形试样进行抗剪强度各向异性特征的对比分析。选取1 m×1 m的天然板岩结构面作为研究对象,采用三维激光扫描法获取结构面三维形貌特征,截取研究位置处圆形、方形和旋转方形试样进行对比评价。研究表明:直接截取的方形试样沿不同剪切方向产生的剪切面积不同,选用圆形试样更能反映结构面抗剪强度的各向异性特征;旋转方形与圆形试样的抗剪强度统计误差小于直接截取的方形与圆形试样的统计误差值,且旋转方形与等面积圆试样的抗剪强度各向异性特征最接近,两者统计误差最小、相似度最大,大于0.9987,当圆形试样不具备实验条件时,可选用旋转截取的等面积方形试样进行结构面抗剪强度的各向异性研究。该研究可为选取结构面试样获取方法提供参考和借鉴,为准确开展抗剪强度各向异性研究提供依据。