Disc-cutter induced rock breakage mechanism for TBM excavation in rock masses with different joint shear strengths

When tunnel boring machines(TBMs)excavate through jointed rock masses,the cutting efficiency is strongly affected by the shear strength of joints,the mechanism of which,however,remains poorly understood.In this study,a series of disc-cutter indentation tests were conducted on granite rock mass specimens with different joint shear strengths.During the indentation,the cracking process was recorded by a digital image correlation(DIC)system.The deformation and strength of specimens,cracking behavior,rock breakage mode and cutting efficiency were quantitatively investigated.In addition,to investigate the combined effects of joint shear strength,orientation and spacing on the rock breakage mechanism,numerical rock mass models were established based on a particle flow code PFC2D.Experimental results reveal that the cracking of primary and secondary cracks changes from the mixed shear-tensile to tensile mode in the initial stage,while the joint shear strength does not affect the cracking mode in the subsequent propagation process.The rock breakage mode is classified to an internal block breakage mode,a cross-joint breakage mode and a cutters-dependent breakage mode.The cross-joint breakage mode is optimal for improving the cutting efficiency.Numerical simulation results reveal that the increase in the joint shear strength changes the internal block breakage mode to cross-joint breakage mode for rock masses of particular ranges of joint orientation and spacing.These findings provide basis for improving the TBM cutting efficiency through jointed rock masses.

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.

A new integrated intelligent computing paradigm for predicting joints shear strength

Joints shear strength is a critical parameter during the design and construction of geotechnical engineering structures.The prevailing models mostly adopt the form of empirical functions,employing mathematical regression techniques to represent experimental data.As an alternative approach,this paper proposes a new integrated intelligent computing paradigm that aims to predict joints shear strength.Five metaheuristic optimization algorithms,including the chameleon swarm algorithm(CSA),slime mold algorithm,transient search optimization algorithm,equilibrium optimizer and social network search algorithm,were employed to enhance the performance of the multilayered perception(MLP)model.Efficiency comparisons were conducted between the proposed CSA-MLP model and twelve classical models,employing statistical indicators such as root mean square error(RMSE),correlation coefficient(R2),mean absolute error(MAE),and variance accounted for(VAF)to evaluate the performance of each model.The sensitivity analysis of parameters that impact joints shear strength was conducted.Finally,the feasibility and limitations of this study were discussed.The results revealed that,in comparison to other models,the CSA-MLP model exhibited the most appropriate performance in terms of R2(0.88),RMSE(0.19),MAE(0.15),and VAF(90.32%)values.The result of sensitivity analysis showed that the normal stress and the joint roughness coefficient were the most critical factors influencing joints shear strength.This paper presented an efficacious attempt toward swift prediction of joints shear strength,thus avoiding the need for costly in-site and laboratory tests.

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.

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。

水/CO_(2)对平直节理砂岩剪切力学特性的影响

【目的】注水和CO_(2)会改变平直节理岩石的剪切特性,从而直接影响封存地层的稳定性。探究注水和注CO_(2)对节理岩体剪切特性的影响规律。【方法】对平直节理砂岩进行干燥、注水和注CO_(2)三种试验条件下的剪切试验,分析剪切强度随法向应力、注入流体类型和注入流体压力的变化规律。【结果】试验结果表明:平直节理砂岩在干燥、注水和注CO_(2)三种试验条件下的剪切应力-剪切位移曲线都属于滑动破坏型曲线;注水/CO_(2)会弱化平直节理岩石剪切强度,且水的弱化作用强于CO_(2);提出了平直节理岩石剪切强度与法向应力、注入流体类型和流体注入压力之间的经验公式,试验数据与理论数据的平均误差为5.69%.【结论】研究结果对预测砂岩在注入水/CO_(2)后的剪切强度以及优化施工方案和提高施工安全性具有重要意义。

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。

不同控制方式和卸荷应力路径下类岩石节理剪切强度特征研究

为研究不同控制方式和卸荷应力路径下岩石节理失稳剪切强度特征,借助RDS-200型岩石节理剪切试验系统开展了2种控制方式和3种卸荷应力路径下的类岩石节理直剪试验。结果表明:(1)采用应力控制方式进行切向加卸载时,各剪切加卸载路径下,节理剪切应力-时间曲线多存在失稳跌落现象,剪切应力-位移曲线失稳后多表现为折线形式。(2)与采用应力控制方式进行切向加载的试样结果相比,采用位移控制方式进行切向加载、采用应力控制方式进行法向卸载并保持剪切应力恒定、采用应力控制方式进行法向卸载和切向加载、采用应力控制方式同时进行法向和切向卸载的试样失稳剪切强度均有所降低;节理黏聚力减小百分比平均值为36.07%,而内摩擦角变化范围仅在4.12%以内,所以黏聚力降低是节理失稳剪切强度降低的主要原因。(3)以采用应力控制方式进行切向加载试样的拟合摩尔-库仑公式为基准,位移控制方式或其他采用应力控制方式的加卸载应力路径下试样失稳剪切强度的相对误差平均值最大可达55.78%。经引入黏聚力修正系数k对摩尔-库仑准则进行修正后,相对误差平均值的最大值减小到5.23%,说明对经受位移控制方式或采用应力控制方式的其他加卸载应力路径的工程节理来说,进行黏聚力的折减修正是必要和可行的。研究结果对不同控制方式及卸荷应力路径下岩体节理剪切承载能力的准确估计具有一定参考价值。

不同充填度结构面抗剪强度尺寸效应试验研究

针对现有岩体结构面抗剪强度尺寸效应研究较少考虑充填程度差异影响的不足,首先基于渐进扩大法选取平均起伏差和平均起伏角相近的4种连续尺寸天然结构面形貌,并通过3D打印技术制作结构面面板,然后结合类岩石材料配制方法和毫米级充填装置制备5种充填度的充填结构面,最后通过课题组自主研发的大型结构面直剪试验系统开展3种法向应力条件下的室内直剪试验,获得充填结构面的剪切力学与破坏特征,并分析临界充填度和抗剪强度尺寸效应特性。结果表明:随着充填度的增加,结构面剪切破坏模式主要由“岩-岩”剪断破坏逐渐转变为“岩-土界面”滑动破坏,且结构面归一化峰值抗剪强度逐渐减小,直至达到临界充填度时趋于稳定;随着结构面尺寸的增加,临界充填度逐渐增加,变化范围为1.6~2.0;无充填结构面抗剪强度存在较明显的正尺寸效应,而充填结构面抗剪强度尺寸效应较不明显。通过尺寸效应机理分析发现,在不考虑结构面平均起伏角的影响下,平均起伏高度是影响临界充填度和抗剪强度尺寸效应的主要因素。上述研究可为充填结构面抗剪强度尺寸效应模型建立奠定试验基础。

加锚岩体结构面抗剪强度尺寸效应试验及机理研究

为揭示加锚岩体结构面抗剪强度尺寸效应机理,开展了系列尺寸(100~400 mm)加锚结构面室内剪切试验,对比分析结构面锚固前后抗剪强度及锚杆抗剪力的变化规律,研究了锚杆和结构面的变形破坏特征。研究结果表明:(1)结构面锚固前后抗剪强度均呈负尺寸效应,且尺寸效应随着法向应力增加而逐渐减弱;锚杆抗剪力随着结构面尺寸增加呈非线性增加。(2)锚杆呈“S”形变形特征,变形角度、剪切变形量和轴向变形量随着结构面尺寸增加呈非线性增加;结构面受锚杆挤压发生破裂的程度随着结构面尺寸增加而减弱。(3)引用“锚杆作用面积”概念,结构面尺寸较小时,锚杆作用面积超出结构面产生“边缘效应”,导致锚杆未充分发挥抗剪性能;随着结构面尺寸增大,锚杆性能得到充分发挥,锚杆抗剪力先迅速增加后保持稳定。

倒装LED微小焊点空洞对其性能的影响

利用SnCu0.7锡膏回流工艺制备倒装LED,针对于焊点微小化引起的焊接缺陷。采用X-ray检测分选出不同空洞率焊点,系统研究了空洞率对于接头键合强度的影响规律及回流焊后接头断口特征,通过有限元仿真;探究空洞率对于芯片散热能力影响特性,同时,本文设计了结温测量实验,验证了仿真结果的准确性。最后通过高温老化实验,深入探究不同空洞率LED样品光通量及光电转换效率变化趋势。结果表明:焊点接头键合强度不完全取决于空洞的数量,空洞的大小及其位置对接头键合强度也会有一定的影响,其次随着空洞率逐渐增大,焊点的断裂模式逐渐由韧性断裂转变为脆韧性混合断裂。随着空洞率增大,LED在正常工作下的芯片结温基本呈线性增长趋势,并且空洞率越大,芯片表现出的散热能力越差,当空洞率约为5%,芯片结温为41.472℃,当空洞率为30%时,芯片结温达到45.969℃。随着空洞率增加,光通量与光电转换效率在高温老化开始阶段呈上升趋势,在500 h后光通量开始逐渐下降,在1500 h节点5%空洞率样品光通量维持率为93.22%比30%空洞率样品的82.37%高10.85%,5%空洞率样品光效为42.5%比30%空洞率样品的30.5%高12%。

基于颗粒流的天然多形貌充填节理剪切特性研究

节理剪切特性是影响岩体稳定性的重要因素,因此根据天然原岩节理起伏形貌曲线,采用颗粒流数值计算方法开展了天然多形貌无充填及充填厚度为15 mm和75 mm的节理剪切特性试验,分析节理起伏形貌和充填厚度对剪切特性及抗剪强度的影响.研究结果表明:(1)随着充填厚度的增大,起伏形貌对节理峰前硬化、峰后软化的峰值特征影响降低,峰后软化程度下降.(2)同起伏形貌的节理随着充填厚度的增大,试样由节理剪切破坏向软弱夹层剪切破坏过渡.充填度小于2时,主要是节理剪切破坏和节理-夹层耦合破坏;充填度大于2时,主要发生软弱夹层剪切破坏.(3)充填度小于2时,结构面峰值抗剪强度随充填度增大呈减小规律;当充填度大于2时,结构面峰值抗剪强度无明显变化.

不同法向应力和剪切速率下煤平直节理剪切特性研究

煤体剪切力学参数是分析煤体变形及稳定性的重要参数。为研究不同法向应力和剪切速率下煤平直节理剪切特性,开展剪切数值模拟与实验室试验,研究法向应力、剪切速率对煤平直节理剪切特性的影响规律。结果表明:节理剪切强度随剪切速率增加而减小,随着法向应力增加而增加,且法向应力越高,节理剪切强度受到剪切速率的影响越大;剪切刚度与法向应力呈正相关关系,与剪切速率呈负相关关系,当剪切速率达到一定值之后对剪切刚度的影响不明显;剪切速率增加使模拟节理更容易发生剪切滑移,易引发煤岩动力灾害,这一现象在高法向应力更为严重。

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。