Cyclic shear behavior of en-echelon joints under constant normal stiffness conditions

To reveal the mechanism of shear failure of en-echelon joints under cyclic loading,such as during earthquakes,we conducted a series of cyclic shear tests of en-echelon joints under constant normal stiffness(CNS)conditions.We analyzed the evolution of shear stress,normal stress,stress path,dilatancy characteristics,and friction coefficient and revealed the failure mechanisms of en-echelon joints at different angles.The results show that the cyclic shear behavior of the en-echelon joints is closely related to the joint angle,with the shear strength at a positive angle exceeding that at a negative angle during shear cycles.As the number of cycles increases,the shear strength decreases rapidly,and the difference between the varying angles gradually decreases.Dilation occurs in the early shear cycles(1 and 2),while contraction is the main feature in later cycles(310).The friction coefficient decreases with the number of cycles and exhibits a more significant sensitivity to joint angles than shear cycles.The joint angle determines the asperities on the rupture surfaces and the block size,and thus determines the subsequent shear failure mode(block crushing and asperity degradation).At positive angles,block size is more greater and asperities on the rupture surface are smaller than at nonpositive angles.Therefore,the cyclic shear behavior is controlled by block crushing at positive angles and asperity degradation at negative angles.

Shear sliding of rough-walled fracture surfaces under unloading normal stress

Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted regarding various initial normal stresses(1e7 MPa)and numbers of shearing cycles(1 e5).The peak shear stress of fractures decreased with shear cycles due to progressively smooth surface morphologies,while increased with both JRC and initial normal stress and could be verified using the nonlinear Barton-Bandis failure criterion.The joint friction angle of fractures exponentially increased by 62.22%e64.87%with JRC while decreased by 22.1%e24.85%with shearing cycles.After unloading normal stress,the sliding initiation time of fractures increased with both JRC and initial normal stress due to more tortuous fracture morphologies and enhanced shearing resistance capacity.The surface resistance index(SRI)of fractures decreased by 4.35%e32.02%with increasing shearing cycles due to a more significant reduction of sliding initiation shear stress than that for sliding initiation normal stress,but increased by a factor of 0.41e1.64 with JRC.After sliding initiation,the shear displacement of fractures showed an increase in power function.By defining a sliding rate threshold of 5105 m/s,transition from“quasi-static”to“dynamic”sliding of fractures was identified,and the increase of sliding acceleration steepened with JRC while slowed down with shearing cycles.The normal displacement experienced a slight increase before shear sliding due to deformation recovery as the unloading stress was unloaded,and then enhanced shear dilation after sliding initiation due to climbing effects of surface asperities.Dilation was positively related to the shear sliding velocity of fractures.Wear characteristics of the fracture surfaces after shearing failure were evaluated using binary calculation,indicating an increasing shear area ratio by 45.24%e91.02%with normal stress.

Experimental investigation on shear strength deterioration at the interface between different rock types under cyclic loading

The shear strength deterioration of bedding planes between different rock types induced by cyclic loading is vital to reasonably evaluate the stability of soft and hard interbedded bedding rock slopes under earthquake;however,rare work has been devoted to this subject due to lack of attention.In this study,experimental investigations on shear strength weakening of discontinuities with different joint wall material(DDJM)under cyclic loading were conducted by taking the interface between siltstone and mudstone in the Shaba slope of Yunnan Province,China as research objects.A total of 99 pairs of similar material samples of DDJM(81 pairs)and discontinuities with identical joint wall material(DIJM)(18 pairs)were fabricated by inserting plates,engraved with typical surface morphology obtained by performing three-dimensional laser scanning on natural DDJMs sampled from field,into mold boxes.Cyclic shear tests were conducted on these samples to study their shear strength changes with the cyclic number considering the effects of normal stress,joint surface morphology,shear displacement amplitude and shear rate.The results indicate that the shear stress vs.shear displacement curves under each shear cycle and the peak shear strength vs.cyclic number curves of the studied DDJMs are between those of DIJMs with siltstone and mudstone,while closer to those of DIJMs with mudstone.The peak shear strengths of DDJMs exhibit an initial rapid decline followed by a gradual decrease with the cyclic number and the decrease rate varies from 6%to 55.9%for samples with varied surface morphology under different testing conditions.The normal stress,joint surface morphology,shear displacement amplitude and shear rate collectively influence the shear strength deterioration of DDJM under cyclic shear loading,with the degree of influence being greater for larger normal stress,rougher surface morphology,larger shear displacement amplitude and faster shear rate.

Effect of lateral stress on frictional properties of a fracture in sandstone

The injection of large volumes of natural gas into geological formations,as is required for underground gas storage,leads to alterations in the effective stress exerted on adjacent faults.This increases the potential for their reactivation and subsequent earthquake triggering.Most measurements of the frictional properties of rock fractures have been conducted under normal and shear stresses.However,faults in gas storage facilities exist within a true three-dimensional(3D)stress state.A double-direct shear experiment on rock fractures under both lateral and normal stresses was conducted using a true triaxial loading system.It was observed that the friction coefficient increases with increasing lateral stress,but decreases with increasing normal stress.The impact of lateral and normal stresses on the response is primarily mediated through their influence on the initial friction coefficient.This allows for an empirical modification of the rate-state friction model that considers the influence of lateral and normal stresses.The impact of lateral and normal stresses on observed friction coefficients is related to the propensity for the production of wear products on the fracture surfaces.Lateral stresses enhance the shear strength of rock(e.g.Mogi criterion).This reduces asperity breakage and the generation of wear products,and consequently augments the friction coefficient of the surface.Conversely,increased normal stresses inhibit dilatancy on the fracture surface,increasing the breakage of asperities and the concomitant production of wear products that promote rolling deformation.This ultimately reduces the friction coefficient.

Shear band evolution and acoustic emission characteristics of sandstone containing non-persistent flaws

Direct shear tests were conducted on sandstone specimens under different constant normal stresses to study the coalescence of cracks between non-persistent flaws and the shear sliding characteristics of the shear-formed fault.Digital image correlation and acoustic emission(AE)techniques were used to monitor the evolution of shear bands at the rock bridge area and microcracking behaviors.The experimental results revealed that the shear stresses corresponding to the peak and sub-peak in the stressdisplacement curve are significantly affected by the normal stress.Strain localization bands emerged at both the tip of joints and the rock bridge,and their extension and interaction near the peak stress caused a surge in the AE hit rate and a significant decrease in the AE b value.Short and curvilinear strain bands were detected at low normal stress,while high normal stress generally led to more microcracking events and longer coplanar cracks at the rock bridge area.Furthermore,an increase in normal stress resulted in a higher AE count rate and more energetic AE events during friction sliding along the shearformed fault.It was observed that the elastic energy released during the crack coalescence at the prepeak stage was much greater than that released during friction sliding at the post-peak stage.More than 75%of AE events were located in the low-frequency band(0e100 kHz),and this proportion continued to rise with increasing normal stress.Moreover,more AE events of low AF value and high RA value were observed in specimens subjected to high normal stress,indicating that greater normal stress led to more microcracks of shear nature.

Experimental and Numerical Study on the Shear Strength and Strain Energy of Rock Under Constant Shear Stress and Unloading Normal Stress

Excavation and earth surface processes(e.g.,river incision)always induce the unloading of stress,which can cause the failure of rocks.To study the shear mechanical behavior of a rock sample under unloading normal stress conditions,a new stress path for direct shear tests was proposed to model the unloading of stress caused by excavation and other processes.The effects of the initial stresses(i.e.,the normal stress and shear stress before unloading)on the shear behavior and energy conversion were investigated using laboratory tests and numerical simulations.The shear strength of a rock under constant stress or under unloading normal stress conforms to the Mohr Coulomb criterion.As the initial normal stress increases,the cohesion decreases linearly and the tangent of the internal friction angle increases linearly.Compared with the results of the tests under constant normal stress,the cohesions of the rock samples under unloading normal stress are smaller and their internal friction angles are larger.A strength envelope surface can be used to describe the relationship between the initial stresses and the failure normal stress.Shear dilatancy can decrease the total energy of the direct shear test under constant normal stress or unloading normal stress,particularly when the stress levels(the initial stresses in the test under unloading normal stress or the normal stress in the test under constant normal stress)are high.The ratio of the dissipated energy to the total energy at the moment failure occurs decreases exponentially with increasing initial stresses.The direct shear test under constant normal stress can be considered to be a special case of a direct shear test under unloading normal stress with an unloading amount of zero.

Size dependent free vibration analysis of functionally graded piezoelectric micro/nano shell based on modified couple stress theory with considering thickness stretching effect

Higher-order shear and normal deformation theory is used in this paper to account thickness stretching effect for free vibration analysis of the cylindrical micro/nano shell subjected to an applied voltage and uniform temperature rising.Size dependency is included in governing equations based on the modified couple stress theory.Hamilton’s principle is used to derive governing equations of the cylindrical micro/nano shell.Solution procedure is developed using Navier technique for simply-supported boundary conditions.The numerical results are presented to investigate the effect of significant parameters such as some dimensionless geometric parameters,material properties,applied voltages and temperature rising on the free vibration responses.

Simulation of ductile fracture initiation in steels using a stress triaxiality-shear stress coupled model

Micromechanics-based models provide powerful tools to predict initiation of ductile fracture in steels. A new criterion is presented herein to study the process of ductile fracture when the effects of both stress triaxiality and shear stress on void growth and coalescence are considered. Finite-element analyses of two different kinds of steel, viz. ASTM A992 and AISI 1045, were carried out to monitor the history of stress and strain states and study the methodology for determining fracture initiation. Both the new model and void growth model (VGM) were calibrated for both kinds of steel and their accuracy for predicting fracture initiation evaluated. The results indicated that both models offer good accuracy for predicting fracture of A992 steel. However, use of the VGM leads to a significant deviation for 1045 steel, while the new model presents good performance for predicting fracture over a wide range of stress triaxiality while capturing the effect of shear stress on fracture initiation.

Stress analysis of full-length grouted bolt under shear deformation of anchor interface

The bolt anchoring force is closely related to the shear properties of the anchor interface. The shear stress distribution of full-length grouted bolts is analyzed based on the stress-strain relationship among the bolt, grout, rock mass and bond interface,considering the shear properties of the grout and contact interface bonding behavior. In this case, the interfacial shear stress of the grout and rock mass and the bolt axial force are obtained under pull-out and normal working conditions. The results show that the peak shear stress of the interface with the shear deformation of the bond interface is significantly lower than that without it when the pull-out force is applied. When designing bolt parameters of grade IV and V rock mass, the relative deformation between the rock mass and anchor should be considered, with a “unimodal” to “bimodal” shear stress distribution.In the case of a low elastic modulus of rock masses,both the shear stress concentration and distribution range are obvious, and the neutral point is near the bolt head. As the elastic modulus increases, the shear stress concentration and distribution range are reduced, and the neutral point moves towards the distal end. As a result, the optimum length of fulllength grouted bolts can be determined by in-situ pull-out tests and decreases with the increased elastic modulus of the rock mass.

Critical Cyclic Stress Ratio of Undisturbed Saturated Soft Clay in the Yangtze Estuary under Complex Stress Conditions

There exists a critical cyclic stress ratio when sand or clay is subjected to cyclic loading. It is an index dis-tinguishing stable state or failure state. The soil static and dynamic universal triaxial and torsional shear apparatus de-veloped by Dalian University of Technology in China was employed to perform different types of tests on saturated soft marine clay in the Yangtze estuary. Undisturbed samples were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotropic consolidation with different initial consoli-dation parameters. The effects of initial orientation angle of major principal stress, initial ratio of deviatoric stress,initial coefficient of intermediate principal stress and stress mode of cyclic shear on the critical cyclic stress ratio wereinvestigated. It is found that the critical cyclic stress ratio decreases significantly with increasing initial orientation angle of major principal stress and initial ratio of deviatoric stress. Compared with the effects of the initial orientationangle of major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate prin-cipal stress is less evident. Under the same consolidation condition, the critical cyclic stress ratio from the cyclic cou-pling shear test is lower than that from the cyclic torsional shear test, indicating that the stress mode of cyclic shear has an obvious effect on the critical cyclic stress ratio. The main reason is that the continuous rotation in principal stressdirections during cyclic coupling shear damages the original structure of soil more than the cyclic torsional shear does.

Mathematical Modeling of Shear Stress and Direct Shear Test for Compressible Soil: Case of Soil Bordering the Wouri River

This paper focuses on the development of the mathematical model of shear stress by direct shear test for compressible soil of the littoral region, which will be a great tool in the hand of geotechnical engineers. The most common use of a shear test is to determine the shear strength which is the maximum shear stress that a material can withstand before the failure occurs. This parameter is useful in many engineering designs such as foundations, roads and retaining walls. We carried out an experimental laboratory test of ten samples of undisturbed soil taken at different points of the border of Wouri river of Cameroon. The samples were collected at different depths and a direct shear test was conducted. The investigations have been performed under constant vertical stresses and constant sample volume with the aim to determine the frictional angle and the cohesion of the compressible soil which are so important to establish the conditions of buildings stability. Special care was taken to derive loading conditions actually existing in the ground and to duplicate them in the laboratory. Given that the buildings constructed in this area are subjected to settlement, landslide, and punch break or shear failure, the cohesion and the frictional angle are determined through the rupture line after assessed the mean values of the shear stress for the considered ten samples. The bearing capacity of the soil, which is the fundamental soil parameter, was calculated. From the laboratory experimental results, the least squared method was used to derive an approximated mathematical model of the shearing stress. Many optimizations methods were then considered to reach the best adjustment.

Shear Stress Variation at Scour Hole of Circular Pier

The causes of local scour are generally categorized into flow condition, structure, and riverbed material. A three-dimensional vortex flow generated with the influence of the structure is the main factors of the flow conditions, and the size of the particles is assumed to be the main factor of the riverbed case. Various studies about pier local scour have been carried out by researchers since the 1960s, and a large number of experimental formulas have been suggested. Difficulties were encountered by these past studies, however, in terms of considering the influence of various riverbed materials and scour changes (floods, etc.) on time, with the condition of maximum scour depth. In the case of Korea, especially, scour influenced by various riverbed materials and the frequency of floods have been determined to be very important factors. Therefore, the ultimate purpose of this study on pier scour is to suggest the scour examination method that could consider various riverbed materials and the frequency of floods. In this study, the periodic changes in local scour based on the differences in the diameters of four types of bed materials, and on the hydraulic condition of the initial scour, were determined and compared with those in former studies. Using the results of the comparison, this study aims to determine the changes in the shear-stress around piers for various bed materials through the effect of time on scour depth (S, Smax), the shear-stress around piers, and the particles’ critical shear stress (τc).

Experimental Study on Shear Mechanical Properties and Section Morphology of Coal Samples

The mechanical properties of rocks under cyclic and dynamic loading are important research topics for solving the structural stability of large engineering rocks. As underground mining in coal mines goes deeper, ground stresses are increasing and instability damage of coal rocks by shear loading is frequent. Therefore, in order to investigate the shear mechanical properties and section morphological characteristics of intact coal samples in the direct shear test, the RDS-200 rock direct shear instrument was used to carry out direct shear tests on intact coal samples under different normal stresses, and the shear section was scanned for three-dimensional morphology. The results show that: 1) from the strength characteristics, the peak shear strength of the coal samples increased linearly with increasing normal stress, and the residual shear strength increased logarithmically. 2) In terms of deformation characteristics, the peak shear displacement of the coal sample increases linearly with increasing normal stress, the pre-peak shear stiffness increases logarithmically, and the residual normal displacement decreases linearly. 3) From the morphological characteristics of the shear surface, with the increase of normal stress, the section transitions from high-order undulating to flattening type. The maximum height of the fracture surface profile and kurtosis coefficient of the shear section decreased linearly, and the profile area ratio and root mean square of slope decreased as a power function, i.e. the higher the normal stress, the smaller the undulation of the section, the sharpness of the roughness shape and the roughness coefficient JRC, and the flatter and smoother the section. The findings of this study can help to provide some reference for the evaluation of shear instability occurring in coal bodies under different normal stresses.

天然裂缝产状对水力压裂施工压力的影响

天然裂缝产状是影响裂缝壁面法向应力及切向应力的重要参数,而天然裂缝应力状态是决定地面施工压力大小的重要因素,准确计算、预测施工压力大小对水力压裂设计与施工有着重要的作用。根据天然裂缝产状和地层地应力状态,基于裂缝起裂与延伸准则,建立地面施工压力计算模型,并利用实际裂缝型储层进行计算分析。研究发现:正断层与走滑断层中,倾角越小施工压力越小,正断层中方位角越趋近于0°、180°、360°时施工压力越小;走滑断层中方位角越趋近于90°、270°时施工压力越小。逆断层中,当方位角为0°~45°、135°~225°、315°~360°时,倾角越大施工压力越大,当方位角为45°~135°、225°~315°时,倾角越大施工压力越小;同一倾角下,方位角在0°~90°、180°~270°范围内时,施工压力随方位角增大而减小,方位角在90°~180°、270°~360°范围内时,施工压力随方位角增大而增大。根据天然裂缝产状准确计算施工压力大小,可为现场压裂施工参数选择与优化提供理论依据。

单箱双室变截面波形钢腹板组合梁剪应力分析

为了研究单箱双室变截面波形钢腹板组合箱梁(CBBCSW)的剪应力分布规律,分析了单箱双室变截面CBBCSW的顶底板与波形钢腹板(CSW)的承剪问题。基于微元体平衡和切应力互等定理推导了变截面CSW的剪应力计算公式,并与等截面CSW剪应力计算公式进行了对比。对各项剪应力的性质展开了研究,针对变截面CBBCSW的顶底板和参与承剪机理进行了分析,提出了底板承剪比例计算公式。以室内模型试验梁和一座单箱双室变截面CBBCSW为例,采用ANSYS有限元软件分别建立了试验梁与实桥的有限元模型,研究了不同约束体系下,剪应力在CSW上的分布以及顶底板与CSW参与承剪的规律。对比不同荷载情况下,有限元计算结果与理论公式计算结果的差异,分析不同荷载工况对顶底板与CSW参与承剪的影响。此外,分析了横隔板对单箱多室CBBCSW剪力分配的影响。研究结果表明:所提公式的理论结果与试验实测结果和有限元结果吻合较好,且底板参与承剪的比例不可忽视;变截面引起的“附加剪应力”是影响剪力传递的直接原因,弯矩的大小会直接影响底板参与承剪的比例;对称荷载下中腹板与边腹板的剪力分布几乎相同,而横隔板会改变CSW上剪力分配,靠近横隔板处的部位剪应力分布更均匀。研究结果可以为实际工程中变截面CSW的剪应力和承剪比的分析提供参考依据。

断面有效正应力对断层侧向封闭能力的控制作用

小位移断层的侧向封闭性决定断层圈闭富集油气的能力,随着油气勘探技术理论的发展,曾被忽略的小位移断层在油气勘探和开发过程中的作用日益突出。为了明确小位移断层的内部结构特征及其对断层侧向封闭性的影响,应用自主研发的高压低速环形剪切装置,以断面有效正应力为变量,对高孔隙度纯净砂岩开展环形剪切物理模拟实验,分析断面有效正应力对断层侧向封闭性的影响。结果表明:断裂在高孔隙纯净砂岩内以碎裂作用机制为主,形成典型的碎裂型断层岩,其渗透率明显降低,是导致同层砂岩对接段侧向封闭的根本原因。在其他因素固定的条件下,断面有效正应力越大,断层岩碎裂程度越大,颗粒直径和孔隙度越小;岩石颗粒的破碎及碎屑基质的填充导致断裂带孔隙度和渗透率明显降低。断面有效正应力是小位移断层侧向封闭能力的主要控制因素,小位移断层具有富集油气的潜力。研究结果对裂陷盆地断块油气藏勘探开发具有重要的指导意义。

Behavior of interfacial stresses between RC beams and GFRP sheets

Seven reinforced concrete （RC） beams with epoxy-bonded glass fiber reinforced plastic （GFRP） sheets and two control RC beams were experimentally tested to investigate the bond behavior of the interfaces between RC beams and GFRP sheets. The variable parameters considered in test beams are the layers of GFRP sheets, the bond lengths and the reinforcement ratios. The results indicate that the flexural strength of the repaired beams is increased, but the ultimate load of beams with GFRP sheets debonding failure is reduced relatively. The bond length is the main factor that results in bonding failure of the strengthened beams. An experimental method of interfacial shear stress is proposed to analyze the distribution of shear stress according to experimental results. The analytical method of shear and normal stresses and a simple equation are proposed to predict the peeling loads. The proposed model is applied to experimental beams. The analytical results show a good agreement with the experimental results.

钙质砂三轴排水试验围压与剪切速率对应力特性影响研究

以海南省三沙市西沙群岛钙质砂为研究对象开展不同剪切速率的三轴固结排水试验,研究钙质砂剪切应力特性。试验结果表明:围压越大偏应力峰值出现的越晚,相同围压剪切速率越快应力峰值越大,应力下降也越快应力下降比越大,剪切速率对应力的峰值大小及残余剪切强度有一定影响,低围压下剪切速率对偏应力峰值影响更大。围压越小偏应力比越大,相同围压剪切速率越大偏应力比增长越快,当剪切速率低于一定速度时,偏应力比的变化并不受剪切速度的影响。相同围压条件下,由于排水剪切时围压和孔压保持不变,偏应力和平均有效应力关系为斜率3的直线,不同剪切速率破坏线趋势相同,剪切速率越大破坏线角度越大,但相差不大,破坏线为接近斜率1.75的直线,钙质砂内摩擦角为42°。剪切过程平均有效应力先增大后减小,剪切过程表现出剪缩到剪胀的变化过程,剪缩阶段不同剪切速率下孔隙比下降速率相近,剪切速率在剪胀阶段对孔隙比影响较大,剪切速率越大,孔隙比变化越大。

循环荷载下饱和黄土的剪切特性与破坏模式

饱和黄土的破坏模式和剪切特性是分析黄土地基及其构筑物地震变形的基础,为了揭示循环荷载下黄土独特的强度衰减和变形劣化规律,本文开展了饱和黄土不排水动三轴试验,控制初始剪应力(q_(s))和循环剪应力(q_(c)),研究了不同初始应力状态下饱和黄土的动强度、孔压发展规律及破坏模式。试验结果表明:当q_(s)<q_(c)时,重塑和原状黄土试样表现为循环迁移型破坏,当q_(s)≥q_(c)时,重塑黄土试样表现为流动型破坏,而原状黄土试样则表现为塑性应变累积型破坏;黄土试样的循环应力比(R_(CS))、循环阻力比(R_(CR,10))随着初始剪应力比(R_(SS))的增大呈先增大后减小的趋势;循环迁移型和塑性应变累积型破坏在循环加载的初始阶段孔压上升迅速,孔压增长曲线分别呈“风琴”状和“喇叭”状,而流动型破坏的初始阶段孔压增长速率较慢,当到达某一振次后孔压突然增长,孔压增长曲线呈“镰刀”状。基于初始阶段孔压发展指标(K_(1-2))与初始应力状态(q_(s)/q_(c))提出了图表法,可用于预测循环荷载下饱和黄土的破坏模式。

K_(0)固结软黄土的不排水抗剪强度模型

为准确预测工程建设场地中饱和软黄土的不排水抗剪强度,采用考虑结构特性及各向异性特征的K_(0)固结软黄土模型,在e-lnp′空间下推导得出考虑结构性影响的体应变通式。引入结构应力比来反映土体固结历史对不排水强度的影响,遵从不排水条件下的体应变规律,推导得出K_(0)固结软黄土的不排水抗剪强度理论计算模型。研究结果表明:本文所提出的计算模型能较准确预测饱和软黄土的不排水抗剪强度。通过参数分析探讨模型主要参数与软黄土不排水抗剪强度的关系,发现软黄土的不排水抗剪强度比与塑性参数负相关,与结构性参数和结构应力比正相关。