Experimental Studies on Cyclic Shear Behavior of Steel-Clay Interface Under Constant Normal Load

The degradation of the shear stress between pile-clay interface caused by undrained cyclic jacking affects the jacking force.A series of large displacement monotonic shear,cyclic shear and post-cyclic monotonic steel plate-clay interface shear te sts were performed under the constant normal load(CNL)condition to inve stigate the effects of normal stre ss,cyclic amplitude,and number of cycles on a steel plate-clay interface using the GDS multi-function interface shear tester.Based on the experimental results,in monotonic shear tests,change of shear stress took place in the specimen,the shear stress rapidly reached the peak value at shear displacement of 1 mm,and then abruptly decreased to the residual value.In cyclic shear te sts,accumulated displacement was a better parameter to describe the soil degradation characteristics,and the degradation degree of shear stress became greater with the increasing of normal stress and accumulated displacement.Shear stress in post-cyclic monotonic shear tests did not generate a peak value and was lower than that in monotonic shear tests under the same normal stress.The soil was completely disturbed and reached the residual strength when the cumulative displacement approached 6 m.An empirical equation to evaluate shear stress degradation mechanism was formulated and the procedure of parameter identification was presented.

Microscratch of copper by a Rockwell C diamond indenter under a constant load

The scratch test is used for quality control mostly in phenomenological ways,and whether fracture toughness can be obtained from this test is still a matter of debate requiring further elucidation.In this paper,values of the fracture toughness of copper obtained by different scratch-based approaches are compared in order to examine the applicability of scratch-based methodologies to characterize the fracture toughness of soft metals.The scratch response of copper to a Rockwell C diamond indenter is studied under a constant normal load condition.The variations of penetration depth,residual depth,and residual scratch width with applied normal load are quantified from spherical to sphero-conical contact regimes by piecewise functions.A newly proposed size effect law is found to be the most suitable for scratch-based approaches to characterizing the fracture toughness of soft metallic materials with significant plasticity.A simple expression relating the nominal stress to the penetration depth is proposed for the spherical contact regime and gives almost the same value of fracture toughness.The residual scratch width provides useful information on pile-up of material and on the spherical tip radius of the indenter.It is found that the values of the fracture toughness obtained from the microscratch test are influenced by the data range for analysis.

Degradation of joint surface morphology, shear behavior and closure characteristics during cyclic loading

In order to investigate the failure mechanism of rock joint,a series of laboratory tests including cyclic direct shear tests under constant normal load(CNL)conditions were conducted.Morphology parameters of the rock joint surface were precisely calculated by means of a three-dimensional laser scanning machine.All test results were analyzed to investigate the shear behavior and normal displacement behavior of rock joints under CNL conditions.Degradation of rock joint surface during cyclic shear tests was also analyzed.The comparison results of the height parameters and the hybrid parameters of the joint surface during cyclic tests show that the degradation of the surface mostly happens in the first shear and the constant normal loads imposed on the joints have significant promotion effects on the morphology degradation.During cyclic shear tests,joints surfaces evolve from rough state to smooth state but keep an overall undulation.Dilatancy of rock joints degrades with the degradation of joint surface and the increase of normal loads.The closure deformation of joint is larger than that of the intact rock,and the normal stiffness increases with the increase of shearing times.

恒定法向刚度边界下含陶粒砂砂岩裂隙剪切力学特性研究

含支撑剂砂岩裂隙的剪切行为对致密气开采具有重要影响。通过开展不同边界条件下两种粗糙砂岩剪切试验,探究法向应力(1~6 MPa),法向刚度(0~5 GPa/m),初始法向应力(2~6 MPa),支撑剂尺寸(16/30目,20/40目,30/50目)对含陶粒砂砂岩裂隙剪切特性的影响。结果表明:加入陶粒砂后,裂隙面的峰值摩擦角减小,剪切刚度及峰值剪应力降低,并且剪缩量增加,剪胀量减小;随着法向应力的增加,剪切强度提高。在恒定法向刚度(CNS)边界下,随着法向刚度的增加,残余剪应力提高,但峰值剪应力、最终剪胀量和表观摩擦角降低。随着初始法向应力增加,峰值剪应力、残余剪应力及剪切刚度均提高,且变化幅度高于法向刚度的影响;初始法向应力的增加抑制了裂隙面的剪胀,提高了裂隙面的表观摩擦角。随着陶粒砂尺寸的减小,剪切强度提高,剪胀位移增加。

UDEC中Saeb-Amadei节理本构模型的二次开发

控制岩体水力学特性的节理在常法向应力边界条件和常法向刚度边界条件2种情况下的力学变形行为不同。由于Saeb-Amadei节理模型考虑了节理法向变形对节理剪切变形及剪胀的影响及节理吻合度对结果的影响,且适用于常法向应力边界与常法向刚度边界。引入Saeb-Amadei节理模型以研究裂隙岩体水力耦合特性。结合UDEC中的User-defined joint constitutive models接口,采用C++语言,编写Saeb-Amadei节理模型的数值嵌入程序Userjsa,进行单裂隙岩体的数值直剪和数值单轴压缩试验,对比计算结果与解析结果表明,计算结果与解析结果有很好的一致性,验证了数值嵌入程序Userjsa的正确性,为裂隙岩体水力耦合计算提供基础。

Effect of bolt inclination angle on shear behavior ofbolted joints under CNL and CNS conditions

Rock bolts are widely used in rock engineering projects to improve the shear capacity of the jointed rock mass.The bolt inclination angle with respect to the shear plane has a remarkable influence on the bolting performance.In this study,a new artificial molding method based on 3D scanning and printing technology was first proposed to prepare bolted joints with an inclined bolt.Then,the effects of the bolt inclination angle and boundary conditions on the shear behavior and failure characteristic of bolted joints were addressed by conducting direct shear tests under both CNL and CNS conditions.Results indicated that rock bolt could significantly improve the shear behavior of rock joints,especially in the post-yield deformation region.With the increase of bolt inclination angle,both the maximum shear stress and the maximum friction coefficient increased first and then decreased,while the maximum normal displacement decreased monotonously.Compared with CNL conditions,the maximum shear stress was larger,whereas the maximum normal displacement and friction coefficient were smaller under the CNS conditions.Furthermore,more asperity damage was observed under the CNS conditions due to the increased normal stress on the shear plane.

循环剪切对岩石节理形貌与力学行为影响实验研究

为研究岩石节理在循环剪切作用下的物理力学特性变化规律,对节理试件进行了不同法向荷载下的循环剪切试验,并对节理表面形貌进行了测量研究。结果表明:随着循环剪切次数的增加,节理表面最高点与最低点之间的垂直距离迅速减小,试件所施加的法向荷载越大则会增加节理表面形貌的劣化程度,节理表面的劣化主要发生在第1次剪切过程。节理峰值抗剪强度与残余强度均随剪切次数增加而减小,第2次剪切以后节理剪切曲线峰值点消失,峰值抗剪强度几乎与残余抗剪强度相等。同时,随着形貌特征的劣化,节理剪胀随着剪切次数的增加而减小。

循环荷载下砂与钢板界面的弱化机制

针对桩-土界面的循环弱化机制,采用自主研发的大尺度恒刚度界面剪切仪,进行不同剪切位移幅值和初始法向应力下砂与钢板界面的循环剪切试验研究.研究结果表明,界面剪切应力、法向应力与剪切位移关系曲线分别呈"滞回环"和"阶梯"状发展,且随循环次数的增加趋于"扁平",相邻循环应力应变曲线趋于重合,两者的循环弱化速率均随初始法向应力和剪切位移幅值的增加而增大;同组试验不同循环下的最大剪切应力随循环次数的增加近似呈对数型弱化;循环剪切过程中界面摩擦角的发展趋势与剪切位移幅值密切相关;循环剪切形成毗邻界面1cm厚剪切带,剪切带内砂土颗粒破碎率达5%以上.

试样倾斜和正压力对球形压头微米划痕测试紫铜的影响

采用球形压头对紫铜进行微米划痕试验,研究了不同恒定正压力下试样倾斜对划痕测试紫铜的影响.结果表明:试验测得的名义摩擦系数与试样倾斜角度线性相关,且斜率不受正压力的影响.建立了球形压头与试样倾斜状态的位置关系模型,发现试样倾斜对名义摩擦系数中的黏着组分无影响,而犁耕组分随倾斜角度线性变化.试样倾斜时的摩擦系数可通过球-面接触力学模型进行校正,获得试样无倾斜时的摩擦系数.摩擦系数与正压力之间呈非线性关系,其中犁耕组分线性增大,黏着组分非线性增大并趋于稳定.通过划痕形貌测量的残余划痕宽度几乎不受试样倾斜的影响,但与正压力的0.5次方存在线性关系.分析了划痕硬度随正压力的变化,发现由残余划痕宽度计算得到的划痕硬度几乎为恒定值,不受正压力影响,而由接触投影面积计算得到的划痕硬度随正压力增大而增大,随后趋于稳定.

CNL及CNS条件下结构面剪切特性试验研究

为比较CNL及CNS边界条件下结构面剪切力学特性的差异,开展2种法向边界条件下不同粗糙度结构面的直剪试验研究,系统阐述法向刚度、初始法向应力及结构面粗糙度对结构面剪切力学特性的影响。试验结果表明,低法向应力水平下,法向刚度效果显著,CNS条件下剪切位移曲线展现出明显的硬化特征,剪胀曲线被抑制,峰值及残余剪切强度、峰值剪切位移及峰值法向位移均大于CNL试验结果。随着法向应力增大,法向刚度效果逐渐减弱,高法向应力水平下,CNS和CNL试验结果相近。试验成果有助于进一步认识CNL和CNS条件下结构面剪切力学特性及其差异。

岩石节理剪切力学特性的非线性理论模型研究

针对人工节理试样开展不同法向应力条件下的直剪试验,基于试验结果的非线性特征,对Goodman线性剪切模型进行改进,提出一种更接近实际情况的非线性全量剪切模型,该模型大部分参数均可从试验数据获取,并能准确描述节理剪切过程中剪切应力与剪切位移之间的非线性关系。借鉴Saeb和Amadei切向与法向耦合刚度系数的计算方法,结合提出的非线性全量剪切模型,推导出可以反映节理剪切非线性特性的切向耦合刚度系数表达式,并建立基于非线性力学行为的节理增量剪切模型,该增量模型可以对节理的切向和法向变形行为同时进行描述。在常法向应力边界和常法向刚度边界条件下分别对增量剪切模型进行编程求解。将全量模型的计算值与已有直剪试验结果进行对比分析,两者吻合得很好,验证提出的全量模型的准确性与可靠性。分别考虑常法向刚度和常法向应力两种边界条件,将增量模型模拟值与已有的试验结果及Saeb-Amadei模型进行对比分析,增量模型计算的剪应力与试验结果吻合得更好,说明提出的增量模型比Saeb-Amadei模型准确度更高;增量模型计算的法向应力、法向位移与试验值大体上吻合,总体上可以反映节理剪切的法向力学行为,说明增量模型可以较好地反映节理试样剪切过程中的切向和法向变形行为,验证了增量模型在不同边界条件下的适宜性。