不同应力路径下砂岩能耗变化规律试验研究

为研究不同应力路径下岩石的能耗变化规律,采用MTS815岩石力学试验系统开展了砂岩的单轴压缩、常规三轴和卸荷三轴试验。结果表明:耗散能曲线变化是岩石内部损伤和破裂产生的表现,在弹塑性变形阶段,常规三轴耗散的能量占岩石吸收总能量的比例最大,卸荷三轴次之,单轴压缩最小;岩石的储能极限与围压具有明显的线性关系,单轴压缩试验中岩石的储能极限最低,卸荷三轴次之,常规三轴试验岩石的储能极限最高;岩石峰前和峰后的能量耗散速率与围压也具有良好的线性关系,峰后应力跌落阶段能量耗散速率明显较峰前能量耗散速率大数倍至数十倍,说明岩石峰前损伤速率较小,而峰后却快速损伤破裂,耗散能曲线的突然变陡表明岩石破坏发生。     

Laboratory and theoretical investigations on the deformation and strength behaviors of artificial frozen soil

In order to study the mechanical characteristic of artificial frozen soils, such as strength, and stress-strain relationship, a series of triaxial compression tests of frozen sand has been conducted under confining pressures varying from 0.0 to 14.0 MPa with different water contents at − 6 °C. Frozen sand presents strain softening during shearing process under low confining pressures; but with increasing confining pressure, the strain softening decreases, and even presents strain hardening under high confining pressures. The strength of frozen sand is affected by water content and confining pressure. The strength with low water content always increases with increasing confining pressure; however, for frozen sand with a high water content, the strength experiences an increase followed by a decrease with increasing confining pressure. To describe the strength characteristic of frozen sand, the non-linear Mohr-Coulomb criterion, in which the generalized internal friction angle and cohesion under various confining pressures are obtained from experimental results, has been presented. The result shows that the non-linear Mohr-Coulomb criterion can reflect the decrease of strength of frozen sand under high confining pressures. The stress-strain relationships of frozen sand are represented by hyperbolic functions, which can describe both the strain hardening behavior of frozen sand under high confining pressure and the strain softening behavior under a low confining pressure.     

渗透压循环加卸载作用下页岩渗透率演化规律研究

为研究页岩在渗透压循环加卸载作用下渗透率演化规律,利用改进的脉冲衰减气体渗透率测量装置对页岩进行不同围压、不同渗透压循环加卸载试验。试验结果表明:初始围压的大小会影响页岩的初始渗透率,初始围压越大初始渗透率越小;在定围压循环加卸载渗透压的过程中,渗透率随着渗透压的加载而增大,随着渗透压的卸载而减小;在同一渗透压加卸载循环中,加载阶段和卸载阶段的渗透率曲线不重合,且卸载阶段渗透率曲线位于加载阶段渗透率曲线的上方;对渗透率随渗透压循环加卸载的变化关系进行拟合,拟合结果符合二次函数关系,相关系数大于95%。     

混凝土三轴蠕变统计损伤模型研究

为探讨混凝土三轴蠕变特性,采用SAM-2000微机控制电液伺服材料三轴试验机并配备压力自平衡式三轴压力室对混凝土进行了三轴压缩试验和三轴蠕变试验,结果表明:围压的存在可以显著提高混凝土的承载能力,围压越大,混凝土的轴向承载能力越高,蠕变变形量和变形速率越小。假设混凝土破坏为其内部微元破坏累积所致,微元破坏概率与混凝土应变存在一定关系P[#x003b5;],且服从Weibull分布,结合三轴压缩试验结果,计算得到了描述混凝土损伤特性的统计损伤变量;基于Burgers流变模型,通过引入损伤变量建立了混凝土三轴蠕变统计损伤模型,利用MATLAB软件进行了模型参数拟合,经对比分析,模型曲线与试验曲线拟合结果较理想,主偏应力越大,拟合结果越精确。此模型可供今后研究参考。     

三轴SHPB加载下砂岩力学特性和破坏模式的试验研究

利用改造的三轴SHPB动静组合加载实验装置,对均质砂岩进行了不同围压和不同应变率下的三轴冲击压缩试验,作为对比利用RMT-150C试验机也进行了部分准静态下的三轴压缩实验。根据实验结果,分析了围压对砂岩动态冲击性能的影响,并重点讨论了冲击过程中岩石的破坏模式。研究结果表明,在围压一定的情况下,岩石的动态压缩强度随应变率的提高而提高;在应变率相同的情况下,岩石的动态压缩强度和弹性模量会随着围压的增大而增大。岩石发生破坏的临界入射能,随着围压的增大而增大。岩石单位体积吸收能与应变率之间呈线性递增关系,而且递增的程度随着围压的增加而增加。三轴冲击加载下,应变率较低时岩石内部形成压剪破裂面但整体不失稳,应变率很大时岩石破碎形成锥形块体形式。     

脆性材料水泥砂浆多轴应力下的动态响应分析

在实际的结构应用中,混凝土类材料一般处于复杂工作应力状态,且可能承受动态荷载的作用。据此,本文采用Instron3421液压伺服试验机和具有主动围压加压装置的SHPB研究了混凝土材料-砂浆宽应变率范围多轴应力下的动态力学行为;基于Johnson-Cook强度模型框架,确定了等效强度模型的率相关参数及其他材料常数;提出了适用于描述主动围压下砂浆受冲击荷载的损伤演化规律,并确定了损伤演化常数,实验数据与理论值吻合较好     

多围压下三轴压缩试验与不可破裂颗粒离散元法分析

采用大型三轴试验仪器对道砟散体材料进行试验,重点研究不同围压条件下三轴试验力学特性与影响规律。同时基于离散单元法(DEM),采用非破裂不规则颗粒簇,在伺服条件下建立道砟三轴试验散粒体模型,对道砟颗粒在不同围压下进行相关数值三轴模拟,探讨偏应力、体应变、围压、体积应变与轴应变之间的关系,反映道砟基本力学特性。研究结果表明:采用离散单元方法进行数值模拟与室内三轴试验得到的一系列应力-应变规律基本一致,验证了离散单元法在道砟研究中的可行性与可靠性。同时,围压对道砟散体变形、最大偏应力、偏应力比值具有影响,其中随着围压提高道砟变形降低,因此在有砟道床设计和养护维修中,增大道床围压可降低道床沉降与变形。     

温度和损伤程度对砂岩渗透特性影响的试验研究

岩石的渗透特性与许多地下工程的稳定与安全有关,而深部岩石工程环境所处的温度、应力以及自身损伤情况对渗透特性也有较大的影响。因此,有必要开展损伤岩石在不同温度、应力情况下渗透特性研究。利用GCTS高温高压动态岩石三轴仪、岩石全自动气体渗透率测试系统,对不同损伤程度砂岩进行了不同围压和实时温度的渗透特性试验。试验结果表明:不同损伤程度砂岩的渗透率与围压之间呈现幂函数负增长关系,在低围压向高围压过渡过程中,高损伤试样渗透率出现较大的跌落;不同损伤程度砂岩渗透率与温度关系为负相关,高损伤试样渗透率随着温度增加下降得更加缓慢;随着损伤程度的增加,砂岩渗透率呈现出低围压情况下先略微下降再上升,高围压情况下逐渐上升的特征。低围压状况下,损伤程度较高的试样之间渗透率差异较大。     

Experimental behaviour of high-performance concrete in confined tension

Prestressed concrete is commonly used in a wide range of applications, yet on rare occasions spalling problems have been reported for concrete structures prestressed in two directions. The tensile behaviour of concrete may be affected by prestressing; consequently a number of experiments have been performed to reproduce this load combination on laboratory samples. Tests were carried out in a compression-tension triaxial cell suitable for the independent application of a lateral confining pressure, as well as a direct axial tension load. Results reveal a measurable but limited reduction in concrete tensile strength with a confining pressure of a similar magnitude as the prestress (order of magnitude of 10 MPa). Further experiments were conducted at higher confining pressures, in both confined tension and in triaxial extension, in order to generate an overview of limit state behaviour for the studied high performance concrete.     

Experimental study on dynamic damage evolution of concrete under multi-axial stresses

The effect of stress state on the dynamic compressive strength and the dynamic damage evolution process of concretes are investigated by use of a Spilt Hopkinson Pressure Bar (SHPB) and the ultrasonic technique. The columned concrete specimen is encircled by a steel sleeve. The multi-axial loading includes the axial and the radial loadings. The axial loading is supplied by the incidence bar, and the radial ones are produced by the steel sleeve. Analysis of the dynamic damage evolution of the samples is based on the measurement of the changes of ultrasonic wave velocities before and after the impact tests. The waveforms in the test bars, the stress strain curves, the confining pressure of the specimen, the dynamic compressive strength and other information about the samples are obtained during the SHPB experiments. The results of the tests show that the loading rate and stress states of the specimen apparently influence the damage evolution process in concretes. The dynamic damage evolutions are accelerated with the increase of the strain rate and are delayed significantly under the confined pressure.     

Viscoplastic modeling of asphalt mixes with the effects of anisotropy, damage and aggregate characteristics

This paper presents an approach for constitutive modeling of the viscoplastic behavior of asphalt mixes. This approach utilizes an anisotropic non-associated flow rule based on the Drucker–Prager yield surface. The selection of this yield surface is motivated by the field stress paths and material properties associated with permanent deformation at high temperatures. The efficacy of the model is demonstrated by analyzing data from compressive triaxial tests conducted at different confining pressures and strain rates for three different mixes. The model parameters are related to the experimental measurements of aggregate shape characteristics, aggregate surface energy, inherent anisotropic distribution of aggregates, and microstructure damage measured using X-ray computed tomography and image analysis techniques. Establishing the relationship between the model parameters and material properties is important in order to optimize the mix properties, and achieve desirable mix performance.     

被动围压条件下岩石材料冲击压缩试验研究

为研究煤矿岩石材料被动围压条件下动态力学性能和变形破坏规律,利用Ø50mm变截面分离式Hopkinson压杆(SHPB)试验装置,对45#钢质套筒环向约束状态下煤矿岩石试件进行了不同加载速率冲击压缩试验。试验结果表明：被动围压条件下SHPB试验中,岩石试件的材料延性和抗破坏能力均得到增强,试件轴向应力是采用同种加载条件无围压SHPB试验时的1.2倍,破坏应变比无围压SHPB试验提高2～3倍,且径向应力随轴向应变增大总体呈上升趋势,试件破坏为压剪破坏模式,与无围压SHPB试验有所不同。     

不同升温-降温路径下中空圆柱饱和粉质黏土的热固结

利用自行研制的适用于中空圆柱体试样并可控制试样内、外边界温度的热固结试验装置,进行了一种饱和粉质黏土的热固结试验。温度路径包括试样内、外边界等温单级和多级升温-降温以及内、外边界不等温多级升温-降温,温度荷载的施加范围为25℃~75℃,围压为50 kPa、100 kPa、150 kPa、200 kPa四种情况。论文研究了不同升温-降温路径和不同固结压力条件下饱和粉质黏土的孔隙水压力和固结体应变随时间的演化规律,并对不同温度路径下的试验结果进行了比较。分析表明:同一温度路径下,围压越大,升温或降温所产生的最大归一化孔隙水压力的绝对值越小,固结体应变的大小与围压大小并不成单调关系;同一围压下,升温或降温幅度越大,所产生的最大归一化孔隙水压力的绝对值越大,相应的固结体应变也越大;经过不同温度路径升、降温到同一温度值,设置的温度等级越多,产生的体应变也就越大,并且相同温度等级路径,内、外等温的情况要比不等温情况产生的体应变大。     

A discrete element model of concrete under high triaxial loading

A numerical model based on a three dimensional Discrete Element Method (DEM) has been used to study the behavior of concrete under high-confining pressures (up to 650 MPa). At this range of pressures, irreversible compaction of the material occurs and needs to be considered. Within the discontinuous nature of the model, a local constitutive law has been developed to reproduce this phenomenon quantitatively. Local parameters to be used in this constitutive law are identified by simulating reference uniaxial and triaxial experimental tests in compression. Once these parameters have been obtained, the model is used to predict the response of concrete sample for triaxial compressive tests at different levels of confinement. Beyond the macroscopic volumetric and stress–strain response, the model gives interesting insights on the local evolution of the nature of the interaction forces between the discrete elements. The computational implementation has been carried out in the discrete element and open source code YADE (http://yade-dem.org[20]).     

围压对Ⅱ型CA砂浆力学性能的影响

对不同围压下的CRTS-Ⅱ型CA砂浆进行了三轴压缩试验,并测定CA砂浆的应力应变曲线。分析了峰值应力、峰值应变、残余强度及弹性模量随围压的变化规律,并对其进行线性拟合。结果表明,随着围压的增大,应力应变曲线峰值处逐渐平缓;峰值应力、峰值应变及残余强度逐渐变大,弹性模量基本不变;CA砂浆试件破坏时的裂缝与轴线的夹角逐渐增大。     

三向应力状态下混凝土强度和变形特性研究

对设计强度为１０MPa的混凝土立方体试件进行三轴压缩试验,系统研究了等围压条件下混凝土的强 度和变形特性,围压分别为０,４,８,１２,１６MPa５个量级;同时研究了不等围压条件下混凝土强度变化特性, 试验表明,随着围压增加,混凝土的极限抗压强度有明显增强的趋势;随着小主应力的增加,中主应力的影响 有减弱的趋势;峰值应力处应变随围压增加幅度显著。得出了在不同恒定围压下混凝土的应力应变全过程曲线; 通过与当前文献资料的对比分析,指出了在混凝土结构计算中适用的强度及变形表达式,为工程实践提供了 依据。     

A constitutive model for the uplift behavior of anchors in cohesionless soils

Abstract

A series of triaxial tests has been performed to establish the stress‐strain curves for I‐Lan sand and Taipei silty sand. A constitutive model for the continuous strain hardening‐softening and volumetric dilatancy of these two soils is proposed, based on the results of triaxial tests. Using this model, a numerical program is then established, with FLAC software, to analyze the uplift behavior of model anchors in sand and field anchors in silty soil.

It was found from triaxial tests, that the peak friction angle increases with relative density of soil and decreases with confining pressure. A non‐associated flow rule between plastic strain increment and stress tensor was found. As accumulative plastic strain, relative density and confining pressure were changed, the mobilized friction angle and mobilized dilatancy angle also changed. All parameters needed for the proposed model can be expressed as functions of relative density and confining pressure. This model can calculate the stress‐ strain curves of cohesionless soils determined from triaxial tests accurately.

The load‐displacement behaviors determined from anchor tests are compared with those calculated from this numerical program, the numerical results are in good agreement with the test results not only for model anchors in sand but also for different types of field anchors in silty soil.     

Discrete element simulation of railway ballast: modelling cell pressure effects in triaxial tests

The paper investigates reproducing the effects of confining pressure on the behaviour of scaled railway ballast in triaxial tests in discrete element models (DEM). Previous DEM work, using a standard Hertzian elastic contact law with an elastic–perfectly plastic tangential slip model, has been unable to replicate the behaviour observed in laboratory tests across a range of confining pressures without altering both the material stiffness and the inter-particle friction. A new contact law modelling damage at the contacts between particles is introduced. Particle contact is via spherically-capped conical asperities, which reduce in height if over-stressed. This introduces plasticity to the behaviour normal to the contact surface. In addition, the inter-particle friction angle is varied as a function of normalized contact normal force. At relatively low normal forces the friction angle must be increased for peak mobilized friction angles to match the laboratory data, an effect that is attributed to interlocking at the scale of surface roughness. Simulation results show close agreement with laboratory data.     

冲击荷载下花岗岩残积土的滞回曲线特征与损伤定量评价

为评价冲击荷载下花岗岩残积土的损伤发展规律,基于不同振幅(A = 100 ～400 kPa)、频率(f=3 ～15 Hz)和围压(σ'3 =50 ～500 kPa)下室内循环冲击试验得到滞回曲线的形态特征,提出4个反映试样在冲击荷载下的能量消耗、损伤程度、刚度衰减和塑性应变发展特性的定量结构损伤参数:累积耗散能量EN、...     

砂岩渗透性演化特性的孔隙率分布细观模拟分析

通过不同围压下的三轴渗透试验,获得砂岩全应力-应变过程中渗透率变化数据。基于试验结果,建立砂岩试件离散元颗粒流数值模型,模拟得到砂岩压缩变形过程中体应变、孔隙率分布和黏结断裂特征。考虑围压效应,从细观机制上解释了动态孔隙率分布对渗透性演化特性的影响。研究结果表明,低围压下砂岩出现贯通的剪切带,裂隙易张开,对渗流具有显著的促进作用。围压越高,砂岩试样的黏结断裂数量和范围越大,裂隙数量增多,但裂隙和孔洞也容易被压密,从而对渗流起抑制作用。贯通的张开裂隙会大幅增加渗透性,而曲折间断的张开裂隙只会小幅增加渗透性,周围被闭合裂隙和压密的孔洞包裹的完全孤立的张开裂隙则对渗透性贡献很小。