ECC三轴压缩特性试验研究

为探究纤维增强水泥基复合材料(ECC)在复杂应力状态下的受压特性,通过常规三轴压缩试验得到了不同围压下的ECC应力-应变曲线,分析了ECC的强度特性、变形规律以及破坏模式.试验结果表明:PVA纤维的掺入、施加围压能够有效约束ECC的径向变形并提高其韧性,且ECC的轴向峰值抗压强度和峰值应变随围压的提高而增大.ECC的三轴受压变形过程表现为弹性变形-应变硬化-应变软化三阶段,且围压越大,应力-应变曲线的软化段变化越缓和.在三轴受压状态下,ECC的主压应力、轴向峰值应变与围压有较好的线性相关性.     

基于三轴压缩试验的软岩遇水强度软化规律研究

针对软岩遇水软化导致强度降低的现象,对云南省富源县的典型软岩试样进行了三轴压缩试验,讨论了软岩遇水软化的强度变化规律。试验首先获取了该软岩的矿物成分、扫描电镜图片和浸水前后的结构状态变化;然后分别对天然状态和饱水状态的两种试样开展了常规三轴压缩试验,测定不同围压(1、3、5 MPa)下的应力-应变关系曲线。试验结果表明:软岩在浸水12h后,其岩体结构出现明显的崩解现象,表明软岩的水敏性较强;分析软岩的三轴应力-应变曲线可以看出该试样主要经历初始压密、线弹性变形、塑性变形和破坏后软化4个阶段;通过对比两种状态试样的峰值强度发现该软岩试样的软化程度随围压的增加有一定程度的减弱。研究结果对于围岩支护工程中软岩的长期稳定性分析具有一定的参考价值。     

基于三轴压缩试验的水泥土力学特性研究

处于不同环境和条件下的水泥土力学特性直接影响水泥土工程的安全性和耐久性.因此,通过室内三轴试验,设计了不同的水泥掺入比,不同围压,对分别在海水和清水环境下养护的试件进行了水泥土的三轴压缩试验,研究了海水环境、水泥掺量、围压对水泥土的力学特性的影响规律.试验结果表明,水泥土三轴压缩的应力-应变曲线总体呈现软化型,水泥掺量愈大,其水泥土的脆性特征越显著;随着围压的增加,试件破坏时的延性特征越明显;随着水泥掺量的增加,水泥土的极限应力差值逐步增大,而极限应力差值所对应的应变逐步变小;随着围压的增大,水泥土的极限应力差值逐步增大,而水泥土破坏时峰值所对应的应变越大;水泥土的黏聚力和内摩擦角随着水泥掺量的增加均逐步增大;与清水环境相比较,海水环境下水泥土的抗剪强度、黏聚力和内摩擦角均有降低.其研究成果为水泥土工程的安全性和耐久性设计提供技术参数,具有一定的工程意义和参考价值.     

干湿循环作用对煤矸石稳定膨胀土行为的影响

以煤矸石粉掺入膨胀土中改良膨胀土的胀缩性行为,实现固体废弃物煤矸石再利用为目的.将不同掺量的煤矸石粉加入膨胀土中,进行干湿循环试验,检测试件的含水率、轴向胀缩变形量和强度变化.通过压汞试验获得试样的孔隙特征值,从微观角度揭示膨胀土胀缩行为机理.试验结果表明:吸水膨胀后的试样最终含水率随干湿循环次数的增加而降低,干燥收缩后的试样最终含水率随干湿循环次数的增加而增加;随干湿循环次数的增加,收缩和膨胀试样最终轴向应变逐渐减小,这与含水率变化规律一致;绝对膨胀率和收缩率在第1次循环中达到最大值,此后逐渐减小并趋于稳定.随着干湿循环次数增加黏聚力增强而内摩擦角有所降低,掺加6％煤矸石粉后使膨胀土试样的抗剪强度增大.从孔隙特征分析,试样的膨胀率和收缩率得到抑制,强度增加的主要原因在于掺入煤矸石粉改变了膨胀土的孔径分布.     

非饱和重塑Q3黄土抗剪强度分析

以宝兰客专隧道典型黄土为研究背景,探讨了含水率对重塑Q3黄土抗剪特性的影响,并通过数据拟合得到重塑Q3黄土破坏强度与含水率之间的关系表达式。结果表明:随着含水率的增加,土样的破坏强度随之减小;在同一含水率下,随着围压的增加,其抗剪强度逐渐增大。在低围压下试样有明显的峰值,为脆性破坏;在较高围压下试样为塑性破坏。含水率较低时,剪应变呈现出应变软化及弱硬化状态;含水率较高时,剪应变呈现出硬化及强硬化状态。在轴向应变较小时,重塑Q3黄土的应力-应变呈现出近似线性的关系;当轴向应变较大时,应力-应变呈现出高度的非线性关系。通过线性回归方法以围压和含水率数据回归出重塑黄土的破坏强度计算式,对比计算数据和实测数据,发现它们之间的相对误差较小,实测值与计算值的变化趋势基本一致,本研究成果可以为类似工程提供依据。     

三轴应力条件下泡沫轻质土压缩特性

为探究在复杂应力条件下泡沫轻质土的压缩特性,通过不固结不排水三轴压缩试验研究了三种不同湿密度的泡沫轻质土在多种围压状态下的应力-应变分布规律、抗剪强度特性和破坏形态.结果 表明,泡沫轻质土的三轴压缩变形过程可划分为弹性变形-应变硬化-压密破坏三个阶段.同一湿密度泡沫轻质土的偏应力和峰值应变随加载围压增大均呈现先增大后减小规律.提高泡沫轻质土湿密度,降低孔隙率,能够增大其粘聚力及内摩擦角,粘聚力约为同龄期无侧限抗压强度0.34倍,实测轻质土侧向压力系数与理论计算值一致.轻质土试件破坏形态与其力学强度有关,可根据最终破坏形态分为剪切破坏和压密破坏两类.     

不同方法测定水泥土渗透系数的研究

为研究适合测定水泥土渗透系数的试验方法,结合工程实际,对防渗墙进行现场钻孔注水试验,对钻芯取样的水泥土试样进行室内变水头渗透试验和三轴渗透试验,对比室内试验结果和现场试验结果,探讨适合检测水泥土芯样的室内试验方法,分析三轴渗透试验中孔隙水压和围压对水泥土渗透性的影响。对重新配制的水泥土试样进行室内变水头渗透试验、三轴渗透试验及水泥土渗透试验,研究不同方法测定水泥土渗透系数的差异。试验结果表明:三轴渗透试验比室内变水头渗透试验更适合测定水泥土芯样渗透系数;三轴渗透试验中,围压为0时,水泥土芯样渗透系数会随着孔隙水压的增加而增加,围压改变时,会随着围压的增加而减小;三种室内渗透试验方法测定水泥土试样的渗透系数基本相接近,都可用于测定水泥土试样渗透系数。     

铁尾矿粉改良水泥土的强度与动力特性试验研究

为提高水泥改良土强度,验证尾矿粉能够起到对水泥土的增强效果,对已用水泥改良过的路基土,在尾矿粉掺入量不同的条件下,经改良后水泥土的强度特性,以及对被最佳掺量的尾矿粉改良后的水泥土,在围压和频率不同的条件下进行动力特性的试验研究,制备了不同尾矿粉掺量(0%、2%、4%、6%、8%、10%)的试验土样进行无侧限抗压强度试验和动三轴试验;试验结果表明:随着尾矿粉掺入量的增加,无侧限抗压强度先增大后减小,土的应力应变曲线先急剧增加达到峰值后逐渐趋于平缓.当尾矿粉掺量达到最佳时,分析不同围压和不同频率的条件下G/Gmax~γ、λ~γ变化规律.对于同一动剪应变幅,动剪模量比随围压的增加而增大.阻尼比随动剪应变幅的增大而增大,随围压的增大而减小.G/Gmax~γ、λ~γ关系受频率影响不太敏感,都集中于一狭小范围.该试验能够提高水泥土强度的尾矿粉最佳掺量为6%.     

心墙沥青混凝土静三轴试验的剪胀性研究

沥青混凝土作为悬浮密实型结构,其剪切特性受密度及应力水平的影响。沥青混凝土的剪胀性将会对其内摩擦角,凝聚力,峰值强度及渗透系数有很大的影响。通过静三轴试验后孔隙率的变化情况表明,随着围压的增加,沥青混凝土的剪胀性变得明显,孔隙率增加明显;骨料在围压为2MPa时,骨料并不会发生破碎,通过测定试样的孔隙率得出,增加的孔隙率均为产生的空隙。     

冻融循环影响下水泥、煤矸石改良粉土的力学特性研究

为研究水泥、煤矸石改良粉土路基在冻融循环作用下的力学性质,以高速公路粉土填料为研究对象,制作了含5％水泥和20％煤矸石的改良粉土试件,对其进行了负温度分别为-25℃、-30℃、-35℃的冻融循环试验,并以冻融次数和围压为变量对改良粉土进行三轴压缩研究.研究结果表明:增加围压和冻融次数、降低温度是改良粉土应力-应变曲线由应变软化型向应变硬化型转变的因素;冻融循环次数的增加,改良粉土极限强度、弹性模量、粘聚力及内摩擦角等指标的衰减规律为快速、缓慢、稳定三个阶段,趋于稳定的冻融循环次数为8次;温度的降低对改良粉土极限强度、弹性模量、粘聚力劣化影响较大,而对内摩擦角的劣化影响较小,力学指标劣化程度有收敛趋势.此研究可为粉土在路基工程中的应用和防治灾害提供一定的工程价值.     

Influence of free water on the quasi-static and dynamic strength of concrete in confined compression tests

The behaviour of concrete under high pressure and dynamic loadings is experimentally investigated in the present paper. The specimen is confined in a cylindrical elastic steel ring that insures a quasi-uniaxial strain state of loading. It is subjected to static and dynamic (with strain rates in the range from 1e−6/s to 200/s) axial compressive loadings. Transverse gauges glued on the lateral surface of the ring allow for the measurement of the confining pressure so that the volumetric and the deviatoric response of the specimen can be computed. At high or intermediate strain rates, water saturated and dried specimens show strongly different results: i.e. a continuous increase of strength with pressure in dried specimens and a quasi nil strength enhancement in water-saturated specimens. This difference is not observed with quasi-static loadings. As explained through a basic poromechanics analysis, this dissimilarity is mainly attributed to an increase of pore pressure inside the saturated concrete during fast (quasi-static or dynamic) experiments.     

Dynamic Failure of Borosilicate Glass Under Compression/Shear Loading Experiments

Dynamic compression/shear experiments on a borosilicate glass at an average strain rate of 250 s⁻¹ are conducted using a modified version of a split Hopkinson pressure bar. Instead of applying confining pressure, cuboid specimens with the material axis inclining to the loading direction at different angles (0°, 3°, 5°, and 7°) are used to generate higher shear stresses. A high-speed digital camera, synchronized with the loading stress pulse, is used to record the dynamic crack initiation and propagation. Experimental results show that the equivalent stress at failure decreases with increasing shear portion in the stress. Digital images show that the cracks initiate randomly in the right specimen, whereas cracks initiate from the stress-concentrated corners in the inclined specimens. Subsequent crack propagation, despite specimen inclination angles, is along the specimen axis rather than the compressive loading direction.     

Post-yield behaviour of low density polyethylenes: 2. Plastic fracture

Diamond-shaped cavities have been observed to grow in a stable fashion through oriented low density polyethylenes, linear and high pressure, and account for the ultimate fracture characteristic of these polymers. Their growth has been examined by the application of fine grids on the specimen surface, and (adjacent) elements of material at the diamond tip deform in simple shear in turn, as the maximum shear is attained. Simple shear tests on drawn material confirm that the angle between the linear faces of the diamond cavities is determined by the onset of strain softening followed by strain hardening. The characteristics of these determine the extent of yielding in each element.     

含裂隙煤体抗压性能的试验研究与模拟计算

煤体内部天然存在的裂隙是瓦斯流动的主要通道和影响煤矿安全的重要因素。为考察不同原生裂隙分布形态的煤体抗压缩强度与裂纹演化规律,对原煤试样进行初始裂隙的提取和室内三轴压缩试验,结合数值模拟方法进一步研究了含裂隙试样在围压分别为1, 3, 5, 7, 9 MPa和加载速率在0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0 m/s下的峰值应力及破坏形态。结果表明,煤样的峰值应力随围压增加呈指数形式增大。垂直方向延伸的原生裂隙对裂纹扩展带的影响较小,当原生裂隙的延伸角度倾向于对角线方向时,裂纹容易在裂隙端部萌生和扩展。试样2的“V”型裂纹带在低围压(1, 3, 5 MPa)时受原生裂隙影响较大,高围压(7和9 MPa)下的破坏特征与试样1相似。随着加载速率逐渐递增,试样的抗压强度变化经历先缓慢上升,后迅速增加,最终趋于平缓的过程。低加载速率下的裂纹多聚集在原生裂隙延伸方向上,较高的加载速率容易造成裂纹集中于试样的角部位置,裂纹数目也迅速增多。     

Influence of random pore defects on failure mode and mechanical properties of SiC ceramics under uniaxial compression using discrete element method

To investigate the relationship between micro-defects in ceramic materials and macro mechanical properties and behaviours, a computational model of SiC ceramics with randomly oriented elliptical pores was established using the discrete element method (DEM). The effects of pore defect content and its aspect ratio on the failure mode, stress-strain curve and mechanical properties of specimen were investigated under uniaxial compression. The effective Young's modulus which was obtained from DEM simulations was compared with the predictions of Mori-Tanaka scheme (MTS) and Self-Consistent scheme (SCS) at various pore defect densities. The results showed that the compressive strength and crack initiation stress decrease nonlinearly as the pore defect content increases. Furthermore, the smaller the aspect ratio of the elliptical pore defects was, the more obvious the weakening trend was. As the pore defect content increases, the failure mode of the specimen changed from brittle fracture to tensile-shear mixing and then to axial splitting. The stress-strain curves showed a certain “softening” period during the loading process. The effective Young's modulus obtained from the DEM simulations coincides with the approximations of MTS and SCS at low pore densities. However, when the pore defect density became larger, the DEM simulation results were slightly lower than the theoretical results of the Mori-Tanaka scheme, which only considers the weak interaction between defects.     

Nonlinear finite element analysis of the butt-joint elastomer specimen

The practical strength of a butt-joint specimen is of great importance to many industrial applications such as adhesive joints, elastomer mountings, flexible couplings, etc. A butt-joint specimen could fail either cohesively or interfacially, depending on the strength of the materials and the stress distribution in the specimen. In the past, engineering design has been based either on theoretical linear analysis or on empirical rules of thumb. A more realistic analysis based on the nonlinear finite element (FE) method is presented here. The elastomer layer in the butt-joint specimen is modeled by a modified Ogden-Tschoegl strain energy function. The nonlinear axisymmetric FE program is formulated on the total Lagrangian procedure. The nominal strain, the thickness of the rubber layer, the compressibility (or Poisson's ratio), and the strain-hardening (or softening) parameter are taken as the variables in the analysis. The maximum radial and axial stresses are found along the central axis, while the maximum shear stress is near the corner of the bond plane and the free lateral surface. The stiffness as a function of the apparent strains is obtained for various thicknesses, various Poisson's ratios, and various strain-hardening parameters. The lateral contraction and the volume dilatation of the specimen are also calculated and related to the stress distribution in the specimen. A well-defined peak load occurs at a critical strain for thin specimens made of materials with a low strain-hardening parameter and high Poisson's ratio values.     

Experimental Study on Impact‐induced Initiation Thresholds of Polytetrafluoroethylene/Aluminum Composite

Impact experiments were conducted with a gas gun to investigate the impact‐induced initiation thresholds of a polytetrafluoroethylene/aluminum (73.5 wt % and 26.5 wt %) composite. Targets of steel, aluminum, and low‐density polyethylene materials and sample rods of four different lengths were used to decouple the effects of impact pressure and loading strain rate. By subjecting the samples to different loading conditions, it was shown that the impact‐induced initiation of polytetrafluoroethylene/aluminum is decided by the impact pressure and the loading strain rate simultaneously. The impact pressure and strain‐rate thresholds for initiation were arrested by the experiments. A 30° inclined steel target was used to produce a compression‐shear configuration as a comparison with the normal impact experiments. The initiation was more likely to happen; it demonstrated a shear‐induced initiation mechanism, and a lower initiation strain‐rate threshold was observed under oblique impact. Based on the experimental results, two theoretical curves were proposed to predict the impact‐induced initiation of polytetrafluoroethylene/aluminum under normal and 30° oblique impact.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Application of digital image correlation method to biogel

This study adopts the digital image correlation (DIC) method to measure the mechanical properties under tension in agarose gels. A second polynomial stress–strain equation based on a pore model is proposed in this work. It shows excellent agreement with experimental data and was verified by finite element simulation. Evaluation of the planer strain field by DIC allows measurement of strain localization and Poisson's ratio. At high stresses, Poisson's ratio is found to exceed the standard assumption of 0.5 which is shown to be a result of pore water leakage. Local failure strains are found to be approximately twice those determined by crosshead displacements. Viscous properties of agarose gels are investigated by performing the tensile tests at various loading rates. Increases in loading rate do not cause much difference in the shape of stress–strain curves, but result in increases in ultimate stress and strain. POLYM. ENG. SCI., 50:1585–1593, 2010. © 2010 Society of Plastics Engineers