Static and dynamic tensile failure characteristics of rock based on splitting test of circular ring

Static and dynamic splitting tests were conducted on ring marble specimens with different internal diameters to study the tensile strength and failure modes with the change of the ratio of internal radius to external radius （ρ） under different loading rates. The results show that the dynamic tensile strength of disc rock specimen is approximately five times its static tensile strength. The failure modes of ring specimens are related to the dimension of the internal hole and loading rate. Under static loading tests, when the ratio of internal radius to external radius of the rock ring is small enough （ρ〈0.3）, specimens mostly split along the diametral loading line. With the increase of the ratio, the secondary cracks are formed in the direction perpendicular to the loading line. Under dynamic loading tests, specimens usually break up into four pieces. When the ratio ρreaches 0.5, the secondary cracks are formed near the input bar. The tensile strength calculated by Hobbs’ formula is greater than the Brazilian splitting strength. The peak load and the radius ratio show a negative exponential relationship under static test. Using ring specimen to determine tensile strength of rock material is more like a test indicator rather than the material properties.

Tensile strength of sea ice using splitting tests based on the digital image correlation method

The splitting test is a competitive alternative method to study the tensile strength of sea ice owing to its suitability for sampling.However,the approach was questioned to the neglect of local plastic deformation during the tests.In this study,splitting tests were performed on sea ice,with 32 samples subjected to the regular procedure and 8 samples subjected to the digital image correlation method.The salinity,density,and temperature were measured to determine the total porosity.With the advantage of the digital image correlation method,the full-field deformation of the ice samples could be determined.In the loading direction,the samples mainly deformed at the ice-platen contact area.In the direction vertical to the loading,deformation appears along the central line where the splitting crack occurs.Based on the distribution of the sample deformation,a modified solution was derived to calculate the tensile strength with the maximum load.Based on the modified solution,the tensile strength was further calculated together with the splitting test results.The results show that the tensile strength has a negative correlation with the total porosity,which agrees with previous studies based on uniaxial tension tests.

Indirect Tensile Characterization of Graphite Platelet Reinforced Vinyl Ester Nanocomposites at High-Strain Rate

An indirect tensile testing method is proposed for characterizing low strength graphite platelet reinforced vinyl ester nanocomposites at high-strain rate. In this technique, the traditional Brazilian disk (diametrical compression) test method for brittle materials is utilized along with conventional split-Hopkinson pressure bars (SHPB) for evaluating cylindrical disk specimens. The cylindrical disk specimen is held snugly in between two concave end fixtures attached to the incident and transmission bars. To eliminate the complexities of conventional strain gage application, a non-contact Laser Occluding Expansion Gage (LOEG) has been adapted for measuring the diametrical transverse expansion of the specimen under high-strain rate diametrical compressive loading. Failure diagnosis using high-speed digital photography validates the viability of utilizing this indirect test method for characterizing the tensile properties of xGnP (exfoliated graphite nanoplatelets) reinforced and additional CTBN (Carboxyl Terminated Butadiene Nitrile) toughened vinyl ester based nanocomposites. Also, quasi-static indirect tensile response agrees with previous investigations conducted using the traditional dog-bone specimen in direct tensile tests. Investigation of both quasi-static and dynamic indirect tensile test responses shows the strain rate effect on the tensile strength and energy absorbing capacity of the candidate materials. The contribution of reinforcement to the tensile properties of the candidate materials is presented.

Dynamic rock tensile strengths of Laurentian granite: Experimental observation and micromechanical model

Tensile strength is an important material property for rocks. In applications where rocks are subjected to dynamic loads, the dynamic tensile strength is the controlling parameter. Similar to the study of static tensile strength, there are various methods proposed to measure the dynamic tensile strength of rocks.Here we examine dynamic tensile strength values of Laurentian granite(LG) measured from three methods: dynamic direct tension, dynamic Brazilian disc(BD) test, and dynamic semi-circular bending(SCB). We found that the dynamic tensile strength from direct tension has the lowest value, and the dynamic SCB gives the highest strength at a given loading rate. Because the dynamic direct tension measures the intrinsic rock tensile strength, it is thus necessary to reconcile the differences in strength values between the direct tension and the other two methods. We attribute the difference between the dynamic BD results and the direct tension results to the overload and internal friction in BD tests. The difference between the dynamic SCB results and the direct tension results can be understood by invoking the non-local failure theory. It is shown that, after appropriate corrections, the dynamic tensile strengths from the two other tests can be reduced to those from direct tension.

Effects of 3D deformation and nonlinear stress–strain relationship on the Brazilian test for a transversely isotropic rock

To improve the accuracy of indirect tensile strength for a transversely isotropic rock in the Brazilian test, this study considered the three-dimensional (3D) deformation and the nonlinear stress–strain relationship. A parametric study of a numerical Brazilian test was performed for a general range of elastic constants, revealing that the 3D modeling evaluated the indirect tensile strength up to 40% higher than the plane stress modeling. For the actual Asan gneiss, the 3D model evaluated the indirect tensile strength up to 10% higher and slightly enhanced the accuracy of deformation estimation compared with the plane stress model. The nonlinearity in stress–strain curve of Asan gneiss under uniaxial compression was then considered, such that the evaluated indirect tensile strength was affected by up to 10% and its anisotropy agreed well with the physical intuition. The estimation of deformation was significantly enhanced. The further validation on the nonlinear model is expected as future research.

Acoustic emissions evaluation of the dynamic splitting tensile properties of steel fiber reinforced concrete under freeze-thaw cycling

This study empirically investigated the influence of freeze-thaw cycling on the dynamic splitting tensile properties of steel fiber reinforced concrete(SFRC).Brazilian disc splitting tests were conducted using four loading rates(0.002,0.02,0.2,and 2 mm/s)on specimens with four steel fiber contents(0%,0.6%,1.2%,and 1.8%)subjected to 0 and 50 freeze-thaw cycles.The dynamic splitting tensile damage characteristics were evaluated using acoustic emission(AE)parameter analysis and Fourier transform spectral analysis.The results quantified using the freeze-thaw damage factor defined in this paper indicate that the degree of damage to SFRC caused by freeze-thaw cycling was aggravated with increasing loading rate but mitigated by increasing fiber content.The percentage of low-frequency AE signals produced by the SFRC specimens during loading decreased with increasing loading rate,whereas that of high-frequency AE signals increased.Freeze-thaw action had little effect on the crack types observed during the early and middle stages of the loading process;however,the primary crack type observed during the later stage of loading changed from shear to tensile after the SFRC specimens were subjected to freeze-thaw cycling.Notably,the results of this study indicate that the freeze-thaw damage to SFRC reduces AE signal activity at low frequencies.

Uniaxial Compressive and Splitting Tensile Tests of Artificially Frozen Soils in Tunnel Construction of Hong Kong

Three diferent kinds of artificially frozen soils are tested for artificial ground freezing(AGF) project in the tunnel construction of Stonecutters Island Sewage Treatment Works, Hong Kong. Uniaxial compressive test is conducted and uniaxial compressive strength, modulus of elasticity and Poisson's ratio are obtained. Meanwhile, relations of all these three parameters and temperature are fitted by linear function. The linear relationship between the above-mentioned parameters and temperature is suitable for engineering practice. Splitting tensile test of frozen soil is conducted to obtain tensile strength and find out failure pattern in test. All the parameters obtained are necessities in design and practice.

超高性能混凝土-石材界面黏结性能试验研究

超高性能混凝土(UHPC)加固修复既有石结构中,UHPC与石材之间优异的黏结性能是保证加固效果的关键。因此,为探究UHPC-石材界面的黏结性能,通过劈裂抗拉试验和双L型剪切试验,考察不同界面处理方式对UHPC-石材界面黏结性能的影响。结果表明,UHPC-石材界面破坏模式主要分为三种,其中发生模式C破坏(石材UHPC破坏)时界面黏结强度最大。界面键槽处理方式能显著提高UHPC-石材界面黏结强度、界面剪切刚度,且在改变破坏形态的同时有效提升试件的延性,当键槽深度为20 mm时,试件表现出较好的界面黏结性能。另外,试件剪切荷载-位移曲线包含滑移前阶段和滑移后阶段,凿毛试件在滑移后阶段没有经历塑性变形。相同界面处理方式下,UHPC-石材的界面抗剪强度与界面劈裂抗拉强度呈正相关关系。

土体抗拉强度试验研究方法的综述

土体抗拉强度比抗压和抗剪强度数值更小,因此时常被忽略,造成理论及试验的落后。文中系统归纳了土体抗拉强度的间接和直接方法,对比分析了现有试验方法的优缺点,结果表明:(1)土体抗拉强度的单轴拉伸方法按照力的加载方向可以分为垂直法和水平法,而垂直法中土样破坏前上半部分的重量会影响试验的准确性;(2)常用的土样固定方式有粘结、锚固、摩擦力和夹持,它们各有优势,但夹持较前三者更便捷和可操作。最后,提出水平单轴拉伸试验的后续仍需优化的问题。如制样脱模中土样的损耗、土样受拉段应力分布差,无法适用于大骨料砾质土、土样固定方式繁琐、土样与平台间的摩擦力难以消除等。

成型方式对应力吸收层沥青混合料性能的影响

开展了旋转压实与马歇尔击实成型对应力吸收层沥青混合料体积和力学性能的影响分析。试验结果表明,应力吸收层配合比设计中可用马歇尔击实50次代替旋转压实50次成型试件,旋转压实试件与马歇尔试件的劈裂抗拉强度、贯入应力均具有良好相关性,换算系数分别为1.347和0.910。提出了基于马歇尔击实方法的应力吸收层设计要求,可更好地进行配合比设计和工地现场质量控制。

预制裂隙砂岩巴西劈裂特性试验研究

为研究裂隙对岩石宏观破裂形态的影响,开展了单裂隙、共线双裂隙、平行双裂隙3种裂隙类型下砂岩的巴西劈裂试验,分析3种裂隙类型不同裂隙倾角下的破裂形态并对其分类,同时分析不同裂隙倾角下3种裂隙类型抗拉强度的变化。试验结果表明:裂隙会削弱岩石的抗拉强度,裂隙数量、裂隙倾角及裂隙间几何排布对试样的破裂形态及抗拉强度造成影响;单裂隙、共线双裂隙与平行双裂隙砂岩试样分别存在5种、4种与3种类型的破裂形式,岩石非均质性导致同一裂隙倾角下试样的破裂形态有所不同;单裂隙与共线双裂隙砂岩的抗拉强度总体在0°裂隙倾角时最大,随着裂隙倾角的增大,单裂隙砂岩抗拉强度呈先下降后上升再下降的变化规律,共线双裂隙砂岩抗拉强度呈现下降趋势,而平行双裂隙砂岩的抗拉强度在45°裂隙倾角时最大。

沥青混合料低温劈裂虚拟试验影响因素研究

基于离散元法(discrete element method,DEM)建立沥青混合料二维劈裂(indirect tensile,IDT)试验模型,研究集料模型、集料形状、空隙率,以及加载方位角等因素对混合料低温劈裂虚拟试验结果(劈裂强度和最大水平拉应力)的影响。结果表明:沥青混合料低温虚拟劈裂试验时,集料模型对数值模拟计算效率、内部结构中的接触力链和裂纹扩展路径等有很大影响;实际边界模型较等效椭圆形与等效多边形模型其数值模拟结果变异性较小;空隙率大小对劈裂强度及水平向最大拉应力有显著影响,随空隙率的增大两者均有不同程度的减小;不同加载方位角下的劈裂试验数值模拟结果呈各向异性。

Biaxial tensile behavior of CoCrFeNi high-entropy alloy under dynamic and proportional loadings

The dynamic failure behavior of CoCrFeNi High-Entropy Alloy(HEA)under plane biaxial stress was investigated in detail.The dynamic biaxial tensile tests were conducted using an Electromagnetic Biaxial Split Hopkinson Tensile Bar(EBSHTB)system.For comparison,the quasi-static uniaxial and biaxial tensile tests,as well as dynamic uniaxial tensile tests,were per-formed respectively.A cruciform specimen suitable for large plastic deformation was designed and employed in the experiments.The Finite Element Method(FEM)verified that the improved cruciform specimen could satisfy the basic requirements.The feasibility of the proposed specimen was further confirmed through loading tests.Finally,the quasi-static and dynamic yield loci of the HEA in the first quadrant of the principal stress space were plotted.The results indicate that the alloy exhibits obvious strain hardening effect and strain rate strengthening effect,the yield locus and plastic work contours can be accurately described by Hill'48 criterion.

基于间接拉伸试验的沥青-集料界面黏结性研究

为研究沥青-集料界面黏结性能,选取70号基质沥青及SBS改性沥青混合料在不同温度、老化、水分条件下进行间接拉伸试验,通过分析其峰值力、断裂能、开裂容忍指数(CT)来表征沥青与集料间的黏结性,揭示各影响因素对沥青-集料的影响特征。结果表明,沥青混合料的峰值力随温度升高而降低,但断裂能及CT指数随温度升高而增大,温度在本质上改变了沥青的黏弹性质,从而使得沥青与集料的界面黏结性能发生显著改变。随着老化程度的加剧,沥青混合料峰值力增大而断裂能、CT指数降低,在长期老化条件下,70号基质沥青与SBS改性沥青的峰值力分别增长31.1%,15.4%,而断裂能分别减少27.6%,19.1%,老化作用提升了沥青混合料的黏结强度,但抗裂性及延展性遭到削弱。长期高温浸水处理后,沥青混合料的峰值力、断裂能、CT指数均发生显著下降,在高温浸水4 d后,70号基质沥青和SBS改性沥青混合料的峰值力分别下降48.2%,33.1%,CT指数分别降低67.7%,32.5%,水分侵入沥青-集料界面造成了沥青膜的剥离,严重影响沥青-集料界面的黏结性。

The Application of Dynamic Shock Mechanics Test in Engineering Blasting

With the continuous advancement of China’s infrastructure construction to the west,according to the geographic situation in the southwest region,such as mountainous areas and complex terrain,the road construction process is inevitably accompanied by earth and rock blasting.To improve the quality and safety of the project,this paper addresses the problems of land and rock blasting faced in the construction of mountain road projects,taking the research of rock dynamic mechanics test as the starting point,and using a combination of theoretical analysis and experimental research methods.The specific research content includes the following parts:dynamic impact compression test(SHPB),dynamic splitting tensile test,and stress-strain curve analysis of the test results,which provides the theoretical basis and numerical parameters for the numerical simulation of future engineering blasting.

钢纤维增强大掺量再生骨料混凝土力学与断裂性能

通过立方体抗压试验、劈裂抗拉试验与三点弯曲试验,探讨再生粗、细骨料较高取代率和钢纤维掺量对再生混凝土力学性能与断裂性能的影响。测试了试件28 d的立方体抗压强度、劈裂抗拉强度与双K断裂韧度,分析了再生混凝土基本力学性能与断裂性能之间的相关性,提出了钢纤维增强大掺量再生骨料混凝土起裂断裂韧度及失稳断裂韧度与劈裂抗拉强度的计算关系。结果表明:再生粗、细骨料以50%质量取代率分别替换天然碎石与河砂,钢纤维体积掺量为1.0%时,抗压强度与劈裂抗拉强度达到最高,分别达到天然混凝土的77.12%与93.97%。掺加1.0%的钢纤维后,试块的失稳断裂韧度明显增加,并且在再生细骨料掺量为50%时均超过了天然混凝土。

基于PIV劈裂试验膨润土抗拉强度公式的修正

用于高放废物深地质处置库的压实膨润土砌块因绑扎起吊,会受到拉应力的作用产生拉张裂隙进而影响其使用效果,因此系统研究膨润土的抗拉强度具有重要工程实际意义。基于粒子图像测速技术对压实膨润土进行了一系列的劈裂试验和直接拉伸试验,探求快速且准确获得膨润土抗拉强度的试验方法,同时基于PIV劈裂试验对膨润土抗拉强度公式进行了修正。试验结果表明:压实膨润土的抗拉强度随着含水率的增加呈先增加后减小的趋势,当含水率为13%时其抗拉强度最大。应力-位移关系曲线、位移矢量场和裂隙特征在阶段性划分中存在明显的一一对应关系。当压实膨润土的含水率和初始干密度均相同时,劈裂试验获得的抗拉强度大于直接拉伸强度。采用Frydman提出的修正系数2g和PIV劈裂试验获取的变形参数对劈裂抗拉强度进行修正,修正效果较好。同时在经典巴西劈裂公式基础上引入一个简化修正系数K,结合PIV劈裂试验结果也可快速、准确地获得压实膨润土的直接拉伸强度。

缓冲/回填材料砌块抗拉强度试验研究

对半工业化/工业化制备的缓冲/回填材料砌块抗拉强度以及不同位置抗拉强度均匀性的准确评价,可为优化砌块制备工艺、设计砌块起吊装置提供重要参考。基于单轴压实技术制备标准试样,使用径向劈裂法测定抗拉强度,研究高径比、荷载接触方式对砌块抗拉强度测试结果的影响。在此基础上,采用钻孔、取心和切割技术在大型扇形砌块不同位置取样,分析其抗拉强度的均匀性。实验结果表明:使用劈裂法测定砌块抗拉强度时,平板加载和弧形加载结果相近,垫条加载试验结果小于前两者;砌块抗拉强度随高径比增加先呈指数减小,在高径比大于0.6时可忽略其对抗拉强度的影响;大型砌块不同位置处抗拉强度与模具形状无关,而与距模具边界位置相关,距边界越远抗拉强度越小。

金矿尾矿灰混凝土力学及耐久性能试验研究

为进一步探究金矿尾矿灰对混凝土力学及耐久性能的影响,将不同质量的金矿尾矿灰按照一定比例掺入到混凝土中,进行抗压强度、劈裂抗拉强度、抗冻试验、抗碳化试验,并利用微观测试手段(SEM)进一步探究金矿尾矿灰对混凝土的影响机理。结果表明:不同掺量的金矿尾矿灰混凝土抗压强度与劈裂抗拉强度均呈先增加后降低趋势,且强度随养护周期增加而增加,相同养护周期条件下W4=15%的混凝土抗压与抗拉强度均达到最大;掺入金矿尾矿灰可有效提高混凝土抗冻性能与抗碳化性能,这与金矿尾矿灰填充效果和生成的胶凝物质有关;SEM图片显示掺入金矿尾矿灰可减少混凝土孔隙,使颗粒接触更加紧密,而过量金矿尾矿灰易导致团聚,无法有效填充孔隙。

基于三维随机细观模型的沥青混合料断裂行为研究

为了从细观层次研究沥青混合料内部损伤失效机理,采用三维激光扫描技术获取真实集料三维形态信息,根据其形态特征参数提出一种三维随机凹凸型集料建模方法,并构建三维虚拟集料库。使用Monte Carlo方法构建AC-13三维虚拟试件,利用多刚体动力学方法模拟试件压实过程。采用截面算法获取沥青混合料二维细观模型,在模型有效性和准确性得到验证的基础上利用该模型开展虚拟间接拉伸试验,运用内聚力模型和广义Maxwell模型表征沥青混合料断裂行为和黏弹特性,引入断裂损伤指标D_(m)量化其内部损伤,并利用虚拟试验探讨温度对沥青混合料断裂行为的影响。结果表明,在宏观层次,主裂纹不受温度影响,沿着粗集料的边界逐渐发展直至试件破坏;在细观层次,相较于沥青内部界面单元,沥青-集料界面单元更易出现损伤,更快达到损伤失效点,且受温度影响显著。