Experimental research on creep behaviors of sandstone under uniaxial compressive and tensile stresses

The consideration of time dependence is essential for the study of deformation and fracturing processes of rock materials, especially for those subjected to strong compressive and tensile stresses. In this paper, the self-developed direct tension device and creep testing machine RLW-2000M are used to conduct the creep tests on red sandstone under uniaxial compressive and tensile stresses. The short-term and long-term creep behaviors of rocks under compressive and tensile stresses are investigated, as well as the long-term strength of rocks. It is shown that, under low-stress levels, the creep curve of sandstone consists of decay and steady creep stages; while under high-stress levels, it presents the accelerated creep stage and creep fracture presents characteristics of brittle materials. The relationship between tensile stress and time under uniaxial tension is also put forward. Finally, a nonlinear viscoelastoplastic creep model is used to describe the creep behaviors of rocks under uniaxial compressive and tensile stresses.

Effects of Loading Rate on Flexural-tension Properties and Uniaxial Compressive Strength of Micro-surfacing Mixture

The major objective of this research was to discuss the effects of loading rate on the flexural-tension properties and uniaxial compressive strength of micro-surfacing mixture using three-point bending test and uniaxial compressive test respectively. As a preventive maintenance surface treatment on asphalt pavement, micro-surfacing was formed on the basis of the ISSA recommendation of an optimum micro-surfacing design. Tests were conducted over a wide range of temperature to investigate the difference of properties from low loading rate to a relatively high loading rate. Three-point bending test was used to study the flexural strength, strain and modulus of micro-surfacing mixture, and uniaxial compressive test was carried out to obtain the relationship between strength and the loading rate as well as temperature. The experimental results showed that flexural strength at high loading rate was larger than that at low loading rate. The flexural strength difference between low and high loading rate enlarged when the temperature rose. The flexural strain at high loading rate increased compared with results of the low loading rate. Results of the flexural modulus revealed that micro-surfacing mixture exhibited better anti-cracking characteristic at low temperature when given a relatively low loading rate. Results of uniaxial compressive test revealed that the strength difference of micro-surfacing among different loading rates increased with the increase of temperature. The logarithm relationship between the strength and loading rate over a wide range of temperature was obtained to compare the experimental and predicted values, which resulting in a reasonable consistency.

ANALYSIS OF THE LOCALIZATION OF DAMAGE AND THE COMPLETE (STRESS-STRAIN) RELATION FOR MESOSCOPIC HETEROGENEOUS ROCK UNDER UNIAXIAL TENSILE LOADING

The mechanical behavior of rock under uniaxial tensile loading is different from that of rock under compressive loads. A micromechanics-based model was proposed for mesoscopic heterogeneous brittle rock undergoing irreversible changes of their microscopic structures due to microcrack growth. The complete stress-strain relation including linear elasticity, nonlinear hardening,rapid stress drop and strain softening was obtained. The influence of all microcracks with different sizes and orientations were introduced into the constitutive relation by using the probability density function describing the distribution of orientations and the probability density function describing the distribution of sizes. The influence of Weibull distribution describing the distribution of orientations and Rayleigh function describing the distribution of sizes on the constitutive relation were researched. Theoretical predictions have shown to be consistent with the experimental results.

Damage and failure rule of rock undergoing uniaxial compressive load and dynamic load

For understanding the damage and failure rule of rock under different uniaxial compressive loads and dynamic loads, tests on red sandstone were carried out on Instron 1342 electro-servo controlled testing system with different uniaxial compressive loads of 0, 2, 4 and 6 MPa. It is found that peak stress, peak strain, elastic modulus and total strain energy decrease with the increase of static compressive stress. Based on the test results, the mechanism on damage and failure of rock was analyzed, and according to the equivalent strain hypothesis, a new constitutive model of elastic-plastic damage was established, and then the calculated results with the established model were compared with test results to show a good agreement. Furthermore the rule of releasing ratio of damage strain energy was discussed.

The Complete Stress-strain Curve of Recycled Aggregate Concrete under Uniaxial Compression Loading

The mechanical performance of recycled aggregate concrete （RAC） is investigated. An experiment on the complete stress-strain curve under uniaxial compression loading of RAC is carried out. The experimental results indicate that the peak stress, peak strain, secant modulus of the peak point and original point increase with the strength grade of RAC enhanced. On the contrary, the residual stress of RAC decreases with the strength grade enhancing, and the failure of RAC is often broken at the interface between the recycled aggregate and the mortar matrix. Finally, the constitutive model of stress-strain model of RAC has been constituted, and the results from the constitutive model of stress-strain meet the experiment results very well.

Compression Sensibility of Magnetic-concentrated Fly Ash Mortar under Uniaxial Loading

The electrical conductivity, compression sensibility, workability and cost are factors that affect the application of conductive smart materials in civil structures. Consequently, the resistance and compression sensibility of magnetic-concentrated fly ash （MCFA） mortar were investigated using two electrode method, and the difference of compression sensibility between MCFA mortar and carbon fiber reinforced cement （CFRC） under uniaxial loading was studied. Factors affecting the compression sensibility of MCFA mortar, such as MCFA content, loading rate and stress cycles, were analyzed. Results show that fly ash with high content of Fe3O4 can be used to prepare conductive mortar since Fe3O4 is a kind of nonstoichiometric oxide and usually acts as semiconductor. MCFA mortar exhibits the same electrical conductivity to that of CFRC when the content of MCFA is more than 40% by weight of sample. The compression sensibility of mortar is improved with the increase of MCFA content and loading rate. The compression sensibility of MCFA mortar is reversible with the circling of loading. Results show that the application of MCFA in concrete not only provides excellent performances of electrical-functionality and workability, but also reduces the cost of conductive concrete.

Residual Tensile Strength of Plain Concrete Under Tensile Fatigue Loading

The functional relation between the residual tensile strength of plain concrete and number of cycles was determined. 99 tappered prism specimens of plain concrete were tested under uniaxial tensile fatigue loading. Based on the probability distribution of the residual tensile strength, the empirical expressions of the residual tensile strength corresponding to the number of cycles were obtained. The residual tensile strength attenuating curves can be used to predict the residual fatigue life of the specimen under variable-amplitude fatigue loading. There is a good correlation between residual tensile strength and residual secant elastic modulus. The relationship between the residual secant elastic modulus and number of cycles was also established.

Estimate of Uniaxial Compressive Strength of Hydrothermally Altered Soft Rocks Based on Strength Index Tests

The purpose of this study was to clarify the relationships between results of index tests and uniaxial compressive strength (UCS) in hydrothermally altered soft rocks of the Upper Miocene, which are typical of the soft rock found in northeastern Hokkaido, Japan. Index tests were performed using point load testing machine and needle penetrometer with irregular lump specimens under forced-dry, forced-wet, and natural-moist states. The relationships between irregular lump point load strength (IPLS) index and UCS, and needle penetration (NP) index and UCS were “UCS = approximately 19 IPLS index” and “UCS = 0.848 (NP index)0.619”, respectively, in soft rocks with a UCS below 25 MPa. These relationships could be applied to on-site tests of rocks with natural moisture content. The UCS could be calculated from IPLS and NP tests on soft rocks only when UCS was below 25 MPa, using the equations obtained as a result of this study.

Study on Fluid Shear Flow on Osteoblast-like Cells in Their Tensile Loading Experiments

In vitro cell loading experiments are used to investigate stimulation of strain to cellular proliferation. As the flowing conditions of culture fluid in loading systems has been little known, strictly people can not detect the influence of strain to cellular proliferation exactly because shear flow can enhance cell proliferation either. Based on the working principle and cyclic loading parameters, we simplify Navier-Stokes equation to describe the flow of culture fluid on substrates of uniaxial and equi-biaxial flat tensile loading systems and four point bending system. With approximate solutions, the distributions of velocity field and wall shear flow to cells are gained. Results show: shear flows are zero in the middle (or fixed point or line) of substrate for all systems, and they get larger proportionally to distance from middle and substrate elongate; the shear flow on the substrate of four point bending system is much greater than those of others. This shear flow in four point bending system, confirmed by Owan, I., et al with OPN mRNA increase in their experiment, could cause more influence to osteoblast-like cells than that caused by strain. We estimate the average magnitude of shear stress in Owan’s device, the results are consistent with other experimental data about shear flow. In conclusion our study makes it possible to differentiate the influence of strain on cellular proliferation to that of shear flow in loading experiments with the devices mentioned above quantitatively.

Experimental and numerical simulation of loading rate effects on failure and strain energy characteristics of coal-rock composite samples

The deformation and failure of coal and rock is energy-driving results according to thermodynamics.It is important to study the strain energy characteristics of coal-rock composite samples to better understand the deformation and failure mechanism of of coal-rock composite structures.In this research,laboratory tests and numerical simulation of uniaxial compressions of coal-rock composite samples were carried out with five different loading rates.The test results show that strength,deformation,acoustic emission(AE)and energy evolution of coal-rock composite sample all have obvious loading rate effects.The uniaxial compressive strength and elastic modulus increase with the increase of loading rate.And with the increase of loading rate,the AE energy at the peak strength of coal-rock composites increases first,then decreases,and then increases.With the increase of loading rate,the AE cumulative count first decreases and then increases.And the total absorption energy and dissipation energy of coal-rock composite samples show non-linear increasing trends,while release elastic strain energy increases first and then decreases.The laboratory experiments conducted on coal-rock composite samples were simulated numerically using the particle flow code(PFC).With careful selection of suitable material constitutive models for coal and rock,and accurate estimation and calibration of mechanical parameters of coal-rock composite sample,it was possible to obtain a good agreement between the laboratory experimental and numerical results.This research can provide references for understanding failure of underground coalrock composite structure by using energy related measuring methods.

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.

Experimental investigation of electrode cycle performance and electrochemical kinetic performance under stress loading

Lithium-ion batteries suffer from mechano–electrochemical coupling problems that directly determine the battery life. In this paper, we investigate the electrode electrochemical performance under stress conditions, where seven tensile/compressive stresses are designed and loaded on electrodes, thereby decoupling mechanics and electrochemistry through incremental stress loads. Four types of multi-group electrochemical tests under tensile/compressive stress loading and normal package loading are performed to quantitatively characterize the effects of tensile stress and compressive stress on cycle performance and the kinetic performance of a silicon composite electrode. Experiments show that a tensile stress improves the electrochemical performance of a silicon composite electrode, exhibiting increased specific capacity and capacity retention rate, reduced energy dissipation rate and impedances, enhanced reactivity, accelerated ion/electron migration and diffusion, and reduced polarization. Contrarily, a compressive stress has the opposite effect, inhibiting the electrochemical performance. The stress effect is nonlinear, and a more obvious suppression via compressive stress is observed than an enhancement via tensile stress. For example, a tensile stress of 675 k Pa increases diffusion coefficient by 32.5%, while a compressive stress reduces it by 35%. Based on the experimental results, the stress regulation mechanism is analyzed. Tensile stress loads increase the pores of the electrode material microstructure, providing more deformation spaces and ion/electron transport channels. This relieves contact compressive stress, strengthens diffusion/reaction, and reduces the degree of damage and energy dissipation. Thus, the essence of stress enhancement is that it improves and optimizes diffusion, reaction and stress in the microstructure of electrode material as well as their interactions via physical morphology.

Strength Prediction of Cruciform Specimen Under Biaxial Loading

In order to achieve a better understanding of failure behavior of cruciform specimen under different biaxial loading conditions,a three-dimensional finite element model is established with solid and interface elements.Maximum stress criterion,two Hashin-type criteria and the new proposed criteria are used to predict the strength of plain woven textile composites when biaxial loading ratio equals 1.Compared with experimental data,only the new proposed criteria can reach reasonable results.The applicability of the new proposed criteria is also verified by predicting the tensile and compressive strength of cruciform specimen under different biaxial loading ratios.Moreover,the introduction of interface element makes it more intuitive to recognize delamination failure.The shape of the predicted delamination failure region in the interface layer is similar to that of the failure region in neighboring entity layers,but the area of delamination failure region is a little larger.

Prediction of uniaxial compressive strength of rock based on lithology using stacking models

Uniaxial compressive strength(UCS)of rock is an essential parameter in geotechnical engineering.Point load strength(PLS),P-wave velocity,and Schmidt hammer rebound number(SH)are more easily obtained than UCS and are extensively applied for the indirect estimation of UCS.This study collected 1080 datasets consisting of SH,P-wave velocity,PLS,and UCS.All datasets were integrated into three categories(sedimentary,igneous,and metamorphic rocks)according to lithology.Stacking models combined with tree-based models and linear regression were developed based on the datasets of three rock types.Model evaluation showed that the stacking model combined with random forest and linear regression was the optimal model for three rock types.UCS of metamorphic rocks was less predictable than that of sedimentary and igneous rocks.Nonetheless,the proposed stacking models can improve the predictive performance for UCS of metamorphic rocks.The developed predictive models can be applied to quickly predict UCS at engineering sites,which benefits the rapid and intelligent classification of rock masses.Moreover,the importance of SH,P-wave velocity,and PLS were analyzed for the estimation of UCS.SH was a reliable indicator for UCS evaluation across various rock types.P-wave velocity was a valid parameter for evaluating the UCS of igneous rocks,but it was not reliable for assessing the UCS of metamorphic rocks.

层间摩擦系数对非黏结柔性管抗拉铠装层轴压屈曲失效模式的影响研究

在柔性管运输、铺设安装和服役过程中,由于其抗拉铠装层钢带为柔性细长螺旋结构,当外部载荷超过临界值时,抗拉铠装层钢带容易发生屈曲失效。考虑非线性材料、几何大变形、层间接触和摩擦等非线性效应的影响,运用ABAQUS有限元软件建立了63.5mm典型非黏结柔性管8层完整结构模型,研究了在抗拉铠装层不同层间摩擦系数的情况下,非黏结柔性管在轴向压缩载荷作用下的力学性能和屈曲失效模式。研究结果表明,层间摩擦系数对非黏结柔性管的抗压刚度和抗拉铠装层屈曲失效模式有较大影响,层间摩擦系数以0.1和0.2为分界点,抗压刚度和屈曲失效模式均呈现出“三段式”变化规律。研究成果可为海洋非黏结柔性管的结构设计和屈曲失效评估提供技术参考。

花岗岩声发射多期应力记忆方向独立性的试验研究

为了研究花岗岩在压缩荷载阶梯循环下偏转加载时多期应力记忆方向独立性,采用自制的偏转加载装置对花岗岩进行不同偏转角度的阶梯循环加卸载试验,研究岩石在偏转加载循环后记忆先前多次循环中最大应力的凯塞效应(KE)以及抹录不净现象(IEP)。结果表明:花岗岩KE以及IEP均具有方向独立性,KE方向独立性存在的临界角度为10°,IEP方向独立性存在的临界角度为12°;偏转角度达到12°后,IEP会干扰KE对历史最大应力的记忆。

点荷载强度计算模型现场试验修正及效果验证

点荷载试验为常用的岩石单轴抗压强度间接测试方法,对工程设计和施工开展具有指导作用。针对点荷载强度指数计算方法展开研究,在进行大量实际点荷载试验获得相应结果的基础上,对试验数据进行统计分析并在计算公式中引入新的变量以期对现有的点荷载强度计算方法进行优化。对长山壕金矿岩体进行了采样、试样制备与点荷载试验,采用统计方法对点荷载试验结果进行分析得出优化方法,并以同采区同位置岩样的单轴抗压强度作为参考值,将多种计算结果与参考值进行了对比分析。结果表明:改进后方法的计算结果更贴近岩样实际单轴抗压强度,且相同试验、相同数据条件下的计算结果离散性处于较低水平,为现场试验条件有限时估算岩样单轴抗压强度提供了一种可靠的点荷载强度指数计算方法。

基于声发射频谱分析的阴极炭块破裂研究

通过对阴极炭块进行不同加载速率的单轴压缩实验和声发射监测实验,分析炭块破裂过程中的力学特性和声发射特性。研究表明:由对应的应力门槛值可确定炭块破裂过程的不同阶段;声发射波形占比量的大小能反映炭块内部破坏程度;加载速率和炭块种类对炭块裂纹扩展速度均有影响;不同声发射波形频谱特征对应炭块的不同裂纹尺度和类型;不同阶段典型声发射信号频谱特征表现为单峰频谱到多峰频谱的交替变换;平均主频比和信号事件率均具有分形特征,其关联维数整体均呈现先上升后下降再上升最后下降的趋势。

点荷载试验在典型露天采场超高边坡工程地质勘察中的应用

夏日哈木镍钴矿山拟建大型超高边坡露天采场,需进行边坡工程地质勘察工作。岩石抗压强度是重要的工程地质参数,如何在青藏高原山区快速获得各类岩石的单轴抗压强度等参数非常关键,尤其是各类难以进行室内压缩试验的风化或蚀变岩石。本次在野外现场采用济南矿岩HDH-1型岩石点荷载仪对矿区内各类岩石进行点荷载试验,并通过前人总结的点荷载强度与单轴抗压强度转换公式计算出了各类岩石的单轴抗压强度及层状(条带状)岩石的各向异性指数。本次点荷载试验克服了昆仑山区岩石测试加工工序复杂、周期较长、成本高等因素,且适用于风化层、蚀变带内不易进行室内压缩试验的岩石,是一种间接测量估算岩石相关强度参数的快捷方法,对以后西部山区的工程地质勘察工作具有较为广泛的借鉴作用。

基于Hoek-Brown准则的岩体高温-荷载耦合损伤模型及其验证

针对隧道岩体在高温和荷载共同作用下产生的耦合损伤问题,基于连续损伤力学理论,引入高温-荷载耦合损伤因子,建立了考虑高温作用的广义Hoek-Brown(H-B)准则的弹塑性损伤本构模型,解决了模型在数值积分过程中的奇异点问题,并利用Fortran语言编写了模型的有限元程序。结果表明:模型计算的应力-应变曲线与试验所得到的曲线趋势一致且计算结果误差在5%以内,温度作用下隧洞围岩损伤逐渐加剧,验证了模型的准确性和程序的稳健性,可为火灾作用下隧道围岩安全稳定性评价提供一种有效计算方法。