Circumferential strain behavior during creep tests of brittle rocks

Creep tests were carried out on Inada granite under confining pressure and on dry and wet specimens of Kamisunagawa sandstone. Distinctive points Q_A (the point where the axial strain rate reaches a minimum), Q_C (the point where the magnitude of circumferential strain rate reaches a minimum) and R_C (the point where the magnitude of circumferential strain acceleration begins to increase) were defined. Dependency of the strain values at the distinctive points on creep stress, confining pressure and water presence was examined. Circumferential strain at Q_C or R_C was not affected by creep stress for both rocks. Peak load points were observed in reloading stress–strain curves when reloading was carried out before Q_C for Kamisunagawa sandstone. Circumferential strain values at peak load during reloading coincided with critical extensile strain, which is defined as circumferential strain value at peak load during constant strain rate tests and is insensitive to confining pressure, water presence, and anisotropy, although slightly affected by strain rate. Load increased slightly or did not increase when reloading was carried out after R_C for both rocks. R_C in creep tests scattered around unloading stress–strain curves from peak load during constant strain rate tests for Kamisunagawa sandstone. These observations suggest that Q_C and R_C in creep tests are closely related to the peak load points during constant strain rate tests. Circumferential strain would be used as a condition-insensitive damage indicator of rock in creep tests as well as in constant strain rate tests.     

Quantification of pre-peak brittle damage: Correlation between acoustic emission and observed micro-fracturing

Acoustic emission (AE) is a useful non-destructive technique to determine whether damage in rock material has occurred, even though the interpretation of AE can be complex. In this paper, two laboratory situations are studied, to investigate the correlation between micro-fractures and AE. In the first case, damage is induced by pure macro-compressive stresses and in the second case by macro-tensile stress (in one direction). The specific evolution of the recorded cumulative AE energy as a function of the applied load leads, in both cases, to a subdivision of the loading in different phases. Thresholds for the transition of these phases are defined. A thorough petrographical analysis of thin slices of samples, damaged to these different thresholds, allows a physical interpretation of damage phases. This methodology leads to a damage evolution model both for macro-compressive and macro-tensile stresses. This paper demonstrates the advantages of the combined use of AE and detailed petrographic study of thin rock slices. Furthermore, insight is provided on the pre-peak damage evolution caused by macro-compressive and macro-tensile stresses in rock material.     

A pore crack model for the mechanical behaviour of porous granular rocks in the brittle deformation regime

A model is developed for predicting and simulating the mechanical behaviour and failure mode of brittle porous granular rocks loaded in compression. It is based on a fracture mechanics analysis applied to cracks emanating from the surface of cylindrical pores in two-dimensional, which is well suited to the microstructure of such rocks. It is also consistent with the usual experimental procedure used for biaxial compression tests since the numerical scheme is implemented under the assumption of imposed axial strain. The model takes into account interactions between neighbouring cracks, which grow when their stress intensity factor reaches the fracture toughness of the rock. The simulation of crack growth from cylindrical holes, associated with a failure criterion based on the coalescence of interacting cracks, allows one to calculate the critical stress at rupture and to derive theoretical stress–strain curves.The present model is then used to compare theoretical results with laboratory data obtained on four sandstones with porosity ranging between 13% and 25.5% which were deformed under conventional triaxial compression conditions at confining pressures between 0 and 35 MPa. The comparison shows that by using a small number of parameters (pore size, pore density, fracture toughness, and elastic moduli), the model is able to predict the rock behaviour during the compression tests and the stress level at rupture in a quite accurate way, when the microstructural parameters introduced are consistent with the observations.     

An experimental study on creep of welded tuff

This paper presents results of five creep tests conducted in uniaxial compression at room temperature. The test specimens were from the welded Topopah Spring tuff formation at Yucca Mountain, Nevada. The specimens exhibited brittle failure. Multiple stress levels were applied in steps to each specimen. Each stress level was maintained for 3 days or longer. The transient creep for all the stress levels can be well described by power functions. The transient creep is caused by an elastic aftereffect. The tuff does not show real steady-state creep. The so-called steady-state creep rate decreases with time. In general it is very low, typically in the range of 10⁻⁷ h⁻¹. For the first 50 h, the so-called steady-state creep rate may be described as a power function of stress. The specimen failed shortly after it entered into the accelerating creep phase.     

渗水膨胀岩隧洞黏弹塑性蠕变解及变形特性分析

渗水膨胀岩隧洞变形是一个受渗流、膨胀与扩容等多因素影响的复杂时变问题。探讨了其变形机制,建立渗水膨胀围岩受力平衡方程,进而得到反映扩容角影响的弹塑性解,并证明余东明解和侯公羽解是本文解的两个特例。以此为基础,借助改进的西原模型,获得了考虑膨胀应变影响的渗水膨胀岩隧洞黏弹塑性蠕变解,并分析了渗流、膨胀与扩容等因素对洞壁时效变形的影响规律。结果表明：随着内外水头比减小或扩容角增大,时效变形逐渐表现为黏弹塑性;膨胀系数α₂只影响围岩黏弹性变形,位移为α₂的单调递增线性函数;在隧洞开挖扰动初期,封闭应变迅速发展成位移,对渗水膨胀围岩变形影响极大。所得结论对渗水膨胀岩隧洞施工和变形控制具有一定指导意义。     

考虑空隙压密阶段特征的岩石应变软化统计损伤模拟方法

为建立能很好地反映岩石变形破坏全过程模拟方法,针对现有岩石统计损伤本构模型难以反映初始压密阶段变形非线性的局限性与不足,首先,在充分探讨空隙岩石变形机理基础上,采用宏观与微观相结合的分析方法,将空隙岩石抽象为岩石骨架和空隙两部分组成,建立空隙岩石变形分析模型;然后,在探讨空隙部分变形机理基础上,考虑空隙压密引起变形的不可恢复性特征,建立空隙部分变形分析方法,同时,引入统计损伤理论,建立岩石骨架变形分析方法,进而建立模拟空隙岩石变形破坏全过程的统计损伤本构模型,并给出了其参数确定方法。该模型不仅能反映空隙岩石的应变软化特征,而且,还能较好地反映空隙岩石在初始压密阶段的变形非线性特点。最后,通过试验曲线、本文及现有同类模型理论分析曲线的比较,表明了该模型与方法的合理性与优越性。     

Creep and time-dependent damage in argillaceous rocks

This paper presents the results of laboratory tests on the time-dependent behaviour of three rocks characterized by a high proportion of clay particles. The viscosity of these sedimentary rocks was studied under different loading conditions in uniaxial compression: static or cyclic creep tests and quasistatic tests (low-loading strain rate) were performed across various orientations of fabric planes.The quasistatic tests showed similarities in the mechanical response of these three argillaceous rocks: a late phase of dilation and a linear development of volumetric deformation before the beginning of unstable crack propagation. The development of secondary and tertiary creep phases during the creep tests highlighted the existence of a deviatoric stress threshold, below which only primary creep is observed. Long-term creep tests also showed that the volumetric variation is not constant during the development of viscoplastic deformations.A microstructural analysis of thin sections extracted from specimens after the tests, gave evidence of cataclastic and granular creep. Damage to the argillaceous matrix occurs and no cracks were observed in the quartz and carbonate grains. This evidence was also demonstrated for tests with loading at a high strain rate.Finally, this study highlights the significant viscoplasticity of argillaceous rocks. The mechanical properties deteriorate rapidly when crack propagation becomes unstable and the viscoplastic strains seem to be due to clay particle slips, known as granular creep.     

Variation in strength and creep life of six Japanese rocks

In this paper, variations in strength and creep life are investigated for rocks under various conditions: dry and wet, uniaxial and triaxial, and compressional and tensile. A number of parameters are introduced for this purpose; to assess the time-dependent failure under constant and monotonic loading the following parameters are used: the parameter of time dependency δ, coefficient of creep life α and coefficient of strength β. δ explains the rate dependency of strength or stress level dependency of creep life. α and β are related to each other. Variations in β have been evaluated using data from previous experiments. It is confirmed here that δ, β and variations in β determined by creep tests are in most cases identical to those determined using strength tests. Variation in β in the wet condition is almost the same as that in the dry condition; however, variation in tension increases more than in compression. Under confining pressure, variation in β is reduced for Neogene sedimentary rocks, and does not appear to change for igneous rock and welded tuff.     

Development of a transparent triaxial cell and observation of rock deformation in compression and creep tests

A transparent triaxial cell was designed and manufactured using acrylic resin. The cell was used to conduct strength and creep tests. Photographs were taken of the specimens at constant time intervals during the constant strain rate test. Photographs were also taken at constant intervals of strain during the creep test, but this rate was changed to one image per second when the specimens first showed tertiary creep. Comparison of the axial and lateral strains during the constant strain-rate and creep tests indicated no significant differences between the two tests. It is well known that the axial creep strain rate is inversely proportional to remaining life in tertiary creep. This study showed that the lateral creep strain rate is also inversely proportional to remaining life. The constant strain-rate tests were conducted with transparent end pieces attached firmly to the upper and lower ends of the specimens. Three holes were drilled into the end pieces, and water was expelled into the holes when the specimens were compressed. It was clearly observed that the water began flowing from the holes back into the specimen during the volumetric expansion of the specimen. The transparent triaxial cell permitted easy observation of water ejection and re-absorption into the specimens.     

岩石的损伤、黏结和摩擦特性研究

收集文献上多种完好及损伤后岩石的常规三轴压缩强度,研究材料的损伤、黏结和摩擦特性及其对围压与强度之间关系的影响。围压增大使裂隙的摩擦承载能力增大,若达到附近材料或其他弱面承载能力后则不再直接影响试样的强度;材料局部损伤的影响随着围压增大降低,试样承载的主应力差以指数形式趋于极限,即最大剪切力等于材料的真实黏结力。大理岩塑性变形引起局部剪切屈服,但真实黏结力没有降低;加热600℃足够长时间后造成使晶界完全开裂则引起真实黏结力降低。岩石剪切破坏时强度与围压的关系可用指数准则描述;不过,花岗岩在围压20MPa之内可出现沿轴向劈裂破坏而强度随围压线性增加。     

考虑流变参数弱化综合影响的软岩蠕变损伤本构模型及其参数智能辨识

根据损伤的定义和软岩的蠕变破坏特点,提出反映应力水平和时间因素对弹性模量弱化综合影响的软岩蠕变损伤变量的一般表达式,推导并建立软岩蠕变损伤演化方程,探讨损伤变量随应力水平和时间的变化规律。以Burgers模型为基础,建立可考虑参数综合弱化的软岩蠕变损伤本构方程。修正后的Burgers蠕变损伤本构模型体现了参数随时间增长和应力水平增大的弱化现象,反映了岩石材料的损伤劣化过程。基于粒子群优化算法(PSO)具有迭代过程简单、能有效地收敛到全局最优解等优良特性,提出基于PSO的蠕变损伤本构模型参数智能辨识的方法和步骤。采用MATLAB软件,编制基于粒子群优化算法的参数辨识程序。以某深部软岩试样蠕变试验为例进行验证分析,结果表明采用考虑参数综合弱化的蠕变损伤本构模型具有较好的实用性。     

基于累积损伤理论的土石坝地震永久变形分析

以龙冈的在不规则荷载作用下土体应变的累积损伤理论为基础,提出了基于累积损伤理论的土石坝地震永久变形分析方法。采用大型静力和动力三轴仪,对某土石坝筑坝材料的静动变形特性进行了试验研究,确定了筑坝材料的静力和动力应力应变关系、累积应变模型和相应的材料参数。利用该方法,分别对在日本2008年6月14日岩手—宫城内陆M7.2级地震作用下该土石坝的在建坝体和竣工后坝体的地震永久变形特性进行了研究。结果表明：在建坝体的坝内沉降计算值与现场实测值基本吻合,并且通过与其它土石坝实测值的比较,说明竣工后坝体的坝顶沉降预测值在合理范围之内。     

脆性岩石渐进及蠕变失效特性宏细观力学模型研究

压应力作用脆性岩石渐进及蠕变失效特性是其力学性质研究的两个主要研究方向。其对于深部地下开挖围岩稳定性的判断有着重要的指导意义。岩石内部微裂纹扩展对脆性岩石的渐进及蠕变特性有着重要的影响。因此,基于岩石的应力与裂纹扩展关系及裂纹扩张演化法则,并结合宏细观损伤定义之间的关系,提出了一个新的宏细观力学模型,推出了岩石完整的应力–应变关系与蠕变理论表达式。分析了围压对岩石的应力–应变关系的影响。研究了岩石内部初始微裂纹尺寸及裂纹间摩擦系数对应力应变关系及岩石强度的影响。并给出了不同围压下岩石裂纹初始应力与峰值应力,其对蠕变实验中的施加应力初始值选取提供了一定参考。然后,研究了恒定围压、轴压分级加载应力路径下的岩石蠕变应变及应变率变化趋势。通过试验结果验证了理论模型的合理性。进而,对压应力作用下细观裂纹扩展对岩石力学特性影响的理解提供了一定的理论参考。     

考虑围压效应和强度脆性跌落的岩石全应力–应变曲线统计损伤模拟方法

为了建立能够较准确地描述岩石全应力–应变曲线的统计损伤模拟方法,针对目前统计损伤本构模型在模拟围压效应和强度脆性跌落方面存在的严重缺陷,首先,基于三轴压缩条件下脆性岩石基本变形特征,将岩石视为由软物质和硬物质两种细观化材料串联组成,建立宏细观物质变形分析模型;然后,基于瞬时应变分析方法和统计损伤理论,分别提出软物质和硬物质的变形分析方法,从而建立能够考虑围压效应和强度脆性跌落的岩石全应力–应变曲线统计损伤本构模型,给出参数的确定方法;最后,通过模型验证与分析,模型不但能够描述岩石初始宏观变形特征与围压之间的关系,也能够消除岩石轴向峰后理论变形曲线存在的随机性,从而能够较准确地模拟出岩石全应力–应变曲线,表明方法具有一定的合理性和可行性。     

On anisotropic continuous damage of plates in Flexure

Internal damage due to fissuration results in an overall anisotropic material behaviour. A scalar damage parameter does not allow one to model a direction dependent response of continuously damaging solids. Employing the idea that the scalar damage parameter can be associated with an appropriate strain a damage tensor related to strains is introduced. The overall elastic properties are determined using a simplified form of the sensor function representations and the stress-strain relations regarding the overall response are given. For combined stress states an uncoupled damage theory is derived for materials with no lateral deformability under axial stress. Both brittle-ductile and elastic-brittle cases are studied using circular plates, for the elastic-brittle case only the governing differential equation are presented.     

Reliability analysis of geogrid creep data in Japan

The data collected by creep testing carried out in conformity with current Japanese practice and reported in Public Works Research Center product certification reports are interpreted in order to identify and discuss differences between current Japanese practice and European and US practices. A database of 66 different geogrid products from 10 different manufacturers was reviewed comprising of 362 different constant load creep tests. An important outcome from the analysis of the creep test data collected to date is a strong case for the adoption of elevated temperature testing in order to eliminate the excessively long extrapolation times required to compute creep strength reduction factors.     

采矿引发地表变形对建筑物的损坏程度预测评估

以某煤矿为例,经实地野外调查,充分分析了矿区地形地貌、地层岩性、地质构造、工程地质特征等,预测评估了煤矿开采引发地表变形对建筑物的损坏程度,为矿方制定目标、提出决策等提供了科学的理论依据。