Deformation Characteristics of Hydrate-Bearing Sediments

The safe and efficient development of natural gas hydrate requires a deep understanding of the deformation behaviors of reservoirs.In this study,a series of triaxial shearing tests are carried out to investigate the deformation properties of hydrate-bearing sediments.Variations of volumetric and lateral strains versus hydrate saturation are analyzed comprehensively.Results indicate that the sediments with high hydrate saturation show dilative behaviors,which lead to strain-softening characteristics during shearing.The volumetric strain curves have a tendency to transform gradually from dilatation to compression with the increase in effective confining pressure.An easy prediction model is proposed to describe the relationship between volumetric and axial strains.The model coefficientβis the key dominating factor for the shape of volumetric strain curves and can be determined by the hydrate saturation and stress state.Moreover,a modified model is established for the calculation of lateral strain.The corresponding determination method is provided for the easy estimation of model coefficients for medium sand sediments containing hydrate.This study provides a theoretical and experimental reference for deformation estimation in natural gas hydrate development.

Experiment and constitutive modelling of creep deformation in the frozen silt-concrete interface

To ensure the long-term safety and stability of bridge pile foundations in permafrost regions,it is necessary to investigate the rheological effects on the pile tip and pile side bearing capacities.The creep characteristics of the pile-frozen soil interface are critical for determining the long-term stability of permafrost pile foundations.This study utilized a self-developed large stress-controlled shear apparatus to investigate the shear creep characteristics of the frozen silt-concrete interface,and examined the influence of freezing temperatures(−1,−2,and−5°C),contact surface roughness(0,0.60,0.75,and 1.15 mm),normal stress(50,100,and 150 kPa),and shear stress on the creep characteristics of the contact surface.By incorporating the contact surface’s creep behavior and development trends,we established a creep constitutive model for the frozen silt-concrete interface based on the Nishihara model,introducing nonlinear elements and a damage factor.The results revealed significant creep effects on the frozen silt-concrete interface under constant load,with creep displacement at approximately 2-15 times the instantaneous displacement and a failure creep displacement ranging from 6 to 8 mm.Under different experimental conditions,the creep characteristics of the frozen silt-concrete interface varied.A larger roughness,lower freezing temperatures,and higher normal stresses resulted in a longer sample attenuation creep time,a lower steady-state creep rate,higher long-term creep strength,and stronger creep stability.Building upon the Nishihara model,we considered the influence of shear stress and time on the viscoelastic viscosity coefficient and introduced a damage factor to the viscoplasticity.The improved model effectively described the entire creep process of the frozen silt-concrete interface.The results provide theoretical support for the interaction between pile and soil in permafrost regions.

Pore structure of low‑permeability coal and its deformation characteristics during the adsorption–desorption of CH4/N2

The pore structure of coal plays a key role in controlling the storage and migration of CH4/N2.The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement efect of N2 injection.The deformation characteristic of coal during adsorption–desorption of CH4/N2 is an important factor afecting CH4 pumpability and N2 injectability.The pore structure characteristics of low-permeability coal were obtained by fuid intrusion method and photoelectric radiation technology.The multistage and connectivity of coal pores were analyzed.Subsequently,a simultaneous test experiment of CH4/N2 adsorption–desorption and coal deformation was carried out.The deformation characteristics of coal were clarifed and a coal strain model was constructed.Finally,the applicability of low-permeability coal to N2 injection for CH4 displacement technology was investigated.The results show that the micropores and transition pores of coal samples are relatively developed.The pore morphology of coal is dominated by semi-open pores.The pore structure of coal is highly complex and heterogeneous.Transition pores,mesopores and macropores of coal have good connectivity,while micropores have poor connectivity.Under constant triaxial stress,the adsorption capacity of the coal for CH4 is greater than that for N2,and the deformation capacity of the coal for CH4 adsorption is greater than that for N2 adsorption.The axial strain,circumferential strain,and volumetric strain during the entire process of CH4 and N2 adsorption/desorption in the coal can be divided into three stages.Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-diference.A strain model for the adsorption–desorption of CH4/N2 from coal was established by considering the expansion stress of adsorbed gas on the coal matrix,the compression stress of free gas on the coal matrix,and the expansion stress of free gas on micropore fractures.N2 has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas fow,which can signifcantly improve the efectiveness of low-permeability coal seam gas control and promote the efcient utilization of gas resources.

A bounding surface visco-plasticity model considering generalized spacing ratio of soils

The non-unique critical state of soils with time-dependent behaviors is a significant issue in geotechnical engineering problems.However,previous bounding surface plasticity models cannot predict accurately the non-unique critical state of soils,because the distance between the compression line and critical state line charged by strain-rate effect is basically neglected.To fill this gap,a generalized spacing ratio of soils is defined in the elasto-viscoplastic framework,and a bounding surface visco-plasticity model is formulated and verified,which can consider the generalized spacing ratio.Specifically,the generalized spacing ratio of soils reflects the distance between the compression line and the critical state line of soils with time-dependent behaviors.Then,the generalized spacing ratio is introduced into an improved anisotropic bounding surface.A new expression of the visco-plastic multiplier is derived by solving the consistency equation of an anisotropic bounding surface.In the expression,a strain rate index is proposed to account for the strain-rate effect on visco-plastic strain increment,and a visco-plastic hardening modulus is derived to predict the visco-plastic response of soils in overconsolidation conditions.The model is then verified through constant strain rate tests and creep tests.Notably,it can capture the non-unique critical states of soils with time-dependent behaviors due to the generalized spacing ratio and the creep rupture of soils due to the visco-plastic multiplier that considers the stress ratio and visco-plastic strain rate.

Anisotropic dynamic permeability model for porous media

Based on the tortuous capillary network model,the relationship between anisotropic permeability and rock normal strain,namely the anisotropic dynamic permeability model(ADPM),was derived and established.The model was verified using pore-scale flow simulation.The uniaxial strain process was calculated and the main factors affecting permeability changes in different directions in the deformation process were analyzed.In the process of uniaxial strain during the exploitation of layered oil and gas reservoirs,the effect of effective surface porosity on the permeability in all directions is consistent.With the decrease of effective surface porosity,the sensitivity of permeability to strain increases.The sensitivity of the permeability perpendicular to the direction of compression to the strain decreases with the increase of the tortuosity,while the sensitivity of the permeability in the direction of compression to the strain increases with the increase of the tortuosity.For layered reservoirs with the same initial tortuosity in all directions,the tortuosity plays a decisive role in the relative relationship between the variations of permeability in all directions during pressure drop.When the tortuosity is less than 1.6,the decrease rate of horizontal permeability is higher than that of vertical permeability,while the opposite is true when the tortuosity is greater than 1.6.This phenomenon cannot be represented by traditional dynamic permeability model.After the verification by experimental data of pore-scale simulation,the new model has high fitting accuracy and can effectively characterize the effects of deformation in different directions on the permeability in all directions.

Deformation characteristics and constitutive model of seafloor massive sulfides

Deformation characteristics and constitutive model of seafloor massive sulfide(SMS)were selected as the research object.Uniaxial/triaxial compression test were carried out on the mineral samples,and the deformation characteristics of specimens under various conditions were studied.According to characteristics of the mineral,a new three stages constitutive equation was proposed.The conclusions are as follows:The axial strain,peak strain and maximum strength of seafloor massive sulfide increase with the confining pressure.The elastic modulus of the metal sulfide samples is decreased sharply with the increase of confining pressure.According to characteristics of seafloor massive sulfide,the constitutive equation is divided into three parts,the comparison between theoretical curves and experimental data shows that both of them are in good agreement,which also proves the correctness of the constitutive equation for uniaxial compression.

Analysis of axial strain in one-dimensional loading by different models

Different phenomenological equations based on plasticity, primary creep （as a viscoplastic mechanism）, secondary creep （as another viscoplastic mechanism） and different combinations of these equations are presented and used to describe the material inelastic deformation in uniaxial test. Agreement of the models with experimental results and with the theoretical concepts and physical realities is the criterion of choosing the most appropriate formulation for uniaxial test. A model is thus proposed in which plastic deformation, primary creep and secondary creep contribute to the inelastic deformation. However, it is believed that the hardening parameter is composed of plastic and primary creep parts. Accordingly, the axial plastic strain in a uniaxial test may no longer be considered as the hardening parameter. Therefore, a proportionality concept is proposed to calculate the plastic contribution of deformation.

Mathematical and numerical modelling of large creep deformations for annular rotating disks

A simulation model is presented for the creep process of the rotating disks under the radial pressure in the presence of body forces. The finite strain theory is applied. The material is described by the Norton-Bailey law generalized for true stresses and logarithmic strains. A mathematical model is formulated in the form of a set of four partial differential equations with respect to the radial coordinate and time. Necessary initial and boundary conditions are also given. To make the model complete, a numerical procedure is proposed. The given example shows the effectiveness of this procedure. The results show that the classical finite element method cannot be used here because both the geometry and the loading （body forces） change with the time in the creep process, and the finite elements need to be redefined at each time step.

Study on Rheological Characteristics of a Grease Used in High Speed Bearing

On a self-made super-high shear strain rate rheometer, the rheological characteristics and apparent viscosity curves of a high-speed bearing grease were obtained under different working conditions. A new grease rheological model suited to a shear strain rate range of 0—3.5×10~6s^(-1) was presented. The results showed that the shear stress increased linearly at first and then increased nonlinearly with the increase in shear strain rate up to 1.5×10~6s^(-1), and finally the shear stress decreased slightly with the successive increase in shear strain rate. The shear stress increased with a decreasing rolling speed and an increasing contact pressure. The apparent viscosity decreased rapidly with the increase of shear strain rate at beginning and could approach the viscosity of the base oil if the shear strain rate surpassed 1.5×10~6s^(-1). The fits between the test data and the predicted values by the new model were fairly good.

粉砂质泥岩蠕变特性与非线性黏弹塑性本构模型

针对边坡、隧道工程中粉砂质泥岩长期稳定性问题,采用DZSZ-150型多场耦合岩石三轴压力试验机,利用粉砂质泥岩相似材料试样,开展不同围压下粉砂质泥岩蠕变试验。根据蠕变试验结果建立可描述粉砂质泥岩蠕变全过程的非线性黏弹塑性本构模型,以7 MPa围压下蠕变试验结果为例,对所提出的蠕变本构模型进行合理性和可行性验证。结果表明:试样的稳态蠕变速率与偏应力大小有关,随着偏应力的增加,试样稳态蠕变速率与偏应力之间符合指数函数关系;在加速蠕变阶段的蠕变速率相对低偏应力水平下减速蠕变和稳态蠕变速率明显要增大很多,且非常短时间内就发生了蠕变破坏;提出一种描述粉砂质泥岩全蠕变过程的非线性黏弹塑性本构模型,该模型能描述粉砂质泥岩蠕变-破坏全过程的蠕变变形规律,反映了粉砂质泥岩在不同偏应力水平作用下的蠕变特性,克服了经典模型中无法准确描述加速蠕变过程的难题。

砂岩冻结/解冻过程蠕变特性研究

寒区岩体长期经受荷载与冻融的共同作用,若不考虑冻融过程对其长期力学行为的影响,将会给寒区工程建设和安全运营带来重大的安全隐患。为此,以寒区某边坡工程砂岩为研究对象,通过开展不同冻结温度下的冻结/解冻过程单轴分级加载蠕变试验,使岩石在同一应力状态下处于冻结和解冻过程,真实再现寒区工程岩体长期力学响应特征。以此研究冻结/解冻过程对岩体长期力学特性的影响,并对其蠕应变、稳态蠕变速率及长期强度等宏观力学指标进行量化分析。结果表明:(1)砂岩冻结过程先后经历冷缩阶段、冻胀阶段和稳态蠕变阶段,解冻过程只经历融缩阶段和稳态蠕变阶段;冷缩阶段和融缩阶段砂岩发生收缩变形,冻胀阶段则发生膨胀变形;(2)冻结/解冻温度为-5℃/25℃、-10℃/25℃、-15℃/25℃时,砂岩蠕应变较常温状态下蠕应变增幅范围分别为102%~193%、81%~126%、105%~194%,解冻后稳态蠕变速率较冻结前最大增长3.65倍、4.31倍、5.56倍,冻结/解冻过程蠕变砂岩的长期强度是常温状态下长期强度的96.33%、88.52%、75.44%;(3)应力对冷缩、冻胀变形的产生起抑制作用而对融缩变形的产生起促进作用;冻结温度越低,冻胀变形和解冻后融缩变形越明显。文章提出的将蠕变与冻融过程相结合的试验方法能较为真实地反映工程实际,该方法为寒区岩体工程长期稳定性评价提供新途径。

剪切大变形条件下心墙沥青混凝土渗透特性研究

为研究心墙沥青混凝土在大剪切变形条件下的渗透特性,基于自主研发的三轴剪切轴向渗透试验系统,对70号、110号沥青混凝土开展了不同围压、渗透压组合的三轴剪切渗透试验,并提出了基于全局优化算法的沥青混凝土渗透系数反演最优模型。试验结果表明:剪胀效应引起裂隙的张开与扩展,使沥青混凝土的渗透系数大幅增加。低围压下,轴向应变从8%增加至30%,渗透系数变化幅度可达2个数量级;应变状态和应变增量影响沥青混凝土的渗透性,渗透系数随应变场的变化而改变。压密阶段,随轴向应变的增加,体积增量比为负值,渗透系数减小。裂隙发育阶段,体积增量比为正值,渗透系数增大;围压对沥青混凝土的渗透性有显著影响。两种标号沥青混凝土试样的防渗安全轴向应变阈值在低围压时较小,随着围压的增加呈递增趋势。当围压加载至1.0 MPa,轴向应变增加至30%后,沥青混凝土的渗透系数仍满足规范要求。

强扰煤岩体蠕变过程中跨尺度非连续结构演化研究进展

【目的和方法】巷道围岩时效变形破坏、蠕变型冲击地压等灾害是深井面临的重大威胁,加之深部采动与结构变化的影响,亟需研究强扰煤岩体蠕变过程中跨尺度非连续结构演化规律,系统回顾煤岩体力学性质的尺度效应、非连续结构与多物理场效应,以及煤岩体长时稳定性、蠕变演化规律与力学模型。【结果和结论】强调不同尺度(细观-宏观-工程尺度)、不同尺寸(REV范围内)煤岩变形破裂过程中非连续结构(包括裂隙和矿物)与物理力学性质存在密切的关联特征。指出煤岩体跨尺度非连续结构导致应力场非均匀分布,造成力学各向异性、尺度效应及尺寸效应,进而出现非协调宏观破裂现象。透明解析与推演煤岩变形破裂过程中跨尺度非连续结构与多物理场演化规律,是认识煤岩灾变内禀机制的关键。明确指出现有蠕变实验与蠕变模型在煤岩表面变形及破裂方面具有优势,但无法预测煤岩内部变形与非连续结构;为此提出了全尺寸CT扫描重构、数字体图像相关(DVC)、跨尺度等效岩体-等效晶质建模(SRM-GBM)相结合的破解方法。对扰动煤岩体蠕变前沿问题进行了讨论,阐明了非连续结构与应力引起内部变形损伤诱发蠕变这一新观点,指出非连续结构和应力是煤岩非协调蠕变的主控因素,最后建立了强扰煤岩体蠕变过程中跨尺度非连续结构与多物理场演化的精细化建模及透明解析新方法。研究成果为相关矿山灾害的发生机理、预警与防控提供理论基础。

基于塑性元件微元化的重塑黄土黏弹塑性本构模型

基于塑性元件微元化和无穷级数的思想,通过加、卸载条件的三轴蠕变试验建模并通过动力三轴试验验证,建立了重塑黄土的黏弹塑性本构模型,并获得了相应的参数指标。研究表明,该模型可以很好地描述重塑黄土的蠕变、静力和动力特性,且能更加合理地解释卸载条件下回弹曲线的变化特征。塑性元件微元化的处理方法弥补了以往黏弹塑性本构模型中塑性变形不易描述的缺点,相比其他的黏弹塑性本构模型,该模型应用更加简单且适用范围更广。

滇中红层泥岩剪切蠕变特性及其本构模型

为保障滇中红层泥岩所在区域隧洞工程的施工及长期运营安全,本文以“滇中引水工程”狮子山隧洞段的红层泥岩为研究对象,首先,采用分级加载剪切蠕变试验方法,研究其蠕变特性;其次,基于非线性流变力学理论,分别构建了触发式非定常加速元件、非线性弹性元件及复合型黏性元件;第三,通过各元件的合理组合,建立了滇中红层泥岩的非线性、非定常蠕变模型;最后,对所建模型进行参数估计,与试验结果进行对比,验证理论模型的适用性。研究结果表明:在较低应力状态下,岩样主要表现为衰减蠕变,其中蠕变变形在总变形中的占比可达85%;而在较高应力状态下,岩样发生从衰减蠕变、等速蠕变直至加速蠕变破坏的全过程蠕变,其中蠕变变形在总变形中的占比达87%。剪应力对岩样蠕变变形具有显著影响,剪应力越高,衰减蠕变达到稳定状态所需的时间越长,蠕变变形在总变形中的占比越高。在全过程蠕变中,岩样的蠕变变形以稳态蠕变为主,其蠕变时间占总蠕变时间的80%,蠕变量占总蠕变量的51%。该蠕变模型既能完整地描述岩样从衰减到等速直至加速蠕变的全过程,同时又能克服传统模型参数取值随应力而不断变化的问题,通过一组参数就可以描述岩样在各应力状态下的蠕变变形特性。理论模型得到的结果与试验结果吻合较好,说明所建模型对于滇中红层泥岩具有较好的适用性。

动态冲击下红砂岩蠕变特性及损伤本构模型

为研究岩石在动态扰动下的蠕变特性,在自行研发的冲击扰动蠕变试验装置上,对细粒红砂岩开展了多级冲击扰动蠕变试验,分析了不同蠕变应力与扰动能量下瞬时变形与蠕变行为,结合间接法得到了扰动能量对长期强度的影响。结果表明:动态扰动在较低的蠕变应力下使试样致密,而当蠕变应力超过一定的应力阈值后加剧试样内部的损伤恶化,其引起的瞬时变形与蠕变应变则随蠕变应力表现出先减后增的规律;随着动态扰动能量的增加,试样的破坏应变越大,但长期强度越低;引入长期强度与扰动能量的相关性、动态扰动单元和扰动-损伤黏性元件,建立了黏塑性扰动损伤蠕变模型,结合试验数据验证了该模型的准确性,该模型可以准确地描述砂岩在静态恒载和动态扰动组合作用下的瞬态变形和蠕变行为。该研究成果对于钻爆法隧道的支护设计以及安全生产具有重要的理论意义与应用前景。

桥梁结构中E-GFRP单向板徐变性能与双尺度均匀化数值评估

由于玻璃纤维增强复合材料(GFRP)组分材料中的树脂属于高分子材料,桥梁工程界对GFRP结构的徐变性能十分担忧,阻碍了其推广应用。该文聚焦于桥梁工程E型玻璃纤维增强复合材料(E-GFRP)单向板,基于双尺度均匀化数值模拟方法,从纤维和树脂的徐变性能评估了E-GFRP单向板的徐变性能,并与试验结果进行比较验证了预测结果的准确性。在此基础上,分析了应力水平、纤维体积率及持荷形式对E-GFRP单层板徐变性能的影响,结果表明,应力水平越高、纤维体积率越小,单层板的徐变粘滞应变越大。在此基础上,提出了不同纤维体积率下E-GFRP单层板粘滞应变模型,准确表述了应力水平、持荷时间和徐变应变之间的关系。最后,提出了E-GFRP单层板徐变断裂时间预测公式,可为E-GFRP结构的长期性能预测提供参考。

压电致动器非线性特征的控制策略研究进展

压电致动器具有位移分辨率高、体积小、响应快、驱动负载能力强、可多自由度输出以及不受电磁干扰等优点,被广泛应用在微纳加工、微机电系统封装、生物医学以及航空航天等领域。然而,由于压电致动器自身存在的迟滞、蠕变等非线性特征,使得输出的稳定性以及精确度受到一定的影响,需采取恰当的控制策略解决上述问题。本文首先对压电致动器的非线性特征进行概述;其次,重点回顾了迟滞模型和蠕变模型的发展历程;然后,介绍了压电致动器的控制补偿策略及研究进展;最后对压电致动器控制技术的未来发展方向进行了讨论与展望。

多孔介质各向异性动态渗透率模型

基于迂曲毛细管网络模型,推导建立了各向异性渗透率与岩石正应变间的关系式,即各向异性动态渗透率模型(ADPM),采用孔隙尺度流动模拟对模型进行了验证,并对单轴应变过程进行了计算,分析了变形过程中影响不同方向渗透率变化的主要因素。研究表明:层状油气藏开采的单轴应变过程中,有效面孔率对各方向的渗透率影响规律一致,随着有效面孔率的减小,渗透率对应变的敏感程度增大;垂直于压缩方向的渗透率对应变的敏感程度随迂曲度的增加而减小,沿压缩方向的渗透率对应变的敏感程度随迂曲度的增加而增大。初始各方向迂曲度相同的层状油气藏,在压力下降的过程中,迂曲度对各方向渗透率变化幅度的相对关系起决定性作用,迂曲度小于1.6时,岩石水平方向渗透率的降幅要高于垂向渗透率,迂曲度大于1.6时则相反,传统动态渗透率模型无法表征这一现象。经孔隙尺度模拟实验数据的验证,新模型拟合精度高,可以有效表征不同方向变形对各方向渗透率的影响。

干湿循环作用下吹填固化轻质土蠕变特性研究

采用新型固化技术对高含水率吹填土进行固化处理,通过三轴流变仪,考虑干湿循环次数、围压和初始静偏应力等因素研究其对吹填固化轻质土蠕变变形和抗剪强度指标的影响规律。试验结果表明:吹填固化轻质土具有密度小、质量轻及强度高等优点。在一定程度上,增大围压可有效缓解吹填固化轻质土的蠕变变形,而初始静偏应力对其影响较小,前7次干湿循环对土体破坏应力的影响最大,而后基本保持不变。抗剪强度指标c、φ值均随初始静偏应力的增加而逐渐降低,随干湿循环次数的增加呈现出先减小而后逐渐稳定的趋势,且前7次干湿循环,二者均呈指数函数降低;7次以后,变化均较小并逐渐稳定。基于c、φ值与时间关系曲线,分别构建了不同偏应力作用下吹填固化轻质土c、φ值的预测公式。根据蠕变曲线发展态势,通过回归分析,引入围压、初始静偏应力和干湿循环次数等因素,建立了吹填固化轻质土蠕变变形预测模型,并验证了其可行性。研究结果可为实际工程场地提供理论依据。