三维粒状脆性材料颗粒破碎的数值三轴试验

针对脆性材料颗粒破碎的三维力学问题,从细观力学角度,以颗粒极限拉应力作为颗粒破碎的判别标准,进行了考虑颗粒破碎的堆石料大三轴排水剪试验的数值模拟,直观揭示了堆石料在三轴应力状态下的变形过程和细观机理。结果表明,基于颗粒流方法的数值模拟可再现堆石料室内试验的应力—应变及体变曲线、围压依赖性、剪胀性及结构变化等。     

特高心墙堆石坝建设中值得关注的几个问题

基于已建工程经验、试验研究、坝料施工填筑质量检测、大坝变形监测数据分析等,并结合300m级RM特高心墙堆石坝工程建设面临的技术挑战,研究认为特高心墙堆石坝建设中的堆石体填筑质量控制标准、超大粒径堆石料室内缩尺试验精度、特高心墙堆石坝长期运行安全等几个问题值得关注。建议堆石体填筑采用孔隙率和相对密度双控标准,将现场试验和室内试验手段相结合研究超大粒径堆石料缩尺效应试验问题,可从理论上实现特高堆石坝变形协调与预测的准确控制;低频高水位变幅条件下特高堆石坝的长期变形机理较为复杂,需进一步深化研究库水位消落带变应力工作条件下往复循环荷载变形、流变和湿化问题。     

土工格栅加筋土石坝的试验和数值模拟研究

为进一步了解土工格栅加筋土石坝的变化特性,对土工格栅和堆石料接触面特性进行了拉拔试验,发现接触面上的剪应力较大,且随上覆压力的增加而增加;试验拉拔位移主要为土工格栅的伸长量,接触面上的相对位移较小,因此加筋土石坝数值模拟应设置土工格栅和堆石料接触面变形协调,并将试验结果应用于加筋土石坝的数值模拟中,分析了土石坝加筋前后应力变形特性和动力响应的变化规律。结果表明,加筋对土石坝变形的抑制效果主要体现在地震工况下;加筋后坝体的地震永久变形有所减小,加筋对顺河向的地震永久变形抑制效果大于震陷;加筋后顺河向永久变形指向下游,最大震陷位于坝顶。     

200 m级面板堆石坝流变分析

针对堆石流变会引起200 m级面板堆石坝面板脱空、压坏等现象,采用沈珠江三参数流变模型对200m级混凝土面板堆石坝进行三维有限元计算,对比了不考虑堆石料流变的计算结果,并分析了堆石料流变特性对坝体和混凝土面板应力变形及面板接缝变形的影响.结果表明,堆石流变较大程度上改变了坝体和面板的变形与应力.     

考虑侧向约束的堆石料颗粒破碎试验研究

堆石料的变形与颗粒破碎所引起的颗粒滑移和填充密切相关,通过室内大型单轴固结试验和三轴剪切试验,以Marsal破碎率为量化指标,分析了堆石料的颗粒破碎情况。结果表明,在单轴固结试验中,破碎率随轴压的增加而增加,两者的关系可用指数函数表示;在三轴试验中,随围压的增加,破碎率不断增加并最终趋于稳定,两者的关系可用幂函数表示。在两种试验条件下颗粒破碎情况差别很大,而这主要是由不同侧向约束所引起的,因此可依据侧压力系数的不同,采用不同函数计算破碎率。     

面板坝次堆石区力学特性对坝体变形协调的影响

以厦门抽水蓄能电站上水库面板堆石坝为研究对象,采用三维非线性有限元法和邓肯E-B模型,设计多组不同力学特性的主次堆石料组合,对比分析坝体变形及面板位移,探究蓄水期坝体各部位变形受堆石材料性能变化的影响程度。建议面板堆石坝主次堆石区变形协调的变形梯度极限指标为3%,同时以地震永久变形量作为抗震校核的指标。对于该工程,在现有填筑料的基础上,适当提高次堆石料力学性能,坝体坝轴线下游侧水平及垂直位移均显著降低,位移的极值部位向坝轴线靠近,高程略有上升;次堆石料的变形模量对面板的挠度影响不大。     

考虑临界状态的堆石料广义塑性本构模型

基于大型静力三轴剪切试验结果,构造了考虑颗粒破碎的塑性模量表达式和考虑临界状态的剪胀方程,建立了适用于堆石料的广义塑性本构模型,并利用本构模型对堆石料三轴试验进行预测。结果表明,考虑和不考虑临界状态的本构模型均能较好地模拟堆石料在高围压下的剪缩性、在低围压下的剪胀性以及由于颗粒破碎引起峰值和剪胀应力比的非线性特性;考虑临界状态的本构模型能模拟应力——应变曲线逐渐逼近临界状态的规律,但其对三轴试验的预测精度略低于不考虑临界状态本构模型。     

缩尺效应对堆石料颗粒破碎特性的影响

受试验仪器在试样尺寸上的限制,室内试验中需要对试验土料进行缩尺处理,因而堆石料的缩尺效应对颗粒破碎的影响成为其本构关系研究中亟待解决的问题。采用相似级配法和混合法对微风化灰岩料进行缩尺,缩尺后试样最大粒径分别为60、40、20、10 mm。通过三轴固结排水剪切试验和筛分试验,利用提出的相对破碎率计算方法,计算了试样剪切前后的相对破碎率。结果表明,制样过程中产生的颗粒破碎不可忽略。相同条件下,试样的相对破碎率随最大粒径的增大而增大;混合法缩尺后试样的相对破碎率大于相似级配法缩尺试样。基于试验结果,提出了考虑缩尺效应的堆石料相对颗粒破碎率估算方法。研究结果可为建立考虑缩尺效应对颗粒破碎影响的堆石料本构模型提供理论依据。     

基于ACC-RBF的水布垭面板堆石坝参数反演分析

蚁群聚类径向基函数(ACC-RBF)神经网络是将蚁群聚类算法和径向基函数神经网络组合运用的一种新型神经网络模型,把该网络用于水布垭高面板坝堆石体的多参数反演问题,在室内试验参数的基础上用有限元计算获得学习样本,采用该网络对坝体堆石料的邓肯E-B模型参数进行反演分析,用反演所得参数结合三维非线性有限元计算坝体应力变形,并...     

K0固结条件下土石坝坝基砂卵砾石料强度与变形分析

选取级配相同的双江口土石坝、长河坝土石坝坝基覆盖层砂卵砾石料,分别进行4种不同围压下的静止土侧压力系数K0固结排水剪切大型三轴试验,分析了K0条件下砂卵砾石料的强度与变形特性。结果表明,K0条件下的砂卵砾石料在低围压下表现出较强的剪胀特性;不同围压下,砂卵砾石料的峰值强度与围压之间近似呈线性关系;两种砂卵砾石料的初始弹性模量Ei、初始泊松比νi与围压之间均存在良好的幂函数关系。结果为土石坝坝基覆盖层砂卵砾石料的强度与变形特性研究提供了参考。     

Gradation of mechanical properties in gas diffusion electrode. Part 1: Influence of nano-scale heterogeneity in catalyst layer on interfacial strength between catalyst layer and membrane

Stress and plastic deformation analyses of catalyst layer have been conducted after experimentally investigating its mechanical properties at nano-scale. Interestingly, catalyst layer is found to have varying mechanical properties as a function of depth and therefore it is classified under graded material. Effect of gradation in catalyst layer on interfacial strength between membrane and catalyst layer is explained with the aid of numerical simulations. Stress redistribution near interface line is observed in graded model, while stresses are found to have concentrated at critical locations throughout the discrete model. However, it is outlined from this study that the gradation in catalyst layer leads to greater amount of plastic energy dissipation—an indication of enhanced ductility. An experimental coupled numerical approach is presented to characterize the effect of transitional variations of mechanical properties in catalyst layer on the interfacial line and membrane.     

Influence of sandblasting and hydrogen on tensile and fatigue properties of pipeline API 5L X52 steel

The evaluation of static properties and lifetime of a pipeline notched under the impact of sand with or without the presence of hydrogen has been performed. The material damage was made by electrolytic hydrogen and projecting corundum particles (aluminium oxide). It has been shown that sandblasting and hydrogen have little affect on the yield stress and ultimate strength. The material lifetime and elongation at fracture are clearly affected by hydrogen, which penetrates into the surface layers of the material and changes the local fracture mechanism. Despite the erosion of these layers, under the sand impacting, failure strain and lifetime are improved. The observation of failure mode shows that the deformation field, after sandblasting, is very important. The crack propagation and the failure seem to be intra granular. The cracks, in the pipeline API 5L X52 steel charged with hydrogen, propagate following the porosity path without any distinct direction. The absorbed hydrogen atoms placed inside the crystalline sites of steel cause the embrittlement of material so that a small effort is sufficient to create cleavage. Modified notch failure assessment diagram was used to evaluate the dangerousness of studied notch defect in different environments: air, hydrogen and sandblasting.     

A stable and flexible carbon black/polyethyleneimine-bacterial cellulose photothermal membrane for high-efficiency solar vapor generation

Although various special materials have been exploited for enhancing evaporation performance of a solar vapor generation system, their practical applications could be greatly limited by structure destruction due to the seawater corrosion and external mechanical forces. In this work, we have developed a carbon black/polyethyleneimine-bacterial cellulose (CPB) membrane with vacuum filtration method to simultaneously enhance the strength and evaporation performance of solar vapor generation system, by applying carbon blacks as the photo-thermal conversion material, bacterial cellulose as the skeleton material for enhancing the structure strength, and polyethyleneimine (PEI) for tailoring the water absorption capacity. Effects of carbon black particle concentrations and PEI concentrations were probed, respectively, for optimizing the light absorption capacity and water absorption capacity in laboratory. Furthermore, the outdoor experiments were carried out to evaluate the strength and evaporation performances of CPB membrane. Results show that the evaporation efficiency of the CPB membrane could reach about 85.05% and 81.89% in the lab and outdoor under one sun irradiation. Additionally, a force of 59.37 MPa and folding more than 100 times will not break the structure of CPB membrane, which confirms the preferable structure strength. This superior CPB membrane, together with its low cost, simple fabrication, excellent mechanical properties, scalability and desalination ability, provides a feasible way for practical applications.     

掺人工砂对磷酸钾镁水泥砂浆性能的影响

磷酸钾镁水泥(MKPC)是一种基于酸碱反应而凝结硬化的新型胶凝材料,掺人工砂可改变其砂浆强度、表观特征和体积稳定性。利用不同人工砂掺量砂浆强度试验,研究了人工砂掺量对MKPC砂浆抗折性能和抗压性能的影响;运用X射线衍射物相分析方法(XRD)与电镜扫描方法(SEM)分析了MKPC砂浆表面的微观特征;利用对比试验分析了人工砂对MKPC砂浆体积稳定性影响。结果表明,人工砂掺量为50%的MKPC砂浆硬化体抗折强度、抗压强度最高;结构致密、界面结构完善;表现出微膨胀性,最大膨胀率为普通砂浆最大收缩率的1/6,体积稳定性良好。     

冷压缩变形对45钢组织性能影响的研究

冷压缩变形对材料组织性能有着较大的影响，而材料的性能对工业制成品的最终质量有着较大的影响。通过对45钢试样进行试验，经冷压缩变形，使45钢的晶粒度得到了细化，从而其强度得到了提高。     

An analytical solution for thermocapillary-driven convection of superimposed fluids at zero Reynolds and Marangoni numbers

This study aims to investigate thermocapillary-driven convection in two superimposed fluids in zero gravity. The fluids occupy the space between the walls of a horizontal microchannel which is heated from below by imposing the top wall to a uniform temperature and the bottom wall to a sinusoidal temperature that is higher (on the average) than the temperature of the top wall. The goal is to mimic thermocapillary convection as a result of the variation of the heights of the fluids along the microchannel and to explore the parameters that affect the fluid flow and interface deformation. This is achieved by solving the equations of conservation of mass and momentum and the balance of thermal energy and negligible analytically in both fluids, in the limit of creeping flow regime and negligible convection of heat. It is shown that the induced flow is characterized by periodic convection cells whose period is the same as the period of the imposed temperature field and extend from the interface to the walls in the vertical direction. The flow strength depends on the relative thicknesses of the fluid layers and the ratio of material properties. The maximum flow strength is achieved at a relative thickness that is set by the competition between the thermal and hydrodynamic effects. An estimate of the interface deformation is provided and it is shown that the sense of interface deformation is set by the relative thickness of the fluid layers and the viscosity ratio.     

吸气过滤器应力有限元分析及优化设计

针对某吸气过滤器各部件结构的应力仿真计算和强度分析问题,根据有限元理论和方法建立了有限元计算模型,采用ANSYS Workbench进行应力计算,得到了各部件的变形云图和应力分布云图。在应力分析的基础上,根据材料力学的相关理论和压力容器的相关标准对模型进行应力评定和强度分析,应力计算结果显示,有两个部件不能满足强度要求。对两个问题部件进行结构优化,优化后的所有部件结构均能满足强度要求,为吸气过滤器材料成本的降低提供了一种有效的方法。     

不同EPS粒径下轻量土变形强度特性三轴试验研究

为确定新型土工合成材料轻量土在不同发泡颗粒(EPS)粒径下的变形机理,通过三轴固结不排水试验系统研究其变形强度特性的影响规律。试验结果表明,轻量土应力-应变关系具有非线性、多阶段性、应变软化和应变硬化特性;抗剪强度破坏包络线有折线型与直线型两种,取决于自身结构强度与围压;粘聚力与内摩擦角均随EPS粒径的增大而递减。     

COUPLED THERMOVISCOPLASTICITY EQUATIONS WITH TEMPERATURE-DEPENDENT MATERIAL PROPERTIES

Coupled thermoviscoplasiicity equations for materials with temperature-dependent material properties are developed based on the rational theory of thermodynamics. It is shown that, in addition to the mechanical work due to viscoplasticity, micro-structural effects, and dilatation, a mechanical work due to the temperature dependency of the material properties contributes to the total rate of mechanical energy generation. Axisymmetric cyclic bending of a circular plate with a hole clamped at the inner edge has been solved, using fmite~element analysis, in order to investigate the effect of coupling and the temperature-dependent material properties on the thermomechanical behavior of the plate. For the material chosen (steel 1070) and under the conditions considered (infinitesimal deformation and constant ambient temperature), the effect of coupling as well as the contribution of the temperature-dependent material properties is negligible.     

Behavior of Ca(OH)2/CaO pellet under dehydration and hydration

With construction of a thermochemical energy conversion prototype system to store solar heat, thermal dissociation of pellets of Ca(OH)₂ and hydration of CaO have been investigated in some detail for its application to the system. The inorganic substance is very attractive as a material for long term heat storage, but molar density changes associated with the reaction are fairly large. Therefore, this factor has been taken into account in the kinetic equation. The importance of additives and pellet size has been discussed considering reactivity and strength of pellets. An analysis has been attempted when chemical reaction is important. The deformation of pellets was observed during hydration.