Numerical analysis on the influence of rock specimen size on crack stress field

In the simulation of rupture processes of seismic sources by using either numerical method or rock mechanics experiments, improper setting of the specimen size will influence the stress field near the faults. In this study, 2D finite element method (FEM) was used to calculate the stress field of rock specimens in different sizes with fixed-size elliptic holes. The calculated stress field was compared with analytic solution for elliptic-hole problem in an infinite medium. Numerical results showed that boundary effect of a rock specimen with an elliptic hole on stress field under uniaxial compression cannot be neglected. Critical aspect ratio of the specimen is about 3:2, and critical ratio of distance between the tip of the hole and the border of specimen (d) to the major axis of the elliptic hole (l) is about 7.3. Numerical analysis on rock specimen size can provide theoretical reference for rock specimen experiments, and it is also helpful for setting of model sizes in numerical simulations of fault movement.

Experimental study on complete stress-deformation curves of larger-size concrete specimens subjected to uniaxial tension

In order to provide parameters for numerical analyses of the huge Three-Gorge concrete dam (2309 m long by 175 m height), complete tensile stress-deformation curves for large-size plain concrete specimens were measured and studied by per-forming uniaxial tensile tests on large-size unnotched specimens (250 mm×250 mm×1400 mm). The specimens were prepared with the three-graded-aggregate materials provided by the client of the Three-Gorge project. To prevent a failure occurring near the ends of the unnotched specimens, both the ends of each specimen (450 mm in length) were cast using a higher-strength concrete than the middle part (i.e., active part). Tensile tests were completed on a specially-designed tensile testing machine, which can be easily re-assembled to accommodate different-size specimens. To make the specimens fail stably, a cyclic loading scheme was adopted after the peak strength was reached. Four of five tests in this study were successful, and four complete tensile stress-deformation curves were obtained. It was found that the post-peak curve of the large-size specimens used in this study is more gradual than those for the small-size specimens reported in the literature.

Finite size specimens with cracks of icosahedral Al Pd Mn quasicrystals

Icosahedral quasicrystals are the most important and thermodynamically stable in all about 200 kinds of quasicrystals currently observed. Beyond the scope of classical elasticity, apart from a phonon displacement field, there is a phason displacement field in the elasticity of the quasicrystal, which induces an important effect on the mechanical properties of the material and makes an analytical solution difficult to obtain. In this paper, a finite element algorithm for the static elasticity of icosahedral quasicrystals is developed by transforming the elastic boundary value problem of the icosahedral quasicrystals into an equivalent variational problem. Analytical and numerical solutions for an icosahedral A1-Pd-Mn quasicrystal cuboid subjected to a uniaxial tension with different phonon-phason coupling parameters are given to verify the validity of the numerical approach. A comparison between the analytical and numerical solutions of the specimen demonstrates the accuracy and efficiency of the present algorithm. Finally, in order to reveal the fracture behavior of the icosahedral A1-Pd-Mn quasicrystal, a cracked specimen with a finite size of matter is investigated, both with and without phonon-phason coupling. Meanwhile, the geometry factors are calculated, including the stress intensity factor and the crack opening displacement for the finite-size specimen. Computational results reveal the importance of pbonon-phason coupling effect on the icosahedral A1-Pd-Mn quasicrystal. Furthermore, the finite element procedure can be used to solve more complicated boundary value problems.

Effect of size on characteristic of acoustic emission in rock specimen

A series of uniaxial-compression tests were conducted on some granite speci- mens.A multi-channel,high-speed AE signal acquiring and analyzing system was em- ployed to acquire and record the characteristics of AE events and demonstrate the tem- poral and spatial distribution of these events during the failure process.The test results show that in the primary stage,many low amplitude AE events are developed rapidly and distributed randomly throughout the entire specimens.In the second stage,the number of AE increases much slower than that in the first stage,while the amplitude of most AE events became greater.Contrarily to the primary stage,AE events clusteres in the middle area of the specimen and distributes vertically conformed to the orientation of compres- sion.The most distinct characteristic of this stage is a vacant gap formed approximately in the central part of the specimen.In the last stage,the number of AE events increases sharply and their magnitude increases accordingly.The final failure location coincidently inhabited the aforementioned gap.The main conclusion is that most macrocracks are de- veloped from the surrounding microcracks existed earlier and their positions occupy the earlier formed gaps,and the AE activity usually becomes quite acute before the main rup- ture occurs.

辉长岩骨料全级配碾压混凝土力学特性研究

辉长岩坚硬致密,具有高强度、高弹模特性,但将辉长岩骨料应用于大型水电工程在国内外尚无先例,缺乏可借鉴的工程实践经验,且湿筛碾压混凝土试件的力学性能无法真实反映全级配碾压混凝土的力学特性。对辉长岩骨料最大粒径为80 mm的全级配碾压混凝土的力学特性演化规律开展了系统研究,包括抗压强度、劈拉强度、轴压性能、轴拉性能、抗剪性能等,对比分析了全级配碾压混凝土抗压、劈拉强度及其湿筛试件的尺寸效应,并将试验结果与国内典型工程所用碾压混凝土的力学性能发展规律进行对比分析,此外还建立了辉长岩骨料全级配碾压混凝土抗压强度、劈拉强度、轴压性能、轴拉性能等力学性能-养护龄期演化函数模型。结果表明:辉长岩骨料全级配碾压混凝土的抗压强度、劈拉强度、抗剪等力学性能随龄期的发展规律符合国内同类工程碾压混凝土力学性能发展的一般规律;C_(90)20W8F100辉长岩全级配碾压混凝土试件与其湿筛试件抗压强度平均尺寸效应系数为0.80、劈拉强度平均尺寸效应系数为0.66;相关力学性能演化模型相关性高(R^(2)=0.9643~0.9992)。可见辉长岩骨料用于大型水电工程碾压混凝土坝建设具有可行性。

碳纤维增强复合材料准静态电弧损伤的电-热耦合分析

为有效预测碳纤维增强复合材料(carbon fibre reinforced polymer,CFRP)层压板的雷击电弧附着响应,并探讨阳极试件材料对雷电电弧参数空间分布的影响,基于商业有限元分析软件ABAQUS,分析了CFRP层压板雷电流注入后的热-电-结构耦合,建立了CFRP层压板电弧附着损伤的三维有限元模型。模拟了复合材料层压板在雷电流作用下的冲击响应,进行了复合材料层压板雷击附着机理和作用模式的分析,并探讨了试件尺寸对电弧附着响应特性的影响。研究结果表明,当CFRP试件的长宽比接近1时,CFRP损伤区域往往集中在准静态电弧附着处,与其他长宽比相比,损伤面积较小,但损伤深度较深。

粒径尺寸对红黏土持水曲线及孔径分布影响研究

为探究粒径尺寸对红黏土的持水特性及孔径分布的影响,用轴平移技术和蒸汽平衡技术对不同粒径尺寸制备的红黏土压实样开展广吸力范围内的持水特性试验和压汞试验,基于不同粒径尺寸试样的持水曲线和孔径分布曲线,揭示粒径尺寸影响土体持水特性的细观机理。具体结论如下:(1)粒径尺寸只对压实样在低吸力范围的持水曲线产生影响,表现为粒径尺寸越小,制备试样进气值则越大。(2)不同粒径制备的饱和压实样均为单峰型孔径结构,且粒径尺寸越小,制备试样的集中孔径分布的峰值越高,分布的范围也越小,表明粒径越小的试样的孔径分布越均匀。(3)预测和实测持水曲线的比较结果显示,由于无法考虑土体干化路径下的孔隙比减小,孔径分布曲线可用于预测土体在对应吸力下的含水率,而预测饱和度低于实测的饱和度。

Effects of Specimen Height on the Acoustic Emission Rate Value 'a' for Cement Mortar

In order to study the size effect on the AE rate ’a’ value,three kinds of mix ratios were set up by different particle sizes and water cement ratios,45 cement mortar specimens with five different heights were tested under axial compression.And the whole damage processes were monitored by full-digital acoustic emission acquisition system,followed by an analysis of mechanical behavior and AE activity.The experimental results show that the height of the cement specimen has significant effects on the compressive strength and the acoustic emission rate ’a’ value,but a slight effect on the accumulated AE hits number,which is analyzed from aspects of failure process of cement mortar specimens.

黏性泥沙粒径对高速水流空蚀影响的试验研究

在小型循环式水洞中对黏性泥沙粒径对高速水流空蚀影响的机理开展了试验研究。在水洞的水箱内筒中人工配制了含沙量s为12 kg/m^(3),中值粒径d 50分别为0.087,0.240,0.403 mm的3种黏性泥沙水样。为探究中值粒径对空蚀的影响,制作了不同配合比的混凝土试件,在空蚀区安装混凝土试件,使混凝土试件在不同中值粒径工况下进行4 h的空蚀试验,通过混凝土试件每小时的质量损失来表征空蚀量。试验结果表明:在含沙量s为12 kg/m^(3)工况下,水洞空蚀区各测点的压强和空化数随中值粒径的增加而升高,促进了空蚀的发生;当混凝土试件配合比、龄期不变时,混凝土试件的空蚀程度随中值粒径的增大而加深,空蚀范围扩大;当喉部流速提高时,空蚀则进一步扩大。较高强度配合比(水灰比=0.40,灰砂比=1.5)的混凝土试件在不同中值粒径时的抗空蚀能力优于较低强度配合比(水灰比=0.45,灰砂比=2.0)的混凝土试件。

室内缩尺级配堆石料力学参数的表征单元体积

为了寻找室内缩尺级配堆石料强度和变形参数的表征单元体积(REV),利用离散元方法(DEM)研究试样尺寸对玄武岩堆石料力学行为的影响.通过室内单颗粒破碎试验,分析玄武岩颗粒的破碎特性,确定DEM模拟所需的颗粒破碎强度参数.开展一系列试样尺寸、颗粒排列和围压不同的三轴压缩DEM试验.分析堆石料强度和变形参数随试样尺寸的变化规律,建议了各参数的REV.结果表明,颗粒排列和试样尺寸均会影响堆石料的力学行为.各力学参数的变异系数随着试样尺寸的增大而减小.不同力学参数的REV尺寸不同,为了提高DEM计算效率,可以根据研究对象选择合适的试样尺寸.在模拟工况下,要使所有参数均达到稳定,数值试样尺寸应不小于直径300 mm×高度600 mm.

制样含水率影响红黏土强度特性的微观机理研究

将制样含水率分别小于和大于最优含水率制备得到的红黏土(称为干侧样和湿侧样),通过轴平移技术(ATT)或蒸汽平衡技术(VET)施加吸力后,用非饱和土三轴仪开展同一净应力下的吸力控制三轴剪切试验,分析制样含水率对压实红黏土强度特性的影响。试验结果表明:无论是干侧样还是湿侧样,土体的峰值强度和剪胀量都随着吸力的增大而增大,且高吸力试样表现出明显的剪胀和软化力学特性。此外,相同吸力下,干侧样的峰值强度和剪胀量均大于相应湿侧样的试验结果,证实了制样含水率会对压实红黏土的力学特性产生重要影响。相同吸力和孔隙比的干侧样和湿侧样压汞试验结果表明,干侧样和湿侧样的孔径分布曲线存在明显差异,干侧样为双峰型孔径结构,湿侧样则为典型的单峰型孔径结构。进一步量化分析后确定,干侧样峰值强度大于相应的湿侧样,主要是由于干侧样的大孔径(d>0.2μm)累计体积量小于相应湿侧样的大孔径累计量引起的。而且,土体的峰值强度随大孔径累积体积量的增大而呈指数减少。

烟草保润性能测试方法

为了得到一套过程简便、结果准确的烟草保润性能测试方法,为增香保润重大专项的实施奠定基础,考察了测试方法、湿度控制手段、取样量及烟样形态4个因素对保润性能测试结果可靠性的影响。结果表明:在进行烟草保润性能测试时,尽量增大取样量,采用差量法、将形态均匀的烟丝放入硫酸干燥器内,并将硫酸干燥器放入相同湿度下的恒温恒湿箱中进行烟丝解湿或吸湿过程水分变化情况的测试,可以得到相对准确的测试结果。

经历常温降至-196℃再回温混凝土温度场试验研究

通过不同形状和尺寸的混凝土试件经历常温降至-196℃再回温的作用,探讨混凝土内温度在降温和回温过程中的变化规律以及降温回温作用速率、试件的形状和尺寸等对其的影响。结果表明,混凝土内温度在降温回温过程中呈现出显著的滞后性且降温与回温过程存在较大的差异,反映出混凝土在超低温环境下仍具有明显的热惰性,但与高温时不尽相同。混凝土内温度的变化以及与作用温度间的温差受降温回温作用速率和试件尺寸的影响明显且呈现非线性关系。在常温至-196℃温度作用范围最小边长不小于100 mm试件的内外最大温差可达100℃,并且在降温过程中混凝土内温度低于反转温度时,其温度变化速率将达到甚至超过降温作用速率,但混凝土内各处温度变化的回温速率始终低于作用速率。这些试验结果可为(LNG)储罐类混凝土结构设计提供参考。

各种级配大坝混凝土双K断裂参数试验研究——兼对《水工混凝土断裂试验规程》制定的建议

为配合水工混凝土断裂试验规程在我国的制定,提供更充足的基础数据,受规程编制组委托,进行了最大尺寸为S×D×B=2200 mm×550 mm×240 mm的三点弯曲梁试件和最大尺寸为2H×D×B=1200 mm×1200 mm×250mm的楔入劈拉试件共140个试件的断裂试验。通过电阻应变片,测得不同布置测点荷载应变变化曲线,由此确定了不同级配及湿筛法大坝混凝土实测的起裂荷载和失稳荷载,计算了双K断裂韧度参数,讨论分析了双K断裂参数的尺寸效应和形状效应。结果表明:电阻应变片法测定起裂荷载的方法可行,Pini/Pmax=0.6￣0.85;三点弯曲梁法所得混凝土双K断裂韧度值无明显的尺寸效应;楔入劈拉试件由于竖向加载荷载PV引起附加力矩MV对裂缝尖端应力场的影响,导致所得混凝土断裂韧度值表现出一定程度的尺寸效应,但当采取措施消除此影响或此影响较小时,楔入劈拉试件的断裂韧度值也将无明显的尺寸效应;在相同配合比、相同高度条件下,不同试件形式的双K断裂韧度值无明显的差异。因此,可认为混凝土双K断裂参数是描述混凝土结构裂缝起裂及失稳断裂过程的材料常数,与试样类型和尺寸无关。

岩石动态劈裂试验的最优试件尺寸分析

利用有限元分析软件LS－DYNA对常规巴西圆盘、平台巴西圆盘和带孔巴西圆盘这三种最常用的劈拉试件，进行岩石 SHPB 动态劈裂试验的数值模拟，并依据模拟结果，采用平台巴西圆盘法，对花岗岩试样进行了动态劈裂试验。对比模拟与试验结果，得到了加载应变率与岩石动态抗拉强度的关系，探讨了用于岩石动态劈裂试验的最优试件尺寸。结果表明：平台巴西圆盘法可以较好地满足中心起裂条件，试验结果的可信度更高，而对于所采用的岩石参数，建议平台巴西圆盘试件的中心角取20°～24°；如采用带孔巴西圆盘试件进行动态劈裂试验，建议内外孔径比取0．4～0．5；岩石的动态劈拉强度随应变率的增大而增大，但增大速率越来越小，且这种增大趋势受到试件尺寸的显著影响。

AA7050高强铝合金在固溶淬火过程中的温度变化及固溶制度(英文)

采用温度记录系统、组织观察、力学性能测试、电导率测试和DSC热分析研究7050高强铝合金固溶过程中的温度变化,确定合理的固溶热处理制度。采用了2种不同尺寸规格的试样进行试验。研究表明,在本试验条件下的固溶热处理过程中,大尺寸试样固溶温度相对较低,固溶时间较短,导致合金中强化析出相的体积分数减少。对于尺寸为25mm×25mm×2.5mm和70mm×60mm×20mm的试样,合理的固溶热处理制度分别为(480℃,30min)和(480℃,90min)。小尺寸试样中的GP区和η′相密度大于大尺寸试样中相应的密度,这与合金性能测试的结果一致。

盐渍土地区混凝土耐久性试验用试件尺寸的评价

采用2种不同尺寸的混凝土试件进行室内混凝土耐久性快速破坏试验,寻求适用于室内混凝土耐久性快速破坏试验的最合理的混凝土试件尺寸。结果表明:采用尺寸为100 mm×100 mm×400 mm的混凝土试件比采用尺寸为40 mm×40 mm×160 mm的混凝土试件进行耐久性快速破坏试验的试验结果具有较好的准确性。

试样尺寸及制样粒径大小对粗粒土三轴试验抗剪强度的影响

三轴试验是确定土的抗剪强度的主要方法,试样尺寸是影响三轴试验结果的重要因素之一.粗粒土的三轴试验需要采用大尺寸的试样进行,相应需要采用大尺寸试验装置及更多的制样用土.与大尺寸三轴试验相比,采用小尺寸三轴压力室,所需的试样较小,制样用土较少,且小尺寸三轴试验过程更快,试验成本更低.但采用小尺寸试样对粗粒土进行三轴试验时,往往得到偏小的抗剪强度参数,导致实际工程设计过于保守而不经济.为了降低试验成本,可以采用小尺寸试样进行试验,通过对试验结果进行修正来获得大尺寸试样试验结果,进而使工程设计具有良好的经济性.针对以上问题,本文对比研究了不良级配砾质砂混合物在采用去粗法前后制样试验所得的抗剪强度,提出了采用不同尺寸试样进行试验时对试验结果进行修正的方法及计算公式.

不同小试样测量蠕变性能的比较研究

小试样蠕变试验技术,在服役构件力学性能测试中得到了广泛的发展与应用。目前已有小尺寸单轴试验、小冲杆试验、剪切冲压试验、压痕试验、三点弯试验、悬臂梁试验、固支直杆弯曲试验、圆环试验和双杆试样等方法。基于上述小试样方法,重点从试样尺寸、应力状态、关联式类型、蠕变性能、变形测量和影响因素等方面,全面地综述不同小试样测量材料蠕变性能的方法和结果的异同。通过对比发现,不同类型试样所得蠕变曲线差异较大。小冲杆试样的数据转换过程较为复杂,试验过程中影响因素较多。悬臂梁试样和圆环试样的等效标距较大,变形测量误差小。三点弯、悬臂梁和圆环试样,蠕变试验过程中存在大变形效应,可采用临界载荷法预先控制试验载荷范围。固支直杆弯曲试样方法对摩擦的敏感性较低,可获得较为准确的断裂数据。所得结论为选择合适的小试样方法测试材料的蠕变性能提供理论指导和依据。

岩石巴西劈裂强度与裂纹扩展颗粒尺寸效应研究

基于一组经室内巴西试验结果标定的类岩石材料细观参数,通过颗粒流程序(PFC)建立圆盘试样离散元模型,对含不同颗粒粒度中心直切槽圆盘试样进行巴西劈裂模拟,分析颗粒尺寸对荷载-位移曲线、拉伸强度和破裂模式的影响,揭示裂纹扩展过程中细观力场、微裂纹以及能量演化规律。研究结果表明:切槽圆盘试样荷载-位移曲线可分为单峰值(type I)、峰值之后软化(type II)以及峰值之后强化(type III)3种;切槽圆盘试样拉伸强度显著比完整圆盘的低,降幅与切槽倾角和颗粒半径相关;当切槽倾角不变时,拉伸强度总体上随着颗粒半径的增大而增大;而当颗粒半径不变时,拉伸强度随着切槽倾角的增大而减小;当切槽倾角相同时,不同颗粒半径中心直切槽圆盘试样破裂模式显著不同,颗粒尺寸主要影响中心直切槽圆盘试样次生裂纹的萌生和扩展;边界对试样作的功首先用于克服颗粒间黏结以产生裂纹,裂纹在应变能的作用不断扩展,在裂纹产生之后,摩擦能才开始起作用;颗粒的运动程度很低,因此动能很小;边界能与抗拉强度总体上呈正比关系,即边界能越大,拉伸强度越大。