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1.
1 IntroductionThefractureofdenturebasemadefromacrylicresin(polymethylmethacrylatePMMA)isanunresolvedprob lem[1] .Onemethodofreinforcingdenturebasematerialistousefibercompositereinforcement[2 -6] .Intheearly196 0s ,glassfibersweretestedasareinforcementforPM MAdenturebase .Vallittuetal[8] reportedthatthelongfibersimprovedtheproperties .Themostsignificantrein forcingeffectwas + 14 5 % (ontransversestrength) .Inthefollowing 10years ,manyinvestigationswerefocusedoncarbon graphite (CGF PMMA)…  相似文献   

2.
1 INTRODUCTIONChemicalvapordeposition (CVD)diamondholdsgreatpromiseinsolvingthermalmanagementprobleminhigh performancemulti chipmodules (MCMs) .However ,thismaterialdoespresentsometechnologi calchallenges ,onebeingthedevelopmentofreliablemetallizationsystems[1] .Thedesirablecharacteristicsofametallizationsystemare goodadhesiontothesubstrate ,lowstress ,goodelectricalconductivityandminimalreactionsatsubsequentprocessingtempera tures (upto 4 0 0℃ ) [2 ] .Goldisusuallychosenasthemetalfor…  相似文献   

3.
1 IntroductionCarbonnanotubes (CNTs ) ,firstobservedin1991[1] ,haveattractedmoreattentionbecauseofmuchnewexcellentperformance ,andshowapromisingfutureashydrogenstoragemedia[2 ,3 ] .However ,comparedto5wt % - 10wt %hydrogenbytheoreticalcalculation[4] ,thecapacityofexperimentalstorageacceptedtodateisonly4 .2wt% [5] .Thelowstoragecanbeattributedtothehigh lygraphitizedsurfaceofCNTs ,whichhashigherresis tanceeffectsonhydrogenmoleculesandpreventshydrogenfromenteringthenanotubes[6] .Furthermore ,…  相似文献   

4.
1 INTRODUCTIONCobalt basedcatalystsarehighlyattractiveduetotheirhighselectivityforheavyhydrocarbons[1 3 ]andlowCO2 selectivity[4 ] inFischer Tropschsynthesis (FTS ) .Variousstudieshavebeenperformedontheinfluenceofthesupportmaterial,suchassilica[5,6] ,titania[1 ] ,alumina[7,8] .Conventionally ,SiO2 isusuallyusedforthesupportofcobalt basedcatalysts ,andAl2 O3 supportedcobaltcatalystisalsoemployedinoilrefinery .AluminasupportisinfavorofdispersingmetalCo[8] oncobaltaluminacatalysts ,wh…  相似文献   

5.
1 INTRODUCTIONBayerprocessiswidelyusedtotreatthebauxitewithmoleratioofAl2 O3toSiO2 (n (Al) /n (Si) )morethan 8.AlthoughthereareabundantbauxiteresourcesinChina ,mostofthemarediasporicbaux itewithhighaluminumcontent ,butlown(Al) /n(Si) ,usually 5 8[1] .ThistypeofbauxiteisdifficulttobetreateddirectlybytheBayerprocess .Itisthereforehighlydesirabletoincreasen(Al) /n(Si)ofbauxitebyphysicalcleaning[2 ] .Forthisreason ,flotationdesilicate Bayerprocesswasdeveloped[3] .Flotationdesilicationc…  相似文献   

6.
Microstructure of Selective Laser Sintered Polyamide   总被引:5,自引:0,他引:5  
1 IntroductionSelectivelasersintering (SLS)isanadvancedrapidprototypingtechnologythatcanshortenthedesignandmanufacturingcycle ,hencereducetheproductioncostandincreasethecompetitivenessofnewproducts[1] .SLSisathermalprocess ,creatinglayersbyascanninginfraredla serbeammeltingpowder[2 -4] .Eachlayerisdrawnonthepartcylinderusingthelasertosinterthematerial.Thenthepartbedisloweredandpowder feedingcartridgeraised .Anewcoveringofpowderisnextspreadbyaroll er.Thesinteredmaterialformsthepartwhiletheu…  相似文献   

7.
1 INTRODUCTIONZincoxideoresarethemainsourceofzincmetalaf terzincsulfideores .Withtheescalatingdepletionofzincsulfideores,zincoxideoresincludingwillemite(Zn2 SiO4 ) ,hemimorphite[Zn4 (Si2 O7) (OH)·H2 O]andsmithsonite (ZnCO3) ,becameanimportantsourceofzinc[1] .InChina,zincoxideoresarerelativelyabundant,whicharemainlystoredinsouthwestandnorth westChi na,suchasYunnan ,Sichuan ,GuangxiandGansuprovince[2 ] .Manystudieshavebeendoneontheconcentrationofzincoxideores,yettheprogressesaresli…  相似文献   

8.
1 IntroductionInrecentyears ,manyresearcheshavebeenfocusedonnanoparticles[1,2 ] .Thestable ,isolablemonolayer pro tectedclusters (MPCs)representanevenexpandingfieldofresearchduetotheirpotentialuseinapplicationssuchasnano reactors[3 ] ,molecularmachines ,electronicinks ,bio sensing[4] andthinfilms .Schhiffrinandhisco wor kers[5] firstlydevelopedanewmethodcombiningtheclas sicalmethodofFaradaywiththesimultaneousphasetransfer .Particlespreparedinthiswayareextremelysta bleanddonotshowanysignsofd…  相似文献   

9.
1 IntroductionInfrared(IR)diffusereflectionspectroscopyisusedextensivelyintheanalysesandmeasurementsoforganicandinorganicsubstances ,especiallyforpowderma terials[1] .ButtheinfluenceofparticlesizeonIRspectraistypicallyconsideredasanuisancefactorwhichmanyscattercorrectionmethodsattempttoeliminate[2 ,3 ] .Insomereferences[4-6] ,ithasbeenalludedthattheparticlesizesweredeterminedbyIRdiffusereflectionspectra ,ap pliedtothetestingofpharmaceuticalproductsandrawmaterialswhoseparticlesizesarefromsev…  相似文献   

10.
Preparation and microstructure of AI-Ni-Y powder by rapid solidification   总被引:1,自引:0,他引:1  
1 INTRODUCTIONAluminium basedamorphoushasattractedmoreandmoreattentionforitsgoodmechanicalproper ties[13] .Al basedamorphouswereoftenpreparedbymeltspinningonasinglerollerordoublerollers[4 8] .SincetheglassformingabilityofAlalloyisrelativelylow[9] ,theAl basedamorphouscanonlybepreparedunderhighsolidificationvelocity ,whichresultsinnubAl basedamorphousalloyorfilm(preparedbyionbeamsputtering[10 ] )withorderofμm .Itisverydif ficultforustoproducebulkyAl basedamorphousal loy ,whichalsolimits…  相似文献   

11.
Fault rockburst is treated as a strain localization problem under dynamic loading condition considering strain gradient and strain rate. As a kind of dynamic fracture phenomena, rockburst has characteristics of strain localization, which is considered as a one-dimensional shear problem subjected to normal compressive stress and tangential shear stress. The constitutive relation of rock material is bilinear (elastic and strain softening) and sensitive to shear strain rate. The solutions proposed based on gradientdependent plasticity show that intense plastic strain is concentrated in fault band and the thickness of the band depends on the characteristic length of rock material. The post-peak stiffness of the fault band was determined according to the constitutive parameters of rock material and shear strain rate. Fault band undergoing strain softening and elastic rock mass outside the band constitute a system and the instability criterion of the system was proposed based on energy theory. The criterion depends on the constitutive relation of rock material, the structural size and the strain rate. The static result regardless of the strain rate is the special case of the present analytical solution. High strain rate can lead to instability of the system.  相似文献   

12.
The capacity of energy absorption by fault bands after rock burst was calculated quantitatively according to shear stressshear deformation curves considering the interactions and interplaying among microstructures due to the heterogeneity of strain softening rock materials. The post~peak stiffness of rock specimens subjected to direct shear was derived strictly based on gradientdependent plasticity, which can not be obtained from the classical elastoplastic theory. Analytical solutions for the dissipated energy of rock burst were proposed whether the slope of the post-peak shear stress-shear deformation curve is positive or not. The analytical solutions show that shear stress level, confining pressure, shear strength, brittleness, strain rate and heterogeneity of rock materials have important influence on the dissipated energy. The larger value of the dissipated energy means that the capacity of energy dissipation in the form of shear bands is superior and a lower magnitude of rock burst is expected under the condition of the same work done by external shear force. The possibility of rock burst is reduced for a lower softening modulus or a larger thickness of shear bands.  相似文献   

13.
Considering strain localization in the form of a narrow band initiated just at peak stress, three analytical expressions for stress - strain curves of quasibrittle geomaterial (such as rock and concrete) in uniaxial tension, direct shear and uniaxial compression were presented, respectively. The three derived stress - strain curves were generalized as a unified formula. Beyond the onset of strain localization, a linear strain-softening constitutive relation for localized band was assigned. The size of the band was controlled by internal or characteristic length according to gradient-dependent plasticity. Elastic strain within the entire specimen was assumed to be uniform and decreased with the increase of plastic strain in localized band. Total strain of the specimen was decomposed into elastic and plastic parts. Plastic strain of the specimen was the average value of plastic strains in localized band over the entire specimen. For different heights, the predicted softening branches of the relative stress- strain curves in uniaxial compression are consistent with the previously experimental results for normal concrete specimens. The present expressions for the post-peak stress - deformation curves in uniaxial tension and direct shear agree with the previously numerical results based on gradient-dependent plasticity.  相似文献   

14.
A mechanical model for strain softening pillar is proposed considering the characteristics of progressive shear failure and strain localization. The pillar undergoes elastic, strain softening and slabbing stages. In the elastic stage, vertical compressive stress and deformation at upper end of pillar are uniform, while in the strain softening stage there appears nonuniform due to occurrence of shear bands, leading to the decrease of load-carrying capacity.In addition, the size of failure zone increases in the strain softening stage and reaches its maximum value when slabbing begins. In the latter two stages, the size of elastic core always decreases. In the slabbing stage, the size of failure zone remains a constant and the pillar becomes thinner. Total deformation of the pillar is derived by linearly elastic Hooke‘s law and gradient-dependent plasticity where thickness of localization band is determined according to the characteristic length. Post-peak stiffness is proposed according to analytical solution of averaged compressive stressaverage deformation curve. Instability criterion of the pillar and roof strata system is proposed analytically using instability condition given by Salamon. It is found that the constitutive parameters of material of pillar, the geometrical size of pillar and the number of shear bands influence the stability of the system; stress gradient controls the starting time of slabbing, however it has no influence on the post-peak stiffness of the pillar.  相似文献   

15.
The volumetric strain was categorized into elastic and plastic parts. The farmer camposed of axial and lateral strains is uniform and determined by Hooke's law ; however, the latter consisting of axial and lateral strains is a fuaction af thickness af shear band determined by grndieat-dependeat plasticity by cansidering the heterngeneity of quasi- brittle materials. The non- uniform lateral strain due to the fact that shear band was farmed in the middle of specimen was averaged within specimen to precisely assess the volumetric strain. Then, the analytical expression for volumetric strain was verified by comparison with two earlier experimental results for concrete and rack. Finally, a detailed parametric study was carried out to investigate effects of constitutive parameters ( shear band thickness, elastic and softening rnoduli ) and geometrical size of specimen( height and width of specimen ) on the volume dilatancy.  相似文献   

16.
土的非线性应力-应变关系及一般数学模型   总被引:3,自引:0,他引:3  
将土的剪切试验测得的应力—应变关系分成逐渐硬化类型和峰值后软化类型,提出了可以统一表示这两种类型应力—应变关系的表达式.以三轴剪切试验测得的应力—应变关系为基础,从统一的表达式推出了割线模量和切线模量表达式.按逐渐硬化类型和峰值后软化类型两种情况分别给出了确定参数的方法.该表达式为发展一个更一般的非线性力学模型提供了基础。  相似文献   

17.
为解决混凝土多孔砖砌体墙的裂缝问题,提出了一种基于竖向灰缝模型的求解其等效断裂韧度(线弹性断裂力学中裂缝发生失稳扩展的临界值)的实用解析方法.首先,运用修正的剪滞理论,分区引入变异层,建立了分层剪滞模型;然后,根据能量法则,推导出了求解其等效断裂韧度的解析计算模式.通过与3组11个试件的试验结果对比发现,该解析解的均方差和变异系数更小,且其等效断裂韧度是与试件尺寸无关的等效断裂参数,这为研究混凝土多孔砖砌体墙的开裂与破坏的预测预报提供了新的方法,在工程上具有较强的适用性.  相似文献   

18.
为揭示深厚表土冻融土-混凝土结构接触面剪切力学特性,利用改进的DRS-1高压直残剪试验系统,开展系列高法向应力、升温条件下冻(融)土-结构接触面直剪试验,总结高应力作用下不同融化程度冻(融)土-结构接触面剪切应力-应变关系曲线的基本特征.通过试验分别建立峰值前和考虑应变软化特征的高应力、升温条件下冻(融)土-结构接触面剪切应力-应变经验方程,探讨接触面剪切应力极值和初始剪切模量随法向应力水平、融化程度等的变化规律.结果表明:随融化程度的加深,冻(融)土-结构接触面剪切应力-应变关系曲线形态由具有应变软化特征逐渐转变为具有应变硬化特征;当接触面剪切应变较小时(峰值前),标准的双曲线模型对高应力、升温条件下冻(融)土-结构接触面剪切应力-应变关系仍具有良好的适用性;而具有应变软化特征的全过程剪切应力-应变关系可采用改进的双曲线模型描述.  相似文献   

19.
剪胀对岩样全部变形特征的影响   总被引:11,自引:0,他引:11  
采用FLAC内嵌语言FISH编制了计算平面应变压缩岩样轴向、侧向、体积应变及泊松比的FISH函数,研究了剪切扩容对剪切带图案及岩样全部变形特征的影响。在峰值强度之前及之后,岩石的本构模型分别取为线弹性及莫尔库仑剪破坏与拉破坏复合的应变软化模型。分析表明,增加剪胀角使岩样由单一向共轭剪切破坏转变,并使接近Arthur倾角的剪切带倾角增加。剪切带宽度随剪胀角增加,可由基于梯度塑性理论且考虑剪胀后的剪切带宽度公式进行解释。剪胀角增加导致峰值强度及对应的轴向、侧向及体积应变增加。在峰后,由于剪胀引起剪切带条数及宽度增加,因而,轴向应力-轴向及侧向应变曲线软化段都变平缓。剪胀角较高时,岩样可获得更大的侧向变形量及泊松比,甚至是负的体积应变;岩样失稳破坏的前兆更加明显。  相似文献   

20.
The distributed shear stress and the displacement across shear band, the evolution of plastic zones, and the load-carrying capacity of rock specimen were investigated in plane strain direct shear test according to Fast Lagrangian Analysis of Continua (FLAC). And then the shear displacement distribution in normal direction of system composed of localized shear band and elastic rock was analyzed based on gradient-dependent plasticity. The adopted failure criterion was a composite of Mohr-Coulomb criterion, that is, the relation between tension cut-off and postpeak constitutive of rock was linear strain-softening. Numerical results show that shear stress field approximately undergoes three different stages. At first, shear stress is only concentrated in the middle of top and base of specimen. Next, shear stress in the middle of specimen tends to increase, owing to superposition of shear stresses. Interestingly, two peaks of shear stress appear far from the loading ends of specimen, and the peaks approach with the increase in timestep until elements at the center of specimen yield. Finally, relatively lower shear stress level is reached in large part of specimen except in the regions near the two ends. As flow stress decreases, the analytical shear displacement distribution in shear band based on gradient-dependent plasticity becomes steeps outside the band, it is linear and its slope tends to decrease. These theoretical results qualitatively agree with that of the present numerical predicted results. Main advantage of the analytical solution over the numerical results according to FLAC is that it is continuous, smooth and non-linear (except at elastic stage).  相似文献   

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