用声发射预测静疲劳下陶瓷材料的断裂

运用声发射检测技术，检测出受静载荷作用的陶瓷材料内部形成微裂纹和亚临界裂纹扩展的声发射信号，研究了裂纹形成、扩展时所激发的声发射计数率ｄＮ／ｄｔ与静载荷作用时间ｔ的关系。实验发现试样在断裂前２２～３０ｓ所检测到的亚临界裂纹扩展的声发射信号具有明显不同的变化规律，试样所受的静载荷越小，初次检测到亚临界裂纹扩展的声发射信号离发生断裂的时间越长。     

高强空心粒子增韧增强陶瓷材料

提出采用高模量高强度空心粒子增韧增强陶瓷材料的陶瓷基体裂纹尖端钝化增韧的新机制。力学分析表明,采用高模量空心粒子能够同时增韧增强陶瓷材料,裂纹尖端应力的释放提高了材料的断裂强度;陶瓷材料中裂纹在空心粒子的空心处发生钝化继而再次形核扩展,从而提高了材料的韧性,上述观点在热压ＴｉＣ／ＴｉＢ２／ＭｏＳｉ２复合材料中得到初步证明。     

热应力下陶瓷材料中裂纹扩展的声发射研究

用声发射技术监测了刚玉－莫来石质陶瓷材料在缓慢加热、冷却和骤冷时微裂纹的形成、扩展过程，发现其在冷却过程中声发射计数率的峰值约为加热过程的４００倍。材料在热应力下微裂纹的形成、生长主要发生在冷却过程中。当晶粒尺寸减小时，材料的微裂纹扩展、蔓延逐渐被抑制在较小的范围内。骤冷时，由热应力引起的微裂纹稳态扩展和失稳扩展的声发射特性与材料残余强度的变化趋势是一致的。     

陶瓷材料预压应力加工的力学模型

基于弹性力学和压痕断裂力学,建立了陶瓷材料预压应力下的加工力学模型,分析了预压应力及载荷比对材料内部第一主应力及最大剪应力的影响.随着预压应力的增大,第一主应力由拉应力逐步转变为压应力,最大剪应力先减小而后随之增大.若施加合适的预压应力,则能降低材料内部的最大剪应力,改变材料内部裂纹的扩展方向,从而有效降低陶瓷加工过程中的损伤.随着载荷比增大,第一主应力在压头后方迅速变为拉应力,最大剪应力也随之增大,但压头正下方的应力状态不受载荷比的影响.最后对碳化硅陶瓷进行预压应力划痕实验,验证了上述分析结果,进一步表明陶瓷材料预压应力加工是可行的.     

一种新型可加工陶瓷材料: CeZrO2/CePO4

对可加工Ce-ZrO2／CePO4陶瓷材料的设计制备和性能进行了研究。结果表明：在载荷作用下弱界面处易形成微裂纹，并发生裂纹联接，使材料在加工过程中以晶粒去除形式为主，实现了该材料的金属刀具加工，且加工损伤变小。弱界面处微裂纹的形成、偏转和桥联等形式，耗散了主裂纹扩展的能量，增加了材料的断裂功，可在一定程度上改善材料的强度等力学性能。随着CePO4加入量的增加，弱界面增多，Ce-ZrO2／CePO4陶瓷材料的可加工性变好但力学性能却变差。烧成温度等工艺因素对Ce-ZrO2／25％CePO4(质量分数)陶瓷的强度等力学性能影响较大，在1550℃下加热2h，颗粒尺寸与发生裂纹桥联等形式相适应，可充分发挥弱界面的作用。     

复合材料加筋壁板装配应力对结构失效影响的试验与数值分析

复合材料层压板由于各向异性及沿厚度方向的不连续性的特点,在承受面外载荷作用下,会产生层间应力。层间应力值超过层压板层间开裂强度时,层压板会发生层间分层现象。为了研究装配应力对加筋壁板破坏行为的影响,本文进行了相应的试验和数值分析。首先对加筋壁板结构进行静力加载试验,得到了试验件的破坏载荷和破坏模式。其次通过螺栓加载至工况载荷并保载一定时间,实现了保载试验。最后利用有限元分析了结构的应力分布规律。结果显示,试验件在静力载荷的作用下会发生共胶接区脱粘破坏并伴随缘条分层破坏,在保载作用下长桁下缘条产生了微分层现象,数值分析获取的应力分布规律与试验结果吻合良好。研究结果可为复合材料壁板的装配作业规范和铺层方案设计验证提供参考。     

变幅载荷疲劳裂纹扩展行为的试验研究

用某压力容器实际工况载荷波动所编制的载荷谱,对材料为16MnR的CCT试件进行了变幅载荷谱下的疲劳裂纹扩展试验。根据试验数据,分析了裁荷谱中大载荷级和小载荷级分别对裂纹扩展量的贡献,并对考虑和不考虑超载迟滞效应对小载荷裂纹扩展行为的影响进行了对比。作为这方面的初步探索,本研究获得了一些有意义的结论。     

变幅载荷谱中加载顺序对疲劳裂纹扩展速率的影响

根据某高温高压换热器实际工况载荷波动所编制的载荷谱,用材料为2 1/4Cr-1Mo钢的CT试件,不同的载荷谱加载顺序,进行变幅载荷谱下的疲劳裂纹扩展速率试验研究。根据试验数据和超载迟滞效应理论,讨论了不同载荷谱加载顺序对疲劳裂纹扩展速率的影响。     

高温热处理对石英陶瓷材料的影响研究

本文考察了不同温度和不同次数的高温热处理对石英陶瓷材料强度、晶相和微观形貌的影响。在对石英陶瓷材料单次热处理时,650~850℃下材料强度高于62MPa,强度变化不明显;在950℃热处理后,材料强度降为57.9MPa,较未热处理样品降低8.8%。在850℃、保温4h条件下,材料经过4次反复热处理后,强度降低为55.1MPa,较未处理试样降低13.2%,1000℃下抗热震处理后强度降低至51.7MPa。XRD和SEM对材料的晶相和微观形貌变化的测试结果表明,热处理温度过高和热处理次数越多,材料出现的析晶越多,造成石英颗粒出现微裂纹缺陷也越多,导致材料的强度降低。     

高强混凝土裂纹扩展规律的CT观察

随着我国煤矿开采深度的不断加大,对深厚表土层立井井壁所使用的高性能混凝土的强度、耐久性等性质的要求也越来越高.基于工业CT测试技术,对深厚表土层立井使用的C100高性能混凝土试样进行了细观力学试验,分析了在单轴压缩条件下其内部微裂纹的孕育、萌生、发展及其贯通过程.并利用盒维数法,计算了其内部微裂纹演化的分形维数,在此基础上,获得了载荷、裂纹扩展及分形维数三者之间的对应关系.结果表明:C100高性能混凝土胶凝材料具有较高的强度;随着加载应力的提高,混凝土的破碎程度加深,裂纹的弯折程度加大,导致裂纹演化的分形维数也增大.     

The Mechanics of Indentation Induced Lateral Cracking

The mechanics governing the lateral cracks that form when a hard object plastically penetrates a ceramic is presented. The roles of indentation load, penetration depth, and work of indentation are all highlighted, as are the influences of the mechanical properties of the material. A closed form solution for cracking induced by expansion of a two-dimensional cavity is used to bring out essential features related to parametric dependence and scaling. The three-dimensional axisymmetric problem for an annular crack driven by a rigid spherical or conical indenter is solved using numerical methods. The region of highest tensile stress is identified corresponding to the location where a crack is most likely to nucleate. This location coincides with the depth below the surface where the crack will expand parallel to the surface under mode I conditions. The solutions have been substantiated by comparison with measurements of the cracks that form upon Vickers indentation. The basic formula for the crack radius has been used to predict trends in cracking upon static penetration and impact by a projectile. In both cases, the extent of the cracking is substantially diminished by increasing the toughness of the material. The yield strength has a much smaller effect. The cracks caused by penetration and the volume removed per impact both decrease marginally at higher yield strength.     

Mechanical behavior of ceramic-metal joint under quasi-static and dynamic four point bending: Microstructures,damage and mechanisms

This paper studies the mechanical behavior of Alumina ceramic-Kovar joint under quasi-static and dynamic four-point bending (FPB). The joint is fabricated by molybdenum-manganese (Mo-Mn) metallization method with extra additions. The bend strength of the joint is improved by glass phase migration. Electronic universal testing machine and modified split Hopkinson pressure bar (SHPB) are employed to realize the loading process. The microstructure of the joint is investigated by scanning electron microscope (SEM) and the chemical composition is determined by energy dispersive spectrometer (EDS). Digital image correlation (DIC) technique is used to determine the displacement contours and ultra-high speed camera is used to monitor the deformation and crack evolution around the joint. It is found that the specimen will slip due to the different Young's modulus of base material. The dynamic bend strength of the joint is lower than the quasi-static bend strength. The failure mechanism of the ceramic-metal joint is mainly intergranular failure for the dynamic bend but mixed transgranular/intergranular failure for the quasi-static case. The crack starts from the inherent voids inside the ceramic and then expands along the metallization band between solder and ceramic.     

Crack healing of ferrosilicochromium-filled polymer-derived ceramic composites

In this study, SiOC/FeSiCr/SiC-ceramics were derived by using polymethylsilsesquioxane as a polymeric precursor and FeSiCr (iron-chromium-silicide) and SiC powders as ceramic fillers in a polymer-extrusion process followed by pyrolytic conversion in a nitrogen atmosphere. The crack healing properties of the ceramic in an oxidising atmosphere were subsequently investigated. The SiOC/FeSiCr/SiC-ceramics showed crack closure and strength recovery behaviour after oxidation treatments in air from 600 to 1300?°C with various holding times from 2 to 1000?min. The crack healing mechanisms at different oxidation temperatures and with various dwell times are discussed.     

Mechanics of penetration of ceramic targets

A model for the penetration of a ceramic target with the formation of an expanding conical hole is proposed that can be used to determine the apex angle of the cone knocked out from the target. The main cause of this fracture of ceramic targets is that the tensile strength of a ceramic material is much lower than its compressive strength. A method for calculating the maximum penetration velocity in a ceramic target without substrate was developed using the energy fracture criterion for ceramics. Two-dimensional numerical simulation of the impact of a steel projectile on a corundum plate showed satisfactory agreement between the calculated and experimental configurations of the resulting conical crack provided that the ratio between the compressive and tensile strengths corresponds to the recommendations of the developed model.     

Film‐forming behavior and mechanical properties of colloidal silica/polymer latex blends with high silica load

In this article, silica sol (diameter: 8–100 nm) and polymer latex (T_g < 25°C) were mixed and dried at room temperature to prepare nanocomposite films with high silica load (≥50 wt %). Effects of silica size, silica load, and the T_g of the polymer on the film‐forming behavior of the silica/polymer latex blend were investigated. The transparency, morphology, and mechanical properties of the nanocomposite films were examined by UV–Vis spectroscopy, SEM, and nanoindentation tests, respectively. Transparent and crack‐free films were produced with silica loads as high as 70 wt %. Thirty nanometers was found to be the critical silica size for the evolution of film‐forming behavior, surface morphology, and mechanical properties. Colloidal silica particles smaller than this critical size act as binders to form strong silica skeleton. This gives the final silica/polymer nanocomposite film its porous surface and high mechanical strength. However, silica particles with sizes of 30 nm or larger tend to work as nanofillers rather than binders, causing poor mechanical strength. We also determined the critical silica load appeared for the mechanical strength of silica/polymer film at high silica load. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Fabrication and properties of monolayer graphene reinforced Al2O3/TiN ceramic tool material

Al₂O₃/TiN/graphene ceramic tool materials were prepared by spark plasma sintering technology and the strengthening and toughening mechanisms were studied. The influence of monolayer graphene content on the mechanical properties and microstructure of the composite material were analyzed and the strengthening and toughening mechanisms were researched. The results showed that with an addition of .5 vol.% graphene the mechanical properties of the material reached the best. The bending strength, hardness, and fracture toughness were 624 MPa, 23.24 GPa, and 6.53 MPa·m^1/2, respectively. Graphene existed in the forms of few-layer and multilayer. The toughening mechanism of few-layer graphene was mainly graphene breaking, and that of multilayer graphene included graphene breaking and pulling-out. Graphene could contribute to the uniform growth of grains due to the excellent electrical conductivity and the high thermal conductivity. The addition of nano-TiN introduced many endocrystalline structures and graphene promoted this phenomenon. Micro-TiN grains made the crack extension show a combination of transgranular fracture, intergranular fracture, crack bridging, and crack deflection, while graphene introduced weak grain interfaces and made the crack appear more branches. The layered graphene made the material fracture change from two-dimension to three-dimension.     

Fracture mechanics properties of fused silicas used for international space station windowpanes

The fracture toughness and slow crack growth (SCG) parameters of a quartz-based silica and a high-purity fused silica were measured as part of a program to review the reliability of the International Space Station windows. The materials exhibit the same fracture toughness (.75 MPa m^1/2 in N₂) and very similar SCG parameters. The literature on fused silica indicates excellent agreement of fracture toughness, but a very wide range of SCG parameters, even from the same institution, with strength-based methods usually yielding a lower power law exponent than direct crack velocity measurements. Use of the exponential function is shown to provide better agreement between test methods, with velocity curves derived from strength tests of bare fiber and polished or ground test specimens paralleling those from wide-range, direct crack velocity observations, implying that constant stress rate tests can predict long lifetime via the exponential function. However, much variation still exists. SCG parameters for soda–lime silicate are much less sensitive to the test method than fused silica. Static load tests and stress intensity measurements resulted in a fatigue threshold of .3 MPa m^1/2 for fused silica.     

Some strength characteristics of ceramics made from vitreous silica

Conclusions The form of the original material and the production parameters during its treatment have an important effect on the properties of quartz ceramic.The strength of the ceramic increases with an increase in the fineness to which the original material is milled and with a decrease in the moisture content of the suspension.Ceramic made from vitreous silica shows no significant shrinkage or increase in strength after drying and firing at up to 800°C. The maximum strength (on the whole) and best thermal-shock resistance are characteristic of specimens fired at 1200°C while maximum shrinkage and minimum water absorption is found in specimens fired at 1300–1350°C. This difference in temperatures is probably because under the chosen experimental firing regimes, the beginning of cristobalite-formation slightly outstrips the sintering of the material.A real possibility of using cullet from previously fired articles made of vitreous silica, particularly of synthetic silica, has been established.Translated from Ogneupory, No. 8, pp. 55–60, August, 1980.     

凝胶法制备熔石英纳米复合陶瓷工艺及性能的研究

对凝胶法制备熔石英纳米复合陶瓷的工艺进行了探讨,并对不同工艺参数下的材料性能进行了分析和对比,讨论了影响材料性能的主要因素,寻求其最佳工艺参数.研究结果表明,用凝胶法可以使纳米相粒子均匀分散于陶瓷基体中,并能方便快速制备熔石英纳米复合陶瓷,纳米相引入后明显改善了材料的烧结性能,增加了材料的密度和强度.     

Preparation and properties of ceramics from a zirconia nanopowder

The possibilities of preparing ceramics from a zirconia nanopowder have been investigated. A ceramic material with a fine-grained (of the order of 2–3 μm) structure is synthesized. A ceramic material with zircon predominant is produced using a silica nanopowder (tarkosil). The properties of the initial nanopowder and the ceramic material synthesized are investigated using transmission electron and scanning electron microscopies. The microhardness and strength of the ceramics are determined.