Surface Cracking in Layers Under Biaxial, Residual Compressive Stress

Thin two-phase, Al₂O₃/ t -Zr(3Y)O₂ layers bounded by much thicker Zr(3Y)O₂ layers were fabricated by co-sintering powders. After cooling, cracks were observed along the center of the two-phase, Al₂O₃/ t -Zr(3Y)O₂ layers. Although the Al₂O₃/ t -Zr(3Y)O₂ layers are under residual, biaxial compression far from the surface, tensile stresses, normal to the center line, exist at and near the surface. These highly localized tensile stresses can cause cracks to extend parallel to the layer, to a depth proportional to the layer thickness. A tunneling/edge cracking energy release rate function is developed for these cracks. It shows that for a given residual stress, crack extension will take place only when the layer thickness is greater than a critical value. A value of the critical thickness is computed and compared with an available experimental datum point. In addition, the behavior of the energy release rate function due to elastic mismatch is calculated via the finite element method (FEM). It is also shown how this solution for crack extension can be applied to explain cracking associated with other phenomena, e.g., joining, reaction couples, etc.     

Mechanical Behavior of a Laminar Ceramic/Fiber-Reinforced Epoxy Composite

The mechanical properties of a noval laminar composite are investigated. The composite consists of dense alumina sheets bonded between sheets of a uniaxial carbon-fiber-reinforced epoxy tape. The behavior of the composite in both flexural and tensile loading is characterized and the results are related to the properties of the constituents. The role of the interlaminar interface in composite behavior is also examined. Implications for the design of laminar composites with complex shapes are briefly discussed.     

机械性面裂的分析

在建陶生产中,坯体的机械性面裂极易被忽略,笔者根据生产实践经验,对该缺陷进行了全面分析,并提出了解决办法。     

Application of a Tough Surface Layer to a Fiber-Bonded Composite

The feasibility of creating "tough surface material" using oxide-fiber-reinforced oxide matrix ceramics was studied. Al₂O₃ fiber/(ZrO₂, Al₂O₃) matrix composite was used as the surface material of a Si–Ti–C–O-fiber-bonded composite. The sintering of the matrix (ZrO₂ and Al₂O₃) of the surface composite layer (SCL) and its bonding to the fiber-bonded composite (FBC) were done simultaneously by vacuum hot pressing. A spherical indentation test demonstrated the advantage of the SCL in reducing the damage of the base FBC from an indenter, because the high fracture resistance of the surface composite layer could reduce the stress concentration by the cumulative microfracture process.     

Toughening of a Cordierite Glass–Ceramic by Compressive Surface Layers

An indentation fracture mechanics analysis is developed to characterize the toughening effects of a compressive surface layer in brittle materials. The analysis is used to describe the enhanced toughness of cordierite glass–ceramic laminate composites, in which thermal-expansion mismatch effects induced uniform stress in the exterior layers of the symmetric exterior:interior:exterior structures. Interpretation of indentation crack length and inert strength tests via the analysis shows that cracks can be viewed as experiencing discrete regions of decreasing stabilization on propagation from small cracks and complete containment within the compressive layer to large cracks and partial extension into the compensating tensile interior. The observations are described using a stress-intensity factor for circular cracks in linear stress fields that includes different base and surface values for extended cracks. Deconvolution of inert strength data for the model cordierite system studied suggested an increase in toughness from 1.4 MPa·m^1/2 for the base material to a peak of about 5 MPa·m^1/2 for a 1:18:1 composite structure, with attendant increases in strength and flaw tolerance.     

Crack Bifurcation in Laminar Ceramic Composites

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al₂O₃ layers sandwiched between thicker layers of Zr(12Ce)O₂. The Al₂O₃ layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O₂ layers were nearly free of residual, tensile stresses because they were much thicker than the Al₂O₃ layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al₂O₃ layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al₂O₃ layer, it bifurcated as it approached the Zr(12Ce)O₂/ Al₂O₃ interface. After the crack bifurcated, it continued to propagate close to the center line of the Al₂O₃ layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O₂ layer, where it bifurcated again as it entered the next Al₂O₃ layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al₂O₃ layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.     

Change in Ceramic Object Surface Layer Structure During Operation. Part 1

Refractories and Industrial Ceramics - An original method is used to study the surface layer structure of a VOK-71 ceramic object after diamond grinding. Kinetics of changes in ceramic structure in...     

铺层结构对三维中空夹层复合材料压缩性能的影响

采用手糊成型制备了双夹层三维中空夹层复合材料,重点研究了厚度为5 mm和8 mm的三维中空织物ZK5和ZK8铺层顺序对复合材料压缩强度的影响,并从树脂含量、织物厚度匹配性角度进行分析。结果表明,随树脂含量增加,三维中空夹层复合材料的压缩强度明显提高;双夹层三维中空夹层复合材料中,下层的树脂含量明显高于上层;ZK5和ZK8织物铺层顺序对双夹层中空复合材料的压缩强度影响较大,其中将ZK8置于ZK5下层时复合材料的压缩强度为ZK8置于ZK5上层时的1.7倍,即在三维中空夹层复合材料总厚度不变的情况下,将高厚度中空织物置于下层的结构明显优于将其置于上层的结构。通过这一概念可以在保证三维中空夹层复合材料整体厚度尺寸不变、质量不增加的条件下通过铺层结构的匹配设计,最大限度地提高复合材料的力学性能。     

内衬SHS陶瓷复合油管抗压性能有限元分析与试验研究

内衬SHS陶瓷使油管的抗防腐性能显著提高,但考虑到衬里管的性能与普通油管存在差异,本文选用抗压强度作为衡量管体性能的指标展开研究。采取有限元分析与试验研究相结合的方法对内衬SHS陶瓷复合油管的压溃过程进行分析与研究。掌握了油管压溃过程的破坏规律,得到陶瓷内衬管承载能力明显要高于普通油管,表现出较好的抗压性能。     

Factors Affecting Threshold Strength in Laminar Ceramics Containing Thin Compressive Layers

Compressive layers placed within a laminate can arrest cracks. With increasing applied stress, the arrested crack can propagate through the compressive layer. These phenomena produce a material with threshold strength, i.e., failure cannot occur below a critical applied stress. A previously reported stress intensity function describes different variables, e.g., magnitude of compressive stress, thickness of compressive layer, and distance between compressive layers, which govern threshold strength. Laminar composites composed of thicker Al₂O₃ layers separated by thinner Al₂O₃/mullite layers were fabricated to test the different variables that are predicted to govern threshold strength. The data agreed well with the predicted values only when the magnitude of compressive stress and/or the thickness of the compressive layer were low. For these conditions, the crack extended straight through the compressive layers, as assumed by the model used to predict threshold strength. On the other hand, when the compressive stress and/or layer thickness were large, threshold strength was larger than the predicted value. In addition, for these conditions, the crack bifurcated through the compressive layer. The angle between the bifurcated cracks increased with increasing compressive stress.     

Surface Modification of Graphite Particles Coated by Atomic Layer Deposition and Advances in Ceramic Composites

Graphite particles have been coated with Al₂O₃ via atomic layer deposition. Alumina content was measured via inductively coupled plasma spectrometry (ICP), LECO combustion analysis, and thermogravimetric analysis (TGA). While alumina was present, adherence was limited, and nonconformal films were deposited on the graphite particles. Coatings produced changes in particle interactions and dispersability. These changes were observed via sedimentation rates of particle suspensions in water, Zeta potential values, and particle size distributions. Alumina‐Graphite composites were sintered using coated and uncoated particles. Differences in bulk thermal properties are ascribed to enhanced dispersability of the coated particles in presintered powder mixtures. EDS mapping of the sintered composites confirms the enhanced dispersion of the coated graphite particles. Particle coating through atomic layer deposition provides a means to improve particle dispersion with low material loadings. It has been shown that changes in particle interaction characteristics can be achieved even without uniform and conformal coatings. These particle interaction changes can result in sintered composites with enhanced thermal properties.     

陶瓷材料加工表面完整性及其对材料可靠性的影响

采用磨削-抛光加工、金刚石锯片切割加工、超声波加工和电火花线切割加工技术对Al2O3/TiC陶瓷材料进行加工.研究了各加工方法对陶瓷材料的加工表面完整性及其可靠性的影响.结果表明:加工方法对陶瓷材料加工表面完整性和可靠性有很大的影响.磨削-抛光加工的陶瓷试样表面粗糙度小,硬度高,抗弯强度及其可靠性最高.金刚石锯片切割加工和超声波加工次之;电火花加工陶瓷试样表面粗糙大,硬度低,抗弯强度及其可靠性最低.电火花加工陶瓷表面产生一硬度低且表面粗糙的约8μm厚的热影响层,加工表面含有大量的电火花腐蚀凹坑,随电流的增大,电火花加工陶瓷试样的抗弯强度及其可靠性降低.超声波加工陶瓷试样的表面完整性与磨料粒度有关,随磨料粒度的减小,陶瓷试样的抗弯强度及其可靠性增加.     

深度改革创新规避瓷砖行业同质化现象

当前,我国瓷砖的发展已经进入了饱和期,市场上瓷砖品种繁多,鱼龙混杂,同质化现象日益严重。消费者在选购瓷砖产品时更注重新颖独特的设计,因此,瓷砖企业要加强自身创新,形成自己的独特优势。即为了规避同质化现象带来的发展阻碍,瓷砖企业唯有在产品、品牌、渠道等多方面同时着手进行改革创新,才能摆脱同质化的红海;另外,在同质化局势下,瓷砖企业必须进行营销渠道创新,拓宽推广销售的领域,才能完成营销渠道的升级,增加企业产品的销售额。     

Mechanisms Responsible for Strain-Rate-Dependent Compressive Strength in Ceramic Materials

Compressive strength of SiC and Al₂O₃ is determined over a wide range in loading rate. The results are interpreted in terms of the tensile growth of axial microcracks, controlled by two distinct mechanisms. The first is a material-dependent, thermally activated process operative at strain rates 10²s; the other is a relatively material-independent, strain-rate-sensitive inertial process which controls failure at higher loading rates.     

Determining Residual Stresses in Thin-Walled Ceramic/Ceramic Composite Pipes

Fiber-reinforced ceramic materials exhibit excellent strength, toughness, and resistance to high temperatures and thermal shocks. To determine the service stress limit of a material, it is very important to determine residual stresses that are developed during the manufacturing processes. This work uses the laser speckle technique as a sensitive, noncontact, quantitative tool to determine residual stresses in thin-walled pipes made of ceramic/ceramic composite. The laser measurement is combined with an elastic solution to obtain both the radial and the tangential stress distributions along the thickness of the pipe. The proposed approach, suitable only for thin-walled cylinders, possesses different advantages with respect to simplicity and sensitivity. The results obtained show that this approach is more reliable and accurate than the strain gauge method.     

Delamination Cracking in a Laminated Ceramic-Matrix Composite

Delamination crack propagation has been investigated in a laminated fiber-reinforced ceramic-matrix composite. The crack growth initiation resistance has been shown to be dominated by the critical strain energy release rate for the matrix. However, the resistance increases with crack extension because of bridging effects associated with intact fibers and, in some cases, intact segments of matrix. The delamination cracks also assume a steady-state trajectory within a 0° layer close to the 0°/90° interface.