Effect of Alumina on the Strength, Fracture Toughness, and Crystal Structure of Fluorcanasite Glass-Ceramics

The influence of alumina content (0-15 wt% Al₂O₃) on the indentation strength, fracture toughness ( K _{I c} ), and crystal structure of fluorcanasite (Al₂O₃-CaO-F-K₂O-Na₂O-SiO₂) glass-ceramics was analyzed. Increasing the Al₂O₃ content from 0 wt% (CAN0) to 8 wt% (CAN8) caused the mean indentation strength and K _{I c} values to decrease from 213 ± 14 MPa and 2.7 ± 0.1 MPa·m^1/2, respectively, for the CAN0 glass-ceramic to 78 ± 16 MPa and 1.3 ± 0.2 MPa·m^1/2, respectively, for the CAN8 glass-ceramic. Increased Al₂O₃ concentrations (0-15 wt%) significantly affected the crystal size, crystal shape, aspect ratio, and crystal aggregation characteristics of the fluorcanasite glass-ceramics. The addition of greaterthan equal to8 wt% of Al₂O₃ to fluorcanasite glass caused a transformation from canasite to leucite.     

Chevron-Notched Toughness of Materials with Rising Fracture Resistance Curves

The effect of rising resistance curves on the fracture tougness determined from chevron-notched beam tests is estimated via a set of simple numerical calculations. It is shown that for materials with a rising resistance curve, the toughnesses determined by the peak load method and by the work of fracture method are both higher than the initiation toughness, and both depend on the sample size relative to the length over which the resistance curve increases. It is also found that the toughness based on the work of fracture is higher than that based on the peak load. Fracture toughness data obtained from tests of a nickel–alumina composite are discussed in light of the results of the numerical calculation.     

Fracture Toughness Enhancement for Alumina Systems: A Review

Investigations have been carried out to determine ways of tailoring ceramic materials in order that one or more toughening mechanisms are activated in service. Microstructural manipulations, as well as composite formulations involving metallic, intermetallics, and ceramic phases have been used with ceramic matrices. Macrostructurally, laminated structures and functional gradient materials (FGMs) have also been formulated to enhance mechanical properties. Although significant improvements in material properties have been reported, ceramics are still below their projected positions on the materials map. This article presents a review of research activities pursuant to improving fracture toughness of alumina matrix systems and the enhancements achieved.     

Effect of Contact Residual Stress Relaxation on Fracture Strength of Indented Soda-Lime Glass

The strength of Vickers-indented soda-lime glass measured in air at room temperature steadily increases with time after indentation, whereas optical retardation steadily decreases in the same interval. Annealing after indentation causes further strength increase and retardation decrease. The results are consistent with Marshall and Lawn's treatment of the slow crack growth of indentation flaws driven by the combined influence of residual contact stress and applied stress. Post-indentation strengthening of indentation flaws can be explained without recourse to flaw blunting.     

Fracture Toughness Anisotropy of Sodium β- Alumina

The fracture toughness anisotropy was determined for sodium p-alumina single crystals, using a hardness indent method. For cracks with a habit plane normal to the 00.1 planes, the fracture toughness is ∼2 MPa.m¹¹², whereas for cracks running parallel to the 00.1 planes, it is ∼0.16 MPa.m¹¹². This extreme anisotropy may partly explain the difference between calculated and observed critical current densities jor mode I failure initiation of poly crystalline solid electrolytes .     

Effect of Notch-Root Radius on the Fracture Toughness of a Fine-Grained Alumina

The dependence of K _Ic on the notch-root radius has been examined for a notch radius as small as a few micrometers in a dense, fine-grained, polycrystalline alumina ceramic. The notch radius can be systematically varied by using a semimanual procedure in a special jig which polishes out rather than cuts the specimen. K _Ic is independent of the notch sharpness for notch-root radii < 10 μm. The results are critically compared with those obtained by other standard techniques and discussed in terms of residual compressive stresses introduced during the notching procedure.     

Flexure Strength, Fracture Toughness, and Slow Crack Growth of YSZ/Alumina Composites at High Temperatures

Flexure strength and fracture toughness of zirconia–alumina composites, fabricated by hot pressing 10 mol% yttria-stabilized zirconia (10-YSZ) reinforced with 0–30 mol% alumina particulates or platelets, were determined as a function of alumina content at 1000°C in air. Both strength and fracture toughness of the two composite systems increased with increasing alumina content. For a given alumina content, flexure strength of the particulate composites was greater than that of the platelet composites at higher alumina contents (≥20 mol%); whereas, fracture toughness of the platelet composites was greater than that of the particulate counterparts, regardless of the alumina content. The susceptibility to slow crack growth (SCG), determined at 1000°C via constant stress-rate testing, was greatest for 30 mol% particulate composite with SCG parameter n =5–8 and was least for 30 mol% platelet composite with n =33. Elastic modulus of both composite systems decreased below 400°C and then remained almost unchanged up to 1000°C, forming a unique transition around 400°C, irrespective of alumina content.     

Fracture Origin and Strength in Advanced Pressureless-Sintered Alumina

Advanced raw materials and shaping approaches enable the production of pressureless-sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450–600 MPa), failure often originates at volume flaws, as known for hot-pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot-pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain-size dependence of grinding-induced surface damage contributes to a grain-size effect for the strength.     

Strength Variability of Indented Soda-Lime Glass

Studies of annealed indentation flaws show that specimen strength is inversely proportional to the square root of the surface trace dimension of the strength-controlling radial crack over a range of indentation loads. Using the surface trace as the characteristic dimension in the appropriate fracture mechanics equations, however, underestimates the measured strengths by about 30%. There is also a lack of correlation between the individual strengths and the surface traces at a given indentation load. Despite the complex nature of indentation flaws, strength predictions of individual indentation cracks based on the radial crack depth (not the surface trace) are in agreement with measured strengths. Strength variability of indented glass specimens at a given indentation load is due mainly to the spectrum of crack depths.     

Dependence of Fracture Toughness of Alumina on Grain Size and Test Technique

The fracture toughness ( K_Ic ) of sintered alumina was measured using notched beam (NB) and indentation/strength-in-bending (ISB) techniques. K_Ic (NB) decreases with increasing grain size. For fine-grained materials (<5 μ m ), NB results overestimate K_Ic , and exhibit a substantial notch-radius sensitivity. A stress-intensity-derived model is used to explain this notch sensitivity. The ISB results are very similar to those obtained using the double-cantilever-beam (DCB) method and show an increasing fracture toughness with increasing grain size. The differences between the NB and ISB (DCB) results for coarser-grained materials are thought to be related to R -curve behavior.     

Spark Plasma Sintering of Magnesia-Doped Alumina with High Hardness and Fracture Toughness

The effect of grain size of magnesia and its content as well as spark plasma sintering conditions on the density, grain size, strength, hardness, and toughness of alumina was investigated. Spark plasma sintering conditions were optimized at 1150°C/5 min/175°C/min. Addition of 100 nm magnesia gave higher density levels (99.5%), while better strength (600 MPa), hardness (25 GPa), and fracture toughness (4.5 MPa·m^1/2) were obtained with 15 nm magnesia. The good strength and hardness is attributed to the submicrometer grain size of the matrix, and the improved toughness to the presence of Mg-rich nanoparticles and nanopores at grain boundaries.     

Crack-Shape–Wake-Area Effects on Ceramic Fracture Toughness and Strength

It is argued that, where crack wake-area effects are a significant factor in the toughness of ceramics, the rate of change of wake area with crack growth normalized by the instantaneous wake area should be a factor in their mechanical behavior. Simple geometrical considerations show that differences in such wake area changes can vary by a factor of two or more for different test/crack geometries. This raises key questions regarding the (1) applicability of certain toughness tests for predicting strengths, (2) possible favoring of certain initial crack shapes or shape changes during failure, and (3) interaction with fracture mechanics, e.g., how stress intensity and such geometrical wake effects balance to control crack shape and failure.     

断裂韧度对PVA纤维水泥基材料弯曲性能的影响

对加入超强吸水性能SAP颗粒形成的聚乙烯醇(PVA)纤维增强水泥基材料,通过楔劈拉伸试验研究了材料的断裂韧度,三点弯曲试验研究了材料的弯曲性能,同时基于微观力学机理分析了断裂韧度对PVA纤维水泥基材料延性的影响.断裂试验结果表明:水泥基体断裂韧度随SAP颗粒掺量增大而减小;弯曲试验结果表明:开裂强度与弯曲强度随SAP颗粒掺量增大而降低,但弯曲韧度随SAP颗粒掺量增大而提高;依据能量准则与强度准则发现:掺入SAP颗粒后的PVA纤维水泥基材料的延性提高与高的E值与S值相关.     

Evaluation of Tensile Strength and Fracture Toughness of Yttria-Stabilized Zirconia Polycrystals with Fracture Surface Analysis

The tensile strength of yttria-stabilized tetragonal zirconia polycrystals (Y-TZPs) was measured and the fracture surfaces were analyzed with the scanning electron microscope and X-ray microanalyzer. The fracture origins of the pressureless-sintered samples were voids or inclusions such as A1₂O₃, A1₂O₃ with SiO₂, and cubic-ZrO₂, while the fracture origins of the hot isostatically pressed samples were inclusions; no voids were detected at fracture origins. The higher strengths of the hot isostatically pressed samples versus those of the pressureless-sintered samples were consistent with the change in fracture origins. The fracture toughness of the samples calculated from the tensile strength and analysis of the fracture origins was 3.4 to 3.7 MPa √m. These values are lower than those measured with the SEPB method. These discrepancies might be caused by the difference in the state of the fracture origin and its neighborhood, such as the size of the fracture origin and interaction between two surfaces in the precrack.     

Effect of Surface Flaw Size on Fracture Strength of Alumina Ceramics

Semielliptical surface flaws of different sizes were introduced into Al₂O₃ by Knoop microhardness indentation. The specimens were fractured by four-point bending and the profiles of the indentation flaws were determined by observing the fracture surfaces with a scanning electron microscope. The relation between the indentation flaw size and the fracture strength could be well explained by applying the fracture-mechanics analysis for semielliptical surface flaw in bending. The calculated values of the as-indented critical stress intensity factor, K_IC, were lower than previously reported presumably because of the influence of the residual stresses produced by the indenter.     

钢纤维高强混凝土断裂韧度及影响因素

通过钢纤维高强混凝土切口梁三点弯曲试验,探讨了相对切口深度（a0/h）、粗骨料最大粒径（dmax）、水灰比（W/C）和钢纤维体积率（ρf）等因素对钢纤维高强混凝土断裂韧度（KfIC）和断裂韧度增益比（KfIC/KIC）的影响。结果表明：ρf一定时,KfIC随a0/h的增加逐渐减小,随dmax的增加呈增大趋势,随W/C的减小逐渐增加;a0/h一定时,KfIC随ρf的增加逐渐增大;钢纤维能够显著提高高强混凝土断裂韧度,KfIC/KIC大于1;影响因素不同,KfIC/KIC变化趋势不同。在分析试验结果的基础上,建立了钢纤维高强混凝土断裂韧度计算模式。     

超高韧性水泥基复合材料弯曲韧性研究

为研究超高韧性水泥基复合材料(UHTCC)的弯曲韧性,开展了薄板试件四点弯曲加载试验,结合对试件弯拉荷载和跨中挠度的分析,探究了纤维体积掺量、粉煤灰掺量、水胶比三因素对UHTCC弯曲韧性的影响规律,分析了韧性指数IU与极限跨中挠度的变化规律.结果 表明,纤维掺量越大,材料弯曲韧性越好;粉煤灰掺量在F/C为1.2时各阶段韧性指数最大,表明粉煤灰掺量对UHTCC弯曲韧性的影响存在一个最佳值;水胶比为0.24、0.28时韧性指数未达到I50,且水胶比为0.26时峰值荷载对应的韧性指数IU最大,表明过大或过小水胶比都会降低UHTCC持续耗能的能力;在不同因素影响下,峰值荷载所对应的韧性指数IU与极限跨中挠度具有良好的相关性,且峰值荷载处于应变硬化阶段末期,几乎包括荷载-挠度曲线的全部,使用IU能够较好地反映UHTCC的能量吸收能力.     

Fracture Toughness Anisotropy and Toughening Mechanisms of a Hot-pressed Alumina Reinforced with Silicon Carbide Whiskers

The fracture toughness of a hot-pressed alumina and that of a hot-pressed alumina/SiC-whisker composite containing 33 vol% SiC whiskers were measured by four-point bending on single-edge precracked bend bars having sharp precracks created by "bridge indentation." Two batches of the composite were examined, one exhibiting a greater degree of whisker clustering than the other. The fracture toughness of the alumina was around 4 MN·m^-3/2 whereas that of the composite varied between 5 and 8 MN·m^-3/2 depending on microstructural uniformity and crack-propagation direction. Crack deflection in combination with a change in fracture from intergranular to transgranular fracture is proposed as an explanation of the superior fracture toughness of SiC-whisker-reinforced alumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack-propagation direction in identical crack planes. This effect could with good accuracy be described in terms of crack deflection for the composite with uniform whisker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack-growth direction was greater and could not entirely be explained by crack deflection.     

发泡剂对多孔氧化铝陶瓷孔结构和抗弯强度的影响

以十二烷基苯磺酸钠(SDBS)作为发泡剂,凝胶注模结合发泡法制备多孔氧化铝陶瓷.研究了发泡剂SDBS的添加量对多孔氧化铝陶瓷的气孔率、孔径尺寸及气孔分布和抗弯强度的影响.研究结果表明,在一定范围内随发泡剂SDBS添加量的增加,总气孔率和闭气孔率会有明显的上升趋势,孔径尺寸差异逐渐变小,气孔分布的均匀性逐渐变好.当SDBS的添加量超过1.0wt％ 后,气孔率虽然无明显变化,但是样品孔径尺寸及气孔分布的均匀性均变差,样品的抗弯强度随着SDBS用量的增加有明显降低的趋势.当SDBS的添加量为1.0wt％ 时,可以制备出闭气孔率为49％,抗弯强度为35 MPa,孔径尺寸及气孔分布均匀的多孔氧化铝陶瓷.