玻璃及微晶玻璃的脆性表征

脆性是玻璃和陶瓷等材料的一个重要力学性能,但是目前对于脆性材料的脆性表征,没有统一的明确的表征方法.简述了玻璃及微晶玻璃的脆性断裂机理,对文献中可见的各种脆性表征手段进行了综述和比较,认为压痕法是一种方便、快捷、高效和低成本的实验方法.进一步,本文用压痕法对不同的微晶玻璃的脆性进行了测试,并和抗冲击法进行了对比,结果表明,压痕法对样品脆性的区分能力可以满足对微晶玻璃进行脆性评价的需求,可用于玻璃与微晶玻璃的脆性评判.     

脆性材料的亚临界裂纹扩展和双向应力的影响

分析了陶瓷,玻璃等脆性材料的断裂机理和表征.指出了脆性材料的失效过程的三种不同的裂纹发展模式.实验研究了玻璃在静载下的亚临界裂纹扩展特征及其受双向应力的影响.从而为脆性材料的可靠性和寿命评价提供理论基础和手段.     

纳米陶瓷及其发展趋势

纳米陶瓷改变了传统陶瓷的脆性，大幅度提高了材料的强度、韧性和超塑性，同时对传统陶瓷材料的电学、热学、磁学、光学等性能产生了重要影响。本文综述了纳米陶瓷的性能以及发展趋势。     

利用废玻璃研制瓷质砖及陶瓷透水砖

以陶瓷废料,废玻璃和粘土为主要原料研制瓷质砖及陶瓷透水砖,通过对该材料的吸水率、显气孔率、抗折强度、体积密度等性能的测试及SEM分析,结果表明:在合适的工艺参数条件下,可得透水系数为0.10cm/s,压缩强度为12.1Mpa透水瓷砖及瓷质砖。     

白榴石增强的牙科玻璃陶瓷的热处理制度研究

本文研究了白榴石增强的牙科玻璃陶瓷的热处理制度对白榴石晶体的生成、玻璃陶瓷的耐腐蚀性和力学性能的影响,采用热分析、X射线衍射分析、力学性能表征及金相显微观察等方法对热处理后的牙科玻璃陶瓷进行测试和分析.热分析表明,玻璃陶瓷晶核的生成温度为760℃,晶核的成长温度为1010℃.试验采用阶梯式升温热处理制度,成功地在玻璃陶瓷中析出白榴石晶体,随着白榴石含量的增加,材料的力学性能也获得了提高,热处理后材料的三点弯曲强度为102.2 MPa,显微硬度为5.45 GPa,断裂韧性为2.41 MPa·m1/2,强度达到ISO6872牙科陶瓷材料的标准.     

玻璃陶瓷增韧研究进展

玻璃陶瓷(即微晶玻璃)由于原料易获取、易制备、性能兼具高硬度、高耐酸碱度等特点而在研究与应用领域备受瞩目。然而,其自身的脆性严重限制着该材料的服役表现。为解决这一问题,目前已能通过析晶增韧、相变增韧和纤维增韧三种主要方法显著提高玻璃陶瓷的韧性。旨在综述上述三种玻璃陶瓷增韧方法的基础上,展望未来可能的研究方向。为目前及将来相关领域的研究者规划相关研究方案提供参考和借鉴。     

损伤容限评价硫系玻璃退火温度选择的合理性

硫系玻璃具有塑性变形缺乏以及强度对微缺陷敏感的性能缺点,机械性能差是其作为红外成像系统窗口结构材料的弱点和障碍。退火处理在降低应力、改善光学性能、提高强度、降低脆性以及改善机械性能等方面一直是相对有效的方法。但是,硫系玻璃退火温度选择的合理性一直缺乏行之有效的评价手段。本文以市场认可度高的As40Se60(mol%)硫系玻璃的为例,通过研究退火温度与缺陷容忍能力、能量耗散能力、损伤容限、折射率等参数关系,在不影响材料光学性能的前提下,用损伤容限定量评价退火温度选择的合理性,为硫系玻璃可加工性能的提高及评价提供一种全新的思路。     

陶瓷砖装饰用釉料——微晶玻璃釉

近年来，微晶玻璃材料引起众多行业的关注，主要原因在于其具有一些特殊的性能，如良好的耐腐蚀性能、机械强度高、表面硬度高、热膨胀系数低以及适应性强等特性。微晶玻璃铀应用陶瓷行业的变化是，传统的二次烧成工艺逐渐被—次快速烧成替代和传统的铀料配方有了很大的改变。     

利用废玻璃制备环保型陶瓷透水砖

以废玻璃，陶瓷废料和粘土为主要原料研制环保型陶瓷透水砖，通过对该材料的吸水率、显气孔率、抗折强度、体积密度等性能的测试及SEM分析，结果表明：在合适的工艺参数条件下，可得透水系数为0．10cm／s，压缩强度为12．1Mpa陶瓷透水砖。通过对该砖的显微结构分析可知：气孔分布比较均匀，透水性较好，满足当前环保的要求。     

利用废玻璃制备环保型陶瓷透水砖

利用废玻璃、陶瓷废料和粘土为主要原料，研制环保型陶瓷透水砖。通过对该材料的吸水率、显气孔率、抗折强度、体积密度等性能的测试及SEM分析，结果表明：在合适的工艺参数条件下。可得透水系数为0．10cm／s，压缩强度为12．1Mpa陶瓷透水砖。通过对该砖的显微结构分析可知：气孔分布比较均匀，透水性较好，满足当前环保的要求。     

On the relationship between ceramic strength and the requirements for mechanical design

During the last fifty years the mechanical properties of ceramic materials have been greatly improved, their toughness and strength have been increased and the scatter of strength decreased. Adequate statistical design procedures for brittle materials exist but cracking and brittle fracture of ceramic components still occur very often.In this review the theory of brittle fracture and the underlying assumptions are critically discussed and the measurement procedures of strength are reviewed. It is shown that the strength of materials, the strength of specimens and the strength of components are often quite different properties. Three main factors are identified which – in order to avoid unexpected failure of components – have to be considered much more than in the past: (i) hidden stresses, i.e. stresses caused by thermal strain mismatch, by contact (for example in joints) and internal stresses, (ii) the quality of the component's surfaces and edges and (iii) proper handling of ceramic materials and components.It can clearly be stated that the mechanical properties of many ceramic materials are appropriate even for applications under severe loading conditions but bad or incomplete mechanical design, insufficient surface finish and mishandling are the main reasons for unexpected failure of ceramic components.     

脆性材料强度统计分析中Weibull模数估计的试样数量的优化

为了全面的评价脆性材料强度特性指标,研究了强度统计方法中Weibull模数估计的试样数的优化问题。文中利用无偏极大似然估计理论结果,用置信度和相对误差双参数,计算出Weibull模数估计中的最优试样数,以便在Weibull模数估计试验中根据研究对象的要求,正确选用试样数量。文中结果在脆性材料强度特性评价与可靠性评价及其它适合Weibull统计的对象均可应用。     

Single and dual adhesive bond strength analysis of single lap joint between dissimilar adherends

The emerging trends for joining of aircraft structural parts made up of different materials are essential for structural optimization. Adhesively bonded joints are widely used in the aircraft structural constructions for joining of the similar and dissimilar materials. The bond strength mainly depends on the type of adhesive and its properties. Dual adhesive bonded single lap joint concept is preferred where there is large difference in properties of the two dissimilar adherends and demanding environmental conditions. In this work, Araldite-2015 ductile and AV138 brittle adhesives have been used separately between the dissimilar adherends such as, CFRP and aluminium adherends. In the dual adhesive case, the ductile adhesive Araldite-2015 has been used at the ends of the overlap because of high shear and peel strength, whereas in the middle of the bonded region the brittle adhesive AV138 has been used at different dimensions. The bond strength and corresponding failure patterns have been evaluated. The Digital Image Correlation (DIC) method has been used to monitor the relative displacements between the dissimilar adherends. Finite element analysis (FEA) has been carried-out using ABAQUS software. The variation of peel and shear stresses along the single and dual adhesive bond length have been captured. Comparison of experimental and numerical studies have been carried-out and the results of numerical values are closely matching with the experimental values. From the studies it is found that, the use of dual adhesive helps in increasing the bond strength.     

Impact resistance of thermoplastics Prediction from bulk properties

Abstract

Although neither the ‘impact strength’ nor the brittle–tough transition temperature measured using notched impact bend (Charpy and Izod) tests can be used for quantitative design, both are widely accepted as realistic indices on which to assess and select plastics. However, polymer suppliers cannot easily tailor materials for impact strength, since it is not a single property but a convolution of several. The thermal decohesion criterion allows two of these properties – resistance to the initiation and rapid propagation of brittle fracture – to be predicted explicitly from bulk material properties. This paper demonstrates the strengths and the limitations of impact test simulation, using this criterion, to predict an inferred G_c . Methods of isolating and measuring a third property, shear lip resistance, and of evaluating its contributions to toughness and transition temperature, are discussed. A potential scheme for building this into the simulation is assessed using a simplified analytical model. Using data from the ‘inverted Charpy’ test, this model itself yields a useful account of the brittle–tough transition temperature in impact.     

Micromechanical Stress Concentrations in Two-Phase Brittle-Matrix Ceramic Composites

A quantitative investigation was conducted on the effect of micromechanical stress concentrations on the strength of two-phase brittle-matrix ceramic systems. The materials consisted of a continuous brittle matrix containing dispersions with elastic properties different from those of the matrix. A soda borosilicate glass was used as the matrix and the dispersions consisted of spherical alumina particles 60μ in diameter and spherical pores 60μ in diameter. Stress concentrations were varied by measuring the strength of the composite under uniaxial and biaxial tensile stress conditions. The experimental results showed that micromechanical stress concentrations strongly affect the macroscopic strength of the composite. Under biaxial tensile stress, additions of either alumina microspheres or spherical porosity to the glass matrix resulted in a decrease in strength equal to the maximum calculated stress concentration factor. Under uniaxial tensile stress conditions, however, the reduction in strength for the glass-alumina system was negligible. The glass-porosity system gave a reduction in uniaxial strength which was not equal to the maximum calculated stress concentration factor. Experimental results suggest that differences in strength of brittle multi-component systems under uniaxial and biaxial stress states can be attributed in part to micro-structural features. On the basis of the experimental work, a hypothesis is developed relating the relative size of the region in the glass matrix over which stress concentrations act to the size of the Griffith flaws responsible for failure. This hypothesis is extended to the effect of porosity on the strength of polycrystalline brittle ceramic materials.     

Low temperature impact properties of long fiber thermoplastic composite molding materials

Four long fiber thermoplastic resin matrices, nylon 6, polypropylene, polyethylene terephthalate, and styrene maleic anhydride, containing differing amounts of long fiber glass reinforcement, were tested for notched Izod impact strength over the temperature range of 22 to −32°C. The notched impact properties of the long fiber thermoplastic composite molding materials are substantially greater than literature values for short fiber analogues. The fiber dominant performance of the long fiber materials is evidenced by increasing impact values with corresponding increases in weight percent fiber content. No apparent ductile/brittle transition in the fracture mode was observed for the long fiber materials that were tested.     

Toughening of high strength cementitious matrix reinforced by discontinuous short fibers

High strength cementitious materials are more brittle than conventional cement-based materials. The brittleness of high strength cementitious materials can be reduced by using fibers. Toughening of high strength cementitious matrices reinforced by discontinuous short fibers is studied in this paper using nonlinear fracture mechanics. An R-curve-based approach is proposed to describe the matrix toughening due to fiber reinforcement. The effect of fiber bridging is modeled by a closing pressure which depends on length, diameter, and volume fraction of fibers, and the fiber-matrix interfacial bond. Results predicted by the proposed R-curve approach match quite well with experimental data from different studies. It is found that use of fibers in relatively high volume fractions not only reduces the brittle natuer of high strength cemenitious matrices, but also increases the maximum matrix strength.     

脆性材料强度评价

本文介绍以先进结构陶瓷为主要对象的脆性材料抗张强度的各种评价方法。包括直接单轴拉伸法和各种间接测定法。     

Random glass mat reinforced thermoplastic composities. Part VII: A statistical approach to strength

Large macroscopic-scale variations in the tensile moduli and tensile strengths are charcteristic of random glass mat composites (GMT). The large-scale, point-to-point variations in the local stiffness is characterized by a probability density function that can be used to predict the stiffness of parts only in a statistical sense. Weibull statistics widely used for modeling the scatter in the strength of brittle materials cannot be applied to the large variations in the strength of GMTs: The macroscopic stress field in brittle materials is assumed to be deterministic, while the stress field in GMTs varies randomly on a macroscopic scale. A statistical approach for characterizing the strength of GMTs is developed by combining an empirically established strength-modulus correlation with the statistical characterization of the tensile modulus.