EFFECTS OF CRACK LENGTH, NOTCH ROOT RADIUS AND GRAIN SIZE ON FRACTURE TOUGHNESS OF FINE CERAMICS

Generally, fracture toughness and fracture stress of ceramics depend on crack length, notch root radius and grain size. These three parameters are most important when assessing the integrity of structural ceramic members and developing high-performance ceramics. A new failure criterion called the process zone size failure criterion, has been proposed based on the existence of a crack-tip process zone. Using this criterion, it is shown that theoretical values are in good agreement with many test results quoted from many papers. It is concluded that this failure criterion is useful when evaluating crack length and notch root radius problems. The effect of grain size on both the fracture toughness and on the toughening mechanism is also considered.     

SIZE EFFECTS AND A MEAN-STRENGTH CRITERION FOR CERAMICS

On the basis of a large amount of research on the fracture behaviour of ceramics, a new failure criterion is advanced. It is pointed out that the rupture of structural ceramics is governed by the mean stress in a specific zone, rather than by the peak stress value. The size of the process zone was derived as a material constant, and reflects the relation between the toughness and the strength of the material. The results show that the critical maximum stress varies with the stress gradient: the steeper the stress gradient, the higher the critical peak stress. Different experiments concerning the size effect have been carried out and the reason for size effects has been explained by a mean strength theory.     

A FRACTURE AND SEVERITY ANALYSIS OF COMPOSITE CERAMICS

Ceramic composite crucibles failed when they were operated under extreme conditions of temperature and heating rate. To analyze the failure causes, fracture stress and fracture toughness values were ascertained and previously reported. Moreover, a new failure criterion for ceramics was proposed based on the existence of a crack-tip process zone. A probabilistic failure assessment curve was proposed to evaluate quantitatively the reliability of ceramics. By using the above data and theories, a fracture and severity analysis for crucibles has been completed. Some manufacturing techniques are proposed to improve reliability during service.     

PROCESS ZONE SIZE FAILURE CRITERION AND PROBABILISTIC FRACTURE ASSESSMENT CURVES FOR CERAMICS

Data of fracture toughness and fracture stress of ceramics, generally show considerable size effects and scatter. This presents problems for the integrity of structural ceramics. Fracture mechanics test results on Si₃N₄ and A1₂O₃ reveal a sensitivity to crack size and the necessity to invoke probabilistic concepts to evaluate their integrity. A new failure criterion is proposed here, based on the existence of a crack-tip process zone. A probabilistic failure assessment curve is also proposed to quantitatively evaluate reliability.     

Subsurface damage in alumina induced by single-point scratching

Damage-free processes of grinding of brittle materials have been widely used in industry for producing electronic and optical components with high surface integrity. It has been found that the transition threshold from ductile flow to brittle fracture during the process of material removal plays a central role in the quality control of a machined surface. However, the precise microscopic mechanism which governs the formation of dislocation structure and micro-cracking when machining a brittle material such as alumina, remains unclear. The mechanism of formation and structure of the plastic or damage zones in alumina of two different grain sizes (1 and 25 m) subjected to single-point scratching with sharp and blunt indenters were studied in this paper. Using transmission electron microscopy, characteristic features of the plastic/damage zone in terms of loading conditions and microstructure of the materials were carefully investigated. It was found that the grain size and the geometry of the indenter had a great effect on the dislocation structure of the plastic zone and that the subsurface damage could be very severe, even though the machined surfaces appeared damage-free. These results indicate that the ductile flow to brittle fracture transition in machining brittle ceramics is more complicated than previously thought and that a reliable criterion has yet to be established to predict a real damage-free grinding process.     

马氏体不锈钢切刀失效分析

对某切刀矩时工作后出现的裂纹失效进行了分析。着重讨论了在磨削加工中形变应力、热应力和组织应力对切刀成形后的影响，提示了切刀在磨削加工后产生应力集中从而导致失效，提出了预防措施。     

结构陶瓷磨削表面微裂纹的研究

结构陶瓷的磨削表面微裂纹是较常见和较危险的磨削损伤，本文应用压痕断裂力学。磨削表面热应务陶瓷微观结构的有关知识分析了结构陶瓷表面裂纹的形成机理、形状特征及奖表面的延伸，并对几种典型结构民磨削表面层的微观状态进行了大量的ＳＥＭ观察，结果表明：（１）磨削微裂纹与陶瓷的断裂韧性Ｋ１Ｃ和显微硬度Ｈ之比、陶瓷的热特性及磨削工艺参数有关；（２）磨削微裂纹的 压痕效应径向裂纹、热裂纹、（３）细晶陶瓷多为穿晶裂纹     

A STUDY ON STRENGTH CHARACTERISTICS OF Si3N4 COIL SPRINGS

Coil springs made of silicon nitride (Si₃N₄), which has higher strength and heat resistance than other ceramics, have recently been developed. By examining the range of strengths and the heat resistance of the coil springs, it is shown that the coil springs can be used at temperatures up to 1000°C. We derive an equation to calculate the effective volumes of coil springs and examine the influence of size on the strength of coil springs. In addition, using a process zone size failure criterion, proof tests were conducted and analysed. The results show that the mean strength of coil springs decreases with increasing effective volume. Therefore the strength of coil springs can be estimated from the value of the effective volume, and this relationship can be applied in design calculations. Furthermore, by conducting proof tests, coil springs that have relatively large defects can be eliminated and so only highly reliable coil springs can be placed in service.     

Ductile Regime Mirror Finish Grinding of Ceramics with Electrolytic In-process Dressing (ELED) Grinding

Advanced structural ceramics, such as silicon nitride based materials, are of interest owing to their unique physical and mechanical properties. However the cost of grinding these ceramics, which is an integral part of their fabrication, is very high. Moreover, grinding can result in surface and sub-surface damage in the material and these defects can significantly reduce the strength and reliability of the finished components. Grinding damage is sensitive to grinding parameters. Two types of silicon nitride based ceramic materials were ground with Electrolytic In-Process Dressing (ELID) using different grit sized metal bonded diamond grinding wheels. With the application of ELID technology, mirror surface finish was realized with a #4000 mesh size wheel (average grain size = 4μm). Differences in ground surface topography caused by wheel grain size were analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM and AFM studies reveal that material was predominantly removed in the ductile mode when ELID grinding was performed with a #4000 grit size wheel or finer.     

Fracture behavior of shear key structures with a softening process zone

The objective of this paper is to investigate the fracture behavior of short fiber reinforced ceramic structures by means of the fracture mechanics approach. In this paper, structural stability in relation to crack growth in shear key structures with a softening process zone under effectively bending loads is studied through residual strength diagrams and load deflection curves. In addition, the behavior of process zone size preceding traction free crack in the shear key structure is investigated.Shear key structures with a softening process zone can behave stably under loading in the presence of a crack. Results of this analytical study potentially indicate that short fiber reinforcement for ceramics could eliminate catastrophic failure or unstable fracture behavior of ceramic structures, and also indicate the plausible applicability of ceramics as construction materials.     

飞机桨壳螺帽磨削裂纹分析

采用金相分析方法，对12CrNi3A钢制飞机浆壳螺帽裂纹进行了分析。结果表明，因磨削工艺控制不当，出现局部过热，使工件表面产生二次淬火马氏体，造成组织内应力增大，以至产生磨削裂纹。     

Performance Improvement of Grinding of SiC Using Graphite as a Solid Lubricant

Grinding is a widely employed finishing process for different materials. It is inherently characterized by its high specific energy requirement unlike other machining processes. This leads to a high grinding zone temperature, which impairs the workpiece quality by inducing thermal damage in the form of surface and subsurface cracks, phase transformations, tensile residual stresses, etc. The microcracks and residual stresses induced in the surface of the ceramics during grinding can severely limit the application of ceramic components. This article deals with an investigation on using graphite as a solid lubricant to reduce friction and thereby improve the surface integrity of ground silicon carbide (SiC). An experimental setup has been developed, and experiments have been conducted to study the effect of using a graphite lubricant on the surface roughness, grinding forces, and specific energy while grinding SiC material. Results indicate that there is a considerable improvement in the performance of grinding SiC using graphite as a solid lubricant when compared with dry grinding in terms of specific energy requirements, surface roughness, and damage. The employment of graphite as a solid lubricant in ceramic grinding makes the process more economical and reliable.     

Strength calculation of composite single lap joints with Fiber-Tear-Failure

The Fiber-Tear-Failure (FTF) is a common mode of failure in the adhesively bonded single lap joint having continuous fiber reinforced composite. Within single lap joints, presence of material and geometric discontinuities restrict the applicability of strength of material based approach for failure load prediction. Zone based approach appears promising in tackling discontinuity issues provided the right failure criterion and critical zone size are known. In this paper, a right failure prediction criterion is identified that can be used for the Fiber-Tear-Failure load prediction using zone based approach. Using a carbon fiber composite and an epoxy paste adhesive, failure modes were generated experimentally using different dimensions of lap joints. Several stress/strain based failure criteria for composite were tested. Critical zone size was calculated by performing the finite element analysis on a single lap joint with known failure load. For other joints, failure loads were calculated by adjusting the input loads in the analysis such that the failed zone size became the same as the critical zone size. The result reveals that Azzi–Tsai (Norris) criterion is capable of predicting failure loads of single lap joints with FTF.     

电机行星减速器动凸轮断裂分析

介绍了电机行星减速器动凸轮的断裂情况,对断裂动凸轮进行了宏、微现检验。结构表明,淬火过热物磨削加工不当,致使零件中形成了许多淬火裂纹和磨削裂纹,最终导致动凸轮断裂。     

超声振动辅助磨削完全烧结氧化锆陶瓷牙冠的实验研究

针对现有全锆牙在制作过程中存在二次烧结、收缩精度难以控制等问题,提出了采用超声振动辅助磨削完全烧结氧化锆陶瓷牙冠的方法。从理论分析的角度对其运动学特性进行了研究,并通过超声振动辅助磨削和普通金刚石磨削实验,对该方法的可行性进行了分析。结合牙冠的加工特点,重点研究了主轴转速对材料去除率、表面粗糙度以及最大边缘碎裂的影响规律。实验结果表明,超声振动辅助磨削不仅能提升材料的去除率,有效抑制出口边缘碎裂,同时降低了工件表面的粗糙度,是实现完全烧结氧化锆陶瓷牙冠高效低损伤加工的新方法。     

溶胶-凝胶法制备PbTiO3铁电玻璃陶瓷的高频晶粒尺寸效应

首次报道了用溶胶－凝胶法制备的钛酸铅铁电玻璃陶瓷的晶粒尺寸对其高频介电性能的影响。研究表明,铁电玻璃陶瓷的高频介电弛豫主要是由材料内部的畴壁共振引起的。晶粒尺寸增加,畴壁宽度增大,介电弛豫向低频移动;反之,晶粒尺寸减小,畴壁宽度变小,介电弛豫向高频移动,弛豫频率提高,介电损耗变小。     

从微孔洞起始的热冲击裂纹的数值研究——Ⅰ: 热应力分析

陶瓷材料在热冲击荷载作用下的典型破坏模式是裂纹的起始和扩展。当高温的材料或构件突然处于低温环境时,裂纹将由材料或构件的表面开始产生和发展。因此,材料表面及其表面附近的材料微结构对裂纹的起始有着非常重要的影响。实验表明:某些具有微孔洞结构的陶瓷材料比相应的密实材料的抗热冲击性能有所提高,但相关的机制目前还没有得到定量的解释。本文作者利用商业有限元软件ABAQUS,分析了受热冲击表面上的开口微孔洞、处于表面附近的闭口微孔洞周围的热应力场,以及微孔洞的大小、微孔洞与表面的距离对热应力场的影响,其目的是为进一步分析裂纹的起始和扩展做准备。数值结果表明:一般情况下,闭口微孔洞引起的应力集中比开口微孔洞引起的应力集中更为严重,特别当闭口微孔洞与表面之间的距离很小时,和没有微孔洞情况相比,闭口微孔洞引起的最大的应力集中系数超过6。     

New Vitrified Bond Diamond Grinding Wheel for Grinding the Cylinder of Polycrystalline Diamond Compacts

In this work, a kind of new vitrified bond based on Li2O-Al2O3-SiO2 glass ceramics was used to bond the diamond grains, which is made into grinding wheel and the cylindrical grinding process of polycrystalline diamond compacts （PDCs） by using the new vitrified bond diamond grinding wheel was discussed. Several factors which influence the properties of grinding wheel such as amount of vitrified bond and the kinds and amount of stuff in grinding wheel were also investigated. It was found that the new vitrified bond can firmly combine diamond grains, when there are only diamonds and vitrified bond in the structure of grinding wheel, the longevity of the grinding wheel is about 2.5-3 times as that of resin bond grinding wheel for processing PDCs. The grinding size precision of PDCs can be improved from 4-0.03 mm to 4-0.01 mm because of larger Young＇s modulus of vitrified bond than resin bond. The grinding time of a PDC product can be 1.75-2.0 min from 3.25-3.5 min, so this kind of grinding wheel can save much time for processing PDCs. Also, there is hardly noise when using this new vitrified bond diamond grinding wheel to process PDCs. The amount of vitrified bond in grinding wheel influences the longevity of grinding wheel. When the size of diamond grains is 90-107 μm, the optimal amount of vitrified bond in grinding wheel is 21% （wt pct）. When the amount of vitrified bond exceeds 21%, there are many pores in grinding block, which will decrease the longevity of grinding wheel. The existence of addition stuff such as Al2O3 or SiC can reduce the longevity of grinding wheel.     

二维超声振动磨削陶瓷的表面质量试验研究

采用金刚石砂轮对陶瓷材料进行了不同参数下的普通磨削和二维超声振动磨削试验,对获得的不同表面质量特征及不同的加工参数对表面质量的影响进行了分析。实验结果表明,在同样的加工条件下,超声振动磨削表面的沟槽浅而宽,在超声振动下砂轮不易堵塞,利于使用细砂轮磨削,因此超声振动磨削可以提高陶瓷材料的表面质量。     

COHESIVE MODELLING OF PROCESS REGIONS FOR CRACKS IN LINEAR ELASTIC STRUCTURES—FUNDAMENTAL ASPECTS

The process region at the tip of a crack in a linear elastic structure has been modelled by a cohesive zone. Growth of the front end of the cohesive zone is governed by a critical stress intensity factor criterion, and advance of the original traction free crack is determined by a critical crack opening at the rear end of the cohesive zone. Damage resistance curves relating the applied stress intensity factor to the growth of the cohesive zone have been calculated for an idealized structure containing two characteristic dimensions. Instability resulting in failure of the structure is found to occur either by unstable growth of the front end of the cohesive zone, without a fully developed cohesive zone, or by unstable growth of the original flaw, when the crack opening displacement at the rear end of the cohesive zone reaches a critical value. The influence of the size of the structure compared to the length of the cohesive zone is investigated, and conditions for the limits of validity of the small scale yielding assumption are discussed. Comparisons are made between the maximum load and the length of the cohesive zone at instability resulting from the present analysis, and the values predicted by linear elastic fracture mechanics.