工程陶瓷的断裂特性与评价

综述了陶瓷材料的强度与断裂理论的现状、发展及问题,讨论了脆性陶瓷失效研究的几个主要方面及相应的技术。     

Fracture Behavior of Open-Cell Ceramics

Cellular structures are commonly used by nature as a means of optimizing the strength-to-weight ratio of a body. Cellular ceramics may have potential in structural applications requiring a high modulus-to-weight ratio. The models that are used to predict the mechanical behavior of these materials describe them as an interconnected network of columns and plates and predict a dependence on density and cell size. The fracture toughness and strength of several open-cell alumina materials were measured and compared to a theoretical model. Image analysis was used to fully characterize the macrostructure of the samples and provided a basis for evaluating the toughness and strength behavior. It was observed that the properties of the bulk material depend on the strength of the individual struts in the structure. The fit of the data to an open-cell model was found to be dependent on the agreement between the actual macrostructure and that assumed in the model. Both microscopic and macroscopic flaws contribute to the variability of the strength in these materials and it was found that there was a wide distribution of bulk and strut strengths. Both of these flaw populations must be controlled to maximize the mechanical properties of these open-cell ceramics.     

Fracture Criteria for Piezoelectric Ceramics

Fracture criteria for piezoelectric materials were investigated. Mode I and mixed mode fracture tests were performed on PZT-4 piezoelectric ceramics to verify the validity of the mechanical strain energy release rate as a fracture criterion. Experimental results indicated that crack extension could be aided or impeded by an electric field, depending on the field direction. Further, the direction of crack extension was studied. A crack closure method, together with finite element analysis, was introduced to calculate the mechanical strain energy release rate. The maximum mechanical strain energy release rate was used to predict fracture loads under combined mechanical and electrical loads. It was found that the mechanical strain energy release rate criterion is superior to other fracture criteria and predicts fracture loads fairly accurately.     

Inclusion-Initiated Fracture Model for Ceramics

The fracture of ceramics initiating from a typical inclusion is analyzed. The inclusion is considered to have a thermal expansion coefficient and fracture toughness lower than those of the matrix and a Young's modulus higher than that of the matrix. Inclusion-initiated fracture is modeled for a spherical inclusion using a weight function method to compute the residual stress intensity factor for a part-through elliptical crack. The results are applied to an α-Al₂O₃ inclusion embedded in a tetragonal ZrO₂ ceramic. The strength predictions agree well with experimental data.     

Fracture Behavior of Low-Density Fibrous Ceramics

It was established that subcritical crack growth occurs in the silica fiber-based tiles that are currently in use as part of the thermal protection system (TPS) of the Space Shuttle. By measuring the tensile strength distribution and the subcritical crack-growth parameters, it was possible to predict the material's behavior during proof-testing. A similar time dependence of dynamic strength was also found for structural systems using this ceramic tile, suggesting that the ceramic is controlling their strength behavior. It was possible, therefore, to predict the influence of the proof testing currently in use for the Shuttle. These predictions were found to be in reasonable agreement with the experimentally determined strength distributions after proof-testing. This study confirmed that proof-testing and tile densification should lead to improved reliability of the TPS. The research further suggests that the subcritical crack-growth behavior in these fibrous tiles is simiilar to that observed in bulk glass of similar composition and that failure occurs by the tensile fracture and fragmentation of individual fibers.     

Dynamic Fracture Toughness of Piezoelectric Ceramics

Both dynamic and static three‐point bending fracture testings of 32 piezoelectric ceramic samples were performed in four different poling directions. A modified split Hopkinson pressure bar method with a polyvinylidene fluoride (PVDF)‐based gauge was utilized for the dynamic experiments. The loading rate greatly influenced the fracture toughness in two ways: the dynamic fracture toughness values were much higher than those of the static fracture toughness, and unlike the static fracture toughness, the influence of poling direction on the dynamic fracture toughness was not obvious.     

Tensile Fracture Toughness Measurements in Ceramics

A novel procedure is outlined whereby fracture toughness values for ceramics can be measured under uniaxial tension (mode I) in specimens containing a fatigue crack. Circumferentially notched rods of rapolycrystalline aluminum oxide were precracked in cyclic compression to introduce a fatigue crack at room temperature, following the technique proposed by Suresh and co-workers.^7,10,11 Subsequently, the precracked rods were fractured in pure tension. Highly reproducible values of fracture toughness were obtained using this method.     

Anisotropy of Fracture Toughness of Piezoelectric Ceramics

The anisotropy of fracture toughness of PZT- and barium titanate-based piezoelectric ceramics was studied using doubletorsion and indentation methods. The influence of elastic anisotropy and the piezoelectric effect in calculating the K_c value are discussed. The fracture toughness in the polarization plane is greater than that in the perpendicular plane. A model is suggested to explain the observed anisotropy; it is based on assumption of a stress-induced domain reorientation zone near the crack tip.     

Surface Engineering of Structural Ceramics

The role of surface engineering for controlling structural ceramic properties is reviewed. Initially, a perspective of the effects of an unmodified surface state on fracture, adhesion, tribological properties, and degradation is given. Ceramic surface modification techniques including laser, ion, ion-beam-assisted deposition, and plasma-assisted deposition are then discussed with regard to how each non-equilibrium technique affects bulk and surface properties. The importance of experiments, process and materials characterization, and modeling for understanding changes in localized bonding, chemistry, and structure is emphasized. Although not exhaustive, a broad guide to important literature is given.     

陶瓷机械的反求工程

针对我国陶瓷机械的现状，提出开反求工程研究是开发陶瓷机械产品，提高陶瓷机械装备水平的迅捷之路，以及提出了实施陶瓷机械反求工程研究应注意的问题。     

Dynamic Fracture of Ceramics in Armor Applications

Ceramic materials have been extensively used in armor applications for both personnel and vehicle protection. As the types of threats have diversified recently, e.g., improvised explosive devices and explosively formed projectiles, a proper set of ceramic material selection criteria is needed to design and optimize corresponding mitigating structures. However, the dynamic fracture and failure behavior of engineering ceramics is still not well understood. Using examples of thin ceramic plates and confined thick ceramics subjected to kinetic energy projectile impact, this article provides a brief summary on the current understanding of dynamic failure processes of ceramics under dynamic penetration loading conditions. Laboratory examination of dynamic fracture of ceramics is conducted using split Hopkinson bars with various loading rates, stress states, and loading histories.     

工程陶瓷的电火花加工技术

本文回顾了近20年来工程陶瓷的电火花加工技术的产生与发展历程,并论述了国内外研究现状。     

Mixed-Mode Fracture of Ceramics in Diametral Compression

Mixed-mode fracture of a glass-ceramic and an alumina ceramic from inclined Knoop indenter flaws was studied in diametral compression and four-point-bend tests. In annealed specimens the directions of extension of the cracks and mode I and mode II stress-intensity factors at fracture were analyzed and compared to the predictions of a maximum crack-tip hoop stress theory. Knoop flaws in all cases extended in directions normal to the principal maximum tension rather than in the direction of maximum hoop stress near the crack tip. Mixed-mode fracture envelopes assessed in experiments, particularly with the diametral-compression test, showed significant deviation to higher K_II values relative to the fracture-mechanics predictions. As a consequence, an apparent K _IIc value assessed in the diametral-compression test was approximately twice the value of K _Ic.     

Electrical-Impulse-Induced Fracture of Zinc Oxide Varistor Ceramics

When varistors are subject to very short (˜1 μ s), high-current pulses, they occasionally fracture. We attribute the occurrence of fracture to stress waves, created by inertial forces in response to the rapid Joule heating, causing the propagation of preexisting microstructural flaws. The amplitude of the stress oscillation is shown to be proportional to the second derivative of temperature with respect to time which, in turn, is directly related to the rate of increase of power. An upper limit for the magnitude of the peak stress is derived in terms of material parameters, varistor geometry, and the maximum electrical power. From the magnitude of the peak stress, a critical defect size is estimated which will cause fracture under given pulse conditions. Conclusions for the design and testing of varistors are drawn.     

Grain-Size Dependence of Fracture Energy in Ceramics

A simple model incorporating thermal elastic anisotropy stresses is used to calculate the microcrack zone size around cracks in Al₂O₃. It is found that the ratio of microcrack zone size to grain size is almost constant for notched beam tests, but increases with grain size for double cantilever beam data. It is suggested that notched-beam ratios of fracture toughness are related to crack initiation, whereas double cantilever beam values are related to propagation and reflect R-curve behavior of the material.     

可加工陶瓷及工程陶瓷加工技术现状及发展

陶瓷材料具有轻质、高强、超硬、耐高温、耐磨、耐腐蚀、化学稳定性好等优良特性。但陶瓷烧结体难以机加工,同时加工成本高、精度低限制了其在工程领域的广泛应用。本文综述了可加工陶瓷的分类、加工技术、陶瓷材料加工性能的表征,以及影响陶瓷材料可加工性能的因素等。     

工程陶瓷材料磨削加工技术研究

工程陶瓷材料具有优良的物理、化学、力学性能,在许多领域得到广泛的应用。目前,使用金刚石工具(主要是砂轮)的磨削加工是工程陶瓷去除加工的基本途径。本文概述了陶瓷材料磨削加工技术的进展,并对磨削后的陶瓷工件表面损伤进行了分析。     

莫来石/刚玉复相陶瓷热断裂特性研究

通过调整复相陶瓷中颗粒相的组成与级配,利用引入的热膨胀失配机制,控制莫来石/刚玉复相陶瓷显微结构,改善高温复相陶瓷热稳定性,着重研究了莫来石/刚玉复相陶瓷的热断裂特性.研究表明,全部采用莫来石颗粒相时,由于柱状莫来石颗粒极易发生热冲击穿晶断裂,且热膨胀失配导致的微裂纹较长,密度较小,故复相陶瓷热稳定性较差;全部采用刚玉颗粒相时,虽然基质相裂纹扩展至刚玉颗粒表面时穿晶断裂与沿晶的裂纹扩展同时发生,有利于改善复相陶瓷热稳定性,但由于热膨胀失配过度,未经热冲击时已产生宏观裂纹,故热稳定性最差.而采用莫来石/刚玉复合颗粒相,相比例为75/25制备的复相陶瓷试样,热膨胀失配形成的微裂纹较短,密度较大,其热稳定性最优.     

Mechanisms of Improvement of Fracture Strength in Laser-Surface-Modified Ceramics

Three mechanisms of improvement in fracture strength of laser-surface-modied ceramic materials are proposed to explain the experimental observations of more than 50% increase in fracture strength. First, the improvement in fracture strength by about 50% is considered to arise from either a complete or partial closure of crack surfaces at the interface of the laser-modified overlayer. The second mechanism of improvement in fracture strength is derived from the physical displacement of the crack tip away from the free surface when a laser-modified layer is introduced. Thus, the critical crack size, defmed as a crack that propagates with decreasing energy, is increased almost 100%. The Wid mechanism is based upon the compressive stresses introduced in the laser-modified region. The fast cooling rates attained after laser irradiation are responsible for development of regions of compressive internal stresses. These sources of improvement in fracture strength are analyzed and the results of the calculations compared with experimental results. Through the present understanding of the mechanisms of improvement in fracture strength, it has become possible to calculate a critical thickness of the laser-modifed layer. It is concluded that the possible improvement of fracture strength is achieved when the thickness of the laser-modified overlayer is equal to or greater than this critical value.