几种生物陶瓷材料的裂纹扩展特性

采用破坏力学中的双扭矩实验法，研究了玻璃陶瓷、云母陶瓷、氧化铝陶瓷、氧化锆陶瓷材料分别在大气、水环境中的静负荷和循环负荷下的裂纹扩展特征，阐明了水环境和循环负荷对材料裂纹扩展特性（KI－V特征）的影响。所研究的材料在水环境下的裂纹扩展速度均加快，但玻璃陶瓷和氧化锆陶瓷材料更为明显。在静负荷下这几种材料的裂纹扩展阻力由小到大的次序为：玻璃陶瓷（N－0），玻璃陶瓷（N－11），云母陶瓷，氧化锆陶瓷和氧化铝陶瓷，对于氧化铝和氧化锆陶瓷材料在循环负荷下的裂纹扩展速度均明显加快。     

Mechanical behavior of a cellular composite under quasi-static, static, and cyclic compression loading

The quasi-static, static, and cyclic compressive behavior of a novel epoxy matrix cellular composite reinforced with glass foam granules is investigated. Three different grain-size fractions of the granules are used: 0.5–1, 1–2, and 2–4 mm. The density of the cellular composite varies between 0.65 and 0.82 g/cm3. The material exhibits high specific compressive strength and stiffness within the class of cellular materials; these properties can be varied using appropriate size of granules. The glass foam granules increase the stiffness of the cellular composite compared to neat epoxy foam with the same weight. The measured elastic properties are in good agreement with results obtained from analytical and numerical homogenization methods. The fatigue behavior is determined in static tests and in cyclic tests at 1 and 20 Hz on one type of cellular composite. The fatigue process for cyclic loading is a result of an interaction between static and cyclic damage. The sensitivity to static damage is found to be higher than to cyclic damage. The damage behavior is investigated by evaluation specimen’s stiffness and using scanning electron microscopy.     

生物陶瓷材料的疲劳寿命预测

采用断裂力学中的四点弯曲试验法,研究并预测了氧化铝和氧化锆陶瓷材料在大气和水环境中的循环疲劳破坏特性.结果表明,在相同的应力条件下,氧化铝和氧化锆陶瓷材料,尤其是氧化锆陶瓷,在水环境中的疲劳寿命比大气中的低.通过将预测结果与实验结果比较和对人造股关节的应用,验证了这种疲劳寿命预测方法的有效性和适用性.     

Role of crack size in the bi-modal static fatigue failure of a cordierite glass and glass-ceramic

A study was conducted of the static fatigue behaviour at room temperature of a cordierite glass and glass-ceramic with model flaws of two different sizes introduced by the indentation fracture method. For the cordierite glass the stress intensity exponent, N, for sub-critical crack growth inferred from the dependence. of time-to-failure on stress was the same for the two different crack sizes and showed good agreement with the value of N for data obtained in an earlier study for dynamic fatigue and by the double-torsion method. For the cordierite glass-ceramic, which consisted of larger crystallites (a few micrometres in size) embedded in a primarily crystalline fine-grained matrix, the static fatigue response depended strongly on crack size, also observed during dynamic fatigue. This effect was attributed to the relative size difference between the cracks and the crystallites. The small cracks could propagate in a planar fashion, whereas the larger cracks were subject to toughening by crack-deflection around the crystallites and a corresponding decrease in the rate of sub-critical crack growth. An analysis for the case of idealized bi-modal crack-size dependent sub-critical crack-growth indicated that, for a cordierite glass-ceramic with small cracks, the role of the larger crystallites serves to introduce a pseudo fatigue-limit.On sabbatical leave from Department of Ceramic Engineering, Yonsei University, Seoul, Korea.     

CYCLIC LOADING BEHAVIOUR OF SODA-LIME SILICATE GLASS USING INDENTATION CRACKS

Abstract— Cyclic loading behaviour of soda-lime silicate glass was studied using Vickers indentation flaws. Glass bars were indented in air using a load of 9.8 N. Some samples were annealed in order to remove the indentation residual stress field. Both as-indented and annealed specimens were subjected to static and cyclic bending tests in a water environment using a four-point bending configuration. The influence of maximum stress on lifetime was considered. Crack propagation was examined by fractography and fracture patterns were similar to those observed in static fatigue. Characteristic fracture markers were seen on samples subjected to cyclic load. Experimental results were compared with theoretical predictions which were obtained on the basis of typical sub-critical crack growth behaviour. Precise determination of crack shape and residual stress factors were shown to play an important role in lifetime predictions.     

Indentation fatigure testing of soda-lime silicate glass

Dynamic, cyclic and static fatigue testing was performed on soda lime silicate glass using indentation strength measurements. Using the conventional analysis, the cyclic and static fatigue data were inconsistent with the remaining data for the case when the indentations were annealed prior to testing. Using an analysis that included the measured variations in the fracture mechanics geometric parameters, all three data sets were consistent. Using a numerical analysis, the lifetime for materials in the static and cyclic fatigue tests was shown to be sensitive to variations in the stress intensity factor at short crack lengths. It is therefore very important to understand any crack size dependent of the fracture mechanics parameters in this crack size region for accurate lifetime predictions.     

Fatigue and life-time of bioactive glass-ceramic A-W containing apatite and wollastonite

High-strength bioactive glass-ceramic A-W containing apatite and wollastonite shows the least dynamic fatigue among glass and glass-ceramics of the same composition and of different structure in a simulated body fluid at 38.5° C. An avenge life-time estimated from the fatigue of glass-ceramic A-W is 10 yews under continuous loading of bending stress of 65 MPa in the simulated body fluid, whereas that of a sintered dense hydroxyapatite ceramic is only 1 min. Articles of the glass-ceramic which withstand the stress of 215 M Pa in an inert atmosphere are guaranteed for 10 years life-time in the body environment. The glass-ceramic shows an increase in strength, without having an appreciable change in fatigue, when placed in the simulated body fluid without being loaded. Its practical life-time can therefore be expected to be much longer than that estimated above.     

Effect of Notches and Environment on Cyclic Life of Alumina

Four-point bending cyclic fatigue testing has been performed for alumina specimens with two kinds of notch radius in corrosive environments. Cyclic life is found to be dependent on notch radius and environments, the tests also show that the fatigue notch factor Kf is roughly equal to the theoretical notch factor Kt, the fatigue strength and fatigue limit σc are reduced in environment in the sequence: formamide-room air-distilled water     

钠钙/铝硅酸盐玻璃疲劳行为研究进展

钠钙硅酸盐玻璃和铝硅酸盐玻璃因高强度和高硬度等优异的物理性能被广泛用于民用和国防军工等领域。疲劳断裂是钠钙/铝硅酸盐玻璃失效的主要形式之一。研究钠钙/铝硅酸盐玻璃的疲劳行为对指导其加工工艺、预测寿命与防止失效具有重要意义。本文综述了硅酸盐玻璃疲劳行为的基本原理、实验方法以及钠钙硅酸盐玻璃和铝硅酸盐玻璃静态疲劳与动态疲劳行为的国内外研究进展,通过对比钠钙硅酸盐玻璃与铝硅酸盐玻璃的疲劳行为发现:钠钙硅酸盐玻璃原片的裂纹萌生门槛值远低于铝硅酸盐原片玻璃的裂纹萌生门槛值,且化学强化钠钙硅酸盐玻璃与铝硅酸盐玻璃的裂纹萌生门槛值均随表面压应力的增大而增大;钠钙硅酸盐玻璃与铝硅酸盐玻璃的裂纹扩展速率均随假想温度的升高而增加,且铝硅酸盐玻璃的裂纹扩展速率随假想温度的变化更大;钠钙硅酸盐原片玻璃与铝硅酸盐原片玻璃的断裂应力均具有加载速率依赖性,而化学强化铝硅酸盐玻璃的动态疲劳断裂应力不具有加载速率依赖性。因此,应力状态和环境因素对化学强化钠钙/铝硅酸盐玻璃静态疲劳与动态疲劳行为的影响将成为未来研究方向。     

Corrosion fatigue of bio-ceramic sapphire in isotonic sodium chloride solution

Alumina (sapphire) is considered as a candidate material for bio-ceramics, because of its superior strength characteristics, corrosion resistance and bio-inert properties. These biomaterials are used in aggressive corrosive environments such as the human body under severe mechanical conditions superposing cyclic loading on constant load. Therefore, it is necessary to evaluate the static and cyclic fatigue characteristics in an environment as severe as the human body. In this paper, the effects of human body environment corrosion upon cyclic fatigue characteristics of single crystal sapphire and polycrystalline alumina have been investigated using in vitro testing in isotonic sodium chloride solution, whose composition is closely similar to body fluid. Also, degraded corrosion morphologies and fracture surfaces of single crystal sapphire specimens were examined in detail by SEM and AFM. The fatigue limit of single crystal sapphire was found to be extremely degraded in the isotonic sodium chloride solution compared with that in air. The importance of environment assisted slip deformation is indicated.     

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m^1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al₂O₃-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.     

Corrosion fatigue behavior of FV520B steel in water and salt-spray environments

The corrosion fatigue behavior of FV520B steel in water and salt-spray environments at different temperatures is the focus of this study. Temperature has a significant effect on the fatigue behavior of FV520B steel in water and salt-spray environments, but has only a negligible effect in an air environment. Fatigue life decreases as temperature increases in water and salt-spray environments, and this trend is much more remarkable at a low stress amplitude level. As compared to air and water-spray, it was found that the chloride ion in salt-spray accelerated the fatigue damage of FV520B steel. Micro observation revealed that the fatigue cracks were initiated on the surfaces of the specimens in all experimental environments, and significant evidence of corrosion pits and micro cracks was found on the surfaces of the specimens in the salt-spray environment. Based on the pitting corrosion mechanism, a modified fatigue life prediction model that considered the combined effects of stress, temperature and corrosion was proposed. The study results show that the model predictions are in good agreement with the test data.     

Flexural Fatigue Behavior of Cross-Ply Laminates: An Experimental Approach

Within an experimental approach we describe the mechanical behavior of different resin-epoxy laminates reinforced with cross-ply Kevlar and glass fibers under conditions of static and cyclic three-point bending. In static tests, we consider the effect of stacking sequence, the thickness of 90°-oriented layers, reinforcement type on the mechanical behavior of laminates under loading and on realization of various damage modes leading to rupture. Cyclic loading studies have been performed in two steps. In the first stage, we inquire into the dependence of the behavior and durability of four glass fiber-reinforced laminate-types on the stacking sequence; the second stage is devoted to studying the dependence of cyclic strength and fatigue behavior of laminates on the reinforcement type. Fatigue tests are carried out in load-control regime for glass and hybrid (Kevlar + glass) fiber laminates. Fatigue curves are constructed in coordinates stress – number of cycles until fracture from the criteria corresponding to a drop in stiffness by 5 and 10%. Analysis of the results obtained permits evaluation of the effect of the stacking sequence and the reinforcement type on the behavior of cross-ply laminates in cyclic loading. The presence of Kevlar fibers accounts for nonlinear behavior of laminates in static tests and for low cyclic strength in fatigue tests under three-point bending.     

Fracture and fatigue of a Nicalon/CAS continuous fibre-reinforced glass-ceramic matrix composite

The fracture and fatigue behaviour of Nicalon/CAS continuous fibre-reinforced glass-ceramic matrix composite was studied at temperatures of up to 1000°C in both air and vacuum. Using chevron-notched test-pieces in bending, high nominal toughness values are measured at ambient temperature and at 1000°C in vacuum. In contrast, toughness values obtained in air decrease progressively with test temperature increase from 600 to 1000°C, and at 1000°C they are reduced by a factor of three from their values at ambient temperature. Marked changes in micromechanisms of crack growth under cyclic loading are also observed in air as the test temperature is increased: multiple cracking occurs at ambient temperature, while dominant Mode I cracks can be produced at 1000°C. A further study has been carried out in air on plane-sided test-pieces at a temperature at 1000°C, under both monotonic and cyclic loading. At ambient temperature, effects of cyclic loading have been deduced, while crack growth at 1000°C in air appears to be dominated by static loading with little effect of cyclic loading. These subcritical crack growth and toughness observations are consistent with changes that occur in the fibre/matrix interfaces at elevated temperatures in air reported in the literature.     

Cyclic oxidation behavior of glass-ceramic composite coatings on superalloy K38G at 1100 °C

The SiO₂-Al₂O₃-ZrO₂-CaO-ZnO glass-ceramic composite coatings (GC), nanocrystalline NiCoCrAlY coating, and their combinations (bi-layer GC/NiCoCrAlY) were prepared on K38G specimens. The thicknesses of the glass-ceramic coatings and the NiCoCrAlY coatings were about 10 μm and 20 μm, respectively. Cyclic oxidation tests were carried out at 1100 °C for 120 cycles. Microstructures of the specimens before and after oxidation tests were characterized by SEM, EDS and XRD. The glass-ceramic coatings with or without a NiCoCrAlY intermediate layer improved the isothermal and cyclic oxidation resistance of the Ni-base superalloy K38G at 1100 °C, and performed better than the NiCoCrAlY coatings. An alumina layer formed at the glass/metal interfaces of the specimens coated by the glass-ceramic coatings with or without a NiCoCrAlY intermediate layer. The NiCoCrAlY intermediate layer was beneficial to the cyclic oxidation resistance of the glass-ceramic coatings.     

STATIC AND CYCLIC FRETTING FATIGUE BEHAVIOUR OF SILICON NITRIDE

A cyclic fretting fatigue test machine was constructed. The piezoelectric bimorphs were used as actuators for cyclic loading and fretting motion at the resonance frequency of the system. Fretting fatigue tests under static and cyclic loading conditions were carried out using HIP-sintered silicon nitride. From the experimental results, it was found that fretting fatigue strengths under the two test conditions were identical and hence the effect of cyclic loading on fretting fatigue strength of silicon nitride was almost negligible. A fretting crack initiated in a very early stage of the fatigue life at the position of the maximum frictional stress in the contact area. Fretting fatigue life prediction based on fracture mechanics was also carried out. The predicted lives were in good agreement with the experimental results.     

Mechanical properties of a transformation-toughened glass-ceramic

The fracture toughness of a transformation-toughened glass-ceramic containing 12 vol% t-ZrO₂ was measured. Heat-treatment conditions were selected to produce from the same glass composition a glass-ceramic which contained 12 vol% m-ZrO₂ in a nearly identical matrix. The transformation-toughened material was found to have a fracture toughness 65% greater than the m-ZrO₂-containing material at room temperature. However, at 77 K both materials were found to have approximately the same fracture toughness. Additionally, the susceptibility of the two glass-ceramics to fatigue was determined. No improvement in fatigue behaviour of the transformation-toughened glass-ceramic over the other glass-ceramic was observed.     

BASIC DIAGRAMS OF CYCLIC CRACK GROWTH RESISTANCE OF TITANIUM ALLOYS

Abstract— Basic diagrams of the cyclic crack growth resistance of two of the most investigated titanium alloys, namely Ti-6A1–4V and Ti-6A1–6V-2Sn, are presented. Diagrams are plotted for, in-air, distilled water and 3.5% NaCl solution, which are necessary for lifetime calculations of structural elements made of these metals. The dependency of cyclic crack growth resistance on the yield strength is established. It is shown that cyclic crack growth resistance of titanium alloys in corrosive environments is determined not only by the stress-strain state but also by the electrochemical conditions at the corrosion fatigue crack tip, which for aqueous environments can be characterized integrally by the hydrogen index of the environment and the electrode potential of the metal. Therefore, cyclic corrosion crack growth resistance testing should be performed under constant electrochemical conditions at the corrosion fatigue crack tip or these conditions should be taken into account. A new method of plotting the basic cyclic corrosion crack growth resistance diagrams of titanium alloys is considered.     

Simulation of cyclic stress/strain evolutions for multiaxial fatigue life prediction

This study deals with simulation for cyclic stress/strain evolutions and redistributions, and evaluation of fatigue parameters suitable for estimating fatigue lives under multiaxial loadings. The local cyclic elastic–plastic stress–strain responses were analyzed using the incremental plasticity procedures of ABAQUS finite element code for both smooth and notched specimens made of three materials: a medium carbon steel in the normalized condition, an alloy steel quenched and tempered and a stainless steel, respectively. Emphasis is on the studying of ‘intelligent’ material behaviors to resist fracture, such as stress redistribution and relaxation through plastic deformations, etc. For experimental verifications, a series of tests of biaxial low cycle fatigue composed of tension/compression with static and cyclic torsion were carried out on a biaxial servo-hydraulic testing machine (Instron 8800). Different multiaxial loading paths were used to verify their effects on the additional cyclic hardening. The comparisons between numerical simulations and experimental observations show that the FEM simulations allow better understanding on the evolutions of the local cyclic stress–strain and it is shown that strong interactions exist between the most stressed material element and its neighboring material elements in the plastic deformations and stress redistributions. Based on the local cyclic elastic–plastic stress–strain responses, the energy-based multiaxial fatigue damage parameters are applied to correlating the experimentally obtained lives. Improved correlations between the predicted and the experimental results are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elasto-plastic stress/strain responses.     

Fatigue crack growth behaviour of gamma base titanium aluminides under different loading conditions

Static and cyclic fatigue crack growth behaviour of gamma base titanium aluminides with three different microstructures were investigated. Influence of cyclic test frequency on fatigue crack growth behaviour was also studied at room temperature under a controlled humidity condition. The crack growth behaviour both under static and cyclic loading was strongly influenced by the microstructure. The threshold stress intensity and crack growth behaviour under cyclic loading were much inferior than that under static loading indicating the ‘true-cyclic fatigue’ effect exhibited in gamma base titanium aluminides. No significant effect of test frequency on the crack growth behaviour was observed for the equiaxed and duplex microstructure materials.