THE ASSESSMENT OF SURFACE FATIGUE EFFECTS IN TOUGHENED ZIRCONIA USING A NEW POINT CONTACT METHOD

The effects of repeated point contact loading of a toughened ceria-stabilised zirconia ceramic, CeTZP, by softer metallic cones, have been investigated. All tests were conducted on a purpose-designed and built computer-controlled apparatus. The results clearly indicate that zirconia is susceptible to cyclic loading. The evolution of the cyclic deformation and cracking processes have been analysed in terms of contact pressures, applied loading conditions and number of cycles. The ground tips of both quenched, and quenched and tempered, silver steel cones are plastically deformed during the initial loading cycle to produce a conforming planar surface approximately 50-100 μm in diameter. In all cases, plastic deformation in the CeTZP was observed within, and adjacent to, the contact zones. The degree of plastic deformation increased as the number of cycles increased. The expansion associated with the tetragonal-to-martensitic phase transformation in the zirconia caused granular lifting from the surface, at the edge of the contact zone. This resulted in localised spalling at the peripheries of the contact zones, where the tensile stresses are the highest, followed by a radial expansion at higher cycle numbers. This investigation has identified that the rate of evolution of the fatigue damage, in decreasing order of significance, was controlled by: (a) cone hardness, (b) cyclic load amplitude and (c) mean load.     

MECHANISMS AND FATIGUE PERFORMANCE OF TWO STEELS IN CYCLIC TORSION WITH AXIAL STATIC TENSION/COMPRESSION

Abstract— Fatigue tests conducted under fully reversed cyclic torsion, with and without superimposed axial static tension/compression loads, were carried out using hour-glass smooth specimens in laboratory air. A high strength spring steel and a 316L stainless steel, were employed to evaluate the effects of mean stress on fatigue performance. Experimental test results show that a biaxial tensile/compressive mean stress had no influence on the cyclic stress-strain response in both materials. However a biaxial tensile mean stress was found to be detrimental to fatigue life of the high strength spring steel but had no effect on the total fatigue life of 316L stainless steel. A compressive mean stress was found to be beneficial to the life of both steels. The fatigue behaviour of the two materials was investigated by experimental observations and the application of theoretical analyses of short crack growth behaviour. Based upon the analysis of surface acetate replicas it has been found that fatigue crack growth is material/stress-state dependent. A biaxial tensile static stress promoted a change in the direction of the Stage I (mode II) crack from the longitudinal direction to a plane normal to the specimen axis in the high strength steel but not in the stainless steel. Consequently a different growth behaviour of Stage I (mode II) cracks was observed for the two materials. The effect of a biaxial mean stress on fatigue crack growth behaviour of the two materials is analysed and described in some detail.     

应变疲劳循环软化机制的观察,确证及定量估测

通过透射电镜观察和循环应变加时效的试验方法，对淬火回火态地低碳合金钢的循环软件机制进行了研究。     

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.     

CYCLIC FATIGUE CRACK GROWTH AND CLOSURE BEHAVIOUR IN ALUMINA CERAMICS

Cyclic fatigue crack growth behaviour in alumina ceramics is investigated and the effect of grain size discussed. Special attention is given to crack closure effects. Cyclic fatigue tests were carried out using four-point bend specimens, and the load–strain and load–differential strain curves were monitored. These curves show hysteretic behaviour probably related to frictional sliding of bridging grains, and also include non-linearities due to crack closure. The crack opening load is determined from the load–differential strain curve by using a method introduced in this study. Growth rates can be successfully described by the relationship da/dN = C[ΔK_{eff /(1 ? Kmax /KIC )]m} which is proposed in this study to account for the effects of crack closure and the maximum stress intensity factor. Irrespective of grain size, growth rates can be well represented by the above relationship, implying that the grain size exerts an influence on growth rates not only because of crack closure behaviour but also the material fracture toughness. The growth rate curve based on the proposed relationship shows a sigmoidal form for ceramic materials, which is similar to metals.     

基于材料低周疲劳的裂纹扩展预测模型

该文基于疲劳裂纹尖端循环应力-应变场, 定义了基于应变幅的平均单位循环损伤参量D, 并引入Miner累积损伤率, 从而从理论上建立起材料低周疲劳性能和疲劳裂纹扩展行为之间的联系。以裂尖扩展方向上的单调塑性区尺寸作为疲劳过程区大小, 并提出了基于弹塑性应变疲劳累积损伤的疲劳裂纹扩展预测模型。模型改进了前人提出的疲劳裂纹扩展预测模型, 考虑了单调塑性区内所有材料的弹塑性应变疲劳损伤贡献;模型中参数均有物理意义, 不需要人为调试。基于完成的Cr2Ni2MoV 材料的低周疲劳结果所建立的该文新模型对该材料裂纹扩展速率的预测结果与实验结果有良好一致性。并且, 借助手册数据, 在TC4钛合金材料上进一步得到了验证。     

PREDICTION OF THE FATIGUE LIMIT OF CRACKED SPECIMENS BASED ON THE CYCLIC R-CURVE METHOD

Abstract— The propagation behaviour of fatigue cracks emanating from pre-cracks was numerically simulated to evaluate the development of crack closure with crack growth. The crack opening stress intensity factor at the threshold was approximated as a function of the applied stress and the amount of crack extension. Pre-cracked specimens of a medium-carbon steel with a small surface crack and a single-edge crack were fatigued to investigate experimentally the initiation and propagation of cracks from pre-cracks. Crack closure was dynamically measured by using an interferometric strain/displacement gauge. The threshold condition of crack initiation from pre-cracks was given by a constant value of the effective stress intensity range which was equal to the threshold value for long cracks. The cyclic R -curve was constructed in terms of the threshold value of the maximum stress intensity factor as a function of crack extension approximated on the basis of the experimental and numerical results. The cyclic R -curve method was used to predict the fatigue thresholds of pre-cracked specimens. The predicted values of the fatigue limits for crack initiation and fracture, and the length of non-propagating cracks agreed very well with the experimental results.     

不同温度下混凝土抗拉疲劳性能试验研究

进行了不同温度下混凝土在等幅重复荷载作用下的抗拉疲劳性能试验研究,分析了不同温度下砼抗拉疲劳强度、刚度、变形规律,建立了相应的疲劳方程及考虑温度影响的统一疲劳方程,并将常温下的砼疲劳性能试验结果同其他研究者的结果作了比较.给出了纵向总应变、割线模量的经验公式,及其第二阶段总应变增长率、割线模量衰减率分别与疲劳寿命的关系式.研究结果,为高温环境下受重复循环荷载作用的砼结构设计和分析提供了试验依据.     

混凝土双轴拉压、三轴拉压压变幅疲劳性能研究

利用大型多轴疲劳试验机,进行了拉压双轴和拉压压三轴荷载作用下混凝土两级、三级变幅疲劳试验研究,重点分析了残余应变的变化规律。试验结果表明：混凝土在拉压和拉压压加载工况下的残余应变主要与相对疲劳次数和侧压应力比有关,基本不受加载历程因素的影响。定义相对残余应变为损伤变量,建立了损伤演变方程,并进行了疲劳损伤分析和剩余疲劳寿命预测,为混凝土在多轴变幅疲劳试验研究及疲劳损伤评价提供参考。     

一个新的低周疲劳失效准则

利用修正的塑怀循环应变能记响应数学模型，给出了一个新的低周疲劳失效准则，由此得出了低周疲劳寿命的估算公式，并且采用低周疲劳试验验证该失效准则的合理性和有效性。     

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.     

周期载荷下聚酯/橡胶复合材料和人造丝/橡胶复合材料的热生成对疲劳行为的影响

以单向帘线增强橡胶复合材料为研究对象,研究了周期载荷作用下橡胶复合材料的热生成对疲劳行为的影响。橡胶复合材料在疲劳过程中伴随较多的热生成,SEM观察表明:热生成温度与断裂表面密切联系。橡胶复合材料的滞后损失并不是全部转换为热,而是其中一部分被内能变化所消耗,不转换为有效热生成的滞后损失与疲劳损伤导致的结构变化有关。在疲劳试验中,直到疲劳破坏所消耗的平均有效能耗与疲劳寿命之间存在定量关系。     

LOW-CYCLE FATIGUE AND CYCLIC CREEP OF METALS

Abstract— This paper deals with the effect of accumulated (oriented) plastic deformation (cyclic creep) on the life of metals under conditions of cyclic loading typically in the range of low-cycle fatigue. Specific features of cyclic creep of aluminium, titanium alloys and structural steels were investigated for the lives ranging from 05 to 2 × 10⁵ cycles to rupture as well as specific features of transition from quasistatic to fatigue fracture associated with the variation in the mode of deformation of metals. It is shown that under conditions of repeated stress-controlled loading in a wide range of low temperatures, the cyclic creep processes are the predominant ones determining life of metals over a wide range of cycles, and when calculating life under these conditions it is necessary to take into account the rate of these processes irrespective of the type of fracture: quasistatic or fatigue. An equation was obtained for calculating life under low-cycle loading conditions which takes into account the interrelationship between the life, the accumulated plastic strain, steady-state cyclic creep rate and the resistance of the material to cyclic creep.     

MIXED MODE FATIGUE CRACK THRESHOLDS FROM BENDING AND CYCLIC TORSION OF A PLATE

The effects of plate thickness and crack shape on the threshold condition for crack propagation under cyclic bending or torsion, was studied using a through or a surface pre-crack in a carbon steel plate of various thicknesses. It was concluded that the magnitude of the frictional stress acting on the crack surface at the propagation threshold under cyclic torsion was significantly affected by the plate thickness. An estimation method for the threshold condition for crack propagation under mixed-mode loading for a particular plate thickness was proposed on the basis of the strain energy density criterion.     

A GENERALIZED FATIGUE LIMIT CRITERION AND A UNIFIED THEORY OF LOW-CYCLE FATIGUE DAMAGE

Abstract— A generalized fatigue limit criterion for multiaxial stress state conditions of isotropic materials is presented. This criterion includes four material parameters and uses two invariants of stress amplitudes and furthermore two invariants of mean stresses. It is shown that the fatigue criteria of Sines and Crossland are particular cases of the formulated criterion. Practical recommendations for the use of different fatigue limit criteria are established. Theoretical predictions are compared with experimental data. Finally a continuum damage mechanics theory for low cycle fatigue of isotropic materials is proposed. This theory describes simultaneously the influence of the stress amplitude and the mean stress on the fatigue damage suffered by materials. The proposed theory is based on four material parameters. Special damage theories with a smaller number of material parameters are obtained. Practical recommendations for the use of these fatigue damage theories are presented.     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.     

用铝液抽氧法测量氧化锆的电子特征氧分压

用从铝液和银液中抽氧的库仑滴定法测定了自制氧化钇稳定的氧化锆固体电解质的电子特征氧化压力Pθ，结果表明，900度时电子特征氧分压的范围是10^-26-10^-28kPa，自制氧化锆电解质可用于渗碳气氛氧浓度的测量而不必考虑电子电导对浓差电势的影响。用铝液能实现低温下电子 特征分压的测量。     

INFLUENCE OF SURFACE DEFORMATION AND ELECTROCHEMICAL VARIABLES ON CORROSION AND CORROSION FATIGUE CRACK DEVELOPMENT

This paper presents experimental data for a 0.2%C steel/artificial seawater system showing the influence of shear loading on corrosion response, via measurements of electrochemical variables, e.g. anodic/cathodic Tafel slopes and polarization resistance. Based on the results of these tests, several corrosion fatigue tests were conducted at different stress levels under potentiostatic control. Analysis of the results shows there to be a dependence of corrosion rate on the ratio of applied/yield strain and test frequency. In addition, the corrosion current associated with corrosion fatigue (CF) damage appears to be dependent upon the crack size, which in turn shows a relationship with fatigue crack growth rate. This paper sets out to determine the influence of stress on electrochemical parameters, i.e. free corrosion potential, E_corr , polarization resistance, R_p , anodic, τ_a and cathodic, τ_c Tafel constants. Based upon these results, it is found that a simple linear relationship between stress and corrosion damage does not exist. Furthermore, analysis of the corrosion current fluctuations during corrosion fatigue crack growth shows a minimum current coincident with the point at which a crack is growing at its slowest rate.     

IN SITU SCANNING ELECTRON MICROSCOPE OBSERVATIONS OF FATIGUE IN MAGNESIA-PARTIALLY-STABILIZED ZIRCONIA

Abstract— The response of magnesia-partially-stabilized zirconia to cyclic loading was investigated using a purpose-built loading stage, designed to operate in the vacuum chamber of a scanning electron microscope. Crack tip/microstructural interactions, along with fatigue crack growth rates were studied and recorded photographically and via a video cassette recorder. For the experimental conditions examined, crack bridging, deflection and closure are discussed as possible mechanisms of fatigue crack growth resistance.     

FATIGUE PROPAGATION OF SURFACE FLAWS IN ROUND BARS: A THREE-PARAMETER THEORETICAL MODEL

A three-parameter fracture mechanics model is proposed to theoretically analyse the propagation of an elliptical-arc part-through flaw in a round bar subjected to constant cyclic amplitude axial or bending loads. The edge flaw presents an aspect ratio α=a_el/b_el (a_el, b_el= ellipse semi-axes) and a relative crack depth ζ=a/D, where a and D are the depth of the deepest point on the crack front and the bar diameter, respectively. Additionally a parameter s=a_el/a (ellipse shifting) defines the distance of the ellipse centre from the bar circumference. The surface flaw growth occurs according to preferred patterns which tend to converge to an inclined asymptotic plane in the diagram of α against s and ζ.