Theoretical Estimation of Fatigue Crack Initiation Life for Metallic Materials

The main purpose of this paper is to theoretically estimate fatigue crack initiation life of metallic materials using the three-parameter model. For this purpose, correlations between the three parameters and conventional mechanical properties are established. The three parameters are fatigue ductility coefficient, fatigue ductility exponent, and theoretical strain endurance limit. Based on the correlations, the three parameters can be theoretically estimated, and thus the fatigue crack initiation life of metallic materials can be predicted in the absence of fatigue test data.     

不同润滑条件下齿轮材料的裂纹萌生及扩展研究

斜齿轮常选用黄铜/钢摩擦副,但采用其它替代材料能够获得更好的抗磨损性能。对GJS700球墨铸铁和42Cr Mo4V调质钢在润滑条件下进行了滑动-滚动测试。结果表明,在调质钢和铸铁试样中点蚀破坏的演变并不相同。润滑条件和材料显微组织对裂纹的萌生和扩展具有显著影响,它们影响导致形成点蚀和剥落坑的裂纹倾向和分叉。     

Crystallographic Analysis of Fatigue Crack Initiation Behavior in Coarse-Grained Magnesium Alloy Under Tension-Tension Loading Cycles

Plane bending fatigue tests are conducted to investigate fatigue crack initiation mechanisms in coarse-grained magnesium alloy, AZ31, under the stress ratios R = ?1 and 0.1. The initial crystallographic structures are analyzed by an electron backscatter diffraction method. The slip or twin operation during fatigue tests is identified from the line angle analyses based on Euler angles of the grains. Under the stress ratio R = ?1, relatively thick tension twin bands are formed in coarse grains. Subsequently, compression twin or secondary pyramidal slip operates within the tension twin band, resulting in the fatigue crack initiation. On the other hand, under R = 0.1 with tension-tension loading cycles, twin bands are formed on the specimen surface, but the angles of those bands do not correspond to tension twins. Misorientation analyses of c-axes in the matrix grain and twin band reveal that double twins are activated. Under R = 0.1, fatigue crack initiates along the double twin boundaries. The different manners of fatigue crack initiation at R = ?1 and 0.1 are related to the asymmetricity of twining under tension and compression loadings. The fatigue strengths under different stress ratios cannot be estimated by the modified Goodman diagram due to the effect of stress ratio on crack initiation mechanisms.     

Initiation and Inhibition of Pitting Corrosion in Alkali Solutions under potentiostatic polarization conditions

The variation of the critical pitting potential of a zincalloy was studied in aerated NaOH solutions as a function of the concentration of the aggressive ions, Cl^?, Br^? and I^?. Curves with segmented nature were obtained when E_pitting was plotted versus logarithm of the halogen ion concentrations. Initiation of pitting corrosion was discussed on the basis of formation of complex halo-compounds with the oxides/hydroxides that constitute the passive surface film. Addition of chromate, phosphate and carbonate ions to the halogen-containing solutions causes the shift of the critical pitting potential in the noble direction, accounting for increased resistance to pitting attack. Nitrite-ion additions contribute with the halogen ions in the destruction of the passive film. Sufficient concentrations of the carbonate ions cause complete inhibition of pitting corrosion.     

Interfacial Debonding of FRC Under Cyclic Loading

With the increasingly use of FRC (fiber-reinforced composite) in urban lifelines, mechanical properties investigation is very important for disaster resistance, especially the investigation of fatigue properties. Based on the shear-lag model, an usual composite model under cyclic loading is established. According to the Paris formula, the relationship between interracial fatigue parameters and the number of cycles is obtained under the cyclic loading. Interracial fatigue properties of this model and the growth of the interracial fatigue crack are analyzed. And the Poisson ratio is considered also.     

Effect of Acidic Gelatinous Materials on Pitting Corrosion of Aluminium Hollow-Ware

Abstract

It has been reported that acid gelatinous foodstuffs can attack aluminium very rapidly.This has been confirmed for synthetic solutions simulating these foods. By changing onevariable at a time it has been shown that the corrosion is strongly affected by the type of acid present, the pH of the solution and the gelatine concentration. The results are discussed but it is not possible to postulate a corrosion mechanism.     

The Effect of Weld Profile and Geometries of Butt Weld Joints on Fatigue Life Under Cyclic Tensile Loading

The fatigue life of welded joint was calculated based on numerical integration of simple Paris’ law and a reliable solution of the stress intensity factor (SIF). The initial crack length (a _i) was assumed to be equal to 0.1 mm in case of weld toe. This length was satisfactory for different butt joints geometries. The comparisons with the available data from standards and literature were demonstrated. It was shown numerically that the machining of weld reinforcements will increase the fatigue life. The increase of plate thickness decreases the fatigue strength (FAT) and the number of cycles to failure when using the proportional scaling of crack length. The validation processes of the current calculations have been shown. Therefore, it can be concluded that it will prevent the unnecessary waste of time consumed to carry out the experiments.     

A New Three-Parameter Model for Predicting Fatigue Crack Initiation Life

Based on the idea that the fatigue damage is caused by the cyclic damage strain, a concept of the critical damage quantity is introduced and a new three-parameter model is developed. The model contains three material performance parameters, i.e., the fatigue ductility coefficient, the fatigue ductility exponent, and the theoretical strain endurance limit. The fatigue ductility coefficient reflects the existence of the critical damage quantity. The fatigue ductility exponent shows the damage resistance ability of the material. And the theoretical strain endurance limit represents the existence of the critical cyclic strain. By using the proposed model, the fatigue crack initiation life of metallic materials can be predicted.     

穿透型疲劳裂纹扩展与铝合金局部点蚀损伤特征参数的依存性分析

铝合金的腐蚀以点蚀特征为代表,在腐蚀介质的侵袭下,其构件表面会出现大量的腐蚀坑。这些腐蚀损伤特征对于局部的应力应变产生影响,并且伴随着氢的渗入材料局部的脆性增强,对于临近裂纹的扩展会产生较大的影响。通过建立与局部点蚀损伤特征参数相关的裂纹扩展表征模型表征局部腐蚀对于临近的裂纹扩展影响程度,对于铝合金结构的腐蚀损伤容限评估具有参考价值。     

钛的熔模铸造的选择强化技术

介绍了金属材料在高速高压状态下的一些物理特性，如在强冲击载荷下的本构关系、控制方程，加载和卸载的波动特征，动态屈服效应及动态破坏特征等以及轻气炮试验装置和测量技术的应用。文章对金属材料的动态形变方式也进行了讨论。     

Cyclic Elastoplastic Performance of Aluminum 7075-T6 Under Strain- and Stress-Controlled Loading

Elastoplastic investigations of aerospace aluminum are important in the development of an understanding of the possible cyclic transient effects and their contribution to the material performance under cyclic loading. Cyclic plasticity can occur in an aerospace aluminum component or structure depending on the loading conditions and the presence of external and internal discontinuities. Therefore, it is vital that the cyclic transient effects of aerospace aluminum are recognized and understood. This study investigates experimentally the cyclic elastoplastic performance of aluminum 7075-T6 loaded in symmetric strain control, and asymmetric stress and strain control. A combination of cyclic hardening and softening was noticed from high strain amplitude symmetric strain-controlled tests and at low stress amplitude asymmetric stress-controlled tests. From asymmetric strain control results, the extent of mean stress relaxation depended on the size of the strain amplitude. Additionally, saturation of the ratcheting strain (plastic shakedown) was also found to occur during asymmetric stress control tests. The experimental results were further analyzed using published microstructure research from the past two decades to provide added explanation of the micro-mechanism contribution to the cyclic transient behavior.     

Magnetic Property Changes of CoNiCrAlY Coating Under Cyclic Oxidation and Hot Corrosion

Oxidation of Metals - In this paper, a non-destructive method for evaluation of a CoNiCrAlY coating applied to gas turbine blades has been studied. The specimens from cast Ni-base superalloy...     

加载波形对D36钢腐蚀疲劳裂纹扩展速率的影响

对标准单边缺口三点弯曲试样分别施加三角波、正弦波、方波3种波形,得到了不同波形作用下的腐蚀疲劳裂纹扩展曲线,研究了在不同电位条件下加载波形对海水中D36钢腐蚀疲劳裂纹扩展速率(da/d N)的影响。结果表明:无阴极保护时,三角波和正弦波作用下试样的疲劳裂纹扩展速率相当,并小于方波作用下的疲劳裂纹扩展速率,且方波的加速作用体现在低应力强度因子幅度时;在阴极保护条件下,这种现象更为明显;而在阳极极化条件下,方波对试件的疲劳裂纹扩展的加速作用减弱。     

加载波形对X65钢腐蚀疲劳裂纹萌生及扩展的影响

采用疲劳实验研究了不同加载波形下X65钢在空气和海水中的疲劳行为。结合SEM结果,对疲劳断口和次生裂纹进行了观察。结果表明,在空气和海水中,正锯齿波加载下X65钢的疲劳寿命最大,三角波次之,正弦波最短。与在空气中相比,X65钢在海水中的疲劳寿命显著降低。正弦波与三角波加载时应力上升时间较短,有利于位错开动,加快裂纹萌生。其中正弦波在σ_max的保载时间最长,有利于位错滑移形成,疲劳裂纹扩展速度最快。在海水环境中,Cl^-促进了X65钢表面点蚀萌生,成为腐蚀疲劳裂纹源。当裂纹形成后,电解质进入裂纹间隙,在交变应力作用下裂纹反复张开与闭合,导致裂纹快速扩展。在海水中,当加载波形为正锯齿波时,X65钢的腐蚀疲劳裂纹的扩展机制为阳极溶解,而当加载波形为正弦波和三角波时,X65钢的腐蚀疲劳的扩展机制均为氢脆+阳极溶解混合机制,其中加载正弦波时腐蚀疲劳开裂敏感性最大。     

Effect of Vacuum Pressure on the Initiation and Propagation of Pitting Corrosion of 2205 Duplex Stainless Steel in Concentrated Seawater

Pitting initiation and stable propagation behaviors of 2205 duplex stainless steel were investigated in the hot concentrated seawater under vacuum pressures by potentiostatic polarization. Both applied potentials and vacuum pressures greatly influence the pitting corrosion. Higher potentials lead to much faster stable pitting initiation and growth rates in both static (101.3 kPa) and dynamic (28.4 kPa) solutions. The pressure reduction can also accelerate the pitting initiation rate. However, the boiling of solution can influence the pitting propagation mechanism. The dynamic actions of boiling bubbles are unfavorable to the pit propagation and result in the formation of relatively smooth pit bottom without secondary pits.     

Ti—6.5Al—2Zr—1Mo—1V钛合金简介

平板撞击载荷造成靶样品层裂过程包括孔洞成核，微裂纹生长成宏观裂纹，裂纹切变扩展导致靶局部或完全剥离。本文重点介绍了Ｔａ和Ｔａ－Ｗ合金靶在中低入射应力的平板撞击载荷下裂纹萌生和发展情况，采用金相和扫描电镱观察和分析了Ｔａ和Ｔａ－Ｗ合金靶的原始显微组织和气炮加载试验后回收靶的显微组织变化．研究了靶样品在平板冲击载荷下裂纹萌生和发展同原始晶粒尺寸及靶中Ｗ含量的关系。结果表明，随着合金元素Ｗ的添加及Ｗ含量     

划伤690TT合金在高温含氧水中应力腐蚀裂纹萌生的研究

利用划伤技术研究了690TT合金在325 ℃高温含氧硼锂水中的裂纹萌生和生长情况。试样表面和截面显微分析的结果表明,划伤沟槽底部局部萌生了典型的沿晶应力腐蚀裂纹。由于应力集中,在慢速率拉伸阶段划伤沟槽底部产生了机械裂纹,而机械裂纹成为恒载过程中690TT合金沿晶应力腐蚀裂纹萌生和生长的先导。尖端非常接近晶界或者沿着晶界的机械裂纹可继续形成沿晶应力腐蚀裂纹。690TT合金在恒载荷条件下对应力腐蚀开裂仍有一定的敏感性。     

Ni-Cr-Mo-Cu合金耐晶间腐蚀及点蚀性能研究

在Cr、Mo成分一定的情况下,通过改变铜的质量分数(1％～5％),制备新型Ni-Cr-Mo-Cu耐蚀合金.利用化学浸泡法、电化学法(极化曲线法、循环伏安法)对其耐晶间腐蚀以及耐点蚀能力进行分析研究.实验结果表明,w(cu)=1％～5％的Ni-Cr-Mo-Cu合金具有优良的耐晶间腐蚀以及点蚀性能,相对而言w(cu)=2％的镍基合金耐晶间腐蚀以及点蚀性能较差,因此说明合适的铜含量可以提高镍基合金的耐晶间腐蚀和点蚀性能;并且试验合金晶间腐蚀与点蚀的电化学行为和特征与其浸泡腐蚀的结果是吻合的.     

Recent Developments in Pitting Corrosion of Aluminium

Abstract

This paper draws attention to the practical importance of pitting. The environmental and metallurgical factors controlling pit initiation and propagation are reviewed in the light of recent researches. It is proposed that pits start because of the presence of flaws in the oxide film exposing anodic and cathodic areas and that these pits proceed because the conditions within the pit favour dissolution of the metal. The importance of copper and chloride ions in solution is stressed, as well as the lesser importance of sulphate.