A7N01S-T5铝合金焊接热影响区腐蚀疲劳裂纹萌生行为研究

以高速列车常用的A7N01S-T5铝合金型材焊接热影响区为研究对象,分析析出相的SEM组织及化学成分,采用超景深显微镜观察不同时间节点腐蚀疲劳裂纹萌生过程,并统计分析点蚀坑的面积及最大径长。结果表明,热影响区的腐蚀面积率、腐蚀速率、蚀坑最大径长均随着试验时间的增加而增加,腐蚀坑主要沿着挤压方向分布的大颗粒、富含Fe的二相粒子周围萌生,在交变应力的促进作用下,热影响区腐蚀疲劳裂纹在腐蚀坑最大径向方向沿着挤压方向萌生、扩展。     

点蚀坑的形貌与腐蚀疲劳裂纹萌生

将预产生点蚀的1Cr18Ni9Ti疲劳试样进行长期(N>1×10~6cyc.)疲劳试验。其中一些在6％FeCl_3水溶液中进行,另一些在空气中进行。用定量金相方法逐个测量点蚀坑的直径和深度。发现点蚀坑的深径比和深度都与裂纹萌生有关,且在不锈钢点蚀坑的裂纹萌生过程中力学因素起主导作用而与碳钢完全不同。通过蚀坑底部的扫描电镜观察,分别确定了不锈钢和碳钢点蚀坑中裂纹萌生的初始位置,并依此讨论了腐蚀疲劳裂纹萌生机理。     

A7N01S-T5铝合金焊缝金属腐蚀疲劳裂纹萌生行为

以高速列车常用的A7N01S-T5铝合金型材ER5356焊缝金属为研究对象,采用扫描电镜观察不同时间节点焊缝部位腐蚀疲劳裂纹的萌生过程,并分析第二相在腐蚀疲劳裂纹萌生过程中的变化。结果表明,点蚀坑在第二相周围优先萌生,点蚀机理为阳极溶解,在交变应力的促进作用下,焊缝区腐蚀疲劳"短裂纹"沿着点蚀坑周围萌生。     

30CrNi2MoV钢的腐蚀疲劳裂纹萌生与扩展行为研究

通过不同循环应力和不同浓度的盐酸腐蚀液的疲劳腐蚀实验,对30CrNi2MoV钢腐蚀疲劳裂纹萌生与扩展行为进行研究。结果表明:30CrNi2MoV钢试样腐蚀疲劳裂纹起源于试样表面的点蚀坑。在循环应力作用下,点蚀坑由于应力集中,萌生裂纹,在应力和盐酸的共同作用下,裂纹不断加深、增多,直至发生断裂。随着循环应力的增大,30CrNi2MoV钢的疲劳寿命明显下降。随着盐酸浓度的增加,试样的疲劳寿命有小幅下降。循环应力对30CrNi2MoV钢腐蚀疲劳裂纹的产生起主要作用,盐酸腐蚀液起辅助作用。     

环境对双相钢疲劳性能的影响

本文研究了自来水和3.5％NaCl水溶液分别在全浸和干湿交替状态下对热轧双相钢疲劳性能的影响.结果表明:与空气中的疲劳寿命相比,腐蚀环境有不同程度的降低作用,并以喷雾盐水影响最大.在相同腐蚀条件下,环境的影响随应力幅的降低而增强.马氏体抗疲劳断裂的能力高于铁素体.试样表面和断口的观察结果发现,腐蚀疲劳裂纹萌生与腐蚀坑密切相关,蚀坑以阳极溶解方式形成.     

基于动态贝叶斯网络的海底管道点蚀疲劳损伤失效模型研究

目的研究海底管道在点腐蚀和腐蚀疲劳双重影响下的整个破坏过程,基于动态贝叶斯网络构建系统失效模型,对海底管道系统不同疲劳寿命下的失效概率进行预测。方法将点蚀疲劳损伤过程分为腐蚀点成核、腐蚀坑增长、短裂纹扩展和长裂纹扩展四个阶段,采用蒙特卡洛模拟方法对腐蚀点形成到短裂纹发生前的管道破坏过程进行分析,结合疲劳裂纹扩展的动态贝叶斯网络结构图,在充分考虑相关影响因素不确定性的基础上,为海底点蚀管道系统提出一种创新性的概率分析方法,对点蚀管道疲劳寿命的失效概率进行科学预测。结果结合实例分析,通过蒙特卡洛模拟方法,求解得出腐蚀坑增长转变为短裂纹扩展状态的临界裂纹尺寸为0.8mm。采用动态贝叶斯网络分析方法,对未经受维修保养的点蚀管道进行疲劳寿命预测,当管道运行到第35年时将会面临失效风险。结论所构建的模型可以对海底点蚀管道腐蚀疲劳寿命失效概率进行合理预测,通过观测相关影响参数的变化,及时更新预测结果,有助于为海底管道系统制定有效的维修策略。     

316L不锈钢在模拟人体液内腐蚀疲劳裂纹萌生和扩展过程中应力的作用

探讨应力在316L不锈钢腐蚀疲劳过程中的作用。应力降低了不锈钢的点蚀电位,促进了腐蚀疲劳裂纹的萌生和扩展。初始疲劳裂纹在点蚀孔中的晶界上萌生,在应力的作用下,裂纹由沿晶变为穿晶扩展,并在断口上产生二次裂纹和疲劳辉纹。     

316L不锈钢在模拟人体液内腐蚀疲劳裂纹萌生和扩展过程中…

探讨应力在３１６Ｌ不锈钢腐蚀疲劳过程中的作用，应力降低了不锈钢的点蚀电位，促进了腐蚀疲劳裂纹的萌生和扩展，初始疲劳裂纹在点蚀孔中的晶界上萌生，在应力的作用下，裂纹由沿晶变为穿晶扩展，并在断口上产生二次裂纹和疲劳辉纹。     

高速列车车轴用EA4T钢腐蚀疲劳性能研究

目的 研究腐蚀环境中EA4T车轴钢疲劳性能,为车轴的腐蚀检测和使用寿命评估提供依据.方法 采用旋转弯曲疲劳试验机,在人造雨水模拟的腐蚀环境和空气环境中,对EA4T车轴钢试样进行疲劳试验,以获得不同环境下试样的疲劳S-N曲线、表面损伤以及裂纹扩展规律.然后对扩展裂纹进行概率统计,通过扫描电镜对疲劳失效的断口进行观察,并分析对比不同环境中裂纹扩展门槛值的变化.结果 空气环境中,试样的疲劳极限为355 MPa,而在腐蚀环境中,试样不存在疲劳极限,107循环周次对应的疲劳强度降低到245 MPa,相比空气环境中降低了31％.Gumbel分布统计与Weibull双参数分布统计相比,更适合描述EA4T车轴钢试样表面腐蚀裂纹长度随加载次数的变化.腐蚀环境中,疲劳裂纹萌生于表面腐蚀坑,并存在多个裂纹源.腐蚀环境显著降低了试样裂纹扩展门槛值,空气环境下,该值为6.29 MPa·m1/2,腐蚀环境下降低到4.1 MPa·m1/2.结论 腐蚀环境降低EA4T钢疲劳寿命的主要原因是,腐蚀环境降低了裂纹扩展门槛值,加快了裂纹萌生以及短裂纹扩展.而当裂纹达到一定长度时,腐蚀环境对裂纹扩展几乎没有影响.     

航空装备铝合金结构材料点蚀失效行为

对航空装备铝合金结构材料腐蚀疲劳行为中点蚀及点蚀萌生裂纹2个关键环节的研究进行综述。介绍了点蚀萌生机理、点蚀行为影响因素、点蚀萌生裂纹行为的研究方法，以及点蚀萌生裂纹行为机制等方面的研究结论。研究认为:航空装备铝合金点蚀的本质是受环境条件、结构微观因素、载体条件等多种因素影响的随机性电化学行为，点蚀损伤可通过其几何尺寸、点蚀率、失重率等多种指标进行描述；点蚀损伤在疲劳载荷作用下萌生裂纹，该行为可通过原位观测、断口分析等方式进行研究，并受点蚀形貌、载荷条件、材料微观结构等因素影响。在此基础上提出后续研究趋势:研究对象聚焦在新型航空铝合金材料上，点蚀试验方式注重模拟装备服役环境，研究内容则应深化点蚀萌生裂纹行为的量化分析。     

EVOLUTION OF STATISTICAL DISTRIBUTION OF PIT GEOMETRIC PARAMETERS UNDER FATIGUE LOADING

The pits and pit-associated fatigue crack initiation of the offshore structure steel A537cLlhave been investigated under salt water spray condition.The tests were interrupted on sched-ule to make plastic replica of corroded specimen surface on which many randomly distributedpits can be found.The development of the geometric parameters of pits was obtained and thestatistical distributions of these parameters were determined.Instead of depth,the ratio ofdepth to width is the most dominant parameter in relation to crack initiation.Also,the pittingwas enhanced when low frequency or high stress range was applied and although stress rangehas no effect on the nucleation of pits,it can accelerate the initiation of cracks from pits.     

CORROSION FATIGUE CRACK INITIATION BEHAVIOR OF 316L STAINLESS STEEL IN HANK’S SOLUTION

ＣＯＲＲＯＳＩＯＮＦＡＴＩＧＵＥＣＲＡＣＫＩＮＩＴＩＡＴＩＯＮＢＥＨＡＶＩＯＲＯＦ３１６ＬＳＴＡＩＮＬＥＳＳＳＴＥＥＬＩＮＨＡＮＫ’ＳＳＯＬＵＴＩＯＮ￥Ｊ．Ｈ．Ｘｉｅ;Ｙ．Ｓ．Ｗｕ;Ｊ．Ｑ．Ｈｅ;Ｘ．Ｚ．ＹａｎｇａｎｄＲ．Ｚ．Ｚｈｕ（Ｄｅｐａｒｔｍｅ...     

X80管线钢在近中性pH溶液中腐蚀行为研究

目的研究X80管线钢在近中性p H溶液中的腐蚀与应力腐蚀裂纹萌生行为。方法采用电化学实验和浸泡实验研究X80管线钢在近中性p H溶液中的腐蚀行为,采用慢应变速率拉伸实验研究X80管线钢在近中性p H溶液中,在自腐蚀电位和外加电位下的应力腐蚀裂纹萌生行为。结果 X80管线钢在近中性p H溶液中的极化曲线只有活化区,没有钝化区,其自腐蚀电位约为-750 m V,浸泡195天后,试样表面没有氧化膜出现,但是观察到点蚀坑。在自腐蚀电位下,X80管线钢试验表面有大量的应力腐蚀裂纹;在-500 m V阳极外加电位下,X80管线钢试验表面几乎没有观察到应力腐蚀裂纹;在-850 m V阴极外加电位下,X80管线钢试验表面的应力腐蚀裂纹很少,但是随着外加阴极电位负移到-1300 m V时,X80管线钢试验表面的应力腐蚀裂纹增多。结论 X80管线钢在近中性p H溶液中发生均匀腐蚀,但是夹杂物剥落能在X80管线钢表面形成点蚀坑。在近中性p H溶液中,在自腐蚀电位下,X80管线钢应力腐蚀裂纹萌生敏感性最强;外加阴极电位抑制应力腐蚀裂纹萌生,但是随着外加阴极电位的负移,应力腐蚀裂纹萌生敏感性增强;外加阳极电位下,由于均匀腐蚀的作用,应力腐蚀裂纹萌生敏感性较弱。     

Crack Initiation Life of Materials Under Combined Pitting Corrosion and Cyclic Loading

Pitting corrosion triggered damage is responsible for the degradation of many metallic materials affecting structural integrity. As pitting and crack initiation processes govern the overall life of such structures and components, particularly at nominal cyclic stresses, there is a need to develop simple models to estimate crack initiation life of materials. This paper presents a simple deterministic model that considers the effect of cyclic stressing under pitting corrosion conditions. The developed model is validated on an aluminum alloy 2024-T3, and 12% Cr stainless steel used in aircraft and steam turbines, respectively. The predicted critical pit depth values are in fair agreement with the limited experimental data available in the literature. The model indicates that at high stresses, the crack initiation can occur very rapidly even from relatively small pits. The crack initiation life predictions when compared with the available experimental data, suggest a probable stress-level dependency with regard to the form and extent of the influence of cyclic stresses on pit growth and subsequent crack formation.     

螺栓底座裂纹分析

本文分析了飞机前襟翼作动筒连接螺栓底座表面裂纹的形成原因。对底座表面裂纹形貌以及渗层组织进行了观察,并且对螺栓的化学成分以及硬度进行了测定。结果表明,螺栓底座的表面裂纹为渗碳层裂纹,该裂纹性质为疲劳裂纹,裂纹起始于微动腐蚀形成的表面点蚀坑。采用显微硬度法检查渗碳层深度为1．2mm,超过了渗碳层深度的设计要求（0．6—1mm）。分析认为,螺栓底座表面裂纹的形成与渗碳层深度超标有关,而微动磨损产生的点蚀坑促进了渗碳层疲劳裂纹的萌生。     

Behavior of 316L stainless steel containing corrosion pits under cyclic loading

The environmental performance of 316L grade stainless steel, in the form of tensile specimens containing a single corrosion pit with various aspect ratios, under cyclic loading in aerated chloride solutions is investigated in this study. Results from environmental tests were compared and contrasted with those obtained using finite element analysis (FEA). Fractography of the failed specimens obtained from experiments revealed that fatigue crack initiation took place at the base of the shallow pit. The crack initiation shifted towards the shoulder and the mouth of the pit for pits of increasing depth. This process is well predicted by FEA, as the strain contour maps show that strain is the highest around the centric strip of the pit. However, for shallow pits, local strain is uniformly distributed around that strip but begins to concentrate more towards the shoulder and the mouth region for increasingly deep pits.     

The effects of film formation and mechanical factors on the initiation of stress corrosion cracking of unalloyed steels in carbonate solutions

Investigations into the intergranular stress corrosion cracking (SCC) of unalloyed steel St 36 in carbonate-bicarbonate solutions have shown that SCC susceptibility is limited to the potential range of magnetite formation for both constant load and constant strain rate tests. Experiments to determine the creep behaviour of the steel in the corrosive solution show that creep rates corresponding to the critical strain rates in the constant strain rate test occur during the crack initiation state after loading of the tensile specimen with a constant crack initiating load. These experimental findings establish, therefore, that critical crack initiating strain rates of the unalloyed steel are always present during the initial stage for SCC regardless of the test method used. The cause of crack initiation at grain boundaries can be attributed to the fact that the magnetite layers feature faults and grooves in the oxide along the grain boundaries in the entire critical potential range. These oxide grooves, which are also observed on unstressed steel, cause a notch-like localized corrosion attack. The depth and crack tip radius of the resultant grain boundary notches appear to be sufficient to initiate propagating cracks under the single action of predominately pure stresses without macroscopic deformation of the steel. This does not necessarily mean, however, that a purely stress induced corrosion process is involved, for it is not out of question that the concentration of stress in the base of grain boundary notches is sufficient to cause localized deformation processes required for crack initiation.     

板状构件的疲劳裂纹形成及疲劳寿命预估

采用虚功原理分析了大范围损伤条件下平面应力板状构件的疲劳裂纹形成及疲劳寿命预估问题。导出了平面应力板状构件在大范围损伤条件下的疲劳裂纹形成及疲劳寿命预估的封闭解答。该方法力学概念清晰、明确,可广泛用于分析各类板状构件在大范围损伤条件下的疲劳裂纹形成及疲劳寿命预估问题。为预估板状构件的疲劳裂纹形成及疲劳寿命提供了新思路和新方法。     

Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454- H32 was studied under laboratory air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long- life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long- life (low- stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long- cycle region.