基于直流电位法的疲劳裂纹长度测量的技术及应用

航空发动机设计用材料的疲劳裂纹扩展速率测量是其性能测试的重要环节,而金属材料在600℃以上温度时合金表面由于氧化或者高温热辐射,无法使用基于光学原理的测量方法测量裂纹长度,所以需要使用自动测量方法——直流电位法。在室温条下使用铝合金对电位法系统进行测试,通过直流电位法与目测法测量的裂纹长度进行对比,发现使用参考试样测量的裂纹长度测量要好于没有使用参考试样的测量结果,说明参考试样可以很好地消除温度波动对测量结果的影响;同时在高温实验中,使用勾线法标定电位法测量的裂纹长度数据。结果表明,电位法测量的裂纹长度与勾线法测量平均裂纹长度之间的平均相对误差为1.79%,说明在高温实验环境下,试样出现表面氧化,可以使用直流电位法进行裂纹长度的测量。应用电位法技术完成650～800℃实验环境下,获得钛合金材料负应力比疲劳裂纹扩展试验数据。     

直流电位法检测高温合金的疲劳裂纹扩展性能

介绍了在高温下使用直流电位法测定疲劳裂纹长度的基本原理、试验方法及所需配置的仪器,并对有关影响因素进行了分析;并利用Johnson的分析型关系式,研究了一种适用于自动测定材料高温疲劳裂纹长度的直流电位法,该方法已成功应用于高温合金疲劳裂纹扩展试验,其测得的疲劳裂纹扩展速率da/dN-ΔK数据与长焦聚显微镜法测得的数据完全一致。     

高温下直流电位法测量裂纹长度的应用

介绍了用直流电位法测量疲劳裂纹长度方法的基本原理、实现方法及所需配置的仪器,并对有关影响因素作了分析。该测量系统主要用于高温下的各种材料在恒幅载荷下的疲劳裂纹扩展和门槛值试验中的疲劳裂纹长度的测量,可提高测量疲劳裂纹长度的自动化水平。     

高温合金热疲劳实验机的研制及实验研究

介绍了用于测试高温合金热疲劳性能的热疲劳实验机。该实验机主要用于定性地测量合金在某一条件下循环规定的次数后热疲劳裂纹扩展的长度,或热疲劳裂纹在扩展一定长度下循环的次数。该实验机的优点是:完全自动化、运行灵活、定位准确。通过对DZ951定向凝固镍基高温合金进行实验研究,表明该热疲劳实验机具有较好的稳定性和可靠性。DZ951合金主要通过铝和铬元素氧化,产生氧化孔洞。热疲劳裂纹通过孔洞相互连接萌生和扩展。合金具有两条热疲劳裂纹,且扩展具有一定方向,与枝晶生长方向成45°扩展。     

利用原位观察技术测定TC4钛合金的疲劳裂纹扩展门槛值

针对目前金属材料疲劳裂纹扩展门槛值测定中存在的测定周期长、试样尺寸较大以及单位周次裂纹扩展长度测量误差较大等问题,采用扫描电镜原位观察技术测定了TC4钛合金的疲劳裂纹扩展门槛值,即通过扫描电镜对裂纹扩展长度的实时精确测定实现快速获得材料的疲劳裂纹扩展门槛值。结果表明：利用该方法测得的TC4钛合金的疲劳裂纹扩展门槛值与已报道的数据基本一致,并且具有快速简便等优点,表明该方法可以用来测定金属材料的疲劳裂纹扩展门槛值。     

用直流电压降法研究高温下GH3230合金的疲劳裂纹扩展行为

采用直流电压降法对GH3230合金进行了高温下的疲劳裂纹扩展试验,分析了温度及应力强度因子对GH3230合金疲劳裂纹扩展速率的影响,并利用扫描电子显微镜对断口进行分析。结果表明：在相同的应力强度因子下,随着温度的升高,合金的裂纹扩展速率增大;温度从750℃升高到850℃时,裂纹扩展速率明显增大,从850℃升高到950℃时,小应力强度因子下的裂纹扩展速率相差不大,随着应力强度因子的增大和温度的升高,裂纹扩展速率的差距增大;观察断口表面可知,在裂纹扩展区和瞬断区,断口表面呈现典型的疲劳辉纹和韧窝特征,随着温度的升高,断口表面的氧化物颗粒增多,裂纹扩展区的疲劳辉纹不明显。     

不同温度环境下CrMoW转子钢超高周疲劳行为研究

利用旋转弯曲疲劳试验机,研究CrMoW转子钢在常温与600℃条件下的超高周疲劳特性。对试验数据采用三参数模型拟合,用扫描电子显微镜(SEM)对疲劳断口进行分析。研究结果发现,600℃下的S-N曲线呈现直线下降的趋势,不存在传统意义上的疲劳极限。高温会加速试样的氧化,促进裂纹的萌生与扩展,降低材料的疲劳寿命。断口分析表明:疲劳裂纹主要萌生于试样表面,很少发现裂纹萌生于内部的情形。在600℃下,裂纹萌生区普遍发现有夹杂物。对试验前后转子钢硬度值进行测量,没有发生明显变化。     

挤压SiCw／LD2复合材料的疲劳裂纹扩展

本文测量了挤压19％(Vol)SiCw/LD2复合材料疲劳裂纹扩展的门槛值ΔK_(th)和扩展速率da/dN。并在扫描电镜下观察了疲劳断口形貌。结果表明,复合材料在门槛值附近和中速扩展区的疲劳裂纹扩展抗力高于基体材料LD2合金。复合材料纵向试样于170℃时效后的疲劳裂纹扩展抗力高于150℃时效,疲劳裂纹扩展扩展过程包括不在一个面内的微裂纹长大和联接这些微裂纹的(Ⅰ+Ⅱ)复合型裂纹的扩展。与裂纹相垂直的晶须有效地阻碍了微裂纹的长大。     

COD法在检测疲劳裂纹长度中的应用

本文描述了在计算机控制下,利用柔度技术在疲劳门槛值和裂纹扩展速率试验中,CT试样上自动检测疲劳裂纹长度的方法。对用目测法和COD法所测疲劳裂纹长度的试验结果进行了分析比较,讨论了引起误差的主要原因。     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

燃机压气机叶片热强钛合金TC11疲劳行为研究

在不同温度对TC11钛合金进行旋转弯曲疲劳性能测试及组织形貌观察,研究了温度对合金疲劳行为的影响。结果表明,在150—250℃,合金的旋转弯曲疲劳强度随着温度的升高而降低。随着温度的升高,合金的屈服与抗拉强度降低,等轴组织的含量也降低,这是合金疲劳强度降低的主要原因。高温下合金的疲劳裂纹从表面萌生,未发现内部萌生的特征。     

Multiaxial fatigue — An overview

Many engineering components are subjected to multiaxial type of fatigue loading and the fatigue life relation based on uniaxial testing needs modifications before it could be used for multiaxial condition. In this paper a brief review of the different approaches based on stress and strain is presented for the correlation of bulk parameters with the fatigue life and the merits and limitations are discussed.     

316L不锈钢焊接接头高温低周期疲劳显微结构变化和断裂特征

本研究对316L奥氏体不锈钢母材和焊缝分别进行高温低周疲劳试验,对试样的微观结构和裂纹扩展形貌进行观察,分析母材和焊缝在高温低周疲劳循环应力响应下的位错结构和损伤机制。结果表明,316L奥氏体不锈钢母材在试验过程中由于位错增殖和位错湮灭导致发生循环硬化和循环稳定,在焊缝中由于位错湮灭导致发生循环软化。母材和焊缝在连续低周疲劳试验中裂纹主要以穿晶方式扩展,焊接接头处孔洞的连接是最终导致焊接接头疲劳断裂的主要机制。     

Effect of temperature on high-cycle fatigue and very high cycle fatigue behaviours of a low-strength Cr–Ni–Mo–V steel welded joint

Axially push–pull fatigue tests of a low-strength Cr–Ni–Mo–V steel welded joint were conducted up to very high cycle fatigue regime at room temperature and 370 °C. The S–N curve at room temperature shows a duplex shape, while the S–N curve at 370 °C is continuously decreasing with lower fatigue strength. The welds at 370 °C undergoes dynamic strain ageing and has an enhanced load–defects interaction, leading to equal distribution of failures among different parts of the welds. The Z parameter model, with micro-defect location incorporated, having sound physical representation, is life-controlling of the welds at high temperature.     

3D-C/SiC复合材料在室温和1300℃的拉-拉疲劳行为

采用应力比为0.1，频率为60Hz的正弦波在室温和1300℃，10^-4Pa真空中对3D－C／SiC复合材料进行了拉－拉疲劳试验。同时用SEM分析了疲劳断口特征。结果表明：若取循环基数为10^6，1300℃疲劳极限为285MPa，约为抗拉强度的94％；室温疲劳极限为235MPa，约为抗拉强度的85％。1300℃疲劳断口的纤维拔出长度比室温短。疲劳损伤主要起源于纤维束编织交叉部位，随着疲劳循环次数的增加，纤维束周围基体的损伤也不断加剧。     

3D-Cf/SiC复合材料在1500℃的拉-拉疲劳行为

在1500℃,10<'-4>Pa真空中,采用应力比0.1和0.5,频率60Hz和20Hz的正弦波对三维编织炭纤维增强碳化硅基复合材料(3D-C<,f>/SiC)进行了拉-拉疲劳实验,利用SEM和HRTEM分别观察了疲劳试样的断口形貌和热解炭界面相的微结构.结果表明:若取循环基数为10<'6>次,当应力比为0.1时,20...     

Effect of notch on fatigue behaviour of a directionally solidified superalloy at high temperature

The effect of notch types and stress concentration factors (K_t) on low cycle fatigue life and cracking of the DZ125 directionally solidified superalloy has been experimentally investigated. Single‐edge notched specimens with V and U type geometries were tested at 850 °C with stress ratio R = 0.1. High temperature in situ optical method was used to observe crack initiation and short crack propagation. Scanning electron microscope observation of fracture was used to analyse the failure mechanism. The results reveal that fatigue resistance decreases with K_t increasing from 1.76 to 4.35. The ratcheting is found to be affected by both K_t and the nominal stress from the displacement–force curve. In situ observations indicate that the cracking does not occur at the notch apex but at the location where the max principal stress or Hill's stress is the highest. According to the scanning electron microscope observations, the failure of the notched specimens strongly depends on the anisotropy microstructures.     

核电主管道不锈钢在高温高压水环境下的疲劳裂纹萌生行为

利用腐蚀疲劳测试系统研究了高温高压水环境下两种压水堆核电站一回路主管道用不锈钢的腐蚀疲劳裂纹萌生行为。结果表明,316LN奥氏体不锈钢的裂纹主要在材料表面的驻留滑移带处萌生,少量裂纹在两簇驻留滑移带交界的亚晶界面处。含有少量铁素体的Z3CN20.09M奥氏体不锈钢的疲劳裂纹依次在试样表面的驻留滑移带处、相界处和点蚀坑处萌生,但主要是在驻留滑移带处。通过研究高温高压水环境下氧化膜的组成和腐蚀疲劳试样横截面的形貌,分析了疲劳裂纹在滑移带处萌生的机理。最后对比分析两种不锈钢裂纹萌生机制的异同,并讨论了铁素体对材料腐蚀疲劳性能的影响。     

Microstructures of modified RR2072 single crystal superalloys and their effects on LCF response at 950 8°C

Effects of microstructural modifications, that are a consequence of adding minor grain boundary strengthening elements (C, Hf), on the fatigue response of an experimental single crystal superalloy have been studied. Investigations show that in the modified alloys, as a consequence of the casting process, the population of pores is reduced, but the average pore size increases and larger pores occur close to the fatigue sample surface. Such porosity changes in the modified alloys are an important contributor to their low cycle fatigue (LCF) properties. At the high stress range of LCF tests undertaken, the growth of cracks initiates from pores and their location in the modified alloys are responsible for decreased fatigue lives. At lower stress ranges, crack initiation consumes a considerable proportion of fatigue life. Since crack initiation involves strain localisation at or near the specimen surface where there is interaction with the environment, the population of pores near the specimen surface and the increased length of the tests plays a significant role in governing fatigue lives. The modified alloys benefit from their lower density of pores in the vicinity of the free surface, and tend to have comparable fatigue lives to that of the base alloy at the lower stress level.     

Recent developments in ultrasonic fatigue

The recently increased interest in very high cycle fatigue properties of materials has led to extended use and further development of the ultrasonic fatigue testing technique. Specimens are stimulated to resonance vibrations at ultrasonic frequency, where the high frequency allows collecting lifetime data of up to 10¹⁰ cycles and measuring crack propagation rates down to 10^?12 m per cycle within reasonable testing times. New capabilities and methods of ultrasonic testing and outstanding results obtained since the year 1999 are reviewed. Ultrasonic tests at load ratios other than R = ?1, variable amplitude tests, cyclic torsion tests and methods for in situ observation of fatigue damage are described. Advances in testing at very high temperatures or in corrosive environments and experiments with other than bulk metallic materials are summarized. Fundamental studies with copper and duplex steel became possible and allowed new insights into the process of very high cycle fatigue damage. Higher cyclic strength of mild steels measured at ultrasonic frequency because of plastic strain rate effects are described. High‐strength steels and high‐alloy steels are less prone to frequency influences. Environmental effects that can lead to prolonged lifetimes in some aluminium alloys and possible frequency effects in titanium and nickel and their alloys are reviewed.