DAMAGE MECHANISMS OF INCONEL 738LC UNDER CREEP-FATIGUE LOADING CONDITIONS

1.IntroductionFirststagebladesofhigh--temperaturelandbasegasturbinesaredamagedbyacombinationofcreepandfatigue.Theleadingedgeofthebladeinparticularissubjectedtoacompressivestrainholdconditionduringgasturbineoperation.Suchconditionwasknownasthesevereconditionsfornickel-basedsuperalloysandsignificantreductionofcreep-fatiguelifehasbeenreported['--'].Althoughsomelifepredictionmethodsforsuchconditionshavebeenproposed[3],theyarenotsufficienttopredictthecreep--fatiguelifeingeneralduetolackofknowledgec…     

Normalized coffin-manson plot in terms of a new life function based on stress relaxation under creep-fatigue conditions

A new life prediction function based on a model formulated in terms of stress relaxation during hold time under creep-fatigue conditions is proposed. From the idea that reduction in fatigue life with hold is due to the creep effect of stress relaxation that results in additional energy dissipation in the hysteresis loop, it is suggested that the relaxed stress range may be a creep-fatigue damage function. Creep-fatigue data from the present and other investigators are used to check the validity of the proposed life prediction equation. It is shown that the data satisfy the applicability of the life relation model. Accordingly, using this life prediction model, one may realize that all the Coffin-Manson plots at various levels of hold time in strain-controlled creep-fatigue tests can be normalized to make one straight line.     

新型马氏体耐热钢蠕变-疲劳性能与寿命预测

针对P92和G115新型马氏体耐热钢进行了应力控制下蠕变-疲劳性能研究,分析了保载时间和峰值应力对应力控制下蠕变-疲劳性能的影响规律.?结果表明,应力控制下蠕变-疲劳过程中蠕变变形和损伤主导了蠕变-疲劳寿命.?蠕变-疲劳寿命随峰值应力和保载时间的增加而下降,同时相同峰值应力下,G115钢的蠕变-疲劳寿命约是P92钢的8...     

GH720Li镍基高温合金蠕变-疲劳试验研究

对先进航空发动机涡轮盘的典型材料镍基高温合金GH720Li的蠕变-疲劳特性进行了试验研究。通过对真实涡轮盘上取样的圆棒试验件,开展了650℃不同保持时间下的GH720Li合金蠕变-疲劳试验。结果表明,650℃下保持时间对GH720Li合金的蠕变-疲劳特性影响较大,即,在较长的保持时间(30 min)下,蠕变损伤占主导地位,降低了材料的蠕变-疲劳寿命。SEM断口分析表明:随着保持时间的增加,裂纹形成区由穿晶和沿晶混合断裂向沿晶断裂转化;裂纹扩展区由穿晶断裂向穿晶、沿晶的混合模式转化。最后,基于GH720Li合金的蠕变-疲劳试验数据,分别选取载荷谱转换法和机械功密度法进行GH720Li合金的蠕变-疲劳寿命预测,并研究了2种模型的工程适用性。     

MULTIAXIAL CREEP-FATIGUE LIFE EVALUATION UNDER PROPORTIONAL LOADING

A new method was proposed for the multiaxial creep-fatigue life evaluation under proportional loadings. Because this method was derived from the strain range partitioning method with a multiaxiality factor, it was possible to consider the influence of both creep-fatigue interaction and multiaxial stress state on fatigue life. In order to predict the combined axial-torsional fatigue life the damage under combined loading was defined as linear summation of the damages under axial loading and torsional loading. Axial-torsional creep-fatigue tests were carried out using tubular specimens of 316LC austenitic stainless steel and the ferritic rotor steel. This rotor steel was developed for the permanent magnet type eddy current retarder in heavy trucks. Experimentally obtained lives of both steels were well corresponded with the lives predicted by the proposed method. It was found that the proposed method was effective in multiaxial fatigue life evaluation under proportional creep-fatigue Ioadings.     

蠕变疲劳交互作用下裂纹萌生的有限元模拟

通过结合初始应力应变场与连续损伤力学理论以及单元失效和裂纹萌生准则,构建了蠕变-疲劳交互作用下裂纹萌生的预测模型,将模型编写为UMAT耦合到ABAQUS有限元分析软件中,实现了初始无缺陷结构蠕变-疲劳交互作用下裂纹萌生的有限元模拟,并分析了影响裂纹萌生寿命的因素.通过与线性累计损伤理论对比发现,裂纹萌生位置蠕变损伤和疲劳损伤具有相互促进作用,蠕变疲劳的交互作用使裂纹萌生寿命减小;蠕变和疲劳载荷加载顺序对损伤的累积具有很大影响,模拟发现承受蠕变载荷的结构在承受后续循环载荷下总损伤值更大,裂纹萌生寿命更短.     

An investigation of the reduction in tensile strength and fatigue life of pre-corroded 7075-T6 aluminum alloy

In aging aircraft, the synergetic interaction between corrosion and fatigue has been shown to reduce the life expectancy of aluminum alloys. The objective of this study was to quantify the effects of corrosion, in terms of mass loss per unit area, on the static strength and fatigue life of 7075-T6 aluminum alloy. This was an experimental study in which test specimens were corroded in a laboratory environment. The corrosion process was accelerated by use of a corrosion cell. Test specimens were cut from flat sheets of aluminum and covered with masking material to restrict corrosion to a confined area. After testing, the fatigue life, ultimate tensile strength (UTS), and hardness of the specimens were observed to drop significantly with small amounts of corrosion. After the initial decrease, the UTS was observed to decrease linearly with increasing corrosion levels. The fatigue life of the specimens decreased in an inverse exponential fashion as mass loss per unit area increased. The hardness values of the corroded surfaces were also observed to drop. The topology of the pits and the related subsurface damage produced areas of high stress concentration resulting in the immediate reduction of UTS and fatigue life of the specimens. Subsurface corrosion damage was responsible for the reduction in hardness.     

焊后热处理对308L奥氏体不锈钢焊缝蠕变-疲劳损伤机制的影响

对钨极氩弧焊308L奥氏体不锈钢焊接接头在高温下进行应变控制的蠕变—疲劳试验,使用光学显微镜和扫描电镜分别对焊态试样和焊后热处理试样进行观察,同时用能谱议(EDS)对析出相进行判定,分析微观结构演变和裂纹扩展机制,研究焊后热处理对焊缝金属微观组织以及焊接接头蠕变—疲劳破坏行为的影响.结果表明,经高温焊后热处理,焊缝金属中δ-铁素体的连续网状结构被打破,裂纹扩展模式由沿晶扩展向混合型扩展转变,从而提高了焊接接头的蠕变—疲劳抗力.     

老龄飞行器结构腐蚀损伤评价参数研究

对遭受腐蚀损伤的铝合金结构性能进行了研究.将铝合金平板试验件在不同腐蚀条件下进行预腐蚀,对腐蚀后的试验件进行疲劳加载试验.对描述铝合金腐蚀损伤性能的两个关键参数,腐蚀损伤度和腐蚀损伤平均深度进行了显微镜观测和有限元模拟分析.结果表明,这两个参数可以作为铝合金结构腐蚀损伤性能的评价参数.     

晶粒尺寸对GH720Li镍基合金蠕变-疲劳寿命的影响

为探明晶粒尺寸对涡轮盘材料—GH720Li镍基高温合金的蠕变-疲劳寿命的影响机理,开展了650℃下不同晶粒尺寸的GH720Li高温合金圆棒蠕变-疲劳试验研究。GH720Li高温合金的蠕变-疲劳寿命随着晶粒尺寸的减小而降低。而后,通过断口的SEM分析研究晶粒尺寸对GH720Li高温合金的蠕变-疲劳损伤影响机制。其失效机制主要为晶界的氧化起裂,而晶界长度随晶粒尺寸变小而增加。最后,对基于迟滞能量的损伤方程和机械功密度2种方法进行了修正,GH720Li合金的蠕变-疲劳寿命预测与试验结果吻合较好,验证了方法的准确性。     

铁磁材料磁记忆信号与切口宽度的相关性

为探索裂纹宽度与磁记忆信号之间的关系,以45CrNiMoVA钢为试验材料,对具有不同预置切口宽度的试件进行静载拉伸试验,并采用磁记忆检测仪检测试件表面的磁记忆信号,研究了不同外加载荷下试件表面磁记忆信号的变化,并对各预置切口位置处磁记忆信号梯度变化进行分析。结果表明:随着外加载荷的增加,试件表面磁记忆信号不断增强,矩形槽位置处磁记忆信号梯度变化加剧,磁记忆信号梯度值随外加载荷的增加而单调增大;外加载荷值相同时,磁记忆信号梯度随裂纹宽度先减小后增大,而非简单的线性关系。     

A study of creep-fatigue life prediction using an artificial neural network

In this study, using AISI 316 stainless steel, creep-fatigue tests were carried out under various test conditions (different total strain ranges and hold times) to verify the applicability of the artificial neural network method to creep-fatigue life prediction. Life prediction was also made by the modified Coffin-Manson method and the modified Ostegren method using 21 data points out of a total 27 experimental data points. The six verification data points were carefully chosen for the purpose of evaluating the predictability of each method. The predicted lives were compared with the experimental results and the following conclusions were obtained within the scope of this study. While the creep-fatigue life prediction by the modified Coffin-Manson method and the modified Ostegren method had average errors of 35.8% and 47.7% respectively, the artificial neural network method had only 15.6%. As a result, the artificial neural network method with the adaptive learning rate was found to be far more accurate and effective than any of the others. The validity of the artificial neural network method for life prediction checked with the six verification data points also proved to be very satisfactory.     

高线轧机辊箱油膜轴承润滑特性分析

高线轧机油膜轴承工况条件极为恶劣,为了保证轧机油膜轴承在多种工况下的安全性,采用有限元方法耦合求解润滑方程和温黏方程,分析了不同工况下轧机油膜轴承的关键静动特性润滑性能参数,并对多种工况条件下轴承的润滑特性进行了校核。在高速重载工况下,轴承的温升偏高,可以通过增加冷却润滑油的流量或采用具有高热导率的轴承材料来加强散热;在低速重载工况下,轴承的比压较高,油膜厚度较小,可以通过增大轴承宽度来提高轴承的安全性。同时,应当减小轧机在恶劣工况条件下的连续运行时间,降低对轴承的损伤,延长其使用寿命。     

EFFECTS OF MODIFICATION OF THE CARBIDE CHARACTERISTICS THROUGH GRAIN BOUNDARY SERRATION ON CREEP-FATIGUE LIFE IN AUSTENITIC STAINLESS STEELS

Modification of the carbide characteristics through the grain boundary serration is investigated, using an AISI 316 and 304 stainless steels. In both steels, triangular carbides were observed at straight grain boundaries while planar carbides were observed at the serrated grain boundaries. The serrated grain boundary energy is observed to be much lower than that of the straight one. Therefore, the carbide morphology is found to be changed from triangular to planar along the serrated boundary to reduce the interfacial energy between the carbide and the matrix. The creep-fatigue properties of these steels at 873K have been investigated. The creep-fatigue life of the sample with planar carbide at the serrated grain boundary was found to be much longer than that with triangular carbide at the straight one. These results imply that the planar carbides with lower interfacial energy have higher cavitation resistance, resulting in the retardation of cavity nucleation and growth to increase creep-fatigue life.     

CRACK GROWTH, FRACTURE FEATURE AND RUPTURE LIFEDURING CYCLIC CREEP OF SUPERALLOY GH2132

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CRACK GROWTH BEHAVIOR AND FRACTURE FEATURE OF GH2132 UNDER CREEP-FATIGUE CONDITIONS

1.IntroductionSuperalloysareirreplaceableinengines[llandenginesundergostaticandcyclicloading[2].Thereforeitisimportanttostudycrackgrowthbehaviorofsuperalloysundercreepfatigueconditions.Itisnoticedthattheeffectofcyclicloadingoncrackgrowthisconsiderablel3'4],butdifferentatalowstresslevelfromthatatahighoneIS'6].Itisfoundthattheyieldstrengthofmaterialsisanimportantparameterinstudyingfatiguepropertiesl7,8]andcrackgrowthic,lo].Soitisofmuchimportancetocorrelatecrackgrowthbehaviorandfracturefeaturew…     

TC4钛合金的多轴高周疲劳损伤及强度退化研究

不同工况下TC4(Ti-6Al-4V)钛合金疲劳累积损伤及强度退化存在较大差别。为了充分表征载荷参数的影响,基于Chaboche损伤模型以及改进的多轴疲劳损伤准则,提出新的强度退化模型,开展了TC4钛合金的多轴高周疲劳(HCF)寿命预测和强度退化评估。首先,开展TC4合金在一系列加载路径下的多轴比例和非比例疲劳试验。将Chaboche非线性损伤准则和临界平面法与提出的损伤控制参数相结合,描述了TC4合金的非线性疲劳损伤计算和寿命预测。其次,进一步建立了基于累积损伤的非线性强度退化模型,并证明了该模型在不同载荷工况下均可以获得更高的精度。实验结果表明,由于考虑载荷参数的影响,提出的TC4钛合金疲劳寿命与强度退化预测结果精度远高于其他的预测模型。     

蠕变疲劳裂纹扩展对汽轮机转子寿命的影响

依据汽轮机转子安全性评价准则和蠕变疲劳裂纹扩展引起寿命损耗的计算模型,实际计算了含缺陷的超临界汽轮机转子的蠕变疲劳裂纹扩展寿命损耗,分析了缺陷当量和工作应力对转子寿命损耗的影响大小,对汽轮机转子寿命评估工作方面有一定的理论指导意义.     

Creep and creep-fatigue behavior of high chromium steel weldment

Strength of welded joints of high chromium steels is one of the important concerns for fabricators and operators of ultra supercritical thermal power plants.A number of creep as well as creep-fatigue tests with tensile hold have been carried out on the welded joints of two types of high chromium steels widely used in Japan,i.e.Grade 91 and 122 steels.It was found that failure occurred in fine grain heat-affected zone in all the creep-fatigue tests,even at a relatively low temperature and fairly short time where failure occurred in plain base metal region in simple creep testing.Four procedures were used to predict failure lives and their results were compared with the test results.A newly proposed energy-based approach gave the best estimation of failure life,without respect of the material and temperature.     

围压卸载下高应力岩石动力扰动的破坏特性(英文)

利用改造后的动?静组合加载SHPB装置,系统研究砂岩预先经三维加载再围压卸载的动?静组合加载的破坏特性。结果表明:采用动?静组合加载,当动载荷较大时,试样整体失稳,应力—应变曲线为典型的I型曲线;随着动载荷的减小,应力—应变曲线逐渐向II型曲线转变,即向岩爆曲线过渡,揭示了高应力下动力扰动诱发岩爆,释放弹性储能的现象。使用高速摄像仪拍摄试样破坏过程,直观反映试样的动态破坏过程。剥落碎片的断口表面形貌特征分析显示,用较低能量冲击时,试样受到张剪性破坏和膨胀性破坏的共同作用;当用较高能量冲击时,试样以膨胀破坏为主。碎块分形结果表明,采用高应力动?静组合加载,当用小能量冲击时,引发弹性储能释放能提高试样的破碎程度。