高铌TiAl合金在疲劳蠕变作用下的裂纹萌生及扩展

利用SEM原位观察技术研究了近片层Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金在750℃疲劳蠕变交互作用下的裂纹萌生及扩展行为,循环实验采用在最大拉应力保载的梯形波.结果表明,裂纹主要在片层团界面萌生,裂纹萌生方式包括蠕变空洞和疲劳微裂纹.片层团界面处的微裂纹先通过吞并蠕变空洞或在裂纹尖端应力集中作用下沿片层团界面进行扩展,然后相互连接长大;当裂纹扩展受到不同取向的片层团界面阻碍时,受阻的裂纹开始沿试样厚度方向扩展,且附近伴随出现垂直于载荷方向的微裂纹;最终受阻的裂纹相互连接直至合金断裂.将实验结果与该合金在相同条件下疲劳变形和蠕变变形的原位观察结果进行了比较.结合实验结果建立了高铌TiAl合金在疲劳蠕变交互作用下裂纹萌生及扩展示意模型.     

GH169蠕变／疲劳交互作用下裂纹扩展研究

本文采用一种新型高温裂纹检测方法——翻转电位法[3,4],研究了航空发动机的涡轮盘材料GH169在650℃时不同保持时间的蠕变/疲劳交互作用下的裂纹扩展行为,进行了两种不同预测模型的比较,并用扫描电镜对断口作了微观分析。实验结果表明,利用Paris公式拟合初始裂纹扩展阶段实验数据时相关性差,但双曲正弦寿命预测模型具有良好的相关性和预测性,且这种模型更为方便实用。     

1.25Cr-0.5Mo钢在蠕变疲劳交互作用下的裂纹扩展分析

利用大型有限元分析软件ANSYS,模拟分析了石油化工高温设备中广泛使用的1.25Cr-0.5Mo钢在蠕变疲劳交互作用下的裂纹扩展行为,得出了裂纹在不同的条件下的扩展规律。分析过程中,采用了损伤累积的方法,对每个载荷步的裂纹扩展进行累积,提高了结果的可信度。     

[112]铝单晶体的疲劳损伤与裂纹萌生

对「１１２」双滑移取向铝单晶体进行了对移压疲劳试验，详细观察和对比研究了正表面和侧表面上的疲劳损伤与裂纹萌生过程，在正表面上两个主滑移系滑移台阶是相互平行行的，呈典型的持滑转移带损伤形貌；     

基于Tanaka-Mura位错模型的疲劳裂纹萌生寿命预测

为了准确预测材料的疲劳寿命,提高结构疲劳寿命预测精度,对ABAQUS有限元数值模拟预测试样疲劳寿命的方法进行了研究. 基于Tanaka-Mura位错理论,利用python语言对ABAQUS进行二次开发,模拟预测了S960QL马氏体钢和Ti₂AlNb钛合金接头各区域疲劳裂纹萌生寿命. 利用泰森多边形法生成了晶体特征单元建立了微观子模型,考虑了体心立方结构相互垂直的两条滑移带作为潜在的裂纹萌生位置,并对具有相同取向的多条平行滑移带都进行了模拟计算. 通过计算得到的裂纹扩展速率变化,给出了裂纹萌生阶段过渡到裂纹扩展阶段的临界点处的裂纹萌生寿命. 模拟结果表明,除焊缝柱状晶组织外裂纹萌生寿命与试验数据吻合良好.     

多轴应力下蠕变—疲劳交互作用的数值模拟

高温和带保持时间条件下低周疲劳研究是近代工程结构十分重要而又复杂的问题。揭示疲劳蠕变交互作用是其中重要问题。本文从其重要因素即不同应力状态、在CP波型下不同应变控制下进行研究,并在取得结果的基础上用数值模拟揭示各因素对交互作用的影响规律。     

7449合金高周疲劳及裂纹萌生行为

研究7449-T7951合金的高周疲劳及裂纹萌生行为。在室温下,采用光滑及缺口试样进行疲劳寿命测试,应力比(R)分别为0.5和1.0。采用金相显微镜、扫描电镜及透射电镜对该合金的微观组织及疲劳试样断口进行分析,以揭示其疲劳裂纹萌生机理与合金微观组织之间的关系。结果表明:7449-T7951合金具有优异的疲劳性能;应力比为0.5和1.0时,光滑试样的疲劳寿命极限(σN)分别为349和134 MPa,而缺口试样的σN(缺口系数Kt=3.0)分别为138和70 MPa。其裂纹萌生行为受合金中粗大第二相、析出相、晶界和位错(滑移带)的共同影响。     

[12]铝单晶体的疲劳损伤与裂纹萌生

对[112]双滑移取向铝单晶体进行了对称拉压疲劳试验，详细观察和对比研究了正表面和侧表面（包含两个主滑移系Burgers矢量）上的疲劳损伤与裂纹萌生过程.在正表面上两个主滑移系滑移台阶是相互平行的，呈典型的持续滑移带（PSB）损伤形貌；而在S面上则始终未见PSB,但有晶体学关系不明确的呈波纹形不均匀分布的形变花样产生，并发展导致微裂纹的萌生,其尺寸可与正表面上的PSB裂纹相匹比。由此进一步证实了循环形变累积所致的内部组织损伤是疲劳裂纹萌生的基本因素．和单滑移取向单晶体相比，在复滑移条件下的显著交滑移过程明显地促进了试样的疲劳损伤和裂纹萌生.     

不锈钢管道焊缝金属疲劳短裂纹行为的实验研究Ⅱ.裂纹萌生、扩展与交互作用

基于"有效短裂纹准则",用复型研究了1Crl8Ni9Ti不锈钢管道焊缝金属光滑试样表面的短裂纹萌生、扩展与交互作用行为.结果表明:有效短裂纹(ESFCs)萌生于δ铁素体与奥氏体基体的交界处.ESFCs扩展具有明显微观结构短裂纹(MSC)和物理短裂纹(PSC)两阶段特征.主导有效短裂纹(DESFC)行为是ESFCs交互作用的结果,适于表征短裂纹行为.DESFC扩展率的分散性与ESFCs密度的分散性有关,说明DESFC裂尖前沿扩展条件的差异及其演化是DESFC随机扩展行为的本质原因.这一差异也是材料疲劳寿命和循环应力-应变响应存在分散性的本质原因.     

接触疲劳裂纹的萌生与扩张行为的研究

本文通过力分析和扫描电镜观察，研究了电点接触条件下有表现滑动和无表面动摩擦进地萌扩张行为，指出了接触疲劳破坏的典型特征和一般特征，结果表明，在边界润滑条件下，接触疲劳裂纹萌生于表面，以Ⅱ型方式向纵深扩张，裂纹扩张走向与裂纹尖端处最大切应力走向一致，有表面滑动摩擦时，典型的裂纹扩张路径（或剥落坑形状）由呈Ｕ字四个阶段组成，无表面轻摩擦时，扩张路径则由Ｖ字形的三阶段组成：一般的破坏以连续剥落的方式发展     

Characterization of deformation behavior for waspaloy under creep-fatigue interaction

Hysteresis loops of strain controlled LCF tests for Waspaloy were characterized within the temperature range of 350°C and 600°C. Materials deformation behaviors under symmetric and asymmetric loading conditions were assessed using Chaboche’s viscoplasticity model. The strain hardening and stress relaxation occurring in the loading cycles were estimated. Hardening plays an important role at an early stage of loading, but the stress relaxation becomes dominant at a later stage. The observed change in the slope of hysteresis loops between the first cycle and the second cycle and mean stress relaxation under asymmetric loading conditions may be explained by the evolution of the kinematic hardening variable which depends on the inelastic strain range. The effect of creep on the cyclic shake down is expected to be limited.     

P92钢焊接接头蠕变损伤与裂纹扩展数值模拟

高温焊接接头由于蠕变损伤而提前失效的案例频频发生,准确预测焊接接头的蠕变损伤和裂纹扩展行为对于保证高温装备的结构完整性具有重要意义.文中基于延性耗竭模型并结合有限元方法,考察了结构因素对厚壁圆管焊接接头蠕变失效行为的影响.结果表明,蠕变裂纹萌生/扩展行为受热影响区宽度影响,细晶热影响区宽度对蠕变裂纹的萌生时间影响不大,但会改变裂纹萌生位置;相比之下,粗晶热影响区的宽度变化对裂纹萌生时间影响略大.不同坡口形式展现出不同的裂纹萌生/扩展行为,而X形坡口是四种坡口形式中的较优选择.     

基于Normalized C&L准则的ECAP裂纹萌生趋势的数值模拟

根据Normalized Cockcroft&Latham韧性断裂准则,利用Deform-2D对6061Al等径角挤压过程进行了裂纹萌生趋势的模拟,在一道次模拟中,得出了模具拐角φ、外转角半径r与背压对试样裂纹损伤分布的影响:模具拐角越大,裂纹损伤值越小;外转角半径r=6 mm为最佳,大于6 mm时裂纹损伤值增大;施加背压有利于减少裂纹的产生,背压越大,裂纹损伤值越小,但是,考虑到ECAP的效率,背压值不能选择太大的数值。对多道次ECAP进行数值模拟,并与实验结果进行对比验证。结果表明,三道次、A路线挤压后裂纹损伤值在上表层最大。     

热疲劳裂纹扩展的数值模拟

本文根据热疲劳损伤力学和有限元技术,研究了复杂温度环境下的热疲劳损伤现象,建立了复杂热力学边界条件下的热应力分析模型,研究了多变量耦合作用下的温度场、热应力场等的分布规律及高温交变热冲击条件热疲劳裂纹的扩展特性,并进行了实验验证,对于类似条件下工作设备的热损伤问题具有重要指导意义.     

Numerical simulation of central shrinkage crack formation in a 234-t steel ingot

Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.     

锻钢活塞预成形过程的数值模拟分析

锻造成形过程是一个非常复杂的弹塑性大变形过程,有限元法是用于锻造成形过程模拟中一种有效的数值计算方法。通过数值模拟技术分析锻钢活塞的预成形过程,优化预成形模具型腔,获得了合理的生产工艺,达到减少成形工步、提高原材料利用率并获得合格终锻件的目的。     

新型耐磨钢NM400斜Y形坡口焊接抗裂性分析

采用斜Y形坡口焊接抗裂性试验对NM400钢的焊接抗裂性进行了研究,通过测定其表面裂纹率、断面裂纹率及根部裂纹率,评定NM400的抗裂性能,并分析焊前预热对材料抗裂性的影响.同时,应用热弹塑性有限元法对斜Y形坡口试样进行了焊接过程的有限元数值模拟计算,得出横向焊接残余应力的分布情况及峰值位置.结果表明,常温下NM400的根部裂纹率较高,预热150℃时各裂纹率均降至0.因此,焊前预热能够提高NM400的焊接抗裂性.另斜Y形坡口根部产生较大的应力集中,局部横向残余应力超过材料的强度,是致使其产生裂纹的力学因素.     

Effect of temperature on fatigue crack growth in P92 steel

Fracture at high temperature has become a critical problem for such high temperature components as those used in power plants or oil refinery plants, because both high operating temperature and pressure are required for better thermal efficiency. Therefore, it is very important to approach such problems from the viewpoint of high temperature material properties. Since fatigue and creep are closely related to such components failures, the fracture behavior in high temperature components must be evaluated through fatigue and creep crack growth tests, and based on these results, better operating conditions can be determined. In this study, recently developed P92 (9Cr-2W) alloy steel, which is a high strength material for high temperature use, is investigated and its fatigue crack growth has been characterized by Paris law. A series of high temperature fatigue tests were carried out at 400, 500, 550, 600, 625, 650, and 700°C to verify the temperature effect. The results indicated that the Paris exponent remained at approximately the same value up to a certain temperature. From 600 to 700°C, creep rupture tests were conducted in order to investigate the creep behavior with temperature. Further analysis has also been carried out to investigate the effect of temperature on fracture mode shift, dimple formation, and its role in crack growth rate and deformability at high temperature.     

Fatigue crack growth behavior of heat affected zone in P92 steel weldment

In this study, the effect of microstructure on fatigue crack growth behavior of HAZ was investigated using newly developed P92 steel weldment. Fatigue crack growth rates of the fusion line and fusion line+2 mm in HAZ were found to be faster than that of welds or base metal, while fatigue crack growth rate of the fusion line+1 mm was the slowest. The fracture appearance of the fusion line and fusion line+2 mm revealed mainly transgranular, cleavage-like faceted fracture surfaces and the fusion line+1 mm showed quasi-cleavage fractures with scattered dimples, which increased resistance to fatigue crack growth.