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.     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

DZ40M合金的热疲劳性能

采用扫描电子显微镜研究DZ40M定向凝固钴基高温合金的热疲劳性能。结果表明,随着上限温度的升高,裂纹扩展速率增加,热疲劳抗力降低。在上限温度相同时,随着保温时间的延长,热疲劳性能提高,在合金表面形成均匀致密的氧化物保护膜有利于热疲劳性能的提高。由于碳化物与基体的热膨胀系数不同,合金在受温度交替变化时,易在基体和碳化物界面处产生裂纹孔洞,从而萌生裂纹。热疲劳裂纹通过裂纹孔洞的相互连接向前扩展     

含20%Ni金属基陶瓷的热冲击疲劳

研究了含２０％Ｎｉ的Ｔｉ（Ｃ，Ｎ）基金属陶瓷的热冲击疲劳，试验结果表明：随循环温度的上升和冷却速率的加快，热冲击疲劳裂纹的形成孕育期缩短，裂纹扩展速率加快。缺口也使裂纹形成的孕育期缩短，观察还表明，热冲击疲劳断口中存在的疲劳条纹。     

氧化对WC_p/钢基表层复合材料热裂纹萌生扩展的影响

利用负压实型铸渗工艺,通过涂覆预制块法,成功地制备了以高铬钢为基材,WC为增强颗粒的表层复合材料,通过氧化增重法、热震实验法及扫描电镜等分析测试方法重点研究了氧化对WC/钢基表层复合材料热裂纹萌生及扩展的影响。研究结果表明:WC颗粒在高于600℃时,会氧化成为结构疏松的WO3,并且随着温度的升高,氧化反应的速度加快,而WC的氧化,对热疲劳裂纹的萌生和扩展产生重要的影响。在500℃以上的空气环境中,复合材料基体会在裂纹源的尖端处形成氧化物。结合环境中的氧对裂纹扩展影响的分析可知,生成的氧化物为裂纹的扩展提供了途径,并且使复合材料极易在热应力的作用下导致开裂。     

Transition of crack propagation from a transgranular to an intergranular path in an overaged Al-Zn-Mg-Cu alloy during cyclic loading

The fatigue crack propagation behavior in the overaged Al-Zn-Mg-Cu alloy was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results revealed that a fatigue crack tended to transgranularly propagate in the near-threshold regime, whereas intergranular crack propagation was dominant at the high ΔK regime. The transition of crack propagation from a transgranular to an intergranular path that occurred in the Paris regime was strongly influenced by the misorientation of adjacent grains and precipitate free zones. In addition, a crystallographic model of crack propagation was proposed to interpret the transition. The fatigue short crack propagation on a single slip plane was responsible for the formation of a transgranular propagation path in the near-threshold regime. The fatigue long crack propagation, which was conducted by a duplex slip mechanism in the Paris regime, led to the formation of fatigue striations. The formation of a zigzag crack in the near-threshold regime was ascribed to the high misorientation of adjacent grains.     

热应力作用下碳化钨基钢结硬质合金梯形裂纹的形成机理

在自约束型热疲劳试验机上对碳化钨含量不同的钢结硬质合金进行了热疲劳试验，用金相显微镜和扫描电镜对热疲劳裂纹扩展行为进行了研究。结果表明：在热应力作用下，不仅在合金的表面，在合金的内部都形成了“梯形”裂纹。这种形态的裂纹是由试样缺口处的热疲劳裂纹扩展而引起的，合金中的钢基体相阻碍裂纹扩展，而WC硬质相诱导裂纹扩展，这两种因素的综合作用促使热疲劳裂纹成“梯形”扩展。分析认为：使WC粒子在钢基体中均匀分布，提高钢基体的热疲劳抗力，从而来阻止梯形裂纹的形成，是提高碳化钨钢锆硬质合金抗热疲劳性能的有效途径。     

不同类型过载下Ⅰ型疲劳裂纹的扩展行为

研究了Ⅱ、Ⅲ型过载对Ⅰ型疲劳裂纹扩展行为的影响并与Ⅰ型过载进行比较．结果表明：单周Ⅱ、Ⅲ型过载也延迟了其后的Ⅰ型疲劳裂纹扩展，但作用程度小于Ⅰ型过载；过载延迟效应随过载时基础载荷的降低和过载比的加大而增强，在近门槛区过载表现为疲劳裂纹扩展停止、疲劳门槛提高，在高基础载荷过载时疲劳裂纹则延缓、减速扩展．过载延迟行为主要由裂尖尾迹塑性诱导裂纹闭合和裂尖附近残余压应力状态引起。     

气孔对铝合金焊接接头超长疲劳寿命的影响

采用超声疲劳试验方法对铝合金5052-H32焊接接头进行疲劳试验,研究其在超长寿命区间(107~109周次)的疲劳强度及失效机理.结果表明,相同疲劳寿命下焊接接头疲劳强度较母材下降了73.3%,在超高周疲劳区间仍然会发生疲劳破坏;断口观察发现焊接缺陷(气孔)是诱发疲劳裂纹萌生的主要原因.为揭示焊接缺陷对焊接接头疲劳裂纹萌生及扩展的影响,采用有限元技术分析了气孔对应力集中系数和裂纹尖端应力强度因子的影响.最后讨论了焊接缺陷对疲劳寿命的影响机制及裂纹扩展特性.     

变质处理后Fe-V-W-Mo-Cr合金热疲劳性能的研究

采用自约束热疲劳试验机对Fe-V-W-Mo-Cr合金进行了热疲劳试验,采用金相显微镜对热疲劳裂纹扩展行为进行了研究,同时用RE-Mg对Fe-V-W-Mo-Cr合金进行变质处理。结果表明,经RE—Mg变质处理后,碳化物和基体组织明显细化,并产生较多的小块状和颗粒状碳化物,促使合金的导热系数增加,热膨胀系数减小,热疲劳裂纹萌生所需热循环次数增加,裂纹扩展速率减小,在热循环700周次后的热疲劳裂纹长度减小约49％。     

热疲劳裂纹张开过程的有限元模拟

用有限元方法并考虑材料的多线性随动强化性质对带有裂纹的简单试件模型进行热疲劳分析,研究热疲劳裂纹张开的过程和规律。试件上下端固定,左侧温度按照60℃-240℃-60℃循环,右侧恒温60℃,裂纹位于左侧中部。热疲劳裂纹张开、扩展的直接动力是垂直裂纹面的拉应力,其产生是由于在升温半循环中左侧材料膨胀受压产生压缩塑性变形,经历降温半循环温度回到初始温度60℃时,试件左侧塑性变形不能完全恢复,于是拉应力产生,裂纹张开。在稳定的温度循环下,由于试件左侧塑性变形不断累积,热疲劳裂纹的张开位移和应力强度因子增加,并趋于稳定。     

一种Al-Cu-Mg-Zr合金的疲劳裂纹扩展行为研究

研究热处理状态、应力比和试样取向等对含Zr的Al-Cu-Mg系铝合金疲劳裂纹扩展行为的影响.结果表明,T3、T4两种热处理状态试样的疲劳裂纹扩展速率没有明显差别;L-T方向和T-L方向的试样也有大致相同的疲劳裂纹扩展速率;应力比R对疲劳裂纹扩展行为有明显影响,高应力比下的疲劳裂纹扩展速率明显快于低应力比条件下的扩展速率,并且高应力比下的疲劳裂纹扩展在较小的△K值下进入快速扩展阶段,并很快断裂.在较低的△K水平下应力比的影响与裂纹闭合效应有关.     

喷丸强化残余应力对AISI304不锈钢疲劳裂纹扩展行为的影响

目的 探究喷丸强化残余压应力对AISI 304不锈钢疲劳裂纹扩展行为的影响规律.方法 建立并联合紧凑拉伸(CT)试样三维有限元模型和对称胞元喷丸有限元模型,发展一套多步骤数值模拟方法.首先,建立AISI 304不锈钢CT试样的三维有限元模型,模拟不同外加交变载荷工况下的疲劳裂纹扩展过程.基于线弹性断裂力学理论,利用裂纹闭合技术,计算不同裂纹长度对应的应力强度因子范围,采用修正的Paris公式计算疲劳裂纹扩展速率,并通过试验数据对计算结果进行考核.其次,建立多弹丸分层逐次冲击靶面的对称胞元喷丸有限元模型,模拟100％和200％喷丸覆盖率下的残余应力场,并通过试验数据对该对称胞元喷丸有限元模型的有效性进行验证.最后,将喷丸强化诱导的残余应力场以读写外部文件的方式导入CT试样三维有限元模型,模拟在内部残余应力场和外部交变载荷共同作用下的疲劳裂纹扩展行为.结果 对于相同的喷丸工况,保持外加载荷比不变而减小最大外加载荷,或者保持最大外加载荷不变而减小外加载荷比,喷丸强化诱导的残余压应力对疲劳裂纹扩展的抑制作用愈加显著.对于相同的外加载荷工况,200％喷丸覆盖率工况比100％喷丸覆盖率工况更能有效降低AISI 304不锈钢的疲劳裂纹扩展速率.结论 喷丸强化诱导的残余压应力场能够有效抑制AISI 304不锈钢的疲劳裂纹扩展.     

碳化钨钢结硬质合金表面热疲劳裂纹扩展的观察

本文借助于光学显微镜和扫描电镜，全面观察了在热应力的作用下，热疲劳裂纹在GJW50碳化钨钢结理质合金（未经锻造）表面扩展的全过程．观察结果表明，热疲劳扩展方式与硬质合金中的硬质相分布状态有关．小颗粒的硬质相聚集区与钢基体的界面是热疲劳裂纹的主要扩展地；热疲劳裂纹前沿已经开裂的大颗粒硬质相为热疲劳裂纹扩展的必经地．     

保温时间和缺口半径对DZ951合金热疲劳性能的影响

通过扫描电子显微镜研究了保温时间和缺口半径对DZ951合金热疲劳性能的影响。结果表明,在相同上限温度下,随着保温时间的延长,合金裂纹萌生次数增加,热疲劳抗力提高。铸态合金热疲劳裂纹在碳化物处萌生。热处理合金通过氧化萌生裂纹。裂纹萌生后,主要通过氧化产生孔洞并通过孔洞相互连接使裂纹扩展。热疲劳裂纹沿着与枝晶生长方向呈45°角扩展。热处理合金具有优异的热疲劳性能。随着缺口半径的增加,应力集中减小,合金热疲劳性能提高。     

316不锈钢的裂纹扩展性能

从实用出发对3168不锈钢管材进行了热循环下的裂纹扩展试验,试验表明该材料在交变热应力下的裂纹扩展速率缓慢,带裂纹的管道在监督下可以继续运行。     

烧结机篦条用高硅Fe-Cr-Mn基耐热钢的热疲劳性能

为了解耐热钢的热疲劳性能,结合篦条的实际工况,设计材料成分并浇注和制备试样,对篦条材料进行900℃热疲劳试验,通过扫描电镜观察裂纹形貌和裂纹扩展情况,分析了材料成分和组织对热疲劳性能的影响。结果表明,经热循环处理30O次以下,w（C）=0．4％耐热钢的热疲劳性能最好。     

热作模具钢热疲劳机理及性能改善的研究现状

热作模具钢的热疲劳性能是影响模具使用寿命的主要因素。对热作模具钢热疲劳性能的研究现状和评定方法进行了阐述,分析了组织演变、裂纹萌生和扩展对热疲劳性能的影响。同时对影响热疲劳性能的因素和改善热疲劳性能的方法进行了探讨和总结,并针对热作模具钢热疲劳研究的趋势进行了展望。     

H13钢热疲劳性能研究

以盐浴法为手段,对H13钢进行了热疲劳实验,探讨了淬火温度、回火温度、表面处理对其热疲劳失效裂纹的影响并确定了合理的热处理工艺。实验得出:试样热疲劳循环次数在400个周期时,裂纹形成后扩展迅速,裂纹平均生长速度约为2.51×10-3mm/周,而采用1 020℃淬火、640℃回火热处理工艺,与裂纹平均生长速度相比,热疲劳裂纹生长速度减缓了0.45×10-3mm/周,表面渗氮的试样裂纹源可推迟50个热疲劳循环周期。     

The Study on Plasma Boriding of H13 Steel at Low Temperature Assisted by Surface Nanocrystallization Technology

Plasma boriding treatment was carried out at low temperature for the hot work die steel H13 assisted by surface nanocrystallization technology in this paper.At the same time,the thermal fatigue property of it was investigated through thermal fatigue testing with 3000 continuous cycles from room temperature to 700℃.The changes of structure and grain size in surface layer were characterized by high-resolution transmission electron microscopy(HRTEM).After plasma boriding at 580℃ for 4 h,the phase composition,morphology and in-situ nanomechanical property of boride layer were investigated by X-ray diffraction spectroscopy(XRD),scanning electron microscope(SEM),nanoindentation test,respectively.The results show that the boride layer with about thickness of 5μm is composed with two phases of Fe2B and FeB.The nanohardness of boride layer is as high as 21 GPa.Furthermore,thermal fatigue testing shows that the boride layer with excellent oxidation resistance and mechanical strength at elevated temperatures could effectively delay the crack initiation and impede the crack propagation.Therefore,the thermal fatigue property of H13 can be remarkably improved.