Study of crack propagation behavior during low cycle fatigue in spheroidal graphite cast iron based on observation of surface and fracture sections

In this paper, in order to clarify the effect of crack coalescence for crack growth rate, low cycle fatigue tests were carried out using two kinds of spheroidal graphite cast iron (SGI). Crack propagation behavior from crack initiation to fracture was investigated by observing the surface of specimens and their fracture section. The main results obtained are as follows: (1) The variation range in the crack growth curves in SGI is larger than that in its matrix material. This tendency is caused by the occurrence of crack coalescence and the difference in crack initiation length. (2) In most of the cases where the crack growth rate accelerated, there was microshrinkage on the neighboring surface. (3) Crack coalescence is not the main factor in accelerating crack growth rate.     

激光表面熔凝处理对共晶铝硅合金疲劳裂纹扩展的影响

研究了共晶铝硅合金在激光表面熔凝处理后的显微组织变化及激光表面熔凝处理对合金疲劳裂纹扩展的影响.结果表明,激光表面熔凝处理使试验合金的表层微观组织发生显著变化,熔凝层中的初生α-Al枝晶相和共晶Si相均有明显细化,熔凝层的硬度显著提高;在应力比R为0.1和0.5的试验条件下,激光表面熔凝处理能显著提高共晶铝硅合金的疲劳裂纹扩展抗力.     

2524-T34合金疲劳裂纹的萌生和扩展行为

通过四点弯曲疲劳试验研究2524-T34板材的疲劳性能,借助金相和扫描电镜观察疲劳裂纹的萌生和扩展行为。结果表明:2524合金具有良好的疲劳性能,疲劳强度达到屈服强度的80%以上;疲劳裂纹主要在第二相粒子以及第二相粒子/基体界面萌生,裂纹扩展过程中的偏转与晶界的阻碍有关;相邻晶粒内两个有利滑移面之间的位向差是控制裂纹通过晶界扩展的重要因素。     

一种镍基高温合金的低周疲劳性能

研究了一种镍基高温合金在不同温度下的低周疲劳性能,分析了疲劳断口。结果表明,该合金循环应力响应行为表现出对温度和外加总应变幅很强的依赖关系,不同的循环应力响应行为可归因于位错、强化相和合金元素间复杂的交互作用。合金疲劳寿命与温度、外加总应变幅、氧化损伤程度有关。疲劳断裂行为受外加应变幅和氧化影响很大。     

A356铸造铝合金的疲劳裂纹萌生及短裂纹扩展研究

研究了A356-T6铸造铝合金的缺口疲劳裂纹萌生与早期扩展行为及机制.结果表明,热等静压试样的疲劳抗力优于非热等静压试样.对于钝缺口试样,疲劳裂纹萌生于缺口根部附近的多个平面,最终哪个裂纹源扩展成主裂纹取决于局部微观组织.对于缺口几何形状不同的热等静压和非热等静压疲劳试样,在疲劳过程中,不管是在高应力状态下,还是在低应力状态下,都出现了铝基体的循环塑性变形和共晶硅粒子断裂导致疲劳裂纹萌生.对于非热等静压试样,铸造缩孔在构件的疲劳过程中起着重要作用,但即使缺口根部存在较大尺度的铸造缩孔,导致了疲劳裂纹萌生,但也同时观察到疲劳裂纹从共晶硅粒子、金属间化合物、铝基体的滑移带和铁基金属间化合物等处萌生.对于脆性的A356铸造铝合金可采用修正的断裂力学参量ΔKn、局部应力范围Δσ或局部应变幅Δε/2作为控制参量来表征疲劳裂纹萌生行为,而缺口有效应力强度因子范围ΔKneff和ΔJs参量可用来表征缺口场中短裂纹扩展行为.     

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.     

管材高温低周疲劳实验方法及数据处理

采用径向应变控制法 ,在MTS880 /810试验机上用d 13mm× 1.5mm的单相α Ti合金管材 ,在 (35 0±1)℃下进行了 5个不同应变量的高温低周疲劳 (HTLCF)实验 ;记录了各自的循环应力—应变滞后环 ,得出了试样的循环响应特征和各试样的高温低周疲劳寿命。对实验数据按Coffin Manson公式拟合出疲劳曲线方程 ,从而绘出了该试材的Δεt— 2Nf 倍寿命关系曲线。考虑到低周疲劳数据的离散性 ,对其中的Δεt=1.0 %和Δεt=1.5 %两个应变量的各 6个试验数据按双参数Weibull分布进行了统计分析。     

Effects of recrystallization on the low cycle fatigue behavior of directionally solidified superalloy DZ40M

The effects of recrystallization on low cycle fatigue behavior were investigated on directionally solidified Co-base superalloy DZAOM.Optical microscopy and SEM were used to examine the mierostructure and fracture surface of the specimens.The mechanical testing results demonstrated that the low cycle fatigue property of DZ40M significantly decreased with the partial reerystallization.Fatigue cracks initiate near the carbides and the grain boundaries with slip-bands.Both the fatigue crack initiation and propagation can be accelerated with the occurrences of recrystallized grain boundaries.     

2A97铝锂合金的疲劳裂纹萌生及早期扩展行为(英文)

研究2A97铝锂合金的疲劳裂纹萌生及早期扩展行为。在室温条件下,采用光滑试样进行疲劳测试,其中最大应力为恒定值,应力比R为0.1,频率f为40 Hz。利用金相显微镜、透射电镜、扫描电镜及电子背散射衍射等手段对合金的微观组织进行分析,研究合金的疲劳裂纹萌生及早期扩展行为与其微观组织的关系。结果表明:2A97合金的疲劳裂纹主要萌生于试样表面的杂质相和粗大第二相处;其疲劳裂纹的早期扩展行为主要受晶粒结构与位错或滑移带共同作用的影响。当相邻晶粒的错配度接近于其晶内的最优滑移面的位向差时,大角度晶界强烈阻碍滑移带的运动,从而导致裂纹分叉和偏折。     

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.     

小角度晶界对单晶高温合金高周疲劳性能的影响

用双籽晶法制备了带有9°小角度晶界的DD6单晶高温合金试板,研究了小角度晶界对合金700℃高周疲劳性能的影响,并与[001]取向合金的疲劳性能进行了对比分析,用扫描电镜和透射电镜分析了其断口形貌和断裂机制。结果表明,带有9°小角度晶界合金的高周疲劳极限比[001]取向合金的稍有降低。疲劳裂纹萌生于试样表面或亚表面,而不在晶界上形成。小角度晶界试样的疲劳断裂机制有两种情况,大部分试样为类解理断裂,其它试样为类解理与沿晶混合断裂。     

High temperature fatigue behavior of wrought nickel-base superailoy GH4049

Push-pull total strain-controlled fatigue tests without and with a hold period were carried at elevated temperatures for wrought nickel base superailoy GH4049. The influence of the testing temperature and strain hold period on fatigue behavior was determined. The alloy would exhibit either cyclic strain hardening, softening or stability during cyclic straining. Fatigue life depends strongly on the testing temperature and the introduction of the strain hold period. Observations on fatigue specimens using transmission electron microscopy (TEM) showed that the dislocations were distributed mostly in the γ matrix. It was observed by scanning electron microscopy (SEM) that cracks initiated always in a transgranular mode, but their propagation mode was closely related to the testing temperature. In addition, the fatigue life was predicted by linear damage summation (LDS), strain range partitioning (SRP) and the strain energy partitioning (SEP) method. The results of life prediction indicated that the SRP and SEP methods were in a good agreement as to the measured and predicted life at lower temperatures, while the LDS method showed better predictability at higher temperature as compared to the SRP and SEP methods.     

Effect of pre-strain on fatigue crack growth of 2E 12 aluminum alloy

The influences of pre-strain on the mechanical property and fatigue crack growth of 2E12 aluminum alloy were evaluated by SEM, TEM, mechanical property and fatigue tests. The axial fatigue tests were conducted under a constant amplitude sinusoidal wave loading at stress ratio of 0.1 in laboratory air and salt fog at room temperature. The results show that the yield stress of pre strain material is higher than that of the material without undergoing pre-strain, but pre-strain can not make the increase of the growth rate of fatigue crack. Fatigue crack growth rates of the alloy in salt fog are higher than those in air. The increased fatigue crack growth of the alloy in a given environment and more brittle striations can be observed in salt fog.     

Low cycle fatigue properties and cyclic deformation behavior of as-extruded AZ31 magnesium alloy

The low cycle fatigue(LCF)properties of as-extruded AZ31 Mg alloy were investigated under total strain amplitudes in the range of 0.4%-1.2%with strain rate of 1×10- 2s -1.Due to the twinning effect in compression during loading and the detwinning effect during unloading,the alloy showed an asymmetric hysteresis loop.The cyclic stress response exhibited cyclic hardening at high total strain amplitudes.The cyclic deformation behaviors were discussed using the Coffin-Manson plot,which divided the plastic strain amplitudes into the tension side and the compression side.Through the LCF tests that were started from either tension or compression under a total strain amplitude of 1.0%,the interaction between the twinning effect and dislocation was analyzed.The twinning effect during the LCF test and the variation of the dislocation density were investigated using optical microscopy and transmission electron microscopy,respectively.     

Kinetics of γ’ precipitation and its influence on fatigue crack growth behavior of a new single-crystal nickel- based superalloy (cmsx-4g) at room temperature

An investigation was carried out to determine the growth kinetics of γ precipitates in a newly developed single-crystal nickel-base superalloy containing rhenium (CMSX- 4G). The investigation also examined the influence of γ’ precipitates (size and distribution) on fatigue crack growth behavior of the material in a room-temperature ambient atmosphere. The influence of load ratio on fatigue threshold of the material and crack growth mechanisms in fatigue was also studied. Compact tension specimens were prepared from a single-crystal nickel-base superalloy, CMSX-4G, with the (001) crystallographic direction. These specimens were given two different heat treatments to produce two different γ’ size precipitates. Fatigue crack growth behavior of these materials was studied at three different load ratios (R = 0.10, 0.50, and 0.90) in room-temperature ambient atmosphere. The results of the present investigation demonstrate that rhenium additions in CMSX-4G substantially lowers the γ coarsening kinetics of this alloy. The smaller γ’ precipitate size was found to be beneficial for fatigue resistance and has resulted in a higher fatigue threshold and lower fatigue crack growth rate in the threshold region. The fatigue threshold was found to decrease with an increase in load ratio. The crack growth mechanism in the threshold region was found to occur by a combination of microvoid coalescence and striations.     

退火温度对Al-4.2Cu-1.4Mg合金的微观结构和力学性能的影响

针对航空用高强Al-4.2Cu-1.4Mg铝合金,研究了退火温度对微观组织结构、力学性能和疲劳裂纹扩展速率的影响。采用金相显微镜、扫描电镜和能谱分析手段对合金的微观组织和析出相种类进行了分析。结果表明:退火温度从380℃提高到440℃后,T3态板材中的晶粒尺寸不断细化,第二相Al2CuMg和Al2Cu的体积分数逐渐减少,因此疲劳裂纹扩展速率呈下降趋势。另外,随着中间退火温度的升高,成品板材的拉伸强度有少量降低,而伸长率则有所提高。     

电子束局部热处理对TC4合金接头疲劳性能影响

通过低周轴向应变疲劳试验,研究了电子束局部热处理对TC4钛合金接头疲劳性能的影响.结果表明,电子束局部热处理TC4钛合金接头在疲劳过程中存在一定的软化行为;在一定的载荷条件下,与焊后未热处理状态相比,经过电子束局部热处理后,TC4钛合金电子束焊接接头的疲劳寿命可以提高30%以上.     

回火温度对Si-Mn低碳贝氏体钢塑韧性的影响

采用扫描电镜、X射线衍射仪、透射电镜研究了不同回火温度对Si-Mn低碳贝氏体钢的力学性能、微观组织、残留奥氏体、冲击断裂裂纹扩展的影响。结果表明,提高低碳贝氏体钢回火温度,降低残留奥氏体量,增加残留奥氏体的稳定性,有利于塑韧性的改善。但回火温度达到500℃以上,残留奥氏体量都发生转变或分解,塑韧性会变差。稳定的残留奥氏体会增大裂纹扩展能量,从而改善塑韧性。     

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。