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。     

SLOW PROPAGATION OF FATIGUE CRACK NEAR THRESHOLD

The effect of pearlite interlamellar spacing(d)from 0.31 to 0.77 μm of T8 steel on fatiguecrack propagation near threshold has been investigated at different stress ratios(R).Theclosure stress intensity factor(K_(cl)),the threshold value(ΔK_(th))and crack tip opening dis-placement range(ΔCOD)were measured.The results show that pearlite interlamellar spac-ing in this range has no effect on K_(cl),but has effect on the threshold value in low stress ratio.The threshold value is higher for materials with larger interlamellar spacing.No matter whatthe stress ratio is,during the propagation near threshold,the propagation rate will be thesame,provieded the crack tip opening displacement range is the same.Metallurgical examinations were also carried out to show that the crack propagationpath is sensitive to mierostructure.The crack always propagates along either boundary ofpearlite colony or ferrite lamellar in pearlite.     

应力比对25Cr2Ni2MoV钢疲劳裂纹扩展门槛值的影响

对25Cr2Ni2MoV钢进行疲劳裂纹扩展门槛值试验,试样尺寸为紧凑拉伸试样,试验的频率为20 Hz,波形为正弦波,试验环境介质为空气,试验的温度为23 ℃,试验过程采用连续的降K程序进行测试。试验结果表明,应力比R分别为0.05,0.1,0.3,0.5,0.7和0.8时,有效门槛值?Keff分别为5.30、5.10、3.68、3.34、2.72和2.43 MPa·[m],随着应力比的增加,有效门槛值逐渐降低,da/dN曲线向较小的?K方向移动;随着应力比的增加,裂纹闭合效应的影响是逐渐降低的,当应力比R≥0.7时,裂纹闭合效应消失,实际测量的门槛值即为有效门槛值,采用修正后的McEvily-Groeger公式,可以得到25Cr2Ni2MoV钢有效门槛值的经验方程。     

喷丸残余应力对裂纹闭合效应影响的数值仿真

基于裂纹闭合效应,利用ABAQUS软件建立用于预测残余应力场中疲劳裂纹扩展特性的弹塑性有限元模型。考虑塑性和残余应力场对裂纹闭合的作用,分析残余应力、应力比和裂尖单元尺寸对裂纹闭合效应的影响。研究结果表明:未喷丸试样的裂纹闭合类型为塑性诱导裂纹闭合,喷丸残余应力场中的裂纹闭合为塑性和残余压应力共同作用,且裂纹张开力的大小与残余应力的分布相对应;正应力比越大,裂纹闭合效应越不明显,疲劳裂纹扩展速率越快;裂尖单元尺寸小于塑性区范围时可以真实反映裂尖的闭合状态;喷丸残余压应力通过提高裂纹闭合力,增强裂纹闭合效应,抑制疲劳裂纹扩展。     

EFFECT OF CRACK CLOSURE ON SLOW PROPAGATION IN DUAL-PHASE STEEL

The effect of ferrite content in ferrite-martensite dual-phase steel on the initiation and prop-agation of fatigue crack and the plastic deformation at crack tip has been studied.In the rangeof ferrite content from 24.2 to 41.5％,the optimum seems to be 33.8％,of which the crack ini-tiation will be prolonged,the threshold value increased,the propagation rate decreased and theclosure stress intensity factor increased.As the propagation force is described by effectivestress intensity factor,three steels with various ferrite contents will show the same propagationbehaviour on da/dN vs △ K_(eff)curve.It is shown that the closure effect increases with thedecrease in △K at the fatigue crack tip.When △K equals to △K_(th),the closure effect reachesa maximum value of0.7 in a dual-phase steel with 33.8％ferrite.     

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

It is obtained in this paper that the fatigue threshold value of mode H was 1.9 times of that ofmode Ⅰ in dual-phase steel(DPS),and the normal stress intensity factor range oJ mode Ⅱbranch crack tip was 2.2 times of that of mode Ⅰ.Above results illustrate that the resistance ofmode Ⅱ crack growth was higher than that of mode Ⅰ,the former resulting fromroughness-induced shear resistance,the latter,crack closure.The mode Ⅱ component can play two important roles in near-threshold fatigue crackgrowth:(1)increasing crack tip plasticity which accelerates the crack growth and(2)intro-ducing crack surface contact and rubbing to reduce the crack propagation rate.By means ofcrack closure,the quantity of shear resistance was easily solved in this paper.The frictionshear stress strength factor range of mode Ⅱ,K_,is still much higher than the closure stressstrength factor range of mode Ⅰ,K_(Ⅰ,cl).This illustrated that the roughness enlarged the secondrole and played a role of shielding crack tip from mode Ⅱ crack.     

Fatigue crack propagation behavior of AZ31 Mg alloy in ultra-high vacuum

The fatigue crack propagation (FCP) behavior of extruded AZ31-F was examined in conjunction with crack closure phenomena. FCP experiments were carried out in an ultra-high vacuum system using a fatigue test machine under a constant load amplitude. To determine the effect of crack closure on the FCP rates, the crack closure levels were measured using the compliance method. The δK_th values of the AZ31-F decreased with increasing load ratio. It was found that ΔK_eff,th showed an almost constant value regardless of the load ratio.     

腐蚀疲劳裂尖材料损伤研究

金属环境断裂过程依赖于材料,应力与介质参数间的复杂交互作用,这些交互作用的澄清是实现材料断裂控制的重要基础,而裂纹尖端单元损伤过程的研究则是联接微观机制与宏观断裂规律的桥梁。通过对钝化和活化典型腐蚀体系下恒速长大腐蚀疲劳裂纹受力学与电化学参数扰动后的扩展动力学响应规律的系统研究,并结合多种其它分析手段,考察了不同腐蚀因素对裂尖材料的损伤作用机制。在实验基础上,对腐蚀导致裂纹闭合变化,裂纹扩展过载延     

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.     

2050铝合金的疲劳裂纹扩展行为研究

研究取样方向、应力比以及微观组织对2050铝合金疲劳裂纹扩展速率的影响。结果表明:取样方向和应力比对疲劳裂纹扩展速率在近门槛区和快速扩展区的影响显著,而在稳态扩展区的影响则相对较小;L-T取样方向的速率最低,T-L取样方向的速率次之,S-L取样方向的速率最高,这主要与合金的晶粒取向、织构取向和第二相粒子取向有关;应力比为0.5的裂纹扩展速率高于应力比为0.1的速率,这在近门槛区主要与裂纹闭合效应有关,在快速扩展区主要受最大应力场强度因子Kmax的影响。     

预应变对工业纯钛TA2疲劳裂纹尖端应变场的影响

钛制承压设备在制造和服役过程中会产生塑性变形,影响其抗疲劳性能。为了研究预应变对工业纯钛TA2疲劳裂纹扩展行为的影响,本文对原始材料、预应变量为10%、20%和30%的材料进行疲劳裂纹扩展试验,结合数字图像相关(Digital image correlation, DIC)技术获取裂纹尖端应变场,研究预应变对裂纹尖端应变场的影响。结果表明：随着预应变量的增加,稳态疲劳裂纹扩展阶段的扩展速率减小,裂纹张开位移减小,裂纹张开载荷增大,裂纹尖端塑性区、塑性变形量以及循环塑性应变累积量均减小。因此预应变抑制了裂纹尖端的塑性变形及循环塑性累积,致使裂纹闭合效应愈加明显,从而抑制了裂纹的扩展。研究结果对钛制承压设备的安全评定具有参考意义。     

高速列车车轴用EA4T钢腐蚀疲劳性能研究

目的 研究腐蚀环境中EA4T车轴钢疲劳性能,为车轴的腐蚀检测和使用寿命评估提供依据.方法 采用旋转弯曲疲劳试验机,在人造雨水模拟的腐蚀环境和空气环境中,对EA4T车轴钢试样进行疲劳试验,以获得不同环境下试样的疲劳S-N曲线、表面损伤以及裂纹扩展规律.然后对扩展裂纹进行概率统计,通过扫描电镜对疲劳失效的断口进行观察,并分析对比不同环境中裂纹扩展门槛值的变化.结果 空气环境中,试样的疲劳极限为355 MPa,而在腐蚀环境中,试样不存在疲劳极限,107循环周次对应的疲劳强度降低到245 MPa,相比空气环境中降低了31％.Gumbel分布统计与Weibull双参数分布统计相比,更适合描述EA4T车轴钢试样表面腐蚀裂纹长度随加载次数的变化.腐蚀环境中,疲劳裂纹萌生于表面腐蚀坑,并存在多个裂纹源.腐蚀环境显著降低了试样裂纹扩展门槛值,空气环境下,该值为6.29 MPa·m1/2,腐蚀环境下降低到4.1 MPa·m1/2.结论 腐蚀环境降低EA4T钢疲劳寿命的主要原因是,腐蚀环境降低了裂纹扩展门槛值,加快了裂纹萌生以及短裂纹扩展.而当裂纹达到一定长度时,腐蚀环境对裂纹扩展几乎没有影响.     

Environment and R-ratio effects on fatigue crack propagation transitions in Ti-6A1-4V (ELI)

The multiple-sloped relationship between da/dN and AK in stage II fatigue crack propagation regime in most engineering alloys has been a subject of considerable research efforts. Even though the crack closure is recognized as a crucial factor affecting FCP rates, closure effect on transition behavior is often ignored due to the uncertainties associated with ΔK_eff value and the difficulties in measuring closure especially in aqueous environment. The present study therefore focused on the closure-free transition behavior in mill annealed Ti-6A1-4V in moist air and 1% NaCl at a fixed potential of -500 mV_SCE as a function ofR ratio. The complex da/dN-ΔK relationship in Ti-6A1-4V remains after closure-correction for eachR ratio and environment, suggesting that closure is not a controlling factor for a transition, while it may complicate transition behavior. After closure-correction, the transition points and slopes are insensitive to R ratio in moist air, suggesting that R ratio effect on transition behavior in moist air is mostly due to crack closure. The 1% NaCl solution further complicates the da/dN-ΔK_eff shape in Ti-6A1-4V at R ratios of 0.1 and 0.4 by superimposing cyclic stress corrosion cracking (SCC)-induced transition at intermediate ΔK regime. Varying R ratio in 1% NaCl is complex and appears to modify the cyclic and/or static SCC contribution.     

EFFECTS OF MICROSTRVCTURE ON FATIGUE CRACK GROWTH BEHMIOR IN Ni-BASE SUPERALLOY GH586

1.IntroductionInmetalsandalloys,thenearthresholdfatiguecrackgrowthisfrequentlytermed'microstructurallysensitive'.Thisisduebothtotheappearanceofthefracturesllrface,whichisoftenfaceted,andtotilelargevariationsincrackgrowthratewhichcanbeobtainedbyalteringparameterssllchasgrainsize,yieldstrength,ortherelativeproportionsanddistributionsofphasesinthemicrostructure[1].Thisimpliesthatsignificantimprotremelltintotalfatiguelifecouldbeachievedbychangingthemicrostructureofalloys.Therefore,itwouldbeveryadv…     

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)* was investigated. The investigation also examined the influence of heat treatment on resultant microstructures and the near-threshold fatigue crack growth behavior. In addition, the influence of load ratios(R), material strength, and grain size on fatigue threshold was studied. Compact tension specimens prepared from Incoloy 825 with transverse-longitudinal (TL) orientation in the as-received, as well as two different heat treated conditions, were used. The heat treatment studies revealed a peak hardness condition after solution treatment at 1200 °C for 1/2 hr, followed by aging at 600 °C for 434 hr. Among all the heat treated conditions, the fatigue threshold was the highest and the near-threshold crack growth rate was lowest in this peak aged condition. Fatigue threshold values were observed to decrease with an increase in load ratio, whereas an increased grain diameter resulted in a higher fatigue threshold. An earlier mathematical model was found applicable to characterize the relationship between load ratio and fatigue threshold. Preferential etching of grain boundary suggests formation of a thin film of carbide precipitation along the grain boundary region in the aged specimens. This carbide precipitation facilitated intergranular crack growth in these samples, resulting in higher roughness-induced crack closure. The highest fatigue threshold in the peak aged condition can be attributed to this large roughness-induced crack closure process. Incoloy 825 is a trademark for products of Huntington Alloys, Inc.     

A dislocation model for the two critical stress intensities required for threshold fatigue crack propagation

We have examined the variations, with decreasing load ratio, of threshold peak and cyclic stress intensities required for fatigue crack growth in stage I (mainly mode II loading) using a simple model simulating dislocation motion near a crack tip. In this model the crack grows by dislocations running into the crack during loading and unloading phases. Initially we have studied the behaviour of a crack with a dislocation source relatively far away from the crack tip. Crack propagation rates showed a Paris regime at high ΔK, and an abrupt threshold value ΔK_th below which no crack growth occurred. The variation with load ratio of the peak (K_th) and cyclic (ΔK_th) stress intensities at the fatigue threshold showed that two different processes controlled the behaviour. At high load ratios dislocations are generated readily during loading and the threshold is controlled by the need for sufficient unloading to allow dislocations to run back into the crack, so that the criterion ΔK ≥ ΔK∗ results. At negative load ratios it is the generation of dislocations during the loading phase that controls the threshold condition, since once generated, the large unloading and reversed loading easily forces dislocations back to the crack. Under these conditions the threshold criterion becomes K_max ≥ K∗.     

Effect of microstructure and load ratio on fatigue crack growth behavior of advanced Al-Cu-Li-Mg-Ag Alloys

Fatigue crack growth (FCG) behavior of recently developed three Al-Cu-Li-Mg-Ag alloys, Weldalite 049, X 2095 and MD 345, was examined in air at load ratios of 0.1 and 0.75. It was found that all three alloys showed better resistance to fatigue crack growth than conventional high strength Al alloys. The morphologies of crack growth paths were generally linear, but some showed deflection and branching. And the alloys revealed rough and transgranular fracture surfaces. Among the factors contributing to the excellent resistance of Al-Cu-Li alloys to fatigue crack growth are enhanced slip reversibility and high surface roughness causing a high crack closure level, thus reducing ΔK_eff for crack extension. The fatigue threshold decreased and fatigue crack growth rates increased significantly with increasing the load ratio. This is caused by the decrease in crack closure level at high load ratio. But the fracture mode did not show a significant change with increasing the load ratio.     

淬火条件对砂铸A356合金疲劳裂纹扩展速率的影响

在相同固溶和时效条件下，选择3种不同温度的水对砂铸A356合金进行了淬火处理，并对这3种不同淬火态下的合金进行了拉伸及疲劳裂纹扩展性能研究。结果表明，随淬火介质温度的降低，淬火冷却速率提高，合金的强度增加，塑性下降；当应力比R较低时，淬火条件对合金门槛区的裂纹扩展有明显影响，降低淬火介质温度可以提高合金的疲劳裂纹扩展阻力。此外，不同淬火态合金的疲劳扩展均明显地表现出与应力比R的相关性，这种相关性可以用裂纹闭合来说明。     

疲劳裂纹扩展行为的研究现状及钛合金的疲劳裂纹扩展特征

疲劳裂纹扩展行为是现代材料研究中重要的内容之一。论述了组织结构、环境温度、腐蚀条件以及载荷应力比、频率变化对材料疲劳裂纹扩展行为的影响。总结出疲劳裂纹扩展研究的常用方法和理论模型,并讨论了"塑性钝化模型"和"裂纹闭合效应"与实际观察结果存在的矛盾。最后,对钛合金疲劳裂纹扩展研究的内容和研究结果进行了概述。     

Influence of closure on the 3D propagation of fatigue cracks in a nodular cast iron investigated by X-ray tomography and 3D volume correlation

Synchrotron X-ray tomography was performed during in situ fatigue crack propagation in two small-size specimens made of nodular graphite cast iron. While direct image analysis allows us to retrieve the successive positions of the crack front, and to detect local crack retardation, volume correlation allows for the measurement of displacement fields in the bulk of the specimen. The stress intensity factors (SIFs), which are extracted from the measured displacement fields and the corresponding local crack growth rate all along the front, are in good agreement with published results. In particular, it is possible to link the non-propagation of a crack with crack closure in the crack opening displacement maps or with a local value of the measured SIF range. It is shown that a non-uniform closure process along the crack front induces an asymmetric arrest/growth of the crack.