8090Al—Li合金疲劳短裂纹门槛值的研究

本文研究了8090Al-Li合金穿透疲劳短裂纹的门槛值,测定了不同长度短裂纹的门槛值和裂纹闭合效应,探讨了该合金疲劳短裂纹门槛值、裂纹尾迹和裂尖屏蔽之间的关系,给出了该合金穿透疲劳短裂纹的上界值和理论最低门槛值。     

海洋用结构钢在NaCl溶液中疲劳短裂纹的扩展

本文用海洋平台结构钢A537CL1的单边裂纹样品,在空气和3.5％ NaCl水溶液中进行了腐蚀疲劳短裂纹扩展实验,比较不同裂纹长度及环境介质对疲劳短裂纹扩展行为的影响。结果表明,在门槛值附近,对于相同的应力强度因子范围,短裂纹扩展速率比长裂纹扩展快。短裂纹扩展的门槛值随裂纹长度的减小而减小。与空气介质相比较,盐水对疲劳短裂纹扩展加速的程度远大于长裂纹。     

GROWTH OF SHORT CRACKS OF A537CLI STEEL UNDER FATIGUE LOADING IN 3.5% NaCl SOLUTION

The effects of environmental media,both in atmosphere and in 3.5% NaCl aqueous solution,on the growth of short cracks in comparison with the long ones have been investigated underfatigue loading on the specimens of single-edge crack about 0.05—0.15 mm for A537CLIsteel.The growth rate of short cracks is faster than those of long ones around the threshohtstress intensity.The threshold stress intensity for short crack growth decreases with de-creasing crack length.The acceleration effect of the salt water,as compared with atmosphereenvironment,on the growth of short cracks under fatigue is much greater than that on thegrowth of long cracks.     

组合加载下碳素工具钢中疲劳小裂纹的门槛区行为特征

通过考察碳素工具钢的表面滑移、疲劳破坏及表面裂纹的扩展条件，以统一的局部应力讨论了光滑试样和裂纹试样的疲劳特征。分析和实验结果表明，在裂纹试样的名义应力小于光滑试样表面滑移门槛应力的范围内，线弹性断裂力学是有效的，对应裂纹扩展门槛值的裂尖性区为一常数。     

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.     

BEHAVIOUR OF INITIATION AND GROWTH OF SHORT FATIGUE CRACKS

ＢＥＨＡＶＩＯＵＲＯＦＩＮＩＴＩＡＴＩＯＮＡＮＤＧＲＯＷＴＨＯＦＳＨＯＲＴＦＡＴＩＧＵＥＣＲＡＣＫＳＷＵＺｈｉｘｕｅ;ＸＵＨａｏ（ＩｎｓｔｉｔｕｔｅｏｆＭｅｃｈａｎｉｃａｌＥｎｇｉｎｅｅｒｉｎｇ,ＮｏｒｔｈｅａｓｔｅｒｎＵｎｉｖｅｒｓｉｔｙ,Ｓｈｅｎ...     

30CrMnSiNi2A高强钢的疲劳小裂纹扩展特性及寿命预测

研究了３０ＣｒＭｎＳｉＮｉ２Ａ高强钢在恒幅载荷下的小裂纹起始特征和裂纹扩展特性。试验结果表明，小裂纹起始于缺口表面初始缺陷（如夹杂或孔洞）处。在应力比Ｒ＝０的恒幅载荷下，未显示小裂纹效应：在Ｒ＝－１的恒幅载荷下，则显示出小裂纹效应的存在。利用裂纹闭合模型和微观结构缺陷作为初始裂纹尺寸的方法预测了小裂纹扩展速率和疲劳全寿命，预测值与实验值符合良好。     

Influence Mechanism of Cerium on the Threshold of Short Fatigue Crack for Aluminum-lithium Alloy 2090

The short fatigUe cracks for aluminum-lithitim alloys are mainly in the field of mechanicalcharacteristic research [l--31. It is found thataluminum-lithium alloys eallibit the strongershort crack effect, which is difficult to overcome. In this icicle, the thresholds of short fatigUe cracks for alUminUm-lithium alloys 2090and 2090 Ce have been quantitatively stUdiedand evaluated. By adding Ce into alloy 2090, thebeneficial influence over the microstrUcture andproperties can be achieved [4, 5…     

Integrated fracture mechanics approach to analyse fatigue behaviour of welded joints

Abstract

Current fracture mechanics methods for fatigue assessment, including those that consider thresholds for crack propagation, are based on long crack behaviour. The present work is concerned with an attempt to predict the fatigue strength of welded joints using a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, which is a function of crack length. The fatigue strength of butt welded specimens stressed transversely was analysed. Experimental results from the literature were used for comparison. Estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks, and the applied stress distribution along the crack path obtained from simple finite element models. The influence of plate thickness, initial crack length, and reinforcement angle on fatigue strength of butt welded joints was analysed. Results show good agreement with experimental trends.     

BEHAVIOR OF SHORT FATIGUE CRACKS AT BLUNT NOTCHESIN A MEDIUM CARBON STEEL

1.IntroductionTheinitiati0nandpropagation0ffatiguec=acksatn0tchesareofgreatpracticalim-portancesincethemaj0rity0ffatiguefailuresinengineeringc0mponentsandstructuresoccurduetol0calstressconcentrationsatnotches.Thefatiguelifeisusuallyseparatedintotwoparts:0neportion0flifespentincrackinitiationandtheotherp0rti0nspentinpropagati0n.However,t0talfatiguelifecannotbeevaluatedbecausetherearepresentlyn0proceduresf0revaluatingthecrackinitiati0nlength.Crackinitiati0nisusuallydefinedasthepr0cessesofl0calpl…     

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 …

基于“有效短裂纹准则”,用复型研究了１Ｃｒ１８Ｎｉ９Ｔｉ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     

THE CHARACTERISTICS OF SHORT CRACKS AND EQUIVALENT PARAMETER OF FATIGUE DAMAGE

1. IntroductionThe initiation and propagation behavior of short cracks occupy a sigllificant fractionof the fatigue life of unflawed ..ie.ialll--3]. FOr this reason all understanding of the basiccharacteristics of short cracks in a smooth specimen is very importallt. There have beensome studies concerned with the multiplication of cracks dllring fatiglle ill carbon steels[3--4]and a stainless steeds]. In particular Goto reported that there is a colltilluous formationof microcracks during the …     

SHORT FATIGUE CRACK BEHAVIOUR IN ISO-STRESS SPECIMENS

Fatigue tests were made on a vibration machine using specially designed specimens to investi-gate the initiation and propagation features of short fatigue cracks in a weld metal.The spec-imens were triangular in shape so that when loaded as a cantilever beam the surface tensilestress is constant.Test results show that short fatigue cracks mainly originate from slip bandswithin ferrite grains.The development of short fatigue cracks is marked by a gradual increasein crack density with fatigue cycles.Coalescence of short cracks leads to crack propagationand the crack path is predominantly transgranular.     

45钢疲劳短裂纹的形成和扩展

本文对４５钢光滑试样疲劳短裂的纹的形成和扩展进行了试验，利用复型技术和光学显微镜观测了裂纹尺寸演化全过程，发现疲劳短裂纹形成寿命小于失效寿命的１０％，根据裂纹的扩展速度将短裂纹形成分成微观短裂纹和物理短裂纹两个阶段，给出了区分两种短裂纹的裂纹特征尺寸值和两种短裂纹的扩展速度公式。     

一种等应力试样中的疲劳短裂纹行为

设计了一种三角形试样,当施加弯曲载荷时,该试样等腰三角形段内的表面拉应力为常数。采用振动试验机对所设计的试样进行疲劳试验,以研究一种焊接金属表面疲劳短裂纹萌生和发展特征。结果表明,疲劳短裂纹起源于铁素体晶内的滑移带。随疲劳周次增加,短裂纹发展以裂纹密度不断增加为主要特征。短裂纹的汇合造成裂纹发展且裂纹路径以穿晶为主。     

8090Al—Li合金的疲劳裂纹扩展

研究了8090型Al—Li合?疲劳裂纹扩展行为以及组织结构和环境的影响,结果表明,在空气中,自然时效和欠时效?现出最好的疲劳裂纹扩展阻力,其次是峰时效状态,而过时效状态的疲劳裂纹扩展阻力最低,在3.5%NaCl水溶液中,时效状态对疲劳裂纹扩展行为有着同空气中相同的影响规律,但在同样的时效条件下,在3.5%NaCl水溶液中的疲劳裂纹扩展阻力要比空气中的低,在相同的应力强度因子范围△K作用下,缺口短裂纹和物理短裂纹均表现出比长裂纹高的扩展速率,用位错的平面滑移性和循环滑移可逆性解释了时效的影响,用裂纹的闭合效应和裂纹尖端塑性区尺寸说明了长短裂纹扩展行为的差别。     

两平行热疲劳裂纹的屏蔽效应

受热疲劳损伤的构件表面易产生热疲劳裂纹网。裂纹网中主裂纹的应力强度因子可反映构件的损伤程度,然而由于裂纹问的屏蔽效应,直接计算裂纹网中主裂纹的应力强度因子是困难的。用有限元方法,计算了若干组平行裂纹发生屏蔽效应时的应力强度因子。定义了热疲劳裂纹的比间距n、屏蔽剩余百分数s和裂纹长度比值f,发现此三者有确定关系且与边界条件及裂纹长度无关。给出了n—s,的关系曲线,根据此曲线可按单条裂纹的应力强度因子推算两条裂纹发生屏蔽效应时主裂纹的应力强度因子。     

钛合金焊缝表面疲劳短裂纹的扩展行为

对TC2钛合金焊缝金属疲劳表面短裂纹扩展行为进行了实验研究,表明裂纹的扩展过程是多裂纹系统的演化过程.基于实验结果,考虑到短裂纹扩展过程中的合并、干涉作用等因素,用应力松弛区模糊化的方法,建立了仿真模型,对疲劳短裂纹演化的物理过程进行了数值模拟.将短裂纹演化行为模拟结果与短裂纹扩展复型的实验结果比较,可以看出两者吻合较好,表明所建立的准则合理、实用.     

FATIGUE CRACK PROPAGATION IN Al-Li ALLOY 8090

Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up-on the aging conditions.The naturally aged and underaged specimens showed the highest re-sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overagedspecimen is the lowest and that of the peakaged specimen in between.As compared to air,3.5% NaCl solution does not change the effect of aging conditions on the fatigue crackgrowth,but causes a decrease of the resistance to the fatigue crack growth under the same ag-ing condition.Both short cracks from notch and physically short cracks showed much higherrates of fatigue crack propagation in comparison with long cracks under the same aging condi-tion and stress intensity level.The growth behavior of the short crack depends on its type.Thegrowth rate of short crack from notch decreases first to a minimum and then increases with in-creasing △K.However,the physically short crack grows at a progressively increasing rate.The effects of aging conditions are explained in terms of the slip planarity of dislocations andthe cyclic slip reversibility.The observed short crack behavior is considered to be dependent onthe crack closure and the local plasticity near the crack tip.     

SHORT FATIGUE CRACK PARAMETER BASED ON THE TOTAL CRACK AREA

1. Introduction The initiation and propagation behavior of short cracks occupies a significant fraction of the fatigue life of unflawed material[13]. For this reason an understanding of the basic characteristics of short cracks in a smooth specimen is very important. There have been some studies concerned with the multiplication of cracks during fatigue in carbon steels[3~1 and a stainless steel1~J. In particular Goto reported that there is a continuous formation of microcracks during the en…