一种等应力试样中的疲劳短裂纹行为SCIEI

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。     

一种亚稳β钛合金中疲劳短裂纹穿晶扩展晶体学特征的EBSD研究

简单介绍一种疲劳短裂纹扩展跨越晶界的晶体学模型和利用EBSD技术判定活动滑移系的简便方法.作为实际应用的示例,利用EBSD技术应用晶体学模型研究了一种亚稳β钛合金--TIMETAL LCB中疲劳短裂纹穿晶扩展的晶体学特征.实现了穿晶裂纹活动滑移系的判定.结果表明除在试样表面晶粒两侧的裂纹走向偏折因素外,裂纹面(活动滑移面)间角是更重要的控制短裂纹穿晶扩展的晶体学因素.     

激光熔化沉积TC18钛合金组织及缺口高周疲劳性能

研究了激光熔化沉积TC18钛合金沉积态及热处理后的组织,以及室温缺口高周疲劳裂纹萌生和扩展特性.激光熔化沉积态组织由具有沿沉积方向定向生长特征的短柱晶及等轴晶组成,晶粒内部为超细的网篮组织;通过双重退火处理后,组织转变为较粗大的片层状网篮组织.研究表明,随着试样最大循环应力的升高,主裂纹长度降低,疲劳寿命也降低;激光熔化沉积TC18钛合金缺口疲劳裂纹源唯一,且起源于缺口根部表面处;疲劳裂纹扩展区发现两种类型的二次裂纹,二次裂纹有助于消耗裂纹扩展能,有助于提高疲劳极限.     

原位XRD法研究电疲劳过程中铁电陶瓷PLZT的畴变

利用SEM观察了未疲劳和疲劳铁电陶瓷极化试样的断口形貌.利用原位XRD观测了电疲劳过程中极化试样表面铁电电畴的变化,并记录了相应的剩余极化强度.实验结果表明:疲劳试样的主要断裂模式为沿晶断裂;随疲劳次数的增加,每次疲劳后试样表面I(002)减小,I(200)增加;每次疲劳后在外加电场作用下试样表面I(002)增加幅度和I(200)减小幅度随疲劳次数的增加而减少.这些现象说明:可翻转电畴的减少是剩余极化强度降低的主要原因;非协调高畴变应变形成大量沿晶微裂纹并导致疲劳试样主要断裂模式为沿晶断裂;沿晶微裂纹的形成和伸长主要发生在电疲劳的初期.     

铜铝复合接触线用银铜合金疲劳试验

通过采用不同应变幅控制,对不同变形量冷拉银铜合金进行室温低周疲劳试验.结果表明,随着应变幅的增大,滞后环面积也随之增大;随着循环周次的增加,循环应力逐渐降低,从而产生疲劳软化;加工率为38%的银铜合金循环周期大于加工率为19%的银铜合金循环周期;试样断裂后存在三个明显疲劳特征区:裂纹源、裂纹扩展区、瞬断区;银铜合金的裂纹扩展具有穿晶和沿晶两种方式.     

AZ31镁合金在高周疲劳过程中微观组织和孪晶演变特征

利用扫描电子显微镜（SEM）和电子背散射衍射（EBSD）技术研究了室温条件下AZ31镁合金在不同加载频率（3和30 Hz）和不同应力幅值（90,95,100,105,110 MPa）疲劳变形后的组织演变规律及断口形貌特征。结果表明：随着加载应力增加,基体内残余孪晶数量增加,残余孪晶主要以拉伸孪晶形式存在。随着应力幅值的增加晶粒逐渐细化,这是由于在循环过程中,拉伸孪晶演变诱导晶粒细化。随着应力幅值的增加,织构强度显著减弱,这与试样疲劳后的再结晶机制有关。通过对试样疲劳断口的分析,发现孪晶片层处容易引起裂纹萌生,随着应力的增加,试样中裂纹扩展区面积逐渐减小,在疲劳裂纹扩展区观察到明显的疲劳辉纹。最终断裂区表面粗糙,主要存在韧窝、撕裂脊以及二次裂纹等形貌。在最终断裂区可观察到韧窝,韧窝尺寸随着循环应力的增加,在较高加载频率下,韧窝的尺寸与数量均减小。     

工业纯镁内部疲劳微裂纹的热扩散性愈合

采用单向对称拉压低周疲劳疗法在具有等轴晶的工业纯镁试样中引入了微米级的内部裂纹,裂纹主要位于晶内和穿晶位置,宽度为0.5-15μm,长宽比分布在10-35之间,在623K对疲劳试样进行2,4和6.5h的真空退火处理,SEM观察表明：晶内疲劳微裂纹纵剖面二维形态发生了主要由表面扩散控制的形态变化,由初始的扁椭圆形演化成多个球洞定向排列的形态;穿晶疲劳微裂纹纵剖面二维形态则在表面和晶界扩散的耦合作用下首先在晶界处分隔成两部分,并在晶界上留下了一个空洞,在退火处理的后期阶段,空洞收缩消失,引起空洞间距的增大并引起试样密度少量恢复。     

T6态ZM51变形镁合金的高周疲劳行为

以工业化挤压生产大截面尺寸的ZM51镁合金为研究对象,测试挤压态和T6态合金沿挤压方向(ED)的静态力学性能,并在应力比R=-1的条件下,研究T6态ED试样的高周疲劳行为。结果表明:T6处理后,ED试样的拉伸和压缩强度均得到提升,但伸长率大幅降低,压缩屈服强度的提高幅度远低于拉伸屈服强度的,合金拉压屈服不对称性增加。T6态ZM51镁合金循环周次为1×10~7时的疲劳极限为103 MPa,约为其抗拉强度的30%。发生在裂纹尖端塑性区内的"拉伸孪生-退孪生"是主要的疲劳损伤机制。合金疲劳裂纹的萌生位置以试样表面附近的孪晶界为主,疲劳裂纹穿晶路径扩展。疲劳断口两侧存在覆盖整个断口的针状孪晶层,应力越大,孪晶层厚度越小,而单个晶粒内的针状孪晶的数量越多,且间距越小。     

表面滚压对LZ50车轴钢疲劳短裂纹行为的影响

完成了6根表面经滚压处理的漏斗形圆棒试样疲劳短裂纹复型实验,结果表明其短裂纹行为表现出明显的微观结构短裂纹(MSC)和物理短裂纹(PSC)2阶段特征.在MSC阶段,由于不同微观结构障碍的阻碍,主导短裂纹出现2次较明显的扩展减速,进入PSC阶段后,随着主导短裂纹尺度的增长,该现象不再显著.与表面未经滚压的试样相比,在给定主导短裂纹尺度条件下,滚压试样的裂纹扩展速率低得多,这种差异在MSC阶段可达1个数量级.滚压试样平均疲劳寿命达到未滚压试样的6.4倍.表面滚压处理有效抑制了微裂纹的形核与合并,改善了局部微观结构条件,并推迟了MSC与PSC过渡阶段的到来,提高了材料的抗疲劳性能.对主导短裂纹尺度、疲劳寿命分数和有效短裂纹密度3种特征参量进行分析,确定了其良好假设分布和统计演化规律.     

316LN不锈钢管状试样高温高压水的腐蚀疲劳行为

设计了一种管状疲劳试样,高温高压水流经试样内部,试样外部与空气接触。利用管状试样研究了316LN不锈钢高温高压水腐蚀疲劳性能,重点关注了应变速率对其疲劳性能的影响。实验结果表明,高温高压水环境降低了316LN不锈钢的疲劳强度,且疲劳寿命随应变速率降低而降低;管状试样与标准棒状试样获得的疲劳寿命相差不大,表明利用管状试样研究核电结构材料高温高压水环境疲劳性能是合理可行的。在低应变速率条件下,疲劳裂纹源区域为典型的扇形花样,呈现准解理开裂特征。疲劳裂纹扩展区为典型的疲劳辉纹特征。疲劳裂纹萌生阶段高温高压水环境效应更加显著。同时讨论了316LN不锈钢在高温高压水环境中的疲劳损伤机理。     

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.     

BEHAVIOUR OF INITIATION AND GROWTH OF SHORT FATIGUE CRACKS

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

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。     

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.     

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.     

X80管线钢焊接热模拟热影响区疲劳裂纹扩展行为的分析及预测

通过焊接热模拟方法对X80双相管线钢不同焊接热影响区疲劳裂纹扩展行为进行了研究,并分析实际焊接试样与焊接热模拟试样的热影响区疲劳裂纹扩展行为的差异,讨论了焊接热模拟技术应用于疲劳裂纹扩展寿命预测的可行性。结果表明,当裂纹在粗晶热影响区(CGHAZ)和临界热影响区(ICHAZ)上扩展时,由实际焊接试样与焊接热模拟试样热影响区所测得的疲劳裂纹扩展速率曲线(da/dN-DK曲线)存在明显差异,其原因与显微组织变化引起的裂纹扩展阻力大小不同有关。因此,建议采用实际焊接试样完整的热影响区所测得的da/dN-DK曲线来评估疲劳裂纹扩展寿命。     

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.     

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

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

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 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…