基于拘束的X80钢裂纹扩展阻力曲线构建

对于管道中的裂纹,由于其裂纹尖端拘束水平低,使用单边缺口弯曲试样、紧凑拉伸试样所测得的裂纹扩展阻力曲线对管道进行断裂评估,得到的结果趋于保守. 文中采用裂纹深度不同的单边缺口拉伸试样,探讨了裂纹深度对裂纹扩展阻力曲线的影响. 采用有限元分析,计算了裂纹深度不同的单边缺口试样的裂纹尖端拘束水平. 最后,基于裂纹尖端的拘束水平,所构建的裂纹扩展阻力曲线能更加准确地表征管道中裂纹的扩展阻力曲线.     

喷丸对H13钢单边带缺口试样疲劳裂纹扩展行为的影响

喷丸工艺通过改变试件的表面形貌、微观组织及残余应力等表面完整性可以影响裂纹的萌生和扩展;对于存在缺陷的试件,喷丸的作用机制和影响结果有所不同。采用白光干涉仪、SEM、XRD及显微硬度计等对单边带缺口的H13钢薄板试样喷丸前后的表面完整性进行了测定。并借助原位SEM开展了系列裂纹扩展试验,分析了喷丸对试样疲劳寿命、裂纹扩展速率以及疲劳断口特征的影响。研究结果表明,虽然残余压应力诱发裂纹闭合,但由于喷丸后表面粗糙度的大幅提高增强了缺口效应,表面加工硬化使得韧性有所降低,以及残余应力在加载过程中发生松弛等因素,喷丸后单边带缺口试样的裂纹萌生过程缩短,疲劳寿命降低,且裂纹扩展速率的变化较小。喷丸前后疲劳断口形貌均为准解理特征,喷丸后断口近表面处的撕裂棱特征消失。     

电弧增材制造钛合金界面处残余应力及其影响研究

采用电弧增材制造方法制备了含增材/基材界面钛合金板,采用轮廓法测量了其残余应力分布。建立了模拟紧凑拉伸(C(T))试样加工和裂纹扩展过程中残余应力发展的有限元模型,缺口状态C(T)试样内残余应力分布与轮廓法测试结果吻合良好。采用该模型讨论了试样内残余应力随裂纹扩展的变化规律及对裂纹扩展的影响。试验和数值分析结果表明:2种类型试样缺口状态的残余应力分布有很大差别,A类试样(缺口位于基材)残余压应力区域靠近缺口根部,C类试样(缺口位于增材)残余压应力区域远离缺口根部;A类试样内残余应力随裂纹扩展迅速释放,残余应力引起的应力强度因子较小;C类试样内残余应力随裂纹扩展变化较小,残余应力引起的应力强度因子较高,降低了疲劳裂纹扩展寿命。     

单边缺口拉伸试样的断裂韧性计算方法对比

在工程临界评估（engineering critical assessment）中,裂纹尖端张开位移（CTOD值）的精确性将极大影响设计安全裕度与服役寿命.单边缺口拉伸（SENT）试样裂纹尖端的应力应变场与管道在实际服役状况下相似,被认为比较适用于测量管道CTOD值.目前有几种针对SENT断裂韧性的计算方法,但是并没有统一的标准.文中采用API X70管线钢,进行SENT试样断裂韧性试验并对比各计算方法与双刀口法之间的区别.采用Crackwise 4.0评估了不同计算方法得到韧性值对裂纹极限尺寸的影响.结果表明,几种CTOD计算方法相较于双引伸计法都有较大误差,断裂韧性的精确度对于ECA评估极限裂纹尺寸有极大影响.     

拉伸超载对超高强度钢A-100冲击疲劳性能的影响

研究了A-100钢单边缺口三点弯曲试样经拉伸超载处理后,冲击疲劳性能的变化。在室温下测得拉伸超载处理前后,冲击能量-冲击疲劳寿命曲线,并对冲击疲劳断口进行微观分析。结果表明:拉伸超载可延长A-100钢的冲击疲劳裂纹起始寿命而对裂纹扩展寿命无影响;试样超载后裂纹源区微观形貌发生变化,缺口根部残余压应力的存在是造成这些变化的主要原因。     

焊接残余应力对7N01铝合金疲劳裂纹扩展影响

焊接残余应力作为平均应力影响裂纹扩展. 将残余应力与外载平均应力分离,通过构建典型焊接残余应力场,借助扩展有限元计算焊接残余应力场的应力强度因子. 开展了紧凑拉伸（CT）试样的疲劳扩展试验,基于Walker公式将裂纹尖端平均应力强度因子K_m（静态量）和应力强度因子幅值ΔK（动态量）分离,获得疲劳裂纹扩展速率da/dN与K_m及ΔK的非线性关系. 结果表明,不同外载荷下,应力比与裂纹长度为非线性关系;残余应力对裂纹扩展存在尺度效应:CT试样裂纹长度小于2 mm时,残余应力场明显影响疲劳裂纹扩展速率;当裂纹长度大于2 mm,外载荷为主导因素.     

基于单边缺口拉伸试样的环缝接头阻力曲线确定方法对比

近些年单边缺口拉伸(SENT)试样被用于管道基于应变的设计与评估,并且相关机构发展了一些断裂韧性的测试标准,包括DNVGL-RP-F108,CANMET,BMT,USP以及英国国家标准BS 8571等. 但相关测试标准之间在测试原理、测试方法以及断裂韧性计算方法上还存在较大差异,有必要对它们之间的区别进行进一步的研究. 文中以API X80管线钢环缝接头为研究对象,对夹持式单边缺口拉伸(SENT)试样的裂纹尖端张开位移(δ)和J积分阻力曲线不同确定方法进行了对比. 结果表明,通过拟合计算J-R曲线、δ-R曲线以及初始断裂韧性值J_0.2和δ_0.2,给出了SENT试样断裂韧性阻力曲线计算的推荐公式.     

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.     

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。     

加载波形对D36钢腐蚀疲劳裂纹扩展速率的影响

对标准单边缺口三点弯曲试样分别施加三角波、正弦波、方波3种波形,得到了不同波形作用下的腐蚀疲劳裂纹扩展曲线,研究了在不同电位条件下加载波形对海水中D36钢腐蚀疲劳裂纹扩展速率(da/d N)的影响。结果表明:无阴极保护时,三角波和正弦波作用下试样的疲劳裂纹扩展速率相当,并小于方波作用下的疲劳裂纹扩展速率,且方波的加速作用体现在低应力强度因子幅度时;在阴极保护条件下,这种现象更为明显;而在阳极极化条件下,方波对试件的疲劳裂纹扩展的加速作用减弱。     

考虑材料微观塑性损伤的焊接接头延性裂纹扩展行为分析

通过考虑材料微观塑性损伤，定量研究了焊接接头中强度匹配、试件几何形状对接头抗延性裂纹扩展行为的影响。对管线用钢焊接接头，测试了标准三点弯曲试样、同种接头及不同匹配状态的双边裂纹拉伸试样的阻力曲线。通过考虑材料塑性损伤的数值模拟，由标准三点弯曲试样的阻力曲线测试结果，得出反映材料微观损伤的特殊单元模型控制参量，再根据该参量对同种接头及不同接头强度匹配下双边缺口拉伸试件的阻力曲线进行了定量预测，其结果与试验结果相当吻合。结果表明，考虑塑性损伤的特殊单元模型能很好地描述接头的抗延性裂纹扩展阻力特性。     

Effect of surface rolling on fatigue behavior of a pearlitic ductile cast iron

Surface rolling is a mechanical treatment usually used in parts fabricated with steel and ductile cast iron, specifically in stress concentration regions, to improve fatigue properties. This process hardens and introduces compressive residual stresses to the surface of the material through the application of controlled strains, thus provoking a reduction of resulting tensile stress at its surface under cyclic loading. This work deals with the effect of surface rolling on high cycle fatigue behavior of a pearlitic ductile cast iron used in crankshafts by the automotive industry. Rotating bending fatigue tests were performed in both smooth and notched specimens, the latter either with or without a surface rolling treatment. Compressive residual stresses and heavy plastic deformation imposed on the surface grains due to cold work made difficult the nucleation and propagation of the crack at the rolled surface of the notch. As a consequence, surface-rolled notch testpieces presented a higher endurance limit (478 MPa) than both smooth (299 MPa) and notched (166 MPa) testpieces did. The surface rolling apparatus developed for this work proved to be very efficient and simple, providing good control of parameters involved in the process (i.e., rolling load, frequency, and number of revolutions).     

NUMERICAL MODELING OF DUCTILE CRACK EXTENSION USING COMPUTATIONAL CELL ELEMENTS FOR WELDED JOINTS

1.IntroductionConstrainteffectsonfracturetoughnessofferritesteelsremainakeyissueforthesafetyaJssessmentofstructures.Strllcturalandpressurevesselsteelsgenerallyexhibitincreaseinfracturetoughnessoverthefirstfewdistance0fstablecrackgr0wth.Laborat0rytestingoffracturespecimenstomeasureresistancecurves(R-curves)c0nsistentlyrevealsamarkedeffect0fabsolutespecimensize,ge0metry,andloadingmode0nRcurvesI1-3].Forthesamematerial,deep-notchbendspecimensyieldl0wRcurvewhiletheshallow-notchbendspecimenortensi…     

Fatigue Crack Growth of Peened Friction Stir-Welded 7075 Aluminum Alloy under Different Load Ratios

The effect of peening on the fatigue crack growth performance of friction stir-welded 7075 aluminum alloy was investigated. The fatigue crack growth rates were assessed for laser- and shot-peening conditions at stress ratios (R) of 0.1 and 0.7. The surface and through thickness residual stress distributions were characterized for the different regions in the weld. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and the as-welded condition. The effect of the compressive stresses obtained through laser peening was deemed responsible for increasing the resistance to fatigue crack growth of the welds.     

裂解炉管焊接残余应力场中裂纹扩展行为的数值分析

针对裂解炉管对接焊接热影响区中常见的环向与斜裂纹在焊接残余应力的作用下的扩展行为进行数值预测.采用基于VNA算法的有限单元交互法(FEAM),结合有限元方法研究焊接残余拉应力对炉管裂纹扩展的影响,通过研究裂纹尖端区域的应力强度因子的变化来判断裂纹的准静态扩展倾向.结果表明,在多道焊焊接残余应力的影响下,在炉管焊缝热影响区内表面的环向裂纹有沿环向变长的倾向,而斜裂纹则同时有变长、变深的倾向.     

Residual stresses and FCP prediction in FSW through a continuous FE model

Residual stresses prediction is definitively a relevant and open issue in welding processes. In the last years, different numerical models are proposed by many researchers to predict the residual stress field in FSW butt joint. However, most of these works are based on properly tuned analytical models and neglect the effect of the mechanical action due to the presence of the tool pin.In the present paper, a continuous 3D FE model, which was previously developed by some of the authors, was used to simulate the FSW process of butt joints with a single block approach, and predict the residual stress field by considering both thermal and mechanical actions. Then, the residual stress effect on the fatigue life was estimated through fatigue crack propagation (FCP) tests in compact tension specimens. Encouraging results were found out leading to a few interesting conclusions that have to be properly verified in the future.     

Fatigue crack/residual stress field interactions and their implications for damage-tolerant design

Residual stress fields are now widely accepted to have significant influence on fatigue crack growth. Tensile stresses have detrimental effects on fatigue lives, whereas compressive residual stresses can be beneficial. Control of fatigue lives via residual stress is now established in many industrial applications, using techniques such as shot peening or cold expansion. However, knowledge of the processes that occur when a fatigue crack grows through a pre-existing stress field is far from complete. Although the residual stress field will clearly have an effect on crack growth, the crack will equally have an effect on the residual stress field. The determination of this effect is not trivial, and direct measurement may be the designer’s best safeguard. This article outlines the complementary effects that a growing fatigue crack and a residual stress field have on each other. Two types of residual stress field are considered: mechanically induced and thermally induced. The results are discussed in terms of the implications that residual stress interactions have for damage-tolerant-based design.     

Long fatigue crack growth behaviour of ferritic steel weld metal

Abstract

Single edge notched weld joint specimens were tested at different stress levels to study the long fatigue crack growth behaviour of a ferritic steel (nuclear grade SA333 Grade 6 steel) weld metal at two stress ratios R of 0·1 and – 1. A two slope behaviour was noticed in the crack growth rate versus stress intensity factor range plots at both stress ratios. Different parameters were employed to generalise the load ratio effect on fatigue crack growth rate.