夹杂物对超高强度钢低周疲劳裂纹萌生及扩展影响的原位观测

采用扫描电镜原位观测了数微米大小夹杂物对超高强度钢疲劳裂纹萌生及扩展的影响,并用有限元法解释了夹杂物尺寸和形状对疲劳裂纹萌生及初期扩展的影响程度,得到了超高强度钢低周疲劳裂纹萌生与扩展特性和夹杂物对疲劳破坏的关键尺寸的估计值．同时,讨论了现行实验方法对确定铁基材料裂纹扩展速率的作用．     

基于解析算法与J积分法的耐候钢接头疲劳裂纹扩展分析

为准确描述焊接接头疲劳裂纹在扩展中的动态过程,借助多阵元超声相控阵设备对12 mm厚耐候钢对接接头的表面裂纹扩展动态进行实时监测,得到半椭圆形裂纹深度、长度与寿命的演化关系.借助Abaqus建立有限元分析模型,将裂纹几何变化历程输入,计算裂纹尖端7个方向的能量释放率,获得裂纹尖端应力强度因子幅值.将裂纹尖端(θ = 90o)的K值与BS7910标准中推荐解析公式对比分析,其结果吻合性好.在此基础上,分别采用简化公式与两阶段扩展模型计算试样的疲劳寿命. 结果表明,两阶段裂纹扩展模型能准确地预测对接接头裂纹的疲劳寿命,且与试验结果高度吻合.     

热不匹配应力对热障涂层表面裂纹扩展的影响

本文采用扩展有限元方（XFEM）法研究了高温下热失配应力对热障涂层表面裂纹扩展的影响。结果表明在热失配应力的影响下,表面裂纹的位置、倾斜角度和长度对裂纹扩展的长度、能量释放率和裂尖的应力水平有着显著的影响;对于初始长度相同的裂纹,界面波谷处裂纹扩展长度最长,能量释放率最大;倾斜角度越大,裂纹扩张长度越短,应变能越大;裂纹初始长度越大,裂纹的扩展长度越长且扩展速率越快,能量释放率越大。存在多条表面裂纹的情况下其裂纹扩展相互影响。     

Mechanism of electric fatigue crack growth in lead zirconate titanate

A series of experiments was performed with through-thickness cracks in ferroelectric double cantilever beam (DCB) specimens. Cyclic electric fields of different amplitudes were applied which resulted in cyclic crack propagation perpendicular to the electric field direction. Crack propagation was observed optically and three regimes were identified: a pop-in from a notch, steady-state crack growth and a decrease of the crack growth rate with increasing cycle number. Crack growth only occurred if the applied field exceeded the coercive field strength of the material. Furthermore, the crack extended during each field reversal and the crack growth rate increased with increasing field. Based on the experimental observations, a mechanistic understanding was developed and contrasted with a nonlinear finite element analysis which quantified the stress intensity in the DCB specimens. The driving forces for crack formation at the notch and subsequent fatigue crack growth were computed based on the distribution of residual stresses due to ferroelectric switching. The finite element results are in good agreement with the experimental observations and support the proposed mechanism.     

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.     

含铒铝合金显微组织对疲劳裂纹扩展的影响

对含铒铝合金板材进行了不同的热处理,分析了各板材的显微组织对疲劳裂纹扩展和力学性能的影响。结果表明:晶粒的长/径比越小,板材的疲劳裂纹扩展速率越慢;且随着疲劳裂纹扩展速率的降低,裂纹宽度增加且裂纹路径曲折;稀土元素Er的加入,在合金中形成Al_3(Er,Zr)第二相粒子,其强烈地钉扎位错,阻碍位错运动,以减少位错在晶界处的聚集产生的应力集中,降低板材的疲劳裂纹扩展速率。     

微动工况对疲劳裂纹扩展影响的数值模拟研究

文章研究了不同微动工况对疲劳裂纹扩展的影响,在Hypermesh中建立带有预置裂纹的有限元模型,把单元和节点信息导入Matlab,利用向量式有限元法研究薄板在恒定拉载荷、恒定压载荷、拉压载荷和交变载荷作用时的裂纹扩展规律。研究表明,恒定拉载荷和交变载荷作用下,裂纹开口形状和程度大致相同;裂纹在恒定压载荷作用下发生交错;在拉载荷作用时裂纹开口幅度达到最大值,拉压载荷作用下裂纹扩展开口幅度最小,其次,在交变载荷下裂纹扩展速度最快,拉载荷和压载荷作用下裂纹扩展最慢。     

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.     

A7N01铝合金复合加载下的疲劳裂纹扩展行为

利用CTS试样,研究了A7N01P-T4铝合金母材在I-II型复合加载下,不同加载角度时疲劳裂纹的扩展行为,利用有限元数值计算复合加载下裂纹尖端的应力强度因子(SIF, stress intensity factor)得到了加载角度与裂纹开裂方向的关系,并与由最大周向应力准则导出的关系进行了对比,二者吻合良好;根据疲劳试验和有限元计算的结果,并引入当量应力强度因子,分析了不同加载角下疲劳裂纹的扩展速率. 结果表明,经当量化处理后,各加载角下的裂纹扩展速率曲线基本重合,并且满足Paris公式.     

Crack initiation and kinking behaviours of spot welded coach peel specimens under cyclic loading

Abstract

The present study is carried out to investigate fatigue crack initiation and kinking behaviours of spot welded coach peel (CP) specimens of low carbon steel sheets subjected to cyclic loading by experimental and finite element analysis methods. Evaluations of fatigue crack growth stages were performed by crack tip plastic strains and J integral analyses and also by microhardness measurements on process zone. According to the experimental and analytical results, fatigue crack initiation and growing stages in the spot welded CP specimens can be divided to three stages. Stage I corresponds to 'gap sharpening stage' observed at the beginning steps of cyclic loading with crack growing on the interface plane between the overlapped sheets. Stage II corresponds to kinked crack initiation and propagation through the sheet thickness observed after applying a certain number of loading cycles. Stage III corresponds to crack propagation through the width of the specimens observed at the final step of the fatigue crack propagation. The FE results of the crack paths and crack locations are in good agreement with those of experimental observations.     

T形接头承载角焊缝根部裂纹扩展角度

针对Q355B低合金钢T形接头承载角焊缝根部疲劳失效的扩展路径问题,提出了一种基于受力分析计算的等效应力强度因子法(KEQ法)预测根部裂纹的扩展角度. 经有限元模拟验证,其求解应力强度因子的最大误差小于5%. 与基于有限元分析计算的最大周向应力法(MCS法)和有效结构应力法(ETS法)相比较,并结合3种不同应力水平下弯曲疲劳试验结果发现,基于等效应力强度因子法、最大周向应力法与有效结构应力法求解的裂纹扩展角度分别为25.6°,25.9°和32.2°,相较于实际疲劳试验的根部裂纹扩展角度24°,误差分别为6.67%,7.92%和34.17%. 结果表明,基于KEQ法求解裂纹扩展角度准确度最高,更适于预测承受弯曲疲劳载荷下T形接头角焊缝根部裂纹的扩展角度.     

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.     

涡轮盘中心孔三维疲劳裂纹扩展分析

基于有限元软件ABAQUS和三维裂纹扩展分析软件Franc3D,对涡轮盘中心孔三维疲劳裂纹扩展进行研究分析。首先,对平板试样表面裂纹进行裂纹扩展模拟计算研究,对比手册中Gross/Brown理论模型验证裂纹扩展应力强度因子数值模拟的准确性;其次,针对涡扇发动机涡轮盘结构,对轮盘不同外缘等效应力、转速情况的应力强度因子以及考虑初始缺陷的三维疲劳裂纹扩展寿命进行计算;最后,讨论发动机载荷差异对应力强度因子和裂纹扩展寿命影响规律。结果表明:在相同裂纹长度时,应力强度因子随着轮盘外缘等效应力和转速增加而增大,载荷越大疲劳寿命则越短,且裂纹越长,影响越大。为工程上三维裂纹扩展计算以及寿命评估提供参考。     

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 利用实验测定和有限元模拟计算分析方法,研究了过载对铝合金疲劳裂纹扩展速率的影响。通过测定疲劳裂纹扩展速率和裂纹尖端塑性区半径计算,结果表明过载使裂纹尖端产生较大的塑性区,而导致裂纹在过载后一段时间内停止扩展,即过载对铝合金的裂纹扩展有延迟效应。     

Fatigue crack growth of titanium alloy joints by electron beam welding

Electron beam welding（EBW） has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding（EBW） joints, mechanism of fracture was investigated under scanning electron microscope（SEM）. The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam firstly.Then, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture mechanism.Crack growth was simulated by conventional finite element method and extended finite element method.     

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。     

加载频率对中温环境下疲劳裂纹扩展的影响

由于材料或环境的因素,加载频率对疲劳裂纹扩展速率将产生很大的影响,以工业Ｔｉ为对象,研究了中等温度环境下疲劳裂纹扩展性能及加载频率的影响,并用弹、粘塑性理论对其进行了理论上的探讨,基于该理论导出的本构关系和利用有限元方法,对裂纹尖端的应力应变进行了分析,结果表明,粘塑性应变范围和Ｊ积分范围可作为裂纹扩展的控制参数,能很好地反映加载频率对裂纹扩展的影响。     

LY12爆炸复合板疲劳性能研究

考察了ＬＹ１２爆炸复合板不同取向的拉伸强度、分层韧必及疲劳裂纹扩展行为。观测了疲劳长裂纹的扩展路径形态,并利用断裂力学理论材料的层状结构与其疲劳性能之间的关系。结果表明层合板中的层商界面性能对其疲劳性能具有重要影响,在ＬＹ１２爆炸复合板中,垂直板向Ｌ－Ｓ取向和Ｌ－Ｔ取向的疲劳裂纹在扩展过程中都发生了明显的止裂。     

激光冲击TC17钛合金疲劳裂纹扩展试验

为研究激光冲击强化对钛合金试件疲劳性能的影响,在标准试件的裂纹扩展路径上设计了全强化和间隔强化两种不同的强化方案,研究激光冲击强化对试件疲劳寿命和裂纹稳定扩展时速率的影响规律,利用有限元数值模拟和X射线残余应力测试获得了试件的残余应力场分布状态,并对比分析了试件的断口形貌和微观组织特征。结果表明:相比于未强化试件,激光冲击强化后试件的平均疲劳寿命分别提高了2.14倍和1.90倍,两种不同的冲击强化方法分别降低钛合金试件的裂纹扩展速率24%和15%。间隔强化后试件表面产生-512 MPa的最大残余压应力,裂纹扩展的C′值为-7.3,m值为2.6,而强化间隔区引入最大值为82.4 MPa的残余拉应力,裂纹扩展速率急剧升高,C′值减小至-13.6,m值为8.0。当裂纹扩展到强化区时,扩展速率再次降低,激光冲击强化对TC17钛合金疲劳裂纹扩展有显著的抑制作用。     

STUDIES OF FATIGUE CRACK PROPAGATION AND MICROFRACTUREGRAPHY IN HIGH STRENGTH STEELS

本文用宏观和微观方法测量了几种高强度钢的疲劳裂纹扩展速率,用金相和断口复型法观察了疲劳断口的微观形态以及与显微组织的关系。结果表明,宏观与微观测量的结果基本一致;疲劳裂纹扩展速率随平均应力的增加而增加;18CrMn2MoBA钢显微组织中的颗粒相(M+A)可以降低疲劳裂纹扩展速率。