焊接弧坑热裂纹的力学机制分析

应用断裂力学物理量和有限元方法,分析并揭示焊缝弧坑热裂纹开裂力学机理. 通过应力的无量纲化和正交变换,将热—弹塑性有限元分析获得的节点应力等效为应力强度因子计算模型的外载荷,建立了基于应力强度因子参量的焊缝弧坑热裂纹数值分析模型. 计算并比较焊接热应力作用下焊缝含裂纹单元的张开、剪切和撕裂三类应力强度因子,揭示弧坑裂纹断裂机制并断口试验验证. 结果表明,弧坑热裂纹以拉裂模式主导,在热裂纹敏感区间1100~1000 ℃扩展速度最快,且呈“之”字形开裂取向.     

Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy

The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of ΔK and K_max on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.     

CRACK GROWTH BEHAVIOR IN TITANIUM ALLOY TC11AT ROOM AND HIGH TEMPERATURES

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Comments on spread of plasticity from edge crack close to interface of dissimilar metal joint

Abstract

The situation encountered by a crack in a dissimilar metal joint is analysed using a weight function method applied to a Dugdale crack model. The stress intensity factor at an effective crack tip located in the vicinity of the interface can be calculated from the weight function for a constant pressure load system, based on the principle of elastic stress superposition. The effects on the stress intensity factor and the crack tip opening displacement of a hard transitional zone, which is developed near the interface, are discussed.     

焊接残余应力对2024铝合金薄板疲劳寿命的影响

对搅拌磨擦焊、TIG焊和激光焊进行数值模拟,得到焊接残余应力场.将残余应力场施加到线弹性断裂力学模型之中,运用J积分方法计算残余应力强度因子,并计算裂纹扩展速率,通过与试验结果和虚拟裂纹闭合法计算结果进行对比,验证了文中所使用方法的正确性.研究发现,残余应力强度因子的分布与残余应力分布形式相似.残余应力的引入,对应力比有较大影响,但随着应力比的增大,残余应力对应力比的影响逐渐减弱.焊接残余应力的引入缩短了焊接构件的使用寿命,当裂纹长度较小时,TIG焊接构件使用寿命比搅拌摩擦焊接构件和激光焊接构件使用寿命短.     

Loss of constraint on fracture in thin film structures due to creep

Fracture in thin films is normally constrained by the substrates. If the substrate creeps, however, the constraint will be lost over time. This paper presents a two-dimensional model for channel cracks in an elastic film on a viscous layer, and implements an extended finite element method to evolve the displacement field and the stress intensity factor with relatively coarse meshes. Solutions are obtained for stress intensity factors of channel cracks with several in-plane geometries. The stress intensity factor increases with time, indicating the loss of constraint. Several scaling laws are obtained. Extensions of the present model are outlined for nonlinear creeping and viscoelastic layers, as well as a thick viscous substrate. Fracture in thin film structures subjected to ratcheting deformation under cyclic temperatures using the analogy between creeping and ratcheting is also discussed.     

陶瓷涂层断裂韧性的表征

由于陶瓷本身的脆性,陶瓷涂层的应用受到了极大的制约。断裂韧性是反映材料抵抗裂纹失稳扩展能力的力学性能指标,陶瓷涂层因为脆性和低维特点,对其断裂韧性的表征存在较大困难,目前主要有临界应力强度因子KⅠC、临界裂纹扩展能量释放率GⅠC和裂纹密度β三种表征方法。本文对上述方法进行了总结、分析。     

Application of Paris’s law to thermal cycling-induced failure in semiconductor device patterns

The present work provides a semi-empirical model to show that the growth rate of thermal displacement-induced cracks in semiconductor devices depends on pattern thickness. Paris’s law is adopted to characterize the growth rate of cracks during thermal-cycling. The crack propagation rate is estimated from the semi-empirical relation (dc/dN)_v=C(ΔK)ⁿ, where ΔK indicates the range of an applied stress intensity factor, and C is a scaling constant. The applied stress intensity factor is related to the initial crack length as ΔK=YΔσ(πc)^1/2, where σ represents the thermal displacement-induced normal stress, c describes the pre-existing crack length, and Y is a geometrical factor. The resulting crack growth rate can be expressed as a function of device pattern thickness: dc/dN∞(1/t)^m, where t describes the pattern thickness, and m is another constant. The present semi-empirical results showing the relationship between the crack growth rate and pattern thickness indicate that if a semiconductor device pattern becomes thinner by 68%, its susceptibility to thermal cycling-induced damage will be enhanced by 76%.     

Effects of the Electron Beam Welding Process on the Microstructure,Tensile, Fatigue and Fracture Properties of Nickel Alloy Nimonic 80A

The purpose of this study was to evaluate rotary bending high-cycle fatigue properties and crack growth of Nimonic 80A-based metal and electron beam-welded joints. All the tests were performed at room temperature. Fracture surfaces under high-cycle fatigue and fatigue crack growth were observed by scanning electron microscopy. Microstructure, hardness and tensile properties were also evaluated in order to understand the effects on the fatigue results obtained. It was found that the tensile properties, hardness and high-cycle fatigue properties of the welded joint are lower than the base metal. The fracture surface of the high-cycle fatigue shows that fatigue crack initiated from the surface under the high stress amplitude and from the subsurface under the low stress amplitude. The effect of the welding process on the statistical fatigue data was studied with a special focus on probabilistic life prediction and probabilistic lifetime limits. The fatigue crack growth rate versus stress intensity factor range data were obtained from the fatigue crack growth tests. From the results, it was evident that the fatigue crack growth rates of the welded are higher than the base metal. The mechanisms and fracture modes of fatigue crack growth of welded specimens were found to be related to the stress intensity factor range ΔK. In addition, the effective fatigue crack propagation thresholds and mismatch of welded joints were described and discussed.     

钢轨表面裂纹扩展方向研究

目的研究钢轨表面裂纹的扩展方向。方法采用有限元分析软件ANSYS,在不同轴重和不同角度裂纹的工况下,获得不同裂纹位置的应力强度因子。结果裂纹在接触斑边缘的位置时,应力强度因子K最大。随着轴重的增加,应力强度因子KI和KII均增大;随着裂纹角度的增加,KI增加,而KII减小。当裂纹角度为60°时,其等效应力强度因子幅值ΔKeff最大。结论钢轨表面的裂纹在扩展初期,以斜裂纹为主,扩展角度趋向于60°。     

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.     

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.     

Residual Stress Evaluation of AA2024-T3 Friction Stir Welded Joints

The main aim of this study was to evaluate the residual stress field in friction stir welded joints of 2024-T3 aluminum alloy plates using the slitting method. This is based on the fact that when a cut, simulating a growing crack, is incrementally introduced into a part, residual stresses are relieved on the slot surfaces created, causing the part to deform. Such deformation can be measured by strain gages attached to specific regions of the part and the residual stress profile that originally existed can be evaluated. Cuts were introduced by wire electro discharge machining (WEDM), in finishing mode, either perpendicularly or longitudinally to the weld nugget, in 3.2 × 60 × 120 mm³ rectangular testpieces. For the longitudinal testpieces, the slot was introduced in two different positions: on the center of the weld nugget and 5 mm distant from the weld center line, in order to sample the thermomechanically/heat affected zone. The residual stress intensity factor, K _r, was calculated using a fracture mechanics approach and the inverse weight function method was employed to obtain the initial residual stress profile. Residual stress redistribution profiles ahead of the slot tip could also be derived using the inverse weight function method. However, for cracked components subjected to compressive residual stress fields, when the crack faces are in contact, a non-linear problem arises and the zero displacement condition has to be taken into account in order to provide a more accurate solution of the residual stress field.     

Deformation near parallel cracks close to interface in bonded dissimilar plate

Tensile tests are carried out by using rectangular plate specimens extracted from the explosion clad plate. Artificial parallel cracks are made perpendicularly to the explosive interface in each specimen. When a tensile load is applied perpendicularly to the crack plane, strain, plastic deformation near cracks and crack opening displacement are examined by experiments as well as elasto-plastic finite element analysis. The effects of the material inhomogeneity, the change of material characteristics, the residual stress and the interaction between parallel cracks on the deformation behavior are revealed. The plastic zone around parallel cracks, at relatively low applied stress level, grows mainly ahead of and between two cracks. The interaction between parallel cracks on the crack opening displacement can be explained from the square of stress intensity factor at the low applied stress level and from the plastic deformation at the high applied stress level.     

工业结构钢疲劳极限的断裂力学研究

文章提出一种预测材料疲劳极限的简便方法。该方法仅需获得一个材料参数(如材料硬度)便可确定材料的裂纹扩展抵抗阻力曲线。由于有效应力强度因子范围门槛值ΔKeffth∞是一个与裂纹长度及应力比无关的材料常数,只需将裂纹张开应力强度因子门槛值Kopth表示为裂纹长度的函数,作出材料的疲劳断裂曲线,就可以预测任意应力比下光滑试样的疲劳极限。利用这种方法作出的预测值与试验结果一致。     

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.     

一种基于FRANC3D的高压气瓶安全性评定的新方法

介绍了边界元法计算应力强度因子的方法和原理,利用FRANC3D软件建立了不同尺寸的半圆形表面裂纹的三维模型,计算了裂纹尖端的应力强度因子值,并与Newman-Raju理论公式的计算结果进行比较.结果表明了该软件具有较高的精度.结合蒙特卡罗方法,举例说明15 MnMoVN钢高压球形气瓶的疲劳失效评定和疲劳寿命预测过程,为高压气瓶的安全性评定提供了一种新方法.     

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。     

A study of competitive anion adsorption at the tip of a crack in stress corrosion cracking of AI 27-1 alloy

A method to determine chloride ion concentration adsorbed at the tip of crack in stress corrosion cracking of AI27-1 aluminium alloy in chromic acid based solutions is discussed. The dependance of hydrogen permeability of passivating film at the tip of crack on the concentration of adsorbed chloride ions is the backbone of the method. Critical stress intensity factor, K_HE, and critical crack growth rate, V_cr data form an experimental base for the method and are a necessary prerequisite of hydrogen embrittlement as the main mechanism of cracking in stress corrosion conditions.     

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.