焊接接头非均匀力学性能对J积分和失效评定曲线的影响

由于焊缝填充金属的不同和焊接热循环的作用,焊接接头不同区域有着不同的力学性能。本文将焊接接头模化为由两种材料组成,即母材金属和焊缝金属,用弹塑性有限元分析法计算了焊接接头强度组配对Ｊ积分和失效评定曲线的影响。结果表明,焊接接头不均匀强度组配对裂纹驱动力－Ｊ积分和失效评定曲线的形状与位置有很大影响。对于一个特定的焊接接头,如果接头为低组配或等组配,则以母材金属的极限载荷作为无因次载荷而建立的安全评定     

核级管道材料高温疲劳裂纹扩展行为实验研究

为研究核级管道材料在500℃以上的高温疲劳裂纹扩展性能,对管道母材、焊缝和热影响区材料进行了高温条件下的疲劳裂纹扩展速率试验,基于概率分析方法获得了考虑不同存活率的概率疲劳裂纹扩展曲线。研究结果表明,高温条件下,管道不同位置区域材料的疲劳裂纹扩展性能存在较为明显的差异,焊缝和热影响区的抗疲劳裂纹扩展能力明显优于母材。试验研究结果可用于核反应堆管道结构安全评估和断裂力学分析。     

直流电压降法测量核电结构材料在空气中的疲劳裂纹扩展速率

介绍了直流电压降（DCPD）方法测量材料裂纹扩展的原理,并采用DCPD方法测量了反应堆典型结构材料在空气中的疲劳裂纹扩展速率（CGR）,分析了载荷、频率和载荷比（R＝K_max/K_min, K为应力强度因子）对疲劳裂纹扩展速率的影响。实验结果表明,材料的硬度与疲劳裂纹扩展速率有密切关系,即材料硬度越高,裂纹扩展速率越高。     

温度及氢化物对Zr-Sn-Nb合金焊缝疲劳裂纹扩展行为的影响研究

采用小尺寸三点弯曲试样完成了渗氢和未渗氢Zr-Sn-Nb合金母材和焊缝在室温和360 ℃下的疲劳裂纹扩展速率试验,研究了温度和氢化物对焊接薄板的疲劳裂纹扩展行为的影响。结果表明,腐蚀吸氢后,在母材和焊缝区均析出了呈水平向分布的片状氢化物。相比母材区,焊缝区析出的氢化物更为致密。在相同温度下,未渗氢母材的抗疲劳裂纹扩展性能均优于未渗氢焊缝。腐蚀吸氢后,母材在相同温度下的抗疲劳裂纹扩展性能也优于焊缝。在室温下,腐蚀吸氢后的母材和焊缝的抗疲劳裂纹扩展性能相比吸氢前明显下降。360 ℃下,渗氢母材和焊缝中的氢化物部分溶解,使得其抗疲劳裂纹扩展性能得到一定程度提升。     

氢及氢化物对锆合金疲劳裂纹扩展速率的影响及估算

采用紧凑拉伸（CT）试样．研究了不同氢含量的Zr-4及Zr-Sn-Nb合金在室温下疲劳加载裂纹扩展（dα/dN）行为．用扫描电镜观察了断口形貌。结果表明，氢含量对疲劳裂纹扩展速率影响微弱，疲劳断裂受通常的裂纹萌生、稳态扩展和瞬间断裂机制控制。根据疲劳裂纹扩展机理．导出了裂纹扩展门槛值△Kth的关系式．得出了一个描述疲劳裂纹扩展速率油（dα/dN）与材料性能常数之间的关系式，该关系式可用于预测材料的疲劳裂纹扩展速率。用锆合金实验数据对（dα/dN）预测表达式进行验证．结果表明，预测值与实验值吻合较好。     

焊接接头疲劳裂纹随机扩展的概率模型

焊接接头微观组织的不均匀性，使得结构疲劳裂纹萌生和扩展寿命的随机性很大。本文采用概率断裂力学方法和可靠性分析方法，针对压力容器焊接接头的组织不均匀特点，研究了焊缝区疲劳失效概率模型。Monte-Carlo数值模拟结果表明，这一模型对裂纹亚临界扩展规律的描述是令人满意的，在安全判定上是安定的。     

核压力容器异种金属焊接接头延性断裂行为数值研究

使用GTN损伤模型与有限元计算相结合的方法,对核电安全端异种金属焊接接头试样裂纹扩展路径及J-R阻力曲线进行数值研究.结果表明:异种金属接头中的堆焊层及焊缝中心裂纹扩展路径和均质材料基本相同,而界面及近界面裂纹的扩展路径显著地偏离初始裂纹位置.界面裂纹及近界面裂纹倾向于向屈服强度低的软材料一侧扩展,这是由裂尖前区域不同材料的强度失配所致.不同初始位置和不同尺寸的裂纹具有不同的材料拘束和几何拘束,其J-R裂纹扩展阻力曲线不同.宏观断裂力学无法预测这种裂纹的扩展路径和扩展阻力,基于GTN损伤模型的数值计算方法可以预测这种裂纹的扩展路径及扩展阻力,可望用于安全端焊接结构的完整性评定和破前漏分析.     

包含过热区的焊接接头强度组配对CTOD的影响

利用试验研究和数值分析方法，对模拟焊接接头三点弯曲试样不同强度组配对裂纹尖端张开位移（CTOD）的影响进行了研究，本研究的模拟焊接接头试样是通过电子束焊接法焊制的，数值分析中采用了三金属模型，较以往的二金属模型更接近于实际，结果表明，不同的焊接接头强度组配对CTOD有不同程度的影响；无量纲载荷P／PY和CTOD裂纹扩展驱动力关系曲线与极限载荷的选取有很大关系，二维数值计算与试验结果在无量纲载荷P／PY较小时具有良好的一致性，但随P／PY的增加，二者的偏差增大。     

316不锈钢室温和350℃低周疲劳性能研究

采用MTS材料试验机研究作为反应堆结构材料的316奥氏体不锈钢母材在350℃和室温,以及焊缝在室温,±0.3%~1.5%应变幅的低周疲劳性能试验,并采用扫描电镜对试验后样品进行了断口分析。研究结果表明,316不锈钢疲劳性能较好,室温下疲劳寿命高出350℃同一应变幅的30%~50%以上,且母材的疲劳寿命显著高出焊缝同一应变幅的一倍以上。随应变幅的增加,材料疲劳寿命相应下降,峰值应力增加。室温下母材和焊缝均呈现出随循环周次增加、峰值随应力逐渐下降的规律。母材在高温下,随应变幅的增加,逐渐由循环硬化过渡到饱和行为。低周疲劳试验后,断口表面可观察到裂纹源和疲劳条带。随应变幅增加,疲劳条带间距增大,且同一应变幅下,焊缝的间距大于母材,高温的疲劳间距大于室温,与疲劳试验结果相吻合。     

压水堆核电厂主管道窄间隙自动焊焊丝研究

根据窄间隙自动焊工艺及主管道母材特点,在ER316L焊材基础上研究与主管道自动焊技术相匹配的专用焊材。通过模拟焊接试验和热裂纹试验验证了自动焊焊材的稳定性、可焊性,并对其焊缝疲劳寿命进行试验。研究结果证明新开发的自动焊焊材与主管道窄间隙自动焊工艺相匹配,焊缝接头综合性能良好。     

Low cycle fatigue of welded joints: new experimental approach

To take into account the reduced fatigue strength of welded joints, a reduction life factor applied on fatigue curves (J_f value) was introduced into the RCC-MR [Design and Construction Rules for Mechanical Components of FBR Nuclear Islands, AFCEN, 1993], for the design and construction of fast breeder reactors. To better assess this factor, previous work showed that mechanical behavior of a welded assembly is influenced by the geometry of the weld and by the interaction of the different cyclic plastic behavior of the two materials: base metal (BM) and weld metal (WM). A new procedure (named FFAST) was performed on welded joint specimens extracted from butt-welded pipe connections (uniaxial tensile–compressive load). An innovative experimental approach is proposed to study the local mechanical behavior of the welded joint specimens and then determine the J_f parameter. The main advantage of the method is to avoid problems due to the relative stiffness of weld part versus the BM part of the specimen. A continuous recording of the stress and strain in the weld allows an estimation of the mechanical behavior and finally the fatigue life of the joint. Observations of the crack surface show two different crack initiation zones near the weld depending on the load level. Calculations of the tests and comparison with experimental results are presented. These studies make it possible to assess in a practical way the J_f design method. It appears that J_f value cannot be considered as a single value for it is influenced by several factors depending on the weldment and on the load level.     

Effect of welded mechanical heterogeneity on local stress and strain ahead of stationary and growing crack tips

The crack tip stress and strain condition is one of main factors affecting environmentally assisted cracking (EAC) behaviors in light water reactor (LWR). The mechanical and material properties of the base metal, weld metal and heat-affected zone (HAZ) in welded joints are heterogeneous because of the inherent characteristics of welded joint. Since the welded joint is more susceptible to EAC, to understand the effect of the strength mismatch in the welded joint on EAC growth rate, the stress and strain state in both stationary and growing crack tips of the welded joint specimen are investigated by the elastic-plastic finite element method (EPFEM) in this paper. The results indicate that the strength mismatch and sampling position in the welded joint would observably affect the stress and strain ahead of the stationary and growing crack tip, and accordingly affect the EAC growth rate.     

Environmentally-assisted cracking behaviour in the transition region of an Alloy182/SA 508 Cl.2 dissimilar metal weld joint in simulated boiling water reactor normal water chemistry environment

The stress corrosion cracking (SCC) and corrosion fatigue behaviour perpendicular and parallel to the fusion line in the transition region between the Alloy 182 Nickel-base weld metal and the adjacent SA 508 Cl.2 low-alloy reactor pressure vessel (RPV) steel of a simulated dissimilar metal weld joint was investigated under boiling water reactor normal water chemistry conditions. A special emphasis was placed to the question whether a fast growing interdendritic SCC crack in the highly susceptible Alloy 182 weld metal can easily cross the fusion line and significantly propagate into the adjacent low-alloy RPV steel. Cessation of interdendritic SCC crack growth was observed in high-purity or sulphate-containing oxygenated water under constant or periodical partial unloading conditions for those parts of the crack front, which reached the fusion line. In chloride containing water, on the other hand, the interdendritic SCC crack in the Alloy 182 weld metal very easily crossed the fusion line and further propagated with a very high rate as a transgranular crack into the heat-affected zone and base metal of the adjacent low-alloy steel. The observed SCC cracking behaviour at the interface correlates excellently with the field experience of such dissimilar metal weld joints, where SCC cracking was usually confined to the Alloy 182 weld metal.     

Effect of notch dimension on the fatigue life of V-notched structure

The stress singularity degree associated to a V-notch has a great influence on the fatigue life of V-notched structure. The growth rate of the crack initiated at the tip of a V-notch depends on the stress singularity of the V-notch. The fatigue life accompanying with this small crack will represent a large amount of the total fatigue life. In this work, boundary element method (BEM) is used to study the propagation of the crack emanating from a V-notch tip under fatigue loading. A comparison of the fatigue life between the crack initiated from V-notch tip and a lateral crack is done by a crack propagation law until these two cracks have the same stress intensity factors (SIFs). The effect of initial crack length, notch opening angle and notch depth on the crack extension and propagation is analyzed. As an example of engineering application, the fatigue life of a welded joint is investigated by the present method. The influence of weld toe angle and initial crack length on the fatigue life of the welded structure is studied. Some suggestions are given as an attempt to improve the fatigue life of welded structures at the end.     

Elastic-plastic finite element analyses of stable crack growth and fracture instability in a girth welded pipe

Detailed elastic-plastic finite element fracture mechanics analyses were conducted on a 16 inch diameter Type 304 stainless steel pipe containing a circumferential through-wall crack located in a girth weld. Calculations were performed to analyze the welded pipe treated as (1) a monolithic pipe entirely composed of the base metal, and (2) a composite of base metal and weldment. In the latter, each constituent was assigned distinct mechanical and fracture properties. In both solutions applied J values were calculated for a fixed axial load combined with a monotonically increasing applied bending moment. The material J-resistance curves appropriate for the two problems were each used to initiate and grow the initial crack in a stable manner until fracture instability occurred under load control. It was found that the extent of stable crack growth and the applied loads at fracture instability are distinctly different in the two analyses. It is concluded that more precise fracture mechanics approaches than those now in current use are required for accurate assessments of weld cracking problems.     

Creep crack initiation and creep crack growth assessments in welded structures

In this study, creep crack initiation and creep crack growth in welded structure are analysed. An interaction phenomenon between base metal and weld metal in a cross weld plate is highlighted with a finite element model. A simplified method based on the reference stress approach is proposed to evaluate C* in welded structures. This simplified method is applied for creep crack initiation and creep crack growth assessments in the case of a double edge notched tension cross-welded plate. Correlations between crack initiation time and C* on the one hand, and between creep crack growth rate and C* on the other hand are used. Creep crack initiation time estimation for the full size welded plate is very conservative when crack initiation properties of CT specimen are used. Concerning creep crack growth evaluation, simplified estimation is in good agreement with experimental results when CT specimen crack propagation properties are used.     

Predicting welding residual stresses in a dissimilar metal girth welded pipe using 3D finite element model with a simplified heat source

Dissimilar metal welds are commonly used in nuclear power plants to connect low alloy steel components and austenitic stainless steel piping systems. The integrity assessment and life estimation for such welded structures require consideration of residual stresses induced by manufacturing processes. Because the fabrication process of dissimilar metal weld joints is considerably complex, it is very difficult to accurately predict residual stresses. In this study, both numerical simulation technology and experimental method were used to investigate welding residual stress distribution in a dissimilar metal pipe joint with a medium diameter, which were performed by a multi-pass welding process. Firstly, an experimental mock-up was fabricated to measure the residual stress distributions on the inside and the outside surfaces. Then, a time-effective 3-D finite element model was developed to simulate welding residual stresses through using a simplified moving heat source. The simplified heat source method could complete the thermo-mechanical analysis in an acceptable time, and the simulation results generally matched the measured data near the weld zone. Through comparing the simulation results and the experimental measurements, we can infer that besides the multi-pass welding process other key manufacturing processes such as cladding, buttering and heat treatment should also be taken into account to accurately predict residual stresses in the whole range of the dissimilar metal pipe.     

Fatigue crack growth behavior of weld heat-affected zone of type 304 stainless steel in high temperature water

The fatigue crack growth behavior of the weld heat-affected zone (HAZ) of type 304 stainless steel in high temperature water which simulates the boiling-water reactor environment was investigated to clarify the effects of welding residual stress, cyclic frequency f and thermal aging on crack growth rate. A lower crack growth rate of the HAZ than of the base metal was observed in both the high temperature water and the ambient air caused by the compressive residual stress. The crack closure point was measured in the high temperature water. The effect of the welding residual stress on the crack growth rate of the HAZ can be evaluated separately from the environmental effect through the crack closure behavior. The high temperature water increased the crack growth rate at a cyclic frequency of 0.0167 Hz but did not affect it much at 3 and 5 Hz. The crack growth behavior of the thermally aged HAZ at 400 °C for 1800 h was almost the same as that of the unaged material tested at 0.0167 and 5 Hz in the high temperature water.