20钢热水管焊缝爆裂的失效分析

采用宏观形貌、金相组织和断口观察以及成分分析和力学性能测定等方法,分析了某20钢高温输水管爆裂的原因.结果表明,断裂失效发生在焊缝部位,焊缝组织存在明显蠕变现象,断口起裂处具有典型脆性断裂特征,裂纹内有腐蚀产物,属于典型应力腐蚀裂纹.裂纹扩展后期近外表面处具有蠕变疲劳裂纹特征.应力腐蚀裂纹是该热水管爆裂失效的主要原因.     

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.     

基于全场法的航空Ti-2Al-1.5Mn钛合金疲劳裂纹扩展各向异性研究

准确定量表征航空重要承载结构材料抗疲劳裂纹扩展能力是实施结构件服役寿命评估的基础。本文针对航空用Ti-2Al-1.5Mn钛合金,沿着板材不同取向制备CT试样开展疲劳裂纹扩展速率试验,分别基于全场法及传统方法定量表征了裂纹扩展各阶段应力强度因子幅DK。结果表明：疲劳裂纹扩展速率da/dN-DK关系及裂纹扩展路径显著受到材料取向的影响。相比较于传统表征应力强度因子幅方法,基于全场法一方面能够直接考虑裂尖塑性变形引起裂纹闭合的影响,另一方面能够有效避免因裂纹扩展路径偏折带来的有效裂纹长度测量偏差,从而不能准确获取有效应力强度因子幅的问题,其具有显著优势。基于全场法的疲劳裂纹扩展应力强度因子幅表征具有广泛应用全景。     

TC4-DT钛合金电子束焊接接头的疲劳断口研究

采用单边缺口试样,用扫描电镜观察了TC4-DT钛合金母材及电子束焊接接头焊缝区与热影响区的疲劳断口,分别分析了疲劳裂纹起裂源、扩展区及瞬断区的断口形貌特征.结果表明,疲劳裂纹均起裂于试样缺口根部,并由试样的一侧扩展到另一侧,裂纹扩展初期均沿一定的结晶面扩展,属于脆性穿晶断裂;母材和热影响区的瞬断部位均有明显的韧窝出现,属于塑性沿晶断裂,而焊缝区断口上韧窝不明显;热影响区疲劳断口有疲劳条带出现,但在母材和焊缝区断口上很难看见.     

Hastelloy C-276薄板激光焊接接头疲劳性能EI北大核心CSCD

对0.5 mm厚Hastelloy C-276薄板激光焊接接头进行疲劳试验,结合应力-寿命(S-N)曲线和疲劳断口形貌,研究母材及焊接接头的疲劳性能,分析母材和焊接接头的疲劳断裂机理。结果表明:0.5 mm Hastelloy C-276薄板焊接接头和母材的S-N曲线斜率基本相同,焊接接头疲劳性能和母材的基本相当;母材疲劳断口疲劳裂纹起源于试样侧表面,主要沿宽度方向扩展,随着应力的减小,疲劳源数目减少,疲劳裂纹扩展速率减小;焊接接头在母材和焊缝处随机断裂,焊接接头母材区断口形貌和母材断口形貌基本一致,而焊接接头焊缝区断口的疲劳裂纹起源于侧表面棱角处和焊缝表面,焊缝表面是主要疲劳源,裂纹主要沿厚度方向进行扩展,疲劳裂纹扩展区呈现出准解理断裂特征。     

Hastelloy C-276薄板激光焊接接头疲劳性能

对0.5 mm厚Hastelloy C-276薄板激光焊接接头进行疲劳试验,结合应力-寿命(S-N)曲线和疲劳断口形貌,研究母材及焊接接头的疲劳性能,分析母材和焊接接头的疲劳断裂机理。结果表明:0.5 mm Hastelloy C-276薄板焊接接头和母材的S-N曲线斜率基本相同,焊接接头疲劳性能和母材的基本相当;母材疲劳断口疲劳裂纹起源于试样侧表面,主要沿宽度方向扩展,随着应力的减小,疲劳源数目减少,疲劳裂纹扩展速率减小;焊接接头在母材和焊缝处随机断裂,焊接接头母材区断口形貌和母材断口形貌基本一致,而焊接接头焊缝区断口的疲劳裂纹起源于侧表面棱角处和焊缝表面,焊缝表面是主要疲劳源,裂纹主要沿厚度方向进行扩展,疲劳裂纹扩展区呈现出准解理断裂特征。     

除氧器蒸汽斜管管座裂纹失效分析

通过失效焊缝断口形貌观察,材料成分、组织及力学性能测试,并结合蒸汽斜管和管座焊缝的运行工况和应力分布,系统研究了除氧器蒸汽斜管管座焊缝的失效原因。结果表明,焊缝处的焊接缺陷在外界环境、运行应力的共同作用下发生应力集中,造成缺陷处疲劳裂纹萌生和扩展,导致疲劳断裂。因此,工程实际中应加强焊接质量的控制、定期进行管道焊缝的无损检验,可减少后续失效的发生。     

TiAl基合金弯曲疲劳的断裂机制

通过对弯曲疲劳断裂宏观试验结果以及相应的卸载表面观察和断口观察分析研究,发现在疲劳加载的过程中,首先在缺口根部产生裂纹,裂纹在应力循环的作用下不断扩展,直至疲劳裂纹的长度达到与疲劳外加力所匹配的临界裂纹长度时,突然发生整体解理断裂.在一定应力下的疲劳弯曲加载试验中,随着循环次数的增加,产生的裂纹变长,即产生的损伤严重,疲劳区域变宽,其断裂机制是疲劳区各裂纹单向扩展,解理区起裂源分散扩展直至断裂.对于循环次数较小的材料,其断裂机制是具有发散扩展路径的起裂源直接产生于缺口根部,然后分散扩展直至断裂,在其扩展的路径上并不因疲劳区与解理区而有任何的不同.     

JB46-400大型双点压力机中曲轴断裂失效分析

通过断口形貌分析、成分分析、金相分析、断口硬度分析、应力分析等方法对曲轴断裂失效原因进行分析.结果表明,曲轴中频感应淬火后未能及时回火,表面残余应力较大,在工况条件下表面产生微裂纹;同时曲轴轴肩根部产生应力集中,诱发裂纹进一步扩展,导致曲轴最终疲劳断裂.另外调质处理效果较差、曲轴芯部铁素体含量较多、整体强度不足也是造成曲轴疲劳断裂失效的原因之一.     

不锈钢EGR冷却器钎焊结构断裂分析

通过对钎焊失效件进行断裂裂纹的宏观分析及断口分析,确定了断裂发生的位置,并分析了断裂形式及疲劳裂纹萌生和扩展过程的特点.在对失效件裂纹的宏观观察中发现,断裂多发生在钎焊圆角根部,裂纹沿钎焊圆角根部萌生和扩展.对钎焊圆角表面微观形貌进行观察后发现,在钎焊圆角根部存在着连续分布的钎料瘤,容易引起局部应力集中,促进疲劳裂纹的萌生.在对失效件断口进行的分析中找到了疲劳断裂的典型特征--疲劳辉纹,有力地证明了失效模式为疲劳断裂.     

径向间隙对304L不锈钢振动疲劳性能的影响

基于疲劳试验和有限元分析，研究径向间隙对插套焊疲劳性能的影响. 疲劳试验表明，应力比较大时，试样易在焊趾处失效，焊趾处裂纹由外向内扩展；应力比较小时，易在焊根处失效，焊根处裂纹由内向外扩展. 未失效试样径向间隙高于失效试样. 利用有限元软件建立了插套焊有限元模型，分析了径向间隙对焊根和焊点处疲劳性能的影响. 结果表明，相对于0.1 mm的径向间隙，0.4 mm径向间隙能降低焊根和焊趾处的应力，降低幅度分别为14%和1.4%，径向间隙越大，焊根和焊趾处的应力峰值越小，疲劳性能越好.     

TC4钛合金薄板激光焊接接头的疲劳寿命及断口分析

研究了TC4钛合金薄板激光焊接接头及母材的疲劳性能,并对其疲劳断口进行了观察.结果表明,接头的疲劳寿命在低应力水平时高于母材,在高应力水平时低于母材.母材疲劳裂纹萌生于试样表面;在裂纹扩展区有平行排列的弯曲的二次裂纹和期间更细的疲劳辉纹,瞬断区为细小的等轴韧窝.焊缝疲劳裂纹起源于表面的气孔,源区有笔直且平行排列的二次裂纹;在裂纹扩展区,断口形貌与组织有关,细晶区为韧窝,在细晶与柱状晶交界处为敞口浅韧窝,柱状晶和粗晶区为晶粒大小的刻面,上有大量的微剪切滑移带,断裂机理为滑移带形成及开裂.     

AZ31B镁合金及其焊接接头的疲劳断裂机理

对AZ31B镁合金进行疲劳实验,在2×106循环次数下,母材、对接接头、横向十字接头和侧面连接接头的疲劳强度分别为66.72,39.00,24.38和24.40MPa。采用光学显微镜对裂纹扩展特征进行分析,结果表明,AZ31B母材的疲劳裂纹宏观扩展路径平滑,但微观观察发现疲劳裂纹扩展方向曲弯,有些裂纹分成两岔;裂纹尖端扩展均为沿晶扩展。焊接接头裂纹均在焊趾部位起裂,对接接头和横线十字接头的裂纹沿着热影响区扩展;侧面连接接头的裂纹起裂位于焊脚部位。采用扫描电子显微镜对疲劳断裂机理进行分析。疲劳断口由准解理或解理台阶组成,均为脆性断裂,断口中存在二次裂纹,对接接头中存在疲劳条纹,其间距约为5μm。     

Computer-aided experiment on arc welding

Fatigue properties of cast aluminium welded joints by friction stir welding (FSW) and MIG welding were investigated, comparing with that of the base plate. Fatigue crack propagation tests for the da/dN ? ΔK relation and bending fatigue tests for the S–N relation were carried out. Fatigue cracks in both FSW and MIG specimens were accelerated, when the fatigue crack tip reached the stir zone or the weld metal. This behaviour was discussed based on the crack closure induced by the crack surface roughness and the residual stress. In the S–N properties, the influence of specimen surface finishing on fatigue life was also examined. Fatigue lives of the FSW and MIG specimens in the ‘as weld’ condition were in the range of the largely scattered base plate fatigue lives, in spite of the different fatigue crack initiation sites in each specimen such as the porosity in the base plate, the tool mark bottom in the FSW and the weld toe in the MIG. The FSW specimens with the polished surface showed the particular improvement in fatigue strength for finite fatigue life.     

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.     

感应器安装座断裂分析与结构设计优化

运营3年的铁路机车日常检修时发现极少部分安装座加强筋焊趾位置焊缝产生裂纹,部分底板因裂纹扩展导致断裂。对安装座各焊接组件在机车运营过程中的受力状态以及裂纹焊接组件的宏观断口、化学成分和显微组织进行了分析。结果表明：由于各部件组焊后内应力未经退火消除且结构上的缺口易产生应力集中,缺口又正处于较为薄弱的焊缝热影响区和最大受力力矩位置,在运营过程中极易产生疲劳裂纹。为减少焊接应力,应从结构上减少焊缝数量并对焊缝去应力退火或结构优化。     

Fatigue behaviour of bolted joints

This paper analyzes the tensile fatigue behaviour of bolted joints constituted by commercial steel bolts. They were tested under both monotonic and fatigue tensile loading, with different R-ratio. Results show that under increasing monotonic tensile loading the bolted joint is not the failure zone of the bolt, whereas such a bolted joint is the failure region under cyclic loading. The fatigue life decreases with the increase of the stress range and with the maximum stress, and pre-loading enlarges the fatigue life. Fatigue fracture surface shows a geometry of crescent moon in the case of short cracks and such a shape evolves towards a quasi-straight crack front in the case of long cracks. Fatigue fracture usually happens at the root of the first notch inside the bolted joint, although fracture initiation may happen in several consecutive notch roots, increasing the initiation angle of the fatigue crack as the applied stress diminishes.     

Fatigue characteristics of patch plate joints by fillet welding assisted with bonding

A series of fatigue experiments and elastic analysis were carried out for investigating fatigue characteristics of patch plate joints assembled by fillet welding assisted with bonding. In the case that fatigue cracks occurred at the weld toe by the four-point bending fatigue experiment, the fatigue life of joints assembled by welding and bonding (WB) specimens were almost the same as those by only welding (W) specimens. The elastic analysis simulating the four-point bending loaded situation on W and WB specimens was performed for elucidating its reason. The stress concentration at the weld toe was even high in the WB specimens. Therefore, the fatigue life of WB specimens was not longer than that of W specimens. On the other hand, the stress around the weld root of WB specimen was around 30% of that of W specimen. The possibility of stress reduction effect by bonding was indicated around the weld root rather than around the weld toe. In order to verify this possibility, the four-point bending fatigue experiment was performed by setting the specimens so that the tensile stress was applied on the weld root. It was confirmed that the fatigue cracks occurred from the weld root in both W and WB specimens. The fatigue life defined in this study of WB specimens was from 4 to 8 times longer than that of W specimens when the applied nominal stress range was under 175 MPa. The fatigue life defined in this study of WB specimens was from 2 to 3 times longer than that of W specimens when the applied nominal stress range was over 200 MPa. The results indicated the fatigue life improvement of patch plate joints by fillet welding assisted with bonding when the fatigue cracks occurred at the weld root.     

储罐角焊缝裂纹漏磁检测有限元分析

针对石油石化行业中的储罐角焊缝不易检测的特点,使用漏磁检测技术对储罐角焊缝裂纹进行检测。应用有限元软件建立了角焊缝裂纹缺陷的分析模型,得到了角焊缝裂纹缺陷漏磁场的空间分布,提取出了角焊缝裂纹缺陷的漏磁信号,分析了角焊缝裂纹缺陷影响漏磁信号的因素;在此基础上对储罐角焊缝裂纹缺陷进行了漏磁检测试验,对得到的漏磁信号进行分析,分析结果与有限元分析提取的漏磁信号结果相符,证明了所建立的检测系统的可行性与有效性。     

Fatigue behaviour of out-of-plane gusset joints with one-side fillet weld

In previous studies, many fatigue tests have been performed to investigate the fatigue behaviour of out-of-plane gusset welded joints. However, fatigue behaviour of out-of-plane gusset joints with a one-side fillet weld has not yet been clarified sufficiently. In this research, in order to quantitatively clarify the behaviour, welding residual stress at the weld toes of the joint was measured by a cutting method. In addition to this, fatigue tests were carried out on the joints with a one-side weld and turn-around weld. The plate thickness is 9 mm and the weld size is 10 mm. Moreover, finite element stress analyses were conducted on the test specimen models and parametric models with different main plate and gusset plate thickness. Their results show that fatigue cracks for out-of-plane gusset joints with a one-side fillet weld initiate from both weld toes at the start and end points of weld beads and non-penetration weld of the gusset side. Also, the crack initiation points are different depending on the weld arrangement; however, their fatigue strengths are almost same.