Numerical Investigation on Fracture Initiation Properties of Interface Crack in Dissimilar Steel Welded Joints

Fracture toughness property is of significant importance when evaluating structural safety. The current research of fracture toughness mainly focused on crack in homogeneous material and experimental results. When the crack is located in a welded joint with high-gradient microstructure and mechanical property distribution, it becomes difficult to evaluate the fracture toughness behavior since the stress distribution may be affected by various factors. In recent years, numerical method has become an ideal approach to reveal the essence and mechanism of fracture toughness behavior. This study focuses on the crack initiation behavior and driving force at different interfaces in dissimilar steel welded joints. The stress and strain fields around the crack tip lying at the interfaces of ductile-ductile, ductile-brittle and brittle-brittle materials are analyzed by the numerical simulation. For the interface of ductile-ductile materials, the strain concentration on the softer material side is responsible for ductile fracture initiation. For the ductile-brittle interface, the shielding effect of the ductile material plays an important role in decreasing the fracture driving force on the brittle material side. In the case of brittle-brittle interface, a careful matching is required, because the strength mismatch decreases the fracture driving force in one side, whereas the driving force in another side is increased. The results are deemed to offer support for the safety assessment of welded structures.     

Characterisation of joint properties through spatial mapping of cracks in fatigue specimens,extracted from the linearly friction welded steel coupon

Linear friction welding (LFW) is widely used for the fabrication of high-value components in the aero-industry. The versatility of use and high integrity of joints produced by LFW, suggest that this technology could contribute greatly to sustainable engineering of the future. For example, in near-net-shape manufacturing, LFW is predicted to offer a substantial reduction in material waste and processing time, in addition to offering the capability of bonding material pairs currently difficult to fusion weld. LFW of steel components is seldom researched in published literature, with little information available on fatigue strength and fracture behaviour of such joints. This may hamper the confidence needed to popularise LFW in wider industry. Thusly, authors conducted a discovery-oriented fatigue study, followed by metallography and micro-hardness tests, conducted on a 100Cr steel LFW coupon and corresponding batch of parent material. The weld coupon was dissected into a set of fatigue specimens, to allow mapping of the joint's internal properties through individual fatigue estimates and fracture morphology of each specimen in the set. A prototype fatigue machine was constructed, designed to deliver independent displacement and force controlled bending or torsional loadings, representing complex real-world conditions more accurately than common, uniaxial tests. The hour-glass shape of fatigue specimens allowed for targeting strictly the weld region, inducing fracture at the weakest material section, which was found to lay far outside the immediate contact interface. Referencing distances from fracture points to the theoretical weld interface plane in each fatigue trial, resulted in discovery of a macroscopic pattern. Interpolated to a crack initiation plane, this pattern is proposed to reflect the heat-affected zone, across the greater LFW coupon. Authors also studied crack paths and suggest a link between fracture direction, residual stresses in joint and angular orientation of the crack initiation plane in respect to the theoretical weld interface.     

钝裂纹或缺口下失效评定图的变化及修正方法研究

应用增量理论弹塑性有限元方法以及弹塑性断裂力学 Ｊ积分理论, 深入研究和全面探讨了一种用于含裂纹结构完整性评定的工程方法——失效评定图, 以及钝裂纹或缺口根部曲率半径、缺口长度、缺口端部椭圆离心率、试样形式和材料等对其的影响。提出了钝裂纹或缺口应力强度因子和极限载荷修正方法。通过这一方法建立的失效评定图可有效地消除缺口因素对失效评定图的影响, 由此, 可完全等效地将裂纹失效评定图用于含缺口结构的完整性评定。     

含裂纹结构剩余强度的一种估算方法

剩余强度分析对结构断裂安全性评价十分重要。现有的剩余强度分析方法中,Feddersen工程分析法具有代表性,它对简单含裂纹结构比较有效。但这类方法一般需要预先得到弹性断裂应力的连续曲线,才有可能进行短裂纹阶段的弹塑性修正和长裂纹时的净截面屈服修正,使用不方便,有时甚至不适用。本文认为含裂纹结构的破坏是静力学模式破坏和断裂力学模式破坏共同作用的结果,从而建议一种结构剩余强度的实用估算方法。通过表观断裂韧度准则确定的弹性断裂强度和净截面全面屈服后的静力学破坏强度的结合,估算剩余强度许用值。该方法可以方便地估算任意时刻的剩余强度。分别进行含中心裂纹的有限宽板和含裂纹耳片结构的剩余强度试验,并与估算结果进行比较,表明本文方法预测精度良好,有望推广到其他结构形式。文中强调,表观断裂韧度Kapp值的合理与否对估算结果有明显影响。     

SPFC440钢/5052铝自冲铆接头与胶铆复合接头腐蚀性能对比

为研究SPFC440钢与5052铝合金自冲铆接头的腐蚀性能,分别制备了自冲铆接和胶铆复合两种接头。通过对不同腐蚀时长下的两种接头试样进行疲劳性能测试,分析了粘胶剂对铆接接头腐蚀疲劳性能的影响,并对疲劳断口、裂纹扩展进行SEM和EDS检测。结果表明：粘胶剂的存在不仅提高了接头强度,还保护了金属界面,减小了电偶腐蚀的影响;随着腐蚀时间的增长,复合接头的最大静载荷值不断减小,自冲铆接接头最大静载荷先增大后减小;复合接头的拉伸失效模式与自冲铆接接头的失效形式相似;自冲铆接接头的疲劳裂纹主要集中在下板料的铆钉周围,复合接头主要为下基板的断裂;铆钉与下板相互接触区域以及两板间接触面出现了严重微动磨损。     

Effects of strain rate and temperature on fracture strength of ceramic/metal joint brazed with Ti-Ag-Cu alloy

Ceramics are significantly used in many industrial applications due to their excellent mechanical and thermal properties such as high temperature strength, low density, high hardness, low thermal expansion, and good corrosion resistive properties, while their disadvantages are brittleness, poor formability and high manufacturing cost. To combine advantages of ceramics with those of metals, they are often used together as one composite component, which necessiates reliable joining methods between metal and ceramic. Direct brazing using an active filler metal has been found to be a reliable and simple technique, producing strong and reliable joints. In this study, the fracture characteristics of Si₃N₄ ceramic joined to ANSI 304L stainless steel with a Ti-Ag-Cu filler and a Cu (0.25–0.3 mm) interlayer are investigated as a function of strain rate and temperature. In order to evaluate a local strain a couple of strain gages are pasted at the ceramic and metal sides near joint interface. As a result the 4-point bending strength and the deflection of interlayer increased at room temperature with increasing strain rate. However bending strength decreased with temperature while deflection of interlayer was almost same. The fracture shapes were classified into three groups ; cracks grow into the metal-brazing filler line, the ceramic-brazing filler line or the ceramic inside.     

含裂纹核级管道三维有限元分析与 LBB 评定

三维裂纹仿真分析和防断裂设计问题一直是断裂理论和工程应用研究的主要课题。管道系统是核动力工程中的关键部件。运用三维有限元分析软件 ANSYS 和裂纹分析专用程序 Franc3D,针对中国实验快堆（CEFR）一段余热排放系统中的含裂纹管道进行了三维裂纹仿真模拟分析和LBB（Leak －Before －Break）评定。快堆管道工作时处于高温状态,因此,采用适用于高温环境下的材料和法国结构安全设计规范 RCC －MR．A16标准文件。研究的管道材料为9Cr1Mo 钢的改良型———T91／P91管材,分析模型为含裂纹的三维结构,且包括管道高温工况和蠕变效应。仿真计算结果表明,针对所研究的工况,蠕变较疲劳对三维裂纹扩展的影响更大。采用 T91钢的管路系统能够满足相关规范中的 LBB 设计准则。该研究为快堆管道选材、含缺陷管道安全评估和高温完整性评定提供设计参考。     

直接由KIC／KIa～（T—RTNDT）的实测数据统计分析反应堆压力容器用钢断裂韧性值的概率分布

在反应堆压力容器的结构完整性评定中,ASME规范中选择材料大量断裂韧性数据的下包络线来确定断裂韧度,这在反应堆压力容器结构完整性的概率评定中会产生过于保守的结果。为此,利用三参数威布尔分布模型和对数正态分布模型分别对起裂韧性玩数据和动态止裂韧性KIa数据进行统计分析,并给出了断裂韧性的统计分布模型。     

运行条件对不锈钢点焊疲劳性能影响及开裂分析

以不锈钢车体底架电阻点焊接头为对象,研究了温度（-40℃、室温和70 ℃）、应力比（0.1、0.5）以及3.5%NaCl腐蚀介质对接头疲劳寿命的影响;借助电子背散射衍射（EBSD）技术、扫描电镜（SEM）、仪表球压痕系统(IBIS)和拉伸剪切试验,对晶粒取向、残余应力分布、疲劳断口形貌和力学性能进行分析。结果表明：低温条件下材料强度的提高可获得更好的疲劳性能,70 ℃条件下塑性性能的改善可使高周疲劳性能更优;应力比为0.1时的疲劳性能优于应力比为0.5时的疲劳性能;腐蚀介质和拉应力促使应力腐蚀开裂和裂纹尖端的阳极区快速溶解,加速裂纹的萌生和扩展,显著缩短了点焊接头的疲劳寿命;疲劳裂纹在界面应力集中尖角处形核,沿熔核软化区长大,逐渐向表面扩张,直至断裂失效。     

基于统一强度理论的复合型裂纹断裂准则

提出一个复合型裂纹等效应力断裂准则，用以解决工程上普遍存在的复合型裂纹的断裂问题。该准则通过借鉴强度理论对复杂应力状态的处理方法，将等效应力作为度量复合型裂纹开裂的基本物理量。该准则在预测发生临界扩展时提出两个基本假设，(1)裂纹沿着等效应力最小的方向开始扩展。(2)等效应力达到临界值时裂纹开始扩展。根据选取的等效应力计算公式的不同，等效应力准则有不同的形式，当等效应力的计算采用总应变能理论时，等效应力准则等价于应变能密度准则；当等效应力采用σr时，等效应力准则可近似逼近最大周向应力理论。采用统一强度理论计算等效应力。最后通过与实验结果及其他理论的计算结果对比，验证基于统一强度理论的复合型裂纹断裂准则满足工程精度要求，且适用性好。     

优化组织球铁的力学性能及其微观断裂行为

根据球铁断裂过程中石墨及石墨-基体界面的微观力学行为,对球铁组织进行优化设计.以强相(马氏体)或强韧相(奥氏体-贝氏体组织)环包围石墨,基体组织为马氏体或奥氏体-贝氏体组织,加上适量的铁素体,并通过快速加热短时保温后淬火或等温淬火获得.实验结果表明,优化组织球铁具有很高的强韧性.采用扫描电镜动态拉伸观察优化组织球铁断裂过程,发现微裂纹在石墨-基体界面萌生并沿界面扩展,马氏体环或奥-贝环阻碍界面裂纹的萌生与扩展,基体中的硬相和软相分别提高球铁的强度和韧性,这些都有利于提高优化组织球铁的强韧性.     

钢液固态力学性能的实验研究

本文采用凝固法,在Gleeble-1500动态热力模拟实验机上首次对0.11%C网液态模锻法兰材料进行了凝固过程液固态力学性能的实验测定。结果表明：0.11%C钢液固态存在三个力学性能行为不同的区域：抗拉强度为零,断裂应变较高但随温度降低而急剧下降的Ⅰ区;抗拉强度逐渐形成,断裂应变进一步减小的Ⅱ区;抗拉强度和断裂应变随温度下降而急剧增加的Ⅲ区。很明显Ⅱ区（约1495～1505℃）是热裂纹最易形成的热脆性区。对试样断口形貌的观察和分析进一步证实了实验结果。     

980MPa级低碳贝氏体高强钢MAG焊接接头不同试样的拉伸断裂机理

在980MPa级深海用低碳贝氏体高强钢MAG焊接接头上,制取5种不同形状和缺口位置的试样进行拉伸试验,原位观察了每种试样的动态断裂过程,并对其断裂机理进行了分析,确定了焊接接头的薄弱部位。结果表明:直缺口试样的断裂经历了塑性变形、裂纹起裂、裂纹扩展、裂纹尖端钝化,直至断裂的过程,并且在裂纹扩展过程中,裂纹尖端重复钝化、扩展、新裂纹产生、再钝化、再扩展;圆弧和平板试样以剪切方式断裂,经历了塑性变形、颈缩、出现微裂纹、微裂纹扩展,直至瞬间断裂的过程;所有试样的薄弱部位都为焊缝金属。     

Finite element analysis of coating adhesion failure in pre-existing crack field

Abstract

The influence of a pre-existing crack field on coating adhesion failure in a steel surface coated with a 2 μm thick titanium nitride (TiN) coating was investigated by finite element method modelling and simulation. The stress and strain fields were determined in contact conditions with a spherical diamond tip sliding over the coated surface at a loading of 8 N. One crack in or at the coating increased the maximum tensile stresses with six times from 82 to 540 MPa when the crack was vertical through the coating or L shaped and with nine times when the crack was horizontal at the coating/substrate interface. A simulated multicrack pattern relaxed the tensile stresses compared to single cracks. The results indicate that a cracked coated surface needs to have about five to nine times higher adhesive and cohesive bonds to resist the same loading without crack growth compared to a crack free surface. For optimal coated surface design, the strength of the adhesive bonds between the coating and the substrate in the vertical direction needs to be 50% higher than the cohesive bonds within the coating and the substrate in the horizontal direction. The first crack is prone to start at the top of the coating and grows vertically down to coating/substrate interface, and there it stops due to the bigger cohesion within the steel material. After this, there are two effects influencing that the crack will grow in the lateral direction. One is that steel cohesion is normally bigger than the coating/interface adhesion, and the second is that there are higher tensile stresses in the horizontal than in the vertical cracks. Several vertical cracks can stop the horizontal crack growth due to stress relaxation.     

X80管线钢焊接接头TEM观察下的断裂行为对比

对X80管线钢焊接接头3个区域的试样在透射电子显微镜（TEM）下进行原位拉伸试验,通过观察各个区域裂纹的萌生与扩展过程,对其微观断裂行为进行了研究和分析。结果表明：母材区中,裂纹萌生扩展主要以多裂纹起裂为主,表现为沿晶扩展的断裂形式,最后造成在切应力作用下的剪切脆性断裂;热影响区内,主裂纹首先在切应力作用下以45°方向起裂,整个扩展过程是主裂纹钝化、位错发射、主裂纹前方无位错区形成、微裂纹形核、主裂纹与微裂纹连接扩展这一多尺度过程不断重复的过程,最后导致准解理穿晶断裂;焊缝区中,从主裂纹分支出多条裂纹,裂纹均以与拉伸轴成45°的方向起裂,表现为沿晶与穿晶混合的断裂模式,最后造成穿晶脆性解理断裂。     

钢丝微动疲劳裂纹萌生寿命预测研究*

钢丝微动疲劳过程中,钢丝裂纹萌生特性直接影响其裂纹扩展特性,进而制约钢丝微动疲劳寿命,因此开展钢丝微动疲劳裂纹萌生寿命预测研究具有重要意义。基于有限元法、摩擦学理论和断裂力学理论,运用Smith-Watson-Topper（SWT）多轴疲劳寿命准则建立考虑磨损的钢丝微动疲劳裂纹萌生寿命预测模型,基于多种不同的钢丝疲劳参数估算方法对钢丝的微动疲劳裂纹萌生寿命进行了预测,并探究接触载荷、疲劳载荷、交叉角度及钢丝直径等微动疲劳参数对钢丝微动疲劳裂纹萌生寿命的影响规律。结果表明：基于中值法的预测结果最接近实际值;在微动疲劳过程中,钢丝微动疲劳裂纹萌生寿命主要与接触载荷和疲劳载荷相关。通过引入微动损伤参数建立简化的适用于钢丝绳的钢丝微动疲劳裂纹萌生寿命预测模型,通过与考虑磨损的预测模型计算结果进行对比验证了该模型的准确性。     

Recent development in low-constraint fracture toughness testing for structural integrity assessment of pipelines

Fracture toughness measurement is an integral part of structural integrity assessment of pipelines. Traditionally, a single-edge-notched bend (SE(B)) specimen with a deep crack is recommended in many existing pipeline structural integrity assessment procedures. Such a test provides high constraint and therefore conservative fracture toughness results. However, for girth welds in service, defects are usually subjected to primarily tensile loading where the constraint is usually much lower than in the three-point bend case. Moreover, there is increasing use of strain-based design of pipelines that allows applied strains above yield. Low-constraint toughness tests represent more realistic loading conditions for girth weld defects, and the corresponding increased toughness can minimize unnecessary conservatism in assessments. In this review, we present recent developments in low-constraint fracture toughness testing, specifically using single-edge-notched tension specimens, SENT or SE(T). We focus our review on the test procedure development and automation, round-robin test results and some common concerns such as the effect of crack tip, crack size monitoring techniques, and testing at low temperatures. Examples are also given of the integration of fracture toughness data from SE(T) tests into structural integrity assessment.     

含裂纹强度失配焊接接头弹塑性变形分析

采用有限元方法对平面应变条件下含裂纹不同强度失配的平板拉伸焊接接头的弹塑性变形进行了分析,研究了焊缝与母材强度失配系数、焊缝硬化性能、裂纹长度等对接头塑性变形的影响。结果表明：强度失配系数对接头塑性变形有显著影响。低匹配时,变形首先集中在焊缝中,而高匹配则对焊缝起到了一定的保护作用。失配系数一定时,不同焊缝硬化性能接头的塑性变形发展趋势是一致的;但当外载较大时,局部塑性变形程度不同,焊缝的硬化性能降低,裂尖塑性变形增大。
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化学机械抛光中Si/Cu/Ta/low-k界面剥离和断裂特性研究

超大规模集成电路随着布线层数增加和线宽的缩小,low-k材料的介电常数进一步降低,多孔low-k材料力学性能随之降低,使得晶圆在化学机械抛光(Chemical mechanical polishing,CMP)中面临的主要问题是Cu/Ta/low-k或者超低k材料的界面剥离。针对Si/Cu/Ta/low-k在CMP过程中的承载特性,建立单层布线和多层布线体系的界面力学模型,采用断裂力学理论和有限元法研究Si/Cu/Ta/low-k界面在CMP过程中界面的应力分布规律和界面裂纹的断裂强度。采用能量释放率来描述裂纹的扩展情况,根据界面裂纹能量释放率数值计算方法对裂纹长度、材料性质及不同的布线层数对裂纹扩展时的界面断裂/剥离特性进行仿真分析,得到界面应力分布、能量释放率、相位角与裂纹长度、材料性能、布线层数之间的关系变化曲线。结果表明:随着low-k力学性能降低和布线层数增加,裂纹能量释放率升高,界面裂纹更容易扩展。     

全层状TiAl基合金拉伸试验断裂过程及机理

通过对全层组织TiAl基合金不同缺口试样进行原位拉伸试验,结合数据分析以及断口形貌的观察,研究TiAl基合金拉伸的断裂过程和断裂机理.研究发现在原位拉伸试验中,对于直缺口试样,裂纹起裂于缺口根部,其断裂过程主要是主裂纹(沿层裂纹)首先起裂、扩展、连接,然后形成解理断裂起裂源.这一起裂源形成的同时,在刚好能满足Griffiths脆性解理断裂所需要的临界裂纹尺寸时,引起整个试件发生脆性解理断裂.裂纹首先起裂于层间,层间是薄弱环节,断裂方式是穿层断裂和沿层断裂的混合体,以穿层断裂为主.对于大圆弧和平板试样,这种材料的断裂过程是:首先在较高外加载荷下,在标距范围内比较薄弱的区域沿层开裂,随着外加载荷的进一步增加,这些微裂纹扩展,当微裂纹引起的应力集中和外加载荷共同作用足以引起微裂纹进一步贯穿的瞬间,即能量积累到一定程度时,试样整体发生解理断裂.很多试样的断裂是直接起裂的过程,一旦产生裂纹试样立即解理断裂,即这种材料的强度较高,材料很难产生裂纹,也不易损伤.