疲劳裂纹扩展时焊接残余应力分布的测定

本文采用切割法测定了沿焊缝横向残余应力的分布规律．由于切割试样可以近似地模拟疲劳裂纹扩展，通过测量释放的应力，计算出残余应力的分布，可发现残余应力对疲劳裂纹扩展速率的影响规律．     

厚钢板焊缝断裂试样疲劳裂纹预制的"高K比法"

提出了预制厚钢板焊缝断裂韧度试样疲劳裂纹的“高K比法”。用“高K比法”预制疲劳裂纹,不需要对焊缝试样进行局部韧带压缩或反向弯曲等预处理,简化了焊接接头断裂韧度试验方法,而且此法不改变原焊缝残余应力,所得到的焊缝断裂韧度值更准确,有效地解决了厚钢板焊接接头韧度评定技术的关键问题。“高K比法”可以缩短断裂韧度试验所需的时间、减少备用试样、节约人力物力。介绍了“高K比法”的试验原理、试验方法、工程应用和意义。     

大厚度钢板不同取样方向下的CTOD韧性研究

裂纹尖端张开位移(CTOD)是弹塑性断裂力学中重要参数,其大小直接反映了材料裂纹尖端抗开裂的能力,作为大厚度钢板低温断裂韧性的主要评定标准之一,应用广泛。本文在深入研究各规范对CTOD实验试样取样方向的规定后,依据规范BS7448:Part1:1991和API RP 2Z对某钢厂100mm厚API 2W Gr 60钢母材及热处理后母材进行-10℃、两种取样方式下的CTOD实验。结果表明所有试样实验结果均满足规范验收标准,但热处理降低了材料的韧性。对比不同取样方向下的实验结果,发现厚钢板母材中裂纹尖端在厚度方向上的抗开裂能力小于其在晶粒流动方向上的抗开裂能力,而在热处理后母材中表现正好相反,为CTOD实验中合理选取试样取样方向提供参考。     

利用焊接残余应力及深冷预制裂纹技术的探索

本文探索了利用焊接残余应力及低温深冷技术预制裂纹的方法，并用带纵向焊缝的试板进行试验，证明了该方法的可行性。     

海底管线钢焊接缺陷裂纹尺寸容许值研究

裂纹尖端张开位移(CTOD)作为评定海洋工程用钢断裂韧性的重要指标,已广泛应用于现代工业中.根据BS7448:part4标准,采用多式样法对海底管线钢X70进行低温韧度试验,分别建立了焊缝和热影响区的CTOD-R阻力曲线,从而确定了焊缝和热影响区的CTOD启裂值.考虑到铺设之后的安全性问题,采用较小的焊缝CTOD启裂值为参考依据.以此为基础,分别采用SINTAP标准中的一级评定和三级评定对其埋藏裂纹进行安全评估(ECA)考虑铺设环境和残余应力影响,分别得出在铺设条件下的一级评定和三级评定埋藏裂纹容许尺寸.以试验数据为依托,以BS7448和SINTAP规范为理论指导,为海底管线铺设工程中的焊接缺陷验收和拒收提供了依据,为海底管线钢质量的优劣提供参考.     

基于多因素的焊接结构疲劳裂纹扩展速率分析

以Donahue等提出的疲劳裂纹扩展速率计算模型为基础,通过引入形状系数、张开比和残余应力等参数,建立了适用于焊接结构的疲劳裂纹扩展速率计算模型,分析了多种因素对焊接结构疲劳裂纹扩展速率的影响规律。结果表明,焊板厚度和焊缝余高的变化均会对焊接结构疲劳裂纹的扩展速率产生影响,在对焊接结构表面形状进行设计时应保有一定的焊缝余高;有效应力比的增大会降低焊接结构疲劳裂纹的扩展速率,且裂纹深度的变化不会改变有效应力比对焊接结构疲劳裂纹扩展速率的影响;残余应力的增大会提高焊接结构疲劳裂纹的扩展速率,且残余应力对疲劳裂纹扩展速率的促进作用随着裂纹深度的增加而增大,在对焊接结构的疲劳性能进行设计时须考虑残余应力对结构性能的影响。     

缺口根部和裂纹尖端残余应力的X射线法测定

X射线法用于缺口根部和裂纹尖端等徽区的残余应力测试的先决条件是解决缩小光束直径、提高衍射束的强度和准确设置试样等技术问题.在X射线衍射仪上借助于自行设计制造的限束对光装置和侧倾对中附件,成功地测定了缺口根部半径为1mm的喷丸残余应力场和板形试样压-压周期载荷下裂纹尖端的残余拉应力场.     

焊接残余应力对含缺陷管道 TK?值的影响研究

为给含缺陷焊接管道脆性断裂的进一步研究提供基础,本文采用本征应变与局部应力相结合的方法模拟了两种典型的焊接残余应力,并计算了含内表面环向裂纹的管道分别在这两种残余应力作用下的应力强度因子K和裂尖约束应力T .计算结果表明:管道厚度方向的应力分布决定了裂纹根部的K ?值,沿着管道厚度呈拉应力分布的残余应力对应较大的K值与较小的T 值, T而沿着管道厚度总体呈弯曲分布的残余应力对应较小的K值与较大的T 值;在残余应力分布和管道厚度都相同的条件下,管道内径对K ?值的影响随着裂纹深度的增大而加强.T     

焊接残余应力平板多个共面表面裂纹的应力强度因子

采用线弹簧模型求解含焊接残余应力平板多个共面任意分布表面裂纹的应力强度因子.利用边裂纹权函数给出了裂纹表面上沿厚度非线性分布的残余应力向线性分布的转化公式.基于Reissner板理论和连续分布位错思想,将含多个共面任意分布表面裂纹的无限平板问题归结为一组Cauchy型奇异积分方程,并采用Gauss-Chebyshev方法获得了奇异积分方程的数值解.以三共面表面裂纹为例,计算了表面裂纹的应力强度因子,并讨论了裂纹间距、裂纹几何形状等因素对应力强度因子的影响.     

裂纹尖端张开角准则的稳态裂纹扩展实验研究

ASTM和ISO已发布获得低约束条件下断裂性能和裂纹稳态扩展阻力曲线的标准,标准中采用多种试验方法确定临界裂纹张开角度来表征板材的裂纹稳态扩展。本文采用准静态裂纹扩展试验方法,通过摄像技术测定裂纹尖端张开角(CTOA),验证标准对1mm厚度2024-T3铝合金板稳态裂纹扩展行为的有效性。研究了试样构型、裂纹尺寸和是否疲劳预制裂纹对裂纹稳态扩展过程中CTOA的影响。试验结果表明:裂纹扩展2mm后,距离裂纹尖端1mm处的CTOA趋于一个恒定值6°,这个值与试样构型和裂纹长度无关;同时,不预制疲劳裂纹对CTOA值和试样的最大剩余强度影响较小。     

Measurement of the Residual Stress Tensor in a Compact Tension Weld Specimen

Neutron diffraction measurements have been performed to determine the full residual stress tensor along the expected crack path in an austenitic stainless steel (Esshete 1250) compact tension weld specimen. A destructive slitting method was then implemented on the same specimen to measure the stress intensity factor profile associated with the residual stress field as a function of crack length. Finally deformations of the cut surfaces were measured to determine a contour map of the residual stresses in the specimen prior to the cut. The distributions of transverse residual stress measured by the three techniques are in close agreement. A peak tensile stress in excess of 600 MPa was found to be associated with an electron beam weld used to attach an extension piece to the test sample, which had been extracted from a pipe manual metal arc butt weld. The neutron diffraction measurements show that exceptionally high residual stress triaxiality is present at crack depths likely to be used for creep crack growth testing and where a peak stress intensity factor of 35 MPa√m was measured (crack depth of 21 mm). The neutron diffraction measurements identified maximum values of shear stress in the order of 50 MPa and showed that the principal stress directions were aligned to within ~20° of the specimen orthogonal axes. Furthermore it was confirmed that measurement of strains by neutron diffraction in just the three specimen orthogonal directions would have been sufficient to provide a reasonably accurate characterisation of the stress state in welded CT specimens.     

Neutron Diffraction Analysis of Complete Residual Stress Tensors in Conventional and Rolled Gas Metal Arc Welds

Mitigation of residual stress in an arc weld by high-pressure rolling of the weld seam has been investigated using neutron diffraction. Rolling was found to greatly improve the residual stress distribution, causing significant compressive stresses at the weld line. A novel aspect of the data presented is that at each measurement location, normal strains in nine separate directions were evaluated, enabling calculation of the complete strain and stress tensors. It is thus confirmed that the principal stress directions generally lay close to the specimen coordinate axes (i.e. that they are well-aligned with the direction of welding and rolling), and that rolling does not cause any significant additional residual stresses which could have detrimental effects. Methods of uncertainty estimation and the applications of full-tensor residual stress measurements are also discussed.     

Correlation of modified crack tip opening distance with heat input to the heat affected zone of high-strength low-alloy steels

Investigated is the dependency of the crack tip opening displacement (CTOD) on the local microstructure of the heat affected zone in high-strength low-alloy (HSLA) microalloyed steel. Since the initiation of the crack tip location could not be controlled in fatigue, any possible correlation between heat iput in welding and fracture toughness could be smeared. Modified CTOD data are defined; they show that the fracture resistance of the weld joint decreased as the heat input increased.     

爆炸法消除焊接接头残余应力的数值模拟

利用非线性动力有限元法对爆炸处理消除焊接接头残余应力的全过程进行了数值模拟。首先，采用温度场与位移场的间接耦合方法计算了钢板对接焊的焊后冷却及残余应力的生成过程，求得焊接接头处由高温冷却到室温由于变形受到阻碍而产生的不均匀的残余塑性变形和应力。然后，在焊缝区引入移动的爆炸载荷，计算了爆炸波作用下该钢板焊接接头附近应力的变化。计算结果表明，爆炸处理可引起板内应力的重新分布，从而有效地释放超过塑性极限的残余应力。利用炸药爆炸消除大型焊接结构残余应力是一种经济有效的方法，本文的数值模拟为研究炸药爆炸消除焊接结构残余应力的机理提供了有力的工具。     

CTS试样I/II混合型断裂特性数值计算研究

采用CTS试样研究I/II混合型断裂特性计算裂纹前缘应力强度因子时可采用解析公式,一旦裂纹发生扩展,解析公式便不再适用。文中采用有限元法研究紧凑拉伸剪切（CTS）试样在I/II平面混合型加载下的裂纹扩展行为。采用ANSYS建立CTS试样I/II混合型测试系统有限元模型,为模拟真实受力状态,在CTS试样-销-扇型夹具以及扇型夹具-销-U型夹具之间分别建立接触对进行接触力学分析。通过与解析公式结果进行对比验证了该数值方法的可靠性。采用最大环向应力准则（MTS）,模拟了CTS试样不同加载角度下的裂纹扩展路径,获得了裂纹扩展路径中应力强度因子随裂纹长度的变化曲线,解释了裂纹扩展路径不与外载荷方向垂直的原因。结合文中计算结果,在CTS试样I/II混合型裂纹扩展速率实验测得裂纹长度与寿命的关系曲线a-N的基础上,便可得到材料I/II型混合型裂纹扩展速率曲线。     

缺口件疲劳寿命分布预测的有效应力法

本文提出了一种由光滑件疲劳寿命试验数据预测缺口件疲劳寿命分布的有效应力法。该方法中缺口件的裂纹可能萌生表面被分解成一个个微元,整个表面可看成是这些微元组成的一个串联模型,按照串联概率失效模型,缺口件的疲劳强度失效概率就可以由各微元的疲劳强度失效概率计算得到,其中微元的疲劳强度失效概率是由光滑件的疲劳强度失效概率通过最弱环节理论计算得到的。在缺口件的疲劳强度失效概率表达式中,引入了有效应力的概念,用它查取光滑件的疲劳寿命试验数据就可以直接得到缺口件的疲劳寿命分布。该方法可以同时考虑到应力梯度和试件尺寸对缺口件疲劳寿命分布的影响。进行了材料LY12CZ的带中心孔缺口件的寿命算例分析,预测结果和试验结果吻合良好,表明该方法是有效的。     

On the use of crack-tip-opening displacement to predict the fracture strength of notched graphite/epoxy laminates

The fracture strength and crack-opening displacement of notched graphite/epoxy laminates were measured experimentally using the center-cracked tension-specimen geometry. Four replicate tests were conducted for a variety of laminate stacking sequences, thicknesses, and notch lengths. Most laminates exhibited extensive notch-tip damage prior to fracture. Values of crack-tip-opening displacement (CTOD) at fracture were estimated from values of crack-opening displacement measured at the crack center line. CTOD was independent of specimen crack length for the [0/±45/90] _s , [0/±45/90]_15s , [0/±45] _s , [0/±45/]_15s , and [0/90]_24s laminates. In addition, notched laminate strength was accurately predicted using a Dugdale-type model along with the estimated CTOD.Paper was presented at V International Congress on Experimental Mechanics held in Montreal, Quebec, Canada on June 10–15, 1984.     

Effect of cracked weld joint and yield strength dissimilarity on crack tip stress triaxiality

This paper is concerned with an elasto-plastic analysis of a weld joint containing a central crack in the weld material (WM) whose yield strength may differ from that of the base material (BM). Stress triaxiality along the path of expected crack extension is found to be influenced not only by the applied tensile load level and crack length relative to the specimen width but also by the degree of BM/WM mismatch in the yield strength.Three different cases are analyzed by application of a two-dimensional finite element analysis. They are referred to as under-match, even-match and over-match which correspond, respectively, to the WM yield strength being less than, equal to and greater than that of the BM. In general, the stress triaxiality along the crack front tends to increase for an under-matched weld and decrease for an over-matched weld using the even-matched case as a reference. As the crack length is reduced for a given specimen width, the stress triaxiality decreases accordingly and the BM/WM material dissimilarity becomes more obvious. Displayed graphically are also the crack front plastic zone size that increases with the applied tensile load level and suffers a discontinuity across the weld line.     

Analysis to the stress and strain structures of mode I 3-D crack in elastic-plastic state

In this paper, the stress and strain structures of Mode I 3-D crack in power hardening material are studied by analyzing the fundamental equations of elastic-plastic mechanics. It is shown that three regions, Z1,Z2 and Z3 can be divided in the thickness direction according to the stress characteristic. In region Z1, the stress components in the plane Perpendicular to z axis (thickness direction) can be solved first using the fundamental equations of plane strain state; in region Z3, they can be solved first by the equations of plane stress state. The region Z2 is defined as a transition layer. It is shown that the transition layer is the characteristic of Mode I 3-D crack in elastic-plastic state, and it is significant to the research on 3-D fracture. The crack tip opening displacement CTOD is chosen to describe the amplitude coefficient of the local stress field, and the distribution of CTOD in 3-D state is investigated.The project supported by National Natural Science Foundation of China.     

基于大型炉体焊接残余应力测试的盲孔法研究

为提高炉体焊接残余应力测试精度,对传统的盲孔法进行改进,即在测点打孔后多次扩孔并增加孔深;通过对SM400ZL钢焊接残余应力测试时的应变释放系数A,B进行实验标定,得出多组应变释放系数值,计算可得残余应力的估算值,并对不同状况下的残余应力值进行比较.通过大拉力标定试验得出SM400ZL钢盲孔的孔边塑性变形对测量精度的影响和塑性修正公式.结果表明,改进的盲孔法能够更准确地测得焊接残余应力值,测量结果经塑性修正,最大误差减小到2%以下.测试方法和结果可直接在相关的工程实际中应用.