高组配焊接接头表面裂纹J积分试验研究

实际构件中大量存在着半穿型的焊接表面裂纹,对于这种力学性能不均匀三维裂纹体的弹塑性断裂力学参量的探讨还比较缺乏,利用Ｊ积分直接测试方法以及适当的理论弹性修正,对存在不同组配比的高组配焊接接头焊缝区的三维表面裂纹的Ｊ积分进行试验研究,着重讨论了接头组配比对焊接接头试样加载过程中裂纹扩展驱动力（Ｊ积分、裂纹嘴张开位移ＣＭＯＤ）、应变沿积分路径分布的影响。结果表明,焊接接头试样上应变的分布趋势取决于裂纹     

焊接接头高应变率下的动态断裂韧度分析

对采用Hopkinson气炮加载－Charpy V形冲击试样试验方法对焊接接头高应变率下的动态断裂韧度数据进行了分析研究，通过对试样断口形貌、冲击吸收功、断裂韧度及其影响因素进行分析，表明在高应变率加载条件下，焊接接头冲击试样断口形貌有特殊的表现，延伸带或钝化区的尺寸大小反映了裂纹萌生前的塑性变形大小，也反映了裂纹萌生功的变化趋势。同时说明焊接接头在高速动态冲击载荷作用下，其断裂韧度参量受组配强度、冲击速度及环境温度等因素影响显著，焊接热影响区的断裂韧度值关系到焊接接头断裂韧度的好坏。     

复合加载方式作用下熔合线上含裂纹焊接接头的裂端场与断裂参量

针对复合载荷作用下熔合线含裂纹的焊接接头,应用弹塑性有限元方法分析了其裂端应力场的分布规律.并对不同组配焊接接头的COD断裂参量及其复合角进行了数值计算,讨论了加载角度和接头强度组配对焊接接头断裂行为及断裂参量的影响机制.研究结果表明,对于复合载荷作用下的熔合线含裂纹焊接接头,其裂端应力场同时受到加载角度以及材料非均匀性的影响.当加载角度较小时,复合载荷对接头性能的影响相对于接头组配的影响更为显著.而当加载角度达到并超过30°时,情况与之相反.     

复合材料界面裂纹焊接接头塑性发展方向与复合角依存关系的数值分析

针对熔合线含裂纹的不同强度组配焊接接头,应用弹塑性有限元方法对其裂端场塑性区和复合角进行数值计算,讨论了强度组配与裂端场塑性区的发展规律以及裂端场塑性区发展方向与复合角的关系.结果表明,强度匹配对其裂端场塑性区发展规律有很大影响,即裂端场塑性区优先向屈服强度较小的材料发展;在同一载荷作用下,低组配接头裂端场塑性区比高组配大;裂端场塑性区随着复合角的增大而减小,对于不等组配接头,在载荷小于裂纹两边材料中较小的屈服强度值作用下,其塑性区发展方向与复合角呈现线性关系,而在载荷大于裂纹两边材料中较小的屈服强度值作用下则呈现非线性关系.     

不同组配P91钢焊接接头断裂性能分析

作为马氏体耐热钢，P91钢具有很强的抗氧化和抗高温蒸汽腐蚀性能，在大功率火力发电设备中得到广泛应用，而其接头的断裂性能对该材料的推广及应用至关重要。采用数值分析及实验的方法，讨论裂纹位于焊接接头的不同位置时，组配发生变化的焊接接头的断裂驱动力和断裂抵抗力。结果表明，材料组配的不同会对焊接接头的断裂驱动力产生一定的影响。从断裂阻力的角度来看，低组配焊接接头的断裂抵抗力要高于高组配。实际工程结构应采用低组配接头形式，以提高焊接接头的抗断性能。     

基于焊接接头断裂性能要求的焊条强度选择

焊接接头大量存在的裂纹及类裂纹缺陷是焊接接头破坏的主要原因之一，本文通过研究焊接接头强度组配对焊接接头断裂性能的影响，指出了不同位置的裂纹对焊接接头强度组配的要求是不同的，就焊条材料强度选择的问题提出了新的见解。     

A6N01铝合金焊接接头疲劳损伤和断口形貌分析

采用高频疲劳试验机对A6N01铝合金MIG焊接试样进行低周拉伸疲劳试验,研究了A6N01铝合金焊接接头疲劳过程中表面形貌变化和疲劳断裂后的断口形貌。结果表明:随着循环应力的加载,在试样产生裂纹之前,晶界处位错塞积严重,产生永久滑移带,裂纹源萌生于永久滑移带。气孔、夹杂等缺陷处应力集中较大,也容易称为裂纹源。在裂纹扩展过程中,主裂纹对次裂纹的扩展具有屏蔽作用。裂纹在相邻的不同位向的晶粒中扩展时,裂纹扩展方向发生变化。A6N01铝合金焊接接头疲劳断口有较多浅显的韧窝,说明A6N01铝合金焊接接头具有良好塑性。     

考虑材料微观塑性损伤的焊接接头延性裂纹扩展行为分析

通过考虑材料微观塑性损伤，定量研究了焊接接头中强度匹配、试件几何形状对接头抗延性裂纹扩展行为的影响。对管线用钢焊接接头，测试了标准三点弯曲试样、同种接头及不同匹配状态的双边裂纹拉伸试样的阻力曲线。通过考虑材料塑性损伤的数值模拟，由标准三点弯曲试样的阻力曲线测试结果，得出反映材料微观损伤的特殊单元模型控制参量，再根据该参量对同种接头及不同接头强度匹配下双边缺口拉伸试件的阻力曲线进行了定量预测，其结果与试验结果相当吻合。结果表明，考虑塑性损伤的特殊单元模型能很好地描述接头的抗延性裂纹扩展阻力特性。     

疲劳损伤对核电汽轮机焊接转子接头应力腐蚀开裂敏感性的影响

采用慢应变速率拉伸试验(SSRT)研究了不同程度疲劳损伤对核电汽轮机焊接转子接头应力腐蚀开裂敏感性的影响，利用扫描电子显微镜等观察手段讨论了疲劳损伤对汽轮机焊接接头应力腐蚀开裂敏感性和二次裂纹的作用机理. 结果表明，疲劳损伤增强了核电汽轮机焊接转子接头应力腐蚀开裂敏感性. 此外，疲劳损伤的作用使空气中试样的塑性提高而在腐蚀溶液中塑性降低，也影响了试样内部二次裂纹的产生和扩展.     

裂纹长度、焊缝宽度及强度组配对焊接接头J积分的影响

采用虚拟裂纹扩展法对焊接接头力学不均匀体的断裂参量J积分进行了数值模拟分析，考察了焊接接头J积分的守恒性以及不同的裂纹长度（a/W=0.1-0.5)，焊缝宽度（H／c=0.1-0.5)和强度组配（M＝0.6-1.4)在加载过程中对J积分的影响，计算结果表明：焊接接头裂纹尖端近场的J积分具有路径依赖性，而在远离裂尖的J积分表现出路径无关性。强度组配因子M对J积分值有很大的影响，焊缝宽度，裂纹长度对J积分值也有影响。     

Effects of strength matching and crack depth on the fracture parameters for welded joints

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Determination of lower bound fracture toughness of a high strength low alloy steel welded joint

Abstract

The use of high strength low alloy steels for high performance structures (e.g. pressure vessels and pipelines) requires high strength consumables to produce an overmatched welded joint. This globally overmatched multipass welded joint contains two significantly different microstructures, as-welded and reheated. In this paper, the influence of weld metal microstructure on fracture behaviour is estimated in comparison with the fracture behaviour of composite microstructures (as-welded and reheated). The lower bound of fracture toughness for different microstructures was evaluated by using the modified Weibull distribution. The results, obtained using specimens with crack front through the thickness, indicated low fracture toughness, caused by strength mismatching interaction along the crack front. In the case of through thickness specimens, at least one local brittle microstructure is incorporated in the process zone at the vicinity of the crack tip. Hence, unstable fracture occurred with small, or without, stable crack propagation. Despite the fact that the differences between the impact toughness of a weld metal and the that of base metal are insignificant, the fracture toughness of a weld metal can be significantly lower.     

STUDY ON DYNAMIC J-INTEGRAL OF MECHANICAL HETEROGENEOUS WELDED JOINT

Welded joint is a mechanical heterogeneous body, and mechanical heterogeneity has great effect on dynamic fracture behaviour of welded joints. In the present investigation, dynamic response curve and dynamic J-integral of practical undermatched welded joint and whole base and whole weld three-point-bend (TPB) models containing longitudinal crack are computed. Dynamic J-integral is evaluated using virtual crack extension (VCE) method and the computation is performed using MARC finite element code. Because of the effect of inertia,dynamic load response curve of computed model waves periodically. Dynamic J-integral evaluated by VCE method is path independent. The effect of inertia has little influence on dynamic J-integral curve. The value of dynamic J-integral of undermatched welded joint is lower than that of whole base metal and higher than that of whole weld metal. The results establish the foundation of safety evaluation for dynamic loaded welded structures.     

FATIGUE CRACK GSOWTH AND CRACK CLOSURE OF SIMULATED OVER-MATCHED WELDED JOINTS

1.~onItiswellknownthatthefatigUefailureisoneofthecornmonfailuresinmetalstructuresincludingweldedstmctures.VariousformulaehavebeensuggestedtopredictfatigUecrackgrowthrates,andthosewidelyusedinpracticeareasfollows:da/dN=cI(AK)acda/dN=c2(ACOD)?/dN=c3(AJ)%FortnulaeaboverelatedfatigUecrackgrowthrate(da/dN)eitherwiththerangeofstressintensityfactor(isK),orwiththerangeofcrackopeningdisplacement(ACOD),orwiththerangeofJ--intendvalue(6J)ofafatigUeloadcycle.However,sincethediscoverybyW.Elb..['…     

力学失配对电子束焊接接头疲劳裂纹扩展行为的影响

研究了高温合金电子束焊接接头区的疲劳裂纹扩展行为,深入分析了接头区力学失配对疲劳裂纹扩展方向及疲劳裂纹扩展速率的影响规律。结果表明,由于力学不均匀性的影响,电子束焊接接头的疲劳裂纹扩展具有不稳定性,位于高匹配焊接区的疲劳裂纹偏离原裂纹扩展方向,跨越焊接区与母材,呈复合型扩展;焊接区疲劳裂纹的集团使得裂纹尖端有效应力强度因子减小,因而焊缝区裂尖局部有效裂纹扩展驱动力下降,导致接头的疲劳裂纹扩展速率下     

A model of heating a flux-cored wire in arc metallizing and analysis of the structure of the coating

Summary

This paper describes HAZ‐notched CTOD tests of multipass welds in SMYS = 420–460 MPa class high‐strength steels for offshore structural applications. The weld metal strength overmatch causes different fracture behaviour depending on the actual CGHAZ toughness. When the CGHAZ is completely embrittled, the weld metal strength overmatch leads to the lower bound critical CTOD value. This is due to elevation of the local stress in the CGHAZ caused by the restraint effect of the overmatched weld metal. The fracture surface is generally flat, and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. A larger fraction of the CGHAZ along the crack front gives a smaller critical CTOD value. When the CGHAZ has moderate toughness, however, the weld metal strength overmatch may produce a higher critical CTOD value at brittle fracture initiation. This is due to crack growth path deviation towards the base metal. Plastic deformation preferentially accumulates to a greater extent on the softer base metal side before the critical stress conditions for brittle fracture initiation occur in the CGHAZ. This asymmetrical plastic deformation promotes deviation of ductile crack growth from the crack tip CGHAZ. In this case, the critical CTOD value does not always reflect the CGHAZ toughness itself.

A notch location nearer the weld metal sometimes causes fracture initiation in the weld metal if the fatigue crack tip samples the CGHAZ. Such experimental data do not reflect the real CGHAZ toughness.

The significance of the critical CTOD value obtained in the tests must be determined in the fracture toughness evaluation of the weld CGHAZ. This paper presents a procedure for evaluation of CTOD test results obtained for HAZ‐notched welds that considers the strength mismatch effect.     

Effect of notch location on fatigue crack growth behavior of strength-mismatched high-strength low-alloy steel weldments

Welding of high-strength low-alloy (HSLA) steels involves the use of low-strength, equal-strength, and high-strength filler materials (electrodes) compared with the parent material, depending on the application of the welded structures and the availability of filler material. In the present investigation, the fatigue crack growth behavior of weld metal (WM) and the heat-affected zone (HAZ) of undermatched (UM), equally matched (EM), and overmatched (OM) joints has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. Shielded metal arc welding (SMAW) has been used to fabricate the butt joints. A center-cracked tension (CCT) specimen has been used to evaluate the fatigue crack growth behavior of welded joints, utilizing a servo-hydraulic-controlled fatigue-testing machine at constant amplitude loading (R=0). The effect of notch location on the fatigue crack growth behavior of strength mismatched HSLA steel weldments also has been analyzed.     

焊接接头损伤条件下的断裂力学参量数值模拟

利用弹塑性损伤-应变耦合有限元方法研究了不同强度匹配焊接接头中母材区断裂应变的变化对焊缝区裂纹断裂力学参量J积分的影响规律.结果表明,对于强度高匹配、等匹配和低匹配的焊接接头,在焊缝区各项力学性能参数保持不变的条件下,随着母材区断裂应变的减小,焊缝区的塑性应变及裂纹扩展驱动力J积分均有明显的提高;随着接头强度匹配比M的降低(母材屈服强度的升高),母材区断裂应变的变化对J积分值的影响作用减小.     

A J-INTEGRAL ANALYSIS OF CRACKS IN OVERMATCHED WELDED JOINTS

The effects of material inhomogeneity and weld width of the overmatched welded joints with alongitudinal crack or flaw upon the J-interal,a characterizing parameter for elastoplasticfracture,have been studied.The overmatched weld is idealized to be a parallelly cracked andsandwiched hard layer,the yieht stress of which is higher than that of the base metal.Numeri-cal solutions were obtained by using elastoplastic finite element method under the assumptionof plane stress.The resuits show that the values of J-integral are remarkably affected by thematerial inhomogeneity ant the weld width especially when the nominal stress or nominalstrain is comparatively high,which,therfore,should be taken into account when evaluatingthe crack driving force for an overmatched joint.