高温合金焊接接头疲劳损伤演化试验

该文采用应力控制的方式对GH4169焊接接头试件进行疲劳试验,得到了GH4169焊接接头在不同载荷下的寿命。通过对焊接接头疲劳应变幅变化的测量,了解了GH4169焊接接头的疲劳损伤演化过程。该文还对不同载荷下焊接接头宏观损伤演化曲线进行了分析。研究结果可为GH4169焊接结构疲劳寿命预测提供参考。     

基于剩余应变的复合材料结构寿命预测

为实现对复合材料结构的寿命预测,对已有的复合材料疲劳寿命预测模型进行了研究,确定了基本的刚度降模型,提出了剩余应变的概念,并将其应用到渐进疲劳损伤方法中,以Abaqus为平台,编写UMAT子程序,实现了对复合材料结构的寿命预测及疲劳损伤扩展分析.针对某碳纤维增强复合材料TS800开展相关试验,试验结果与预测结果吻合较好.研究表明,本文所改进的渐进疲劳损伤方法能较好地完成对复合材料结构的寿命预测.     

船舶工程焊接接头完整性评估与疲劳寿命预测技术综述

介绍了船体结构中含缺陷的焊接接头完整性评估以及疲劳寿命预测方法，对结构完整性评估的K准则、COD准则、J积分准则、SINTAP/FITNET安全性评定方法以及评定流程、国内外标准进行介绍。并对焊接接头的疲劳强度影响因素，如平均应力、残余应力以及接头几何形状应力集中等进行了分析；介绍了焊接接头疲劳寿命预测的常用方法，如基于名义应力的评估方法、基于断裂力学的评估方法、基于损伤力学的评估方法以及其它疲劳评估方法等。此外，还对基于断裂力学法建立的焊接残余应力与疲劳裂纹扩展寿命之间的函数关系进行了介绍，该方法实现了对含残余应力的焊接结构疲劳寿命的精确预测。     

考虑平均应变的低周疲劳寿命本征损伤耗散预测方法

金属构件疲劳破坏是工业中常见的破坏形式。为了提高构件疲劳寿命的预测精度，针对低周疲劳载荷下平均应变对疲劳寿命的影响，基于连续介质损伤力学及其不可逆热力学框架，并引入Ramberg-Osgood循环本构模型，以等本征损伤耗散功作为等寿命条件，建立了一种考虑平均应变的低周疲劳寿命预测模型。为对比验证新建模型的有效性和先进性，采用新建模型、修正的Ohji模型、Sandor模型和Wei-Wong模型分别对叠加平均应变的45钢和2124-T851铝合金的低周疲劳寿命进行了预测，并与对应的试验结果进行对比。结果表明：新建模型的预测结果与试验结果吻合较好，其预测效果优于现有模型。基于本征损伤耗散理论的疲劳寿命预测方法为金属材料疲劳寿命的预测提供了新思路。     

复合材料单钉接头疲劳累积损伤破坏分析

基于时间增量原理 , 推导了层合板接头疲劳加载累积损伤应力2应变分析的虚功方程。同时 , 引入Hashin三维疲劳失效准则进行材料的损伤判定 , 并结合建立的疲劳加载材料退化模型、 4种基本损伤机制相互关联作用的材料性能退化方法及复合材料接头最终失效判据 , 建立了层合板接头疲劳载荷作用下三维累积损伤分析的寿命预测方法。最后 , 对层合板接头拉2拉疲劳载荷作用下的损伤累积扩展与失效规律进行了仿真分析 , 并与试验结果进行了对比 , 结果表明 : 本文中建立的寿命预测方法能够很好地预测层合板接头的寿命以及损伤发生、扩展及最终失效。      

基于随机振动理论的焊接结构疲劳寿命概率预测方法研究

为了能够在设计阶段对承受随机载荷作用的焊接结构的焊缝进行疲劳寿命预测,该文以随机振动理论为基础,结合主S-N曲线法,通过推导证明了用于焊接结构疲劳寿命评估的等效结构应力与激励载荷之间存在线性传递关系,从而提出了一个频域的焊接结构随机振动疲劳寿命概率预测新方法。该方法能够解决多随机载荷作用下复杂焊接结构振动疲劳寿命预测问题。通过T型接头的算例对比分析,证明了该方法能够考虑激励载荷的频率对焊缝疲劳寿命的影响。直接以线路实测随机加速度谱做为激励载荷,对某轨道集装箱平车车体结构关键焊缝的振动疲劳寿命进行预测,结果表明:该方法的预测结果与实际情况吻合,便于工程应用。     

1Cr18Ni9Ti不锈钢管焊接接头断裂原因分析

某飞机用1Cr18Ni9Ti不锈钢管在进行弯曲疲劳试验时焊接接头发生断裂。通过宏观分析、微观分析、金相检验等方法,对焊接接头的断裂原因进行了分析。结果表明:焊接结构不合理(焊接热影响区处于截面过渡位置),是导致该1Cr18Ni9Ti不锈钢管焊接接头疲劳寿命降低的主要原因;焊接区域存在未焊透情况,需要对焊接工艺进行优化。     

单个冲击对不锈钢管道焊接头低周疲劳寿命的影响

完成了单个冲击对１Ｃｒ１８Ｎｉ９１不锈钢管道焊接头试样低周疲劳寿命影响的试验研究。单个试验最大瞬时峰值应变率达４８０ｓ＾－１。试样未经消除焊接残余应力。采用成组法试验（每组７个试样）,对称加载模式,总应变幅为０．００２２８。结果表明,冲击影响受到焊接残余应力和冲击塑性导入机制的耦合作用。焊接残余应力与冲击应力叠加将增加材料损伤,而冲击塑性导入将减缓疲劳损伤和降低疲劳寿命分散性。前者扼制后者。考虑疲     

基于背应力功的疲劳能量模型及其在轮槽构件寿命预测中的应用

基于局部应力-应变法与疲劳损伤能耗结构,以疲劳过程中背应力塑性功累积为基础,建立了一种新的缺口构件疲劳寿命预测能量模型,并将其应用于某型汽轮机轮槽结构件的疲劳寿命预测。通过与试验结果相比较,初步验证了模型的预测精度(预测寿命与试验寿命误差小于20%)。此外,还进一步将上述能量模型与传统疲劳寿命预测能量方法进行了比较。结果表明,基于背应力塑性功累积的疲劳损伤描述可进一步提高疲劳寿命预测精度,值得工程疲劳设计者重视。     

混凝土双轴拉压、三轴拉压压变幅疲劳性能研究

利用大型多轴疲劳试验机,进行了拉压双轴和拉压压三轴荷载作用下混凝土两级、三级变幅疲劳试验研究,重点分析了残余应变的变化规律。试验结果表明:混凝土在拉压和拉压压加载工况下的残余应变主要与相对疲劳次数和侧压应力比有关,基本不受加载历程因素的影响。定义相对残余应变为损伤变量,建立了损伤演变方程,并进行了疲劳损伤分析和剩余疲劳寿命预测,为混凝土在多轴变幅疲劳试验研究及疲劳损伤评价提供参考。     

Experimental study on the whole‐life heterogeneous ratchetting and ratchetting‐fatigue interaction of SUS301L stainless steel butt‐welded joint

Uniaxial fatigue tests of butt‐welded joint, made from SUS301L stainless steel, were carried out under asymmetric stress‐controlled cyclic loading conditions in this work. The effects of stress amplitude and mean stress on the whole‐life heterogeneous ratchetting and fatigue life of the butt‐welded joint were investigated, respectively, for the specified subzones. The experimental observations show that the whole‐life inhomogeneous ratchetting strain concentrating in a specific fusion zone (denoted as the FZ‐1 subzone) of the welded joint becomes more significant as the stress level increases; the fatigue failure also occurs in the FZ‐1 subzone, and the fatigue life depends on both the applied mean stress and stress amplitude and is determined by the combination of ratchetting damage and fatigue one in the localized FZ‐1 subzone.     

Approach for fatigue damage assessment of welded structure considering coupling effect between stress and corrosion

This paper presents a new approach to assess time-dependent corrosion fatigue damage of welded joint considering the coupling effect between mechanical factor and corrosion factor. The high stress region around weld will accelerate corrosion and be more likely to induce nonuniform corrosion of welded joint. And the effect of loading on corrosion behavior of the steel in NaCl solution was investigated. The synergistic effect between applied elastic stress and chemical attack on Q235 steel was evaluated by electrochemical experiments. A side longitudinal of ship structure is selected as a case study. Time-dependent stress concentration factor of welded joint as a function of corrosion deterioration was analyzed, and the iterative process of stress and corrosion degeneration of plate thickness was used to simulate coupling effect basing on the results of experiment. The hot spot stress approach was adopted to calculate the fatigue damage. It is revealed that the nonuniform corrosion could influence fatigue damage of welded joint, and that impact will be more and more significant with the growth of corrosion year.     

Fatigue life prediction of friction stir spot welds based on cyclic strain range with hardness distribution and finite element analysis

The main aim of the present study is to investigate the fatigue behavior of single friction stir spot welds (FSSW) using strain-based modified Morrow’s damage equation. The correlation between microhardness, cyclic material constants, and mechanical strength of different zones around the FSSW are assumed to be proportional to the base material hardness. Experimental fatigue tests of friction stir spot welded specimens have been carried out using a constant amplitude load control servo-hydraulic fatigue testing machine. ANSYS finite element code has been used to simulate a single tensile shear friction stir spot welded joint, and non-linear elastic-plastic finite element analysis has been employed to obtain the values of local equivalent stress and strain near the notch roots of the joints. The results based on the numerical predictions have been compared with the experimental fatigue test data. It has been shown that the strain-based approach does a very good job for estimating the fatigue life of friction stir spot welded joints.     

Fatigue crack initiation life estimation in a steel welded joint by the use of a two-scale damage model

This work deals with the fatigue behaviour of S355NL steel welded joints classically used in naval structures. The approach suggested here, in order to estimate the fatigue crack initiation life, can be split into two stages. First, stabilized stress–strain cycles are obtained in all points of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. This calculation takes account of: base metal elastic–plastic behaviour, variable yield stress based on hardness measurements in various zones of the weld, local geometry at the weld toe and residual stresses if any. Second, if a fast elastic shakedown occurs, a two-scale damage model based on Lemaitre et al. 's work is used as a post-processor in order to estimate the fatigue crack initiation life. Material parameters for this model were identified from two Wöhler curves established for base metal. As a validation, four-point bending fatigue tests were carried out on welded specimens supplied by 'DCNS company'. Two load ratios were considered: 0.1 and 0.3. Residual stress measurements by X-ray diffraction completed this analysis. Comparisons between experimental and calculated fatigue lives are promising for the considered loadings. An exploitation of this method is planned for another welding process.     

钢箱梁桥海量应变监测数据分析与 疲劳评估方法研究

海量监测数据的分析与应用是桥梁结构健康监测研究中的重要问题之一,以润扬大桥钢箱梁的长期应变监测数据为研究对象,研究了海量应变监测数据分析与疲劳评估方法。首先,基于Eurocode3规范的S-N曲线和Palmgren-Miner准则建立了焊缝疲劳损伤分析及寿命预测方法。其次,研究了应力循环的快速提取及应变数据中随机干扰成分的剔除方法。在此基础上,讨论了对公路钢箱梁桥开展长期持续疲劳监测的必要性,并给出了润扬大桥钢箱梁焊缝的疲劳寿命预测值。研究发现,采用MATLAB与C语言联合编程的雨流计数法可快速处理海量应变监测数据;根据对焊缝疲劳损伤的贡献可确定有效应力循环阀值,从而剔除由随机干扰引起的数量极多但幅值较小的应力循环;焊缝疲劳损伤在一年内会发生大幅度的变化,短期应变监测数据可能导致焊缝疲劳寿命评估值出现较大的偏差。     

固溶氢对TA15钛合金电子束焊接头疲劳扩展寿命的影响

对TA15钛合金电子束焊接试样充氢,研究了氢对其显微组织形态和接头疲劳扩展寿命的影响。结果表明:充氢含量低于0.105%时,氢以固溶态存在于合金中,并未形成氢化物;母材抗疲劳扩展的能力高于焊缝区;随着充氢含量的增加,接头的疲劳寿命大幅下降。其原因是,微量氢的存在降低了TA15合金的韧性,固溶氢提高了疲劳裂纹的扩展速率。氢在边界处的聚集加速了焊接接头内裂纹沿马氏体束边界的扩展,导致在断口形成了团结构这一特殊组织形态。     

Combined effect of strength & sheet thickness on fatigue behaviour of resistance spot welded joint

Fatigue performance of spot welded lap shear joint is primarily dependent on weld nugget size, sheet thickness and corresponding joint stiffness. Two automotive steel sheets having higher strength lower thickness and lower strength higher thickness are resistance spot welded with established optimum welding condition. The tensile‐shear strength and fatigue strength of lap shear joint of the two automotive steel sheets are determined and compared. Experimental fatigue life of spot welded lap shear joint of each steel are compared with predicted fatigue lives using different stress intensity factor solutions for kinked crack and spot weld available in literature. Micrographs of fatigue fractured surfaces are examined to understand fracture micro‐mechanisms.     

Stochastic fatigue damage accumulation in a T-welded joint accounting for the residual stress fields

The residual stresses that occur as a result of nonhomogeneous heating and cooling during welding may have a significant effect on the accumulation of fatigue damage in a welded joint. The problem is complicated not because of the complex spatial distribution of the residual stress fields, but because those fields typically change under an applied load. The present study considers the effect of residual stresses on fatigue damage accumulation in a welded joint subjected to stochastic loading.The influence of residual stresses on stochastic fatigue damage accumulation is accounted for by a simple approach based on an elastic–perfectly-plastic material model and the Gerber correction factor. The model assumes that the residual stress remaining at the critical location depends on the largest nominal stress ever endured by a welded joint. The model predicts that the residual stresses during stochastic loading randomly decay to zero. The effect of material yielding is additionally investigated by considering an elastic–plastic material model with linear kinematic hardening. The residual stresses in this case are computed through Monte Carlo simulations. It is demonstrated that the effect of material hardening is to reduce the rate of residual stress decay and thus to accelerate the rate of fatigue damage accumulation.     

Analysis of fatigue crack behaviour based on dynamic response simulations and experiments for tensile-shear spot-welded joints

Fatigue crack initiation and propagation behaviours were studied based on the dynamic response simulation by the three‐dimensional finite‐element analysis (FEA) and dynamic response experiments for tensile‐shear spot‐welded joints. The entire fatigue propagation behaviour from the surface elliptical cracks at the initiation stage to the through thickness cracks at the final stage was taken into consideration during the three‐dimensional FEA dynamic response simulations. The results of the simulations and experiments found that the fatigue cracks of spot‐welded joint from initial detectable crack sizes to crack propagation behaviour could be described by three stages. Approximately one‐half of the total fatigue life was taken in stage I, which includes micro‐crack nucleation and the small crack growth process; 20% of the total fatigue life in stage II, in which the existing surface crack propagates through the thickness of sheet and 30% of the total fatigue life in stage III, during which the through thickness crack propagates along the direction of plate width to the final failure. According to the relationship between the crack length and depth and the dynamic response frequency during the simulated fatigue damage process, the definition of fatigue crack initiation and propagation stages was proposed. The analysis will provide some information for the fatigue life prediction of the spot‐welded structures.