焊接过程对焊接残余应力及残余变形的影响

采用热弹塑性有限元法，对直通焊和多人同时分段焊两种焊接过程的平板对接焊时焊接应力变形和圆柱壳板环缝的焊接变形进行比较，认为多人同时分段焊直通焊无论在降低焊接残余应力还是焊接残余变形都具有非常显著的效果。     

残余应力与胶粘剂对ARALL层板疲劳裂纹扩展特性的影响

研究了粘剂性质、含量及残余应力状态对纤维－铝合金胶接层板（ＡＲＡＬＬ）疲劳裂纹扩展特性的影响,分析了裂纹扩展过程中的分层状态的变化,结果表明,ＡＲＡＬＬ层板内富胶层的剪切形变和伴随裂纹扩展的分层区越大,即这两方面耗散能量越多,则疲劳裂纺扩展速率越低;胶粘剂含量的影响不明显,给层板施加预应力极大降低了层板的疲劳裂纹扩展速率,其本质在于裂尖在同样的疲劳载荷下实际所受到的有效应力降低。     

残余压应力对45钢疲劳裂纹萌生寿命的影响

本文研究了４５钢不同缺口半径及经预压,喷丸处理后对试样弯曲疲劳裂纹萌生寿命的影响。研究结果表明：随着试样缺口根部半径的增大,试样的弯曲疲劳裂纹萌生寿命增加。     

残余应力和粗糙度对叶片振动疲劳性能的影响

为了研究叶片表面完整性对其振动疲劳性能的影响,本文模拟分析了某型高温合金叶片在振动疲劳实验过程中的动力学应力响应,获得叶片共振时应力幅值随时间的变化规律,分析了残余应力和粗糙度对叶片振动疲劳寿命和疲劳极限的影响规律.结果表明:叶片共振过程中的应力响应幅值先增大后减小呈周期性变化,属于\"拍\"现象,满足关系σ=1 046sin(242.83t)sin(5 828t);叶片的振动疲劳极限和疲劳寿命均随残余应力和粗糙度的增大而减小,振动疲劳极限和残余应力之间的关系满足σfat=510.9-0.31-70.93σrest;而疲劳极限和粗糙度之间的关系则满足σfat=9.67R2roughness-70.93Rroughness+713.23.     

残余应力与弹簧的疲劳性能

阐述了残余应力与弹簧高周疲劳极限间的关系,讨论了表层残余应力在改善弹簧疲劳品质的重要性以及制造流程中引入弹簧内残余应力的各种加工工艺,建立了残余压应力场特征参量与弹簧高周疲劳极限间的定量关系。以内部疲劳极限理论为基础,提出了弹簧表层优化喷丸残余应力场的工程计算方法。     

风力发电机塔筒服役过程中应力状态数值模拟

目的 针对风力发电机塔筒在服役期间疲劳受损的问题,结合塔筒的受力特征和结构特点进行应力状态数值模拟,分析塔筒服役应力与疲劳分布。方法 以装机2 MW的风力发电机塔筒为研究对象,采用ANSYS有限元分析软件进行仿真,分析服役条件下风力发电机各段塔筒结构的受力情况,并设置S-N曲线计算疲劳损伤。结果 在服役条件下,各段风机塔筒的应力分布不同,但应力都主要集中在塔筒法兰焊缝附近位置,迎风侧受轴向拉应力,背风侧受轴向压应力,迎风侧法兰位置是发生疲劳断裂的风险点位置,风机塔筒最大变形发生在塔筒顶端连接机舱位置。在交变载荷的作用下,塔筒迎风面和背风面的疲劳分布一致,最低安全系数均在塔筒下法兰焊缝位置。结论 在服役条件下,考虑到塔筒环焊缝本身就存在焊接应力,在各段塔筒迎风侧和背风侧最大拉、压应力影响的叠加下,会造成塔筒局部的应力集中。因此,在塔筒安装使用时,应避免以塔筒迎、背风侧为焊接起点。     

表面冶金复合材料残余应力产生及对热疲劳性能影响

本文通过对基体及焊层材料线膨胀系数及焊层表面应力的测试，阐述了热喷焊层残余应力产生的原因主要与第一、二次冷却过程有关，表面冷却速度是影响焊层表面应力的关键因素，分析了表面应力对焊层热疲劳性能的影响，焊层表面残余压应力对焊层抗热疲劳性有利。     

表面冶金复合材料残余应力产生及对热疲劳性能影响

本文通过对基体及焊层材料线膨胀系数及焊层表面应力的测试，阐述了热喷焊层残余应力产生的原因主要与第一、二次冷却过程有关，表面冷却速度是影响焊层表面应力的关键因素，分析了表面应力对焊层热疲劳性能的影响，焊层表面残余压应力对焊层抗热疲劳性有利。     

残余应力测定的基本知识——第六讲残余应力与环境敏感开裂

残余应力(σR)是环境敏感开裂(ESC)的三个重要因素之一。无论是应力腐蚀开裂(SCC)、氢致开裂(HIC),或者是腐蚀疲劳开裂(CFC),都存在一个开裂的阈值或条件阈值(SσCC,KISCC,σHIC,KIHIC或-σ1CF,ΔKth)。而σR可以单独存在,也可以与工作应力(σW)组合导致ESC。叙述了残余应力与环境敏感断裂的关系和实例,提出了残余应力测定对确保工程部件的安全运行及正确寿命评估的必要性和重要性。     

拉-拉疲劳载荷下钛合金湿喷丸的残余应力松弛及再次喷丸工艺EI北大核心CSCD

采用湿法喷丸强化工艺(wet shot-peening)对TC4钛合金表面进行处理,研究高、低周的拉-拉疲劳过程中合金残余应力松弛规律,探讨再次喷丸工艺(re-shot-peening,RSP)对疲劳寿命的影响。结果表明:在拉应力载荷状态下,残余压应力依然发生松弛现象。疲劳载荷水平对喷丸TC4钛合金残余压应力场(CRSF)的松弛速率、松弛程度和松弛范围具有重要影响。高周疲劳(HCF)过程中残余应力松弛主要发生在近表层0~30μm,松弛速率较慢。低周疲劳(LCF)过程中残余应力松弛发生在0~80μm,范围更大,速率更快。RSP周期对于TC4钛合金的疲劳寿命也具有较大影响。在25%和50%初始喷丸疲劳寿命进行RSP处理会显著提高疲劳寿命,而在75%初始喷丸疲劳寿命处进行RSP处理对于疲劳寿命基本没有影响。此外,RSP的强化效果与疲劳载荷水平相关,对于高周疲劳寿命提高明显。     

Influence of stress state on high temperature fatigue crack growth in Inconel 718

The influence of stress state on fatigue crack growth in nickel-base superalloys at high temperature is considered, based on studies in corner crack specimens of Inconel 718 at 600 °C. At high frequency and low R , cycle-dependent trans-granular crack growth occurs along the whole crack front, and growth rates are similar at the surface and within the interior of specimens, maintaining the original quarter-circular shape. For conditions of low frequency and high R , increased crack growth rate per cycle is observed with the crack tunnelling ahead at the centre. A time-dependent intergranular crack propagation mode occurs in the plane strain interior, attributed to an oxidation mechanism, whereas near the surfaces under plane stress, a trans-granular cyclic plasticity mechanism is observed. It is proposed that in addition to frequency and R , that stress state influences the competition between the mechanisms controlling crack growth and the transition between them: plane strain in the interior favouring an oxidation-controlled intergranular cracking mechanism as compared with the plane stress surfaces where cyclic plasticity dominates. An FEM study suggests that this influence of stress state is not associated with variation of Δ K along the crack front.     

An equivalent uniaxial fatigue stress model for analyzing landing gear fuse pins

We study the biaxial stress state conditions in landing gear fuse pins in the fuse groove. This biaxial state comprises a combination of shear stresses which are usually the largest stresses in the fuse pin by design, and compressive stresses which keep the half-section of the fuse pin in equilibrium. Conventional fatigue analysis techniques use an equivalent uniaxial stress, based on the Mises stress of a pure-shear condition. The respective predicted fatigue damages are much higher than those obtained from fuse pin cyclic tests. A new equivalent uniaxial fatigue stress model is proposed that includes the additional compressive stress as a relief on the fatigue damage in the fuse groove, thereby explaining the observations from fuse pin tests. The model is used in conventional uniaxial strain-life fatigue software (Goodrich Aerospace’s Fatigue Life V2) to predict the fatigue damage on a landing gear fuse pin with a sample load spectrum. The results are then compared to the pure shear model, and to a biaxial finite element fatigue analysis. As compared to the equivalent Mises model, the proposed model provides less conservative estimation of the fuse pin fatigue life, the latter value being higher than that provided by the two-dimensional finite element calculation. __________ Translated from Problemy Prochnosti, No. 3, pp. 85–98, May–June, 2006.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

关于残余应力的静载松弛与最佳喷丸残余应力场的研究

本文研究了喷丸残余应力场在疲劳加载初期的静载松弛现象及机理,并对最佳喷丸残余应力场进行了探讨。试验结界表明,残余应力在疲劳过程中的静载松弛是工件表层材料剧烈塑性变形的结果。残余应力的静载松弛会使晶界、相界等障碍物处形成一定数量的微裂纹,给工作表层材料带来损伤,降低疲劳裂纹的形核寿命。为了避免疲劳初期由于残余应力的静载松弛所造成的损伤,在喷丸后采用应力松弛低温回火工艺,预先降低残余应力场中的最大残余压应力值,建立最佳残余应力场。这种通过热激活的方式使残余应力发生的松弛属子非损伤性松弛,因而能够有效地提高材料在S—N曲线上的较高交变应力区的疲劳寿命。     

Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

A phenomenological model of fatigue macrocrack initiation near stress concentrators

Fatigue macrocrack initiation is considered to be a two-parameter process. It is governed by the local or strain amplitude, and a certain linear parameter of the material. Corresponding parameters have been proposed, i.e. the local stress range Δσ*y and a characteristic distance d *, the prefracture zone size. The formation of this zone is conditioned by a decrease in yield strength within the material’s surface layers, microstructure, loading amplitude, cyclic strain hardening and environment. The value of d * is estimated experimentally by several methods and is assumed to be a certain material constant, independent of both notch and specimen geometry. At the prefracture zone boundary, a major barrier exists that retards the growth of a physically small fatigue crack. The moment when the physically small crack overcomes the prefracture zone boundary is assumed to be a quantitative criterion, a_{i = d *}, for the micro- to macrocrack transition. The proposed relationships, Δσ*y versus N_i , and d * versus N_i , can be used as a basis for the establishment of the materials resistance to macrocrack initiation.     

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

The fatigue threshold and high growth rate region properties were investigated on several kinds of welded joints. These properties became unique in spite of the variation of steels (ferrite-pearite, martensite, austenite), welding method, heat input and stress ratio. It was revealed that the unique properties occurred from the fully opened fatigue crack due to the tensile residual stresses. Based on these results, the equation of the fatigue crack growth curve for the design and inspection of welded structures was proposed. It is also suggested that the inducement of compressive residual stress at the fatigue critical zone is effective in improving the fatigue properties of welded structures.     

Physical meaning of ΔKRP and fatigue crack propagation in the residual stress distribution field

Previous papers have shown ΔK_RP to be a useful parameter describing fatigue crack propagation behavior, where ΔK_RP is an effective stress intensity factor range corresponding to the excess RPG load (re-tensile plastic zone's generated load) in which the retensile plastic zone appears under the loading process. In this paper, the relationship between ΔK_RP and the zone size ( ) (which is smaller between the tensile plastic zone at maximum load and the compressive plastic zone at minimum load) was investigated using a crack opening/closing simulation model so as to consider a physical meaning of ΔK_RP. As a result, it becomes clear that ΔK_RP dominates the zone size where fatigue damage mostly occurs. This result supports the following crack propagation equation

where C and m are material constants.Simulation and fatigue crack propagation tests were then carried out for compact tension (CT), center cracked tension (CCT) and four points bend (4PB) specimens under constant amplitude loading to obtain C and m values for HT-50 steel. Fatigue crack propagation tests were also carried out under constant amplitude loading using CCT specimens with residual stress distribution due to flame gas heating at the center line or edge lines. The T specimen introduced tensile residual stress at the tip of a notch, and the C specimen introduced compressive residual stress. It therefore becomes clear that tensile residual stress leads to a decrease in RPG load, while compressive residual stress leads to increase in RPG load, and that the simulation results are in good agreement with the experimental RPG load. It also becomes clear that simulated crack growth curve using the simulated and the above equation is in good agreement with the experimental curve. It is understood that tensile residual stress creates only a slight increase in crack propagation rate and compressive residual stress create a big decrease a crack propagation rate.     

Calculations of Sobol indices for the Gaussian process metamodel

Global sensitivity analysis of complex numerical models can be performed by calculating variance-based importance measures of the input variables, such as the Sobol indices. However, these techniques, requiring a large number of model evaluations, are often unacceptable for time expensive computer codes. A well-known and widely used decision consists in replacing the computer code by a metamodel, predicting the model responses with a negligible computation time and rending straightforward the estimation of Sobol indices. In this paper, we discuss about the Gaussian process model which gives analytical expressions of Sobol indices. Two approaches are studied to compute the Sobol indices: the first based on the predictor of the Gaussian process model and the second based on the global stochastic process model. Comparisons between the two estimates, made on analytical examples, show the superiority of the second approach in terms of convergence and robustness. Moreover, the second approach allows to integrate the modeling error of the Gaussian process model by directly giving some confidence intervals on the Sobol indices. These techniques are finally applied to a real case of hydrogeological modeling.