Effect of welding sequences on residual stresses

Accurately predicting welding residual stresses and developing an available welding sequence for a weld system are pertinent tasks since welding residual stress is inevitably produced in a welded structure. This study analyzes the thermomechanical behavior and evaluates the residual stresses with various types of welding sequence in single-pass, multi-pass butt-welded plates and circular patch welds. This is achieved by performing thermal elasto-plastic analysis using finite element techniques. Furthermore, this investigation provides an available welding sequence to enhance the fabrication process of welded structures.     

基于主S-N曲线的带止裂槽焊接接头疲劳寿命预测及改进

以带止裂槽焊接接头为研究对象,提取可能发生疲劳失效的焊趾截面、焊喉截面和止裂槽截面的节点力,基于主S N曲线法求解截面的等效结构应力,评估焊缝的疲劳寿命,并提出改进接头的建议。分析发现：止裂槽的存在会显著增加局部位置的应力,导致疲劳寿命明显降低。填充止裂槽并适当增大水平方向焊脚尺寸,可以提高该位置疲劳寿命10倍左右。     

Evolutionary fuzzy SVR modeling of weld residual stress

Residual stresses are an integral part of the total stress acting on any component in service. It is important to determine and/or predict the magnitude, nature and direction of the residual stress to estimate the life of important engineering parts, particularly welded components. Researchers have developed many direct measuring techniques for welding residual stress. Intelligent techniques have been developed to predict residual stresses to meet the demands of advanced manufacturing planning. This research paper explores the development of Finite Element model and evolutionary fuzzy support vector regression model for the prediction of residual stress in welding. Residual stress model is developed using Finite Element Simulation. Results from Finite Element Method (FEM) model are used to train and test the developed Fuzzy Support Vector Regression model tuned with Genetic Algorithm (FSVRGA) using K-fold cross validation method. The performance of the developed model is compared with Support Vector Regression model and Fuzzy Support Vector Regression model. The proposed and developed model is superior in terms of computational speed and accuracy. Developed models are validated and reported. The developed model finds scope in setting the initial weld process parameters.     

基于主S-N曲线法的T形接头疲劳评估和试验验证

以T形焊接接头为研究对象,将可能发生疲劳破坏的焊趾截面和焊喉截面的节点力转换为相对于中面焊线的等效节点力和弯矩,并利用平衡等效原理转化为线力和线弯矩,基于材料力学公式求出截面的结构应力,解决应力对网格尺寸敏感的难题。从焊缝疲劳破坏的机理出发,基于Paris断裂力学公式推导以等效结构应力为变量的一条主S-N曲线,评估焊缝的疲劳寿命。分析结果发现：采用主S-N曲线法评估的寿命与试验值最接近且破坏位置准确,该主S N曲线法准确性较高。     

Numerical simulation of the electron beam welding process

Electron beam welding is a highly efficient and precise welding method that is being increasingly used in industrial manufacturing and is of growing importance in industry. Compared to other welding processes it offers the advantage of very low heat input to the weld, resulting in low distortion in components. Modeling and simulation of the laser beam welding process has proven to be highly efficient for research, design development and production engineering. In comparison with experimental studies, a modeling study can give detailed information concerning the characteristics of weld pool and their relationship with the welding process parameters (welding speed, electron beam power, workpiece thickness, etc.) and can be used to reduce the costs of experiments. A simulation of the electron beam welding process enables estimation of weld pool geometry, transient temperature, stresses, residual stresses and distortion. However this simulation is not an easy task since it involves the interaction of thermal, mechanical and metallurgical phenomena. Understanding the heat process of welding is important for the analysis of welding structure, mechanics, microstructure and controlling weld quality.In this paper the results of numerical simulation of electron beam welding of tubes were presented. The tubes were made of 30HGSA steel. The numerical calculation takes into consideration thermomechanical coupling (TMC). The simulation aims at: analysis of the thermal field, which is generated in welding process, determination of the heat-affected zone and residual stresses in the joint. The obtained results allow for determination both the material properties, and stress and strain state in the joint. Furthermore, numerical simulation allows for optimization of the process parameters (welding speed, power of the heat source) and shape of the joint before welding. The numerical simulation of electron beam welding process was carried out with the ADINA System v. 8.6. using finite element method.     

A clustering approach for determining the optimal process parameters in cutting

Residual stresses are normally generated in any cutting operation. However, while compressive residual stresses are sometimes suitable as they may even enhance and improve the piece life, tensile residual stresses can be very detrimental representing a negative feature for fatigue life, corrosion, strength and other functional aspects. Thus, a proper set-up of the process parameters in machining operations has a dramatic importance. Currently, optimization methodologies do not seem to be very effective for industrial needs, for which the optimal setup being a desired residual stresses profile becomes a relevant issue. On the basis of the previous considerations, this work proposes a data mining technique, which has proven to be reliable to identify the analytical relationship between residual stresses and the proper process parameters. In order to ensure the use of the developed technique in practice, it has been integrated in the user-friendly software, Predators^®.     

Numerical–experimental crack growth analysis in AA2024-T3 FSWed butt joints

This paper deals with a numerical and experimental investigation on the influence of residual stresses on fatigue crack growth in AA2024-T3 friction stir welded butt joints. The computational approach is based on the sequential usage of the Finite Element Method (FEM) and the Dual Boundary Element Method (DBEM). Linear elastic FE simulations are performed to evaluate the process induced residual stresses, by means of the contour method. The computed stress field is transferred to a DBEM environment and superimposed to the stress field produced by a remote fatigue traction load applied on a friction stir welded cracked specimen; the crack propagation is then simulated according to a two-parameter growth model. Numerical results have been compared with experimental data showing good agreement and evidencing the predictive capability of the proposed method. The obtained results highlight the influence of the residual stress distribution on crack growth.     

A new technique for producing large-area as-deposited zero-stressLPCVD polysilicon films: the MultiPoly process

Polysilicon films deposited by low-pressure chemical vapor deposition (LPCVD) exhibit tensile or compressive residual stresses, depending on the deposition temperature. Polysilicon films composed of alternating tensile and compressive layers can display any overall stress value between those of the individual layers, including a state of zero overall residual stress, depending on the relative thickness of each layer. The residual stress gradient can be similarly controlled by the layer thicknesses and distribution. This has been demonstrated with a ten-layer near-zero stress (<10 MPa), near-zero stress gradient (⩽0.2 MPa/μm) polysilicon film, containing flat cantilever beams whose length-thickness ratios exceed 150. Using multilayer deposition to control the stresses and stress gradients of polysilicon films is termed the MultiPoly process     

航空发动机叶片激光冲击强化技术的研究

介绍了激光冲击强化航空发动机风扇叶片的国内外研究进展情况。激光冲击强化处理是近年出现的表面改性新工艺,其效果优于传统表面改性技术(如喷丸)。通过在航空发动机叶片表面引入残余压应力,激光冲击强化可以大幅度提高叶片的疲劳寿命。激光冲击强化近年的研究,形成了多学科并存的制备技术体系,开发了一系列新的设备和技术,并很快进入了实际应用。制定正确的工艺规程、探索精确控制激光冲击强化的方法,以及结合现代高新技术,引进交叉学科,是今后的发展方向。     

基于主S-N曲线法的焊接结构疲劳寿命预测系统开发和关键技术

根据ASME标准的计算公式,开发基于主S-N曲线法的焊接结构疲劳寿命预测软件,其核心是采用网格不敏感结构应力及一条主S-N曲线的方法计算焊接结构的疲劳寿命.系统开发选用面向对象的VC++语言和OpenGL开发库;采用模块化程序设计的思想将系统划分为有限元定义模块、焊缝定义及结构应力计算模块、载荷谱管理及雨流计数模块、主S-N数据管理及疲劳计算模块等4个核心模块;开发专用的接口程序和三维交互界面.该软件系统已在我国轨道交通制造行业中得到良好应用.以某货车车体关键焊缝疲劳寿命计算为典型实例,用软件系统进行车体疲劳寿命评估,计算结果表明该货车焊缝满足设计要求,同时也证明该软件系统的实用性和高效性.     

Numerical simulation on the effect of welding parameters on welding residual stresses in T92/S30432 dissimilar welded pipe

Dissimilar welded joints are commonly used in fossil power plants to connect martensitic steel components and austenitic stainless steel piping systems. The integrity for such welded structures is depended on residual stresses induced by manufacturing process. In this paper, the characteristics of residual stresses on the dissimilar welded pipe between T92 steel and S30432 steel were investigated using finite element method. Moreover, the effects of heat input, groove shape and layer number on the residual stress distribution were studied to find the approach to reduce the residual stress. The numerical results revealed that the hoop and axial stress in heat affected zone (HAZ) of T92 steel side of the dissimilar welded joint had sharp gradients. By decreasing the groove angle, the peak values of the hoop and axial stress on T92 steel side were reduced greatly while the peak values in welded metal and HAZ of S30432 steel side differed little. Furthermore, more layer number and less heat input decreased the peak value of the tensile residual stress on welded metal and S30432 steel side, but had little effect on the residual stress in T92 steel side.     

Modelling of residual stresses in the shot peened material C-1020 by artificial neural network

This study consists of two cases: (i) The experimental analysis: Shot peening is a method to improve the resistance of metal pieces to fatigue by creating regions of residual stress. In this study, the residual stresses induced in steel specimen type C-1020 by applying various strengths of shot peening, are investigated using the electrochemical layer removal method. The best result is obtained using 0.26 mm A peening strength and the stress encountered in the shot peened material is ?276 MPa, while the maximum residual stress obtained is ?363 MPa at a peening strength of 0.43 mm A. (ii) The mathematical modelling analysis: The use of ANN has been proposed to determine the residual stresses based on various strengths of shot peening using results of experimental analysis. The back-propagation learning algorithm with two different variants and logistic sigmoid transfer function were used in the network. In order to train the neural network, limited experimental measurements were used as training and test data. The best fitting training data set was obtained with four neurons in the hidden layer, which made it possible to predict residual stress with accuracy at least as good as that of the experimental error, over the whole experimental range. After training, it was found the R² values are 0.996112 and 0.99896 for annealed before peening and shot peened only, respectively. Similarly, these values for testing data are 0.995858 and 0.999143, respectively. As seen from the results of mathematical modelling, the calculated residual stresses are obviously within acceptable uncertainties.     

Neuro evolutionary model for weld residual stress prediction

Welding is an efficient reliable metal joining process in which the coalescence of metals is achieved by fusion. Localized heating during welding, followed by rapid cooling, induce residual stresses in the weld and in the base metal. Determination of magnitude and distribution of welding residual stresses is essential and important. Data sets from finite element method (FEM) model are used to train the developed neural network model trained with genetic algorithm and particle swarm optimization (NN–GA–PSO model). The performance of the developed NN–GA–PSO model is compared neural network model trained with genetic algorithm (NN–GA) and neural network model trained with particle swarm optimization (NN–PSO) model. Among the developed models, performance of NN–GA–PSO model is superior in terms of computational speed and accuracy. Confirmatory experiments are performed using X-ray diffraction method to confirm the accuracy of the developed models. These developed models can be used to set the initial weld process parameters in shop floor welding environment.     

轮辋与轮辐间焊缝参数对钢制车轮弯曲应力影响的仿真分析

为考察轮辋与轮辐间焊缝对钢制车轮强度的影响,建立包括该焊缝在内的钢制车轮的有限元模型,分析在弯曲载荷作用下,钢制车轮轮辋与轮辐间的焊脚高度、焊缝截面形状以及焊缝布置位置对车轮应力的影响．仿真结果表明：对于该钢制车轮,轮辋与轮辐间焊脚高度可以由目前的3.5mm减至2.5mm,这为焊接工艺参数的调整提供一定参考;焊缝截面边界为直边时焊缝处应力较小;当每段焊缝的一半对着通风孔时焊接区域应力较小．该仿真分析可用于钢制车轮轮辋与轮辐间焊缝结构的优化,并为钢制车轮在弯曲载荷作用下的疲劳寿命估算提供更准确的应力计算结果．     

Residual stresses evaluation in precision milling of hardened steel based on the deflection-electrochemical etching technique

Machining operations generate residual stresses in both the surface and subsurface of the workpieces. These residual stresses have a big influence on the functionality of the machined parts, thus their evaluation is of great importance. In terms of the fatigue strength and stress-corrosion cracking resistance, the compressive residual stresses are preferred to the tensile ones. In the present study, the generation of residual stresses in the precision milling of steels is evaluated by using the deflection-electrochemical etching technique. Two different materials were used to compare the obtained results: DIN X210Cr12 and DIN 17210-86. For the first one, two different states were tested: hardened and unhardened. The results showed different trends for the materials tested. Thus, the higher residual stresses were found at the surface for the DIN X210Cr12 while the maximum values were obtained in the subsurface for the DIN 17210-86. Finally, when comparing to the X-ray diffraction method, it is stated that both the deflection-electrochemical etching and X-Ray diffraction methods can be used to evaluate the state of the surfaces, though the destructive one can give more detailed information about the residual stresses distribution.     

燃油箱吊座焊缝的强度和疲劳有限元分析

对某内燃机车燃油箱吊座中T型焊接接头部位焊缝强度和疲劳进行有限元分析.在焊缝熔深为2,3和5 mm(全熔透)时,将吊座有焊缝和无焊缝时应力有限元计算结果与传统方法计算结果进行比较,结果表明:在焊缝全熔透时吊座整体最大应力小于传统方法计算的无焊缝时的最大应力,说明全熔透结构具有更好的连接性能.焊缝全熔透时焊缝处节点应力分布表明几何截面突变明显处的节点应力较大.疲劳计算结果表明该燃油箱吊座满足疲劳强度要求.     

Fatigue constrained topology optimization

We present a contribution to a relatively unexplored application of topology optimization: structural topology optimization with fatigue constraints. A probability based high-cycle fatigue analysis is combined with principal stress calculations in order to find the topology with minimum mass that can withstand prescribed variable-amplitude loading conditions for a specific life time. This allows us to generate optimal conceptual designs of structural components where fatigue life is the dimensioning factor. We describe the fatigue analysis and present ideas that make it possible to separate the fatigue analysis from the topology optimization. The number of constraints is kept low as they are applied to stress clusters, which are created such that they give adequate representations of the local stresses. Optimized designs constrained by fatigue and static stresses are shown and a comparison is also made between stress constraints based on the von Mises criterion and the highest tensile principal stresses. The paper is written with focus on structural parts in the avionic industry, but the method applies to any load carrying structure, made of linear elastic isotropic material, subjected to repeated loading conditions.     

Mechanical properties of thin films from the load deflection oflong clamped plates

A plane-strain load-deflection model for long plates clamped to a rigid support is developed. The analytical model describes the nonlinear deflection of plates with compressive or tensile residual stress and finite flexural rigidity under uniform load. It allows for the extraction of the residual stress and plane-strain modulus of single-layered thin films. Properties of compressively and weakly prestressed materials are extracted with an accuracy achieved previously only with tensile samples. Two approximations of the exact model are derived. The first reduces the plates to membranes by neglecting their flexural rigidity. Considerable errors result from this simplification. The second approximation provides an exact expression for the linear plate response. Using the model, mechanical properties were extracted from two plasma-enhanced chemical-vapor deposition (PECVD) silicon nitride films with weakly tensile and compressive prestress, respectively. Measured residual stresses are 1.3±3.8 and -63±12.4 MPa, respectively. Corresponding plane-strain moduli are 134.4±3.9 and 142±2.6 GPa, respectively     

Shot peening simulation using discrete and finite element methods

Modeling shot peening process is very complex as it involves the interaction of metallic surfaces with a large number of shots of very small diameter. Conventionally such problems are solved using the finite element software (such as ABAQUS) to predict the stresses and strains. However, the number of shots involved and the number of elements required in a real-life components for a 100% coverage that lasts a considerable duration of peening make such an approach impracticable. Ideally, a method that is suitable for obtaining residual compressive stresses (RCS) and the amount of plastic deformations with the least computational effort seems a dire need.In this paper, an attempt has been made to address this issue by using the discrete element method (DEM) in combination with the finite element method (FEM) to obtain reasonably accurate predictions of the residual stresses and plastic strains. In the proposed approach, the spatial information of force versus time from the DEM simulation is utilized in the FE Model to solve the shot peening problem as a transient problem. The results show that the RCS distribution obtained closely matches with that of the computationally intensive direct FEM simulation. It has also been established, in this paper, that this method works well even in the situations where the robust unit cell approaches are found to be difficult to handle.