Analysis of residual stresses in hot-rolled complex beams

This paper deals with the analysis of residual stresses in hot-rolled complex beams. After rolling, residual stresses appear during the cooling period when the temperature is not uniform in the cross-section. These temperatures are calculated by a transient nonlinear program. The thermal stresses are estimated by a two-dimensional thermoelastoplastic or thermoelasto-viscoplastic finite element idealization. Bending effects are introduced in a generalized plane-strain formulation assuming circular curvature along the beam. The method leads to a plane-strain calculation which is very interesting from the computational point of view; the temperature dependence of the physical properties can be taken into account. The technique has important industrial applications. It permits the optimization of the state of residual stress in hot-rolled complex beams by testing different cooling conditions.     

Finite element analysis of residual stress induced by shot peening process

The aircraft industry has only recently begun to explore possible application of welding as an alternative joining method for the design of future large civil airliner wing. One of the main obstacles, encountered in the past years, to welding application within the aircraft industries were due to failure in the weldments, caused by high tensile residual stresses present in the region of the weld, reducing drastically fatigue strength of welded joints. Improvement in the fatigue life of the welded joint can be obtained if compressive residual stresses are introduced at the weld region.Shot peening is a manufacturing process intended to give aircraft structures the final shape and to introduce a compressive residual state of stress inside the material in order to increase fatigue life. This paper presents the modeling and simulation of the residual stress field resulting from the shot peening process. The results achieved show that a significant decrease of welding induced tensile residual stress magnitude can be obtained. Good agreement between experimental and numerical results was achieved.     

A Review on the Mechanical Modeling of Composite Manufacturing Processes

The increased usage of fiber reinforced polymer composites in load bearing applications requires a detailed understanding of the process induced residual stresses and their effect on the shape distortions. This is utmost necessary in order to have more reliable composite manufacturing since the residual stresses alter the internal stress level of the composite part during the service life and the residual shape distortions may lead to not meeting the desired geometrical tolerances. The occurrence of residual stresses during the manufacturing process inherently contains diverse interactions between the involved physical phenomena mainly related to material flow, heat transfer and polymerization or crystallization. Development of numerical process models is required for virtual design and optimization of the composite manufacturing process which avoids the expensive trial-and-error based approaches. The process models as well as applications focusing on the prediction of residual stresses and shape distortions taking place in composite manufacturing are discussed in this study. The applications on both thermoset and thermoplastic based composites are reviewed in detail.     

FEM analysis of multilayered MEMS device under thermal and residual stress

It is essential to ensure reliability to produce a Micro-Electro Mechanical Systems(MEMS) on commercial scale. Reliability problem in inkjet printhead, one of MEMS, is also very important. To eject an ink drop, temperature of heater must be high so that ink contacting with surface reaches above 280° C on the instant. Its heater is embedded in the thin multi-layer in which several materials are deposited. MEMS processes are the main sources of residual stresses development. Residual stress is one of the factors reducing the reliability of MEMS devices. We measure residual stresses of single layers that consist of multilayer. FE analysis is performed using design of experiment. Transient analysis for heat transfer is performed to get a temperature distribution. And then static analysis is performed with the temperature distribution obtained by heat transfer analysis and the measured residual stresses to get a stress distribution in the structure. Although the residual stress is bigger than thermal stress, thermal stress is more influential on fatigue life.     

Optimization of the buckling load for composite structures taking thermal effects into account

This paper deals with optimization of the buckling load for laminated composite structures. A new methodology has been developed where thermal residual stresses introduced in the manufacturing process are included in the buckling analysis. The thermal effects are also included in the calculation of the buckling load sensitivities, and it is therefore possible to “tailor” the thermal residual stresses in order to increase the buckling load. Rectangular plates and circular cylindrical shells subjected to axial compression are considered. The structures are optimized twice; the first time the thermal residual stresses are ignored in the optimization, and the second time the thermal residual stresses are included in the optimization. These two sets of optimizations give two important results. Firstly, it is possible to increase the buckling load for the structures significantly when the thermal residual stresses are taken into account. Secondly, structures which have been optimized ignoring the effects of thermal residual stresses, may have a buckling load which is much less than expected when the effects of the thermal residual stresses are included. Received November 7, 1999     

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.     

Numerical analysis of the tool wear effect in the machining induced residual stresses

Machining is a dynamic process involving coupled phenomena: high strain and strain rate and high temperature. Prediction of machining induced residual stresses is an interesting objective at the manufacturing processes modelling field. Tool wear results in a change of tool geometry affecting thermo-mechanical phenomena and thus has a significant effect on residual stresses. The experimental study of the tool wear influence in residual stresses is difficult due to the need of controlling wear evolution during cutting. Also the involved phenomena make the analysis extremely difficult. On the other hand, Finite Element Analysis (FEA) is a powerful tool used to simulate cutting processes, allowing the analysis of different parameters influent on machining induced residual stresses.The aim of this work is to develop and to validate a numerical model to analyse the tool wear effect in machining induced residual stresses. Main advantages of the model presented in this work are, reduced mesh distortion, the possibility to simulate long length machined surface and time-efficiency. The model was validated with experimental tests carried out with controlled worn geometry generated by electro-discharge machining (EDM). The model was applied to predict machining induced residual stresses in AISI 316 L and reasonable agreement with experimental results were found.     

Sensitivity analysis of material input data influence on machining induced residual stress prediction in Inconel 718

Inconel 718 is commonly used in structural critical components of aircraft engines due to its properties at high temperatures. In order manufacture the final part, these components have to be machined, so the final surface integrity obtained after machining becomes a key issue. Residual stresses, which are included in surface integrity, are an important issue. Although much of the research carried out on machining induced residual stresses has been empirical, finite element modelling appears to be a complementary solution to gain understanding of it. However, some of the major drawbacks still need to be solved before it can become a reliable tool for industry, such us the identification of input parameters and computational cost. This paper deals with the study of machining induced residual stresses. An orthogonal cutting 2D finite element model was used and a sensitivity analysis was conducted to determine the influence of model input data on the predicted residual stresses. The results obtained from the sensitivity analysis showed that material constitutive law was the most relevant input data when predicting residual stress fields. Importantly the material behaviour at a high heating rate in adition to high strain rate must be considered.     

On the effectiveness of early life cycle defect prediction with Bayesian Nets

Standard practice in building models in software engineering normally involves three steps: collecting domain knowledge (previous results, expert knowledge); building a skeleton of the model based on step 1 including as yet unknown parameters; estimating the model parameters using historical data. Our experience shows that it is extremely difficult to obtain reliable data of the required granularity, or of the required volume with which we could later generalize our conclusions. Therefore, in searching for a method for building a model we cannot consider methods requiring large volumes of data. This paper discusses an experiment to develop a causal model (Bayesian net) for predicting the number of residual defects that are likely to be found during independent testing or operational usage. The approach supports (1) and (2), does not require (3), yet still makes accurate defect predictions (an R ² of 0.93 between predicted and actual defects). Since our method does not require detailed domain knowledge it can be applied very early in the process life cycle. The model incorporates a set of quantitative and qualitative factors describing a project and its development process, which are inputs to the model. The model variables, as well as the relationships between them, were identified as part of a major collaborative project. A dataset, elicited from 31 completed software projects in the consumer electronics industry, was gathered using a questionnaire distributed to managers of recent projects. We used this dataset to validate the model by analyzing several popular evaluation measures (R ², measures based on the relative error and Pred). The validation results also confirm the need for using the qualitative factors in the model. The dataset may be of interest to other researchers evaluating models with similar aims. Based on some typical scenarios we demonstrate how the model can be used for better decision support in operational environments. We also performed sensitivity analysis in which we identified the most influential variables on the number of residual defects. This showed that the project size, scale of distributed communication and the project complexity cause the most of variation in number of defects in our model. We make both the dataset and causal model available for research use.     

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.     

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.     

Choosing the kernel parameters for support vector machines by the inter-cluster distance in the feature space

Determining the kernel and error penalty parameters for support vector machines (SVMs) is very problem-dependent in practice. A popular method to deciding the kernel parameters is the grid search method. In the training process, classifiers are trained with different kernel parameters, and only one of the classifiers is required for the testing process. This makes the training process time-consuming. In this paper we propose using the inter-cluster distances in the feature spaces to choose the kernel parameters. Calculating such distance costs much less computation time than training the corresponding SVM classifiers; thus the proper kernel parameters can be chosen much faster. Experiment results show that the inter-cluster distance can choose proper kernel parameters with which the testing accuracy of trained SVMs is competitive to the standard ones, and the training time can be significantly shortened.     

隐半马尔可夫模型在剩余寿命预测中的应用

剩余寿命预测是作出正确的状态维修决策的基础和前提,是设备退化状态识别的重要内容。隐马尔可夫模型（HMM）是一种具有较强模式分类能力的统计分析算法,但是它不能直接用于剩余寿命的预测,而且考虑到隐马尔可夫模型的局限性和剩余寿命预测模型的可解释性,应用隐半马尔可夫模型（HSMM）进行建模和预测。针对HSMM的训练算法极易陷人局部极值点的问题,提出了基于改进微粒群优化算法（MPSO）进行修正。实验结果证明了该方法在设备剩余寿命预测研究上的有效性和可行性。     

Modelling of the cutting tool stresses in machining of Inconel 718 using artificial neural networks

This study covers two main subjects: (i) The experimental and theoretical analysis: the cutting forces and indirectly cutting tool stresses, affecting the cutting tool life during machining in metal cutting, are one of very important parameters to be necessarily known to select the economical cutting conditions and to mount the workpiece on machine tools securely. In this paper, the cutting tool stresses (normal, shear and von Mises) in machining of nickel-based super alloy Inconel 718 have been investigated in respect of the variations in the cutting parameters (cutting speed, feed rate and depth of cut). The cutting forces were measured by a series of experimental measurements and the stress distributions on the cutting tool were analysed by means of the finite element method (FEM) using ANSYS software. ANSYS stress results showed that in point of the cutting tool wear, especially from von Mises stress distributions, the ceramic cutting insert may be possible worn at the distance equal to the depth of cut on the base cutting edge of the cutting tool. Thence, this wear mode will be almost such as the notch wear, and the flank wear on the base cutting edge and grooves in relief face. In terms of the cost of the process of machining, the cutting speed and the feed rate values must be chosen between 225 and 400 m/min, and 0.1 and 0.125 mm/rev, respectively. (ii) The mathematical modelling analysis: the use of artificial neural network (ANN) has been proposed to determine the cutting tool stresses in machining of Inconel 718 as analytic formulas based on working parameters. The best fitting set was obtained with ten neurons in the hidden-layer using back propagation algorithm. After training, it was found the R² values are closely 1.     

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.     

退化数据驱动的设备剩余寿命在线预测

为在线预测单台服役设备的可用剩余寿命,提出一种融合先验退化数据和设备自身现场退化数据的剩余寿命预测方法。建立符合非线性Wiener过程描述的设备退化模型,利用先验数据采用极大似然法估计模型中的未知参数,使用贝叶斯方法融合新增的现场退化数据实时更新模型参数,进一步实现对设备实时剩余寿命评估。数值仿真和实例计算的结果表明,与固定参数法相比,该方法能够根据现场退化数据不断更新设备剩余寿命分布,进而更好地体现设备的个体差异,显著降低剩余寿命分布的不确定性。     

The effects on residual stress in multilayer NTC thermistors during soldering process and bending test (I. soldering process)

A new three-dimensional finite element model to characterize the residual stress distribution in multilayer NTC thermistor during soldering process has been developed. During the soldering process, the effects of inner silver electrode number and lateral margin length on mechanical residual stress are studied. Throughout the weldbonding and heating process, the maximum and minimum principal stresses in the active region of the thermistor ceramic are not zero, which implies that most of the thermistor ceramic is not stress-free. Numerical results show that the increasing of the lateral margin length could effectively decrease the maximum tensile stress.

     

Membrane statics of parachute-like shells

The parachute-like shells studied in this paper may be referred to as scalloped paraboloids. The middle surface of the shell, which lacks rotational symmetry, is defined in cylindrical coordinates by the equation Z=Ar²(1 +βθ²). Of interest to the designer are the membrane forces N_r, N_θ, N_rθ in the shell or their dimensionless plan projections n_r, n_θ, n_rθ. The latter may be found from the consideration of equilibrium, using the stress function approach. The stress function has to satisfy a second order partial differential equation subject to proper boundary conditions. The solution is found by separation of variables and the method of infinite series. The generation of practical results relies heavily on the digital computer. An approximate technique, convenient for practical purposes, is also discussed. For the purpose of illustration, a scalloped paraboloid loaded and supported like a parachute is considered. Adequate numerical and graphical results are given. The necessary mathematical expressions are included in an Appendix. It is shown graphically how the stresses within the shell are influenced by the parameter β. By a proper choice of this parameter the stresses within the shell can be restricted to be either entirely tensile or compressive. The value of the optimum β depends on the height to radius ratio μ. A practical curve that describes this optimum μ, β relationship is included.     

On the machining induced residual stresses in IN718 nickel-based alloy: Experiments and predictions with finite element simulation

Residual stresses after machining processes on nickel-based super alloys is of great interest to industry in controlling surface integrity of the manufactured critical structural components. Therefore, this work is concerned with machining induced residual stresses and predictions with 3-D Finite Element (FE) based simulations for nickel-based alloy IN718. The main methods of measuring residual stresses including diffraction techniques have been reviewed. The prediction of machining induced stresses using 3-D FE simulations and comparison of experimentally measured residual stresses for machining of IN718 have been investigated. The influence of material flow stress and friction parameters employed in FE simulations on the machining induced stress predictions have been also explored. The results indicate that the stress predictions have significant variations with respect to the FE simulation model and these variations can be captured and the resultant surface integrity can be better represented in an interval. Therefore, predicted residual stresses at each depth location are given in an interval with an average and standard deviation.