Theoretical prediction of welding distortion in large and complex structures

Welding technology is widely used to assemble large thin plate structures such as ships, automobiles, and passenger trains because of its high productivity. However, it is impossible to avoid welding-induced distortion during the assembly process. Welding distortion not only reduces the fabrication accuracy of a weldment, but also decreases the productivity due to correction work. If welding distortion can be predicted using a practical method beforehand, the prediction will be useful for taking appropriate measures to control the dimensional accuracy to an acceptable limit. In this study, a two-step computational approach, which is a combination of a thermo-elastic-plastic finite element method (FEM) and an elastic finite element with consideration for large deformation, is developed to estimate welding distortion for large and complex welded structures. Welding distortions in several representative large complex structures, which are often used in shipbuilding, are simulated using the proposed method. By comparing the predictions and the measurements, the effectiveness of the two-step computational approach is verified.     

Hybrid model for prediction of welding distortions in large structures

The paper presents a short overview of the contemporary approaches for calculating welding distortions. In order to meet the existing challenges, an advanced hybrid model for prediction of welding distortions in large structures is described. For the purpose of illustrating the capability of this model, a simulation case is put into discussion. The results are validated by comparison with experimental data, as well as with common simulation technique. Analysis of the calculation costs is also presented. The directions for development of calculation technique, based on the presented model, are also suggested.     

FEM prediction of buckling distortion induced by welding in thin plate panel structures

Welding distortion generated during assembly process has a strongly nonlinear feature, which includes material nonlinearity, geometric nonlinearity, and contact nonlinearity. In order to obtain a precise prediction of welding distortion, these nonlinear phenomena should be carefully considered. In this study, firstly, a prediction method of welding distortion, which combines thermo-elastic-plastic finite element method (FEM) and large deformation elastic FEM based on inherent strain theory and interface element method, was developed. Secondly, the inherent deformations of two typical weld joints involved in a large thin plate panel structure were calculated using the thermo-elastic-plastic FEM and their characteristics were also examined. Thirdly, using the developed elastic FEM and the inherent deformations, the usefulness of the proposed elastic FEM was demonstrated through the prediction of welding distortion in the large thin plate panel structures. Finally, the influences of heat input, welding procedure, welding sequence, thickness of plate, and spacing between the stiffeners on buckling propensity were investigated. The numerical simulation method developed in this study not only can be used to predict welding distortion in manufacturing stage but also can be employed in design or planning stage.     

Numerical simulation of welding distortion in thin plates

A three-dimensional model based on the finite-element method is developed to simulate the temperature field and stress distribution in the welding and heat-affected zones during fusion welding of thin plates. The governing equations are solved using the SYSWELD program commercial code. The model’s predictions are tested and verified against the experiments. Angular distortion and longitudinal bending are measured, the results are compared with those obtained from the mathematical model, and a relatively good agreement between them is found. The verified model is used to evaluate the effects of various parameters on the temperature and stress distributions in the welding and heat-affected zones of a thin austenitic stainless steel plate.     

Burnthrough prediction in pipeline welding

Welding of branch connections on gas pipelines at full line pressure is frequently an operational necessity. Weld parameters must be selected so that heat inputs are low enough to avoid burnthrough yet not so low that hydrogen-assisted cold cracking occurs. Current techniques rely on the observation that burnthrough does not occur if peak temperatures on the inner surface are kept below 980^°C. At these temperatures, rate-dependent flow is the dominant mechanism. The problem is one of creep rupture occurring at temperatures in excess of 980^°C with times of the order of seconds. Material constitutive models for the analysis of welding must include both rate-dependent and rate-independent plastic flow as well as the effects of phase transformations. Material properties at elevated temperatures are usually not available for pipeline steels and must be extrapolated from values at lower temperatures. An exploratory study using 3D thermal-mechanical finite element analysis of welding on pressurized vessels is presented and includes comparisons with experiment. The agreement is encouraging. The material failure does occur in appropriate locations but the predictions are generally overconservative. Estimated material properties, especially damage and rupture properties at high temperatures could be improved. This revised version was published online in July 2006 with corrections to the Cover Date.     

Finite element modelling of wooden structures at large deformations and brittle failure prediction

This paper presents a constitutive wood model that accounts for both hardening associated with material densification at large compressive deformations and brittle failure modes. The model is adapted from previous work by the authors and has been modified to deal with wood behaviours. The main novelty of the model is the coupling between the anisotropic plasticity and the ductile densification. The model developed is successfully implemented in the commercial ABAQUS software. Validation was made for uniaxial compressive loadings and an application on a three-points bending test. The results obtained, for the uniaxial compressive loadings, demonstrate the capability of the model to simulate the wood behaviour at large compressive deformations and show clearly the effect of the densification on the plastic behaviour. The result obtained for the three-points bending test shows a good implementation of the brittle failure criterion and demonstrates the suitability of the developed model to analyse and design wooden structures.     

Application of probabilistic thermomechanical models to large and/or complex structures

An affine probabilistic model is used to show how the cumulative and local probabilities of fracture of yielding can be determined in complex and/or large structures. The theory of probabilistic strength of materials supplies an expression allowing the cumulative probability of fracture, or yielding of a large and/or complex structure to be computed, taking into account the knowledge of the said probabilities in a material similarly manufactured but smaller, using an affine transformation. The result is independent of the stress field and of the analytical form of Weibull's specific risk function. The system must be loaded gradually, and if there is some thermal shock then the time in which the thermal perturbation is dissipated into the structure must be greater than the time occupied by the elastic wave in going through the structure, such that the assumption of gradual loading still holds. Costs in terms of time and money are reduced by using smaller models and by considering a time-transformation that allows the testing of structures subjected to thermal stresses to be speeded up.     

Prediction of welding distortion in butt joint of thin plates

This paper investigates distortions and residual stresses induced in butt joint of thin plates using Metal Inert Gas welding. A moving distributed heat source model based on Goldak’s double-ellipsoid heat flux distribution is implemented in Finite Element (FE) simulation of the welding process. Thermo-elastic–plastic FE methods are applied to modelling thermal and mechanical behaviour of the welded plate during the welding process. Prediction of temperature variations, fusion zone and heat affected zone as well as longitudinal and transverse shrinkage, angular distortion, and residual stress is obtained. FE analysis results of welding distortions are compared with existing experimental and empirical predictions. The welding speed and plate thickness are shown to have considerable effects on welding distortions and residual stresses.     

Theoretical prediction and numerical simulation of honeycomb structures with various cell specifications under axial loading

The axial crushing of honeycomb structures with various cell specifications is studied analytically and numerically. Based on the Super Folding Element theory, a new method for predicting the mean crushing stress of honeycomb structures with various cell specifications under axial loading is developed. In this new theoretical method, two types of simplified folding modes named SFM1 and SFM2 are proposed. The mean crushing stress and the folding wavelength for honeycomb structures with various cell specifications are then determined by a minimum principle. The effective crushing distance and the loading rate effect are both considered. In order to illustrate the effectiveness of the proposed approach, numerical simulations are carried out by employing the explicit finite element code LS-DYNA. The bond of the honeycomb panels is simulated by using a tie-break contact. It can be seen that the analytical solutions are in agreement with the numerical results as well as the Wizerbicki’s solutions.     

复合材料三维损伤模型在大开口结构强度预测中的应用

为了预测复合材料大开口结构的强度及损伤扩展情况,开展了3种不同复合材料三维损伤退化模型(瞬间退化模型、渐进损伤退化模型和连续损伤退化模型)在复合材料大开口结构强度预测中的应用研究,完成了含口框加强件的复合材料大开口结构面内纯剪切试验,并采用3种损伤退化模型对试验件及相关文献算例进行了数值模拟。数值模拟结果与试验结果的对比表明:3种损伤退化模型均可以较为精确地预测复合材料大开口结构的极限强度;与其他模型相比,连续损伤退化模型的预测精度最高、通用性最好且网格依赖度最低。研究结果为复合材料大开口结构力学性能的研究提供了理论指导。     

A study on the influence of clamping on welding distortion

This paper presents numerical case studies on the influence of clamping on welding distortion. For the case studies a 1 mm DP600 overlap joint and a 6 mm S355 T-joint have been selected, and reference simulations have been performed. The results have been compared to experiments in a previous paper and show a good agreement. The influence of the clamping time, the release time and the influence of clamp preheating has been investigated for both geometries. Here, the effect of the clamping on buckling distortion as well as bending and angular distortion has been studied. It is shown that the final residual stresses and distortions depend strongly on the clamping conditions. Adaptive clamping may be helpful as an in-line technique for welding distortion mitigation by forcing the material to deform plastically.     

Prediction of welding buckling distortion in a thin wall aluminum T joint

In this paper, local and global welding buckling distortion of a thin wall aluminum T joint is investigated. A thermo-elastic–viscoplastic model is employed to determine longitudinal residual stresses; analysis of thermal model and elastic–viscoplastic (Anand) model are uncoupled. Molten puddle motion (speed of welding) is modeled by using time dependent birth and death element method. Three dimensional nonlinear-transient heat flow analysis has been used to obtain the temperature distribution, and then by applying thermal results and using three dimensional Anand elastic–viscoplastic model, stress and deformation distributions are obtained during welding and after cooling. Local buckling is investigated by analyzing the history of stress and strain relations. Local buckling is assumed to occur at a point if a small change in the magnitude of stress causes large deformation during of the welding process. By applying residual stresses on a structural model and using eigenvalue methods, global buckling instability of the welded structure is determined.     

铝合金薄板焊件旋转挤压矫形研究

从焊接残余变形的产生机理出发,提出了能够矫正薄板焊件残余变形的旋转挤压法,并阐述了该法的工作原理及其控制焊接变形的机理.利用由铣床改造的试验装置对旋转挤压工艺进行了研究.试验结果表明,旋转挤压法能够显著降低2A12T4铝合金薄板的焊接残余变形,选择适当的工艺参数,该方法能将焊接残余变形控制在常规焊接状态的3%以下.旋转挤压控制焊接变形的效果与一些工艺参数有关,保持其它参数不变,工件的行走速度越慢,焊件的残余变形越小.另外,只有当焊缝和近缝区金属均能被挤压头充分延展时,才能取得良好的焊接变形控制效果.     

基于数字图像相关法的薄板焊接变形研究

针对薄板焊接结构的大变形研究,目前采用数值模拟方法计算量大且误差较大,传统的应变片等测量手段无法实现焊缝处的在线密集测量。该文提出基于数字图像相关法的非接触检测方法对大型Q235薄板进行平板堆焊变形测量,重点研究热输入对薄板面内面外变形规律的影响。实验结果表明:基于数字图像相关技术的焊接变形测量方法能够在线、动态、全场获取薄板变形数据;随着热输入的增大,薄板的Z向面外变形及横向/纵向的面内变形也越大;冷却结束后,在焊缝处及热影响区内,薄板的纵向应变较小,而横向应变较大。热输入影响下的薄板焊接变形规律为揭示薄板焊接变形机理提供可靠的依据,为焊接结构件提供准确的制造数据。     

Laser beam welding of low weight materials and structures

In this presentation an overview will be given about laser beam welding of aluminium. Different aspects regarding process parameters, metallurgical aspects, weld seam properties and possible applications will be discussed.     

Life prediction of butt welds containing welding defect

In the present study, the welding defects are divided into two types, i.e. the crack-like defect and the uncrack-like defect, based on the experimental and analytical results, and the criterion for distinguishing the defect type is tentatively proposed. The life predicting methods are given to accurately compute the fatigue life of butt welds containing different types of defect. The computation and the experiments show that the fatigue crack propagation life of the butt welds containing the crack-like defect is approximately equal to the total fatigue life and the fatigue crack initiation life can be neglected. Therefore, the fatigue life of this kind of welds is greatly decreased. On the other hand, the fatigue life of the butt welds containing uncrack-like defect consists of two phases, i.e. the fatigue crack initiation life and the fatigue crack propagation life and the fatigue crack initiation life occupies a greater portion, which cannot be neglected. In order to accurately predict the fatigue life of welded elements, not only the defect size but also the defect type should be determined. Consequently, the technology and the equipment for the nondestructive detection need to be highly developed.     

Theoretical and practical aspects of seeding technology for fusion welding

In numerous applications, contradictory properties are demanded from welded joints; for example, high seam strength and good formability or high hardness value and wear resistance with the highest possible toughness. Controlling the solidification behaviour by means of seeding the molten weld metal represents a very effective method of fulfilling such contrary demands. For this purpose, seeding methods were modified for welding technology in such a way that microstructures can be specified. The choice of seeding materials is based on a theoretical model which describes the functioning mode of complex inoculants in a melt. The effectiveness of the procedure was investigated on ferritic chromium‐steels by using various seeding methods and complex inoculants. Moreover, pronounced seeding effects were verified on nickel based superalloys and titanium.     

Influences of heat source model on welding residual stress and distortion in a multi-pass J-groove joint

Welding mechanical behaviors including residual stress and distortion are highly non-linear phenomena in nature. When numerical simulation methods such as thermal elastic plastic finite element method (FEM) are used to quantitatively predict welding residual stress and distortion, a long computational time is required especially for multi-pass joints. In real engineering structures, many weldments have large dimensions and complex shapes, and they are usually assembled by a multi-pass welding process. Therefore, it is necessary to develop time-effective computational approaches for practice engineering analysis. In this study, a method based on variable length heat sources was proposed for the analysis of thermo-mechanical behaviors for multi-pass joints. The welding residual stress field in a dissimilar metal J-groove joint with axis-symmetric geometrical shape, which was performed by a semi-circle balanced welding process, was investigated using the proposed method. The simulation results were compared with the measured data as well as the simulation results computed by a moving heat source. Meanwhile, the instantaneous line heat source was also employed to estimate the welding residual stresses in the same joint in an extreme case. The influences of heat source model (type) on welding residual stress and distortion were discussed.     

Impact damage prediction in carbon composite structures

This paper describes a strategy for predicting the extent of internal damage in a brittle carbon fibre laminated composite stucture, when subjected to low velocity impact by a single mass. The success of the predictions, which avoid expensive three-dimensional analysis, is validated by test for a wide range of structures from small stiff plates through to large flexible stiffened compression panels whose residual strength is affected much more by internal delamination than tension structures. It is shown that a numerical model needs to incorporate nonlinear behaviour due to both gross deformations and to in-plane material degradation.     

Prediction and measurement of welding distortion of a spherical structure assembled from multi thin plates

A spherical structure assembled from 14 thin bent plates and two circular polar plates was taken as the research object and its welding distortion was investigated in this study. Firstly, the welding distortion due to each welding line is measured by experiment. Then, the spherical structure was modeled by shell element and its welding distortion produced by each welding line was evaluated using the inherent deformation method proposed by authors. The computational process requires a short time when the inherent deformation method is employed. To determine the inherent deformation existing in the welding lines, a computed model with 3D solid element for a typical butt welded joint was applied and one case of the detail transient analysis by Thermal Elastic Plastic Finite Element Method was performed.