Welded Elements Research Articles

Ultrasonic Measurement of Residual Stresses in Welded Elements of Ship Structure

The objective of the study described in this paper is to identify the residual stress distribution and relaxation in standard welded specimens as well as in a large-scale welded panel imitating the critical, from the fatigue point of view, zones of ship structure. The residual stresses were measured after welding and in the process of fatigue loading of welded elements by the UltraMARS system that is based on using ultrasound. The measurements had shown that the maximum residual stresses near the welds (4-5 mm away from the weld) reach levels 290-320 MPa that are close to the yield strength of considered material both in welded specimens and in the large scale panel. Analysis of residual stress relaxation in the welded panel under the action of cyclic loading confirmed the fact that within the interval of applied stress ranges corresponding to the multi-cycle region of loading of the welded joints, the relaxation of residual stresses occurs mainly during the first cycle.

Study on Solidification Crack Criterion during Laser Welding Pure Aluminum and ZL114A Aluminum Alloy

Laser welding of 1A90 pure aluminum and ZL114A aluminum alloy with powder under the same laser welding parameters were carried out in this work. The relation between weld elements and solidification cracking formation of these two materials was investigated in the perspective of metallurgical factor. And the alloy element contents and distribution near the cracks were determined by scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). Then the concept of weld element equivalent was introduced comparing with the carbon equivalent expression. The weld element equivalent expressions were established by using mathematics regress. And the verified results indicate that the weld element equivalent expressions can properly forecast the formation of solidification cracking.

Full Depth-Resolved Strain Mapping of a Three-Pass Slot Weld Specimen in Austenitic Stainless Steel Using the Spiral Slit Technique

An 18 mm thick three-pass slot weld specimen in austenitic stainless steel, manufactured for the purpose of benchmarking Finite Element weld residual stress simulation codes, is currently undergoing extensive non-destructive characterization within a research network. First results from the non-destructive full three dimensional spatially resolved macro-strain mapping in this specimen are presented here. Focussed high-energy synchrotron radiation together with the spiral slit technique was used to obtain depth-resolved information about the variation of lattice parameters. The results show a strong concentration of tensile strain, transverse to the weld at mid-depth of the weld. The maximum longitudinal strains were observed beneath the weld. Furthermore significant weld start- and stop-effects were observed.

Full 3D spatially resolved mapping of residual strain in a 316L austenitic stainless steel weld specimen

A three-pass slot weld specimen in austenitic stainless steel 316L, manufactured for the purpose of benchmarking Finite Element weld residual stress simulation codes, is currently undergoing extensive characterization within a research network. A comprehensive data set from non-destructive full three-dimensional spatially resolved macro-strain mapping in this specimen is presented here. Focussed high-energy synchrotron radiation together with the spiral slit technique was used to obtain depth-resolved information about the variation of lattice parameters. A novel full-pattern analysis approach, based on the evaluation of distinct diffraction spots from individual grains, was developed. The results show high tensile transverse stresses within the bead deposited first. The maximum longitudinal stresses were found beneath the slot. Furthermore significant weld start- and stop-effects were observed. The validity of the results is discussed with respect to the possible impact of intergranular strains due to plastic deformation.

Control of weld composition when welding high strength aluminium alloy using the tandem process

A new cost effective process for generating different weld element compositions has been examined. Utilising tandem welding technology, different series aluminium filler wires were mixed in a single weld pool with the result that the composition of the principal alloy elements, copper and magnesium were accurately controlled. Thermodynamic modelling was then used to predict an optimum weld bead composition for eliminating solidification cracking when welding Al2024. In order to validate the predicted target composition, the tandem process was used to control the composition of the weld bead. The presented results show that using this system to deposit a controlled ternary composition weld, solidification cracking was eliminated when welding highly constrained test pieces. In contrast, cracking was evident when using commercially available binary filler wires under the same conditions.

Simplified FE welding simulation of fillet welds – 3D effects on the formation residual stresses

In this study two- and three-dimensional finite element welding simulations have been carried out. The welded component studied is a T-type fillet weld which is frequently used in the heavy vehicle machine industry with plate thicknesses of eight and 20 mm, respectively. The software’s used for the welding simulations is MSC.Marc and ANSYS. The objective is to study the formation of the residual stresses due to 3D effect of the welding process. Moreover, welding simulations using solid models and contact models in the un-fused weld roots were carried out in order to investigate the possible effect with respect to the residual stresses. Residual stress measurements were carried out using X-ray diffraction technique on the manufactured T-welded structure. The 2D residual stress predictions shows good agreement with measurements, hence the 2D welding simulation procedure is suitable for residual stress predictions for incorporation in further fatigue crack growth analysis from weld defects emanating from the weld toe and the un-fused root.

Explosive welding of thin sheet component with packet and coil distribution of welded elements

Explosive welding of thin sheet component with packet and coil distribution of welded elements

Thermo-mechanical modelling of a single-bead-on-plate weld using the finite element method

This work describes a three-dimensional thermo-mechanical finite element analysis of a single weld bead-on-plate of austenitic stainless steel performed as part of the NeT programme. The overall aim is to validate the use of finite element (FE) weld simulations to accurately predict residual stress states for use in the assessment of welded components. Here, the final residual stresses in the plate are predicted, which can later be verified through comparison with measured distributions. A one-way coupled analysis is carried out with the thermal and mechanical problems solved in sequence. For the thermal analysis, two approaches are adopted to simulate the welding process. In one case sections of the weld bead's elements are sequentially deposited, while in the other the whole bead is deposited simultaneously. A moving heat source is used to simulate the torch traversal over the weld bead in both cases. Predicted thermal profiles, residual stresses and equivalent plastic strains, due to the welding process are presented at certain locations in the plate.

A new analytical model for the elastic–plastic behaviour of spot welded joints subjected to orthogonal load

An analytical procedure for the evaluation of the elastic–plastic stiffness behaviour of spot welded joints is presented. The procedure is based on a new model of spot weld region: a circular plate having variable thickness with a central rigid nugget, which is resolved using an original analytical method. The closed-form solution allows to describe the displacement of a rigid nugget when an axial orthogonal load is applied on the plate while plasticity and large deflections are present. The goal is to reach a reliable spot weld region model which can be used as the basis to develop a spot weld element in FE analysis even when plasticity and large deflections are in effect. The procedure is as completely original as no other can be found in the technical literature, and it has been applied to some examples of plates usually employed for spot weld analysis. The analytical results obtained by using the new general relations precisely match those obtained modelling spot weld area by FEA.

Local equivalent welding element to predict the welding deformations of plate-type structures

Considering the Heat Affected Zone (HAZ) of welding joint, the residual strain behaviors of material under constraint and temperature circulation, as well as the activating mechanism of welding process, this paper addresses a new type welding element for numerical simulation of welding deformation, which is called the LEWE (the local equivalent welding element). This element can describe the basic characteristics of welded seam: the local position points of inherent strain, the equivalent size, the bending radius (or bending angle) from inherent strain, etc. It could be used to predict the welding deformation of plate-type structure. The comparisons between the computed deflection of welded plate and its experiment measurement are present. The results showed that the LEWE possesses a potential to simulate the deformation of welding process high-efficiently and precisely.

Welding process simulation under varying temperatures and constraints

Materials suffer a complex situation of both high-temperature and constraint during welding process. The paper firstly addresses the mechanical behavior of material under varying temperature and constraints by experiments, for which 16Mnq steel is chosen as the specimen material. The conditions of experiments are set to be 30–1200 °C for varying range of temperature, 5–50 MPa for varying range of stress constraint. As for welded structures with the middle-thickness or thin plates, a new modeling method, called the equivalent inherent bend-moment method based on the original welding position, is proposed. In this method, a new element, called the local equivalent welding element (LEWE), is developed. The simulation based on LEWE has an ability to mainly consider the coupling influences of various conditions: the high-temperature performance of material, the varying constraint condition, and the welding technique parameters. A typical example has been present to show the effect to predict the welding deformations of plate structures.

Study on the weld joint of Mg alloy and steel by laser-GTA hybrid welding

A laser-GTA hybrid welding technique was chosen to study the weld of the dissimilar alloys of AZ31B Mg alloy and 304 steel. A lap joint was formed between the two. The weld penetration, which determines the mechanical properties of the welded joints, depends on the laser power. A transition zone formed at the interface of the Mg–Fe during laser-GTA hybrid welding and Mg element diffused into the Fe matrix by forming oxides and reacting in the transition. During tensile testing, the joints fractured at the interface between the Mg alloy and the steel. Metallic oxides produced at the Mg–Fe interface were the reason for the poor mechanical properties of the weld joints.

Prediction of Ferrite Number in Stainless Steel Welds using Bayesian Neural Network Model

Bayesian neural network analysis has been used to develop an accurate model for predicting the ferrite content in stainless steel welds. The analysis reveals the influence of compositional variations on ferrite content for the stainless steel weld metals, and examines the significance of individual elements on ferrite content in stainless steel welds based on the optimised neural network model. This neural network model for ferrite prediction in stainless steel welds has been developed using the database used for generating the WRC-1992 diagram and our laboratory data. The optimised committee model predicts the ferrite number (FN) in stainless steel welds with better accuracy than the constitution diagrams and the other FN prediction methods. Using this generalised Bayesian Neural Network (BNN) model, the influence of variations of the individual elements on the FN in austenitic and duplex stainless steel welds has been determined. It is found that the change in FN is a non-linear function of the variation in the concentration of the elements, with elements such as chromium, nickel, nitrogen, molybdenum, silicon, titanium and vanadium are found to influence the FN more significantly than the rest of the elements in stainless steel welds. Manganese is found to have less influence on the FN. While titanium influences the FN more significantly than niobium, the WRC-1992 diagram considers only the niobium for calculating chromium equivalent. The role of silicon and titanium in influencing the FN in stainless steel welds has been brought out clearly, while these elements are not given due considerations in the WRC-1992 diagram.

Rehabilitation and Repair of Welded Elements and Structures by Ultrasonic Peening

The Ultrasonic Peening (UP) is the most efficient technique for increasing the fatigue life of welded elements as compared to such existing improvement treatments as grinding, TIG-dressing, shot peening, hammer peening, etc. The results of experimental investigation of the efficiency of UP for rehabilitation and repair of welded elements and structures with the goal of preventing the origination and propagation of fatigue cracks are considered in this document. UP treatment was applied to large-scale welded specimens in as-welded condition, after 50 % of expected fatigue life and after repair of fatigue cracks. Also, different techniques for restraining and repair of fatigue cracks were analyzed and compared: overloading; drilling of the crack tips; drilling of the crack tips with installation of high strength bolts; local explosive treatment; local heat treatment; welding with and without UP of weld toe zones. As an example, the practical application of UP for rehabilitation and repair of welded elements of highway and railway bridges are also discussed.

Stress concentration factor analysis for notched welded tubular T-joints

In this study, the finite element method (FEM) is applied on a welded tubular T-joint, in order to analyse stress distribution in the vicinity of the weld fillet. The weld has been modelled all around the joint. A notch is to be simulated in the weld element, all around the joint as well. Using symmetry, the tubular T-joint is submitted to three loading cases: axial loading, in-plane bending (IPB), and out-of-plane bending (OPB). The stress distribution analysis, conducted using the (ARSEM, API, RP2A) code, has enabled locating the peak hot-spot stresses. The finite element method analysis shows that stresses are very high on the brace member, in the vicinity of the weld, and decrease gradually, with a quasi-stable difference, in the direction of the brace extremity. Both on the brace member and along the weld (from crown to saddle), the stresses are high at the crown toe, decrease in the middle and increase once again at the saddle point. Consequently, these zones are susceptible to fatigue damage and require reinforcement solutions in order to ensure sufficiently-long fatigue life for tubular T-joints. For the V-notch, the stress intensity factor at the notch is the parameter selected when processing crack-related problems. Stress distribution from the notch bottom towards the load-bearing segment indicates that the stress intensity factor rises as the radius increases at the notch bottom. The stress concentration factor is higher in the bottom of the notched weld at the load-bearing segment, which in most instances causes crack problems with an increase in stress intensity factor; this finding explains the harmful character of such defects and reveals the extent to which the presence of a notch in a weld fillet lowers the resistance of offshore structures to fatigue cracks.

Some problems associated with the control of the resistance spot welding process

Resistance welding is a process in which the main source of heat is the electric current flowing through the welding region. The result of the flow of current is the resistance heating of the area of contact between the elements to be welded. This results in either strong plasticisation or liquidity. If the upsetting pressure, exerted both during the flow of current and afterwards, is sufficiently high, a permanent joint between the welded elements will be made.7 The magnitude of the pressure should be sufficient to allow the flow of a high current of, possibly, several thousand amperes. Although the process of resistance welding has been known for over 125 years, it is still being improved.

Dissimilar 2·25Cr/9Cr and 2Cr/0·5CrMoV steel welds: Part 2: Identification of precipitates

The long range diffusion of rapidly diffusing elements in dissimilar welds, which occurs during heat treatment and in service exposure, initiates heavy microstructural changes on both sides of the weld. Various carbides and nitrides either dissolve or precipitate as a function of the distance from the fusion line. The type and amount of these particles can be predicted by a numerical method for solving problems of simultaneous precipitation and diffusion. Predicted precipitate distributions for a 2·25Cr/9Cr and a 2Cr/0·5CrMoV weld are verified using experimental data obtained with analytical and energy filtering transmission electron microscopy. The phase diagrams for the high chromium side and the low chromium side of the welds are evaluated and compared with the experimentally determined precipitate distribution.

Effects of temperature-dependent material properties on welding simulation

Detailed three-dimensional nonlinear thermal and thermo-mechanical analyses are carried out using the finite element welding simulation code––WELDSIM [Chao et al., In: Advances in Computational & Engineering Science, vol. II, Tech Science Press: Paledale, USA: 2000. pp. 1206–1211]. The objective is to investigate the effect of each temperature-dependent material property on the transient temperature, residual stress and distortion in computational simulation of welding process. Welding of an aluminum plate using three sets of material properties, namely, properties that are functions of temperature, room temperature values, and average values over the entire temperature history in welding, are considered in the simulation. Results show that (a) the thermal conductivity has certain effect on the distribution of transient temperature fields during welding, (b) the yield stress and Young's Modulus have significant and small effects, respectively, on the residual stress and distortion, after welding, and (c) except for the yield stress, using material properties at the room temperature gives reasonable predictions for the transient temperature fields, residual stress and distortion. Since high temperature material properties are either difficult to obtain or do not exist for many materials, an engineering approach is proposed based on the results in this study. The engineering approach suggests using simplified properties constituted by a piece-wise linear function with temperature for the yield stress and constant room-temperature values of all other properties for computational weld simulation.

An Inspection System to Identify Fatigue Damage on Steel-Bridge Structures

Crack formation and growth in steel bridge structural elements may be due to loading oscillations. The welded elements are liable to internal discontinuities along welded joints and sensible to stress variations. The evaluation of the remaining life of a bridge is needed to make cost-effective decisions regarding inspection, repair, rehabilitation, and replacement. A steel beam model has been proposed to simulate crack openings due to cyclic loads. Two possible alternatives have been considered to model crack propagation, which the initial phase is based on the linear fracture mechanics. Then, the model is extended to take into account the elastoplastic fracture mechanic concepts. The natural frequency changes are directly related to moment of inertia variation and consequently to a reduction in the flexural stiffness of a steel beam. Thus, it is possible to adopt a nondestructive technique during steel bridge inspection to quantify the structure eigenvalue variation that will be used to localize the grown fracture. A damage detection algorithm is developed for the proposed model and the numerical results are compared with the solutions achieved by using another well know computer code.

Ultimate capacity of completely overlapped tubular joints: I. An experimental investigation

This paper presents the experimental finding of a full-scale completely overlapped joint tested to failure under lap brace axial compression. The failure mode of the joint was due to the through brace face plastification. An ovalising effect of the through brace cross-section and bending on the gap region were noted. The onset of non-linear behaviour occurred at 72% of the ultimate load. The capacity of the joint was found to be 7.3% higher than that of the simple gap K-joint, which was determined from the ultimate strength formula. The deformation limit of the joint was 3.82 times the lap brace yield deformation. In the numerical study, the 8-node thick shell element was used to model the joint. No weld element was modelled, but the material with strain hardening property was necessary in the model. The comparison of the load–displacement curve with the experimental results showed a reasonably good agreement, with 5% less for both the ultimate load and the corresponding lap brace axial displacement. As the finite element method was verified to be reliable and accurate, a detailed parametric study was performed to intensively investigate the ultimate load behaviour of the completely overlapped joint as described in the companion paper on behavioural study.