Weld-induced Imperfections Research Articles

Effects of circumferential weld-induced imperfections on the buckling behavior of fabricated steel toroidal shells

Recently, toroidal shells have increasingly been used in engineering applications owing to their excellent load-resistance characteristics. Weld shrinkage during manufacturing leads to deviations from the ideal shell geometry, which is an inevitable imperfection of fabricated steel toroidal shells. To the best of our knowledge, there are no existing studies on the buckling sensitivity of fabricated steel toroidal shells to circumferential weld-induced imperfections. This study presents a modified technique for modeling circumferential weld-induced imperfections, which is employed in a geometrically and materially nonlinear analysis with imperfections (GMNIA) to investigate the imperfection sensitivity of two steel toroidal shells fabricated using different manufacturing processes. The weld imperfections applied on the finite element models were evaluated by modifying Rotter and Teng's weld depression profiles. The procedures for modeling the weld depressions and solving the critical load using the Riks method in ABAQUS are reviewed in detail herein. The obtained buckling results exhibited good agreement with Blachut's experimental results. Furthermore, parametric studies on the relationship between the buckling knockdown factors and the amplitude-to-shell thickness ratios of the imperfect shells revealed the superiority of the fabricated toroidal shells, which can be very beneficial for engineering applications.

Rationally-Based Structural Design of Welded Plate Panels

This study predicts the behavior of welded plate panels (unstiffened plates) with different geometrical properties (slenderness ratio and aspect ratio) in order to address a rational structural design procedure, as these parameters are of great importance from a structural design perspective. Nonlinear finite element analysis has been used to simulate the butt-welding process of plate panels, giving the three-dimensional distribution of distortion and residual stresses induced by welding through the design of a moving heat source. The numerical results are validated with published experimental measurements. The effect of geometrical properties such as slenderness ratio β and aspect ratio a/b on the creation of welding-induced imperfections (distortion and residual stresses) have been investigated in this work. These geometrical properties influence the creation of the welding-induced imperfections, which in turn affect the load-carrying capacity of the plate panels. Three different plate slenderness ratios with three different aspect ratios have been studied. It is concluded that increasing the plate aspect ratio can highly increase the out-of-plane distortion magnitude as well as the compressive residual stress. The plates with high slenderness ratio (thin thicknesses) are highly affected by increasing plate aspect ratio a/b. As the slenderness ratio β increases, the reduction in the ultimate strength due to the existence of welding-induced imperfections highly decreases. Slenderness ratio β can highly affected the ultimate strength of plates with smaller aspect ratio more than plates with higher aspect ratio.

Numerical Investigation on Weld-Induced Imperfections in Aluminum Ship Plates

The present work aims at better understanding and predicting the thermal and structural responses of aluminum components subjected to welding, contributing to the design and fabrication of aluminum ships such as catamarans, lifesaving boats, tourist ships, and fast ships used in transportation or in military applications. Taken into consideration the moving heat source in metal inert gas (MIG) welding, finite element models of plates made of aluminum alloy are established and validated against published experimental results. Considering the temperature-dependent thermal and mechanical properties of the aluminum alloy, thermo-elasto-plastic finite element analyses are performed to determine the size of the heat-affected zone (HAZ), the temperature histories, the distortions, and the distributions of residual stresses induced by the welding process. The effects of the material properties on the finite element analyses are discussed, and a simplified model is proposed to represent the material properties based on their values at room temperature.

Welding of girders with thick plates — Fabrication, measurement and simulation

This article presents experimental and numerical results of the fabrication of welded plate girders under workshop conditions. Main concerns are the prediction of imperfections with the aid of simulation tools and/or simplified engineering models. Their impact on the component design is evaluated in a case study. Special focus is put on the effect of residual welding stress. For this, different simplified distributions are compared with results from welding simulation. The findings confirm the thesis that present recommendations on the implementation of weld-induced imperfections must be rated conservative. This suggests that it is necessary to establish new models. Guidance on future problems in this context will be given.

Numerical analysis of static performance comparison of friction stir welded versus riveted 2024-T3 aluminum alloy stiffened panels

Most researches on the static performance of stiffened panel joined by friction stir welding(FSW) mainly focus on the compression stability rather than shear stability. To evaluate the potential of FSW as a replacement for traditional rivet fastening for stiffened panel assembly in aviation application, finite element method(FEM) is applied to compare compression and shear stability performances of FSW stiffened panels with stability performances of riveted stiffened panels. FEMs of 2024-T3 aluminum alloy FSW and riveted stiffened panels are developed and nonlinear static analysis method is applied to obtain buckling pattern, buckling load and load carrying capability of each panel model. The accuracy of each FEM of FSW stiffened panel is evaluated by stability experiment of FSW stiffened panel specimens with identical geometry and boundary condition and the accuracy of each FEM of riveted stiffened panel is evaluated by semi-empirical calculation formulas. It is found that FEMs without considering weld-induced initial imperfections notably overestimate the static strengths of FSW stiffened panels. FEM results show that, buckling patterns of both FSW and riveted compression stiffened panels represent local buckling of plate between stiffeners. The initial buckling waves of FSW stiffened panel emerge uniformly in each plate between stiffeners while those of riveted panel mainly emerge in the mid-plate. Buckling patterns of both FSW and riveted shear stiffened panels represent local buckling of plate close to the loading corner. FEM results indicate that, shear buckling of FSW stiffened panel is less sensitive to the initial imperfections than compression buckling. Load carrying capability of FSW stiffened panel is less sensitive to the initial imperfections than initial buckling. It can be concluded that buckling loads of FSW panels are a bit lower than those of riveted panels whereas carrying capabilities of FSW panels are almost equivalent to those of riveted panels with identical geometries. Finite element method for simulating static performances of FSW and riveted stiffened panels is proposed and evaluated and some beneficial conclusions are obtained, which offer useful references for analysis and application of FSW to replace rivet fastening in aviation stiffened panel assembly.

Compressive strength of girth-welded stainless steel circular hollow section members: Stub columns

In recent years, stainless steel circular hollow section members have gained more widespread usage as load bearing constituents in construction due to their combination of corrosion resistance, aesthetic appearance and structural efficiency. This paper investigated the compressive strength of girth-welded stainless steel circular hollow section stub columns by numerical method. At first, finite element simulation of the girth welding process was carried out to obtain the weld-induced imperfections such as residual stresses and deformations employing sequentially coupled three-dimensional thermo-mechanical finite element formulation. Then, nonlinear finite element analysis in which the failure mode, the ultimate load-carrying capacity and the full load-displacement response of the girth-welded circular hollow section stub columns were explored taking the residual stresses and distortions together with the initial global geometric imperfections into consideration was conducted. Seven finite element models with different diameter-to-thickness ratios were developed in order to examine the effects that the diameter-to-thickness ratio has on the compressive strength. Results have shown that the weld-induced imperfections should be taken into account in accurate assessment of the compression behavior of girth-welded stainless steel CHS stub columns.

Form of circumferential weld-induced imperfections for tapered-wall cylindrical shell structures

Initial geometric imperfections have a great effect on the buckling strength of thin-walled cylindrical shells under axial compression, and the circumferential weld-induced imperfection is usually the most deleterious imperfection form. Two axisymmetric imperfection forms proposed by Rotter and Teng have widely been employed in the buckling analysis of cylindrical shells. However, the applicability of the two forms for tapered-wall cylinders needs further study, since they are derived from the elastic bending theory for long thin-walled cylinders with a constant wall thickness. This paper presents a modified form of circumferential imperfection for tapered-wall cylinders. Finite element analyses are carried out by employing the trapezoidal strain field approach to model the welding process, and the obtained circumferential depression shapes are used to evaluate the availability of the modified imperfection form. It is shown that the modified imperfection form is reasonable for any wall thickness ratio bet...

Effect of weld-induced imperfections on the ultimate strength of an aluminium patrol boat determined by the ISFEM rapid assessment method

Marine-grade aluminium alloys are being increasingly used as a hull material of naval vessels, particularly in the case of high-speed patrol vessels for which the requirement for reduced structural weight is critical. Combined with the drive for weight reduction, the demand for operations of aluminium vessels in harsher seaway environments has led to increased interest in the application of limit-state design and analysis procedures, which require evaluation of the ultimate strength of the hull girder. This paper investigates the use of the state-of-the-art rapid assessment procedure ISFEM (intelligent supersize finite element method) to examine the ultimate hull-girder strength under vertical and horizontal bending of a metal inert gas welded aluminium midship section with plate and stiffener scantlings typical of a high-speed patrol vessel. The analysis investigates the effect on ultimate strength of weld-induced imperfections including plate and stiffener distortion, residual stresses and material softening in the weld heat-affected zone.

Influence of the residual stresses and distortions on the structural behavior of girth-welded cylindrical steel members

Abstract The structural response of girth-welded cylindrical steel members is affected by the weld-induced residual stresses and distortions. This paper presents finite element analyses to clarify the effects of the girth weld-induced imperfections on the structural behavior of the cylindrical steel members with medium diameter-to-thickness ratio. Finite element modeling of the girth weld-induced residual stresses and deformations is first described. Nonlinear finite element analyses in which the behavior of the cylindrical steel members in pure compression and in pure bending is explored incorporating the girth weld-induced imperfections are next discussed. Results showed that the weld-induced residual stresses and distortions should be taken into account in assessment of the structural behavior of the girth-welded cylindrical steel members subjected to pure compression or pure bending since the weld-induced imperfections always induce local buckling near the girth weld, which alters the load–displacement behavior and diminishes the ultimate load-carrying capacity.

Investigation of the buckling behavior of conical shells under weld-induced imperfections

The initial depression of shell skins is usually created through various panel processes such as rolling or welding. It is important to create some basic design regulations associated with the existing codes. A longitudinal imperfection caused by the continuous welding of a panel's edge to form a cone is the most important case in this context. The present paper discusses 14 laboratory specimens in 2 groups, labeled Shallow Conical Caps (SCC) and Deep Conical Caps (DCC), loaded under uniform hydrostatic pressure. The samples were modified to include either 1 or 2 line imperfections with amplitudes of 1t, 2t and 3t in depth (t the thickness of conical shell). The results presented here are in general agreement with international codes as well as theories concerning initial and overall buckling and collapse.

Effects of welding-induced residual stress on ultimate strength of plates and stiffened panels

During the welding of steel-plated structures, geometric distortions and residual stresses are developed. The influence of welding-induced geometric distortions and residual stresses on the compressive ultimate strength in longitudinal direction of plates and stiffened panels were investigated using nonlinear finite element analyses considering a range of plate thicknesses with various levels of residual stresses. The residual stresses in plates and stiffened panels have been modelled carefully and their effects on these structures due to welding-induced imperfections only are studied. The results and insights observed from the current study are presented.

Comparison for Different Circumferential Weld-Induced Imperfections on Steel Silos

As the thin-walled structure,the buckling of steel silos is very sensitive to the initial geometric imperfections. However, these imperfections are uncertain to the shape and amplitude, so the studies of the initial geometric imperfections have important practical significance. Over the years, the circumferential imperfections have been known to result in the most important influence on the buckling of steel silos, which is also the most common defect in practical engineering. Using the existing research results, this paper analyzes three different imperfection shape functions and compare to the result of experiment in order to identify a function for the finite element analysis.

Buckling Strength of Tapered Cylindrical Shells under Partial Axial Compression: Effect of Circumferential Weld-Induced Imperfections

Buckling is often the main design consideration for thin cylindrical shells. For most load cases, the stability behavior of the shell is acutely sensitive to circumferential weld-induced imperfections, and the corresponding residual stresses are some beneficial to buckling strength of the shell generally. However, these conclusions are all based on the cylinders with constant wall thickness, and the studies about the effect of residual stresses on buckling strength of tapered cylindrical shells under partial axial compression are few. This paper applies trapezoidal strain field approach to simulate circumferential weld-induced imperfections on tapered cylindrical shellls, and studies the stability behavior of the cylinders with single circumferential weld and multiple circumferential welds under partial axial compression respectively. By comparing the results derived from the models with/without circumferential welds and corresponding residual stresses, the effects of weld depressions and residual stresses on tapered cylindrical shells under partial axial compression are obtained.

Analysis of circumferentially welded thin-walled cylinders to investigate the effects of varying clamping conditions

Arc fusion welding predominates in the welding industry as a reliable joining method and has been the subject of investigations by researchers for decades. To ensure in-service structural integrity, optimization of weld-induced imperfections such as welding deformations and residual stresses is critically desirable in circumferentially welded thin-walled cylinders owing to their wide applications in aerospace and aeronautical structures, pressure vessels, and nuclear engineering fields. In this research, computational methodologies for sequentially coupled non-linear transient thermomechanical analysis of the complex arc welding of thin-walled cylinders of stainless steel (AISI 304) are presented. Detailed three-dimensional finite element (FE) simulations with a single V butt-weld joint configuration of 150 mm outer diameter and 3 mm wall thickness cylinders are carried out to investigate the effects of varying structural boundary conditions (mechanical constraints) on weld-induced distortions and residual stress fields. Basic FE models are validated with carefully recorded and properly instrumented experimental data for temperature distribution, deformations, and residual stresses. Predicted and measured welding distortions and residual stresses are compared and discussed in detail. The results reveal that axial deformations are strongly dependent on the degree of restraints. Low-restraint structures exhibit high axial shrinkage, and vice versa. Diametral/radial shrinkage for different clamping conditions show no significant variation. Further, residual stresses show a weak dependence on the degree of restraints. Although the stress levels slightly vary in magnitude, a similar trend is observed for all the structural clamping conditions studied.

Analysis of circumferentially arc welded thin-walled cylinders to investigate the residual stress fields

The control of weld-induced imperfections like welding deformations and residual stresses is of critical importance in circumferentially welded thin-walled cylinders due to their wide utilization in high tech engineering applications in aerospace and aeronautical structures, pressure vessels and nuclear engineering fields. The paper presents a computational procedure for the analysis of temperature distributions and the subsequent residual stress fields during the course of arc welding of thin-walled cylinders of low carbon steel. Parametric studies based on numerical simulations are conducted to investigate the effects of critical welding process parameter on weld-induced residual stresses. Temperature-dependent thermo-mechanical behavior for low carbon steel, filler metal deposition along with double ellipsoidal heat source model is incorporated. The accuracy of the developed finite element simulation strategy is validated for transient temperature distributions and residual stress fields through full-scale shop floor welding experiments with proper instrumentation for data measurement. The aim is to present data to confirm the validity of in-process circumferential welding technology for thin-walled cylinders so that the in service failures of these structures due to process specific inherent stresses may be minimized.

Fabrication of small models of large cylinders with extensive welding for buckling experiments

Cylindrical shells in large steel silos and tanks are commonly constructed from a large number of curved panels joined by many circumferential and meridional welds (referred to as the panel method hereafter). The extensive use of welding in these shells is a unique feature not previously studied in laboratory buckling experiments due to the great difficulty in fabricating realistic small-scale model shells. This paper presents an innovative technique for the fabrication of small models of such large steel cylindrical shells constructed from many welded panels. The experimental set-ups to implement this technique in the laboratory are also described. The new technique consists of two stages: (a) production of a high quality model by rolling two sheets (or a single sheet) and welding along the meridional seams; and (b) ‘welding’ in the form of controlled heat input in a required pattern of circumferential and meridional ‘welds’ on the central portion of the shell surface. The imperfections in an example specimen are also examined to show that they have a realistic pattern. The observed buckling behavior of this specimen is presented and discussed. The specimen buckled at a very low load, confirming that the welding-induced imperfections in such shells are severely detrimental to the buckling strength.

The influence of a weld-induced axi-symmetric imperfection on the buckling of a medium-length silo under wind loading

For particular geometrical constellations of thin-walled cylindrical structures under wind-loading, a peculiar stability failure mode has been observed. This failure mode is characterised by the occurrence of multiple horizontal ripple-like buckles in an area around the upper half of the windward meridian. A recent case study of a cylinder undergoing this type of buckling revealed the strong influence of localised axi-symmetric imperfections on this particular buckling behaviour. For the present paper a detailed investigation into the nature of this influence of such imperfections on the buckling behaviour under wind loading has been performed using finite elements models. A cylinder with a geometry that displayed the particular buckling pattern in question was considered for this study. The parameters describing the nature of the axi-symmetric weld imperfection were varied and their influence on the buckling of these cylinders was studied in detail. The present study draws from insights gained from similar studies for cylinders under axial loading. Many similarities between the two loading cases can be observed and the influence of weld-induced imperfections on the buckling under the two loading types were compared for this paper.

The shape of circumferential weld-induced imperfections in thin-walled steel silos and tanks

The strength of thin-walled cylindrical shell structures is highly dependent on the nature and magnitude of imperfections. Most importantly, circumferential imperfections have been reported to have an especially detrimental effect on the buckling resistance of these shells under axial load. Due to the manufacturing techniques commonly used during the erection of steel silos and tanks, specific types of imperfections are introduced into these structures, among them circumferential weld-induced imperfections between strakes of steel plates. The shape of such a localised circumferential imperfection has been shown to have a great influence on the degree of strength loss of thin-walled cylindrical shell structures. The results of a survey of imperfections in an existing silo at a location in Port Kembla, Australia in combination with linear elastic shell bending theory was used to develop and calibrate a shape function which accurately describes the geometric features of circumferential weld imperfections. The proposed shape function is the first function to combine shell theory with actual field imperfection measurements. It is a continuous function and incorporates all the necessary features to represent the geometry of a circumferential weld-induced imperfection. It was found that after filtering out the effects of overall imperfections three parameters governed the shape of the surveyed imperfections: the depth, the wavelength and the roundness.

Buckling of Thin-Walled Silos and Tanks under Axial Load—Some New Aspects

The strength of thin-walled cylindrical shell structures is highly dependent on the nature and magnitude of imperfections. Most importantly, circumferential imperfections have been reported to have an especially detrimental effect on the buckling resistance of these shells under axial load. Due to the manufacturing techniques commonly used during the erection of steel silos and tanks, specific types of imperfections are introduced into these structures, among them circumferential weld-induced imperfections between strakes of steel plates. A study on several factors influencing the buckling of silos and tanks was carried out using the finite-element method. The interaction between neighboring circumferential weld imperfections was investigated, and it was found that the influence on the buckling behavior depended on the strake height in relation to the linear meridional bending half-wavelength and the depth of the imperfection. The shape of localized circumferential weld imperfections was found to influence the buckling behavior of silos and tanks. The influence of a recently developed shape function on the buckling behavior has been examined. The strengthening effect of weld-induced residual stress fields was also studied, and the extent of the increase in buckling strength was derived for a large range of cylinder geometries.

The influence of circumferential weld-induced imperfections on the buckling of silos and tanks

The load carrying behaviour of cylindrical thin-walled shell structures under axial load is strongly dependent on imperfections invariably caused by various manufacturing processes. Axisymmetric imperfections have been known to result in particularly severe reductions in strength. Imperfections in the vicinity of circumferential welds in steel silos and tanks fall into this category and therefore deserve special attention. A detailed bifurcation and post-buckling finite element analysis was performed on imperfect cylindrical shells. Special care was taken to model the weld-induced circumferential imperfection. The geometry was calibrated against data gained from measuring such imperfections on existing silos and residual stresses were taken into account. Interaction between neighbouring weld imperfections and the role of the strake height in this interaction was investigated. Weld-induced residual stresses were found to have a small strengthening influence on the buckling load. Interaction between neighbouring imperfections was found to reduce the buckling strength of the structures. A post-buckling analysis was undertaken and an explanation of the load-carrying behaviour of the structure after initial bifurcation was given.