Welded Steel Structures Research Articles

Validated Methodology for Assessment of Welded Steel Structures by Nonlinear Finite Element Analysis

Validated Methodology for Assessment of Welded Steel Structures by Nonlinear Finite Element Analysis

Experimental evaluation of the short and long fatigue crack growth rate of S355 structural steel offshore monopile weldments in air and synthetic seawater

Welded steel structures used in the offshore wind industry are exposed to harsh marine environments, which can result in corrosion-induced fatigue damage. Of particular concern is the heat affected zone (HAZ) of welded joints, a region known for its altered microstructure and mechanical properties, which can significantly influence the initiation and propagation of fatigue cracks. This study investigates the short and long fatigue crack growth rates, and the effect of seawater exposure, for the HAZ in S355 steel weldments. Single-edge notch bend (SENB) specimens are used, with a shallow notch in the HAZ. A series of specimens is immersed in synthetic seawater that is continuously circulated at a controlled temperature to assess the synergistic effects of corrosion and fatigue. The experimental method integrates a novel application of front face strain compliance for monitoring short cracks, alongside an extended back-face strain compliance approach for monitoring long crack propagation. It is concluded that the short fatigue crack growth rate of the HAZ is 2.7 to 3.5 times higher in seawater as compared to air. As the crack propagates and enters into the long crack regime, the ratio decreases to 2.2 times at the transition point of the two-stage crack growth curve and further decreases to 1.5 times when the notch advances towards fracture. The findings indicate that the fatigue crack growth rates documented in standards tend to be on the conservative side. This study significantly enriches the fatigue crack growth data available in literature, which will contribute to a more accurate lifetime assessment offshore wind turbine structures.

Post-fire structural changes in welded joints of unalloyed steels

In the article deals with the issue of the impact of fire on changes in the structure and mechanical properties of welded joints of non-alloy steel elements. The processes occurring during welding, affecting the obtained input structure and properties in the weld and in the heat affected zone, were analyzed. On this basis, a concise analysis of the impact of low- and high-temperature fire and the method of extinguishing it on changes in the structure and mechanical properties of welded joints of non-alloyed steels was made. Recommendations for conducting post-fire expertise of welded steel structures are included

Investigation on the effectivity of nanoparticle elimination of a fume extractor TIG torch

AbstractAlthough tungsten inert gas welding is considered one of the cleanest welding techniques, which is virtually “fumeless”, recent studies show, there is a significant amount of nano-sized particles generated during the process. To protect the welding personnel from possible harmful effects of inhalation of fume particles, fume extractor torches are developed. One such welding fume extractor torch was tested and compared to a conventional one during manual welding of unalloyed structural steel. Using the fume extractor torch, a significantly lower amount of nano-particles was measured in the welder’s personal breathing zone effectively decreasing the exposure to possible harmful effects of those nano-sized particles.

Re-evaluation of fatigue design curves for offshore wind monopile foundations using thick as-welded test specimens

The dominant majority of existing offshore wind turbines are supported using monopile foundations which are fabricated by welding thick steel plates. In the current fatigue design recommendations for welded steel structures, which are commonly used in the design of monopile structures, the initial inverse slope of the S-N curve is fixed to −3. The historic rationales for this assumption are the ease of fatigue life calculations, the effect of long-range residual stresses that were not originally captured in the test data on thin welded specimens, and analogy with fatigue crack growth. While this introduces an unquantified level of conservatism to deterministic fatigue calculations, it was an acceptable (and wise) assumption when the S-N design curves were generated a few decades ago; however, current designs and assessments require more accurate calculations and the need to more confidently quantify the likelihoods attached to these evaluations. In the present study, it is argued that unlike large braced structures (e.g. jackets) in which long-range residual stresses may remain in place as fatigue cracks propagate, in circumferential welds of monopile structures the natural relaxation of residual stresses, in the absence of long-range residual stresses, must be accounted for by performing fatigue tests on thick welded samples. This paper presents a thorough analysis of the fatigue test results obtained from 50 mm thick as-welded samples with representative profiles of residual stresses that exist in monopiles. Basic and advanced statistical approaches were employed in the analysis and higher values of inverse slope were found using the large thickness test data. The results from this study are compared with the recommended fatigue design curves available in international standards. The outcomes from this research draw important observations concerning the need to employ an appropriate value of inverse slope in the design of monopile structures by excluding the conservative approach imposed by a fixed value of −3 which was originally implemented for braced structures with the assumption of large and long-range residual stresses.

Life extension of pre-damaged existing crane runway girders

Crane runway girders are subjected to cyclic loading and must therefore be designed against fatigue failure. For existing structures, however, there are no standards for handling pre-damaged components at the end of their calculated lifetime. The Institute for Material and Building Research of the University of Applied Sciences Munich examines different approaches on how to deal with existing welded steel structures. The research project addresses the following questions:How can pre-damaged components without visible cracks be strengthened?How can components with visible cracks be repaired in a fatigue-proof manner?How can pre-damaged components be reinforced through a low-notch application of steel cover plates?In order to answer these questions several numerical and experimental investigations are carried out. Different innovative fastening techniques like lockbolts, adhesives and self-tapping screws for attaching reinforcement cover plates are tested on small and large specimens. Within this paper the research project will be presented and previous results on components without visible cracks will be summarized.

Influence of post-weld heat treatment on welding residual stress in U-rib-to-deck joint

Welding residual stress (WRS) significantly influences the fatigue life and safety of welded steel structures. Post-weld heat treatment (PWHT) is a common method employed to mitigate WRS. In present study, the WRS evolution law and relaxation mechanism of U-rib-to-deck joints was investigated during PWHT. High-temperature heat treatment tests with varying holding times were conducted on U-rib-to-deck joints to reduce WRS. The WRS distributions were measured using the hole-drilling method before and after PWHT. ABAQUS software, incorporating the Norton-Bailey creep constitutive model, was employed to simulate the WRS evaluation during both welding and PWHT processes. The simulated values were validated against the measured data, and the effects of different holding times, holding temperatures, and heating and cooling rates during PWHT on WRS relaxation were discussed. The results demonstrate that PWHT effectively reduces WRS and leads to a more uniform distribution. Extending the PWHT holding time from 0.5 h to 1 h and 2 h results in a gradual decrease in WRS relaxation at 600 °C, primarily occurring in the high-value stress region of the weld zone with a small amplitude. Elevating the PWHT temperature from 500 °C to 550 °C and subsequently to 600 °C amplifies the significance of WRS relaxation. Variations in heating and cooling rates (200 °C/h and 80 °C/h, 100 °C/h and 40 °C/h, 50 °C/h and 20 °C/h) lead to a gradual decrease in WRS relaxation across the entire model. Furthermore, considering the creep effect, the stress relaxation rate in the weld is approximately 70 %–80 %, which found to be about 2–3 times higher than that without considering the creep effect. By understanding the evolution law and relaxation mechanism, this study provides valuable insights for designing PWHT processes and assessing their impact on structural safety and fatigue life.

Testing the impact energy and determination of the brittleness temperature of the welded joint of steel A516 Gr.70

Most problems with steel structures occur in the welded joint or its immediate vicinity. For this reason, special attention is paid to testing the properties of the welded joint. Fractures of welded steel structures in operating conditions have led to the need to study the influence of temperature on the behavior of steel and welded joints. For the purpose of these tests, the critical test conditions (temperature) that cause brittle fracture are defined. The paper will present the results of experimental tests of the impact energy of the welded joint of steel A516 Gr.70. Impact energy testing at different temperatures was done with the aim of determining the brittleness transition temperature of the base metal and components of the welded joint. Also, these tests should give an assessment of the influence of temperature on the ratio of the crack formation energy Wiu, and the crack propagation energy Wa in the total fracture energy Wt. This test procedure determines the tendency to brittle fracture, that is, the tendency to increase brittleness during exploitation. The obtained results are most often used for ongoing quality control and evaluation of material homogeneity.

Numerical design calculation of structural steel welds

This paper focuses on designing fillet welds made from high-strength steel (HSS). It presents a numerical design calculation (NDC) method for assessing weld strength using the regular inclined shell element model (RISEM) in the finite element method (FEM). NDC uses FEM to design joints according to standardised procedures. The inclined shell element in RISEM represents the geometry and stiffness of welds, controlling its stresses in the plane. The paper recommends using a common inclined shell element with rigid links to accurately represent the geometry and rigidity of fillet welds. A new strength criterion is proposed based on stresses on the inclined shell element, using maximum equivalent stress as an indicator. Validation of the method is confirmed by comparing RISEM and test results using finite element (FE) simulation. Additionally, the weld resistance computed from the equation in prEN 1993-1-8:2020 is also in good agreement.

Comparative analysis of fatigue strength of a freight wagon frame

Freight wagons primarily consist of welded steel structures subjected to time varying loads. Therefore, fatigue associated with the welding process is a common failure mode. Several technical codes for fatigue assessment have been proposed, being DVS 1612–2014 and ERRI B 12/RP 60 two of the most used. Nevertheless, no clear correlation between both codes is yet found in the bibliography. To overcome this, the present paper develops a full comparison over a study case of a freight wagon frame model. Finite element method is used for the stress calculation based on loads according to UNE-EN 12663-2. Weld performances comply with UNE-EN 15085-3. Fatigue post-processing is carried in parallel using DVS and ERRI codes. Finally, fatigue results of both codes are compared at each element of the model and each weld type. The results obtained provide a complete overview of the influence on the fatigue design of choosing a specific technical code for fatigue assessment.

Effects of mixed corrosion damage on constitutive relation and ductile fracture behavior of butt-welded joints under cyclic loading

The corrosion damage of welded connection regions posed a threat to the reliability and durability of welded steel structures. In this paper, the morphology scanning and cyclic loading tests of butt-welded joints exposed to the salt spray corrosion environment were conducted to investigate the effects of corrosion damage on surface characteristics and hysteresis behavior of butt-welded joints. Three-dimension scanning test results showed that the butt-welded joint under the salt spray environment suffered a kind of mixed corrosion damage. Compared with the thickness loss caused by uniform corrosion, pitting damage tended to cause serious local geometric defects, aggravating the geometric discontinuity problem. For the corroded butt-welded joint, the hysteresis loop area and cycles decreased significantly with the increase of corrosion damage, and the ductile fracture behavior under the cyclic loading gradually disappeared. The numerical comparative analysis of models with two different patterns of corrosion damage showed that the pitting damage caused a severer hysteresis behavior degradation of butt-welded joints compared to the uniform corrosion damage under the same volumetric loss ratio. Particularly, the pitting damage had a significant effect on reducing cyclic deformation and energy dissipation. According to the above experimental and numerical analysis, a modified cyclic plastic fracture criteria considering pitting damage and a nonlinear cyclic constitutive model considering mixed corrosion for the corroded butt-welded joint were proposed respectively to equivalently consider the influence of corrosion geometric damage by weakening the material properties.

Welding at Low Temperatures of Structural Steels

In this paper, the effect of direct and pulsed current in the arc welding with the use of UONI 13/Moroz coated electrodes on the structure and properties of 10KhSND structural low-alloy steel joints was investigated. Welding was performed in winter at an ambient temperature of-40oС. This work is a continuation of the cycle of research on welding of structural steels, carried out by the authors for several years.

Grain boundary strengthening of microstructure phases in SS400 structural steel weld zone

Grain boundary strengthening is a method of strengthening materials by changing their average crystalize (grain) size, exhibiting the basic relationship between yield stress and the grain size of the materials. This methodology is based on the observation of grain boundaries that are insurmountable borders for dislocationsand the number of dislocations located within a grain. Applying this methodology to the structural steel weld zone, the mechanical properties of each microstructural phase can be evaluated through the values of grain diameter. For structural steel weld zones, there are not many methods to directly determine the mechanicalproperties of microstructural phases. Thus, in this study, a methodology was created to evaluate the values of the yield stress of materials based on the grain boundary strengthening equation. This method was constructedby observing the average grain size and determining the mechanical properties of three microstructural phasesin the weld zone (i.e. based metal, heat-affected zone, and weld metal). The results from this study provide aneasy way for engineers, architects, and scientists to evaluate the values of mechanical properties of based metal, heat-affected zone, and weld metal in the SS400 structural steel weld zone.

Prediction of Angular Distortion in Gas Metal Arc Welding of Structural Steel Plates Using Artificial Neural Networks

The manufacturing of structures ranging from bridges and machinery to all types of seaborne vehicles to nuclear reactors and space rockets has made considerable use of arc welding technologies. This is as a result of benefits including increased joint efficiency, air and water tightness, no thickness restriction (0.6 to 25 mm), decreased fabrication time and cost, etc. when compared to alternative fabrication methods. Gas metal arc welding (GMAW) is a frequently used welding technology in industries due to its inherent benefits, including deeper penetration, a smooth bead, etc. Local heating and cooling that takes place during the multi-pass welding process causes complicated stresses to develop at the weld zone, which ultimately causes angular distortion in the weldment. Angular distortion is a major flaw that affects the weld’s properties as well as the cracking and misalignment of the welded joints. The issue of angular distortion can be successfully solved by predicting it in relation to certain GMAW process variables. A neural network model was created in this research to predict angular distortion. A fractional factorial approach with 125 runs was used to conduct the exploratory experiments. A neural network model with feed forward and backward propagation was developed using the experimental data. To train the neural network model, the Levenberg–Marquardt method was utilised. The results indicate that the model based on network 4-9-3 is more effective in forecasting angular distortion with time gaps between two, three, and four passes than the other three networks (4-2-3, 4-4-3, 797 and 4-8-3). Prediction accuracy is more than 95 percent. The neural network model developed in this study can be used to manage the welding cycle in structural steel weld plates to achieve the best possible weld quality with the least amount of angular distortion.

Physical Testing Campaign Facilitating Validation of Welded Steel Structures by Non-Linear Finite Element Analysis

Abstract Non-linear finite element (FE) analysis is a powerful tool for determination of ultimate capacities of steel components. To produce reliable predictions of structural strength, it is of fundamental importance that non-linear finite element analyses are based on a validated methodology. This paper focuses on physical test campaigns, which are designed with the purpose of providing an experimental foundation for calibrated and validated non-linear finite element assessments of plated components, which form part of typical offshore structures. The experimental work discussed in the current paper is a continuation of the experiments of tubular members and joints, which were carried out by Maersk Oil in the late nineties. The objective is that the two-test campaigns together shall allow for development of a non-linear finite element methodology with calibrated failure criteria providing either characteristic or mean level of capacity/response for the main details in typical offshore structures. This paper particularly focuses on the testing campaign completed in 2017 which forms the basis for the calibration of the failure criteria covering plated sections.

ІМПУЛЬСНІ СТАБІЛІЗАТОРИ ГОРІННЯ ДУГИ ПРИ ЗВАРЮВАННІ ЗМІННИМ СТРУМОМ ПРОМИСЛОВОЇ ЧАСТОТИ

The work deals with the development of AC welding arc stabilization devices that provide a qualitatively different level of functionality to industrial frequency welding transformers. It is shown that despite the rapid spread and use of inverter sources of direct current for arc welding of metals, welding with alternating current, using simple and unpretentious welding transformers, which work at the frequency of the current of the industrial power supply network, continues to be relevant. With regard to manual arc welding with coated electrodes and a non-fusible electrode on alternating current of industrial frequency, the problem of increasing the stability of arc burning is solved thanks to the use of impulse stabilizers of arc burning. Thepurpose of the study is to increase the energy efficiency of industrial frequency alternating current welding power sources due to the development of arc stabilization devices at the modern level.The choice of the polarity of the pulses significantly affects the parameters of the stabilizing device and the AC welding process. Studies of the influence of the pulse polarity on the parameters of the pulse arc stabilizers themselves have been carried out. The schematic implementation of stabilization devices using stabilizing pulses, the polarity of which is opposite to the polarity of the arc current, is considered.The experience of using the developed stabilizers allows us to conclude that they provide sufficiently high stability of arc burning from an alternating current welding transformer during manual arc welding of low-alloy structural steels covered with electrodes, arc welding of stainless and other special steels, arc welding of cast iron, during non-fusible argon arc welding electrode of stainless steels, aluminum and its alloys with the contact method of initial ignition of the arc

Parameter optimization and microstructure evaluation of welding of structural steel 1020 and AA6062

AA6062 and ST1020 materials are used to perform friction stir spot welding (FSSW). In this experiment, the mechanical and microstructural characteristics of the welded joint were determined using a square-head tip tool. To enhance the life of the welded joint, three essential process parameters, namely tool speed, dwell time, and plunge depth were employed with four levels. The L16 orthogonal array was adapted for experimental work to develop a process parameter table. A tensile test was performed for a larger better value to predict the strength of the welded joint, and the value is 5.65 kN. The outcome shows that dwell time is a more prominent parameter than tool speed and plunge depth. The optimal combination of welding parameters is a dwell time of 24 s, a tool rotational speed of 1200 rpm, and a plunge depth of 0.8 mm; dwell time contributes 39.66%, followed by a plunge depth 35.7%, and at last, tool speed 6.171%. Furthermore, to determine the optimum level of parameters. The microstructure of this joint was observed with respect to heat dissipation and changes in grain size. Microstructure studies were carried out to see the area under stir zone for workpieces with high and low tensile strength. This work was validated using the response surface method (RSM). The RSM results are very close to the experimental results.

Effect of heat input on microstructure and mechanical properties in underwater wet flux-cored arc welding of structural steels

Effect of heat input on microstructure and mechanical properties in underwater wet flux-cored arc welding of structural steels

Heat input effect in a multipass and double-wire GMAW welding of a thick structural steel for industrial applications

Heat input effect in a multipass and double-wire GMAW welding of a thick structural steel for industrial applications

Corrosion-fatigue crack growth behaviour of wire arc additively manufactured ER70S-6 steel parts in marine environments

A crucial part of the structural integrity assessment of marine structures is the analysis of the fatigue crack growth behaviour of the welded joints in seawater environments, where the cracks often initiate and propagate under corrosion-fatigue loading conditions. In recent years, technological developments have facilitated the fabrication of steel components and structures using additive manufacturing technologies. Among the existing technologies, the Wire Arc Additive Manufacturing (WAAM) technique has proven to offer great potentials for fabrication of large-scale structures. The present study investigates the corrosion-fatigue crack growth (CFCG) behaviour of the WAAM parts fabricated using ER70S-6 low carbon steel wire to assess the suitability of this technology for future marine structures. In this experimental study, the cracking behaviour and test duration in corrosion-fatigue tests were investigated and analysed in conjunction with the microstructural examination of the tested specimens. Moreover, the obtained results were compared with the recommended trends available in BS7910 standard for conventional welded joints and the data available in the literature on widely used offshore structural steel weldments. The CFCG results obtained from this study contribute to the overall knowledge and design requirements for the new optimised functionally graded structures made with WAAM technology for marine applications.