Welded Steel Structures Research Articles

Effect of Martensite–Austenite Constituent on Low-Temperature Toughness in YS 500 MPa Grade Steel Welds

The effect of martensite–austenite (M–A) constituents and simulated microstructure on low-temperature toughness was investigated in YS 500 MPa grade structural steel welds. The specimens were fabricated using a direct quenching and tempering process. After simulated weld thermal cycles, the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (IRCGHAZ) were produced using a Gleeble tester and real welded joint to support the simulation results. The largest low-temperature toughness was observed in the fine-grained heat-affected zone (FGHAZ) owing to the fine-ferrite microstructure. However, the toughness decreased in the IRCGHAZ because of the slender morphology of the M–A constituents that formed primarily along the prior austenite grain boundaries in the IRCGHAZ.

Soldadura de aços estruturais de alta resistência

Soldadura de aços estruturais de alta resistência

Vulnerability Assessment of Steel Structures in District 12 of Mashhad City and Prioritizing the Welding Defects Using the Analytic Hierarchy Process

Recently, steel structures have been accounted for a large percentage of the buildings due to their advantages such as higher execution speed and easier construction. In steel structures, welded joints are commonly used and their quality plays a key role in stability of buildings under applied loads. Hence, to increase resiliency of welded steel structures against natural hazards, welded joints quality should be improved. In this article, a statistical study was done on welding defects of different connections in 50 welded steel buildings that were being constructed in district 12 (Mashhad, Iran). The reasons for selecting this district were high rates of construction and high potential of hazards. Actually, initial information about joints importance and weld defects were collected by distributed questionnaires among building designer engineers and weld inspectors. In this paper, we used the Expert Choice software that works according to Analytic Hierarchy Process (AHP) to prioritize weld defects in different connections of the buildings and define management solutions to improve them. The priority results revealed that in the non-rigid connections crater, slag inclusion and spatter are more critical whereas in the rigid connections Lack of Penetration (LoP) and Lack of Fusion (LoF) are more critical than the other welding defects in steel building structures.

Evaluation of Three Support Shapes on Behavior of New Bolted Connection BBCC in Modularized Prefabricated Steel Structures

Bolted connections are suitable due to high quality prefabrication in the factory and erection in the workplace. Prefabrication and modularization cause high speed of erection and fabrication, high quality and quick return of investment. Their technical hitches transportation can be removed by prefabrication of joints and small fabrication of components. Box-columns are suitable members for bolted structures such as welded steel structures with moment frames in two directions etc., but their continual fabrication in multi-story buildings and performing the internal continuity plate in them will cause some practical dilemmas. The details of the proposal technique introduced here, is to remove such problems from the box columns. Besides, some other advantages include new prefabricated bolted beam-to-column connections referred to BBCC. This connection is a set of plates joined to columns, beams, support, and bolts. For a better understanding of its fabrication and erection techniques, two connection and one structural maquettes are made. The present work aims to study the cyclic behavior of connection numerically. To verify the accuracy of model, a similar tested connection was modelled. Its verification was then made through comparison with test results. The behavior of connection was evaluated for an exterior connection using three different support shapes. The effects of support shapes on rigidity, ductility, rotation capacity, maximum strength, four rad rotation strength were compared to those of the AISC seismic provision requirements. It was found that single beams support has all the AISC seismic provision requirements for special moment frames with and without a continuity plate, and box with continuity plate is the best support in the BBCC connection.

Microstructure Analyses and Multiscale Stochastic Modeling of Steel Structures Operated in Extreme Environment

The article primarily is concerned with the problem of damage accumulation and fracture modeling for steel structures operated at low temperature conditions. The inhomogeneity of the weld joints is other issue associated with the need to examining the microstructure and defects in steel at different scale levels. The next problem is that, extreme environment contains not only the extreme temperature conditions but phase transitions also, fluctuations in temperature, inappropriate in service and repair. Last factors have stochastic behavior and are even uncertain in nature. So these problems are observed and discussed in this paper as theoretical and experimental for building the multiscale model of structural damage accumulation, taking into account the following: the inhomogeneity of the weld, the low-temperature brittle-ductile transition for bcc steels, and the uncertainty factors estimation concept. The applications describe the locomotive tire lifetime estimation at low temperature conditions. The modeling approach is based on Kachanov-Rabotnov structural damage accumulation theory and stochastic crack growth modeling. Bayesian probability approach has been used for uncertainty factor estimation. The experimental part includes the internal friction study of low temperature transition mechanism for bcc steel, the mechanical tension and impact toughness tests for locomotive tire steel, and the low-cycling testing and microhardness estimation of mechanical properties for welded steel probes. The experimental testing shows the impact toughness drop at low temperature. The microstructural study for weld joints reveals the small cracks in heat affected zone, so the size and distance between such defects are used for stochastic modeling visualization of crack propagation and crack velocity estimation. The revealing mechanisms and proposed relationships could be used for theoretical and numerical modelling of damage accumulation and fracture in welded steel structures.

Review of Fatigue Assessment Methods for Welded Steel Structures

Due to high stress concentrations, welded joints represent the most common locations of fatigue crack initiation in steel structures that are prone to fatigue. Welding affects material properties by the process of heating, cooling, and combining basic and additional material. Since welding is the primary process of joining elements in steel structures, it is obvious that fatigue assessment during the design and maintenance process becomes inevitable. There are many fatigue assessment methods of welded joints, but their precision remains questionable. This paper represents a review of the most common fatigue assessment methods used for welded steel joints. As a result of this review, areas that require additional research are highlighted.

RETRACTED: Investigation of ductile fracture behavior of lap-welded joints with 460 MPa steel

Fractures in welded connections usually occurred at Earthquake. The lap-welded joints are an important type of welded connections in high strength steel structures. In this article, the ductile fracture behavior of lap-welded joints has been studied experimentally and numerically with 460 MPa steel. A series of coupon tests were used to determine two corresponding weld materials (ER55-D2 and ER55-G) mechanical properties. Two micromechanics models (void growth model and stress-modified critical strain models) had been calibrated by circumferentially notched tension specimens and calculated the fracture parameters numerically, which had been applied in predicting in five lap-welded joints. The experimental study showed that the fracture mode of 460 MPa lap-welded joints exhibited plastic damage under the tension tests. Numerical analysis of the fracture parameters also showed that the ductile fracture behavior of lap-welded joint with ER55-G was better. The study establishes an accurate numerical model for analyzing the ductile fracture behavior of Q460 high strength steel lap-welded joints that is applicable in predicting the fracture failure of welded steel structures.

Fatigue strength of repaired cracks in base material of high strength steels

Fatigue crack formation is an inevitable issue for welded steel structures subjected to cyclic loading. Accordingly, repair of the fatigue crack in welded steel structures is unavoidable to prolong fatigue life. However, there is limited knowledge available about the procedure to be adopted for the repair and the life extension to be expected after the repair. The current paper is focused on the effects of the repaired an artificial crack in the base material of S690 and S890 high strength rolled steels on the fatigue strength of the material. An artificial crack was created in the middle of the plate test specimens by spark machining and subsequently, the crack was repaired by using the FCAW (flux-cored arc welding) process. The repaired specimens were tested in a four point bending test rig with a constant amplitude loading for creating a uniform bending moment at the weld region such that the weld cap to be exposed to tensile stresses. The test results show that most of the fatigue cracks initiated at the start-stop points of the weld cap and the fatigue crack initiation life of the specimens occupy approximately 45% of the total fatigue life. The statistical analysis of the test results revealed that the characteristic fatigue strength of the repaired specimens is very close to the detail category 160 of EN 1993-1-9 [5].

Characteristics of microstructural phases relevant to the mechanical properties in structural steel weld zone studied by using indentation

The volume fraction (f) and properties of microstructural phases, the mechanical properties in the weld zone, and their relevance of commonly used steel, SM520, were investigated by using instrumented indentation. The volume fraction and properties of microstructural phases in fusion zone (weld metal –WM), heat-affected zone (HAZ), and base metal (BM) of the weld zone were identified by applying the statistical analysis for observed frequency density of hardness (H), elastic modulus (E), and yield strength (σy) spectra from nano-indentation tests. The mechanical properties distributed across the weld zone were determined from the micro-indentation tests. Two different stiffness ferrite types can be characterized in each individual weld zone from the analysis of H and E results, while two different strength ferrite types can be identified in WM from the analysis of the σy spectrum obtained from nano-indentations. The results from micro-indentations exhibited that the E, H, σy, and n values in the HAZ decrease in the direction from WM to BM region and all the average values of E, H, σy, and n in HAZ are higher than those in BM. The relevance of f, E, H, and σy of microstructural phases and E, H, and σy in the weld zone and was also discussed.

Dependency of tensile strength of ductile cast iron on strain rate and temperature

The dependency of the tensile strength and the notch strength on strain rate and temperature were investigated for conventional ferrite-pearlite type ductile cast iron (JIS-FCD500) to make clear the applicability of ductile cast iron to components for welded steel structures. High speed tensile tests were conducted on notched and smooth specimens with varying strain rate and temperature. Charpy absorbed energy was also evaluated on notched specimen with varying temperature. It is found that the tensile strength is in a good relationship with strain rate-temperature parameter R for the wide range of strain rate and temperature. With decreasing R parameter, both and increase even when Charpy absorbed energy starts decreasing. It should be noted that the notch strength is always larger than the tensile strength at room temperature in the range of R parameter required for the welded structures. Therefore, the tensile strength is confirmed to be useful for the structural design.

Behaviour of retrofitted steel structures using cost effective retrofitting techniques

Steel structures today are edging towards the end of their design life. Recently, the frequency and magnitude of loadings are becoming significantly greater in comparison to the initial design loads at the time of construction. Deterioration from prolonged exposure to environmental conditions including weathering and climate change, as well as the effects of human error, also influence the design life of these older steel structures. The research focuses on developing a comparison between the fatigue performance of 120years old and new equivalent steel structures. The fatigue resistance of both the old riveted and new welded steel structures is evaluated by investigating and analysing the stresses at critical locations within the structures. Retrofitting techniques are applied to both the old and new structures and analyzed in terms of their capacity to increase resistance to fatigue failure and extend the design life of steel structures. The research is conducted by performing both experimental study and finite element analysis. The experimental research analyses the performance of an old riveted structure, as well as a new equivalent prefabricated hot rolled section, to determine areas which are highly susceptible to fatigue failure. The numerical analysis using the finite element package ABAQUS is conducted to model both the old and new girder. Retrofitting proposals are introduced into the FE model both with and without the fatigue induced cracking to investigate improvements in the fatigue performance of the old and new girders, as well as techniques of repairing existing damage. The retrofitting techniques are cost effective and practical in engineering today to improve the performance and loading capacities to enhance the design life of steel structures. The retrofitting techniques are innovative, cost effective and practical in engineering today to improve the performance and loading capacities to enhance the design life of steel structures. An overall conclusion determines the extent of increasing design life, enhancing profitable engineering and focus on sustainability, in comparative terms of either retrofitting an old structure or replacing it with a new steel structure.

Refined Analysis of Fatigue Crack Initiation Life of Beam-to-Column Welded Connections of Steel Frame under Strong Earthquake

This paper presents a refined analysis for evaluating low-cycle fatigue crack initiation life of welded beam-to-column connections of steel frame structures under strong earthquake excitation. To consider different length scales between typical beam and column components as well as a few crucial beam-to-column welded connections, a multiscale finite element (FE) model having three different length scales is formulated. The model can accurately analyze the inelastic seismic response of a steel frame and then obtain in detail elastoplastic stress and strain field near the welded zone of the connections. It is found that the welded zone is subjected to multiaxial nonproportional loading during strong ground motion and the elastoplastic stress-strain field of the welded zone is three-dimensional. Then, using the correlation of the Fatemi-Socie (FS) parameter versus fatigue life obtained by the experimental crack initiation fatigue data of the structural steel weldment subjected to multiaxial loading, the refined evaluation approach of fatigue crack initiation life is developed based on the equivalent plastic strain at fatigue critical position of beam end seams of crucial welded connections when the steel frame is subjected to the strong earthquake excitation.

CT시험편의 Back Face Strain Compliance 평가

용접강구조물에서는 용접비드와 같은 응력집중부가 많이 존재한다. 또한 용접부에서는 용접결함이 발생할 가능성이 많다. 반복적인 피로하중이 응력집중부에 작용하면 응력집중부에서 피로균열이 발생하고, 발생된 균열의 전파에 의해 피로파괴사고를 일으킨다. 따라서 피로파괴사고를 미연에 방지하기 위하여 균열발생수명과 균열전파수명과 같은 피로수명의 파악이 필요하다. 본 연구에서는 피로균열전파특성 연구에 사용되는 컴플라이언스를 도출하였다. 이 컴플라이언스는 피로균열의 자동화 측정에 활용할 수 있다. 본 연구에서는 CT시험편에 대하여 in-house FEM program을 사용하여 컴플라이언스를 계산하였다. 이 계산결과는 저자의 앞선 연구에서 본 프로그램을 사용하여 계산한 J integral의 계산결과와 대비하여 a/W와의 관계를 제시하였다. 그리고 또한 CT시험편 Back Face의 중앙부로부터 위아래 방향의 스트레인 분포에 계산을 실시하였다. 이 분포에서는 중심으로부터 위아래로 갈수록 스트레인이 감소하는 경향을 나타내었다. 이상의 계산과정으로부터 균열진전시험의 자동수행을 위한 컴플라이언스를 얻을 수 있었다. 이것으로부터 CT 시험편의 피로균열진전시험의 자동화에 활용이 가능하게 되었다. In welded steel structures, there are many stress concentration sites such as weld beads, and welding defects are likely to occur at the welded parts. When a repeated fatigue load acts on a stress concentration site, fatigue crack occurs and propagates, leading to fatigue fracture. Therefore, it is necessary to understand fatigue life, crack initiation life, and crack propagation life in order to prevent fatigue failure. In this study, a compliance method was derived for use in the study of fatigue crack propagation characteristics. This compliance can be used for automated measurement of fatigue cracks. The compliance was calculated using an in-house FEM program for a CT specimen. The results of this calculation are presented in relation to a/W and compared with calculation results using the J integral and a program from a previous study. In addition, the strain distribution in the upward and downward directions was calculated from the center of the back face of the CT specimen. In this distribution, the strain tended to decrease from the center to the top and bottom. The compliance method was achieved from these calculations and can be used for automatic execution of crack propagation tests.

Application of digital image correlation for the study of strain concentrations due to change in geometry in weld beads: parent interface of structural steel welds

The paper aims to characterise the strain concentration of welded junctions subjected to elastic loads using digital image correlation (DIC). When measuring such small strains and heterogeneous strains fields with DIC, a compromise between noise and accuracy arises where a suitable choice of the image processing settings becomes crucial to obtain accurate results. An extensive study of the influence of the different processing parameters was carried out to ensure the best trade-off between accuracy and noise. A practical application was later performed with the found optimal settings; welded specimens were subjected to uniaxial and biaxial loads to assess possible influence of the strain state on the strain concentration. Results show a great variability of the strain concentration along the weld bead base, evidencing a significant influence of local geometrical characteristics. On the other hand, the strain state influence was found to be moderate, or almost inexistent for lower strain concentrations.

Temperature Histories of Structural Steel Welds Calculated Using Solidification-Boundary Constraints

Temperature histories of structural steel deep-penetration welds are presented, which are calculated using numerical-analytical basis functions and solidification-boundary constraints. These weld temperature histories can be adopted as input data to various types of computational procedures, which include numerical models for prediction of solid-state phase transformations and mechanical response. In addition, these temperature histories can be used parametrically for inverse thermal analysis of welds corresponding to other welding processes whose process conditions are within similar regimes. The present study applies an inverse thermal analysis procedure that uses three-dimensional constraint conditions whose two-dimensional projections are mapped within transverse cross sections of experimentally measured solidification boundaries. In addition, the present study uses experimentally measured estimates of the heat effect zone edge to examine the consistency of calculated temperature histories for steel welds.

Book review

Design of Welded Steel Structures – Principles and Practice

A methodology for variable amplitude fatigue analysis of HFMI treated welds based on fracture mechanics and small-scale experiments

A methodology is developed using strain-based fracture mechanics (SBFM) and an effective strain-life (ESL) model for the fatigue analysis of as-received and high frequency mechanical impact (HFMI) treated structural steel welds. The methodology includes a crack closure model and accounts for the residual stress state at the weld toe. Several required input parameters for mild structural (CSA 350W) steel were obtained using relatively simple and inexpensive small-scale fatigue tests of smooth specimens, conventional monotonic and cyclic materials tests, laboratory measurements, and residual stress measurements. The ESL modelling component of the methodology is validated using direct crack growth measurements. The SBFM model is evaluated using the results of fatigue tests on weld specimens and is found to be capable of accurately or conservatively predicting the fatigue performance of the as-received and impact treated welds under one constant and two variable amplitude loading histories.

Stress Engineering of Multi-pass Welds of Structural Steel to Enhance Structural Integrity

In multi-pass welding, the weld metal and the associated heat-affected zone are subjected to repeated thermal cycling from successive deposition of filler metals. The thermal straining results into multi-mode deformation of the weld metal which causes a variably distributed residual stress field through the thickness and across the weld of a multi-pass weldment. In addition to this, the as-welded fusion zone microstructure shows dendritic formation of grains and segregation of alloying element. This may result in formation of micro-corrosion cells and the problem would aggravate in case of highly alloyed materials. Local mechanical tensioning is an effective way of elimination of the weld tensile residual stress. It has been shown that application of cold rolling is capable not only of removing the residual stress, but depending on its magnitude it may also form beneficial compressive stress state. Multi-pass structural steel welds used as structural alloy in general engineering and structural applications. Such alloys are subjected to severe in-service degradation mechanisms e.g., corrosion and stress corrosion cracking. Welds and the locked-in residual stress in the welded area often initiate the defect which finally results in failure. In the present study, a multi-pass structural steel weld metal was first subjected to post-weld cold rolling which was followed by controlled heating by a fiber laser. Cold straining resulted in redistribution of the internal stress through the thickness and controlled laser processing helps in reforming of the grain structure. However, even with controlled laser, processing the residual stress is reinstated. Therefore, a strategy has been adopted to roll the metal post-laser processing so as to obtain a complete stress-free and recrystallized microstructure.

Desarrollo de un método de simulación física de zonas térmicamente afectadas en soldaduras de acero, para estudios de propagación de grietas por fatiga

Se presenta el desarrollo de un método de simulación física, a través del sistema Gleeble, de las zonas térmicamente afectadas de uniones soldadas de aceros estructurales (A283 Gr. C y A106 Gr. B), para el estudio de la propagación de grietas por fatiga. Para ello fue necesario utilizar probetas de 90 mm de largo, 49,5 mm de ancho y 7 mm de espesor, en una configuración que no se encuentra dentro de los estándares del sistema para este tipo de simulaciones. Con este método se reprodujeron satisfactoriamente, y por separado, en diferentes probetas del metal base correspondiente, la zona de grano fino, la zona de grano grueso y una zona de múltiples pasadas presentes en uniones soldadas fabricadas por FCAW. Las zonas simuladas fueron reproducibles, homogéneas, no presentaron defectos ni tensiones residuales y tuvieron un tamaño considerablemente grande, además que las características microestructurales (como el tamaño de grano y el porcentaje de fases) fueron muy similares a su contraparte real, lo que permitió estudiar por primera vez el efecto neto de la microestructura sobre el comportamiento a la fatiga en estas zonas. De esta manera, el desarrollo realizado permitió aumentar considerablemente las aplicaciones y las ventajas que esta técnica puede tener, además de mejorar la comprensión del comportamiento a la fatiga de las soldaduras de acero.

Design of welded steel structures: principles and practice

Design of welded steel structures: principles and practice