Welded Steel Structures Research Articles

面内ガセット継手の疲労き裂発生・成長シミュレーション

Welded built-up steel structures in service encounter many accidents caused by fatigue, and it is important for maintenance of social foundations and safeness to estimate their fatigue lives quantitatively.By considering that fatigue cracks cannot grow without the accumulation of alternating tensile / compressional plastic strain, one of authors identified that the effective stress intensity factor range ΔKRPG based on the Re-tensile Plastic zone Generating (RPG) load which represents the fatigue crack driving forces, and suggests that ΔKRPG should be applied as the parameter in order to describe the fatigue crack growth behavior. For predicting fatigue crack initiation and propagation, numerical simulation code “FLARP” based on ΔKRPG was developed.In this paper, it is confirmed that fatigue life estimation by FLARP gives accurate results by comparing the estimated fatigue crack growth curves with the experimental results for in-plane gusset welded joints which are used in many welded steel structures. Moreover, the effect of induced bending moment due to misalignment for the fatigue strength is investigated by the numerical simulations.

MIG Brazing as a Means of Fatigue Life Improvement

MIG-brazing is examined as a potential fatigue strength improvement method for welded structures. The fatigue strength of joints produced using MIG-brazing as the primary joining technique and for the improvement of joints welded using traditional welding was studied experimentally. The effect of the improvement for a typical structural steel weld is achieved by a final pass along the weld toe using MIG-brazing. In total, 31 fatigue test results are presented. These include four pilot cases of welded cruciform joints and two main cases of plates with transverse attachments. The results of the fatigue tests were compared to IIW recommendations for as-welded and improved welds. The fatigue strength of MIG-brazed joints was good. For transverse non-load-carrying attachments, an improvement of 70–80 % measured as local nominal stress was gained with respect to IIW recommendations. The fatigue strength of solely MIG-brazed joints and joints improved by MIG-brazing was almost equal. As an alternative joining method, MIG-brazing is used in some industrial applications. This study has given clear indications that this method could also potentially be used as a weld improvement method for new or repaired structures. However, additional research is needed in order to gain more confidence that the degree of improvement can reliably be obtained in production.

Design and manufacture of composite high speed machine tool structures

The high transfer speed as well as the high cutting speed of machine tools is important for the productivity improvement in the fabrication of molds/dies because non-machining time, called the air-cutting-time, amounts to 70% of total machining time with complex shape products. One of the primary reasons for low productivity is large mass of the moving parts of machine tools, which cannot afford high acceleration and deceleration encountered during operation. Moreover, the vibrations of the machine tool structure are among the other causes that restrict high speed operations. In this paper, the slides of high speed CNC milling machines were designed with fiber reinforced composite materials to overcome this limitation. The vertical and horizontal slides of a large CNC machine were manufactured by joining high-modulus carbon-fiber epoxy composite sandwiches to welded steel structures using adhesives and bolts. These composite structures reduced the weight of the vertical and horizontal slides by 34% and 26%, respectively, and increased damping by 1.5–5.7 times without sacrificing the stiffness. Without much tuning, this machine had a positional accuracy of ±5 μm per 300 mm of the slide displacement.

오스테나이트계 304 스테인리스강의 Nd:YAG 레이저 맞대기 용접특성

Laser beam welding is increasingly being used in welding of structural steels. The laser welding process is one of the most advanced manufacturing technologies owing to its high speed and deep penetration. The thermal cycles associated with laser welding are generally much faster than those involved in conventional arc welding processes, leading to a rather small weld zone. Experiments are performed for 304 stainless steel plates changing several process parameters such as laser power, welding speed, shielding gas flow rate. presence of surface pollution, with fixed or variable gap and misalignment between the similar and dissimilar plates, etc. The following conclusions can be drawn that laser power and welding speed have a pronounced effect on size and shape of the fusion zone. Increase in welding speed resulted in an increase in weld depth/ aspect ratio and hence a decrease in the fusion zone size. The penetration depth increased with the increase in laser power.br/

異なる板厚を有する十字すみ肉溶接継手の疲労挙動

Load-carrying fillet welded joints in welded steel structures are extensively used for connecting the structural members. Typical fatigue cracks have been observed at the welded joints such as secondary structural members of steel bridges. The object of this research is to clarify the influence of the difference in plate thickness on the fatigue behavior of load-carrying fillet welded cruciform joints. Fatigue tests were carried out on the joints with plate of different thickness. The main plate thickness was 9, 14 or 25 mm and the cross plate thickness was 6 or 14 mm. Fatigue crack propagation and strength analyses using fracture mechanics were also performed on the joints with various plate thickness. The tested specimens indicates that the fatigue cracks are initiated and propagated almost along extended line of non-penetration weld line regardless of the main plate and the cross plate thickness. The experimental and the analytical results show that the larger the difference of thickness in each main plate is, the lower the fatigue strength is, when the cross plate thickness is smaller than that of the main plate.

Control of mechanical properties in structural steel welds by numerical simulation of coupling among temperature, microstructure, and macro-mechanics

The mechanical characteristics of steel welded structures such as tensile strength or fracture toughness depend on welding conditions, and multiple welding heat cycles particularly affect the joint performance. It should be controlled under consideration of the microscopic and macroscopic behaviors in materials. In this paper, numerical simulation method of coupling analysis of temperature, microstructure, and stress–strain fields has been developed and applied to investigate the effects of heat input and interpass temperature at multi-pass welded joint of beam-to-column connections on the strength and fracture. Tensile test, Charpy impact test, and three-point bending CTOD test are performed by using welded joint specimen fabricated with various welding conditions. The results of experiments actually show that there exists a welding condition that decreases the joint performance. Three-dimensional thermal elastic–plastic finite-element analysis is developed and performed by using heat source movement and considering phase transformation effect in order to evaluate the proportion of microstructure, distribution of hardness and strength. The parameters of heat input, interpass temperature, and welding direction are changed in the analyses, and the results are used for the consideration of the effect of multiple heat cycles on joint performance. The analytical and the experimental results have shown that controlling heat input and interpass temperature are important features to assure the integrity of welded joints in beam-to-column connections of steel framed structures, and that mechanical properties of the weld metal and the heat-affected zone could be predicted and controlled by using numerical simulation.

Control of Residual Stresses Using Narrow Gap Technique in SMA Welding of Structural Steel

Welding of structural steel plate having conventional V-Groove and narrow grooves of different designs has been carried out by GTAW ischolar_main pass followed by SMAW filler passes. The residual stresses of the weld and HAZ have been measured by centre drill hole technique using three-element strain gauge rosette. Effect of narrow gap welding of different groove design on residual stresses of weld joint has been studied. The effect of variation in heat input per pass in the SMAW filler passes of the narrow gap welds on residual stresses of the weld joint also has been studied. The residual stresses along with the microstructure and hardness of the conventional and narrow gap weld joints have been compared. It has been observed that the use of narrow gap welding with comparatively low heat input reduces the residual stresses of weld joint significantly compared to that observed in conventional V-Groove weld. This is primarily attributed to reduction in amount of weld deposit lowering the contraction stresses in narrow gap weld.

Bruchmechanische Sicherheitskonzepte bei Schweißkonstruktionen

Cases of Structural Steel Building. Steel building in Germany is ruled by laws which shall effect sufficient reliability of the structure. They rely on technical rules for design and erection of welded steel structures. In cases where the rules do not apply it is necessary to make single case studies which demonstrate the safety of the structure. Within this frame and as a result of the development of the Annex C procedure to avoid brittle fracture in EUROCODE 3 a fracture mechanic based methodology has been developed which allows the calculation of the required safety in such special design cases. The methodology is going together with one single component test which is designed and performed in a way that the theoretical calculations are verified.

Metallurgical failure analysis of cold cracking in a structural steel weldment: Revisiting a classic failure mechanism

Cold cracking of structural steel weldments is a well-characterized, well-documented, and well-understood failure mechanism. Extensive effort has been put forth to recognize the welding and materials selection parameters that are conducive to cold cracking; however, these engineering efforts have not fully eliminated the occurrence of such failures. This article describes cold cracking failure specifically related to the construction industry. This particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked. These actions potentially prevented a serious catastrophic event that could have occurred have occurred either later in the construction process or possibly during the use of the building. Individual welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure, and a number of human factors relating to the actual fabrication practices also contributed to the failure process.

Composite Machine Tool Structures for High Speed Milling Machines

To maximize the productivity in machining molds and dies, machine tools should operate at high speeds. However, the productivity of mold manufacturing has not increased significantly because CNC milling machines have massive slides, which do not allow rapid acceleration and deceleration during the frequent starts/stops encountered in machining molds and dies. This paper presents the use of composites for these slides to overcome this limitation. The vertical and horizontal slides of a large CNC machine were constructed by bonding high-modulus carbon-fiber epoxy composite sandwiches to welded steel structures using adhesives. These composite structures reduced the weight of the vertical and horizontal slides by 34% and 26%, respectively, and increased damping by 1.5 to 5.7 times without sacrificing the stiffness. Without much tuning, this machine had a positional accuracy of ± 5μm per 300 mm of the slide displacement.

Experimental study of corrosion fatigue behaviour of welded steel structures

Earlier research has shown that stress-raisers at weld geometry, misalignment and post-weld tensile residual stresses caused by the welding process are the major sources of fatigue cracks initiations and propagation in welded steel structures. The problem becomes more pronounced in the presence of corrosive environment, and the rate of crack propagation becomes exceedingly high and finally the structure fails. It has been observed in this study that fatigue crack propagation in the corrosive medium is strongly influenced by weld-geometry parameter (weld-toe undercut) which played a significant role in reducing the fatigue life and fatigue strength of butt-welded structures, and accelerated the time to propagate to failure. From the limited experiments performed in this study, it has been found that the threshold stress intensity factor of butt-welded structures attains a much lower value in corrosive environment than in air environment and its effect needs to be included in fatigue assessment. It is observed that a more rigorous mathematical model can be determined only when relevant important parameters (such as parameters relating to stress ratios, stress concentrations caused by weld-joint geometry variations, type and rate of loading, residual stresses and threshold properties of the material strength) are known.

Influence of martensite-austenite constituents on heat affected zone toughness of C-Mn steel welds

"Martensite-retained austenite" constituents play an important role on Heat Affected Zone (HAZ) toughness in the welding of structural steels. The influence of thermal cycles - especially in the case of multipass welding - and of steel composition on the presence of such constituents is discussed here. Fracture initiation mechanisms related to these constituents are also examined.

Interim fatigue design recommendations for fillet welded joints under complex loading

Current fatigue design methods for assessing welded steel structures under complex combined or multiaxial loading are known to be potentially unsafe. This has led to a number of research projects over the past 10 years. Some progress has been made in developing better methods, but they are not yet suitable for general design. This paper presents an interim solution based on a review and analysis of relevant published data; all referring to fatigue failure from a fillet weld toe. These indicate that Eurocode 3/IIW S–N curve FAT80/3 (negative inverse slope of 3) is suitable for combined normal and shear stresses acting in phase, and possibly for out‐of‐phase (i.e. non‐proportional loading) bending and shear if the shear stress is not due to torsion. However, a shallower curve FAT80/5 is necessary for out‐of‐phase torsion and bending or tension. Both curves are used in conjunction with the nominal maximum principal stress range occurring during the loading cycle.

Effect of Long-Time Postweld Heat Treatments on the Mechanical Properties of a Carbon-Manganese Pressure Vessel Steel

Postweld heat treatment is a common practice among building codes for welded steel structures and equipment to reduce the high as-welded residual stress level, improve the fracture toughness, and increase the critical size of acceptable defects. There are many discrepancies among international building codes for storage spheres, pressure vessels, and welded structure components about parameters such as duration and temperature for postweld heat treatments. Furthermore, the codes frequently omit the top number of thermal cycles, which the structure may support to maintain the mechanical properties and toughness in an acceptable level, keeping the physical integrity of the equipment. The present work analyzes the effect of duration of the postweld heat treatments on the mechanical properties and fracture toughness of a carbon-manganese steel of specification ASTM A-516 G 70, which frequently is used to build spheres and pressure vessels in the petrochemical industry. The regions corresponding to the base metal (BM) and heat-affected zone (HAZ) were studied. Through the results obtained from the tensile tests, hardness measurements, Charpy V-notch impact and crack-tip opening displacement (CTOD) testing, and microstructural characterization, it is concluded that the mechanical properties and fracture toughness were reduced by increasing the time of the postweld heat treatment. The degradation of the original properties of the steel is attributed to the changes that occurred in the microstructure. With the welding procedure used, it was verified that the fracture resistance of the HAZ was higher than that of the BM.

Column curves for stainless steel alloys

The paper describes a column curve formulation capable of producing accurate strength curves for cold-formed stainless steel columns. The formulation uses a non-linear expression for the imperfection parameter but is otherwise identical to the Perry–Robertson equation used in Eurocode3, Parts 1.1, 1.3 and 1.4. It is shown that several column curves are necessary for accurately describing the strength of austenitic and austenitic–ferritic stainless steel alloy columns and two curves are proposed. One of these is close to the curve for cold-formed sections currently used in the draft Eurocode3, Part 1.4. A new column curve is also proposed for ferritic alloys and 12% chromium weldable structural steels.

Le Peptide-C

Engineering critical assessment (ECA) or fitness-for-service (FFS) is a fracture mechanics based approach that is increasingly utilised for assessing the structural integrity of welded steel structures considering various degradation mechanisms which may occur during service. However, if sufficient number of inputs and/or accurate input values are not available, the ECA result is often considered overly conservative and for such cases adds potentially unnecessary obstacles to design and maintenance solutions High strength steels such as S690, S890 and S960 are increasingly used in the offshore and lifting applications, thus to enhance our confidence in using such steels, it is essential to develop better understanding into the relationship between the information from ECA, fracture toughness that can be currently achieved in industry and the required toughness from rules and regulations. This study explores the influence of different variables on critical fracture toughness requirement of S690 steel structures. In addition, the correlation between all the studied variables are investigated to provide quantitative indications of the most important variables in a static ECA process for high strength steel applications. Quantified sensitivity and correlations are obtained for the six studied variables. Industrial fracture toughness data of welded S690 with section thickness up to 160 mm are collected. This provides a snapshot of currently achievable fracture toughens of S690, in comparison with the ECA results and current rules and standards.

Cold cracking of structural steel weldments as reversible hydrogen embrittlement effect

The problem of hydrogen induced cold cracking (HICC) during welding is discussed in this paper. This form of cracking is seen as the special case of the reversible hydrogen embrittlement (HE) effect. The main hypotheses for the HE are briefly analyzed. It is shown that the difficulties in understanding of HICC are caused by the uncertainty of the physical factors of hydrogen action and by the absence of a clear knowledge of metal fracture micromechanisms. The new model of HE is described. It is suggested that the microcrack occurs in dislocation cluster by a microcleavage mechanism during deformation and propagates at the initial period analogous to the Griffith’s classic scheme. The hydrogen, transported by moving dislocations, is adsorbed by new crack surfaces in the form of negatively charged atoms and lowers the surface energy promoting crack propagation. Some experimental evidence of this model is presented.

Theoretical and experimental foundations of the creation of secondary-hardening weldable structural steels

Theoretical and experimental foundations of the creation of secondary-hardening weldable structural steels

Structural Integrity of Welded Steel Structures

Structural Integrity of Welded Steel Structures

Cost calculation and optimisation of welded steel structures

The cost optimisation is important in structural design. The welding times of various welding technologies are different. Using the costcomp program we can calculate the welding times. Adding other times, like flattening plates, surface preparation, cutting, electrode changing, deslagging, painting, etc. we can form a complete cost function. Times are usually general, but costs are different in various countries. Introducing the fabrication and material specific cost ratio of k f / k m , which varies between 0 and 2 kg/min, it is possible to build the cost function from the times and to work out optimisation in different economic conditions. The investigated structures are predominantly welded from plates. Examples are shown of applications for the design of welded box beams and stiffened plates. The fabrication cost percentages for welded box beam and stiffened plate are 29–35 and 46–71% of the total costs, respectively, thus they can have a significant effect on optimum dimensions. The discrete optima depend on the manufacturers, on the k f / k m ratio and the welding technologies.