Types Of Welded Joints Research Articles

09.05: Remaining fatigue life of steel railway bridges: Improved models for considering realistic behaviour

ABSTRACTMany steel railway bridges will reach the end of their scheduled design service life within the next years. One can also observe some bridges, considerably younger than their scheduled design service life, with insufficient remaining fatigue life and rarely cracks in structural elements. Mostly because of the early applications of the welding technology and sometimes insufficient consideration of fatigue effects, e.g. neglecting of high local secondary stresses. It is a great challenge and very expensive, if a couple of bridges have to be replaced in a short period. Therefore, it is important to have models for realistic fatigue verification to extend the service life.The focus of this paper is to show different approaches – both on the load and the resistance side – to verify an extended service life of steel railway bridges. For example, the big potential of the use of real, measured stress spectra is presented, instead of the common approach according to the Eurocode. In addition, the application of the consequent Miner's rule results in another significant reduction of equivalent stress ranges for application of the fatigue strength curves of the Eurocode.Furthermore, the simplified and conservative assumptions regarding the fatigue resistance in conventional assessment procedures according to the Eurocode (nominal stress approach) are compared with alternative concepts for typical welded joints. For example, the geometric (hot spot) stress concept, the effective notch stress concept and fracture mechanics. The Eurocode already allows the use of the geometric stress concept, but gives little guidance. The practical implementation of these concepts is shown for a representative detail of a steel bridge, considering a variety of geometries.Fracture mechanics approaches are state of the art in mechanical engineering, but still relatively uncommon in civil engineering. These concepts allow for the explicit consideration of real or assumed “non‐detected” flaws or cracks in the component, thereby eliminating the need of an exact knowledge of the past load history, up to the considered defect size. It is shown that the fracture mechanics concept is suitable for steel bridges to determine the remaining fatigue life of critical details.Finally, a multilevel framework is presented to give guidance for engineers to evaluate the service life of a specific steel railway bridge.

Fatigue life evaluation of welded joints considering the local multiaxial stress/strain states and numerically determined residual stresses/strains

This paper puts forward a kind of calculation method of estimating fatigue crack initiation life by considering welding residual stresses as initial stresses. The numerical method could be used to quantitatively analyze the influence of residual stresses on the cumulative fatigue damage. In order to gain the distribution of weld-induced residual stresses, the FE analyses as well as measurements were carried out. Based on critical plane approach, analysis of damage parameters was performed considering both welding residual stress and biaxial loads. Subsequently, validity of the proposed numerical method is verified by analyzing fatigue test results for several typical welded joints. It is concluded that the result of the present method is closer to the experimental value than that obtained with empirical formula method in the estimation of fatigue crack initiation life of welded structures.

Structure and Mechanical Properties of Welded Joints for Nuclear Power Plants of Type MIR 1200

The paper deals with research, development and verification of production technology of selected welded joints for pressure vessels of primary circuits of nuclear power plants of type MIR 1200. Effect of various welding technology including simulation heat treatment on mechanical and fracture properties have been studied. Four type of homogenous 10GN2MFA – 10GN2MFA type of welded joints have been prepared for experimental programme. Conventional mechanical properties (tensile and impact test) as well as unconventional mechanical properties (fracture mechanics, low-cycle fatigue and stress corrosion cracking in water environment) have been studied. Effect of elevated working temperature on structure and material properties has been evaluated. Temperature dependencies of shear fracture have been plotted and effect of welding procedure on transition temperature shift has been evaluated. Experimental data have been compared with numerical simulation using FEM.

Comparison of the microstructure and mechanical performance of 2A97 Al-Li alloy joints between autogenous and non-autogenous laser welding

2A97 Al-Li joints were achieved by using a fibre laser welding with filler wire (non-autogenous) and without filler wire (autogenous), of which the microstructures, mechanical properties and underlying relationships were examined. The results of microhardness tests showed that the fusion zones (FZ) in both types of joints were relatively soft. The width of the softened zone and the degree of softening with autogenous laser welding (ALW) was approximately 1/3 and 3/4 those with non-autogenous laser welding (NLW), respectively. The main reason for the softening was that the alloying elements were more concentrated in the grain boundary but non-uniformly distributed. Both types of welded joints fractured in the FZ during tensile processes, and the tensile strengths of the NLW joint decreased about 17% that of the ALW joint, while the elongation increased about 30%. By using high-speed imaging in low-cycle fatigue tests, it was found that cracks originated near the pores and slowly propagated in the NLW joint, whereas cracks first appeared in the FZ, and then rapidly propagated to the EQZ in the ALW joint. Satisfactory strength and plasticity in NLW joints can be expected by careful selection of the welding parameters and the alloy elements in the filler wire.

Determination of dimensionless stress intensity factor of plate-to-plate butt welds between axially aligned members of different thickness

British Standard 7910:2013 (BS 7910:2013) includes a set of equations for estimating the effect of misalignment on different types of welded joints with a semi-elliptical surface crack. In this study, extensive finite element (FE) analyses are carried out to determine the dimensionless stress intensity factor (Y) of plate-to-plate butt-welded joints between axially aligned members of different thickness. The Y values at the crack deepest points and the crack ends are determined. It is observed that BS 7910:2013 underestimates the Y results of misaligned butt welds for shallow cracks with percentage difference in values being as high as 56%. An overestimation percentage difference of 26% for deep cracks is given by BS 7910:2013. The new Y equations for misaligned butt welds are proposed using multiple regression analyses.

Application of Hybrid Laser arc Welding for the Joining of Large Offshore Steel Foundations

To reduce the costs of the fabrication of offshore wind turbine foundations it is necessary to investigate new fabrication technologies. Hybrid laser arc welding is a potentially well-suited process for this because it requires less groove preparation to achieve deep weld penetration and lower heat input, compared to traditional arc welding. A skirt section of a suction bucket in 16mm steel was used as a case to investigate the hybrid laser-arc welding in order to demonstrate which types of weld and which weld positions are possible. Three types of weld joints were chosen and welded with different welding positions; a butt joint of a bended section, a butt joint of a flat section and a lap joint. Stable welds with sufficient penetration were achieved for the flat welding position of the butt joint of bended section and butt joint of flat section.

Welding distortion prediction with elastic FE analysis and mitigation practice in fabrication of cantilever beam component of jack-up drilling rig

How to predict and mitigate the welding distortion in fabrication of large offshore structures such as jack-up drilling rig, is always an essential engineering problem. In general, welding distortion can be estimated beforehand with experimental measurement of mock-up or numerical computation, and then practical techniques will be carried out to correct the possible welding distortion in the actual fabrication. In this paper, the possible welding distortion during the fabrication of I section welded structure in cantilever beam of jack-up drilling rig was studied with prediction and mitigation. First, an elastic FE analysis was employed, in which (a) the whole examined welded structure was meshed by coarse shell elements; (b) welding inherent deformations of typical welded joints were evaluated with thermal elastic-plastic FE analysis, and will be applied to the welding lines as load; (c) interface element was used to represent the fitting procedure before full welding. In order to eliminate these predicted welding distortions, some mitigation practices such as application of inverse distortion, optimization of welding sequence and constraint methods, were conducted and examined. In particular, welding sequence will intensively influence the final dimensional accuracy via straightening in fitting procedure and updated stiffness of current main welded structure.

Effect of Stress Relief Annealing on Microstructure & Mechanical Properties of Welded Joints Between Low Alloy Carbon Steel and Stainless Steel

Two types of welded joints were prepared using low alloy carbon steel and austenitic stainless steel as base materials. In one variety, buttering material and weld metal were Inconel 82. In another type, buttering material and weld metal were Inconel 182. In case of Inconel 82, method of welding was GTAW. For Inconel 182, welding was done by SMAW technique. For one set of each joints after buttering, stress relief annealing was done at ~923 K (650 °C) for 90 minutes before further joining with weld metal. Microstructural investigation and sub-size in situ tensile testing in scanning electron microscope were carried out for buttered–welded and buttered–stress relieved–welded specimens. Adjacent to fusion boundary, heat-affected zone of low alloy steel consisted of ferrite–pearlite phase combination. Immediately after fusion boundary in low alloy steel side, there was increase in matrix grain size. Same trend was observed in the region of austenitic stainless steel that was close to fusion boundary between weld metal-stainless steel. Close to interface between low alloy steel-buttering material, the region contained martensite, Type-I boundary and Type-II boundary. Peak hardness was obtained close to fusion boundary between low alloy steel and buttering material. In this respect, a minimum hardness was observed within buttering material. The peak hardness was shifted toward buttering material after stress relief annealing. During tensile testing no deformation occurred within low alloy steel and failure was completely through buttering material. Crack initiated near fusion boundary between low alloy steel-buttering material for welded specimens and the same shifted away from fusion boundary for stress relieved annealed specimens. This observation was at par with the characteristics of microhardness profile. In as welded condition, joints fabricated with Inconel 82 exhibited superior bond strength than the weld produced with Inconel 182. Stress relief annealing reduced the strength of transition joints and the reduction was maximum for specimen welded with Inconel 82.

New weld toe magnification factors for semi‐elliptical cracks in plate‐to‐plate butt‐welded joints

AbstractBritish Standard 7910:2013 includes a single set of equations for estimating the weld toe magnification factor (Mk) of different types of welded joints with a semi‐elliptical surface crack. In this study, extensive finite element analyses are performed to determine the Mk of plate‐to‐plate butt‐welded joints subjected to axial and bending loading. The Mk values at the crack deepest points and the crack ends are determined. It is found that the Mk results of butt welds are different with those of single T‐butt joints with percentage difference in values being as high as 63.8% and 97.4% for axial and bending loading cases, respectively. Percentage differences of 60.4% and 358.2% for axial and bending loading cases are observed between butt welds and X‐joints. New Mk equations for butt welds are proposed using multiple regression analyses.

Fatigue tests of notched specimens made from butt joints at steel

AbstractThe weld toe as well as the weld root of joints acts as a geometrical notch, which decreases the fatigue strength of welded components. Local approaches used for fatigue assessment account for the local stress concentration when referring to the notch stress as a fatigue parameter. This applies also to the approaches based on the notch stress intensity factor like, for example, the averaged strain energy density, neglecting the actual notch radius and considering a sharp notch as a simplification. A uniform S‐N curve valid for different types of welded joints and failure locations was derived from re‐analyses of fatigue test results as documented in literature. The fatigue tests described in this paper aimed at validating that energy‐based S‐N curve by dedicated tests on artificially notched specimens. At first, four parameters were investigated in order to estimate their influence on the fatigue strength and to select appropriate notch geometries for the final step of the test campaign. The advantages of these tests are that both the exact notch geometry and the local stress range at the notch, including misalignment effects, were identified and considered in experimental data analysis. This paper presents the results of the rather comprehensive testing activities and comparisons with the design‐S‐N curve mentioned, yielding unexpected fatigue behaviour. This can be explained by the short crack propagation life.

Validation of the weakest link approach and the proposed Weibull based probability distribution of failure for fatigue design of steel welded joints

Abstract The paper describes validation process of a proposed application of the weakest link concept to fatigue life evaluation of steel welded joints. The weakest link concept was used to evaluate failure probability and the resulting fatigue lives of typical welded joints under the constant amplitude uniaxial loading. The validation of this approach involved comparison between the modeling results and experimental results. Investigation concerned butt- and double T-joints manufactured from three steel grades, the latter of which being of two thicknesses. Two assumptions for the approach have been tested: (i) applicability to joints of different geometry and (ii) ability to predict the effect of decreased fatigue life in joints with higher plate thickness. The results revealed appreciable effect of geometry on cumulative distribution function of failure, changing the kurtosis of failure distribution.

Fatigue behavior and strength evaluation of vertical stiffener welded joint in orthotropic steel decks

Abstract This study focuses on the fatigue strength and crack behaviors at weld toe of vertical stiffener and deck in orthotropic steel decks. A total of 20 specimens have been carried out in fatigue tests in relation to the two types of welded joints and diverse stress conditions. The crack initiation and propagation behaviors were comparative analyzed based on four stages of cracking process. In addition, the geometry properties of welded joints were measured in cross sections, and their effect on stress concentration factors was investigated by test and FE analysis. Based on these, the stress concentration factors were calculated and compared by hot spot stress method. Finally, the fatigue strength of this structural detail had been investigated by comparing the fatigue test results and the corresponding regression S N curves. Fatigue strength of partial penetration welded joint is slightly larger than full penetration welded joint before toe crack initiation. However, based on the crack behaviors in the propagation process on the whole, it is acceptable to evaluate these two kinds of welded joint by the same S N curve in practical engineering application.Using the same S N curve JSSC-G (50 MPa) for the allowable fatigue strength evaluation of this structural detail in China was recommended.

A method of determining structural stress for fatigue strength evaluation of welded joints based on notch stress strength theory

This paper presents a new method to evaluate the structural stress in welded joints. The method is based on notch stress strength theory, 1mm stress method and the ‘equivalent zero point’ stress method. According to notch stress strength theory, a simple method with the singularity strength ‘as’ is used to estimate the stress field distribution at the corner. Moreover, a modified methodology for fatigue strength assessment, has been developed, starting from a cruciform model, and then further developed to realistic ship structural components. Validity of this method is further verified by analyzing the stress results for several typical welded joints reported in literature. The results show that the proposed method has the additional advantage in simplified fatigue strength estimation for welded joints and it is able to account for the thickness effect observed in welded joints.

Review of the fatigue strength of welded joints based on the notch stress intensity factor and SED approaches

Several approaches exist for the fatigue strength assessment of welded joints. In addition to the traditional nominal stress approach, various approaches were developed using a local stress as fatigue parameter. In recent times, the N-SIF based approaches using the notch stress intensity at the weld toe or root have been developed. Based on this, the more practical strain energy density (SED) and the Peak Stress approaches were proposed. This paper reviews the proposed design S–N curves of the N-SIF and SED approaches questioning in particular the consideration of misalignment effects, which should be included on the load side of local approaches in order to consider them individually in different types of welded joints. A re-analysis of fatigue tests evaluated for the effective notch stress approach leads to slight changes of the design S–N curves and of the radius of the control volume used for averaging the SED at the notches. Further, on purpose fatigue tests of artificially notched specimens show that the fatigue assessment using a single-point fatigue parameter might be problematic because the crack propagation phase, being part of the fatigue life, is strongly affected by the stress distribution along the crack path that may vary considerably between different geometries and loading cases.

Novel hot spot stress calculations for welded joints using 3D solid finite elements

This paper presents a novel numerical method through which less mesh-sensitive local stress calculations can be achieved based on the 3D solid finite element, adhering strictly to the original definition of hot spot stress. The traction stress, which is defined at 0.5t and 1.5t away from the weld toe, was calculated using either a force equivalent or work equivalent approach, both of which are based on the internal nodal forces on the imaginary cut planes. To confirm the validity of the proposed method, 5 typical welded joints widely used in ships and offshore structures were analyzed using 5 different solid element types and 4 different mesh sizes. Finally, the methodology was applied to the more complicated pontoon-column connection part of a semi-submersible RIG under a realistic wave load, and the performance of the proposed method was compared with the traditionally used surface stress extrapolation method.

New weld toe magnification factors for semi-elliptical cracks in double-sided T-butt joints and cruciform X-joints

British Standard BS7910:2005 has a single set of equations for estimating the weld toe magnification factor (Mk) of different types of welded joints containing a surface crack. The equations are based on the finite element (FE) results of single sided T-butt joint. In this study, extensive FE analyses are carried out to determine the Mk of double-sided T-butt joint and cruciform X-joint. Both the joints are subjected to axial and bending loading, and Mk at crack ends and deepest points of the surface crack are determined. It is found that Mk results for cruciform X-joint differ with those of double-sided T-butt joint with percentage difference in values being as high as 136% and 76% for axial and bending loading cases, respectively. Finally, a new set of Mk equations for cracked double-sided T-butt joint and X-joint are proposed based on multiple regression analyses.

3차원 솔리드 요소를 이용한 용접부 핫스팟 응력 계산에 대한 연구

Because of the high stress concentration near the toe of a welded joint, the calculation of local stress using the finite element method which is relevant to the fatigue strength of the weld toe crack, is a challenging task. This is mainly caused by the sensitivity of finite element analysis, which usually occurs near the area of a dramatically changing stress field. This paper presents a novel numerical method through which a less mesh-sensitive local stress calculation can be achieved based on the 3D solid finite element, strictly sticking to the original definition of hot spot stress. In order to achieve the goal, a traction stress, defined at 0.5t and 1.5t away from the weld toe, was calculated using either a force-equivalent or work-equivalent approach, both of which are based on the internal nodal forces on the imaginary cut planes. In the force-equivalent approach, the traction stress on the imaginary cut plane was calculated using the simple force and moment equilibrium, whereas the equivalence of the work done by both the nodal forces and linearized traction stress was employed in the work-equivalent approach. In order to confirm the validity of the proposed method, five typical welded joints widely used in ships and offshore structures were analyzed using five different solid element types and four different mesh sizes. Finally, the performance of the proposed method was compared with that of the traditionally used surface stress extrapolation method. It turned out that the sensitivity of the hot spot stress for the analyzed typical welded joints obtained from the proposed method outperformed the traditional extrapolation method by far.

Optimal design of welded structure with structural and local-notch stresses based on International Institute of Welding recommendations

A procedure for optimally designing light-weight and reliable welded structures is proposed and applied to the structural design of an illustrative welded structure used in construction and transportation machinery. To verify the effectiveness of the design procedure, the designed structures were experimentally produced, and then the fatigue lives of them were evaluated. The proposed design procedure is composed of two major processes; one is thickness-design process, and the other one is the process for designing weld type. Since the design for thickness and weld type is conducted sequentially, the thickness of welded structures can be determined in an optimization process without considering weld type; hence, the proposed design procedure is not computational demanding. In the thickness-design process, the thickness of a target welded structure is optimized by constraining structural stress that is estimated on the basis of the International Institute of Welding recommendations. After the thickness optimization, weld type of welded joints is designed by using local-notch stress, which is also defined in the IIW recommendations. Two kinds of test pieces based on the designed welded structure were experimentally produced by changing the weld type of a joint, and the test pieces were fatigue tested to verify their fatigue resistance. Since the produced welded structures had expected fatigue resistance, it can be concluded that welded structures can be optimally designed by using the proposed design procedure.

Fatigue strength assessment of Invar alloy weld joints using the notch stress approach

The aim of this study investigated the fatigue strength of Invar alloy weld joints. Invar steel (Fe-Ni 36%) is widely used in the primary and secondary barriers of membrane-type liquified natural gas (LNG) containment vessels. The fatigue test was carried out for two different types of welded joints with raised edge specimens and with overlap joint specimens based on the nominal and notch stress approaches. The thickness of the Invar plate is less than 1.5mm, so the notch stress approach with rref=0.05mm was applied. Our evaluation of the results in terms of the FAT value and the slopes of the design curves are compared with steel, aluminum, and magnesium weld joints in accordance with International Institute of Welding (IIW) recommendations.

Fracture Assessment of Cracked Welded Structures Considering the Heterogeneity of Welded Joints

The crack-like defects often cause the failure of welded structures. Most of the fracture assessment procedures that are used for solving this problem are based on deterministic approaches, for example Zerbst (2007). But the experimental results show that the mechanical properties and fracture toughness of welded joints are significantly changing with a random variation. Taking this into account the fracture assessment of welded structures requires improved numerical approaches. A numerical model of heterogeneous welded joint with randomly varying yield strength was proposed. The results show that the mechanical heterogeneity leads to higher J-integral values for the defects. This implies that neglecting the mechanical heterogeneity of welded joints results in underrating of the risk of failure of welded structures. The proposed numerical estimation of the fracture toughness is used for some typical welded joints and thin-walled pressure vessel. The relations between the variation of yield strength in cracked welded joints and J-integral are presented as the correction functions for calculated parameter of crack resistance.