A welded rectangular hollow section (RHS) X-joint exposed to tension loading has three typical fracture-related failure modes: Punching shear failure (PSF), Brace failure (BF), and Chord side wall failure (CSWF). Prediction of these failure modes by finite element (FE) simulations requires modelling of the material damage. An appropriate damage model accurately predicts the behaviour of the fracture zone and provides the necessary information to improve design rules for welded high-strength steel (HSS) RHS X-joints based on parametric studies using validated model. In this paper, the parameters of the Gurson-Tvergaard-Needleman (GTN) damage model are calibrated for the base material (BM) and the heat-affected zone (HAZ) of butt-welded cold-formed RHS connections, no fracture appeared in the weld. A computational homogenisation analysis is carried out using representative volume element (RVE) models to calibrate the pressure-dependent yield surface parameters of the GTN damage model, considering the different combinations of the accumulated initial hardening strain and the void volume fraction (VVF) due to a varying stress triaxiality. The critical and final VVFs are calibrated against tensile coupon tests. Finally, the GTN damage models calibrated for BM and HAZ are used in the fracture simulation of nine welded cold-formed RHS X-joints in monotonic tension. The FE model successfully predicts the experimental load-displacement relationships and fractured zone, indicating the calibrated GTN models could effectively be used in parametric study of welded cold-formed RHS X-joints. Finally, possible improvements to the used FE model are outlined for future studies.@en

The heat-affected zone (HAZ) is an unmelted region of a welded joint that has changed in material properties because of high temperatures during the welding process. HAZ has a lower strength than the base (parent) material (BM) and the weld metal (WM). The lower material strength is more significant if BM is made of high strength steel and the undermatching filler metal is used. Therefore, the constitutive model of HAZ is essential for predicting the mechanical behaviour of the welded joint. In this paper, a method for determining the true stress–strain relationship of HAZ is proposed. The effect of the transverse constraint on the longitudinal deformation of HAZ imposed by BM and/or WM is eliminated by a linear modification factor correlating to the true strain. Standard tensile coupon tests were used to obtain the constitutive model of HAZ using digital image correlation (DIC). The modification factor proposed for reducing the true stress is calibrated based on finite element analysis (FEA). The modified constitutive model of HAZ is validated against the experimental results obtained by DIC. The validated HAZ material property can be used in advanced numerical simulation of welded joints.@en

In traditional end plate column splices, bolts are placed double symmetrically on the four sides of square hollow sections (SHS). In order to reduce the required gap between the façade and the column, the end plate could be flushed on one or two sides of SHS for the column along the façade or at the corner of a building, respectively. However, the analytical solution (Component method) for the traditional column splice is not applicable in this case. This paper addresses the tensile behaviour of asymmetric column splices, where a cover plate is used on the end plate flushed side. Columns are dominantly loaded in compression and bending, but to verify the component's interaction, the column splices are tested in tension in this paper. The tensile behaviour is investigated through the experiment, the finite element (FE) analysis, and the component method. Eight tensile tests were conducted. The FE model is validated against the experiment. A bi-linear model is employed to characterise the column splice yield resistance, which shows a good agreement with the ultimate resistance of the FE model using a constitutive model without strain hardening. The effective length measured from the FE model is approximately two times that calculated by equations. Using the measured effective length, the component method predicts the characterised yield resistance well (average 13% lower). In comparison, the resistance is underestimated by 35% on average if the calculated effective length is used.@en

The behaviour of square steel tube confined reinforced concrete columns after fire exposure was studied experimentally and numerically in this paper. Eighteen stub columns were first heated following the ISO 834 standard fire including both heating and cooling phases, and were subsequently loaded to failure after cooling to ambient temperature. Failure modes, temperatures in specimens, axial load versus deformation curves and strains in steel tube were monitored and discussed. A finite element model was developed using the sequentially coupled thermal-stress analysis method and was validated against tests found in literatures and this study. Parametric study was performed to identify influences of key parameters, where are heating time, cross-sectional dimension, strengths of materials, steel tube to concrete area ratio and reinforcement ratio, on residual capacity and compressive stiffness. Finally, a simplified method is proposed for predicting residual cross-sectional capacity and compressive stiffness of square steel tube confined reinforced concrete columns after fire exposure.@en

The coupon specimen with a transverse butt weld in the middle could be used for determining local constitutive properties of the heat-affected zone (HAZ) and the weld metal (WM) based on the digital image correlation (DIC). However, limited research is reported to demonstrate how to identify the boundary of each region in DIC results. Accordingly, it is difficult to determine the adequate gauge length for measuring the strain of each region and establishing a stress–strain curve for Finite Element Analysis (FEA) of structural problems. In this paper, a method for identifying the region's boundary is proposed based on coupon tests using three steel grades, S355, S500, and S700, corresponding to three weld matching types, match, overmatch, and undermatch, respectively. First, the hardness and the microstructure investigation were conducted to determine the region's boundary. Then, the boundary was identified based on the DIC result using the proposed method. Finally, the identified HAZ regions were verified against hardness results. Using the proposed method, the gauge length for measuring the strain of a single region, such as HAZ and WM, are determined based on the DIC results. The primary purpose is to establish the stress–strain relationship for FEA of welded joints.@en

This paper presents an experimental investigation on the dynamic mechanical performance of S30408 austenitic stainless steel (ASS) under elevated temperatures, which is essential for determining the behaviour of structures made with this type of steel subjected to the coupled fire and impact/explosion. For this purpose, the quasi-static and dynamic compression tests using Split Hopkinson Pressure Bar (SHPB) were conducted under temperatures of 20–600 °C and strain rates from 0.001 to 3000 s−1. In addition, the corresponding microstructures of tested samples were observed. The stress–strain responses, strain rate and temperature effects as well as the microstructural evolutions were analyzed. Test results show that the stress–strain responses are sensitive to the strain rate and temperature. The strain-rate sensitivity coefficient increases as the strain rate and temperature rise. The microstructural observation reveals that the grain dimension declines with an increment of strain rate or a decreasing temperature. Finally, the dynamic compressive stress–strain models for S30408 ASS under 20–600 °C were suggested on the basis of the Johnson-Cook (J-C) model and have been proved to give a reasonable prediction.@en

A welded connection consists of three main material zones, the base material (BM), the heat-affect zone (HAZ), and the weld metal (WM). The strength of HAZ depends on the BM grade and manufacturing process, electrode grade, and welding parameters. Under certain conditions, HAZ has the lowest material strength, especially for high-strength steel. Therefore, a semi-empirical methodology is proposed to establish a constitutive model of HAZ necessary for predicting the fracture position of welded connections. This methodology is based on an engineering approach to consider HAZ as an isotropic and homogeneous material, with no consideration of different volumetric fractions of microstructures within a HAZ. The equivalent material properties of HAZ in butt-welded hollow section connections were investigated experimentally and numerically. Hardness tests and microstructure investigations were conducted to determine the boundaries of material variations and the width of HAZ. The stress–strain relationship of HAZ was established and calibrated based on tensile coupon tests and finite element analyses. Using the calibrated HAZ stress–strain relationship, the effect of transverse constraint imposed by the adjacent and stronger material (BM and WM) on HAZ was evaluated in the welded connections. Finally, the new methodology of a semi-empirical constitutive model based on the Swift model was used to propose equivalent characteristics of HAZ as a function of the mechanical properties of BM for a specific welding procedure considered in the project.@en

This paper reports an investigation on the tensile resistance of fully-penetrated butt-welded connections of (ultra-)high-strength steels considering the strength softening of the heat-affected zone (HAZ) and the strengthening due to the transverse constraint. Firstly, a parametric study is carried out to analyse the tensile behaviour of fully-penetrated butt-welded S700 connections using validated finite element (FE) models. Based on the FE results, the effect of the investigated parameters on the ultimate resistance of the connections is quantified. Consequently, a Macro prediction model is proposed, neglecting the detail of HAZ inhomogeneous microstructures but considering five design parameters, which are the width and thickness of the specimen, the width of HAZ, the softening ratio, and the matching ratio. Finally, the proposed Macro model is validated against the available experimental data of fully-penetrated butt-welded (ultra-)high-strength steel connections in literature. The results show that the Macro model is proven successful in accurately quantifying the effect of HAZ, concerning the strength softening and the transverse constraint strengthening, on the tensile resistance of fully-penetrated butt-welded (ultra-)high-strength steel connections. It is also found that 157 and 96 out of 185 experiments are predicted within a 5% deviation range using the Macro model and prEN 1993-1-8:2021, respectively, proving that the Macro model is more consistent with the experimental data compared to the prediction of prEN 1993-1-8:2021.@en

The use of high strength steel (HSS) cold-formed hollow section is of significant interest in the construction sector due to competitive costs. The accurate simulation of ductile fracture failure at the micro and macro scale is critical for improving the validity of predicting structural behaviour of HSS cold-formed hollow sections and welded joints. In this study, the ductile failure of the cold-formed S700 material is studied using the Gurson-Tvergaard-Needleman (GTN) damage model. Representative volume element (RVE) models with the void volume fraction (VVF) between 0.1% and 30% are employed to investigate the pressure dependency of the deviatoric limit stress. Different load conditions corresponding to different stress triaxiality levels are applied to unit cells with random spherical pores. The inelastic response of the unit cells are analysed, and the parameters q1, q2, and q3 in the GTN yield surface are calibrated. An equation is proposed to determine parameter q1 for different VVFs. The parameters critical VVF fc and final VVF ff are calibrated by the coupon test from the literature. Finally, the calibrated GTN model is validated against notched coupon tests. The finite element (FE) results show good agreement with the experimental results, indicating that the identified GTN model could efficiently predict the behaviour of the cold-formed HSS.@en

High-strength steel (HSS) has higher strength but lower ductility than mild steel. The cross-section of the structural members may be reduced using HSS instead of mild steel, provided the buckling of elements does not govern the failure. The reduced member size benefits the environment and economy by means of less energy consumption, less carbon dioxide emission, and less labour work during the fabrication and structure construction stages. The current design rules in prEN 1993-1-8 for welded hollow section joints are developed based on extensive experimental and numerical studies on joints made of mild steel (S235 and S355). A material factor Cf is stipulated to reduce the design resistance of the joint given the lower ductility of HSS than mild steel. In addition, the design yield strength of the material should be lower than 0.8 times the ultimate strength (fu) to calculate the resistance of punching shear failure and tension brace failure. However, these two strength restrictions are proposed based on limited experimental and numerical investigations on welded HSS tubular joints. The mechanical background behind the two restrictions is vague. Applying both Cf and the 0.8fu restriction would eliminate the benefits of using HSS, reducing the competitiveness in the market. Besides, the heat-affected zone (HAZ) often has the lowest strength in a weld region. The strength difference between HAZ and the base material (BM) is more significant for HSS than mild steel, indicating that HAZ plays a more critical role in welded HSS joints. Hence, the HAZ constitutive model should be considered in the numerical study of welded HSS joints in order to predict the load-deformation relationship and failure mode correctly. This dissertation proposes a systematic approach to include HAZ in the finite element (FE) analysis of welded joints considering ductile failure mode. First, the mechanical and geometrical properties of HAZ were obtained from tensile tests on the milled welded coupon specimen, the low-force Vickers hardness test, and the microstructure observation. The full-field deformation of the milled welded coupon specimen was measured using the digital image correlation (DIC) technique. Using the DIC result, a method is proposed to identify the boundaries of different regions in the milled welded coupon specimen. The identified boundary matches the hardness result well. Based on the identified boundaries, the width of HAZ and the weld metal (WM) are determined, which provides geometric information for the measuring range of the virtual extensometer in DIC and for creating the FE model with different partitions (HAZ, BM, and WM). Due to the transverse constraint imposed by BM and WM, HAZ was under a biaxial or triaxial stress state during the tensile coupon test. The measured stress of HAZ is higher than that under the uniaxial stress state at a given strain. Hence, a method is proposed to correct the measured stress-strain relationship of HAZ. The modified stress-strain relationship is successfully validated against the tensile coupon test regarding the load-deformation relationship and the strain distribution on the specimen surface. In order to accurately predict the load-deformation relationship and failure mode of welded joints, the Gurson-Tvergaard-Needleman (GTN) damage model is employed to simulate the failure of HAZ and BM. A computational homogenization analysis using representative volume element models was carried out to calibrate the yield-surface-related parameters (q1, q2, and q3). The effect of the hydrostatic pressure, the accumulated initial hardening strain, and the void volume fraction (VVF) on the yield surface were evaluated. An equation is proposed to describe the relationship between VVF and q1 value with a constant q2. The fracture-related parameters (fc and ff) were calibrated against the tensile coupon test. In addition, as the procedures for modifying the constitutive model and calibrating the damage model are rather complicated, a semi-empirical material damage model for HAZ correlating to the mechanical properties of BM is proposed to facilitate the FE analysis of welded joints. Monotonic tensile tests were conducted on 18 welded cold-formed rectangular hollow section (RHS) X-joints made of S355, S500, and S700 to investigate the validity of Cf and the 0.8fu restriction. The test result shows that a conservative resistance is predicted using the current design rules without applying Cf and the 0.8fu restriction. The calibrated GTN damage model for HAZ and BM was implemented in the fracture simulation of welded X-joints. The FE results agree well with the experimental results concerning the load-deformation relationship and the failure mode. Based on the validated X-joint FE model, the importance of including HAZ in the FE model was revealed by the FE analysis without the HAZ constitutive model. Finally, the semi-empirical material damage model for HAZ was employed to predict the tensile behaviour of all 18 welded X-joints. @en

Bolted end plate connections are traditional solutions for splice joints with square hollow sections (SHS). A cover plate could be used on one side or two adjacent sides of SHS where the end plate is flushed, resulting in two types of asymmetric splice joints. The advantage of the asymmetric joints is that no space is required between the façade panel and the member surface whilst the easy assembly feature remains. The component method is used to design the bolted end plate connection, as stipulated in prEN 1993–1–8. However, the design rule is only available for connections with open cross-sections. In this paper, a modified component method for predicting the tensile resistance (end plate fails with failure mode 1 according to prEN 1993–1–8) and stiffness of the asymmetric bolted SHS end plate splice joints is proposed. First, a parametric study is carried out to develop the component method for the traditional symmetric bolted SHS end plate splices based on the validated finite element (FE) model. Next, a simplified two-dimensional FE model is employed to evaluate the effect of the asymmetric feature on the stiffness of the joint. Finally, a parametric study is carried out on two types of asymmetric joints to validate the interaction model of individual components. The resistance and stiffness of the asymmetric end plate splice joints are well predicted using the modified component method.@en

The newest version of prEN 1993-1-8 (2022) prescribes a material factor (Cf) to reduce the design resistance of welded joints made of high strength steel (HSS) mostly due to the lack of available experiments, given the less ductility of HSS compared to mild steel. Additionally, it is stated that the material design yield strength should not exceed 0.8 times the ultimate strength (fu) for the chord punching shear failure and the tensile brace failure. The mechanical background behind Cf and the 0.8fu restriction for different types of joints and loading conditions is vague. In this paper, the validity of Cf and the 0.8fu restriction is investigated experimentally by considering 18 welded rectangular hollow section X-joints tested in tension. A bi-linear model, which is suitable for an elasto-plastic global analysis considering the post-yielding stiffness, is proposed to characterize the nonlinear behaviour of the joint. The predicted resistance and failure mode, with and without considering the Cf and/or the 0.8fu restriction, are compared to the experimental results. In addition, the predicted resistance corresponding to the experimental failure mode is investigated. It is concluded, based on the tested joints in this paper and literature, that Cf and the 0.8fu restriction are not necessary for the design according to prEN1993-1-8. However, the predicted brace failure resistance is unconservative for tested joints that failed by brace failure if Cf for S700 or the 0.8fu restriction for all steel grades is not considered.@en

Welded joints are wildly used in the construction sector for fabrication of steel and aluminium structures. A welded joint is traditionally divided into three regions: The Base Material (BM), the Heat-Affected Zone (HAZ), and the Weld Material (WM). The mechanical behaviour of each region varies depending on properties of BM, FM and welding parameters. In general, HAZ has a lower material strength compared to BM and WM. The material strength difference is even more significant if BM is made of High Strength Steel (HSS) and welded by using undermatching electrodes. Therefore, it is essential to obtain the constitutive model of HAZ to accurately predict the behaviour (strength, stiffness, and ductility) of the HSS welded joint. In this paper, milled coupon specimens with a transverse butt weld in the middle are used for obtaining the original stress-strain relationship of HAZ and WM based on Digital Image Correlation (DIC) measurements. The original and the modified HAZ constitutive model are validated against the milled and unmilled coupon specimens by Finite Element Analysis (FEA). Comparing the FEA and experimental results, it can be concluded that the modified HAZ constitutive model is successfully validated. Finally, the tensile behaviour of the butt-welded square hollow section is investigated through FEA. It is found that the peak deformation would be significantly overestimated if the modified HAZ constitutive model is not used.@en