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.

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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

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.

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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.

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