This study systematically investigates the mechanical properties and fracture toughness of Q355 steel under varying constraint effects and low-temperature conditions. Tensile tests conducted at room and low temperatures reveal that increasing yield and tensile strengths but decreasing ductility with temperature reduction. Charpy impact tests demonstrate a pronounced ductile-to-brittle transition, with the transition temperature determined as-34.0 degrees C through Boltzmann function fitting. Fracture toughness tests conducted under varying constraints and temperatures were used to analyze the effects of specimen thickness (25 mm and 12.5 mm), crack length (25 mm and 18 mm), and temperature (23 degrees C to-100 degrees C) on the resistance curves and characteristic values. The results indicate that in-plane constraint significantly influences fracture toughness more than out-of-plane constraint. The 12.5 mm specimens effectively meet plane strain fracture toughness evaluation criteria, and the resistance curves exhibit low sensitivity to thickness variation. Temperature-dependent fracture toughness tests show Q355 steel transitions from ductile fracture to brittle fracture modes. The ductile-to-brittle transition temperatures were identified as-73.4 degrees C (based onJ1C and S1C) and-52.7 degrees C (based onJmax and Smax), highlighting the impact of testing methodology and toughness parameter selection on fracture mechanism assessment. Above-70 degrees C, Q355 steel exhibits excellent low-temperature crack growth resistance and fracture toughness. Additionally, fracture toughness test results under varying constraints and temperatures show that the choice of test temperature range significantly influences the reference temperature T0 evaluation. Reduction in specimen thickness and crack length both lead to a decrease in T0, with out-of-plane constraint having a greater effect on T0 than in-plane constraint.

This study investigates the low-temperature fracture behavior of Q355 steel welded joints in the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) over a testing temperature range from 23 degrees C to -100 degrees C. Vnotch Charpy impact tests and Single-Edge Notched Bend (SENB) fracture toughness tests under different constraint conditions were conducted to obtain impact energy (KV2), J-R curves, and characteristic values (J1C and Jmax). The reference temperature T0 was evaluated using the Master Curve method, and the ductile-to-brittle transition temperature Tt was determined by Boltzmann fitting. The results show that KV2 decreases markedly with decreasing temperature in all regions, indicating a clear ductile-to-brittle transition. The BM exhibits the highest stability of low-temperature toughness, followed by the WM, while the HAZ shows the greatest scatter and the highest sensitivity to temperature reduction. For standard SENB specimens, J-R curves and J1C values remain nearly constant in the ductile regime, whereas significant differences emerge in the mixed and brittle regimes, with the BM showing the lowest T0 and the HAZ the highest. For non-standard SENB specimens, reducing crack length and specimen thickness lowers constraint levels and leads to increased fracture toughness and reduced T0. Transition temperatures Tt obtained from Charpy impact and fracture toughness tests differ significantly, with J1C providing a conservative assessment and Jmax reflecting crack-propagation resistance. Overall, the low-temperature fracture behavior of Q355 steel welded joints is strongly influenced by testing temperature, constraint condition, and material region, with the HAZ remaining the most critical zone for lowtemperature fracture resistance.

This study systematically investigates the coupled influence of welding environmental temperature and testing temperature on the fracture behavior and ductile-to-brittle transition characteristics of Q355 steel welded joints. Charpy impact and fracture toughness tests were performed on the weld metal (WM) and heat-affected zone (HAZ) over a testing temperature range from 23 degrees C to-100 degrees C under two welding conditions: room-temperature welding (23 degrees C) and low-temperature welding (-10 degrees C). The transition behavior was evaluated using the Master Curve methodology and Boltzmann fitting from both impact-and fracture-mechanics-based perspectives. The results demonstrate that low-temperature welding significantly enhances low-temperature fracture toughness and reduces data scatter in both WM and HAZ. Both the reference temperature T0 and the ductile-to-brittle transition temperature Ttare shifted to lower values under low-temperature welding, reflecting reduced embrittlement sensitivity. While the WM consistently exhibits higher fracture toughness than the HAZ, the HAZ remains more sensitive to low temperatures. Moreover, discrepancies between impact-based and fracture-mechanics-based transition temperatures are minor in the WM but pronounced in the HAZ, highlighting the different sensitivities of the two methods to fracture mechanisms. These findings provide mechanistic insight into low-temperature embrittlement of welded joints and offer practical guidance for optimizing welding procedures and fracture safety assessment of Q355 steel structures operating in cold environments.

This paper investigated the structural behaviour of stainless-clad (SC) bimetallic steel welded tubular T-joints under brace axial compression. Five T-joint specimens were designed, and the effects of welded section type, steel grade, and brace to chord width ratio were clarified. Extensive data on SC bimetallic steel welded T-joints were obtained through full-scale tests, with comparisons made regarding the testing process, failure modes, loaddeformation curves, and strain development. Furthermore, the study reviewed design methods for conventional mild (CM) steel welded tubular T-joints under brace axial compression as outlined in Design Guide 3 of CIDECT, EN 1993-1-8, GB 50017, and ANSI/AISC 360, and evaluated their applicability to SC bimetallic steel welded tubular T-joints. Subsequent numerical studies employed simplified and layered modelling approaches, with validated models used for parametric analysis. Finally, the study proposed design recommendations that serve as references for designing SC bimetallic steel welded tubular T-joints. Research showed that all T-joint specimens exhibited failure initiated by plastic deformation in the chord face at joint resistance; while all connecting welds remained intact, confirming the applicability and reliability of the welding configurations; the finite element (FE) modelling strategy proposed herein could accurately simulate the failure modes and joint resistance, with simplified and layered modelling approaches yielding comparable results; the design methods for joint resistance in CM steel tubular T-joints under brace axial compression, as outlined in CIDECT, EN 1993-1-8, and GB 50017, can effectively guide the design of SC bimetallic steel welded T-joints with a clad ratio up to 0.3, ensuring accuracy and maintaining appropriate safety margins.

Stainless-clad (SC) bimetallic steel is an advanced material which is composed of a substrate layer and a cladding layer, and they are bonded metallurgically together. This study investigates the residual stress distribution model within SC bimetallic steel cold-formed square hollow sections (SHS) using a numerical simulation approach. A two-dimensional finite element model was developed to simulate the press-braking process and validated against existing experimental data. The results revealed that press-braking induced longitudinal residual stresses followed a Z-shaped antisymmetric pattern through the thickness, with the bonding interface and mid-thickness serving as two inflection points. The stress magnitude was significantly affected by the radius-to-thickness ratio and clad ratio. Based on a parametric study of 500 cases, a through-thickness distribution model for press-braking induced residual stresses was proposed, demonstrating well agreement with experimental measurements. A three-dimensional sequential thermo-mechanical coupling model was established to simulate the welding process, validated against the authors' previous experimental results. Welding induced residual stresses exhibited a tri-segment distribution across the cross-section: tensile stresses approaching yield strength in the weld region, uniform compressive stresses in the non-weld region, and a linear transition region. The compressive stresses in the non-weld region were inversely correlated with width of cross-section and clad ratio, while positively correlated with thickness of plate. Based on a parametric study of 52 cases, a cross-sectional distribution model for welding induced longitudinal residual stresses was proposed, which correlated well with experimental results. Furthermore, the influence of the number of welding passes and the feasibility of decoupling the press-braking and welding processes were examined. The proposed residual stress distribution models provided effective predictive tools for SC bimetallic steel cold-formed square hollow sections.

This paper aims to study the behaviour of square concrete-filled stainless-clad bimetallic steel tubular (CFSCBST) stub columns subjected to combined compression and bending through experimental and numerical investigations. Five CFSCBST stub columns were fabricated and tested, including four specimens subjected to combined loading with different loading eccentricity ratios and one specimen subjected to axial compression. The test results, including failure modes, load versus displacement responses and ultimate loads, were discussed in detail. A three-dimensional finite element (FE) model was established and validated against the test results. Subsequently, parametric studies were conducted to investigate the effect of concrete strengths, material combinations, clad ratios, steel ratios, corner radii, and loading eccentricity ratios. Finally, the applicability of existing design standards was assessed herein through comparisons with the FE results. The evaluation results revealed that the Chinese standard DBJ/T 13-51-2020 gives the most accurate predictions that can be extended to predict the ultimate load of CFSCBST stub columns subjected to combined compression and bending.

This paper investigates the local buckling behaviour of titanium-clad (TC) bimetallic steel circular hollow section (CHS) stub columns under compression through experimental and numerical studies. Five CHS stub columns were tested with measurement of four types of tensile coupons and local geometric imperfections. In parallel with the experimental investigation, FE models were established and validated with the consideration of welding joint construction, element types, and material strategies. After validation, a selected FE model was employed for parametric studies, where a total of 320 numerical simulations were derived with different diameter-to-thickness ratios, material combinations, and clad ratios. The existing design standards were evaluated with the parametric results, and recommendations for design methods for the TC bimetallic steel CHS were proposed. Additionally, the study demonstrates that clad ratios significantly affect the bearing capacity of the TC bimetallic steel CHS stub columns due to the different plastic hardening capability and elastic modulus of cladding and substrate layers. A modified design method based on prEN 1993-1-1:2018 was proposed to accurately account for the influence of clad ratio. The present study provides a foundation for developing more accurate cross-section design rules for TC bimetallic steel member in future research.

This paper presents a numerical investigation to thoroughly quantify the welding-induced residual stresses within stainless-clad (SC) bimetallic steel welded box sections. Sequential thermo-mechanical coupling and moving heat source are utilised in ABAQUS to simulate the welding process, and values for the key modelling parameters are recommended. The sectioning process in measurement is also simulated by using the model change method, and the strains released during sectioning process as well as residual stresses on the outside surface are extracted from the finite element (FE) modelling results. Subsequently, the experimental and numerical values of residual stresses are compared to verify the reliability of the FE model. Based on this validated model, parametric analyses are conducted to explain the distribution patterns and influencing factors of residual stresses within the SC bimetallic steel box sections. The residual stresses patterns within substrate and cladding layers are different and are related to sectional geometry and clad ratio. Ultimately, both a layered model considering differences between the two layers and a simplified model describing the overall characteristics are proposed based on the parametric analyses results. By error assessment in terms of overall buckling behaviour in FE analyses, it is found that both models are highly applicable. This paper facilitates the advancement of structural research on SC bimetallic steel and provides reference for the welding simulation of residual stress in such welded sections and members.

Bimetallic steel is an advanced material which is composed of a substrate layer and a cladding layer, and they are bonded metallurgically together. Combined with the advantages of both metals, bimetallic steel has broad application prospects in the construction industry. The welding technique for bimetallic steel is more complicated, making a new kind of cold-formed square hollow section (SHS) a preferred choice due to its reduced need for weld seams. This paper presents an experimental study of the longitudinal residual stress in cold-formed SHS of stainless-clad (SC) bimetallic steel made from S30408 and Q235B, involving six specimens with different cross-sectional dimensions. Three different measurement methods for the residual stress were employed, including mechanical strain gauge on strips, electrical strain gauge on strips, and electrical strain gauge on slitted edges. The comparison of three measurement methods was made in terms of measuring principles, data characteristics, and operational complexity, followed by practical recommendations for selecting the most appropriate method in various situations. During the cutting procedure, the release of residual stress was tracked, and a simplified cutting procedure for residual stress measurement was proposed. The residual stress was decomposed into membrane and bending stresses, a step that was proved to be necessary for the requirement of force equilibrium. Finally, an experiment-based model with four regions was proposed to effectively describe the residual stress within SC bimetallic steel cold-formed SHS.

Stainless-clad bimetallic steel is composed of a substrate conventional mild steel layer and a cladding stainlesssteel layer. This study investigates the residual stresses within the welded circular sections through experimental measurement and numerical simulation. The longitudinal residual stress within five specimens was measured by sectioning method. Based on theoretical analysis and numerical simulation, it is found that the longitudinal residual stress induced by the cold bending process can be described as a Z-shaped distribution along the thickness, resulting in a stress difference of roughly 100 MPa between the two layers, which is closely related to the clad ratio but has limited correlation with the geometric dimensions of the cross-section. Then the residual stresses induced by cold bending process are input as the pre-field stresses into the welding simulation, and the simulation results are compared with the experimental results to verify the reliability of the numerical model. The results show that the residual stress distributions within both the cladding and substrate layers are in the shape of a trilinear model. Through parametric study, the effects of clad ratio, diameter, and plate thickness on residual stress are clarified. A simplified distribution model and a layered distribution model are proposed in which the simplified one ignores the stress difference along the thickness, while the layered one takes into account the difference. Both models fit well with the simulation results and can be effectively applied in the overall buckling analyses of such bimetallic steel columns, with negligible differences of up to 0.015 in buckling factors, providing a basis for the subsequent research on the related buckling research.

The axial compression behaviour of circular concrete-filled stainless-clad bimetallic steel tubular (CFSCBST) stub columns is addressed in this paper by experimental and numerical investigations. A total of five specimens were fabricated and subjected to axial compression loading. The varying parameters in the experimental study included the clad ratio of steel tube, the diameter-to-thickness ratio and the strength of the concrete. A threedimensional finite element (FE) model was developed and validated against both dependent and independent test data to further study the axial compressive behaviour of circular CFSCBST stub columns in this paper. The parametric studies considering five major parameters, including the strength of the concrete, the substrate steel grade, the clad ratio, the diameter-to-thickness ratio and different bonding conditions of the stainless-clad (SC) bimetallic steel, were carried out by using the verified FE model. The applicability of existing design codes to the circular CFSCBST stub columns was further analysed through comparisons with the numerical results. New design methods based on unified and superposition theory have been proposed herein, which have improved accuracy for predicting the ultimate compression capacity of such circular CFSCBST stub columns.

The local buckling behaviour of SC bimetallic steel cold-formed square hollow section (SHS) stub columns is addressed in this paper by experimental and numerical investigations. A total of six specimens with varying plate slenderness were fabricated though press-braked cold-forming process and subsequent welding, which were tested in axial compression. A three-dimensional finite element (FE) model involving geometric imperfections was established and validated against the test results in terms of failure modes, load versus axial displacement curves and ultimate loads, and the single-layered model (SLM) was recommended for the simplicity. Parametric analyses were carried out considering six major parameters, including the thickness, the initial local imperfection, the clad ratio, the material combination, the inner corner radius and the width-to-thickness ratio. The applicability of existing effective width design methods for such stub columns was assessed herein through comparisons with the FE results. Finally, an effective width design method and a three-stage reduction factor method were proposed herein with high accuracy, in which the proposed effective width method and a constant slenderness limit of 0.540 are recommended for the convenient application in practical engineering.

The corrosive marine environment brings significant challenges to traditional steel materials, prompting the exploration of alternative solutions in terms of advanced structural materials, such as titanium-clad (TC) bimetallic steel. This paper presents an experimental investigation into the basic mechanical properties and high- cycle fatigue behaviour of hot-rolled bonding TC bimetallic steels with different clad ratios and surface roughness. Based on a series of tensile coupon tests, bonding interface shear tests, combine tests, and high-cycle fatigue tests, the failure modes, static strengths of base metal and bonding interface as well as fatigue lives are clarified, and experimental S-N curves of the TC bimetallic steel are obtained and analysed by comparing with those of titanium alloys and conventional structural steels. SEM images are taken and studied to provide insights into fatigue crack propagation mechanisms. Additionally, the effects of clad ratio and surface roughness on fatigue performance are also elucidated. Finally, the paper proposes a reliable design S-N curve with a fatigue strength of 321.7 MPa for the hot-rolled bonding TC bimetallic steel. The research outcomes may contribute to predicting fatigue strength for TC bimetallic steel, and offer valuable insights for structural engineering applications of such advanced bimetallic steel.

For stainless-clad (SC) bimetallic steel plates, the imperfections caused by production, processing and transportation are complicated and can influence the ultimate strength of plates profoundly. The theoretical buckling strength of SC bimetallic steel plates was derived by following the Perry-Robertson approach, and different cases were discussed considering configuration, imperfection direction and failure position. 702 numerical simulations were conducted by the verified numerical model to modify the original theoretical formulae. The effects of normalised plate slenderness ratio, clad ratio, material combination and imperfection can be well considered in proposed formulae and their influence was elucidated by the parametric study. Based on the theoretical study of plates, design methods were proposed to predict the local buckling resistance of SC bimetallic steel square hollow section (SHS) stub columns, which were demonstrated to have an improved accuracy compared to those in current standards.

This paper investigates the increasing demand for improved fire performance of composite floor systems in highrise buildings by constructing a new type of composite slab, which is composed of concrete and closed profiled steel deck fabricated with G550 galvanized steel. Six full-scale standard fire tests of composite slabs are conducted to understand the thermal-mechanical response of the new composite slab. The results reveal that the fire duration of all the novel composite slabs exceeds 60 min, demonstrating significantly improved fire performance comparing to a conventional slab. The failure mode for all tested composite slabs is flexural failure, with limited end slip that indicates a good preservation of composite action during fire exposure. Based on the experimental results, numerical model is established and validated through the comparison of temperature and deformation data. A series of parametric analyses are carried out numerically, where overall slab depth, deck depth, supported span and uniform loads are identified as the dominant effects on the fire resistance. The results indicate that the existing design methods specified in the current standards are not fully applicable to the newly proposed composite slab. Simplified calculating methods for insulation-based and bearing capacity-based fire resistance are suggested.

The material behavior of structural component is critically important to resist the fire and other actions, especially for the structural parts directly exposed to fire. In this paper, the room- and elevated-temperature material properties of a newly developed fire-resistant weathering (FRW) steel with nominal yield strength fy = 235 MPa are investigated by using an innovative heating method and conventional steady-state tensile testing method. The stress-strain curves and typical mechanical properties at elevated temperatures are obtained. Furthermore, the equations for reduction factors of Young's modulus, proof strength and tensile strength are proposed, and the thermal expansion characteristic parameters are determined via in-situ heating method. Based on the measured data, relevant equations for describing critical parameters are provided and compared with the available equations in the current standards. According to the experimental stress-strain curves and mechanical properties, the constitutive relationship at elevated temperatures based on the Ramberg-Osgood model is established and proposed. The goodness of fit for the stress-strain constitutive model is all above 96 %, which is consistent with the experimental curve. Therefore, the proposed constitutive model is sufficiently accurate for predicting the elevated-temperature stress-strain relationship of FRW steel. The research findings presented in this paper provide important reference for determining the material properties of FRW steel, and give valuable recommendation for the fire safety design of steel and composite structures.

Fire-resistant steel offers improved retention in strength and elastic modulus at high temperatures, enhancing the fire performance of steel structures. However, unified constitutive models for the high-temperature behavior of fire-resistant steel are still limited. This study aims to develop a broadly applicable high-temperature constitutive model based on experimental research and comparative analysis. Fifty high-temperature steady-state tensile tests were conducted on Q355FRC fire-resistant steel in accordance with Chinese and American testing standards, covering a temperature range from room temperature to 900 degrees C. The results show that Q355FRC fire-resistant steel does not exhibit a yield plateau at either room or elevated temperatures. Compared with conventional structural steel, Q355FRC fire-resistant steel shows improved high-temperature performance, retaining approximately 78 % of its elastic modulus and about two-thirds of its nominal yield strength even at 600 degrees C. The reduction coefficients specified in current design codes for conventional structural steel tend to be conservative for fire-resistant steel. To address this issue, predictive equations and a modified three-stage Ramberg-Osgood model are proposed. The generality of the proposed three-stage model is validated using experimental data from other studies. Simplified expressions for strain-hardening parameters are developed, enabling convenient estimation of the high-temperature mechanical properties for a broader range of fire-resistant steel grades.

Stainless-clad (SC) bimetallic steel, as an advanced high-performance structural material, has seen growing adoption in bridge engineering and architectural applications. Current research efforts, however, have predominantly centered on characterizing its fundamental material properties rather than evaluating structural member performance. Particularly for welded circular hollow section (CHS) structural members under axial compression, those with elevated diameter-to-thickness ratios present critical stability challenges where localized buckling phenomena may significantly compromise load-bearing efficiency. This necessitates comprehensive investigation into their local stability mechanisms and subsequent development of optimized design methodologies. To this end, this paper investigates the local buckling behavior and design of CHS stub columns made of SC bimetallic steel through experimental and numerical studies. A total of eight stub column specimens were fabricated from hot-rolled bonded SC bimetallic steel plates with various thicknesses. The dimensions and geometrical imperfections of the specimens were precisely measured using a non-contact 3D laser scanner. Two measurement methods were employed to process the scanned data, providing a more accurate analysis of imperfections. Finite-element (FE) models incorporating geometric imperfections derived from the 3D-scanned data were developed and validated against both the test results presented herein and independent results from other researchers. Comprehensive parametric studies were conducted using the verified FE models, considering various clad ratios, diameter-to-thickness ratios and steel grades. The applicability of current codified design methods to the SC bimetallic steel CHS stub columns was evaluated by comparing the standards with the numerical results. Based on the parametric studies and evaluation of existing design methods, a diameter-to-thickness ratio limit and effective cross-section formulae were proposed. The design methods presented in this paper offer improved efficiency and consistency, making them suitable for future applications of SC bimetallic steel structures.

This paper describes a series of structural tests on standardized coupons of stainless-clad (SC) bimetallic steel plates and their welded joints under various actions. Despite the clad layers of the stainless steel are bonded at a metallurgic level onto the substrate plate of carbon steel, it is important to establish that debonding of the clad layer from the substrate plate, i.e. an integrity failure within the SC plates, will not take place prematurely. These structural tests include i) debonding tests along the interfaces of the stainless steel and the carbon steel of the SC plates under shear and tension actions; ii) tension tests on the SC plates; iii) both tensile and bending tests on the welded joints of the SC plates. It is shown that among all these tests on the SC plates and their welded joints, the interfaces of the SC plates are very strong and fully intact under various global and local actions. The SC plates are considered to be highly effective, and enhancement provided by the clad layers of the stainless steel to the substrate plates of carbon steel is totally reliable up to its fracture. Moreover, the welded joints of the SC plates are always stronger than their base plates under the effects of welding. Consequently, it is demonstrated that these SC plates are able to meet various stringent structural requirements in construction, and they are able to sustain large applied loads even up to large deformations.

This paper experimentally investigates the structural behaviour of stainless-clad (SC) bimetallic steel welded tubular T-joints under monotonic brace in-plane bending. Seven T-joint specimens were designed, and the effects of welded section types, welding configurations between chord and brace, steel grades, and brace to chord width ratios were clarified. Full-scale tests generated extensive data on the behaviour of such T-joints, including comparisons of testing phenomenon, failure modes, bonding interface conditions and joint moment resistances. The study also reviewed existing design methods for conventional mild (CM) steel welded tubular T-joints, and assessed their applicability to SC bimetallic steel joints. Finally, welding configuration recommendations were proposed herein to guide their design. Research showed that all T-joint specimens exhibited failure caused by either the chord face failure or a combination of the chord face failure and the chord side wall failure at the joint moment resistance; the two welding configurations between the chord and brace studied herein exhibit similar joint performance; it is preferable to use a welding configuration with pure stainless steel consumables, and if another configuration is used, the removal of local cladding metal in the chord face must be controlled to guarantee the removed region being fully refilled with weld consumables; cold-bent square section is recommended to prevent chord weld failure and undesirable T-joint failure modes when the chord width is close to the brace width. The design methods for joint moment resistance in EN 1993-1-8 shows the best prediction accuracy, and the method in GB 50017 is relatively conservative, particularly for brace to chord width ratios greater than or equal to 0.85.