This research aims to experimentally investigate the ductile fracture characteristics and the level of anisotropy of four plates, 400 mm × 150 mm × 3.72 mm, made by Wire Arc Additive Manufacturing (WAAM) technology with 1 mm thick layers. Relatively small roughness is measured, expressed in maximum peak-to-valley height, measured by scanning, of 98 μm. Calibrated parameters for an advanced computational material model are derived for a finite element mesh size of 0.5 mm. The experimental campaign is based on eight types of short coupon specimens, analysed to explore fracture behaviour exposed to various stress conditions. Sixty-five coupon specimens, 51 milled and 14 tested in as printed conditions, cut out in three directions relative to the printing direction, are examined. The assumption of isotropic mechanical characteristics is confirmed. The mesoscale critical equivalent plastic strain (MCEPS) methodology is used to predict experimental results numerically. Three stages are considered: elastic, plastic, and couple plastic-damaged stages. The accuracy of the calibrated parameters is validated by comparing the engineering stress-strain relationships obtained from experimental tests and finite element (FE) analysis, reaching very good agreement. A list of all material parameters for ductile fracture modelling at various triaxiality levels and Lode parameters is provided for a mesh size of 0.5 mm.

The authors regret that due to errors in typing, Eq. (2) and the corresponding texts should be replaced by: where ∆M is the bending moment range and the second moment of area [Formula presented], both in unit width. The results in the paper are not affected by the typing error. The authors would like to apologise for any inconvenience caused.

Monitoring fatigue damage in mechanical connections is essential for maintaining the safety and structural integrity of offshore wind turbines (OWTs), particularly during the early stage of crack initiation. Recently, the C1 wedge connection (C1-WC) has emerged as a promising innovation for use in OWTs. Acoustic emission (AE) monitoring is a widely used real-time technique for detecting fatigue cracks. The space limitations of the lower segment holes in the C1-WC presents challenges for detecting surface cracks with conventional AE sensors. Thin Piezoelectric Wafer Active Sensors (PWAS), while small and lightweight, face limitations due to their poor signal-to-noise ratio. In this study, we propose a baseline-based approach to enhance the effectiveness of PWAS for accurate AE monitoring in confined spaces. A benchmark model correlating the damage state of specimens is created by breaking pencil leads. Multivariate feature vectors are extracted and then mapped to the Mahalanobis distance for damage identification. The proposed method is validated through testing on compact specimens and C1-WC specimens. To enhance the AE detection results, supplementary monitoring techniques, including digital image correlation, crack propagation gauges, and distributed optical fiber sensors, are employed. The experimental setup, signal acquisition, and detection efficiency of these techniques are briefly outlined. This study demonstrates that the proposed approach is highly effective in detecting early damage in C1-WC specimens using AE monitoring with PWAS.

Acoustic emission (AE) is widely used for identifying source mechanisms and the deformation stage of steel material. The effectiveness of this non-destructive monitoring technique heavily depends on the quality of the measured AE signals. However, the AE signals from deformation are easily contaminated by the signals from noise in a noisy environment. This paper presents a hybrid model for deformation stage identification, which combines a self-adaptive denoising technique and an Artificial neural network (ANN). In pursuit of model generality, AE signals were collected from tensile coupon tests with various steel materials and loading speeds. First, a decomposition-based denoising method is applied based on the singular spectral analysis (SSA) and variational mode decomposition (VMD), which is defined as SSA-VMD. Its effectiveness is demonstrated by simulated signals and experimental results. Following the use of the denoising technique, an ANN is constructed to identify the deformation stage of steel materials with the input of features extracted from the filtered AE signals. The results indicate that the ANN achieves a high prediction accuracy of 0.93 in the test set and 0.87 in unseen data. By applying this denoising method, the ANN-based approach enables accurate correlation of the collected AE signals to deformation stages. The finding can be used as the basis for the creation of new methodologies for monitoring structural health status of in-service steel structures.

The flexural behaviour of two types of asymmetric bolted square hollow section (SHS) splices is investigated in this paper. The asymmetric bolted SHS splices are derived from the traditional bolted end plate SHS joints but with the end plate flushed to the SHS surface on one side or two adjacent sides, where a cover plate connects two SHSs. Firstly, four-point bending tests are conducted for two configurations under different loading conditions. Next, finite element (FE) analysis is carried out to simulate the experiments. To reduce the computational time, a simplified FE model is developed, which is further employed for a parametric study. Finally, the component method, which was extended for the asymmetric bolted SHS splices under tensile load, is verified against the FE models in the parametric study. The results show that the extended component method could effectively predict the stiffness and the resistance of bolted asymmetric SHS splices under different bending loads.

Steel Orthotropic Bridge Decks (OBDs) are widely used in the superstructures of movable and long-span bridges. There is a high concentration of stress at many welded connections, which are governing the fatigue performance of OBDs. The fatigue behaviour of the lower end of weld in crossbeam-to-stiffener connection with cut-outs is studied from multiple experimental results obtained from a 48m2 full-scale specimen, as well as finite element simulations. The fatigue resistance of the detail is recommended based on 8 experiments reported in the current study and experiments in the literature using the hot spot stress method. The influence of the load location on the hot spot stress is analysed. The effective notch stress method is used to evaluate the fatigue resistance with different weld profiles.

The acoustic emission (AE) technique is commonly utilized for identifying source mechanisms and material damage. In applications requiring numerous sensors and limited detection areas, achieving significant cost savings, weight reduction, and miniaturization of AE sensors is crucial. This prevents excessive weight burdens on structures while minimizing interference with structural integrity. Thin Piezoelectric Wafer Active Sensors (PWAS), compared to conventional commercially available sensors, offer a miniature, lightweight, and affordable alternative. The low signal-to-noise ratio (SNR) of PWAS sensors and their limited effectiveness in monitoring thick structures result in the decreased reliability of a single classical PWAS sensor for damage detection. This research aims to enhance the functionality of PWAS in AE applications by employing multiple thin PWAS and performing a data-level fusion of their outputs. To achieve this, as a first step, the selection of the optimal PWAS is performed and a configuration is designed for multiple sensors. Pencil break lead (PBL) tests were performed to investigate the compatibility between selected PWAS and traditional WSα and R15α sensors. The responses of all sensors from different AE sources were compared in both the time and frequency domains. After that, convolutional neural networks (CNNs) combined with principal component analysis (PCA) are proposed for signal processing and data fusion. The signals generated by the PBL tests were used for network training and evaluation. This approach, developed by the authors, fuses the signals from multiple PWAS and reconstructs the signals obtained from conventional bulky AE sensors for damage detection. Three CNNs with different architectures were built and tested to optimize the network. It is found that the proposed methodology can effectively reconstruct and identify the PBL signals with high precision. The results demonstrate the feasibility of using a deep-learning-based method for AE monitoring using PWAS for real engineering structures.

To monitor the growth of fatigue cracks in steel specimens, several methods exists. In this paper, the magnetic stray field, which is generated by the magnetisation of the specimen, was measured during loading to investigate how to utilise this data to reliably monitor fatigue crack initiation and growth. Data was collected in a series of fatigue tests on Compact Tension specimens with different force ratios. The evolution of the mean value of the dominant stray field component displayed a sensitivity to stress, plastic deformation and displacement of the specimen. By analysing the stress field induced by the loading, these three causes were distinguished from another. Crack initiation was marked by a large change of the mean magnetic stray field. Moreover, the amplitude of the magnetic stray field components showed a clear peak, at which moment 20% of the life time of the specimen is remaining, indicating that the magnetic stray field might provide a useful method to monitor the evolution of fatigue cracks.

The welding process induces residual stresses because of the non-uniform heating and cooling of the material. Residual stresses are known to influence fatigue crack growth. However, studies addressing both the formation of residual stress – giving a realistic multi-directional stress and strain field – and redistribution of residual stresses due to crack growth in the case of a surface crack have not been found. The extended finite element method was employed in this study to evaluate the stress intensity factors of a planar, growing crack in a welded T-joint with and without welding induced residual stresses. The initial residual stress field was taken from a welding simulation using the finite element method. The redistribution of the residual stress field due to crack growth was studied in addition to the shape and growth rate of the planar crack. The study shows that, in agreement with experimental evidence, the external stress ratio has a significant influence in the absence of residual stress but it does not have a significant influence in the presence of residual stress. The current study gives insight into the cause of this observation.

Steel Orthotropic Bridge Decks (OBDs) are widely used in long-span and movable bridges. Fatigue resistance analysis plays an important role in the design or assessment of OBDs. One possible fatigue failure is the crack initiating from the weld root of stiffener-to-deck plate connections at crossbeams. A full-scale experimental investigation in this study using a 20 mm thick deck plate with a dimension of 9.4 m × 5.1 m, including three crossbeams, represents the modern designed OBDs. The experiments show an arrest of crack propagation with a final crack depth of approximately 75% of the deck plate thickness. On the contrary, through thickness cracks develop in deck plates of 10 or 12 mm. Hot spot stress based fatigue detail categories (DC) using various failure criteria derived from the tests. Analysis with the effective notch stress shows that the DC has low sensitivity to the amount of weld penetration. The results of analyses with the eXtended Finite Element Method (XFEM), employed to analyse the fatigue crack propagation path and crack arrest, are in line with the experimental study.

Fatigue cracks in the stiffener-to-deck plate connections of orthotropic bridge decks, initiating from the weld toe or root and propagating into the stiffener or weld throat, are experimentally and numerically studied. A statistical analysis of the structural stress is carried out using the experimental data. Automatic welded specimens show a significantly higher fatigue resistance than manual welded ones for both details of the study. Including results in the literature, the characteristic fatigue resistances appear larger than the values in current standards and range between 100 and 160 MPa. A proposal for the fatigue resistance values is given for design purposes. The effective notch stress, averaged strain energy density factor, and fracture mechanics methods are employed to study the sensitivity of the weld toe and root cracks for different (geometrical) variations, such as the lack of weld penetration. Among them, the fracture mechanics method agrees best with the experimental data. With the increase of weld penetration ratios from 75% to 100%, the fracture mechanics predicted fatigue resistances remain approximately equal for the weld toe cracks and increase for the weld root cracks.

Bolted connections are one of the key connection configurations in steel structures. The ductile fracture prediction is one of the challenges in the structural integrity evaluating of steel structures. To guarantee the safety of steel bridge in connections, an accurate assessment of the ultimate resistance of high-strength bolts under combined tensile-shear loads is necessary. However, the impacts of various parameters on high-strength bolts under combined tensile-shear loads are not sufficiently analysed in the existing references. Hence, in this paper, the validated mesoscale critical equivalent plastic strain (MCEPS) method is used to evaluate the ultimate resistance of high-strength bolts with different bolt grades, bolt diameters, bolt types, hole clearance, and preload force when the bolts are exposed to combined tensile-shear loading. The simulation results are compared to existing design specifications. Finally, the formula modifications in the existing design standards are proposed based on statistical analysis on numerical parametric results.

Debonding crack propagation at the composite-to-steel interface has been found to be an important failure mechanism for wrapped composite joints under static and fatigue loads. Friction at the interface behind the crack tip may deviate fatigue debonding of the joints from the linear-fracture-mechanics behaviour. This paper presents static and fatigue tests of axial wrapped composite joints. 3D DIC and optical fiber system is employed to monitor displacements and crack propagation. A finite element model is established and validated against static and fatigue test results, where friction is considered at the cracked interface. Through FE modelling, it is proved that the friction at the interface significantly reduce the strain energy release rate (SERR) at the crack tip, leading to retardations of crack growth and stiffness degradation. Parametric study is conducted finally to investigate the influence of friction coefficient, failure modes as well as Paris relationship parameters on the predicted fatigue behaviour of wrapped composite joints.

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.

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.

To establish the tensile constitutive model of ultra-high performance concrete containing coarse aggregate (CA-UHPC), monotonic and cyclic uniaxial tensile tests for CA-UHPC with fiber volume fractions of 2.5% and 2.0% were conducted. Test results showed that CA-UHPC exhibits approximately linear stress-strain relation up to the tensile strength, and tensile softening response composed of the smeared- and localized-cracking stages, regardless of the tested fiber contents. Based on the monotonic test data, the tensile stress-crack opening model of CA-UHPC was established, and the model was further simplified into tri-linear relation. Based on the cyclic test results, tensile damage evolution laws according to the strain equivalence principle and the energy equivalence principle were developed, respectively. Finally, the proposed tensile constitutive model and the calibrated tensile damage evolution laws were demonstrated to effectively predict the mechanical response of CA-UHPC members under both monotonic tension and cyclic tension through numerical simulations.

In this paper, a numerical simulation method of mixed-mode fatigue crack propagation was explored using the extended finite element method (XFEM) and the Virtual Crack Closure Technique (VCCT). Both 2D and 3D numerical models were selected to simulate the fatigue crack propagation of steel specimens. Two coefficients were proposed to calculate the equivalent energy release rate (G_eq) for a better simulation of the mixed-mode fatigue crack propagation of S355 steel. The Walker equation and the calculation formula of G_eq were realized by a user-defined subroutine. A set of optimal correction coefficients of mode II energy release rate (G_II) and mode III energy release rate (G_III) were quantitatively comparing the simulation results and test data. The results will contribute to fatigue crack propagation prediction of steel structures in the civil engineering field.

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.

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.

The overall competitiveness of offshore wind turbine towers is significantly influenced by the selection of the connection. The following three types of connections: a conventional bolted ring flange (RF) connection, ring flange connection with defined contact surfaces (RFD), and C1 wedge connection (C1-WC) are considered. A quantitative comparison is made to enhance performance in a specific condition and enable further optimization of these connections in engineering practices. The study compares the tensile behaviour and fatigue performance of these connections by validated finite element (FE) simulation and analysis. The proposed FE modelling is based on a realistic geometry including all contacts present in the connections, steel full-range stress–strain relationship and ductile damage model. The efficiency and accuracy of the FE models are validated through the comparison with the performed tests. Then, a series of parametric FE analyses are carried out to examine the impact of the applied boundary conditions, bolt pretension level, and steel grade on the behaviour of connections. Load-displacement curves, bolt evolution curves, and stress responses are analysed to compare their tensile behaviour. The effectiveness of conventional segment specimen testing is evaluated thoroughly. For the fatigue performance of connections, the results indicate that the segment specimen testing substantially underestimates the fatigue performance of C1-WCs. This discrepancy is essential to be considered in the tower design. It is also noted that C1-WCs are rather insensitive to the variation of pretension force level, which show superiority to avoiding the difficulties associated with typically bolted joints. This research provides in-depth knowledge for the practical application of such connections and further optimization.