In wind turbine towers, bolted flange connections are subjected to repeated cyclic loading, making fatigue a critical concern for structural integrity. This study experimentally and numerically investigates the fatigue behaviour of M48 bolted L-flange connections subjected to high preload introduced by tension-tightening. The preload initially reached approximately 95% of the bolt yield strength and stabilised at approximately 79% after tensioner removal. A total of 12 full-scale fatigue tests were conducted to quantify the influence of combined axial and bending loading, as well as preload relaxation during cyclic loading. The results show that high preload significantly improves fatigue life, particularly in the endurance-limit region, where existing design standards provide overly conservative predictions. Cyclic loading was found to accelerate preload relaxation, resulting in an increase in the tensile stress range in the bolt. To account for both bending effects and preload loss, an equivalent effective stress range was proposed, which reduced scatter in the fatigue data and provided a more consistent fatigue-damage parameter. In addition, a three-dimensional fracture-mechanics-based fatigue crack growth framework was developed to predict fatigue life. Bolt preload was introduced in the finite element model using a thermo-mechanical approach, and the model was validated against experimental results. Fatigue crack growth parameters were obtained from dedicated tests on specimens extracted from the bolt material. The numerically predicted S-N curves showed good agreement with experimental results and standard-based predictions in the finite-life regime. The proposed methodology improves fatigue-life prediction and supports more reliable design guidelines for preloaded wind turbine flange connections.

Wire Arc Additive Manufacturing enables efficient fabrication of large-scale steel components with high geometric freedom. However, reliable structural use of WAAM high-strength steels requires an in-depth understanding of their cyclic behaviour, particularly in the low-cycle fatigue regime, which is dominated by plastic deformation. This study investigates the LCF behaviour of WAAM-fabricated ER100S-G high-strength steel using specimens extracted from thin-walled (WAAM-W) and solid deposits (WAAM-S). Strain-controlled fatigue tests were conducted at room temperature under strain ranges Δε, of 0.35% to 2.00%. WAAM-W was tested at strain ratios R_ε= −1, 0 and 0.5 while specimens from the WAAM-S configuration were evaluated at R_ε = 0. Although WAAM-S exhibited higher monotonic and cyclic strength, it underwent more pronounced cyclic softening, with a reduction of approximately 35% in the 0.2% offset cyclic yield stress, compared with 17% for WAAM-W. WAAM-S showed a steeper elastic strain-life slope, indicating that its strength advantage diminished with increasing fatigue life, while WAAM-W exhibited greater fatigue ductility and plastic-strain accommodation. This strength stability trade-off is attributed to the heterogeneous and banded bainitic morphology of WAAM-S, which provides cyclic strain localisation. In contrast, the more homogeneous WAAM-W microstructure and strain-induced transformation of retained austenite contribute to distributed plasticity and improved cyclic stability. For WAAM-W, positive mean strain reduced fatigue life at lower strain amplitudes due to a persistent tensile mean stress, whereas increased plasticity at higher amplitudes contributed to stress relaxation. The SWT formulation captured the mean-strain dependence of fatigue life, while the energy-based analysis linked fatigue life to the elastic and plastic strain-energy contributions.

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.

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 conceptual design of modular floating energy islands involves the integration of renewable energy sources, such as wind turbines and solar panels, onto floating structures. These modular systems offer advantages, such as minimal environmental impact and adaptability to different coastal and environmental conditions. This study focuses on the design of connections between the floating modules. Hinged joints are identified as a promising connector type due to their balance of flexibility and strength. Advanced numerical simulations are employed to assess the hydrodynamic forces and structural responses of floating platforms. A hinged connection model is numerically developed and the simulated performance shows good agreement with published results. Subsequently, three different connection configurations are evaluated, and their responses are discussed. Among these, a three-hinge connection system is found to be efficient, minimising relative displacement between the platforms.

Welding induces microstructural changes in the base metal, forming a heat-affected zone (HAZ) that is especially prone to strength degradation in high-strength steel (HSS) connections. While the mechanical behavior of welded joints is strongly influenced by the softened HAZ, most existing research has focused on its tensile and fatigue properties, with limited attention given to its shear behavior, despite evidence of shear failure mechanisms in certain welded structures. Building on recent advances in damage modeling, this paper calibrates a shear-modified Gurson-Tvergaard-Needleman (GTN) model tailored for the HAZ. The GTN model, which uses void volume fraction as a damage index and accounts for microvoids and stress triaxiality, is extended here to better capture shear-dominated failure modes. In the meantime, shear tests were conducted on coupon specimens extracted from butt-welded cold-formed rectangular hollow sections fabricated from three steel grades and three thicknesses. Load-deformation curves and local strain measurements are obtained from these shear tests. Finite element (FE) simulations of the HAZ, incorporating the shear-modified GTN model, are conducted. The experimentally measured load-deformation curves are used to calibrate the parameters of the shear-modified GTN model, while the measured local strains serve to validate the FE model. Practical values for the key parameters of the shear-modified GTN model are recommended for engineering applications. The estimated ultimate load carrying capacity based on the proposed model is in close agreement (approximately 5 %) with the experimental values. The limitations of the proposed model and directions for future research are also pointed out.

Bolted flange connections in wind turbine towers are subjected to cyclic loading, making fatigue a critical concern for their structural integrity. Bolt preload helps mitigate fatigue damage, but actual preload levels often deviate from design values due to uncertainties in the tightening process and geometric imperfections. This study evaluates the fatigue life of bolts L-flange connections under varying preload levels using a numerical fracture mechanics approach. A comprehensive three-dimensional finite element analysis (FEA) is conducted to assess the effects of preload on the stress intensity factor (SIF), crack propagation behaviour, and load transfer function (LTF). Additionally, the influence of thread helix angle, as well as combined axial and bending loads, on SIF and crack front evolution is examined. Experimental validation of the numerically obtained LTF is performed. A methodology for predicting S-N curves is proposed by deriving normalised solutions for LTF and SIF. The results indicate that increasing preload up to 90 % significantly reduces the SIF range, thereby decelerating crack growth and enhancing fatigue life. However, beyond 90 %, the improvement in fatigue life becomes less pronounced. Furthermore, the findings suggest that Eurocode 3 provides conservative fatigue life predictions, as it neglects bending effects, which are less detrimental than axial loading. Notably, even minor preload loss considerably shortens fatigue life, an effect that becomes more pronounced at higher preload levels. This research contributes to the development of predictive fatigue models for the bolted L-flange connection, providing insights into incorporating preload effects into fatigue life assessments.

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.

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.

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.

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.

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.

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.