The detection of bolt looseness is crucial to ensure the integrity and safety of bolted connection structures. Percussion-based bolt looseness detection provides a simple and cost-effective approach. However, this method has some inherent shortcomings that limit its application. For example, it highly depends on the inspector’s hearing and experience and is more easily affected by ambient noise. In this article, a whole set of signal processing procedures are proposed and a new kind of damage index vector is constructed to strengthen the reliability and robustness of this method. Firstly, a series of audio signal preprocessing algorithms including denoising, segmenting, and smooth filtering are performed in the raw audio signal. Then, the cumulative energy entropy (CEE) and mel frequency cepstrum coefficients (MFCCs) are utilized to extract damage index vectors, which are used as input vectors for generative and discriminative classifier models (Gaussian discriminant analysis and support vector machine), respectively. Finally, multiple repeated experiments are conducted to verify the effectiveness of the proposed method and its ability to detect the bolt looseness in terms of audio signal. The testing accuracy of the trained model approaches 90% and 96.7% under different combinations of torque levels, respectively.

The fatigue crack growth properties of G20Mn5QT cast steel and corresponding butt welds, using compact tension specimens, were monitored and investigated via acoustic emission (AE) techniques. Fatigue crack growth is a combination of cyclic plastic deformations before the crack tip, tensile crack fractures, and shear crack fractures. The cyclic plastic deformations release the maximum amount of energy, which accounts for half of the total energy, and the second-largest number of AE signals, which are of the continuous-wave type. The tensile crack fractures release the second-largest amount of energy and the largest number of AE signals, which are of the burst-wave type. The shear crack fractures release the least amount of energy and the lowest number of AE signals, which are similar to the burst type, albeit with a relatively longer rise time and duration. Crack tip advancement can be regarded as a discontinuous process. The critical area before the crack tip brittlely ruptures when the fatigue damage caused by cyclic plastic deformations reaches critical status. The ruptures produce a large number of tensile crack fractures and rare shear crack fractures. Through fractography observation, the shear crack fractures occur probabilistically around defects caused by casting or welding, which lead to stress and strain in the local complex.

With the continuous utilization of renewable energy, the number of onshore wind turbines is increasing. Small design improvements can save costs and facilitate the maintenance and repair of the wind turbine foundation. In this paper, an existing gravity expansion foundation with an anchor cage is improved. Our improvements further expand the space inside the foundation and reduce the length of the anchor bolt, which could reduce the costs and facilitate construction. To study the performance of the new foundation, a three-dimensional finite element model of the foundation–soil–anchor bolt was established via a finite element simulation. The damage evolution of the foundation was simulated with the concrete damage plasticity model (CDP). The separation ratio, foundation settlement, inclination ratio, reinforcement stress, foundation stress, and foundation damage of the new foundation under ultimate load conditions were analyzed. The influence of parameters h1 and b3 on the performance of the foundation was further studied. The finite element analysis results show that the tensile stress of concrete can be effectively reduced by appropriately increasing the corbel height and ring beam width of the foundation. The results also show that the improved wind turbine foundation force is reasonable and can meet the use of the actual project requirements on the level of finite element analysis.

With the aim of investigating the factors influencing the hysteretic behavior of common steel bars with unbonded sections (CS-US), hysteresis tests of six CS-US specimens were conducted, taking unbonded length, the location of the unbonded section, steel bar diameter, steel bar strength grade, and concrete strength grade as study variables. The results show that there exist necking phenomena in steel bars and different degrees of damage to concrete in all specimens. A 3D fine model was established by Solid works and ABAQUS software and verified according to the experimental results. The results show that the simulated values are close to the experimental values. Subsequent to the validation of the model, a thorough analysis was performed to assess the energy dissipation capacity and ductility of CS-US. The findings indicate that the implementation of an unbonded section can remarkably enhance the energy dissipation capacity and ductility of CS-US. It was demonstrated that the larger the unbonded length, the greater the ductility and energy dissipation capacity of CS-US. An alteration in the bonded length at the loading end exerts minimal influence on the energy dissipation capacity and ductility of CS-US. The energy dissipation capacity and ductility of CS-US decrease with an increasing steel bar diameter or strength grade. Concrete strength grades lower than C40 have minimal impact on the energy dissipation capacity and ductility of CS-US; concrete with a strength grade higher than C40 exhibits a decrease in energy dissipation capacity and ductility initially, followed by an increase. However, the values of these parameters remain lower than those observed in concrete with a strength grade below C40. Finally, the proposed design values of the above parameters are provided as a reference for engineering applications.