To address the issue of damage caused by low-speed impacts on composite material laminates coated with polyurethane coating, a numerical analysis method based on three-dimensional progressive cumulative damage in composite laminates and a yield damage criterion for polyurethane coatings was proposed. Firstly, a damage numerical model of polyurethane coating-carbon fiber reinforced composite laminates under erosion was established, and a Vumat subroutine was written. Subsequently, referring to the ASTM D7136 test standard, impact tests with various energy levels were conducted on samples coated with 1 mm and 2 mm polyurethane coatings and uncoated samples. Simultaneously, the proposed damage model was employed to study the formation reasons and propagation patterns of primary damages such as fiber damage, matrix damage, and delamination, thereby revealing the mechanism of polyurethane coating in absorbing impact energy. The results indicated that the mechanical response results calculated by the proposed damage model showed a high degree of agreement with the test results, validating the correctness of the proposed model. Additionally, comparative tests demonstrated the enhancement effect of polyurethane coating on the impact damage resistance of carbon fiber composite laminates. The findings of this study can provide a reference for the design of protective coatings for aircraft.

Aiming at the problem of the low fault diagnosis accuracy caused by the lack of fault samples for the rolling bearings of doubly fed wind turbines under normal conditions for a long time, an improved generative adversarial network fault diagnosis method based on expanding high-quality fault samples and using dual feature extraction was proposed. Firstly,a finite number of rolling bearing fault samples were expanded through a Wasserstein type generative adversarial network with maximum mean discrepancy and penalty constraints. Secondly, based on the dual feature extraction model, the time-frequency converted temporal features and local features were extracted separately. Finally, the fault diagnosis of the rolling bearing balance data was completed through a classifier. The standard dataset and test results show that the proposed method improves the fault diagnosis performance while lacking fault samples.

A set of anti-impact protection device was designed for the test fracture accident of 30 MN tension sensor calibration device. Firstly, based on the kinematic theory, the kinematic model of each component in the fracture process of test fixture was established. Then, different protective device structures at three impact locations were designed. Finally, the finite element models of three buffer structures were established and verified, calculated and optimized. The results show that the egg-box structure protection device at the top plate of the upper reaction rack and the upper ball joints can effectively solve the problems of small protection space and large impact value. The whole device dissipates 59.5% impact kinetic energy of the upper reaction rack, 60.7% impact kinetic energy of the lower reaction rack and 100% impact kinetic energy of the lower ball joints. After the improvement, the initial peak load of the protective device at the lower ball joints is reduced by 62.7%.

To enhance the computational efficiency of structural lightweight design for complex structures, a structural lightweight design method based on the Kriging surrogate model is proposed. The proposed method incorporates a hybrid addition strategy and a sample deletion strategy considering a distance threshold，aiming to rapidly improve the fitting accuracy of the Kriging surrogate model. This model was then applied to a multi-objective lightweight design model of the truck frame, with the optimization objectives of minimizing frame mass and maximum stress. Subsequently, the multi-objective lightweight model was solved using the non-dominated sorting genetic algorithm-II (NSGA-II). The results demonstrate that the proposed hybrid addition strategy and sample deletion strategy considering the distance threshold effectively enhance the update process of the Kriging surrogate model. The structural lightweight design method based on the Kriging surrogate model exhibits significant advantages in both computational efficiency and lightweight performance.

The aircraft’s inlet structure is connected to the engine sleeve using countersunk rivets. During maintenance,fatigue fractures were discovered in some rivets. It suggests that the inadequate perpendicularity of rivet holes during manufacturing causes the rivet misalignment, reducing the load-bearing capacity, and leading to fatigue fractures under aircraft vibrations. The finite element simulation was used to study the effect of inclined rivet holes on the load-bearing capacity,simulation results show that inclined holes cause uneven stress distribution across the rivet section. The higher the tilt angle,the higher the maximum stress and the more uneven stress distribution on the rivet head section. Fatigue tests under axial loads at different tilt angles demonstrated a reduction in the rivet’s fatigue life due to the hole inclination. The study concludes that non-compliance with perpendicularity standards during hole fabrication results in uneven stress distribution, decreasing load-bearing capacity. Therefore, the strict control over rivet hole perpendicularity during the aircraft manufacturing is crucial to ensure structural reliability.

Rolling bearing looseness faults are likely to induce transmission system fault. Considering the factors such as nonlinear contact force of rolling bearings, rub-impact force, damping force between outer ring and housing, a six-degree-of-freedom nonlinear dynamics model was established under the fault of rolling bearing outer ring looseness, and the vibration characteristics of rolling bearing outer ring looseness were analyzed. The simulation results show that the characteristic frequency of the outer ring looseness fault is presented as the rotational frequency of the rotating shaft and its multiple components, and the actual tested wind power bearing ring fault data verifies the accuracy of the model. The results of this paper show that the rolling bearing outer ring looseness is caused by loosing between the outer ring and the housing, and the cyclic impact and friction between the outer ring and housing are formed under the unbalance force with the rotational speed of the shaft and its harmonics. The research results provide a theoretical basis for realizing the mechanism analysis of rolling bearing outer ring looseness and fault diagnosis of the rolling bearing.

A fault diagnosis method based on improved dung beetle optimizer (IDBO)-time varying filtered empirical mode decomposition (TVFEMD) with improved wavelet threshold functions was proposed aiming at that the vibration signal of rolling bearing fault tends to be disturbed and overwhelmed by strong noise background. IDBO was primarily developed to iteratively optimize B-spline order and bandwidth threshold ξ in TVFEMD，and the optimal parameter combination was obtained. Applying TVFEMD on the original signal, the decomposition for intrinsic mode function (IMF) component series were achieved, among which the irrelevant components were removed by correlation coefficient criterion, and target signals were reconstructed. Then the improved wavelet threshold function was employed on the new signal for further denoising.Finally, the envelope spectrum of the processed signal was calculated, from which the typical fault characteristic frequency was extracted. Through simulation signal and fault simulation test analysis, the fault diagnosis method combined with IDBO-TVFEMD and improved wavelet threshold function was compared with empirical mode decomposition (EMD), ensemble empirical mode decomposition (EEMD) and complete EEMD with adaptive noise (CEEMDAN) denoising methods. The research results show that the algorithm model proposed in this paper has higher efficiency.

In view of the problem that the formulary design load based on the design standard can't truly reflect the actual service conditions of the welded frames of subway vehicles, and a large number of online measured data information has not been fully explored in the structural design of the frame, an anti-fatigue design spectrum compilation method based on the stress-time history sample information measured at the weak position of the subway vehicle frame was proposed.The small stress threshold value was determined by clustering ordered samples using the rain-flow counting method to compile measured stress spectrum reflecting structural damage, determining the stress-frequency-damage relation based on the fatigue damage theory, and using the methods of Bayesian parameter estimation and kernel density estimation to obtain stress extrapolation results. Considering the stress concentration caused by structural geometry changes in the cross-section of the welding site, the hot spot stress method and stress linearization method were used to obtain the stress concentration coefficient, and the measured stress spectrum was corrected to achieve the compilation of the design load spectrum. The research results show that the small stress threshold value is 3.18 MPa determined by extrapolation of stress extremes and the relation analysis between normalized stress-frequency-damage, which is 8.19% higher than the stress threshold value determined by traditional methods, the effect of discarding the number of small stress cycles is significant. Considering the load dispersion and stress concentration factor at the weld seam, while ensuring that the structure meets the current service conditions, the relation between the design mileage and equivalent stress is determined, the necessity of compiling the design spectrum is emphasized further. The above research builds the construction method of load spectrum compilation and the equal strength design of structures.

In order to achieve simulation prediction of the isolation performance of rubber isolators, simulation and testing comparative research on the vibration transmissibility of standard specimens were conducted. Firstly, a compression test was conducted on the standard specimen to obtain pressure deformation data. Based on the theory of the large deformation, the data was processed to obtain the true stress-strain curve. The Mooney-Rivlin model was chosen to define the material properties. Secondly, a vibration transmissibility testing system was established, an exciter to excite the standard sample was used, and the measured vibration transmissibility curve was obtained. Then, a simulation model based on the testing system was built, and the simulation transmissibility curve based on the transient dynamic analysis was obtained.Finally, the test results were compared with the simulation results to achieve the simulation prediction of the rubber isolation performance. The results show that the simulated isolation rate curve is highly consistent with the test results, with a peak frequency error of only 4.8%, which can achieve the simulation prediction and provide a simulation guidance for the optimization design and performance improvement of isolators.

Considering the time-varying stiffness characteristics of mechanical systems, a single-degree-of-freedom time-varying impact vibration system model with clearance stiffness was studied. The dynamic model and Poincaré map were established, and numerical calculation methods were given. The influence of the ratio of time-varying stiffness amplitudes on the dynamic response and characteristics of the system was analyzed using numerical simulation and the maximum Lyapunov exponent. By combining multiple initial value bifurcation diagrams, attraction domains, phase diagrams, and Poincaré mapping diagrams, the evolution and bifurcation of coexisting attractors in the system were studied by applying the continuation shooting method. When the bifurcation parameter changes and the system exhibits the coexistence phenomenon,the reasons for the appearance and disappearance of local attractors and the distribution mechanism of unstable attractors in the attraction domain before and after bifurcation are revealed. The stability change rule of coexisting attractors is obtained.