In this paper, the influence on the multiaxial fatigue damage accumulation caused by loading path variation was studied. For 2024‐T4 aluminium alloy, the damage evolution during the entire life was first observed. On the basis of the observation, the stage I of fatigue damage evolution was further divided into two sub‐stages, and the dominant stress parameters of these two sub‐stages were proposed. Taking the dominant stress parameters into account, a phased accumulative fatigue damage model was proposed. Then, 12 multiaxial two‐stage step spectra constructed by loadings with approximately identical fatigue lives were carried out on 2024‐T4 aluminium alloy. The accumulative fatigue damage was calculated by the proposed model, and another five commonly used models and the calculated results were compared. According to the comparison, the newly proposed model had the most accurate results with the smallest scatter.

When the gearbox body interference is connected to the ring gear, prestressing occurs in the ring gear, which has a significant impact on the strength and life of the gear. Research on the prestressing of the inner ring gear is in the preliminary stage, and the distribution rule of the prestressing and the influence of each parameter on the interference prestressing have not been derived. In this paper, based on the method of calculating the prestressing of the thick cylinder in interference fit, the ring gear is found to be equivalent to a thick cylinder, and the distribution rule of prestressing of the ring gear in the interference fit is inferred. Then, by modeling and analyzing the gearbox body and ring gear in the interference fit using ABAQUS, the distribution rule of prestressing the ring gear in the interference fit is obtained through a numerical simulation. Finally, the prestressing of the ring gear in the interference fit is measured using X-ray diffraction, and the distribution rule of prestressing of the ring gear in the interference fit is obtained through analysis. Compared with the distribution rule of prestressing in theory, numerical simulation, and experiment, the theoretical distribution rule of prestressing is amended through a statistical method, and a more accurate formula of prestressing is obtained. Through the calculation of the stress and bending moment in the dangerous section of the ring gear through prestressing, the formula for checking the tooth root flexural fatigue strength in the interference fit prestressing is inferred. This research proposes a tooth root bending strength conditional formula for the inner ring gear of the interference fit, which serves as a guide for the design and production of the actual interference joint inner ring gear.

This Review summarizes the progress in research on the flow structure and aerodynamic characteristics of an airfoil at a low Reynolds number encountered by near-space low-speed aircrafts and micro-air vehicles. The structures of several kinds of laminar separation bubbles and their effect are discussed by drawing on experimental and numerical results reported in the past few decades. The transition process in the separation bubble is detailed from various perspectives, including the receptive, primary instability, secondary instability, and break-down stage. The process of evolution of a coherent vortex structure that may affect the transition is discussed by analyzing the vortex dynamics in the separation bubble. Combined with the flow characteristics at a low Reynolds number and data on the airfoil, aerodynamic characteristics of the airfoil, such as the nonlinear effect and static hysteresis effect at a low angle of attack, are discussed.

In this paper, based on the process of the fatigue crack initiation and the critical plane theory, a continuous stress parameter was proposed to quantify the driving force of the fatigue crack initiation for the fully reversed multiaxial fatigue loading. In this stress parameter, the shear stress amplitude and normal stress amplitude on the critical plane were combined with the variable coefficients which were affected by the normalized fatigue life and the loading non‐proportionality. Owing to these coefficients, for the multiaxial loadings with different non‐proportionalities, the driving force of the fatigue crack initiation during the whole life could be described. After that, a novel accumulative fatigue damage model was established for the multiaxial two‐stage step spectrum. In this model, the accumulative damage was calculated according to the variation of the proposed stress parameter on the critical plane. Considering the directionality of the multiaxial fatigue damage, for the spectrum in which the loading path was variable, the damage accumulation was carried out on the critical planes of the both loadings, and the larger one was chosen as the final accumulative fatigue damage. In order to verify the new model, up to 41 different multiaxial two‐stage step spectrum loading tests on 2024‐T4 aluminium alloy were collected. The new model, as well as other five commonly used models, was applied to calculate the accumulative fatigue damage. The final results showed that, compared with other commonly used models, the new model had the most accurate results with the smallest scatters.

In this paper, the flow characteristics of a symmetrical airfoil at Re=40000 near a 0{degree sign} angle of attack are investigated numerically, and the nonlinear lift coefficient phenomenon and different types of laminar separation bubble (LSB) structures are clearly observed. It is confirmed that the nonlinear characteristics of the lift coefficient originate from the differently-shaped pressure distributions in the LSB. The causes of the different characteristics of the pressure distributions in different types of LSB are revealed by deriving the Reynolds-averaged pressure gradient equation. It is determined that the viscous stress distribution near the wall is the key to the formation of different pressure distributions. Moreover, in order to suppress the disadvantages associated with the nonlinear lift coefficient of a symmetrical airfoil, an active flow control method based on local oscillation is adopted. By introducing an oscillation disturbance upstream of the separation bubble, the effect of Reynolds stress and convection on the wall is enhanced, the reattachment of the separation flow is promoted, and the formation of an LSB at the trailing edge is suppressed. Thus, the nonlinear characteristics of the lift coefficient due to the switching of the LSB structure are eliminated.