Acta Mechanica Sinica最新文献

The high-speed reentry vehicle operates across a broad range of speeds and spatial domains, where optimal aerodynamic shapes for different speeds are contradictory. This makes it challenging for a single-Mach optimization design to meet aerodynamic performance requirements throughout the vehicle’s flight envelope. Additionally, the strong coupling between aerodynamics and control adds complexity, as fluctuations in aerodynamic parameters due to speed variations complicate control system design. To address these challenges, this study proposes an aerodynamic/control coupling optimization design approach. This method, based on aerodynamic optimization principles, incorporates active control technology, treating aerodynamic layout and control system design as primary components during the conceptual design phase. By integrating the design and evaluation of aerodynamics and control, the approach aims to reduce design iterations and enhance overall flight performance. The comprehensive design of the rotary reentry vehicle, using this optimization strategy, effectively balances performance at supersonic and hypersonic speeds. The results show that the integrated design model meets aerodynamic and control performance requirements over a broader range of Mach numbers, preventing performance degradation due to deviations from the design Mach number, and providing a practical solution for high-speed reentry vehicle design.

Parameterized level-set method (PLSM) has been proposed and developed for many years, and is renowned for its efficacy in addressing topology optimization challenges associated with intricate boundaries and nucleation of new holes. However, most pertinent investigations in the field rely predominantly on fixed background mesh, which is never remeshed. Consequently, the mesh element partitioned by material interface during the optimization process necessitates approximation by using artificial interpolation models to obtain its element stiffness or other properties. This paper introduces a novel approach to topology optimization by integrating the PLSM with body-fitted adaptive mesh and Helmholtz-type filter. Primarily, combining the PLSM with body-fitted adaptive mesh enables the regeneration of mesh based on the zero level-set interface. This not only precludes the direct traversal of the material interface through the mesh element during the topology optimization process, but also improves the accuracy of calculation. Additionally, the incorporation of a Helmholtz-type partial differential equation filter, relying solely on mesh information essential for finite element discretization, serves to regulate the topological complexity and the minimum feature size of the optimized structure. Leveraging these advantages, the topology optimization program demonstrates its versatility by successfully addressing various design problems, encompassing the minimum mean compliance problem and minimum energy dissipation problem. Ultimately, the result of numerical example indicates that the optimized structure exhibits a distinct and smooth boundary, affirming the effective control over both topological complexity and the minimum feature size of the optimized structure.

Although machine Learning has demonstrated exceptional applicability in thermographic inspection, precise defect reconstruction is still challenging, especially for complex defect profiles with limited defect sample diversity. Thus, this paper proposes a self-enhancement defect reconstruction technique based on cycle-consistent generative adversarial network (Cycle-GAN) that accurately characterises complex defect profiles and generates reliable artificial thermal images for dataset augmentation, enhancing defect characterisation. By using a synthetic dataset from simulation and experiments, the network overcomes the limited samples problem by learning the diversity of complex defects from finite element modelling and obtaining the thermography uncertainty patterns from practical experiments. Then, an iterative strategy with a self-enhancement capability optimises the characterisation accuracy and data generation performance. The designed loss function structure with cycle consistency and identity loss constrains the GAN’s transfer variation to guarantee augmented data quality and defect reconstruction accuracy simultaneously, while the self-enhancement results significantly improve accuracy in thermal images and defect profile reconstruction. The experimental results demonstrate the feasibility of the proposed method by attaining high accuracy with optimal loss norm for defect profile reconstruction with a Recall score over 0.92. The scalability investigation of different materials and defect types is also discussed, highlighting its capability for diverse thermography quantification and automated inspection scenarios.

The main goal of this paper is to present the free vibration and buckling of viscoelastic functionally graded porous (FGP) nanosheet based on nonlocal strain gradient (NSGT) and surface elasticity theories. The nanosheets are placed on a visco-Pasternak medium in a hygro-temperature environment with nonlinear rules. The viscoelastic material characteristics of nanosheets are based on Kelvin’s model. The unique point of this study is to consider the change of nonlocal and length-scale coefficients according to thickness, similar to the laws of the material properties. The Galerkin approach based on the Kirchhoff-love plate theory is applied to determine the natural frequency and critical buckling load of the viscoelastic FGP nanosheet with various boundary conditions. The accuracy of the proposed method is verified through reliable publications. The outcome of this study highlights the significant effects of the nonlocal and length-scale parameters on the vibration and buckling behaviors of viscoelastic FGP nanosheets.

This paper proposes a new step-by-step Chebyshev space-time spectral method to analyze the force vibration of functionally graded material structures. Although traditional space-time spectral methods can reduce the accuracy mismatch between temporal low-order finite difference and spatial high-order discretization, their time collocation points must increase dramatically to solve highly oscillatory solutions of structural vibration, which results in a surge in computing time and a decrease in accuracy. To address this problem, we introduced the step-by-step idea in the space-time spectral method. The Chebyshev polynomials and Lagrange’s equation were applied to derive discrete spatial governing equations, and a matrix projection method was used to map the calculation results of previous steps as the initial conditions of the subsequent steps. A series of numerical experiments were carried out. The results of the proposed method were compared with those obtained by traditional space-time spectral methods, which showed that higher accuracy could be achieved in a shorter computation time than the latter in highly oscillatory cases.

The concept of local shock strength and a quantitative measure index str of local shock strength are proposed, derived from the oblique shock relation and the monotonic relationship between total pressure loss ratio and normal Mach number. Utilizing the high density gradient characteristic of shock waves and the oblique shock relation, a post-processing algorithm for two-dimensional flow field data is developed. The objective of the post-processing algorithm is to obtain specific shock wave location coordinates and calculate the corresponding str from flow filed data under the calibration of the oblique shock relation. Validation of this post-processing algorithm is conducted using a standard model example that can be solved analytically. Combining the concept of local shock strength with the post-processing algorithm, a local shock strength quantitative mapping approach is established for the first time. This approach enables a quantitative measure and visualization of local shock strength at distinct locations, represented by color mapping on the shock structures. The approach can be applied to post-processing numerical simulation data of two-dimensional flows. Applications to the intersection of two left-running oblique shock waves (straight shock waves), the bow shock in front of a cylinder (curved shock wave), and Mach reflection (mixed straight and curved shock waves) demonstrate the accuracy, and effectiveness of the mapping approach in investigating diverse shock wave phenomena. The quantitative mapping approach of str may be a valuable tool in the design of supersonic/hypersonic vehicles and the exploration of shock wave evolution.

Rectangular explosive charges are usually used in military or civilian explosive transportation and storage. The effects of shape parameters and detonation positions on the peak overpressure and maximum impulse of blasts lack comprehensive investigation, which is significant for the design of blast-resistant structures. In this paper, the side-length ratio of the rectangle, orientation, and detonation position of the charge are chosen as controlling parameters to investigate their influence on blast loads in the scaled distances of the gauges ranging from 0.63 to 10.54 m/kg^1/3 with well validated 3D numerical simulations. The results show that there is a large difference in the near-field spatial distribution of the blast load of the rectangular charge; if the blast load of the rectangular charge is simply evaluated with the spherical charge, the maximum peak overpressure (maximum impulse) will be underestimated by a factor of 7.46 (4.84). This must be taken seriously by blast-resistant structure designers. With the increase in the scaled distance, when the critical scaled distance is greater than 6.32 (7.38) m/kg^1/3, the influence of the charge shape on the maximum peak overpressure (maximum impulse) of the spatial blast load can be ignored. In general, the impact of detonation of the charge at the end on the maximum peak overpressure is greater compared with central detonation, but for the impact of the maximum impulse, it is necessary to pay attention to the side-length ratio of the rectangular charge and the specific detonation position on the end face. Furthermore, the structural response of steel plates placed at different azimuths under the blast load of a rectangular charge is preliminarily analyzed, and the results show that the deformation and energy of the plates are consistent with the distribution of the blast load. These analysis results provide a reference for the explosion protection design in near-field air explosions.

This work discusses the strain and acceleration suppression of a flexible beam subjected to different supports analytically. As classical protection, the beam is mounted on a vertical linear spring together with a linear damper in parallel. This is called linear isolation. To enhance isolation performance, nonlinearity is employed in the boundary. In addition, quasi-zero isolation is established based on the non-linearly enhanced one by adjusting the installation length of the horizontal spring. To discuss their performance fully and fairly, the amplitude, the acceleration, the potential energy of the beam, the input work of the excitation, the dissipation work of the beam, and the dynamics stress along the beam are investigated based on the same parameters. The comparison shows that all these isolations can protect the beam with high efficiency, even when the basement excitation is tiny. Although the linear isolation and the nonlinearly enhanced one will arouse two resonance peaks on both sides of the primary resonance of the beam without isolation, the maximum amplitudes of them are reduced a lot. But for the low frequency excitation, the quasi-zero isolation has the best performance as it drives the primary resonance to the high frequency region. The simulation shows that the beam needs a relatively soft isolation to avoid the damage caused by the shock vibration, including the quasi-zero one. In general, the quasi-zero isolation shows the best performance. The nonlinearly enhanced one is the suboptimal choice. The present work shows the capacities of three isolations for a flexible beam by the steady-state response and the shock vibration. It provides design suggestions for the isolation of flexible beams.

The contact problem of deformed rough surfaces exists widely in complex engineering structures. How to reveal the influence mechanism of surface deformation on the contact properties is a key issue in evaluating the interface performances of the engineering structures. In this paper, a contact model is established, which is suitable for tensile and bending deformed contact surfaces. Four contact forms of asperities are proposed, and their distribution characteristics are analyzed. This model reveals the mechanism of friction generation from the perspective of the force balance of asperity. The results show the contact behaviors of the deformed contact surface are significantly different from that of the plane contact, which is mainly reflected in the change in the number of contact asperities and the real contact area. This study suggests that the real contact area of the interface can be altered by applying tensile and bending strains, thereby regulating its contact mechanics and conductive behavior.

Creating conditions to implement equilibrium processes of damage accumulation under a predictable scenario enables control over the failure of structural elements in critical states. It improves safety and reduces the probability of catastrophic behavior in case of accidents. Equilibrium damage accumulation in some cases leads to a falling part (called a postcritical stage) on the material’s stress-strain curve. It must be taken into account to assess the strength and deformation limits of composite structures. Digital image correlation method, acoustic emission (AE) signals recording, and optical microscopy were used in this paper to study the deformation and failure processes of an orthogonal-layup composite during tension in various directions to orthotropy axes. An elastic-plastic deformation model was proposed for the composite in a plane stress condition. The evolution of strain fields and neck formation were analyzed. The staging of the postcritical deformation process was described. AE signals obtained during tests were studied; characteristic damage types of a material were defined. The rationality and necessity of polymer composites’ postcritical deformation stage taken into account in refined strength analysis of structures were concluded.