Acta Mechanica Sinica最新文献

Monitoring the shape and deformation of morphing wings is vital for ensuring multi-mission flight and safety operation. During the morphing process, the complex deformation of the flexible skin wing usually involves large amounts of movement, shearing, bending, and distortion. This paper proposes an improved stereo-digital image correlation measurement system designed to characterize full-field complex large deformation of flexible skin shear variable-sweep wings (SVSWs). By minimizing reference image updating frequency using the proposed conditional incremental strategy, effectively addressing the computational failures caused by image decorrelation due to complex large deformations. To improve tracking efficiency and accuracy of uncoded targets in complex backgrounds, an automatic subpixel detection method for circular diagonal targets is presented. A series of experiments are performed on a 1200 mm span flexible skin SVSW to verify the proposed methods. The results show that the length and angle measurement accuracies are better than 0.11 mm and 0.05°, respectively. Based on the measured morphing geometry parameters, displacement and strain fields, the structural integrity and morphing performance of the wing under different loads are discussed. During the shear variable-sweep process, the wingtip load dominates the deflection distribution, while its effect on the strain distribution is relatively minor. The proposed method and system can provide reliable data support for the structural optimization design and safety evaluation of such morphing wings.

In the theory of two-dimensional linear elasticity, an elliptical inclusion is known to attain a constant stress field when perfectly buried in an infinite homogeneous matrix if a uniform eigenstrain is applied to it. The focus of this paper falls on the question: when the initially elliptical inclusion verges on a bi-material interface, what would happen to its configuration if it is required to retain the internal constant stress? Specifically, we explore the anti-plane shear version of this question (the version of plane deformations or three-dimensional deformations seems, however, insoluble at this stage), in which an inclusion undergoing a uniform (anti-plane shear) eigenstrain is embedded in a bi-material structure composed of two infinite elastic half-planes whose interface is straight and perfectly bonded, and the shape of the inclusion is to be determined such that the eigenstrain-induced stress inside the inclusion appears to be a constant. Unlike most optimization methods-driven solution procedures for finding the shape of the inclusion approximately in which huge computation is required, we derive by a rigorous theoretical analysis an exact integral equation with respect to the boundary curve of the inclusion that is sufficiently and necessarily related to the existence of a constant stress inside the inclusion. We solve this integral equation via the use of some analytic techniques and present in several illustrative examples a variety of shapes of the inclusion achieving constant stresses. We discover some interesting phenomena for the evolution of the shape of the uniformly stressed inclusion relative to the stiffness of the nearby interface.

Flat plate impact experiments are crucial in assessing the dynamic mechanical properties of materials. However, yaw angle tolerances always affect the accuracy of the results. To analyze this effect, this study conducted numerical simulations and theoretical derivations of non-ideal plate impacts. By comparing the simulated results of spallation, shock wave propagation, and free surface velocity, laws governing the effect of yaw angle on the plate impact were summarized. We observed that yaw angles influence the wave-action time and the shape of the compression zone, which affects the trigger and location of spallation and the free surface velocity of the target. Additionally, the yaw angle diminishes the kinetic energy of the target. When the yaw angle exceeds 2°, a significant energy reduction occurs as the shock wave propagates, which results in insufficient energy for complete spallation. Our analyses led to proposing methods for determining the critical yaw angle in plate impact experiments and to introducing a multipoint-velocimetry approach to calculate the non-ideal impact posture of the flyer. Notably, the findings revealed that 0.2° could serve as the critical yaw angle in certain scenarios. Leveraging these research outcomes judiciously can aid in assessing experimental deviations effectively and optimizing experimental costs.

The stability of gaseous detonation waves is crucial for the operation of detonation-based propulsion systems and the assessment of industrial explosion hazards. However, research on the stability of detonation waves in complex reactive systems that are composed of actual fuels and oxidants and can be described by numerous elementary chemical reactions, has not been fully carried out. To investigate the relationship between linear and nonlinear stabilities in gaseous detonation wave propagation for complex reactive systems, the linear stability analysis and the one-dimensionally nonlinear numerical simulations of H₂/O₂/Ar (argon) detonations based on the reactive Euler equations and detailed reaction mechanisms are carried out. The results show that in complex reactive systems characterized by elementary chemical reactions, the results of linear stability computation of detonation are consistent with those from one-dimensionally nonlinear oscillations of detonation wave. Utilizing these linear stability results, a neutral stability curve and a perturbation frequency transition curve in the phase plane of initial pressure versus inert gas (Ar) dilution ratio are derived, especially the new frequency transition curve clearly describes the transition of perturbations from low-frequency to high-frequency mode. One-dimensional nonlinear simulations show that near the perturbation frequency transition curve, the oscillations of the detonation wave can also transform between the low-frequency, high-amplitude oscillation mode and the high-frequency, low-amplitude oscillation mode, with the oscillation frequency corresponding to the mode that exhibits the maximum growth rate identified in the linear stability analysis. This investigation into detonation stability in complex reactive gases offers guidance for selecting appropriate initial conditions and gas compositions in practical applications of detonation.

Non-Schmid (NS) effects in body-centered cubic (BCC) single-phase metals have received special attention in recent years. However, a deep understanding of these effects in the BCC phase of dual-phase (DP) steels has not yet been reached. This study explores the NS effects in ferrite-martensite DP steels, where the ferrite phase has a BCC crystallographic structure and exhibits NS effects. The influences of NS stress components on the mechanical response of DP steels are studied, including stress/strain partitioning, plastic flow, and yield surface. To this end, the mechanical behavior of the two phases is described by dislocation density-based crystal plasticity constitutive models, with the NS effect only incorporated into the ferrite phase modeling. The NS stress contribution is revealed for two types of microstructures commonly observed in DP steels: equiaxed phases with random grain orientations, and elongated phases with preferred grain orientations. Our results show that, in the case of a microstructure with equiaxed phases, the normal NS stress components play significant roles in tension-compression asymmetry. By contrast, in microstructures with elongated phases, a combined influence of crystallographic texture and NS effect is evident. These findings advance our knowledge of the intricate interplay between microstructural features and NS effects and help to elucidate the mechanisms underlying anisotropic-asymmetric plastic behavior of DP steels.

In recent decades, capacitive pressure sensors (CPSs) with high sensitivity have demonstrated significant potential in applications such as medical monitoring, artificial intelligence, and soft robotics. Efforts to enhance this sensitivity have predominantly focused on material design and structural optimization, with surface microstructures such as wrinkles, pyramids, and micro-pillars proving effective. Although finite element modeling (FEM) has guided enhancements in CPS sensitivity across various surface designs, a theoretical understanding of sensitivity improvements remains underexplored. This paper employs sinusoidal wavy surfaces as a representative model to analytically elucidate the underlying mechanisms of sensitivity enhancement through contact mechanics. These theoretical insights are corroborated by FEM and experimental validations. Our findings underscore that optimizing material properties, such as Young’s modulus and relative permittivity, alongside adjustments in surface roughness and substrate thickness, can significantly elevate the sensitivity. The optimal performance is achieved when the amplitude-to-wavelength ratio (H/λ) is about 0.2. These results offer critical insights for designing ultrasensitive CPS devices, paving the way for advancements in sensor technology.

This work focuses on the fluid-rigid interaction dynamics in the presence of a magnetic field. A rigid thin rectangular column immersed inside stationary metal liquid vibrates with a fixed small amplitude. The magneto-fluid-solid interaction (MFSI) dynamics issue is studied based on the complex Green’s function method. Considering either the normal or tangential vibration of a column, two types of semi-analytical solutions expressed by stream function integral equations of magnetic corrections, describing the time-displacement history of the column, flow field and electrical potential field of metal fluid and representing transient coupling effects of multi-physics field, are derived, respectively. Nonuniform discretization schemes and an iterative plan are applied to evaluate added damping and inertial loads. The results show that the main factor affecting normal vibration is pressure load, and the main factor affecting tangential vibration is vorticity load. The nonlinear effects of magnetic fields on the dynamics of fluid-rigid thin columns are revealed. The normal vibration exhibits better stability than the tangential vibration under the magnetic field. The induced electrical potential field and current intensity excited by normal vibration are significantly stronger than that of tangential vibration. These semi-analytical solutions can be applied as benchmarks in future validation and verification works for MFSI numerical algorithms for magnetic confinement nuclear fusion science.

This paper explores the applicability of the generalized wave impedance hypothesis in split Hopkinson pressure bar (SHPB) experiments, particularly under non-ideal conditions. The study investigates the effects of changes in wave impedance ratio and cross-sectional area ratio on the dynamic response of materials at high strain rates. Through theoretical analysis and numerical simulation, the impact of different wave impedance and cross-sectional area ratios on stress wave propagation characteristics is discussed in detail. It is found that when the cross-sections of two bars differ, shear strain occurs at the abrupt cross-section, leading to waveform distortion in the transmitted and reflected waves. The force balance condition does not always align with the momentum conservation theorem, and only when the three waveforms and wavelengths are completely consistent do they align. The research shows that when the wave impedance ratio and cross-sectional area ratio are within a specific range, the generalized wave impedance hypothesis can accurately predict changes in Young’s modulus and density. Additionally, the study extends the exploration to key factors such as wave impedance ratio, wave speed, Young’s modulus, density, and area ratio.

The sound field driven by piping systems in enclosures may severely affect living comfort, which is frequently encountered in various engineering applications. Managing this sound field relies heavily on the available prediction tools at hand, e.g., the widely used finite element methods are computationally expensive due to the necessity to discretize entire space, analytical models, based on modal expansion method, may offer substantial advantages in terms of computational cost and efficiency. However, deriving eigenmodes of irregular enclosed spaces may be challenging, which impedes accurate and rapid predictions of the sound field in practical applications. This study presents an analytical framework aimed at rapidly and accurately predicting the interior sound field driven by the piping system vibrations in irregular enclosures. Vibration response of the piping system is obtained using the wave approach, and a line dipole source is idealized as the sound source of the piping system vibration. On the basis of eigenmodes of regular enclosures, the Kirchhoff-Helmholtz integral theorem (modal expansion method for irregular enclosures) is introduced to account for the boundaries of irregular enclosures. This theoretical framework is validated through numerical simulations by finite element method and experiments, demonstrating high accuracy and significant efficiency advantages. The proposed method can be further employed to optimize radiated sound fields by tailoring the impedance of space walls or layout of piping systems. This study provides an efficient tool for predicting radiated sound field in general enclosures driven by vibration of piping systems, paving a new path for indoor acoustical optimization.

Osteoarthritis is one of the most common joint diseases, leading to joint pain, dysfunction, and a reduced quality of life for patients. Therefore, it is particularly important to explore more effective prevention, treatment and management methods to relieve patients’ pain and enhance their quality of life. Among physical therapies, pulsed electrical stimulation (PES) is considered to be a promising treatment method due to its high safety and ease-of-use features. PES provides a non-invasive, safe and effective option for patients. However, there are fewer studies on the biomechanical changes of PES in periarticular tissues, and its effects on the biological behavior of chondrocytes remain unknown. This study investigated the effects of PES on the biomechanical properties of osteoarthritic joints and the biological behavior of chondrocytes. The results showed that PES with an intensity of 10 mA and a frequency of 4 Hz increased the cross-sectional area of muscle fibers, prevented muscle atrophy and loss of function, and restored the mechanical properties of muscle tissue. PES also effectively increases the resistivity of knee osteoarthritis cartilage tissue, as well as the elastic modulus of cartilage, which can enhance the biomechanical characteristics of cartilage tissue. PES also promoted the metabolic activity of chondrocytes and increased cartilage matrix synthesis, thereby improving the overall structure and mechanical properties of cartilage tissue. Additionally, cellular experiments showed that 5 consecutive days of 800 mV PES significantly increased the expression level of Piezo1 gene in chondrocytes. At the same time, the expression of type II collagen and transforming growth factor beta increased, while the expression of matrix metallopeptidase 13 decreased. These changes favored the promotion of cartilage matrix synthesis. This has a positive effect on protecting and improving joint health and reducing the impact of osteoarthritis, and is important for understanding the mechanism of action of PES on chondrocytes and the development of related therapeutic strategies.