Acta Mechanica Sinica最新文献

Creep is an important mechanical property of refractory high-entropy alloys (RHEAs) at high temperatures. The existence of short-range order (SRO) and its ability to improve the strength or plasticity of high-entropy alloys (HEAs) have been experimentally proven. However, there is still little research on the correlation between SRO and creep behavior. The mechanism of SRO influencing creep behavior is not yet clear. In this work, the creep behaviors of TiVTaNb RHEA with and without SRO were simulated at various temperatures and stresses using molecular dynamics methods, and the effects of SRO on creep behavior were analyzed. The results show that the SRO is energetically favorable for occurrence in this RHEA. For polycrystalline RHEAs, grain boundary energy is an important driving force for the formation of SRO. Significantly, under the same conditions, the SRO can reduce the steady-state creep rate and change the creep mechanism of the RHEA. Specifically, the models with SRO will exhibit lower stress exponent and grain-size exponent. A mechanism by which SRO reduces the effects of grain boundaries on creep has been discovered. These phenomena can be well explained by the effects of SRO on atomic diffusion. In addition, by analyzing the diffusion ability of different elements, SRO can induce localization of atomic diffusion, resulting in strain localization under high stresses. This work highlights the importance of SRO on the creep of RHEAs and provides a reference for establishing a reasonable creep model of RHEAs.

Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simulations for a Mach 6 flow over a 5.08 mm nose radius cone. Two frequencies of 33 kHz and 150 kHz are considered to be representative of the first and second instability modes, respectively. For the first mode, wall pressure fluctuations for both entropy and vorticity wave cases exhibit a strong modulation yet without a growing trend, indicating that the first mode is not generated despite its instability predicted by linear stability theory. The potential reason for this is the absence of postshock slow acoustic waves capable of synchronizing with the first mode. By contrast, for the second mode, a typical three-stage boundary-layer response is observed, consistent with that to slow acoustic waves studied previously. Furthermore, the postshock disturbances outside the boundary layer can be decomposed into the entropy (density/temperature fluctuations) and vorticity components (velocity fluctuations), and the latter is shown to play a leading role in generating the second mode, even for the case with entropy waves where the density/temperature fluctuations dominate the postshock regions.

Nonlinear characteristics have demonstrated significant advantages in mitigating vibrations across various engineering applications, particularly in effectively suppressing vibrations over a wide frequency range. This paper introduces a novel nonlinear energy sink with a magnetic inerter (MINES). The MINES features a magnetic lead screw that incorporates a pair of helical permanent magnets. When the inner part undergoes linear motion, it is transformed into the rotation of the outer part at a predetermined conversion ratio. Subsequently, the MINES is incorporated into a system with a single degree of freedom, and the corresponding differential equations of motion are derived. The approximate analytical method and the numerical method are used to validate each other. This process clarifies the effectiveness of the MINES in reducing vibrations when subjected to harmonic excitation. The influence of the parameters of the MINES is analyzed. The findings demonstrate that the MINES offers significant benefits in terms of vibration suppression efficiency when the depths of the three barriers are equal. Furthermore, with the increase in excitation amplitude, the MINES enters the nonlinear range, leading to a reduction in system damping. This can effectively prevent the phenomenon of traditional damping stiffening under conditions of high amplitude excitation. Finally, the vibration reduction capability of this nonlinear energy sink was experimentally demonstrated, enhancing its applicability in vibration mitigation.

The coconut structure exhibits inherent impact resistance, with the macroscopically ordered distribution of variable cross-section fibers in its husk playing a crucial role in stress wave propagation and scaling. Inspired by the natural structure and fibers, this study proposes a stress wave propagation model for a variable cross-section bar considering viscous effects. A theoretical model for stress wave propagation in a fusiform-shaped bar with variable cross-section is established, elucidating the stress wave scaling effect observed in coconut fibers. Additionally, a quasi-one-dimensional method for analyzing and measuring stress wave propagation is introduced, and an experimental setup is assembled. Experimental validation of the stress wave scaling effect confirms the theory’s accuracy for stress wave scaling in variable cross-section bars. This research provides theoretical guidance and measurement methods for the design of space landers, automobile anti-collision beams, stress wave collectors, and scalers, as well as for impact testing of macro and micro materials and the design of sustainable plant-based materials for impact protection.

Based on the theoretical representation of piezoelectric quasicrystal, a generalized dynamic model is built to represent the transmission of wave aspects in surface acoustic pulse nano-devices. Surface elasticity, surface piezoelectricity, and surface permittivity help to include the surface effect, which equals additional thin sheets. It is shown that, under certain assumptions, this generalized dynamic model may be simplified to a few classical examples that are appropriate for both macro and nano-scale applications. In the current work, surface piezoelectricity is used to develop a theoretical model for shear horizontal (SH) waves where it contains the surface piezoelectricity theory and a linear spring model to quantitatively and qualitatively explore SH waves in an orthotropic piezoelectric quasicrystal layer overlying an elastic framework (Model I), a piezoelectric quasi-crystal nano substrate, and an orthotropic piezoelectric quasicrystal half-space (Model II). The theoretical model stimulates the numerical results, which establish the critical thickness. As the piezoelectric layer’s thickness gets closer to nanometres, surface energy must be included when analyzing dispersion properties. Furthermore, the effects of surface elasticity and density on wave velocity are investigated individually. The authors establish a parameter, precisely the ratio of the physical modulus along the width direction to along the direction of wave travel. The surface effect’s impact on the general characteristics of piezoelectric structures is seen as a spring force acting on bulk boundaries. Analytical presentation of frequency equations for both symmetric and anti-symmetric waves pertains to the case of an electrical short circuit in Model II. The project aims to analyze SH waves in orthogonal anisotropic, transversely isotropic piezoelectric layered nanostructures, providing a practical mathematical tool for surface effects analysis and adaptability to other wave types, including Rayleigh waves and acoustic surface waves.

Rubber-like materials that are commonly used in structural applications are modelled using hyperelastic material models. Most of the hyperelastic materials are nearly incompressible, which poses challenges, i.e., volumetric locking during numerical modelling. There exist many formulations in the context of the finite element method, among which the mixed displacement-pressure formulation is robust. However, such a displacement-pressure formulation is less explored in meshfree methods, which mitigates the problem associated with mesh distortion during large deformation. This work addresses this issue of alleviating volumetric locking in the element-free Galerkin method (EFGM), which is one of the popular meshfree methods. A two-field mixed variational formulation using the perturbed Lagrangian approach within the EFGM framework is proposed for modelling nearly incompressible hyperelastic material models, such as Neo-Hookean and Mooney-Rivlin. Taking advantage of the meshless nature of the EFGM, this work introduces a unique approach by randomly distributing pressure nodes across the geometry, following specific guidelines. A wide spectrum of problems involving bending, tension, compression, and contact is solved using two approaches of the proposed displacement-pressure node formulation involving regular and irregular pressure node distribution. It is observed that both approaches give accurate results compared to the reference results, though the latter offers flexibility in the pressure nodal distribution.

Auxetic metamaterials have attracted much attention due to their outstanding advantages over traditional materials in terms of shear capacity, fracture resistance, and energy absorption. However, there are lack of design inspirations for novel auxetic structures. According to the materials databases of atomic lattice, some natural crystals possess negative Poisson’s ratio (NPR). In this paper, the mechanism of auxeticity in microscale Ti crystal is investigated through density functional theory simulation. Then we propose a macroscopic auxetic metamaterial by mimicking the microscopic atomic lattice structure of the body-centered cubic Ti crystal. The NPR property of the macroscopic metamaterial is verified by theoretical, numerical and experimental methods. The auxeticity keeps effective when scaling up to macroscopic Ti crystal-mimic structure, with the similar deformation mechanism. Furthermore, from the geometric parameter investigation, the geometric parameters have great influence on the Poisson’s ratio and Young’s modulus of the macroscopic metamaterial. Importantly, an optimized structure is obtained, which exhibits 2 times enhancement in auxeticity and 25 times enhancement in normalized Young’s modulus, compared to the original architecture. This work establishes a link between the physical properties at micro-nanoscale and macroscale structures, which provides inspirations for high load-bearing auxetic metamaterials.

The cavitation tunnel with controlled background pressure is a pivotal experimental setup for studying the mechanisms of cavitating flows and hydrodynamic loads on cavitating bodies. Existing recirculating cavitation tunnels predominantly feature horizontal test sections for modeling cavity flows in horizontal incoming flow and vertical gravitational acceleration and fail to meet the requirements for long-duration experiments on ventilated cavity flows. This paper introduces the unique gravity-driven vertical water tunnel (GVWT), facilitating hydrodynamic experiments on axisymmetric slender bodies with ventilated cavities in the streamwise gravitational acceleration. It elaborates high-throughput data processing method for synchronously measured high-speed camera images of cavity forms and pressure distribution from sensor arrays on model surfaces in unsteady long-duration ventilation conditions. For the ventilated cavity flow against an axisymmetric slender body with 60° cone headform at zero angle of attack, the developed partial cavity can be divided into four regimes: The sheet cavity, the combined sheet and cloud cavity, the entire cloud cavity, and the shedding cloud cavity. The mean cavity length and thickness are well-defined by the high-speed images. For the unsteady ventilated cavity due to the re-entrant jet, the Strouhal number based on cavity length and pulsation frequency of the cloud cavity equals 0.276. The mean pressure distribution in the ventilated cavity reveals a difference between the pressure within the sheet cavity and the maximum pressure in the cavity closure, which is influenced by the streamwise gravitational effect. The experimental results demonstrate that GVWT provides a novel experimental approach for understanding the physics of ventilated cavity evolution and bubbly flows under the effect of the streamwise gravitational acceleration.

The present study proposes a modified random sequential absorption (RSA) algorithm to generate a representative volume element (RVE) model for predicting the elastic properties of discontinuous curved fiber reinforced composites (DCFRCs) with varying fiber waviness functions and orientations. A small-move method was proposed to modify the traditional RSA algorithm. In comparison with the original RSA algorithm, the generation efficiency of the proposed modified RSA algorithm increased by over 40%, and the achievable maximum fiber volume fraction could reach up to 15% with a fiber aspect ratio of 15. The generated RVE model was utilized in conducting finite element analysis to investigate the effect of fiber waviness and wavy functions on the elastic properties of DCFRCs. Finally, a modified rule-of-mixture was proposed to predict the elastic properties of DCFRCs with various fiber orientations. The results indicated that the elastic properties predicted by the modified rule-of-mixture were in good agreement with those obtained from the RVE model, thereby demonstrating its effectiveness.

Abnormal biomechanics plays a central role in the development of knee osteoarthritis (KOA). Low-intensity laser therapy (LILT) is considered an applicable method for the treatment of osteoarthritis. Current research on LILT for the treatment of KOA has focused on the regeneration of articular cartilage. Its biomechanical changes in periarticular tissues have been less well studied, and its role in improving abnormal joint biomechanics is unclear. This study aimed to investigate the role of LILT in improving the biomechanical properties of muscle and cartilage in KOA joints to alleviate cartilage degradation. In this study, a semiconductor laser with a wavelength of 808 nm was used to perform laser interventions in a KOA rat model 3 days per week for 6 weeks. The results of muscle stretch tests showed that LILT could significantly reduce the modulus of elasticity of KOA soleus muscle. Hematoxylin and eosin staining showed that LILT significantly increased the cross-sectional area of the soleus muscle fibers. This suggests that LILT alleviated KOA-induced soleus muscle atrophy and restored the mechanical properties of the muscle tissue. The results of compressive elastic modulus and electrical impedance characterization of cartilage showed that laser intervention significantly increased the elastic modulus and resistivity of cartilage. Results from safranin o-fast green staining and immunohistochemistry showed that LILT significantly increased the synthesis of type II collagen in the cartilage matrix. This may be one of the potential mechanisms by which LILT improves the mechanical properties of cartilage. In addition, immunohistochemistry also showed that LILT reduced the expression of matrix metalloproteinase-13 in cartilage and effectively inhibited the degradation of the cartilage matrix in KOA. In conclusion, the present study demonstrated that LILT alleviated the abnormal biomechanics of KOA joint tissues by improving the mechanical properties of joint muscles and cartilage, thereby slowing down the degradation of KOA cartilage.