Acta Mechanica最新文献

Surface irregularities such as local cavities can disturb the boundary layer flow, resulting in local peaks of aerodynamic heating. If the boundary layer flow enters the interior of a local surface cavity, the laminar-to-turbulent transition may be enhanced. In this work, an improved γ-Re_θ transition model for predicting cavity-induced transition is developed. Analysis of the flow structures around the cavity indicates that flow separation occurs in the cavity and a strong adverse pressure gradient appears near the trailing edge. The pressure gradient parameter λ_ζ is proposed as an indicator for local susceptibility to the separation instability. The separation intermittency γ_sep,new, which is constructed based on λ_ζ, is used to account for the effect of separation on the transition. The improved transition model is validated by observing the Mach 6 flow across cavities installed on a flat plate and the windward surface of the Shuttle Orbiter configuration. In addition, the Hypersonic Inflatable Aerodynamic Decelerator configuration is used to further substantiate its universality and appropriateness in separated-flow transition around such a complex configuration. The numerical results show that the improved γ-Re_θ transition model simulates the augmentation of heating and the cavity-induced transition from laminar to turbulent flow, and is in reasonable agreement with experimental results.

Graphene oxide (GO) is a promising candidate for enhancing cement-based composites, but stacking of GO is a negative factor that affects its enhancing capacity. This study aims to understand the impact of stacked GO on the interfacial structures and mechanical properties of cement-based composites from an atomistic insight. Structurally, it was observed that GO forms hydrogen bonds with surface-adsorbed water through its functional groups, which subsequently interact with calcium-silicate-hydrate (C-S-H) to create a cohesive structure. Additionally, stacked GO layers are interconnected through hydrogen bonds formed between their oxidized functional groups. During tensile and shear processes, interface failure primarily occurs between the stacked GO layers rather than at the interface between the GO layer and C-S-H. This is due to the relatively weak interlayer forces and lower interfacial energy dissipation capacity between the stacked GO layers compared to the GO/C-S-H interface. Mechanically, a monolayer of GO resulted in significant improvements in the mechanical properties of C-S-H, with the tensile strength, Young's modulus, shear strength, and shear modulus increased by 77.4%, 19.0%, 25.2%, and 7.6%, respectively. Conversely, stacked GO weakened the mechanical properties of C-S-H, with three GO layers causing a 38.5% reduction in tensile strength and a 14.6% reduction in shear strength. These atomic-level insights enhance our understanding of the interfacial structures and mechanical properties of calcium silicate hydrate reinforced with stacked GO.

Modern engineering increasingly utilizes complex curved shell structures made from lightweight materials, especially in high-performance fields such as aerospace and aeronautic engineering, where stability under extreme conditions is essential. Developing a new generation of auxetic metamaterials with enhanced mechanical properties drives the need for innovative sandwich structures. Accordingly, this study assesses the influence of a novel bio-inspired butterfly-shaped auxetic core on the stability of axially compressed sandwich toroidal shell segments (TSSs), aiming to improve upon traditional re-entrant auxetic structures. The primary focus is to evaluate how this new auxetic design enhances shell stability, which is crucial for advancing lightweight, high-performance structures. Inspired by butterfly wing structures, the butterfly-shaped core improves stiffness and exhibits a negative Poisson's ratio (NPR), leading to superior stability. The face sheets are reinforced with carbon nanotubes (CNTs) embedded in a polymer matrix, with either uniform (UD) or functionally graded (FG) distributions. A three-parameter model represents the Kerr-type elastic foundation, consisting of a central shear layer and two spring layers. The governing equations are derived using von Kármán shell theory and Stein and McElman approximations, with the Galerkin method applied to establish nonlinear load–deflection relationships under simply supported boundary conditions. Validation against existing studies confirms the model's accuracy. Results show that the butterfly-shaped auxetic core outperforms traditional re-entrant structures in terms of stability, critical buckling loads, and postbuckling behavior. The effects of core unit cell geometry, elastic foundation parameters, shell geometry, and CNT distribution are also examined. These findings provide valuable insights into the design of lightweight metamaterial TSSs with NPR.

This study investigates the impact of variable thermal conductivity and magnetic field effects on magneto-photo-thermoelastic wave propagation in hydro-microelongated semiconductor media. A novel theoretical framework is developed by integrating microelongation effects with hydrodynamic interactions, which are rarely considered in microstructured semiconductor models. The governing equations are formulated using photo-thermoelasticity theory and solved analytically using the Laplace transform method. Numerical simulations are conducted to evaluate the effects of temperature-dependent thermal conductivity and external magnetic fields on key physical parameters, including temperature distribution, displacement, normal stress, and carrier density. The results demonstrate that hydrodynamic interactions significantly enhance wave oscillations and prolong the persistence of thermal and stress waves, emphasizing the crucial role of microstructural effects in semiconductor materials. These findings contribute to the optimization of semiconductor devices for photonic, optoelectronic, and thermal management applications.

Nanoshell-type semiconductor structures are essential for designing high-performance integrated electronic devices, such as sensing and energy harvesting. In this study, we apply modified couple stress and flexoelectric theories to perform a size-dependent structural analysis of flexoelectric semiconductor (FS) curved nanoshells. A two-dimensional theory for an arbitrary orthogonal curvilinear coordinate system is derived from the three-dimensional macroscopic theory of flexoelectric semiconductors by using the Kirchhoff–Love shell theory. A combination of physical and geometric parameters is introduced to measure the strength of the coupling between mechanical loads and the redistribution of charge carriers. A trigonometric series solution is obtained for a simply supported rectangular shell structure subjected to a localized normal mechanical load, revealing the concentration of mobile charges and the formation of electrical potential barriers near the loading area. These results are fundamental for the mechanical manipulation of mobile carrier transport in such shell structures. The results indicate that the FS curved nanoshell structure facilitates the redistribution of mobile carriers, correlating with an increase in electrical potential. This work serves as a starting point for understanding the significance of geometric structure on flexoelectric coupling and carrier transport, providing an effective approach to address issues related to nanoscale shell structures in multi-physical field coupling.

The stability boundaries of the Ziegler column are established, in a closed-form, for undamped and viscously damped conditions with equal and unequal damping in the joints. These boundaries are determined by the combined use of Routh–Hurwitz Stability Criterion and the root-locus plots to visualize the unique behavior of the dynamics of the Ziegler Column. Such an approach reveals clearly the reasons and the combination of the column design parameters that give rise to the observed and well-known phenomenon of the “Ziegler Paradox”. In that paradox, unequal dissipative damping forces in the joints induce a destabilizing effect even though the magnitude of these forces can be fairly small. The paradox has been reported in numerous studies indicating that this destabilizing effect is contrary to the common believe that damping is expected to generally have a stabilizing effect. For the undamped Ziegler column, it is found that the stability is achieved when the follower force F is less than 2.54 k with k denoting the equal stiffness of the springs in the joints. For Ziegler columns with equally damped joints, it is found that stability can be attained when the follower force F is less than (1.2c^{2} + 1.46k) with c denoting the equal damping coefficient. But, columns with asymmetrical, or unequal, damping in the joints are found to be always unstable. It is envisioned that the use of the stability tools of the control systems theory enables a better understanding and visualization of the interactions of the design parameters that influence the column stability. Furthermore, these tools will further enhance the analysis of Ziegler columns with multi-degrees of freedom and with active/passive control capabilities.

Based on the nonlocal strain gradient theory and high-order surface stress model, the mechanical properties on bending and vibration of functionally graded graphene-reinforced composite curved nanobeam are investigated. The general governing equations for the dynamic behavior of curved nanobeam are formulated. The Halpin–Tsai model and the mixture rule are utilized to estimate the effective Young’s modulus and Poisson’s ratio of composite curved beams. The influences of graphene mass fraction, graphene sheet distribution type, beam radian, and high-order surface effect on the mechanical properties of bending and vibration of curved beam are analyzed. In addition, the dependences of the beam deflection, axial displacement and rotation degree in the process of beam vibration on the width-to-thickness ratio and aspect ratio of graphene sheet are discussed. The rationality and applicability of the present model are validated. It is demonstrated that the graphene sheets, the beam radian, and the high-order surface effects on the bending and vibrational properties of curved beam are significant.

Numerous studies have explored beam bending involving piezoelectric effect and flexoelectric effect. However, a higher-order bending theory for transversely isotropic piezoelectric beam has not yet been established, and the associated independent material parameters remain unclear. In this paper, the higher-order bending theory of transversely isotropic beams is presented based on the general dielectric theory including strain gradient and polarization gradient. The general constitutive equations of transversely isotropic dielectrics are detailed for the first time. A semi-inverse solution for a transversely isotropic beam under plane-strain conditions is developed, and meanwhile, the Bernoulli–Euler bending solution is also obtained. The plane-strain solution for a purely bending beam considering strain gradient and polarization gradient can reduce to that of the Bernoulli–Euler beam when the strain along thickness direction is neglected. The electric potential induced by piezoelectric and flexoelectric effects is examined. We hope that the results of this paper will serve as a reference for verifying the reliability of numerical calculation methods and contribute to a deeper understanding of electromechanical coupling effects.

The work focuses on the transference of Love-type waves which are surface seismic waves that cause horizontal displacement perpendicular to the direction of propagation, in a multiferroic solid cylindrical structure, where the interface is assumed to be imperfect and made of a magneto-electro-elastic (MEE) structure. The analytical solution for the layer is obtained using the spatially variable quasi-classical technique which approximates complicated differential equations while maintaining their key physical properties. The coefficients of wave’s phase velocities and attenuation are greatly affected by different parameters as shown in the numerical example. In addition, a graphical comparison of electrical, magnetic, mechanical, magneto-mechanical, electromechanical, and magneto-electrical imperfections in electrically and magnetically open and short cases is presented. The phase velocity is higher in the electrically and magnetically open case as compared to the short case as shown in the results. Some major outcomes are summarized here: the bonding parameter is highly proportional to the phase velocity and inversely proportional to the attenuation coefficient, and imperfection parameters have a serious influence on the curve of phase velocity and attenuation coefficient. This theoretical study leads to the understanding of piezoelectric and piezomagnetic coupling and its potential application and design to sensors, actuators, energy harvesters, and nano-electronics. The novelty lies in the adoption of the quasi-classical method to approach solving differential equations using a polar coordinate system for the first time.

Nipping analysis of graduated-length leaf springs with uniform rectangular cross section of all leaves is presented. Initial gaps between leaves of the pre-assembled spring are determined such that the maximum bending stresses in all the leaves of the assembled and externally loaded spring become equal to each other. The expressions are derived for leaf springs consisting of two-to-five graduated-length leaves, which are stiffened by an additional full-length leaf placed atop. The reduction of the maximum bending stress achieved by nipping is quantified in each case. The deformed shapes and the residual gaps between the leaves in the loaded spring configuration are determined and discussed.