Acta Mechanica Sinica最新文献

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.

The lamellar microstructure is one of the most typical microstructures of TiAl alloys. There are three γ/γ interfaces with different microstructures in lamellar γ-TiAl alloys. In this work, we investigated the deformation processes of lamellar γ-TiAl alloys with different interfacial spacing (λ) via uniaxial tensile loading using molecular dynamics simulations, including true twin (TT), pseudo-twin (PT), rotational boundary (RB), and the mixed structure (TT ∥ PT ∥ RB). The results show that in all lamellar γ-TiAl samples, the Shockley partial dislocation prefers to nucleate in the region between two neighboring interfaces. Then, dislocations move towards, crossing the γ/γ interface. Finally, the dislocation slippage leads to the destruction of the interface, resulting in cracks and structural failure. With the decrease of λ, the ultimate strength slightly increases in the TT or PT structure of γ-TiAl, which follows the Hall-Petch relation. But in general, the interfacial spacing has a slight effect on the ultimate strengths of these four structures of γ-TiAl.

This work investigates the dynamic response of a monocrystalline nickel-titanium (NiTi) alloy at the atomic scale. The results deduced from non-equilibrium molecular dynamics modeling demonstrate that no shear deformation band (SDB) appears in the sample at an impact velocity of less than 0.75 km/s. As this velocity increases, shear deformations become pronouncedly localized, and the average spacing between SDBs decreases until it stabilizes. Combining shear stress and particle velocity profiles, the survival of SDBs is found to be closely associated with plastic deformation. The dislocations clustering around SDBs predominantly exhibit 〈100〉 partial dislocations, whereas 1/2〈111〉 full dislocations are dominant in those regions without SDBs. Void nucleation always occurs on SDBs. Subsequently, void growth promotes a change in the SDB distribution characteristic. For the case without SDB, voids are randomly nucleated, and the void growth exhibits a non-uniform manner. Thus, there is an interaction between shear localization and void evolution in the NiTi alloy subjected to intensive loading. This study is expected to provide in-depth insights into the microscopic mechanism of NiTi dynamic damage.

Nanomaterials have garnered recognition for their notable surface effects and demonstration of superior mechanical properties. Previous studies on the surface effects of nanomaterials, employing the finite element method, often relied on simplified two-dimensional models due to theoretical complexities. Consequently, these simplified models inadequately represent the mechanical properties of nanomaterials and fail to capture the substantial impact of surface effects, particularly the curvature dependence of nanosurfaces. This study applies the principle of minimum energy and leverages the Steigmann-Ogden surface theory of nanomaterials to formulate a novel finite element surface element that comprehensively accounts for surface effects. We conducted an analysis of the stress distribution and deformation characteristics of four typical 2D and 3D nanomaterial models. The accuracy of the developed surface element and finite element calculation method was verified through comparison with established references. The resulting finite element model provides a robust and compelling scientific approach for accurately predicting the mechanical performance of nanomaterials.

Periodic isolator is well known for its wave filtering characteristic. While in middle and high frequencies, the internal resonances of the periodic isolator are evident especially when damping is small. This study proposes a novel aperiodic vibration isolation for improving the internal resonances control of the periodic isolator. The mechanism of the internal resonances control by the aperiodic isolator is firstly explained. For comparing the internal resonances suppression effect of the aperiodic isolator with the periodic isolator, a dynamic model combing the rigid machine, the isolator, and the flexible plate is derived through multi subsystem modeling method and transfer matrix method, whose accuracy is verified through the finite element method. The influences of the aperiodicity and damping of the isolator on the vibration isolation performance and internal resonances suppression effect are investigated by numerical analysis. The numerical results demonstrate that vibration attenuation performances of the periodic isolator and aperiodic isolator are greatly over than that of the continuous isolator in middle and high frequencies. The aperiodic isolator opens the stop bandgaps comparing with the periodic isolator where the pass bandgaps are periodically existed. The damping of the isolator has the stop bandgap widening effect on both the periodic isolator and the aperiodic isolator. In addition, a parameter optimization algorithm of the aperiodic isolator is presented for improving the internal resonances control effect. It is shown that the vibration peaks within the target frequency band of the aperiodic isolator are effectively reduced after the optimization. Finally, the experiments of the three different vibration isolation systems are conducted for verifying the analysis work.

We propose a hybrid quantum-classical method, the quantum-enriched large-eddy simulation (QELES), for simulating turbulence. The QELES combines the large-scale motion of the large-eddy simulation (LES) and the subgrid motion of the incompressible Schrödinger flow (ISF). The ISF is a possible way to be simulated on a quantum computer, and it generates subgrid scale turbulent structures to enrich the LES field. The enriched LES field can be further used in turbulent combustion and multi-phase flows in which the subgrid scale motion plays an important role. As a conceptual study, we perform the simulations of ISF and LES separately on a classical computer to simulate decaying homogeneous isotropic turbulence. Then, the QELES velocity is obtained by the time matching and the spectral blending methods. The QELES achieves significant improvement in predicting the energy spectrum, probability density functions of velocity and vorticity components, and velocity structure functions, and reconstructs coherent small-scales vortices in the direct numerical simulation (DNS). On the other hand, the vortices in the QELES are less elongated and tangled than those in the DNS, and the magnitude of the third-order structure function in the QELES is less than that in the DNS, due to the different constitutive relations in the viscous flow and ISF.