Acta Mechanica Sinica最新文献

A multiscale nonlocal continuum model is proposed to describe the superelastic deformation of gradient nano-grained NiTi shape memory alloys (SMAs). At the mesoscopic scale, the polycrystalline aggregate is regarded as a composite, i.e., the grain-interior (GI) phase is assumed to be a cuboidal inclusion embedded in a matrix of grain-boundary (GB) phase. An intrinsic energetic length and a gradient energy are introduced into the Helmholtz free energy of the GI phase. The criterion of martensite transformation (MT) is derived based on the principle of virtual power and second law of thermodynamics. The hindering effect of GB on MT in GI phase is addressed. By deriving the analytical solution of the proposed model and introducing a scale transition rule, the overall and local stress-strain responses of the specimen at the macroscopic scale are obtained. The prediction capability of the proposed model is verified by comparing the analytical solution with the experiment. The influences of the distribution form for the grain size (GS) on the superelastic deformation of gradient nano-grained NiTi SMAs are further predicted and discussed. The analytical form and low computational cost of the proposed model make it an appropriate theoretical tool to design the gradient nano-grained SMAs with desired mechanical property.

The demand for lightweight and multifunctional surface structure in high-end equipment is steadily growing. The harmonization between flexibility and electromagnetic tunability has become a significant subject for stealth morphing aircraft. This paper presents a microwave absorbing structure based on the kirigami configuration, aiming at improving the conformality with the negative Poisson’s ratio characteristic and expanding the radar stealth range with tunability. A precise electromagnetic reflectivity model of the impedance surface was established by the inversion method, and an integrated optimization algorithm was employed to optimize the structural parameters based on numerical analysis. Specimens composed of thermoplastic polyurethane elastic colloids and resistive materials were prepared to assess the in-plane mechanical tensile and electromagnetic absorption performances through experimental methods. The results indicate that the original absorption band spans 6.2–11.1 GHz, shifts to 8–18 GHz with stretching at a panel rotation angle of 16°, and remains nearly constant for further stretching. The specimens adhere to complex curved surfaces well in experiments and maintain the electromagnetic absorption performance compared with flat surfaces. This research offers a valuable reference for designing electromagnetic stealth structures that are highly stretchable and adjustable.

The dynamics of beams subjected to moving loads are of practical importance since the responses caused by these loads can be greater than those under equivalent static loads in some cases. In this work, a novel inertial nonlinear energy sink (NES) is applied for the first time to achieve vibration suppression in beams under moving loads. Based on the Timoshenko beam theory, the nonlinear motion equations of a beam with an inertial NES are derived using the energy method and Lagrange equations. The Newmark-β method combined with the Heaviside step function is adopted to calculate the responses of the beam under moving loads of constant amplitude and harmonic excitation. The accuracy of the modelling derivation and solution methodology are validated through comparisons with results from other studies. The results demonstrate that the velocity and excitation frequency of the moving load significantly affect the response of the beam as well as the performance of the inertial NES. To enhance its effectiveness under various moving load conditions, parametric optimization is numerically performed. The optimized inertial NES can achieve good performance by efficiently reducing the maximum deflection of the beam. The findings of this study contribute to advancing the understanding and application of NESs in mitigating structural vibrations caused by moving loads.

A lightweight composite resonator, consisting of a soft material acoustic black hole (SABH) and multiple vibration absorbers, is embedded in a plate to achieve localization and absorption of low-frequency vibration energy. The combination of local and global admissible functions for displacement enhances the accuracy of the Ritz method in predicting vibration localization characteristics within the SABH domain. Utilizing soft materials for the SABH can reduce the mass and frequency of the composite resonator. Due to the lack of orthogonality between global vibration modes and localized modes, the low-frequency localized modes induced by the SABH are used to shape the initial global modes, thereby concentrating the global vibration of the plate in the SABH region. Consequently, the absorbers of the composite resonator only need to be a small fraction of the mass of the local SABH to achieve substantial vibration control of the host plate. This vibration localization strategy can significantly reduce the vibration amplitude of the host plate and enhance the effectiveness of lightweight absorbers in vibration reduction.

This study numerically investigates inclined magneto-hydrodynamic natural convection in a porous cavity filled with nanofluid containing gyrotactic microorganisms. The governing equations are nondimensionalized and solved using the finite volume method. The simulations examine the impact of key parameters such as heat source length and position, Peclet number, porosity, and heat generation/absorption on flow patterns, temperature distribution, concentration profiles, and microorganism rotation. Results indicate that extending the heat source length enhances convective currents and heat transfer efficiency, while optimizing the heat source position reduces entropy generation. Higher Peclet numbers amplify convective currents and microorganism distribution complexity. Variations in porosity and heat generation/absorption significantly influence flow dynamics. Additionally, the artificial neural network model reliably predicts the mean Nusselt and Sherwood numbers ((overline{Nu}) & (overline{Sh})), demonstrating its effectiveness for such analyses. The simulation results reveal that increasing the heat source length significantly enhances heat transfer, as evidenced by a 15% increase in the mean Nusselt number.

Developing lightweight lattice materials that possess exceptional strength, stiffness, and toughness (or energy absorption) simultaneously remains a significant challenge. In this study, we develop a novel design strategy: incorporating nonlocal interactions into lattice beams, creating “nonlocal lattices”. Utilizing simulation experiments, we investigated the bending behaviors of these lattices, with a particular focus on their damage evolution. Interestingly, these nonlocal lattices, categorized as stretch-dominated, exhibit extraordinary peak force (strength) and stiffness (modulus) comparable to traditional stretch-dominated lattices, while maintaining superior energy absorption (toughness). Analysis of damage evolution within the lattice beams reveals a transition from localized to dispersed damage patterns. This transition delays strain localization, thereby improving material utilization efficiency. Furthermore, stronger nonlocal interaction leads to a more dispersed damage zone, further improving materials utilization efficiency. These findings demonstrate that nonlocal lattices achieve excellent energy dissipation (toughness) without compromising strength and stiffness. This highlights the crucial role of nonlocal interactions in governing strain localization within lattice materials. The design strategy here unlocks new inspirations for the development of strong and tough lightweight materials.

Accurately predicting the mechanical behavior of pure metals at different radiation doses and prescribing the microstructure evolutions, such as the dislocation structures, remain challenging. This work introduces a 3D hybrid numerical simulation scheme that integrates finite element (FE) and finite difference (FD) modules. The FE module is used to implement the crystal plasticity model, while the FD module is used to solve the reaction-diffusion model regarding dislocation nucleation and transportation. Our hybrid model successfully replicates the mechanical behavior of pristine Cu single crystals and provides details of dislocation cell structures that agree with the experimental observation. Furthermore, the model effectively reflects the irradiation hardening effects for Cu single crystals and demonstrates the formation of dislocation channels and shear band type of strain localization. Our work offers an effective approach for predicting the mechanical responses and the safety evaluation of pure metals in extreme working conditions.

High-speed flows have consistently presented significant challenges to experimental research due to their complex and unsteady characteristics. This study investigates the use of the megahertz-frequency particle image velocimetry (MHz-PIV) technique to enhance time resolution under high-speed flow conditions. In our experiments, five high-speed cameras were utilized in rapid succession to capture images of the same measurement area, achieving ultra-high time resolution particle image data. Through advanced image processing techniques, we corrected optical distortions and identified common areas among the captured images. The implementation of a sliding average algorithm, along with spectral analysis of the compressible turbulent flow field based on velocity data, facilitated a comprehensive analysis. The results confirm the capability of MHz-PIV for high-frequency sampling, significantly reducing reliance on individual camera performance. This approach offers a refined measurement method with superior spatiotemporal resolution for high-speed flow experiments.

Voids play an important role in the fatigue behaviour of polycrystal materials. In this paper, the effects of three factors affecting the stress concentration factors (SCFs) near voids, i.e., size, depth, and applied load, are investigated by employing crystal plasticity constitutive models in polycrystal bulks. The results indicate that SCF is dominated by the void size, while void depth and stress level play secondary roles. The SCF fluctuates by the orientation differences among grains and increases with increasing the size of the void. Finally, based on sensitivity examination of orientations and configurations of grains surrounding the void, an empirical multivariable-coupled formula is proposed to assess SCF near voids considering anisotropy, and the presented model is in good agreement with the simulation results.

This paper establishes a method for identifying and locating dynamic loads in time-varying systems. The proposed method linearizes time-varying parameters within small time units and uses the Wilson-θ inverse analysis method to solve modal loads of each order at each time step. It then uses an exhaustive method to determine the load position. Finally, it calculates the time history of the load. Simulation examples demonstrate how the number of measuring points and step size affect load identification accuracy, verifying that this algorithm achieves good identification accuracy for loads under resonance conditions. Additionally, it explores how noise affects load position and recognition accuracy, while providing a solution. Simulation examples and experimental results demonstrate that the proposed method can identify both the time history and position of loads simultaneously with high identification accuracy.