Acta Mechanica Sinica最新文献

Size effect is typically expected to play an important role in the performance of low-dimensional materials. Meanwhile, due to thermo-mechanical coupling in shape memory alloys (SMAs), temperature also significantly influences phase transformation. This study investigates the synergistic size-temperature effects on the compression instability of NiTi SMA nanorods (NRs) through molecular dynamics simulations and theoretical modeling. The results indicate that the observed instability modes in NRs, namely phase transformation and buckling, are predominantly determined by their length-to-diameter ratio (α). The critical α for the transition between these two instability modes is dictated by a competitive mechanism involving phase transformation driving stress and buckling stress, both of which depend on the size and temperature of the system. A modified Timoshenko model is developed to theoretically predict the critical α based on this stress competition mechanism, providing a comprehensive understanding of the synergistic size-temperature effects on the modulation of the critical α. These findings could offer valuable insights for the mechanical design and application of micro/nano devices utilizing SMA NRs.

In the present literature, two types of piezoelectric fiber-reinforced composite (PFRC) based cylindrical models are considered to investigate the circumferential shear wave propagation on a cylinder. Model I consists of a pre-stressed PFRC layer imperfectly bonded to a pre-stressed piezoelectric cylinder of infinite length. Model II comprises a pre-stressed PFRC layer that is imperfectly bonded to a fiber-reinforced core cylinder. The dispersion equations have been derived for both models, assuming electrically open and short boundaries. The numerical simulations are carried out, and results are portrayed graphically to show the effects of various parameters. The radius ratio, pre-stress, mechanical imperfect bonding parameter, fiber reinforcement and fiber-matrix volume ratio exert considerable effects on the PFRC cylinder. Comparative analysis of the dispersion behavior reveals that the shear wave’s phase velocity varies differently for Model I and Model II, and the phase velocity for Model I is higher compared to Model II. The phase velocity reaches its minimum when the piezoelectric fiber is 0.5–0.6 by volume fraction in the PFRC layer.

The application of multi-material topology optimization affords greater design flexibility compared to traditional single-material methods. However, density-based topology optimization methods encounter three unique challenges when inertial loads become dominant: non-monotonous behavior of the objective function, possible unconstrained characterization of the optimal solution, and parasitic effects. Herein, an improved Guide-Weight approach is introduced, which effectively addresses the structural topology optimization problem when subjected to inertial loads. Smooth and fast convergence of the compliance is achieved by the approach, while also maintaining the effectiveness of the volume constraints. The rational approximation of material properties model and smooth design are utilized to guarantee clear boundaries of the final structure, facilitating its seamless integration into manufacturing processes. The framework provided by the alternating active-phase algorithm is employed to decompose the multi-material topological problem under inertial loading into a set of sub-problems. The optimization of multi-material under inertial loads is accomplished through the effective resolution of these sub-problems using the improved Guide-Weight method. The effectiveness of the proposed approach is demonstrated through numerical examples involving two-phase and multi-phase materials.

Liquid crystal elastomers (LCEs) are advanced materials characterized by their rubber-like hyperelasticity and liquid crystal phase transitions, offering exceptional mechanical properties. The development of smart mechanical metamaterials (SMMs) from LCEs expands the potential for controlling mechanical responses and achieving nonlinear behaviors not possible with traditional metamaterials. However, the challenge lies in managing the interplay between nonlinear material responses and structural complexity, making the inverse design of LCE-based SMMs exceptionally demanding. In this paper, we introduce a design framework for LCE smart mechanical metamaterials that leverages neural networks and evolution strategies (ES) to optimize designs with nonlinear mechanical responses. Our approach involves constructing a flexible, unit-cell-based metamaterial model that integrates the soft elastic behavior and thermo-mechanical coupling of LCEs. The combination of microscopic liquid crystal molecule rotation and macroscopic block rotation enables highly tunable and nonlinear mechanical behaviors, of which the precise inverse design of stress-stretch responses is obtained via neural networks combined with ES. In addition, stimuli responses in the liquid crystal elastomers enable real-time adaptability and achieve tailored stress plateaus that are not possible with traditional metamaterials. Our findings provide new pathways in the design and optimization of advanced materials in flexible electronic devices, intelligent actuators, and systems for energy absorption and dissipation.

42CrMo steel was studied in this paper on its thermomechanical behavior when subjected to dynamic compression, utilizing in-situ dynamic tests and crystal plasticity finite element method (CPFEM) simulations. A split Hopkinson pressure bar, combined with high-speed infrared thermography, was employed to simultaneously record the mechanical response and corresponding temperature evolution, enabling the derivation of the Taylor-Quinney coefficient (TQC). To explore the impact of texture orientation on thermomechanical behavior, a dislocation density-based CPFEM model was applied to analyze the plastic deformation process. The findings demonstrate a satisfactory consistency between numerical predictions and experimental results achieved by the dislocation density-based CPFEM. Simulations of four typical textures demonstrated that texture, through changes in the activated slip systems, significantly influences the evolution of the TQC. These findings contribute valuable insights to the TQC database, enhancing our understanding of material behavior under dynamic loading conditions.

The energy absorption performance (EAP) of plate-lattices was systematically investigated, both independently and when applied to square-tube filling. Based on this, an optimization design model for the crashworthiness of lattice-filled structures was established. The results indicate that among the three basic plate-lattices, the FCC has the best overall EAP. When subjected to three-point bending loads, the curvature of thin-walled tubes and the number of filling cells do not significantly enhance EAP; increasing the wall thickness can improve the specific energy absorption (SEA), but wall thickness has a significant impact on the peak crushing force (PCF)—as the wall thickness increases, the PCF also increases; increasing the relative density can enhance both SEA and PCF, but its energy absorption stability (EAS) initially improves and then weakens. When considering density distribution, placing more material in the middle part of the structure results in better EAP. Under axial loads, curved-tubes have lower SEA and poorer EAS compared to straight-tubes; when considering oblique loading, smaller tilt angles have less impact on the EAP; increasing the number of cells reduces the EAP but can improve EAS. Additionally, the optimization model proposed in this paper can significantly enhance the EAP of the designed structure.

In this paper, we propose a new structural dynamic topology optimization method based on the solid isotropic material with a penalization model for the viscoelastic material considering the viscoelastic material degradation. In our research scheme, the material degradation constraint is derived to be handled as the dissipation energy upper limit constraint under two assumptions based on entropy-degradation. The finite element method is employed to obtain the structural displacement and velocity fields. Then, the adjoint variable method is brought up to derive the sensitivities of the structural dynamic compliance and the overall dissipation energy with respect to the design variables. Finally, the pseudo density design variables are optimized with the method of moving asymptotes to yield the minima of dynamic compliance. Three numerical examples with different load-cases are carried out to illustrate the validity and the stability of the proposed method, and the obtained structural topology patterns, together with the structural performance functions, are compared with those yielded without the dissipation energy constraints. In the discussion part, the influences of the volume fraction and the dissipation energy constraint values on both of the final structural topology patterns and the objective function are numerically investigated and discussed.

In this study, the penetration experiments on thin metal plates with four elliptical cross-section projectiles (ECSPs) were carried out to explore the deformation and perforation of target under the normal penetration. The projectiles were launched by the 40 mm air gun with impact velocities ranged from 200 to 350 m/s and the residual velocities of projectiles were experimentally obtained. The experimental results showed that the shape ratio of the ECSP had a great influence on the deformation characteristics of the target. In addition, Strain sensors were arranged on the impact surface of the metal plate to capture the dynamic deformation during penetration. The correlation between the peak value of the strain signal and its variation with the azimuth angle, the mean square deviation of the peak value, etc. were analysed in detail. Besides, the correlation numerical simulation was conducted to better understand the response characteristics of target and the accuracy of the numerical simulation method was verified by the above experimental results. The similarities and differences of deformation and damage characteristics of targets under different projectiles were analyzed by comparing the radial displacement and radial/tangential stress of targets. The results showed that for ECSPs, the stress in each direction was closely related to shape ratio and circumferential angle. The target was subjected to the coupling of compression and shear stress, which was greatly different from the result of circular cross section projectile.

Nacre-like structures exhibit excellent mechanical properties under low-velocity impact, but the effectiveness of the nacre-like designs under high-velocity impact remains unclear. In this study, the process of a spherical projectile impacting on a nacre-like plate over a wide range of velocities is simulated using the finite element method. A three-dimensional finite element model is constructed and validated against the test data of the target perforation in terms of residual velocity and fracture morphology. The effects of impact velocity, interface strengths, and geometric sizes on the impact resistance capabilities are systematically investigated, and a dimensionless geometrical parameter is proposed to reveal the mechanism affecting the fracture toughness of nacre-like materials. It is found that the impact resistance of the nacre-like material gradually weakens with impact velocity increasing and is inferior to that of homogeneous plates under high-velocity impact. Moreover, the fracture toughness of nacre-like materials depends on the competition mechanism between interfacial enhancement and strength weakening at different impact velocities. These findings provide significant guidance on applying bio-inspired structures to design protective materials.

This study examines the evolution of damage morphology in carbon fiber reinforced plastic (CFRP) laminate under hypervelocity impact by an 8 mm aluminum projectile. Three failure patterns of the projectile are observed, with the projectile being intact, ruptured, or smashed at different velocities. Additionally, the time-frequency spectrum also reveals three distinct modes: initially showing a rising double peak, transitioning to a monotonically decreasing double peak, and eventually culminating in an isolated single peak as the velocity increases. The first peak in the discrete wavelet transformation frequency spectrum may be associated with an initial shear failure upon penetration into the CFRP laminate, while the second peak may be associated with subsequent fiber breakage behind the target. These findings provide valuable information for engineering robust defense structures and assessing damage scenarios in spacecraft.