Acta Mechanica Sinica最新文献

Component sequence preservation is an intrinsic requirement in typical engineering applications, such as deployable chain-like structures, 3D printing structures with contour-parallel toolpaths, additive manufacturing of continuous fibre-reinforced polymer structures, customized stents, and soft robotics parts. This study presents a feature-driven method that preserves component sequences accounting for engineering requirements. The chain-of-bars design variables setting scheme is developed to realize the sequential component’s layout, which sets the current bar’s end point as the next bar’s start point. The total length of the printing path is constrained to reduce the consumption of material accurately. Also, the angle between adjacent bars is constrained to avoid sharp angles at the turning point of the 3D printing path. Next, the sensitivity analysis considering the inter-dependence of substructures is performed. Several numerical examples are given to demonstrate the validity and merits of the proposed method in designing structures preserving component sequences.

In the past few decades, people have been trying to address the issue of walking instability in bipedal robots in uncertain environments. However, most control methods currently have still failed to achieve robust walking of bipedal robots under uncertain disturbances. Existing research mostly focuses on motion control methods for robots on uneven terrain and under sudden impact forces, with little consideration for the problem of continuous and intense external force disturbances in uncertain environments. In response to this issue, a disturbance-robust control method based on adaptive feedback compensation is proposed. First, based on the Lagrangian method, the dynamic model of a bipedal robot under different types of external force disturbances was established. Subsequently, through dynamic analysis, it was observed that classical control methods based on hybrid zero dynamics failed to consider the continuous and significant external force disturbances in uncertain environments. Therefore, an adaptive feedback compensation controller was designed, and an adaptive parameter adjustment optimization algorithm was proposed based on walking constraints to achieve stable walking of bipedal robots under different external force disturbances. Finally, in numerical simulation experiments, comparative analysis revealed that using only a controller based on hybrid zero dynamics was insufficient to converge the motion of a planar five-link bipedal robot subjected to periodic forces or bounded noise disturbances to a stable state. In contrast, in the adaptive feedback compensation control method, the use of an adaptive parameter adjustment optimization algorithm to generate time-varying control parameters successfully achieved stable walking of the robot under these disturbances. This indicates the effectiveness of the adaptive parameter adjustment algorithm and the robustness of the adaptive feedback compensation control method.

During its evolution to the far field, the wingtip vortex exhibits complex instability behaviors such as long-wave/short-wave instability and vortex wandering. However, the quantification influence of vortex instability on its velocity field statistics has not been well investigated. To this end, experimental measurements of a canonical wingtip vortex generated by an elliptical wing under various angles of attack and Reynolds numbers were conducted using particle image velocimetry. It is found that the streamwise variation of wandering amplitude presents an exponential growth within the middle-to-far wake region and asymptotically saturates to 10⁻¹b in the far wake, which differs from the previous report of a linear growth trend in the near-wake region. Further, two average methods, i.e., time average (TA) and ensemble average (EA), were adopted to compare the velocity field statistics. In both TA- and EA-obtained flow fields, the vortex radius r_c, peak vorticity (Omega_{x}^{p}), and vortex circulation Γ all demonstrate a power-law scaling with respect to the streamwise location, i.e., (r_{c}propto x^{k_{r}},Omega_{x}^{p}propto x^{-k_{omega}}), and (Gammapropto x^{-k_{Gamma}}), respectively. For a full rolling-up wingtip vortex in the middle-to-far wake region, the fact that k_Γ = k_ω − 2k_r demonstrates that the vortex circulation can be scaled as (Gamma=Omega_{x}^{p}(r_{c})^{2}). On the other hand, TA overestimates the decay rate of peak vorticity k_ω and the growth rate of vortex radius k_r. Furthermore, the TA-introduced bias level of the peak vorticity and vortex radius is found to be scaled with an empirical scaling between the wandering amplitude by a power law, respectively. These findings provide significant practical value for detecting wake vortex in wake vortex spacing systems.

Real-time onboard health monitoring systems are critical for the railway industry to maintain high service quality and operational safety. However, the issue with power supplies for monitoring sensors persists, especially for freight trains that lack onboard power. Here, we propose a hybrid piezoelectric-triboelectric rotary generator (HPT-RG) for energy harvesting and vehicle speed sensing. The HPT-RG incorporates a rotational self-adaptive technique that softens the equivalent stiffness, enabling the piezoelectric non-resonant beam to surpass resonance limitations in a low-frequency region. The experiments demonstrate the feasibility of using the HPT-RG as an energy harvesting module to collect the rotational energy of the freight rail transport and power the wireless temperature sensors. To allow multiple monitoring in confined spaces on trains, a triboelectric sensing module is added to the HPT-RG to sense the operation speed and mileage of vehicles. Furthermore, the generator exhibits favorable mechanical durability under more than 600 h of official testing on the train bogie axle. The proposed HPT-RG is essential for creating a truly self-powered, maintenance-free, and zero-carbon onboard wireless monitoring system on freight railways.

Osteocytes, the primary cells in bone, play a crucial role in sensing external load environments and regulating other bone cells. Due to the piezoelectric effect of the mineralized matrix and collagen that make up bone, the mechanical stimulus received is converted into an electrical stimulus to affect the reconstruction of bone. Despite the importance of osteocyte, many studies have focused on the mechanical loading and fluid flow of it, there is still a gap in the study of the piezoelectric effects of various mechanosensors on the microscale. In this paper, we developed a finite element model of osteocytes that incorporates the piezoelectric bone matrix. This model is comprehensive, comprising the osteocyte cell body enclosed by lacuna, osteocyte processes enclosed by canaliculi, and the interposed charged ionic fluid. Additionally, it features mechanosensors such as collagen hillocks and primary cilia. In our study, we subjected the piezoelectric bone matrix model to triaxial displacement, subsequently assessing the electrical signal variations across different mechanosensors within the osteocyte. The observed disparities in mechanical perception by various mechanosensors were primarily attributable to greater liquid velocity changes in the polarization direction as opposed to other directions. Collagen hillocks showed insensitivity to piezoelectric signals, serving predominantly to mechanically transmit signals through solid-to-solid contact. In contrast, processes and primary cilia were highly responsive to piezoelectric signals. Interestingly, the processes oriented in the direction of the electric field demonstrated a differential piezoelectric signal perception compared to those in other directions. Primary cilia were especially sensitive to fluid flow pressure changes, which were influenced both by loading rates and external piezoelectric effects. Overall, our findings illuminate the complexity of mechanical perception within osteocytes in a piezoelectric environment. This adds a new dimension to our understanding and suggests avenues for future research in bone reconstruction and cellular mechanical behavioral transmission.

Based on mathematical orthogonality and mechanical equilibrium, a deformation energy decomposition method for classical isotropic square and cube elements is proposed by considering the physical parameters of materials. By aid of this method, the comprehensive deformation energy of planar discrete elastomers can be decomposed into five basic deformation energies, and the comprehensive deformation energy of spatial discrete elastomers can be decomposed into eighteen basic deformation energies. The quantification and visualization of structural deformation performance can be realized. According to the magnitude of different deformation energy in the same element, the decomposition diagram is drawn, which can visually display the area dominated by each basic deformation energy. The cloud diagram is drawn based on the distribution of specific deformation energy in different elements, which can be used to analyze the gradient change of deformation energy in the structure. Finally, the deformation properties of cantilever beam and four-sided consolidation plate are analyzed by deformation energy decomposition method. The correctness and superiority of this method are verified by comparing with the results of strain energy decomposition.

In the current work, we investigated hydrogen/air flame propagation under supergravity conditions. Results show that when gravity is in the same/opposite direction as flame propagation, it leads to acceleration/deceleration of the flame, and that such an effect could substantially modify the flame propagation and structure at high gravity levels. Furthermore, for the absolute and relative flame propagation speeds, the gravity-affected flame speed shows opposite trends as the absolute flame speed is more affected by the local induced flow field, while the relative flame speeds are controlled by the super-adiabatic or sub-adiabatic flame temperature. The gravity-affected thermal and chemical flame structures are also examined through the influence of the mixture equivalence ratio, pressure, and flame stretch.

This study examines the penetration of 12.7 mm armor piercing incendiary projectiles into SiC ceramic-fiber composite target plates. By observing the recovered projectile and the overall damage morphology of the ceramic-fiber composite target plates. Additionally, multi-level screening and weighing of the recovered projectile and ceramic fragments revealed that the mass distribution of the projectile and ceramic fragments under different backing structures conforms to a power-law distribution. Experimental results indicate that for single laminate as the backing, the fragmentation of the projectile and ceramics is highest when T300 is the material. Incorporating a T300 transition layer between the SiC ceramic and aramid fibers (Kevlar) or ultra-high molecular weight polyethylene (UHMWPE) increases the fragmentation of the projectile and ceramics, leading to increased energy absorption. The projectile’s head mainly exhibits pulverized abrasive fragmentation, while larger projectile fragments primarily result from shear and tensile stress-induced shear-tensile failure fractures. The primary damage mode of ceramics under high-speed impact is the expansion of ceramic cones and radial cracks. The main form of damage in UHMWPE laminate is interlayer separation caused by tensile waves, permanent plastic deformation at the back protrusion, and perforation failure primarily due to shear waves. The damage mode of Kevlar laminate is similar to that of UHMWPE, with the distinction being that Kevlar laminate primarily exhibits perforation failure caused by tensile waves. Carbon fiber T300 laminate damage mainly consists of cross-shaped brittle fractures caused by shear waves.

The spinodal decomposition method emerges as a promising methodology, showcasing its potential in exploring the design space for metamaterial structures. However, spinodal structures design is still largely limited to regular structures, due to their relatively easy parameterization and controllability. Efficiently predicting the mechanical properties of 3D spinodal membrane structure remains a challenge, given that the features of the membrane necessitate adaptive mesh through the modelling process. This paper proposes an integrated approach for morphological design with customized mechanical properties, incorporating the spinodal decomposition method and adaptive coarse-grained modeling, which can produce various morphologies such as lamellar, columnar, and cubic structures. Pseudo-periodic parameter β and orientational parameter Θ(θ₁, θ₂, θ₃) are identified to achieve the optimal goal of anisotropic mechanical properties. Parametric analysis is conducted to reveal the correlation between the customized spinodal structure and mechanical performance. Our work provides an integrated approach for morphological variation and tuning mechanical properties, paving the way for the design and development of customized functional materials similar to 3D spinodal membrane structures.

The recoverable strain of rock is completely classified as elastic strain in the conventional elastic-plastic theory, which often results in poor agreement between theoretical and experimental curves. This work proposes an improved elastoplastic model of rock materials considering the evolutions of crack deformation and elastic modulus to better characterize the nonlinear mechanical behavior of rock in the post-peak stage. In this model, the recoverable strain is assumed to be a combination of elastic and crack strain, and the constitutive relationship between crack strain and rock stress is deduced. Based on the proposed assumption, the evolutions of the mechanical parameters including strength parameters, elastic, plastic, and crack deformation parameters versus the plastic strain and confining stress were investigated. The developed elastoplastic model was validated by comparing the theoretical values with the results of the triaxial cyclic loading and unloading test. The theoretical calculation results show a good agreement with the laboratory test, which indicates that the improved elastoplastic model can effectively reflect the nonlinear mechanical behavior of the rock materials. The research results are expected to provide a valuable reference for further understanding the evolution of rock crack deformation.