Acta Mechanica Sinica最新文献

Blood pump is one of the core components of extracorporeal membrane oxygenation, but non-physiological shear stress it generates can damage red blood cells, leading to hemolysis and life-threatening complications. This study aims to investigate the flow features and blood trauma of blood pumps with different numbers of blades (4, 5, and 6 blades) under different flow rates and pressure head (ΔP) support conditions. Hemodynamic parameters including flow field distribution, scalar shear stress (SSS), exposure time and hemolysis index over a specified threshold, were analyzed using numerical simulation. In vitro experiments using fresh bovine blood were conducted at two flow rates and three ΔP. Plasma free hemoglobin (pfHb) and normalized index of hemolysis (NIH) were measured post-circulation. When ΔP is constant, a higher flow rate from blood pump necessitates a higher rotational speed, which increases shear stress and reduces exposure time. The order of SSS is: 5-blade pump < 4-blade pump < 6-blade pump. A higher flow rate reduces backflow and exposure time in inlet tubing. As ΔP increases from 150 mmHg to 350 mmHg, exposure time in lower secondary flow channel decreases. In vitro studies indicated that the 5-blade pump exhibited the lowest hemolysis, with 20% and 30% lower pfHb than the 4-blade and 6-blade pumps (ΔP = 250 mmHg, Q = 5.0 L/min), respectively. Correspondingly, the NIH of 5-blade pump are 11.2% and 40% lower than 4-blade and 6-blade pumps (ΔP = 250 mmHg, Q = 5.0 L/min), respectively. Under various blade numbers and support conditions, the 5-blade pump has lower shear stress, shorter exposure time, and reduced pfHb and NIH, offering better blood compatibility.

This study presents a configuration design method for a redundantly cable-driven parallel robot under hybrid control, where the selected redundant cables are force-controlled, whereas the other cables perform length control in the joint space. The general wrench-feasibility for the hybrid joint-space control strategy was analyzed based on a space with cable tension vectors as bases. A novel performance index, general force-determined cable adjustable force, which considers not only the external interfering forces and moments but also the cable force control errors for the controllers, is explained in this study. A configuration design methodology, combining collision-free and wrench-feasible detection, is developed to obtain the optimal parameters. A trajectory with external disturbance forces and moments is considered in the experimental case study, in which the center of mass of the end effector remains unknown, and force control errors are also considered under the hybrid joint-space control strategy. We found that the hybrid joint-space control has the ability to explicitly control the cable force and cable length, and it can be applied for carrying loads with high accuracy under uncertain external interfering forces and moments by means of an appropriate configuration design.

The complex characteristics of thin-walled parts fabricated by laser powder bed fusion (LPBF), particularly the dependence of their microstructures on wall thickness and scanning strategies, pose significant challenges for this technology. This paper presents a predictive model for microstructural evolution of LPBF-fabricated thin-walled components, integrating three-dimensional cellular automaton (CA) with finite element (FE) analysis. The FE method is employed to solve the temperature field of thin-walled components during LPBF, and the resulting temperature history is used to predict microstructural evolution in the CA model. Experimental validation via electron back scatter diffraction (EBSD) on a 4 mm-thick specimen confirms a high degree of agreement between model predictions and experimental results. The study reveals that when the thickness of samples prepared by LPBF is reduced from 4 mm to 0.4 mm, there is a significant coarsening of grain size. Additionally, grains at the bottom are observed to be coarser compared to those at the top, which is attributed to epitaxial growth and remelting. Furthermore, the study explores microstructural changes induced by manipulating laser power and scanning speed, while maintaining constant energy density. The findings indicate that grain morphology and size remain consistent across varying parameters, emphasizing the dominant influence of energy density. Within a predefined scanning strategy, an upsurge in laser energy density leads to an enlargement of the average grain size. Notably, the implementation of a cross-scanning strategy alters the melt pool orientation, disrupting the directional grain growth and fostering the formation of finer grains. This underscores the crucial significance of processing techniques in LPBF.

This paper uses experimental and numerical methods to explore the relationship between the strength, thickness, and failure modes of the adhesive layer and the ballistic performance of ceramic/metal composite armor. Damage of the adhesive layer under dynamic impact was modeled using the ABAQUS and the cohesive zone modeling approach, and analyzing the dynamic responses of the projectile, ceramic, and backplate. The results show that as the bonding strength of the adhesive increases, the extent of projectile fracture damage generally increases, enhancing the overall ballistic performance of the composite armor. However, the final protective performance is also influenced by the evolution of projectile damage. When the thickness of the adhesive increases, the ballistic efficiency of the ceramic initially increases and then decreases. This is due to the thicker adhesive layer extending the projectile’s dwell time while excessive thickness causes premature failure of the ceramic. Additionally, the adhesive mainly undergoes shear failure near the impact point, with the damage extending outward in a circular pattern. When the proportion of tensile failure in the adhesive layer is higher, the composite armor exhibits better ballistic performance. For the impact of a T12A steel projectile, the optimal adhesive strength and thickness for better ballistic performance are respectively 80 MPa and 0.8 mm.

Suspended graphene nanopores are widely used in nanofluidic devices, as the machined graphene defects can be downscaled to the angstrom scale. Our recent experimental results showed that the suspended graphene can become delaminated from the edges of SiN nanopore under an applied electrical field, theoretical understanding of this process is still lacking. In this work, we analytically studied the voltage-induced blistering of suspended graphene using an energy approach. The external electric field induces accumulation of ions at the graphene-electrolyte interface, causing Maxwell stress resulting in bending and stretching of the graphene and blister formation. We theoretically derived the angle of the graphene blister to the SiN nanopore by energy approach. We found that once the vertical component of the Maxwell stress on the graphene at the perimeter of SiN nanopore exceeds the van der Waals force between the graphene and substrate, the graphene starts to detach from the edges of SiN nanopore. We derived that the threshold voltage of single-layer graphene detachment is in order of 100 mV, which needs to be cautioned for electrical measurements of suspended graphene nanofluidic devices since the voltage amplitude is just in the range of voltage operation for typical electrochemical measurements. The threshold voltage increases as SiN nanopore becomes smaller and increases with the number of graphene layers. Our work theoretically describes the blister formation and delamination of graphene from its substrate nanopores. We expect this theory to be useful for optimizing and understanding the unexpected conduction phenomena observed in suspended graphene nanofluidic devices.

To improve the suspension performance of high-speed maglev vehicles under complex external disturbance, a composite model predictive control (MPC) algorithm based on a neural network is proposed. Firstly, the nonlinear dynamic response prediction model is constructed utilizing the long short-term memory (LSTM) neural network, and this model is trained by machine learning. Subsequently, a rolling optimization controller of the MPC algorithm is designed according to the vehicle suspension system’s prediction model and the suspension target. To compensate for the error of the prediction model resulting from changes in the control algorithm, a composite MPC algorithm is devised by combining both the proportional-integral-derivative (PID) algorithm and the MPC algorithm. This composite approach enables the suspension system to switch the selection of control algorithms in the suspension system according to the prediction error. Finally, the effectiveness of the composite MPC algorithm is verified by simulation and experiment. The results show that the prediction model based on the LSTM neural network can effectively predict the future dynamic response of the vehicle. Moreover, the proposed MPC algorithm can effectively suppress the suspension gap fluctuation in the high-speed maglev vehicle, thereby fostering improved stability in the suspension system.

We examine the electromechanical field and charge redistribution within a flexoelectric semiconductor (FS) nanobeam, accounting for bending, fundamental thickness-shear, and antisymmetric thickness-stretch deformations. The coupled governing equations include microstructure, flexoelectric, and semiconductor effects, highlighting the interplay between mechanical displacement, electric potential, and charge carriers. For applications in flexoelectronic devices, the static bending of a simply supported FS beam induced by uniform pressure and wave propagation in an unbounded FS beam are analytically addressed using the derived framework. The effects of antisymmetric thickness-stretch on mechanical displacements and electron concentration perturbation, as well as size dependence of microstructure and flexoelectric effects, are identified. An interesting finding reveals that wave frequencies of the antisymmetric thickness-stretch mode, as anticipated by the proposed model, are larger compared to those of the model neglecting flexoelectric and semiconductor effects. For the first time, the cutoff frequency of antisymmetric thickness-stretch impacted by the two features is explained mathematically. These findings are beneficial for enhancing the performance of flexoelectronic sensors and electroacoustic devices.

Self-propelled robots have attracted significant attention due to their remarkable ability to navigate confined terrains. These robots usually have deformable structures while having discontinuous contact forces with the ground, resulting in a complex nonlinear system. To provide a solid foundation for the locomotion prediction and optimization for the self-propelled robots, it is necessary to conduct dynamic modelling and locomotion analysis of the robot. Motivated by these issues, this paper proposes a vibration-driven surrogate dynamic model for a deformable self-propelled robot and presents a detailed dynamic analysis. The surrogate dynamic model is employed to classify various types of stick-slip locomotion. Subsequently, the corresponding experiment demonstrates that the surrogate dynamic model effectively predicts the locomotion of the robot, particularly three types of stick-slip locomotion induced by discontinuous friction. Finally, a multi-objective coordinated optimization regarding the locomotion velocity, the cost of transport, and the energy conversion rate of the self-propelled robot is conducted, aiming to comprehensively enhance the robot’s locomotion performance. Additionally, suggestions for the selection of actuation parameters are presented.

The paper employs the unsteady Reynolds-average Navier-Stokes method to model the flame acceleration (FA) and deflagration-to-detonation transition (DDT) processes of hydrogen/air premixed gases in the channel. By varying the distance (S₁) from the first fluid obstacle to the left wall, its impact on FA and DDT processes is investigated. The results indicate that variations in the jet position significantly influence the processes of FA and DDT. Specifically, during the initial phase of FA, FA is affected by both fluid and solid obstacles, and the FA effect is better only when the value of S₁ is small. Reducing S₁ can effectively shorten the DDT time, but a compromise needs to be considered when attempting to reduce both the time and distance of the DDT process. Although fluid obstacles can facilitate FA, this impact gradually diminishes over time, especially when S₁ exceeds 250 mm. In this paper, the optimal results for DDT time and distance are achieved when S₁ is set to 100 mm. Finally, the process of detonation initiation can be categorized into three types: (I) detonation triggered by the interaction between the leading shock wave and a solid obstacle; (II) detonation resulting from the coupling of the flame surface with a high-pressure point; (III) detonation initiated through the interaction of the flame with the reflected shock wave.