Acta Mechanica Sinica最新文献

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.

Computational modeling is a new approach to optimize Young’s modulus of scaffolds by performing a minimal number of experiments. However, presenting a modeling algorithm to predict Young’s modulus and characterize the governing parameters is a challenging task. Here, a novel modeling approach has been proposed to estimate Young’s modulus of scaffolds, considering particle agglomeration and interphase interactions. Employing the characteristic parameters of these two phenomena, we modified the Maxwell model using a simple three-step algorithm to determine the optimal value of these parameters and predict Young’s modulus. Interestingly, the model provides a precision of more than 95% for all the studied cases and presents a remarkably better performance compared to the two other models. For instance, the proposed model has reduced the average absolute relative error of Young’s modulus of poly (3-hydroxybutyrate)-keratin/hydroxyapatite nanorods from 25.1% to 0.08%, demonstrating the high efficiency of this model in predicting Young’s modulus of scaffolds. The results of this study could lighten the way of fabricating nanobiocomposites with optimal mechanical properties, spending lower cost and energy.

The cohesive zone model (CZM) has been used widely and successfully in fracture propagation, but some basic problems are still to be solved. In this paper, artificial compliance and discontinuous force in CZM are investigated. First, theories about the cohesive element (local coordinate system, stiffness matrix, and internal nodal force) are presented. The local coordinate system is defined to obtain local separation; the stiffness matrix for an eight-node cohesive element is derived from the calculation of strain energy; internal nodal force between the cohesive element and bulk element is obtained from the principle of virtual work. Second, the reason for artificial compliance is explained by the effective stiffnesses of zero-thickness and finite-thickness cohesive elements. Based on the effective stiffness, artificial compliance can be completely removed by adjusting the stiffness of the finite-thickness cohesive element. This conclusion is verified from 1D and 3D simulations. Third, three damage evolution methods (monotonically increasing effective separation, damage factor, and both effective separation and damage factor) are analyzed. Under constant unloading and reloading conditions, the monotonically increasing damage factor method without discontinuous force and healing effect is a better choice than the other two methods. The proposed improvements are coded in LS-DYNA user-defined material, and a drop weight tear test verifies the improvements.

During perfusion culture, the growth of bone tissues in the scaffold was closely related to the locations of initial adhered cells and their density. In this study, the fluid mechanical responses of Voronoi-lattice scaffolds and initial adhered cells on scaffolds were quantitatively investigated. Multiphase fluid-structure interaction (FSI) model was verified by comparing with the results of Diamond scaffolds culture in the literature. Fluid mechanical responses of Voronoi-lattice scaffolds and cells were analyzed by multiphase FSI model. Regression equations were established by response surface method (RSM) to determine relationships between structural design factors of Voronoi-lattice scaffolds and fluid mechanical response parameters of scaffolds and cells. The results showed that the percentage of adhered cells and the locations of initial adhered cells obtained by multiphase FSI model of Diamond scaffolds had the same trend with that obtained by perfusion culture. Regression equations established based on RSM could well predict the fluid mechanical response parameters of Voronoi-scaffolds and cells. The multiphase FSI model closely related the densities of cells and the locations of adhered cells to bone tissue growth. The model could provide a certain theoretical basis for constructing and culturing engineered bone tissues in vitro perfusion.

Ship-bridge collisions happen from time to time globally, and the consequences are often catastrophic. Therefore, this paper proposes a new high-pressure water jet interference (HPWJI) method for bridge pier protection against vessel collision. Unlike traditional methods that absorb energy by anti-collision devices to mitigate the impact force of ships on bridges, this method mainly changes the direction of ship movement by lateral high-pressure water jet impact, so that the ship deviates from the bridge piers and avoids collision. This paper takes China’s Shawan River as the background and simulates the navigation of a ship (weighing about 2000 t) in the HPWJI method in the ANSYS-FLUENT software. The simulation results show that the HPWJI method has a significant impact on the direction of the ship’s movement, enabling the ship to deviate from the pier, which is theoretically feasible for preventing bridge-ship collisions. The faster the ship’s speed, the smaller the lateral displacement and deflection angle of the ship during a certain displacement. When the ship speed is less than 7 m/s, the impact of water flow on the ship’s trajectory is more significant. Finally, this paper constructs a model formula for the relationship between the lateral displacement and speed, and surge displacement of the selected ship. This formula can be used to predict the minimum safe distance of the ship at different speeds.

The alignment of elongated fibers and thin disks is known to be significantly influenced by the presence of fluid coherent structures in near-wall turbulence (Cui et al. 2021). However, this earlier study is confined to the spheroids with infinitely large or small aspect ratio, and the shape effect of finite aspect ratio on the alignment is not considered. The current study investigates the shape-dependent alignment of inertialess spheroids in structure-dominated regions of channel flow. With utilizing an ensemble-averaged approach for identifying the structure-dominated regions, we analyze the eigensystem of the linear term matrix in the Jeffery equation, which is governed by both particle shape and local fluid velocity gradients. In contrast to earlier conventional analysis based on local vorticity and strain rate, our findings demonstrate that the eigensystem of the Jeffery equation offers a convenient, effective, and universal framework for predicting the alignment behavior of inertialess spheroids in turbulent flows. By leveraging the eigensystem of the Jeffery equation, we uncover a diverse effect of fluid coherent structures on spheroid alignment with different particle shapes. Furthermore, we provide explanations for both shape-independent alignments observed in vortical-core regions and shape-dependent alignments around near-wall streamwise vortices.

Mechanical-guided assembly of three-dimensional (3D) mesostructures from pre-defined 2D precursors based on the deterministically controlled buckling has attracted increasing attention in both fundamental and applied research areas, owing to the compelling advantages in developing flexible electronic devices with complex 3D geometries and novel functions. Recently, a buckling-guided strategy was reported to enable assembly of complex 3D mesostructures and electronic devices on cylindrical and cylinder-like substrates, which can be integrated with vascular systems for monitoring of flow rate and other physical signals. A clear understanding of nonlinear buckling deformations of elastic beams assembled on cylindrical substrates is thereby essential for the relevant structural design. In this work, we present a systematic study on the nonlinear deformations of buckled ribbon-type structures on cylindrical substrates. Two representative classes of ribbon-type structures are considered, including arc structures and serpentine structures. Starting with the finite-deformation beam theory, a theoretical model is established to investigate deformed configurations resulted from the controlled buckling, including ribbons assembled on both outer and inner surfaces of the substrate. The structure-substrate contact and self-contact are taken into account in the analyses, which could lead to distinct deformed configurations. Both experimental studies and finite element analyses (FEA) were carried out to validate the developed theoretical model. A demonstrative device design based on the 3D ribbon network outside the cylindrical substrate suggests potential applications in energy harvesting across a broad range of frequency. The theoretical model presented herein could offer insights for the practical design of 3D electronic devices that can be conformally integrated with curvy biological surfaces.

Bicuspid aortic valve (BAV) is a common congenital malformation of the aortic valve with various structural characteristics. Different types of BAV can cause secondary aortic diseases, including calcific aortic valve stenosis and aortic dilation, although their pathogenesis remains unclear. In this study, we first established patient-specific BAV simulation models and silicone models (Type 0 A-P, Type 1 R-N, and Type 1 L-R) based on clinical computed tomography angiography (CTA) and pressure data. Next, we applied a research method combining fluid-structure interaction (FSI) simulation and digital particle image velocimetry (DPIV) experiment to quantitatively analyze the hemodynamic, structural mechanical, and flow field characteristics of patients with different BAV types. Simulation-based hemodynamic parameters and experimental results were consistent with clinical data, affirming the accuracy of the model. The location of the maximum principal strain in the patient-specific model was associated with the calcification site, which characterized the mechanism of secondary aortic valve stenosis. The maximum wall shear stress (WSS) of the patient-specific model (>67.1 Pa) exceeded 37.9 Pa and could cause endothelial surface injury as well as remodeling under long-term exposure, thus increasing the risk of aortic dilation. The distribution of WSS was mainly caused by BAV type, resulting in different degrees of dilation in different parts guided by the type. The patient-specific model revealed a maximum viscous shear stress (VSS) value of 5.23 Pa, which was smaller than the threshold for shear-induced hemolysis of red blood cells (150 Pa) and platelet activation (10 Pa), but close to the threshold for platelet sensitization (6 Pa). The results of flow field characteristics revealed a low risk of hemolysis but a relative high risk of thrombus formation in the patient-specific model. This study not only provides a basis for future comprehensive research on BAV diseases, but also generates relevant insights for theoretical guidance for calcific aortic valve stenosis and aortic dilation caused by different types of BAV, as well as biomechanical evidence for the potential risk of hemolysis and thrombus formation in BAV, which is of great value for clinical diagnosis and treatment of BAV.

Vibration isolation for low frequency excitation and the power supply for low power monitoring sensors are important issues in bridge engineering. The main problem is how to effectively combine the vibration isolator with the energy harvester to form a multi-functional structure. In this paper, a system called quasi-zero stiffness energy harvesting isolator (QZS-EHI) with triple negative stiffness (TNS) is proposed. The TNS structure consists of linear springs, rigid links, sliders, and ring permanent magnets. Newton’s second law and Kirchhoff’s law construct dynamic equations of the QZS-EHI, and a comparison is made to contrast it with other QZS and linear isolators. The comparison field includes the QZS range, amplitude-frequency relationship, force transmissibility, and energy harvested power. The isolator can be applied to many engineering fields such as bridges, automobiles, and railway transportation. This paper selects bridge engineering as the main field for the dynamic analysis of this system. Considering the multi-span beam bridge, this paper compares different situations including the bridge with QZS-EHI support, with linear stiffness isolator support, and with single beam support. All results show that the QZS-EHI is not only better than the traditional isolator with linear stiffness under both harmonic and stochastic excitation, but also better than some QZS isolators with double or single negative stiffness in bridge vibration isolation and energy harvesting. Theoretical analysis is verified to correspond to the simulation analysis, which means the proposed QZS-EHI has practical application value.