Acta Mechanica Sinica最新文献

This paper explores the applicability of the generalized wave impedance hypothesis in split Hopkinson pressure bar (SHPB) experiments, particularly under non-ideal conditions. The study investigates the effects of changes in wave impedance ratio and cross-sectional area ratio on the dynamic response of materials at high strain rates. Through theoretical analysis and numerical simulation, the impact of different wave impedance and cross-sectional area ratios on stress wave propagation characteristics is discussed in detail. It is found that when the cross-sections of two bars differ, shear strain occurs at the abrupt cross-section, leading to waveform distortion in the transmitted and reflected waves. The force balance condition does not always align with the momentum conservation theorem, and only when the three waveforms and wavelengths are completely consistent do they align. The research shows that when the wave impedance ratio and cross-sectional area ratio are within a specific range, the generalized wave impedance hypothesis can accurately predict changes in Young’s modulus and density. Additionally, the study extends the exploration to key factors such as wave impedance ratio, wave speed, Young’s modulus, density, and area ratio.

Surface irregularities, such as hills and ridges, can significantly amplify ground motion caused by earthquakes. Therefore, in this study, we propose an analytical solution model to investigate the interaction between an asymmetric triangular hill on Earth and SH waves. Firstly, based on the development of wave functions and regional matching techniques, we introduce a semi-circular artificial auxiliary boundary, dividing the solution model into a semi-infinite body containing a semi-circular depression and an asymmetric fan-shaped region. Secondly, we derive the domain function form applicable to solving asymmetric problems. Utilizing the theory of complex variables, we establish a well-posed matrix for solving domain functions within the same coordinate system. Numerical results demonstrate that the scattering of SH waves by a protuberance is jointly influenced by the geometric parameters of the hill and the angle of incidence. Additionally, the frequency of the incident wave also has a certain degree of impact on the displacement amplitude. This study elucidates the scattering mechanism of SH waves by complex boundaries, providing a theoretical reference for building site selection and seismic design. In practical problems, the asymmetric assumption is more applicable than the symmetry assumption.

The sound field driven by piping systems in enclosures may severely affect living comfort, which is frequently encountered in various engineering applications. Managing this sound field relies heavily on the available prediction tools at hand, e.g., the widely used finite element methods are computationally expensive due to the necessity to discretize entire space, analytical models, based on modal expansion method, may offer substantial advantages in terms of computational cost and efficiency. However, deriving eigenmodes of irregular enclosed spaces may be challenging, which impedes accurate and rapid predictions of the sound field in practical applications. This study presents an analytical framework aimed at rapidly and accurately predicting the interior sound field driven by the piping system vibrations in irregular enclosures. Vibration response of the piping system is obtained using the wave approach, and a line dipole source is idealized as the sound source of the piping system vibration. On the basis of eigenmodes of regular enclosures, the Kirchhoff-Helmholtz integral theorem (modal expansion method for irregular enclosures) is introduced to account for the boundaries of irregular enclosures. This theoretical framework is validated through numerical simulations by finite element method and experiments, demonstrating high accuracy and significant efficiency advantages. The proposed method can be further employed to optimize radiated sound fields by tailoring the impedance of space walls or layout of piping systems. This study provides an efficient tool for predicting radiated sound field in general enclosures driven by vibration of piping systems, paving a new path for indoor acoustical optimization.

Osteoarthritis is one of the most common joint diseases, leading to joint pain, dysfunction, and a reduced quality of life for patients. Therefore, it is particularly important to explore more effective prevention, treatment and management methods to relieve patients’ pain and enhance their quality of life. Among physical therapies, pulsed electrical stimulation (PES) is considered to be a promising treatment method due to its high safety and ease-of-use features. PES provides a non-invasive, safe and effective option for patients. However, there are fewer studies on the biomechanical changes of PES in periarticular tissues, and its effects on the biological behavior of chondrocytes remain unknown. This study investigated the effects of PES on the biomechanical properties of osteoarthritic joints and the biological behavior of chondrocytes. The results showed that PES with an intensity of 10 mA and a frequency of 4 Hz increased the cross-sectional area of muscle fibers, prevented muscle atrophy and loss of function, and restored the mechanical properties of muscle tissue. PES also effectively increases the resistivity of knee osteoarthritis cartilage tissue, as well as the elastic modulus of cartilage, which can enhance the biomechanical characteristics of cartilage tissue. PES also promoted the metabolic activity of chondrocytes and increased cartilage matrix synthesis, thereby improving the overall structure and mechanical properties of cartilage tissue. Additionally, cellular experiments showed that 5 consecutive days of 800 mV PES significantly increased the expression level of Piezo1 gene in chondrocytes. At the same time, the expression of type II collagen and transforming growth factor beta increased, while the expression of matrix metallopeptidase 13 decreased. These changes favored the promotion of cartilage matrix synthesis. This has a positive effect on protecting and improving joint health and reducing the impact of osteoarthritis, and is important for understanding the mechanism of action of PES on chondrocytes and the development of related therapeutic strategies.

The distribution of exothermic reaction rates is jointly influenced by reduced activation energy and reaction rate constant. This study focuses on the effect of distribution of exothermic reaction rates on detonation wave propagation instability, specifically under conditions where the length of the induction and exothermic reaction remains constant. It is found that the distribution variation of exothermic reaction rates significantly influences the detonation wave propagation characteristics. Specifically, under conditions of high activation energy, the exothermic reaction rate profile exhibits a smoother distribution but becomes more prone to perturbations. This heightened sensitivity, coupled with the augmented overdriven degree associated with pulsating detonation and cellular detonation wave propagation, further exacerbates the instability characteristics of detonation waves. Especially to the two-dimensional detonation waves with high activation energies, the distribution of exothermic reaction rates becomes more sensitive to these displacements, reinforcing the transverse shock wave and leading to a transformation of the wavefront and cellular structure towards more unstable configurations. This research delves into the intricate interactions between the distribution of exothermic reaction rates and detonation wave instability, aiming to provide an explanatory of detonation instability.

Equiatomic NiTi shape memory alloys (SMAs) can exhibit multiple martensitic transformations from a parent phase, significantly influencing the advanced macroscopic properties of SMAs, such as the large deformation/strain ability. A comprehensive atomic-scale understanding of the selection rule of the martensite phase/variant and its impact on the macroscopic mechanical behavior of SMA could be helpful for the development of high-performance SMAs. This work studies the transformation pathway, preferred martensite variant and corresponding macroscopic behavior of single crystal and bicrystal NiTi SMAs based on molecular dynamics and theoretical analysis. It is found that the transformation strain of single crystal NiTi is significantly influenced by the crystal orientation-dependent transformation pathway and martensite variant. The selection rule is that the transformation pathway and preferred martensite variant, leading to maximum transformation strains for each orientation, are energetically preferred. It can be predicted theoretically and agrees well with the molecular dynamic simulations. In addition, the stress-strain response of bicrystal NiTi can be modulated by changing its transformation pathway based on the orientation effect. This work provides atomic insights into the orientation-dependent deformation ability of NiTi and could be helpful for the development of high-performance SMAs through orientation modulation.

Ceramic matrix composites have broad application prospects in the aerospace field due to their high temperature resistance and oxidation resistance. The effect of temperature and environment atmosphere on the fracture toughness and failure mechanisms of two-dimensional plain-woven SiC_f/SiC composites was investigated. The results show that they exhibit pseudo-plastic deformation behavior at different temperatures. The fracture toughness is as high as 48 MPa m^1/2 at room temperature, and gradually decreases with rising temperature. The difference in fracture toughness between argon and air initially increases and then decreases with rising temperature. Furthermore, the high-temperature failure mechanisms of these composites were analyzed through macro and micro analysis. Based on this, a physic-based temperature-dependent fracture toughness model considering matrix toughness, plastic power, fiber pull-out, and residual thermal stress was developed for fiber-reinforced ceramic matrix composites. The model has been well validated by experimental results. An analysis of influencing factors regarding the evolution of fracture toughness was conducted by the proposed model. This work contributes to a better understanding of the mechanical performance evolution and failure mechanisms of ceramic matrix composites under multi-field coupling conditions, thereby promoting their applications.

Creep is an important mechanical property of refractory high-entropy alloys (RHEAs) at high temperatures. The existence of short-range order (SRO) and its ability to improve the strength or plasticity of high-entropy alloys (HEAs) have been experimentally proven. However, there is still little research on the correlation between SRO and creep behavior. The mechanism of SRO influencing creep behavior is not yet clear. In this work, the creep behaviors of TiVTaNb RHEA with and without SRO were simulated at various temperatures and stresses using molecular dynamics methods, and the effects of SRO on creep behavior were analyzed. The results show that the SRO is energetically favorable for occurrence in this RHEA. For polycrystalline RHEAs, grain boundary energy is an important driving force for the formation of SRO. Significantly, under the same conditions, the SRO can reduce the steady-state creep rate and change the creep mechanism of the RHEA. Specifically, the models with SRO will exhibit lower stress exponent and grain-size exponent. A mechanism by which SRO reduces the effects of grain boundaries on creep has been discovered. These phenomena can be well explained by the effects of SRO on atomic diffusion. In addition, by analyzing the diffusion ability of different elements, SRO can induce localization of atomic diffusion, resulting in strain localization under high stresses. This work highlights the importance of SRO on the creep of RHEAs and provides a reference for establishing a reasonable creep model of RHEAs.

To build a thrombosis risk assessment model applicable to oxygenators and investigate the effects of oxygenator external configuration and membrane filaments macroscopic parameters on the performances of cylindrical oxygenators. A thrombosis driven by surface contact, shear stress, and anticoagulant drugs, and considering the effects of these factors on platelet, coagulation factor, and hemostatic protein function risk model was developed and validated with clinical oxygenators. The thrombosis model combined with a pressure loss model and an oxygen partial pressure model was used to assess the effect of the external structure and macroscopic parameters of the membrane filaments (height and thickness) on the performance of the cylindrical oxygenator. The cylindrical oxygenator center circular inflow manner and tangential outflow manner from the middle region of the outside benefit the overall performance of the oxygenator (reduced pressure loss and thrombosis risk). Increasing the radial thickness of the oxygenator membrane filaments significantly increased the oxygen exchange ability of the oxygenator and reduced the thrombosis risk compared to increasing the axial height, but with a smaller increase in pressure loss. Contact activation leading to thrombin production contributes significantly to oxygenator thrombosis. The oxygenator has little effect on platelet receptor function. Thrombosis in cylindrical oxygenators tends to form in the flow-flow/border impingement regions because of the high concentration of coagulation factors and long residence times in these regions. A thrombosis risk assessment model applicable to oxygenators was developed. We disclosed the mechanism of the impact of oxygenator external configuration and membrane filaments macroscopic parameters on its internal flow fields, the risk of thrombosis, and the efficiency of gas exchange, which are useful for the design and optimization of cylindrical oxygenators.