力学进展 -

Experimental technique and research progress of electromagnetic Hopkinson bar

WANG Weibin, SUO Tao, GUO Yazhou, LI Yulong, NIE Hailiang, LIU Huifang, JIN Kanghua, DU Bin, JIANG Bin

2021, 51(4): 729-754. doi: 10.6052/1000-0992-20-024

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Abstract:
The e-Hopkinson bar test technology is a new dynamic loading technology developed by combining the electromagnetic drive technology with the Hopkinson bar test technology, which replaces the traditional Hopkinson bar with the bullet hitting the loading bar to generate stress waves. In this paper, the application status of e-Hopkinson bar test technology in uniaxial unidirectional/biaxial and dynamic biaxial symmetric compression/tension, interlaminar fracture of composites, dynamic bauschinger effect and other fields are reviewed, including experimental research, theoretical analysis and numerical simulation. Finally, the future development direction of e-Hopkinson bar test technology is prospected.

The fatigue failure behaviors of ferroelectric materials

CHEN Yu, ZHOU Huajiang, XIE Shaoxiong, XU Qian, ZHU Jianguo, WANG Qingyuan

2021, 51(4): 755-791. doi: 10.6052/1000-0992-20-030

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In the fields of some major equipment technology such as aerospace and nuclear power generation, as the sensitive elements of high-temperature transforming/actuating/energy harvesting devices, the fatigue failure of bismuth layered ferroelectric (BLSF) ceramics seriously restrict the improvement in the life and reliability of devices. This paper is set in the urgent demand of BLSF ceramics, sticking to the fatigue crack growth, domain polarization switching, and interaction mechanisms. It reviewed the progress in the fatigue failure behaviors of ferroelectric materials subjected to three kinds of loadings, including heat, stress, electricity, and their coupled effects. This paper also discusses the research directions of ferroelectric materials in the future according to their new developments and applications. This work aims at providing references for the design of ferroelectric/piezoelectric devices with long life and high performance.

A review of the study on coherent structures in turbulence by the clustering method

DONG Siwei, CHENG Chen, CHEN Jianqiang, YUAN Xianxu, LI Weipeng

2021, 51(4): 792-830. doi: 10.6052/1000-0992-20-032

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The intrinsic randomness of turbulence requires that the study of coherent structures has to be from the statistical point of view. The data-based clustering method is a powerful tool to realize the deep integration of coherent structures and statistics. It makes each continuous region, i.e., an individual structure, as a statistical sample. In addition, the spatial-temporal tracking method based on the spatial overlap of individual structure between consecutive snapshots makes the evolution of individual structure during its lifetime also as a statistical sample. Thus, both the kinematics and dynamics of coherent structures can be statistically described. We reviewed the history of this method and introduced the progress on Reynolds stress structures, velocity streaks, and cascades with emphasis. These results suggest that the clustering method extraordinarily extended our understanding of turbulence compared with the traditional point-wise statistics. Further prospects are also provided for future research.

Mechanical problems of the large thrust liquid rocket engine

LI Bin, YAN Song, YANG Baofeng

2021, 51(4): 831-864. doi: 10.6052/1000-0992-21-003

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According to the extreme mechanical and thermal environment of high-thrust rocket engines, the mechanical characteristics of high thrust liquid rocket engines, such as strong vibration, large static load, multi-source excitation, and complex load transfer path, are analyzed. In statics, the whole engine structural load analysis and the components static analysis method are introduced. In dynamics, the low-frequency structural dynamic model of the entire engine, the refined model updating technique and multi-source load equivalent method are introduced. For the typical engine components, the mechanical challenges faced in the development of high thrust engine are summarized, including high-temperature and high-pressure gas swing device, rotor dynamics, dynamic and static interference fluid excitation, cavitation oscillation in inducer, large range axial force balance, supersonic turbine aeroelastic, thrust chamber thermal fatigue, nozzle lateral force load, assembly pipeline fatigue fracture, etc. The mechanical requirements and future research directions are pointed out. Finally, the status of probabilistic failure analysis of the engine is briefly introduced, which provides mechanical support for the development of high-thrust liquid rocket engines.

Application of artificial intelligence in composite materials

ZHANG Junming, YANG Weidong, LI Yan

2021, 51(4): 865-900. doi: 10.6052/1000-0992-21-019

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Composite materials have become the major materials of light-weight structure due to their light weight, high strength, high modulus, and strong designability. However, as the component, structure and requirements of capability become increasingly complex, traditional research methods based on experiment, theoretical modeling and numerical simulation meet lots of new problems in the properties prediction, design optimization, manufacturing and processing of composite materials. Insufficient experimental observation, lacking theoretical model, constrained numerical simulation and difficult conclusion validation have seriously restricted the development of advanced composite materials in the future-oriented engineering. Instead of the mathematical models used by mechanics, data-driven models are used in the Artificial Intelligence. It directly establishes the complex relationship between variables from high-dimensional and high-throughput data, then captures the laws that are difficult to be discovered by traditional mechanical method, shows natural advantages in simulation, prediction, optimization in complex systems. It has become the development trend in the field of composite materials to find new solutions to the problems faced by traditional methods through Artificial Intelligence. In this paper, the status of properties prediction, design optimization, manufacturing and health monitoring is introduced. The future development direction of this field is discussed.

Substructuring multi-resolution topology optimization with template

HUANG Mengcheng, HUO Wendong, LIU Chang, YANG Dongsheng, HUANG Jia, DU Zongliang, GUO Xu

2021, 51(4): 901-909. doi: 10.6052/1000-0992-21-030

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In the multi-resolution topology optimization (MTOP) method, by decoupling the finite element mesh and discretization of density field, the finite element analysis is carried out with a coarser mesh (i.e., super-elements), and the computational cost is thus greatly reduced in the process of topology optimization. However, the elemental stiffness matrix is calculated each iteration using the average density of super-elements, and this treatment is actually not only inaccurate but also leads to the checkerboard phenomenon and QR patterns when the filter radius is relatively small. In order to alleviate such issues, the super-element is treated as a substructure and the corresponding elemental stiffness matrix is obtained using static condensation. Furthermore, a template library is developed for the substructure based on its density distribution during the topology optimization process. By this means, the elemental stiffness matrix is not required to be calculated repeatedly, the accuracy of the MTOP method is improved significantly and the checkerboard patterns are effectively inhibited as well.

Progress in the analysis of debris cloud evolution: Fully based on probabilistic methods

SHU Peng, YANG Zhen, LUO Yazhong

2021, 51(4): 910-914. doi: 10.6052/1000-0992-21-002

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In view of the intrinsic uncertainty of the debris cloud, a fully probabilistic framework of debris evolution is constructed. The breakup, evolution and collision of the debris cloud can be analyzed analytically, avoiding the problems of low computational efficiency and poor robustness of numerical methods.

Short-range order in multicomponent alloys

BU Yeqiang, WANG Hongtao

2021, 51(4): 915-919. doi: 10.6052/1000-0992-21-027

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Numerous studies indicates that multicomponent alloys extensively possess short-range order, which place a significant impact on their performance improvement. Employing advanced transmission electron microscopy, the size, composition and structure of short-range order in multicomponent alloys can be analyzed at atomic resolution.

Stability of periodic waves of finite amplitude on the surface of a deep fluid

Zakharov V E

2021, 51(4): 920-930. doi: 10.6052/1000-0992-21-051

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We study the stability of steady nonlinear waves on the surface of an infinitely deep fluid (Lamb 1964, Moiseev 1960). In section 1, the equations of hydrodynamics for an ideal fluid with a free surface are transformed to canonical variables: the shape of the surface η( r , t) and the hydrodynamic potential $\varPsi ({\boldsymbol{r}},t) $ at the surface are expressed in terms of these variables. By introducing canonical variables, we can consider the problem of the stability of surface waves as part of the more general problem of nonlinear waves in media with dispersion (Akhmanov 1964, Zakharov 1965). The resuits of the rest of the paper are also easily applicable to the general case. In section 2, using a method similar to van der Pohl's method, we obtain simplified equations describing nonlinear waves in the small amplitude approximation. These equations are particularly simple if we assume that the wave packet is narrow. The equations have an exact solution which approximates a periodic wave of finite amplitude. In section 3 we investigate the instability of periodic waves of finite amplitude. Instabilities of two types are found. The first type of instability is destructive instability, similar to the destructive instability of waves in a plasma (Oraevskii & Sagdeev 1963, Oraevskii 1964), In this type of instability, a pair of waves is simultaneously excited, the sum of the frequencies of which is a multiple of the frequency of the original wave. The most rapid destructive instability occurs for capillary waves and the slowest for gravitational waves. The second type of instability is the negative-pressure type, which arises because of the dependence of the nonlinear wave velocity on the amplitude; this results in an unbounded increase in the percentage modulation of the wave. This type of instability occurs for nonlinear waves through any media in which the sign of the second derivative in the dispersion law with respect to the wave number (d²ω/d k ²) is different from the sign of the frequency shift due to the nonlinearity. As announced by A. N. Litvak and V. I. Talanov (1967), this type of instability was independently observed for nonlinear electromagnetic waves.

2021 Vol. 51, No. 4

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