力学学报

Preface of special issue in memory of Prof. Che-Min Cheng

Lu Xiyun

2022, 54(8): 2071-2074. doi: 10.6052/0459-1879-22-378

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HISTORICAL SIGNIFICANCE AND DEVELOPMENT PROSPECT OF HYDRO-ELASTO-PLASTIC THEORY MODEL

Deng Guoqiang, Yang Xiumin

The hydro-elasto-plastic theory independently created by Che-Min Cheng and his collaborators is a fundamental engineering science to investigate the mechanical properties of materials under violent dynamic loads. The theory has been in development for nearly 60 years since its establishment, and it has gone through the development stages of modeling, software, and engineering, and become increasingly mature. The series of engineering models developed form it have played a core supporting role in solving scientific computing problems such as strong explosive effects. However, in specific engineering applications, further development is required, and the following aspects should be done, such as improving the theory which can truly reflect the changeable behavior of rock mechanics, developing the mature model which can comprehensively describe the failure process of rock under violent dynamic loads, establishing a new computing framework which can meet the universal requirements of basic theory, conducting a variety of material tests which can provide all-round data requirements for numerical simulations, and establishing refined zones which can describe the damage characteristics of conventional penetration and explosion effects. To achieve the above, it is necessary to further improve the theory model, establish engineering frameworks and standards at the national level, and develop hydro-elasto-plastic model software packages and corresponding databases which can be plugged into the numerical simulation software system.

2022, 54(8): 2075-2084. doi: 10.6052/0459-1879-22-240

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REVIEW ON THE RESEARCHES AND PROSPECT OF THE TRANS-SCALE INDENTATION SCALING LAW OF SOLIDS

Yu Zhijie, Wei Yueguang

Indentation scaling law is a general mechanics and physical conclusion for the determination of mechanical properties of solid materials by indentation test method. It has important theoretical significance and is a methodological study to explore the potential mechanics and physical laws of mechanical properties of materials. In this review paper, the main contents are introduced systematically and briefly as follows: a review of the research on the indentation scaling law of traditional solid materials by using the traditional mechanics theory. A review of the research on the trans-scale indentation scaling law of advanced solid materials by using the theory of trans-scale mechanics. The main conclusions are summarized as follows: the traditional indentation scaling law for solid materials can be completely described by a spatial surface. If the value range of a class of dimensionless independent parameters is known, the spatial surface can degenerate into a family of planar curves. The trans-scale indentation scaling law of advanced solid materials (new materials) can be completely described by a three-dimensional function relationship. If the value range of some independent dimensionless parameter is known, the three-dimensional function relationship will degenerate into a series of spatial surface families. The future research on indentation scaling law for researchers in this research region will be likely still to be focused on the establishment of trans-scale indentation scaling law for new materials, aiming to fundamentally solve the theoretical problems that it is difficult to establish the mechanical properties standard for new materials. In addition, they will be also likely focus on the establishment of multi-scale and trans-scale indentation scaling laws for various functional new materials.

2022, 54(8): 2085-2100. doi: 10.6052/0459-1879-22-273

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FURTHER EXPLORATION ON CHARACTERISTIC REGION OF CRACK INITIATION FOR VERY-HIGH-CYCLE FATIGUE

Hong Youshi

With regard to very-high-cycle fatigue (VHCF) of high-strength metallic materials, we previously proposed the concept of crack initiation characteristic region and the related characteristic parameter (IJFatigue 2014, 58: 144-151), and proposed the numerous cyclic pressing (NCP) model to reveal the formation mechanism of this characteristic region (IJFatigue 2016, 89: 108-118). This crack initiation characteristic region is so-called fine granular area (FGA) on fracture surface for high-strength steels or rough area (RA) on fracture surface for titanium alloys. In recent years, the investigators in fatigue research field have paid great attention to the topic of crack initiation of VHCF for high-strength metallic materials and obtained new results. Therefore, several issues on this topic are of great interests and are necessary to be clearly addressed. These include: Does the microstructure refinement as well as nanograin formation in crack initiation characteristic region happen before or after crack initiation? What is the correlation between applied stress ratio and the formation of crack initiation characteristic region? What are the details of refined microstructure in crack initiation characteristic region including the thickness and the distribution of nanograins? Is vacuum environment the necessary condition for the formation of crack initiation characteristic region? What are the features of crack initiation characteristic region in different materials or with different loading modes? This article will clarify such issues by the comprehensive review of the recent results in the literature. This article will also briefly describe the important implications of the crack initiation characteristic region concept and the NCP model, which include: the assessment and prediction of VHCF properties for high-strength metallic materials, the approach to improve the VHCF properties of additively made metallic materials, and the possibility of manufacturing thin film metallic materials with nanograin microstructure. Specially, this article is dedicated to the memory of my supervisor Prof. Che-Min Cheng who passed away on August 25, 2021.

2022, 54(8): 2101-2118. doi: 10.6052/0459-1879-22-276

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FORMATION AND STABILITY OF SHAPED CHARGE LINER JET OF CrMnFeCoNi HIGH-ENTROPY ALLOY

Yan A’min, Qiao Yu, Dai Lanhong

Recently emerging multi-principal component high-entropy alloy is expected to replace copper as a new generation of shaped charge liner material due to its wide composition/property control range and a series of excellent mechanical properties. Based on the experiments and numerical simulation of dynamic mechanical properties of five-element CrMnFeCoNi high-entropy alloy, the feasibility of this alloy as a shaped charge liner is explored. The mechanical behavior of high-entropy alloys at different strain rates and temperatures was studied via split Hopkinson tensile bar (SHTB) and material testing machine, and a Johnson-Cook thermal viscoplastic dynamic constitutive model of high-entropy alloys was established. The continuity condition of high-entropy alloy jet is explored based on the relationship between flow velocity and critical collapse angle. The continuity condition of high-entropy alloy jet is verified by finite element simulation, and the evolution law of high-speed tensile fracture of high-entropy alloy jet is further investigated. The results show that the jet break-up time is negatively correlated with the material tensile strength, and when the dynamic tensile strength increases, the jet break-up time will decrease. This work was provide references for the structural design of novel high-entropy alloy linev.

2022, 54(8): 2119-2130. doi: 10.6052/0459-1879-22-274

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ELASTIC WAVE PROPAGATION CHARACTERISTICS OF DENSITY GRADIENT CYLINDRICAL SHELL CHAINS

Peng Kefeng, Zheng Zhijun, Zhou Fenghua, Yu Jilin

Uniform cylindrical shell chains can control elastic wave transmission, and introducing density gradient may further improve the ability of waveform control. The propagation behavior of elastic waves in the density gradient cylindrical shell chains was studied by developing a mesoscale finite element model and a continuum-based model. By equivalent the density gradient cylindrical shell chain to a variable density elastic rod, the governing equation of the density gradient chains under a stress pulse excitation was established. Based on the Laplace integral transformation and considering the linear density distribution in the rod, the analytical solution of the equation was obtained. Compared with the meso-finite element simulation results, it is found that the analytical solution can well predict the force evolution trend of the graded cylindrical shell chain under the excitation of a triangular stress pulse. The results show that the peak force in the positive gradient chain gradually increases with the wave propagation, while that of the negative gradient chain gradually decreases with the wave propagation. The peak force at the support end of the negative gradient chain is smaller than that of the uniform chain, while that of the positive gradient chain is greater than that of the uniform one. So the waveform control ability of the density gradient cylindrical shell chains is better than the uniform chain. The linear density gradient parameter has great influence on the waveform control ability of the density gradient cylindrical shell chains. The peak force transmitted to the support end increases with the increase of the density gradient parameter, and thus the density gradient cylindrical shell chain can control the stress pulse in a wider range. The theoretical model and its analytical solution provide a theoretical basis for studying the stress wave propagation law and revealing the force regulation mechanism of the graded cylindrical shell chains.

2022, 54(8): 2131-2139. doi: 10.6052/0459-1879-22-019

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EFFECT OF MICROSTRUCTURE ON FLOW BEHAVIOR DURING PENETRATION OF W₂₅Fe₂₅Ni₂₅Mo₂₅ HIGH-ENTROPY ALLOY PROJECTILE

Chen Haihua, Zhang Xianfeng, Zhao Wenjie, Gao Zhilin, Liu Chuang, Tan Mengting, Xiong Wei, Wang Haiying, Dai Lanhong

In order to explore the relationship between the macro deformation behavior of W₂₅Fe₂₅Ni₂₅Mo₂₅ high-entropy alloy projectile and the micro structure of the material in the penetration, a two-phase flow evolution model of constant cross-section straight pipe is established. The model takes the differences of soft and hard phase density, velocity and concentration into consideration based on the simplification of the two-phase flow model. By analogy with the inflow and outflow characteristics of the materials at the head of the projectile in the macro state, the analysis area is selected. The inflow and outflow relationship of the materials in the analysis area under the two-phase microstructure is established. Combined with the microstructure evolution equation, the concentration evolution results in the analysis area are given. The flow stability coefficient t/l_length characterizing the concentration evolution rate of the materials is proposed. In order to compare the penetration behavior of projectiles with different microstructures, the typical two-phase material tungsten- copper alloy (W₇₀Cu₃₀) was selected to carry out the penetration test of two kinds of projectiles into semi-infinite steel target based on small caliber ballistic gun. The microstructure evolution behavior of the two kinds of alloy projectiles is analyzed. The results show that the distribution of hard phase concentration generally reflects the characteristics of "concentrated in the center and sparse at the edge". The higher the concentration of the hard phase, the higher the density and the faster the driving speed, the smaller the flow stability coefficient t/l_length. The better the flow stability of the projectile in the penetration, and the easier it is for the projectile head material to form a continuous plastic flow zone. The two-phase flow evolution model of constant cross-section straight pipe can be used to describe the flow stability of projectile head material in the process of penetration, and reveal the correlation mechanism between projectile head deformation and two-phase microstructure in the process of penetration.

2022, 54(8): 2140-2151. doi: 10.6052/0459-1879-22-128

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SHOCK WAVE RESPONSE AND SPALL STRENGTH IN CoCrFeMnNi HIGH-ENTROPY ALLOY: A MOLECULAR DYNAMICS STUDY

Du Xin, Yuan Fuping, Xiong Qilin, Zhang Bo, Kan Qianhua, Zhang Xu

High-entropy alloys are expected to be used in aerospace, deep-sea exploration and other fields in the future, and will inevitably be affected by extreme shock loading, even will occur spall fracture. In this work, the molecular dynamics (MD) method is used to study the orientation and shock velocity dependence of the shock wave response, spall strength and microstructure evolution of single-crystal CoCrFeMnNi high-entropy alloys. The simulation results show that the elastoplastic two-wave separation phenomenon occurs when the shocking along the [110] and [111] directions and shows a trend of first strengthening and then weakening with the increase of the shock velocity. However, there is no two-wave separation phenomenon when the shocking along the [100] direction. During the shocking process, a large number of disordered structures are generated and increase with the increase of the shock velocity, which makes the spall strength decreases with the increase of shock velocity. In addition, the spall strength also exhibits orientation dependence. A large number of body-centered cubic (BCC) intermediate phases are generated when the shocking along the [100] direction, which inhibits the generation of stacking faults and disordered structures, making the highest spall strength in the [100] direction; The transformation of the relationship of the content of disordered structure in the nucleation area of microvoids at the early stage of spallation, making the spall strength in the [111] direction is higher than that in the [110] direction when the shocking velocity is low (U_p≤0.9 km/s), and slightly lower than that in the [110] direction when the shocking velocity is large (U_p ≥1.2 km/s). The research results are expected to provide theoretical support and data accumulation for the application of CoCrFeMnNi high-entropy alloys under extreme shock conditions.

2022, 54(8): 2152-2160. doi: 10.6052/0459-1879-22-239

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A COUPLING ANALYSIS MODEL OF CLAMPED MONOLITHIC BEAM IMPACTED BY FOAM PROJECTILES

Zhang Yuanrui, Zhu Yudong, Zheng Zhijun, Yu Jilin

The impact of foam metal projectiles may simulate the effect of explosion load. This loading technology has been applied in the impact resistance test of different protective structures. However, the actual impact load on the tested object and the interaction mechanism between the projectile and the tested object are still unclear. In this paper, the theoretical analysis and numerical simulation of the impact process of a foam projectile on a beam fixed at both ends were carried out. Based on the shock wave model of the foam and the structural dynamic response model of the clamped beam, a coupling analysis model describing the impact process was developed. The governing equations of different response stages were presented, and the numerical solution of the governing equations was obtained by using the Runge-Kutta method. The finite element model of a clamped monolithic beam impacted by a foam projectile was constructed by using the Voronoi technique and the impact process was simulated. Compared with the simulation results, it is found that the coupling analysis model can not only predict the velocity variation of projectiles and beams better than the impulsive loading model, but also obtain the actual impact pressure accurately. When the initial momentum of the foam projectile is identical, the change in the initial velocity, density, and length of the projectile can still affect the impact process due to the crushing behavior of the projectile. Finally, the effects of the density, length, and initial velocity of foam projectiles on the peak value, attenuation velocity, and duration of impact pressure were analyzed through the coupling analysis model, and the selection strategy of foam projectiles was proposed for the target simulation loads with different characteristics. The coupling analysis model provides a theoretical basis for studying the interaction mechanism between foam projectiles and the tested structure and the design guide of foam projectiles.

2022, 54(8): 2161-2172. doi: 10.6052/0459-1879-22-223

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EXPERIMENTAL AND NUMERICAL STUDIES OF H₂/CH₄/AIR EXPLOSION IN OPEN SPACE

Cui Yangyang, Wang Cheng, Qian Chengeng, Gu Gongtian, Gao Yang

In recent years, the use of hydrogen involves many fields. Multi-component mixture gas containing hydrogen is widely used in industrial production and domestic fuels. To ensure the safety of hydrogen containing gas in every links, e.g., production, transportation, and use, an outdoor open space mixed gas explosion test system is built. Five cases of hydrogen mole fraction (100%, 75%, 66.67%, 50%, 33.33%), five cases of equivalence ratio of mixed gas (0.8, 1.0, 1.1, 1.2, 1.4), three cases of initial volume of mixed gas (1 m³, 4 m³, 8 m³) and three kinds of constraints are adopted to study their effects on the mixed gas explosion pressure and flame. An explosion pressure prediction model considering the component ratio of the mixture and the initial volume of mixed gas is established based on the classical TNT equivalent method, and the theoretical model of explosion flame propagation radius is further modified by adding the effect of hydrogen mole fraction on the mixed gas explosion. Finally, the influence of building structure on explosion of H₂/CH₄/air mixtures is illustrated by experiments and large-scale high-resolution simulation. The results show that the addition of hydrogen can significantly enhance the gas explosion intensity. The maximum explosion pressure and flame propagation velocity increase with the hydrogen mole fraction, and first increases and then decreases with the increase of equivalence ratio, where the value increase to its peak value when the equivalence ratio is 1.1-1.2. At the same time, the hydrogen mole fraction and the initial volume of mixed gas seriously affect the accuracy of TNT equivalent method in predicting gas explosion pressure. The key parameters, e.g. explosion flame propagation distance, flame velocity and maximum explosion pressure, are obviously different under different construction conditions in the gas filling station. When the top and back side are restrained at the same time, the scope of explosion injury and consequences of an accident are the most serious. Thus, the influence of different building structures should be fully considered when delineating the safety distance of the gas station.

2022, 54(8): 2173-2193. doi: 10.6052/0459-1879-22-304

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NUMERICAL SIMULATION OF TRANSIENT STRONGLY-NONLINEAR FLUID-STRUCTURE INTERACTION IN NEAR-FIELD UNDERWATER EXPLOSION BASED ON MESHLESS METHOD

Wang Pingping, Zhang A-Man, Peng Yuxiang, Meng Zifei

Near-field underwater explosion involves transient and strongly nonlinear phenomena such as the mixing and coupling of multi-fluid flows, and the large deformation, damage and fracture of the structure. For the simulation of near-field underwater explosion, traditional mesh-based methods often face some difficulties, such as structural mesh distortion, low-accuracy in capturing multiphase interface and so on. In this regard, based on completely meshless methods, a transient strongly-nonlinear fluid-structure interaction numerical model is established for the whole Physical process of near-field underwater explosion including shock wave and bubble. The smooth particle hydrodynamics (SPH) based on Riemann solver is used for solving fluid dynamics, and the reproducing kernel particle method (RKPM) is adopted for structural dynamics. The fluid-structure interaction is realized by using the normal flux boundary condition. In order to improve the accuracy in solving the discontinuities of the flow field, the Riemann problem concept is introduced and combined with the MUSCL reconstruction algorithm. Aiming to solve the problem of accuracy decline caused by the drastic change of particle volume in the flow field, the adaptive particle splitting and merging algorithms are applied. To simulate the damage and fracture caused by underwater explosion, based on Lemaitre damage algorithm, the damage and fracture models for RKPM shell structure is proposed, and the crack initiation algorithm is developed according to the visual criterion. Based on the established SPH-RKPM model, the shock wave propagation, the bubble pulsation and jet, and structural damage in the near-field underwater explosion are simulated. The obtained fluid load and structural response are compared with the experimental data and other numerical solutions to verify the effectiveness and accuracy of the SPH-RKPM fluid-structure interaction model, and the underwater explosion load characteristics as well as the mechanism and law of the fluid-structure interaction structure damage are given. The present work aims to provide technical support for the load prediction of near-field underwater explosion, and to provide reference for the damage power assessment and warship protective structure design.

2022, 54(8): 2194-2209. doi: 10.6052/0459-1879-22-271

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DYNAMIC RESPONSE AND SPATIAL-TEMPORAL EVOLUTIONS OF DEEP-WATER COMPLEX CONFIGURATION CABLES WITH DISTRIBUTED BUOYANCY MODULES

Guo Shuangxi, Chen Weimin, Yan Dingbang, Song Jixiang, Shen Yijun

Long flexible cables is one of important parts of complex systems used in explorations and exploitations of ocean resources, particularly in deep or even ultra-deep water. These flexible cables, with large aspect ratio usually at level of 10³, need to be installed with distributed buoyancy modules along its body length. In that case, these distributed buoyancy modules make deep-sea cable configuration more complex and, moreover, their structural properties, such as structural tension and mass, are axially-changing. Thus structural motion response and its spatial-temporal evolutions become more complicated, which brings serious challenges to structural safety. In this study, a novel structural configuration, i.e. the double-stepped cable, is considered, and the dynamic governing equations of deep-water cable with distributed buoyancy modules are developed, principally based on the particular fluid-solid interaction characteristics and its coupling representation of the loadings, along with the experimental observations and verifications using our experimental water tank. The numerical simulations of the double-stepped cable dynamic response are carried out using the modified finite element approach. The responses and its corresponding propagations of the double-stepped cables, in terms of structural displacement and tensions along cable length, under environmental loadings and top-end excitations are comprehensively examined. In addition, the evolutions of displacement amplitudes and wavelengths of this kind of structure with axially-varying tension are explained based on the WKB theory. Our results show that the response does not change monotonously as it propagates along the cable length, and a local peak value may appear in the region with lower tension. Owing to the distributed buoyancy modules, along with axially-varying and discontinuous structural properties, the response spatial-temporal evolutions becomes more variant. There are mixed effects coming from both standing wave and traveling wave. It is also found that structural tensions not only affect the response amplitude significantly, also cause changes of wavelength during the process of response propagation.

2022, 54(8): 2210-2223. doi: 10.6052/0459-1879-22-238

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THE APPLICATION OF MODIFIED PHYSICS-INFORMED NEURAL NETWORKS IN RAYLEIGH-TAYLOR INSTABILITY

Qiu Rundi, Wang Jingzhu, Huang Renfang, Du Tezhuan, Wang Yiwei, Huang Chenguang

Physics-informed neural networks for phase-field method (PF-PINNs) are successfully applied to the modeling of two-phase flow, and it provides a brand-new way for the high-accuracy direct numerical simulation of two-phase flow. As an emerging interface capturing method, the introduction of the phase-field method in two phase flow ensures mass conservation near the interface and significantly enhances the interface capturing accuracy. However, the high-order derivate introduced by the phase-field method decreases the efficiency of network training. To enhance the efficiency of the training process, this paper regards the chemical energy as an auxiliary parameter and one of the outputs of the proposed neural network and revises the loss functions of physics constrain in the PF-PINNs framework based on the deep mixed residual method (MIM), which transforms the relationship between the auxiliary parameter and the phase-field variable from hard constrain to soft constrain. The proposed improvements decrease the size of the computational graph generated in automatic differentiation significantly and the computational cost of calculating high-order derivates in automatic differentiation is reduced. Meanwhile, the Rayleigh-Taylor (RT) instability is tested to assess the modeling ability of the proposed PF-PINNs when the Reynolds number is high and an enormous amount of calculation is needed. Compared with the spectral element-based phase-field method qualitatively and quantitatively, modified PF-PINNs can capture the strong non-linear evolution process of the interface, and the accuracy of modified PF-PINNs reaches the accuracy of traditional numerical solver. The result of the proposed neural network fits the characteristic of RT instability well. Compared the modified PF-PINNs with the original PF-PINNs, the results indicate that the deep mixed residual method can reduce the training time of the original PF-PINNs notably. The proposed method in this paper is a valuable reference for improving the training speed of the neural network and gives a new insight into exploring the intelligent modeling method with high accuracy.

2022, 54(8): 2224-2234. doi: 10.6052/0459-1879-22-253

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QUANTITATIVE EVALUATION OF STRESS SENSITIVITY IN SHALE RESERVOIRS: IDEAS AND APPLICATIONS

Du Shuheng, Shen Wenhao, Zhao Ya-Pu

Quantitative evaluation of stress sensitivity is one of the recognized key engineering problems in shale oil and gas exploration and development. The problems of shale pore size and permeability decline under the condition of variable stress have not been settled yet, and need to be explored further. Based on Griffith's classical elasticity solution, fine characterization of the pores and microcracks of heterogeneous shale, the formula of the retention of rock permeability under stress is derived by establishing a cylinder tube bundle model with elliptical cross-section. Then the stress sensitivity evaluation method and the calculation formula of overburden permeability suitable for heterogeneous shale oil reservoirs are given, respectively. Finally, it has been applied in typical shale oil reservoirs in western and central China. The results show that: (1) under the same stress, the stress sensitivity of shale oil reservoir is jointly controlled by the ratio between the initial major and minor axes of storage and seepage space, Young's modulus, and Poisson's ratio, and has nothing to do with the initial porosity and permeability of shale; (2) The stress sensitivity of fracture-type shale is slightly higher than that of matrix-type shale due to the development of microcracks with high ratio of major and minor axes, and the smaller the Young's modulus, the greater the difference between above two types of shale; (3) Under the effective stress of 40 MPa, the maximum permeability loss of fracture-type and matrix-type shale oil reservoirs is less than 10% and 8%, respectively, which proves that the stress sensitivity of shale is generally low. The impact of stress sensitivity on in-situ reserves and actual productivity of shale oil needs to be reexamined in engineering practice. The conclusion provides the new theoretical and practical basis for the accurate evaluation of shale oil reserves and the efficient improvement of oil recovery.

2022, 54(8): 2235-2247. doi: 10.6052/0459-1879-22-262

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KEY MECHANICAL PROBLEMS OF WELL DRILLING AND COMPLETION AND HYDRAULIC FRACTURE IN HIGHLY EFFICIENT EXTRACTION OF SHALE GAS

Gao Yue, Wang Tao, Yan Ziming, Liu Zhanli, Zhuang Zhuo

The key mechanical problems of well drilling and completion, and hydraulic fracture network reformation are studied in the highly efficient extraction of shale gas. The anisotropic constitution, strength and fracture toughness of poroelastic medium in shale are demonstrated. The micro-homogenous and micro-isotropic assumptions are discussed, and the material constants measurement of the transversely isotropic poroelastic rock is analyzed. The fracture propagation in the weak plane model with the maximum energy release rate criterion is discussed. The time-related shear failure models of borehole wall stability in the poroelastic medium are discussed during well drilling and completion. Eight failure modes of the well drilling problem are discussed, and the allowable borehole pressures are given with explicit expressions. The technologies of hydraulic fracture network reformation for horizontal wells are shown, including the experimental method of hydraulic fracture in a large physical model, the numerical simulation based on the extended finite element method and finite volume method for fluid/solid/fracture coupling during fracking process, and a field application of hydraulic fracturing on site in Sichuan-Chongqing region. Based on the data-driving method, a prediction model of shale gas recovery ratio is developed, and the effectiveness of the extreme gradient boosting method for the small dataset is discussed,.

2022, 54(8): 2248-2268. doi: 10.6052/0459-1879-22-251

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REVIEW ON THE MECHANICAL-THERMAL COMBINED EXPLOITATION METHODS OF DEEP SEA NATURAL GAS HYDRATE

Li Peng, Zhang Xuhui, Liu Lele, Zhang Yan, Lu Xiaobing, Li Qingping, He Yufa

Gas hydrates are an important sort of strategic energy resource in China because of their enormous reserves and small contamination compared to traditional fossil fuels. Many countries have accelerated the exploitation and research of gas hydrates. Profitable extraction methods, disaster control and environmental protection are two key issues that need to be addressed in the commercial exploitation of natural gas hydrates. Currently, the combined use of heat injection and pressure reduction methods is considered to be the most effective method of recovery of gas hydrates. In the method of depressurization and heat injection, natural gas hydrate production includes physical processes and effects such as heat transfer, phase change, seepage, and deformation. The heat transfer is the slowest and the phase change consumes a lot of heat, so it is impossible to directly use the conventional oil and gas extraction scheme that relies solely on the seepage principle to extract natural gas hydrates. Natural gas hydrates in the South China Sea occur mostly in silty clay and silty sand and other sediment types, with poor cementing and shallow burial depth. Conventional extraction methods are not suitable for hydrated extraction in the Southern Sea, and new methods of extraction have to be envisaged. Among these, the improvement of the efficiency of heat transfer in the sedimentary layer is the key to the exploitation of natural gas hydrate. Che-Min Cheng presents a mechanical-thermal combined recovery method, utilizing the heat of seawater, convective heat transfer, and pipe transportation, considering the stratum safety similar to coal mining. The mechanical-thermal combined method for gas hydrate exploitation is a new concept, which has the advantages of high efficiency and controllability, and meanwhile, the safety of formation can be reduced effectively. This paper presents a comprehensive estimation of the energy, apparatus, and economic feasibility of the new method demonstrates the advances in multiphase flow with phase transformation and stratum safety and gives some suggestions for the potential application of these results.

2022, 54(8): 2269-2286. doi: 10.6052/0459-1879-22-301

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PROGRESS IN INSTRUMENTED INDENTATION METHODS FOR DETERMINATION OF SURFACE RESIDUAL STRESS

Peng Guangjian, Zhang Taihua

By reviewing the instrumented indentation methods for determination of surface residual stress, the basic principle and mechanism for measuring residual stress by instrumented indentation was first expounded. The technical route for establishment of instrumented indentation methods for residual stress determination was also summarized. According to the classification of these methods, six representative instrumented indentation methods were discussed in detail to find out their corresponding advantages and limitations. Then, four commonly used stress-generating jigs to verify the reliability of instrumented indentation methods for residual stress determination were illustrated. Finally, the progress in instrumented indentation methods for surface residual stress determination was summarized. It was concluded that the future goal is to establish an instrumented indentation method for determining the mechanical parameters and non-equibiaxial residual stresses simultaneously without using stress-free reference samples. To establish such method, four research essentials, i.e., clear examination mechanism, reliable analysis process, technical feasibility, and reliable results, were discussed.

2022, 54(8): 2287-2303. doi: 10.6052/0459-1879-22-222

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CHARACTERISTICS OF THE HETEROGENEOUS MECHANICAL RESPONSE OF COAL AT THE NANO AND MICRO-SCALE USING INSTRUMENTED INDENTATION EXPERIMENTS

Deng Bozhi, Nie Baisheng, Liu Xianfeng, Shi Farui

Coal is the main energy resource in China. The occurrence of coal and gas outburst, and rock burst in coal mines seriously impact the safety of coal production. Coal is a typical composite. The significant difference in mechanical properties among the components of coal can easily induce internal stress concentration under external stress disturbance. This may result in the instability and failure of coal and forming dynamic disasters in coal mines. This research focused on heterogeneous coal and studied the heterogeneous microstructures and mechanical properties of coal at the nano and micro-scale using micro-CT, scanning electron microscope, and instrumented nano and micro-indentation experiments. The experimental results indicated that: coal is composed of organics and multiple types of minerals with different mechanical properties. Minerals exist in coal as granular filling, filiform filling, and banded intrusion. The mineral content and structure were different among these mineral filling or intrusion areas. It led to the physical and mechanical heterogeneities of coal at the nano and micro-scale; The indentation experiments at the nano-scale could capture the mineral structures at mineral filling or intrusion areas and measure the individual mechanical parameters of minerals or organics. Their significant difference in mechanical properties could be identified by nano-indentation; The indentation experiments at the micro-scale could characterize the mechanical properties of a whole coal composite. The more mineral filling was, the stronger mechanical properties of a coal composite were. Moreover, the failure mode of a coal composite at the micro-scale depended on its mineral structure. The experimental results revealed the heterogeneous characteristics of the microstructures and mechanical properties of coal. The brittle failure induced by the special filiform structure of coal at the micro-scale was discussed. This study provides the fundamental understanding for the prediction and prevention of rock burst, coal and gas outburst during coal mining.

2022, 54(8): 2304-2317. doi: 10.6052/0459-1879-22-244

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MULTISCALE ANALYSIS OF COMPLEX TIME SERIES

Gao Jianbo

To commemorate the first anniversary of Prof. Zheng Zhemin's death, the author first recalls the teachings that Prof. Zheng gave to himself during his postgraduate studies, then introduces their joint work in nonlinear science, which have been expanded and evolved into some general methods for analyzing nonlinear problems. Specific methods include optimal embedding of chaotic time series, direct dynamical test for chaos, multiscale analysis based on scale-dependent Lyapunov exponent (SDLE), and adaptive fractal analysis. In particular, SDLE can very well characterize all known time series models, and therefore, can unify various measures of complexity developed thus far. Adaptive fractal analysis is based on adaptive filtering, which can optimally determine trends, including various oscillation modes and nonlinear regression curves in regression analysis, and can also optimally reduce noise and decompose time series into intrinsic modes. These methods have been widely used in many fields of natural sciences, engineering, and social sciences. They are particularly useful for fault diagnosis in various fields (including operation and maintenance), analysis of biomedical data, and measurement of uncertainty. Prof. Zheng was never confined to a field of comfort, but constantly expanded into new the ones with time going by. In a period with unprecedented changes not seen in a century, we must carry forward the spirit of Prof. Zheng.

2022, 54(8): 2318-2331. doi: 10.6052/0459-1879-22-213

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THE EXPERIENCE OF PRACTICING THE THOUGHT OF ENGINEERING SCIENCE-ENGINEERING SCIENCE IS THE SOURCE OF TECHNOLOGICAL INNOVATION AND HUMAN UNDERSTANDING

Li Shihai, Zhang Li

Mr. Che-Min Cheng had been actively practicing and advocating Mr. Hsue-Shen Tsien’s engineering science thought throughout his life. He himself had also inherited and carried forward engineering science, which is of great practical significance to the promotion of technological innovation in China. Firstly, this paper expounds the definition, methodology and characteristics of engineering science, emphasizing that proposing new solutions is the core content of engineering science and addressing the role of engineering scientists in technological innovation. Secondly, the paper interprets the role of engineering science in technological innovation by analyzing several engineering cases, e.g. the use of explosion methods to deal with underwater foundations, the related techniques in the construction of the Three Gorges coffer. The paper clarifies the theoretical basis and engineering science nature of numerical simulation, in which how to select generalized variables and construct the structure of the solutions is the extremely creative work. The core of the continuous discontinuous simulation method is to construct mutable generalized variables in time domain. Several simulation cases that embody engineering science methodology are listed. The paper also introduces how to systematically practice the methodology of engineering science in the study of key mechanical problems in landslide disaster prevention, expounds the uniqueness of inversion and the feasibility of combining monitoring and numerical simulation, and gives the concept of progressive failure and rupture degree. Finally, based on the cognition of developing high-pressure unloading ore pulverization technology, the paper proposes several engineering scientific research directions for the development of rock physics.

2022, 54(8): 2332-2342. doi: 10.6052/0459-1879-22-326

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2022 Vol. 54, No. 8

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