Mechanics in Engineeringfirst published in 1979. It is administrated by the Chinese Academy of Sciences, with the Chinese Society of Theoretical and Applied Mechanics and the Institute of Mechanics, Chinese, Academy of Sciences jointly as the responsible institutions. It covers general fields of mechanics, with the widest circulation among journals devoted to mechanics in China. Its focuses are on progresses of mechanics and its interdisciplinary fields, applications in engineering, teaching notes on mechanics, profiles of scientists of mechanics, history of mechanics, interesting anecdotes, book reviews and others. With its unique features as distinguished from similar journals in China, keeps the following regular columns... More>>
08 April 2022, Volume 44 Issue 2
Review
APPLICATION AND CHALLENGES OF CARBON FIBER IN MARINE STRUCTURES AND EQUIPMENT1)
ZHANG Yu, DENG Xin, ZHANG Feifan, ZHANG Peng, GUO Yuhan
2022, 44(2):  249-257.  DOI: 10.6052/1000-0879-21-582
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Carbon fiber has excellent properties such as light weight, high strength, corrosion resistance and fatigue resistance, and it is widely used in the industrial production. Traditionally, steel is the main material of marine structures and equipment. However, with the development of offshore oil and gas into deep water and clean marine energy, carbon fiber begins to replace steel because of its advantages. This paper introduces the application and related researches of carbon fiber in marine pipelines and cables, mooring systems, pressure structures, wind turbine blade, hydrogen storage tanks and repairing. It is found that carbon fiber satisfies the demand of light weight and high strength. However, there are still some challenges, such as the connection strength between carbon fiber and metal materials, the durability of carbon fiber in complex marine environment and so on. It is believed that with the development of marine resources and technologies of materials, production and others, CFRP will play a grand role in marine structures and equipment.

Applied Research
ANALYSIS ON THE FLOW-INDUCED VIBRATIONAL SIGNAL OF THE PUMP TURBINE DURING THE TRANSIENT PROCESS BASED ON THE VARIATIONAL MODE DECOMPOSITION1)
ZHENG Xianghao, ZHANG Yuning, ZHANG Liang, LI Jinwei
2022, 44(2):  258-265.  DOI: 10.6052/1000-0879-21-429
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The flow-induced vibration is an important parameter to evaluate the operational stability of the pump turbine. In the present paper, based on the measured vibrational signal of the top cover of the pump turbine in China during the transient process of shutdown from the generating mode, the time-frequency analyses of the vibrational signal are carried out by adopting the empirical mode decomposition (EMD), the variational mode decomposition (VMD) and the Hilbert transform. The following conclusions are obtained. Firstly, compared with the EMD, the time-frequency information of the vibrational signal can be observed more intuitively from the Hilbert spectrum based on VMD. Then, the time-frequency analysis results show that the vibration of the top cover is mainly induced by the propagation of the abnormal pressure fluctuations caused by the rotor-stator interaction (RSI) between the rotational impeller and the stationary guide vanes within the flow passing components. Finally, with the gradual decrease of the discharge during the shutdown process of the unit, the abnormal pressure fluctuations excited by the fluids gradually weaken, leading to the gradual decreases of the intensity of RSI and the vibration of the top cover.

RESEARCH ON FRACTIONAL DERIVATIVE CREEP CONSTITUTIVE MODEL OF GYPSUM ROCK UNDER ACID CORROSION1)
XIE Senlin, WAN Wen, ZHOU Hongwei, JIA Wenhao, ZHANG Lei, WEI Qing, CHEN Wei
2022, 44(2):  266-275.  DOI: 10.6052/1000-0879-21-365
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To explore the erosion damage of groundwater on rocks, the uniaxial compression creep properties of gypsum rock under different pH values and the microscopic changes of the specimens before and after acid corrosion were studied by uniaxial compression creep test and scanning electron microscope test. The results showed that the creep rate of specimens was accelerated (the stronger the acid, the faster the creep) by acid corrosion. The specimens are mainly in the form of splitting and shearing failure, and damage and failure appeared at the end (the stronger the acidity, the more obvious the damage failure was). From a microscopic point of view, the structure of the specimens in the dry state was complete, but the structure of the specimens was damaged (with the increase of acidity, increased pores and loosed structure) by acid corrosion. To describe the creep process of gypsum rock under acid corrosion conditions, the variable coefficient Abel dashpot was modified by introducing the acid corrosion degradation attenuation coefficient $\beta$, and the modified variable coefficient Abel dashpot was used to replace the Newton dashpot in the viscoplasticity of the Nishihara model, and a fractional creep constitutive model considering acid corrosion was established. The parameters in the model were determined through fitting of the experimental data. The sensitivity of the parameter $\beta$ is also analyzed. The study found that the fractional derivative creep constitutive model proposed here after considering acid corrosion conditions can better reflect the three stages of creep of gypsum rock under acid corrosion, especially the accelerated creep stage.

SIMULATION OPTIMIZATION STUDY ON AERODYNAMIC DRAG REDUCTION OF HIGH-SPEED PANTOGRAPH1)
YAN Yongzhao, XU Xianghong, WANG Xizhou, GENG Haosen, HUANG Sijun
2022, 44(2):  276-284.  DOI: 10.6052/1000-0879-21-372
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With the development of green and environmentally friendly modern rail transit design concept, pantograph aerodynamic drag reduction has become one of the key problems restricting the speed increase of high-speed train. When the high-speed train is running, the pantograph is exposed outside the streamlined body, which is one of the main sources of train aerodynamic drag. With the increase of train speed, the aerodynamic drag reduction problem of pantograph needs to be solved urgently. In this paper, the distribution of aerodynamic drag of the whole pantograph on a high-speed trail is simulated with computational fluid dynamics (CFD). It is found that the pressure drag of the contact strip and base is the main source of the aerodynamic drag. The optimization scheme of the contact strip and the base aerodynamic drag reduction shell is proposed, and the aerodynamic drag reduction is verified by comparison with the original model. Calculation shows that aerodynamic drag can be reduced by 25.13% and 24.19% in the opening and closing state, respectively, when the drag reduction pantograph runs at 350~km/h.

SIMULATION ANALYSIS OF ARRESTING PROCESS FOR CARRIER AIRCRAFT CONSIDERING KINK-WAVE1)
GAO Lele, LIU Rongmei, YAO Niankui, CHEN Jianping
2022, 44(2):  285-292.  DOI: 10.6052/1000-0879-21-408
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Based on nonlinear Euler-Bernoulli beam theory, a discretized model of the arresting cable considering the kink-wave is developed to study the phenomenon of the kink-wave and analyze the influence of the kink-wave during the arresting process of carrier aircraft. The numerical simulation result of the arresting process shows that the kink-wave exists in the whole arresting process. The hook load fluctuates due to the kink-wave. This fluctuation is most intense at the initial period of the arresting process. Then the fluctuation gradually attenuates. The existence of the kink-wave significantly shortens the stopping time and stopping displacement, and also makes the maximum value of the arresting cable tension larger. In addition, there are obvious differences in propagation direction, angle of cable tension and average velocity of the kink-wave between the odd and even numbered kink-waves.

DYNAMIC SIMULATION OF CIVIL AIRCRAFT DITCHING BASED ON CFD METHOD
LUO Wenli, CHEN Haoyu
2022, 44(2):  293-302.  DOI: 10.6052/1000-0879-21-381
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A numerical simulation was carried out to investigate the dynamic impact for civil aircraft ditching. The simulation used Reynolds averaged N-S equation from the computational fluid dynamics (CFD). Water-air interface and ditching process were modeled through the combination of volume of fluid (VOF) method and full-flow moving mesh. The position and attitude of aircraft were determined by solving 6-degrees-of-freedom equations. Firstly, the method was validated through typical examples, and then it was used for ditching simulation and analysis of a regional aircraft. The tail-mounted-engine and high-tail layout was focused when researching ditching characteristics. The results show that ditching movement and load characteristics can be well simulated. During water entry period, the regional aircraft is subjected to a large water impact load, while the suction force generated by rear fuselage makes the aircraft pitching up. Afterwards, the suction force decreases as speed decreases, and the aircraft gradually pitches down and approaches to a stable state.

STRENGTH EVOLUTION OF CARBONACEOUS PHYLLITE UNDER THERMAL MECHANICAL COUPLING
YANG Bingchang
2022, 44(2):  303-309.  DOI: 10.6052/1000-0879-21-331
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Temperature field and stress field are important factors that affect the surrounding rock strength of diversion tunnel in carbonaceous phyllite stratum. 16 groups of experiments with 2 factors and 4 levels were set. Based on the results of uniaxial and triaxial tests, the behavior of stress-strain curve and the failure mode of carbonaceous phyllite under thermal mechanical coupling are discussed, and the variation law of peak strength is obtained. Based on Mohr-Coulomb (M-C) criterion and Heok-Brown (H-B) criterion, two calculation methods of failure strength, Mohr-Coulomb-Thermal (M-C-T) method and Heok-Brown-Thermal (H-B-T) method, are established respectively. The experimental results and theoretical analysis show that with the increase of confining pressure and temperature, the failure of carbonaceous phyllite changes from brittle to ductile. The cohesion, uniaxial compressive strength and parameter $n$ increase linearly with the increase of temperature, but the internal friction angle is not affected remarkably by temperature. The results show that both M-C-T method and H-B-T method can quantify the strength evolution of carbonaceous phyllite under thermal mechanical coupling. Error analysis shows that the calculation error of the former is $-$28.5%~7.7%, and that of the latter is $-$8.9%~5.6%. The results of H-B-T method show less scattering, hence the method is more stable and more safety oriented.

STUDY ON SPINDLE OFFSET FORCE AND DEFLECTION PERFORMANCE OF POINT-BIT STEERABLE TOOL1)
TANG Bo, WANG Peng, ZHANG Hong, FENG Ding
2022, 44(2):  310-316.  DOI: 10.6052/1000-0879-21-363
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In order to study the influence of the installation position of weight on bit (WOB) transfer device on the spindle offset force and deflection performance of point-bit steerable tool, a mechanical model of spindle considering axial load is developed based on the mechanical model of transverse bending beam and the bending beam with combined axial and lateral load. The beam element assembly method is used to solve the string mechanics by using the general solution of differential equation. The geometric equation and mechanical equation of spindle are derived. And the effects of the position of WOB transfer device on the spindle offset fore and deflection performance parameters such as bit side force and bit inclination are investigated. The results show that when increasing the position of the WOB transfer device from the bit, the offset force required for tool spindle deflect will be reduced, and the bit-side force and bit inclination will be increased.

FRACTAL DERIVATIVE MODEL OF WATER ANOMALOUS ADSORPTION IN SWELLING SOIL1)
TIAN Peibo, LIANG Yingjie
2022, 44(2):  317-321.  DOI: 10.6052/1000-0879-21-391
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In this paper the fractal derivative model of water adsorption in swelling soil was constructed based on the material coordinates, which correlates the moisture content with the spatial position. The cumulative adsorption of water in swelling soil was also derived. The cumulative adsorption in swelling soil underlying the fractal derivative model is a function of the fractal derivative of time and the diffusion coefficient. The fractal derivative order can be used to classify the adsorption process and to characterize the heterogeneity of soil. The feasibility of the fractal derivative model is verified by analyzing the experimental data of the water cumulative adsorption in the black soil and sand, and the proposed model exhibits higher accuracy than the traditional integer order model.

SENSITIVITY ANALYSIS OF FACTORS AFFECTING SOIL FROST HEAVE RATE BASED ON SSA-BP METHOD
YAO Zhaoming, QI Jian
2022, 44(2):  322-327.  DOI: 10.6052/1000-0879-21-359
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Sensitivity analysis of factors affecting soil frost heave rate is one of the most important research contents of artificial frozen soil, which is of great significance to engineering construction. There are many factors influencing the frost heave rate of artificial frozen soil, and those influencing factors are coupled together. The experimental data of sandy loam, clay and loam in Gansu area are selected, and the sparrow search algorithm is used to optimize the back propagation (BP) neural network to establish a predictive model. On this basis, a method for calculating the sensitivity of the frost heave rate affecting factors is proposed. The results show that the prediction accuracy of the optimized prediction model is much higher than that of the traditional BP neural network prediction model. It also reals that the most sensitive factors affecting the frost heave rate in sandy loam, clay, and loam are frost penetration rate, initial dry density in the freezing part and initial water content in the freezing part. This research provides an important reference for preventing engineering frost heave hazards.

THE CHARACTERISTICS OF THE CENTER OF MASS OF THE LIQUID IN THE TANKER WITH DIFFERENT CROSS SECTION SHAPES1)
HE Lieyun, LI Guojun, ZHOU Yan
2022, 44(2):  328-336.  DOI: 10.6052/1000-0879-21-303
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In order to investigate the motion of the center of mass of liquid in a turning tank truck, tanks with four common section shapes, i.e., circular, oval, multi-section arc and square, are considered in this research. The liquid in the tank is assumed to be quasi-static during the turning. The liquid center of mass is calculated through the integration of the center of mass for each homogeneous section plane. Using MATLAB, the centroid coordinates and centroid trajectory images are obtained. The lateral stability characteristics of tank trucks with different shape of cross-section is analyzed. The research results provide guidelines for the design and driving stability of liquid tank truck.

STUDY ON DYNAMIC SNAP-THROUGH OF A SIX-LAYER ASYMMETRIC CROSS-PLY BISTABLE COMPOSITE LAMINATED PLATE1)
DONG Ting, ZHANG Wei
2022, 44(2):  337-343.  DOI: 10.6052/1000-0879-21-315
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In this paper, the dynamic snap-through phenomenon of a six-layer asymmetric cross-ply bistable composite laminated plate (0/0/0/90/90/90) is studied and the nonlinear vibrations before, after and during the dynamic snap-through are demonstrated. The boundary conditions are fixed at the center and free at the four sides. From the von Kármán's large deflection theory, the dynamic and governing equations of the six-layer asymmetric cross-ply bistable composite laminated plate are established by using the Reddy's third-order shear deformation theory and the Hamilton's principle. Taking the base excitation amplitude as the control parameter, the diagram for the maximum Lyapunov exponents, the Poincaré maps and the time-history for the dynamic snap-through are obtained. Based on the maximum Lyapunov exponents and the Poincaré maps, the periodic vibration, the quasi-periodic vibration, the chaotic vibration and the dynamic snap-through of the six-layer asymmetric cross-ply bistable composite laminated plate are analyzed. It is found that the chaotic vibration is a favorable condition for the dynamic snap-through.

FINITE ELEMENT ANALYSIS OF J-INTEGRAL PLASTIC FACTOR OF SINGLE EDGE NOTCH TENSION SPECIMEN1)
WU Xu, SHUAI Jian, XIE Bin, HAN Xuliang, MA Chenbo, DENG Xiaokang
2022, 44(2):  344-350.  DOI: 10.6052/1000-0879-21-344
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The fracture toughness test of pipeline girth weld is the basis of pipeline integrity assessment, and the accurate calculation of J-integral plastic factor is the premise of fracture toughness test. In this paper, aiming at the SENT (single edge notch tension) specimen with thickness-to-width ratio $B/W=2$, a three-dimensional finite element model of the SENT specimen of the pipe base metal with/without side grooves depth of 10% is established. The influence of crack length, specimen thickness, hardening index and specimen side groove on J-integral plasticity factor are studied. Through fitting the numerical analysis results after considering the factors such as crack length and material hardening performance, the J-integral plastic factor equations of SENT specimens are developed, which is suitable for pipe base metal with or without side groove of depth of 10%. The equations could be used to test the fracture toughness of SENT specimens.

DEFLECTION CURVE EQUATION OF LINEAR STRENGTHENED ELASTIC-PLASTIC BENDING STRAIGHT BEAM1)
CHEN Yingjie, FENG Yongqiang, DONG Jingbo
2022, 44(2):  351-357.  DOI: 10.6052/1000-0879-21-378
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Aiming at the incomplete application of materials in elastic-plastic stage. In this paper, by using the principle of elastic-plastic partition minimum potential energy, thecriterion of potential energy and Euler equation of bending straight beam under linear strengthening model are derived. The deflection curve equation of cantilever beam and simply supported beam under concentrated load issolved. The deflection curve equation is put into Matlab software for numerical calculation, and the results are compared with ANSYS. The results show that both the numerical solution and the finite element value meet the allowable error range in practical engineering. The proposed method provides a new idea for solving practical engineering problems.

THE MATRIX FORM OF DIFFERENTIAL CONSTITUTIVE EQUATION FOR VISCOELASTIC MODEL1)
PENG Peihuo, HUANG Chaojun
2022, 44(2):  358-367.  DOI: 10.6052/1000-0879-21-267
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This paper derived the general matrix form for the linear viscoelastic model for arbitrarily linked springs and dampers. Firstly, the underlying linear viscoelastic problem is modeled through the standard model composed of basic Maxwell units. Then the standard model is transformed into a directed graph, and the basic stress equation and strain equation are expressed in the form of independent path and closed enclosure. And then the general matrix form of a differential constitutive equation for an arbitrary viscoelastic model is derived. Finally, a universal paradigm suitable for computer programming is developed, and the relevant algorithm is realized with the Python language along with numerical results.

Research on Education
DISCUSSION ON LIMITATION OF VIRTUAL DISPLACEMENT IN UNIT-LOAD METHOD1)
LI Min, LI Yilun, CHEN Weimin
2022, 44(2):  368-372.  DOI: 10.6052/1000-0879-21-358
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Unit-load method is a general method to solve displacement, which plays an important role in the energy method of material mechanics. When using virtual displacement mode in the unit-load method, the main limiting condition for virtual displacement is to satisfy the displacement boundary condition. For statically indeterminate problems, especially for the unit-load system construction with free boundary conditions, the understanding of the limiting condition determines the flexibility of using the unit-load method. In this paper, several classical examples are used to analyze the effect of this restriction, and to provide reference for explaining related problems in teaching activities.

THE CONSTRUCTION AND PRACTICE OF THE INTRODUCTION COURSE OF MECHANICNS1)
ZHAO Yingtao, MA Shaopeng
2022, 44(2):  373-378.  DOI: 10.6052/1000-0879-22-041
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Professional introduction course is the major enlightenment education course for freshmen. It is an introductory course for undergraduates to understand the major connotation and increase the students' interest. However, it is not easy to construct the course because of its characteristic as "the double-edged sword". Based on the practice of the course "introduction to mechanics" in BIT, this paper tries to discuss the key points and difficulties of the course, and summarize the gains and losses in our practice, which might provide some experience and lessons for the development of similar courses.

CONSTRUCTION AND DISCUSSION OF INTERDISCIPLINARY MECHANICS COURSE IN THE CONTEXT OF "NEW ENGINEERING"1)
WAN Chao, YU Yang, HUO Bo
2022, 44(2):  379-384.  DOI: 10.6052/1000-0879-21-266
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During the new technology revolution around the world, China government is promoting the "new engineering" plan. Major of mechanics is one of the critical fundaments for engineering, which means that it requires more urgent improvements on the professional education and disciplinary construction in response to the revolution direction of engineering education in the new era. Based on biomechanics and biomimetics interdisciplinary course, this paper analyzed the main problems during the teaching of the interdisciplinary course (i.e., less knowledge of background for students, unclear teaching systematic content for teachers, and lack of attractive approach for teaching). Some specific attempts were then proposed and summarized for solving these problems, such as performing potential integrations with general education course for fresh students, basic mechanics course, and modern educational technology. We suggest that these teaching practices may offer potentially helpful reference and consulting for the development and improvement of interdisciplinary course in the existing engineering majors.

ANALYSIS ON THE INDEPENDENCE OF EULER ANGLES AND THE COORDINATE TRANSFORMATION MATRIX
ZHANG Yuan, WANG Shimin, WANG Qi
2022, 44(2):  385-389.  DOI: 10.6052/1000-0879-21-288
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The independence of the Eulerian Angles and the rotation sequence in constructing of transformation matrix from body coordinates to fixed coordinate problems are discussed in this paper, by introducing the transition coordinate system. The rigid body position is independent of the rotation sequence corresponding to Eulerian angles. However, there exists a sequence in all of them, in which each of the three rotations is carried out around common coordinate axes of the two coordinate systems associated with each other, and the transformation relation of each rotation is expressed with simple transformation for rotation about a fixed axis. Therefore, this sequence is chosen to construct the transformation matrix from the body coordinate to the fixed coordinate system.

FORMATION AND USAGE OF DIFFERENTIAL MOMENTUM EQUATION1)
HUANG Shuxin
2022, 44(2):  390-392.  DOI: 10.6052/1000-0879-21-277
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The differential momentum equation in fluid mechanics is a fundamental equation, which is usually called the motion equation in textbook. The Navier-Stokes equation can be deduced from the momentum equation by adding some assumptions. The present manuscript shows the formation and usage of the equation. The equation was once reported in the work in 1828 of French Augustin L. Cauchy (1789—1857). George G. Stokes (1819—1903) in England could be the first person who used the equation correctly in the flow problem of constant-viscosity fluid according to the literatures. Moreover, English Ronald S. Rivlin (1915—2005) could use the equation in the viscoelastic flow problem firstly.

A NOTE ON GALERKIN METHOD1)
CHEN Bo, LI Yinghui, LI Xingyu
2022, 44(2):  393-396.  DOI: 10.6052/1000-0879-21-308
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Galerkin method is a numerical method widely used in mathematics, physics, and engineering problems. There are some controversies among textbooks and some literature about the selection of trial functions. To make an example, this paper uses Galerkin method to solve the static and dynamic problems of a cantilever beam under the axial load. It is proved that correct results cannot be obtained when choosing trial functions that satisfy only displacement but not force boundary conditions, even if increasing the number of trial functions.

DISCUSSION ON THE TEACHING METHOD OF THE CONCEPT OF CORE OF CROSS-SECTION1)
WU Zeyan, LUO Wei, TIAN Dongfang, PENG Hui, YE Yong
2022, 44(2):  397-403.  DOI: 10.6052/1000-0879-21-319
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The core of cross-section is an important concept in the design of the columnar structure of concrete non-tensile materials under compression in the field of civil engineering. In order to deepen students' understanding of the concept of the core of cross-section in the course of material mechanics, it first developed a programing code with MATLAB to calulate the core of cross-section. The concrete numerical examples help students to understand the concept more easily. Then the convexity of the core of the cross-section was discussed. The convexity of the core of the section is explained by the static equivalence principle and the superposition principle from the perspective of mechanics; the convexity is proved mathematically by the neutral axis equation and the definition of convex set. The teaching process is helpful in cultivating students thinking skills.

A NEW METHOD FOR GEOMETRIC CONSTRUCTION ANALYSIS OF COMPLEX PLANAR SYSTEMS1)
WANG Mengfu
2022, 44(2):  404-408.  DOI: 10.6052/1000-0879-21-330
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It is well known that the zero-load method is generally used to analyze the geometric construction of complex plane systems in structural mechanics. However, the zero-load method needs involved calculation, and the analysis process is cumbersome. In order to simplify the analysis of geometric construction of complex plane system, based on the substitute member method and the zero-load method, a new method for geometric construction analysis of complex plane systems is developed, and the basic rule of geometric construction analysis of complex plane system is presented. Finally, the feasibility and superiority of this method are demonstrated by several examples.

BUILDING THE LINK BETWEEN BASIC MECHANICS AND PRACTICE BY OPEN EXPERIMENT—STRESS ANALYSIS OF THIN-WALLED CYLINDER AS AN EXAMPLE1)
LU Yulin, CHEN Xiaoran, WANG Li, WEI Jia
2022, 44(2):  409-414.  DOI: 10.6052/1000-0879-21-347
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Taking the stress analysis of a thin-walled cylinder as an example, an open experiment is designed. Based on the knowledge of materials and elasticity mechanics, the stress analysis of the thin-walled cylinder is described to model similar structures such as pop-up can and gas tank. The internal pressure and strength of the thin-walled cylinder are analyzed through theoretical analysis, tension experiment and electrical measurement experiment. The experimental results are in good agreement with those of theoretical analysis. The achievement degree of open experiment shows that this experiment has a certain interest and professionalism. Students have a positive attitude and recognize that the comprehensive application ability has been improved. However, this research also reveals some problems in this experiment. It provides a powerful reference for the further improvement of the open experiment.

A SOlVING EQUATION FOR MOMENT DISTRIBUTION METHOD
ZHU Xiaojiang, ZHANG Linnan, QIN Taiyan
2022, 44(2):  415-418.  DOI: 10.6052/1000-0879-21-271
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This paper transformed the gradual calculation process in the traditional moment distribution method into the solution of equations. The method can improve the calculation efficiency and accuracy. The equations obtained by this method are similar to the equations obtained by the matrix displacement method, but the unknowns are completely different. The derivation process of the equation is more concise and easier to understand. It can inspire students to think about the analysis of the matrix displacement method in teaching.

A NEW PHOTOELASTIC EQUIPMENT FOR CLASSROOM TEACHING1)
SU Fei, LUO Qian
2022, 44(2):  419-422.  DOI: 10.6052/1000-0879-21-352
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Photoelastic experiment is a full field and real-time optical experiment. It can vividly show some abstract mechanical phenomena to students in the form of images, which greatly improves the teaching efficiency and students interest. However, the reality is that the teaching time for basic mechanics theory is compressed, and the time for experimental class is very limited. The advantages of photoelastic experiment in teaching cannot be brought into full play. This paper introduces a new type of photoelastic equipment which can be used in classroom teaching to demonstrate its working principle. It is composed of only LED light source integrated with circular polarizer, loading frame and pixel polarizing camera. It has simple structure and can be folded. It weighs about 5kg together with suitcase. The equipment can be easily carried to the classroom to accomplish photoelastic teaching experiments in ten minutes. The experimental results can be displayed on the large screen of the classroom, and the underlying theories can be explained to students with a well prepared teaching plan. Through the above teaching reform, we give full play to the advantages of photoelastic experiment in teaching without increasing class hours.

DESIGN OF TEACHING AIDS AND EXPERIMENTAL CLASSROOM BASED ON BIONIC HELIX1)
YUAN Yanan, ZHANG Zuoqi
2022, 44(2):  423-426.  DOI: 10.6052/1000-0879-21-245
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For the design of composite microstructure, the authors noticed that nature has given a very good example in the preparation of high-performance composites using two-phase or multi-phase ordinary materials. The course of mechanical design in composite materials is offered in order to enhance students understanding of bionic design. Therefore, this paper proposes a design platform of spiral micro structure for automatic prepreg placement, which enables students to achieve accurate control of prepreg placement angle in class, and provides feasible experimental conditions for optimizing the mechanical properties of laminates by imitating the spiral micro structure in biological tissues.

APPLICATION OF PIV EXPERIMENT IN FLUID MECHANICS TEACHING OF CENTRIFUGAL PUMP AND DEMONSTRATION OF EXAMPLES1)
LI Hongbin, JIANG Shuang, NI Fusheng, GU Lei
2022, 44(2):  427-433.  DOI: 10.6052/1000-0879-21-366
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Centrifugal pump is one typical application of fluid mechanics theory. Many textbooks for fluid mechanics include centrifugal pump as an important chapter. When conveying a single medium, the flow in the pump is the compound movement of circular motion and relative motion. When conveying the solid-liquid two-phase, there is also a velocity difference between the two phases, hence the internal flow is very complicated. If these complex fluid movements can be visualized to demonstrate flow pattern, it will be a very good supplement to theoretical teaching, which will strengthen students understanding and consolidate classroom knowledge. For better visualize the internal flow, the pump and pipelines are made of transparent material, hence the impeller and volute area can be viewed without obstruction. Using particle image velocimetry (PIV) technology to capture the internal flow, a single medium or two-phase medium flow can be observed in the impeller observation experiment from a macroscopic perspective, velocity field of single medium and the relative velocity of two-phase medium can be measured locally.

TEACHING RESEARCH AND STUDY FOR SPACECRAFT RELATIVE MOTION DYNAMICS
WANG Weilin, SONG Xumin, CHAI Hua, LIU Haitao, YUE Zhiyu
2022, 44(2):  434-442.  DOI: 10.6052/1000-0879-21-316
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Spacecraft relative motion dynamics is core content of space service principles and it focuses on two spacecraft relative motion laws based on target orbit coordinate. The relative motion laws in space are contradictory to the relative motion laws on the ground, which pose critical challenge to students' spatial imagination. The teaching team proposed a systematic teaching technique which includes illustration method, interaction method and experimental method. The implicit relative motion model is transferred into explicit expression through formula translation. The statical formula is demonstrated by case application and the abstract relative motion law is represented by vivid pictures and dynamic animation, etc. The practice indicates that the proposed method contributes to student learning enthusiasm and the teaching quality improvement significantly.

TEACHING PRACTICE OF THE CORE GENERAL EDUCATION COURSE OF "UBIQUITOUS MECHANICS"1)
CHU Haijian, CHEN Liqun, ZHANG Tianzhong
2022, 44(2):  443-446.  DOI: 10.6052/1000-0879-21-455
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General education of mechanics plays an important role in cultivating students' scientific and engineering literacy. Based on the multi-round teaching practice of the core general education course "ubiquitous mechanics", this paper explores the organization and implementation of the general education course of mechanics. It has established a general education course management mode of "discipline planning, multi-person teaching and designee management", a curriculum concept of "value guidance, knowledge teaching and ability cultivation" and a multi-teaching mode of "lecturing teaching, discussion teaching and research teaching". Students have been enthusiastic in choosing this course and the teaching team has put lots of energy into it, since its opening. The practice of this course has promoted the students' scientific and engineering literacy and expanded the influence of mechanics discipline among teachers and students.

The Field of Mechanics:Beijing Winter Olympics on Mechanics
IS THE TRAJECTORY DEFLECTION OF A ROTATING CURLING STONE AN UNKNOWN PUZZLE?
LIU Yanzhu
2022, 44(2):  447-449.  DOI: 10.6052/1000-0879-22-108
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Two mechanical explanations of the trajectory deflection of a rotating curling stone are given, based on the asymmetrical friction effect and the scratch-guiding effect.

HOW SKELETON ATHLETES ACHIEVE HIGH-SPEED SLIDING ON THE TILTED TRACK1)
MA Yun, ZHANG Xiao, SUN Qing, HUO Bo, HU Jing, WANG Ning
2022, 44(2):  450-453.  DOI: 10.6052/1000-0879-22-093
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It explains the mechanics why athletes for the skeleton sport in the Winter Olympic Games are able to achieve high-speed sliding on the tilted track, using the d'Alembert principle. Though careful design and implantation of experiments, it reveals that mainly two factors, i.e., appropriate sliding speed and low center of gravity, determine whether athletes can smoothly and safely pass the curved section of the track. In addition, more similar demonstration experiments can be used in the future to promote the teaching work for the theoretical mechanics.

PRINCIPLES OF MECHANICS IN SLAP SHOT IN ICE HOCKEY1)
ZHANG Kai, WANG Jiuling, LI Zhenzhen, HU Jing, WANG Ning, JIN Yanfei
2022, 44(2):  454-457.  DOI: 10.6052/1000-0879-22-091
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Ice hockey, as the only ball team sport in the Winter Olympics, exhibits the perfect combination of skill, strength and speed, and reflects the olympic spirit of "Faster, Higher, Stronger - Together". From the perspective of mechanics, we analyze and discuss the important factors in slap shot in ice hockey, and reveal the underlying mechanics principles. By designing model experiments for slap shot in ice hockey, it is found that the angular velocity and the length of the stick have important effects on the speed of the puck. From the law of conservation of angular momentum and conservation of energy, it is found that the speed of the puck is proportional to the angular velocity before the player hits the ball, and the speed of the puck increases after hitting as the distance from the puck to the axis of rotation increases.

HOW TO REDUCE AIR RESISTANCE IN SPEED SKATING1)
CHEN Xiaodong, HE Guosheng, YANG Xiufeng, HU Jing, WANG Ning
2022, 44(2):  458-465.  DOI: 10.6052/1000-0879-22-095
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This paper mainly introduces how speed skaters reduce air resistance in competition. Firstly, the air resistance of speed skater is introduced, including friction resistance and differential pressure resistance. Then, wind tunnel experiments and computer simulations are carried out to illustrate how speed skaters reduce drag through clothing, posture and alignment. The research on drag reduction in speed skating can provide technical support for winter olympic athletes and help to improve competition results.

PRINCIPLES OF MECHANICS IN JUMP ACTION OF FIGURE SKATING1)
LIU Guangyan, CHENG Xiuyan, LIU Haiyan, BAI Ruoyang, LI Hailong, WU Xia, WANG Ning
2022, 44(2):  466-469.  DOI: 10.6052/1000-0879-22-090
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This paper interpreted the underlying principles of mechanics for the jump actions in figure skating competition in the 2022 Beijing Olympic Winter Games, using the theorem of momentum, the theorem of moment of momentum and the law of conservation of moment of momentum etc.

WHAT POSE DOES THE ATHLETE USE TO FLY STEADILY AND FAR IN SKI JUMPING1)
JIANG Liang, CHEN Xue, GAO Xianzhi, Huang Yi, MA Yun, HUO Bo, WU Xia, WANG Ning
2022, 44(2):  470-473.  DOI: 10.6052/1000-0879-22-109
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It explains the technical characteristics and mechanical principles in the ski jumping sport during the four phases, i.e., approach, takeoff, flight and landing. Relevant experiments have been carried out to investigate the dynamics. It is found that the leg thrust against the ground during takeoff phase and maintaining specific body posture during flight phase are the key factors to ensure the athletes to achieve satisfactory scores during the game. This paper also discusses the possible applications of the relevant experiments for teaching theoretical mechanics.

MECHANICS FOR THE HIGH-DEGREE BODY INCLINATION OF SHORT-TRACK SPEEDING SKATERS WHILE PASSING THROUGH A CURVE1)
SU Yu, LIU Zhanwei, ZHOU Chunyan, YU Yang, DIAO Zhuo, BAI Wenshuo, WU Xia, WANG Ning
2022, 44(2):  474-478.  DOI: 10.6052/1000-0879-22-092
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Short-track speeding skaters need to maintain a high degree of body inclination while passing through the curve. Such phenomenon implies rich mechanics, and it serves as a valuable case for mechanics teaching. In this work we explain the underlying mechanics for the high-degree body inclination phenomenon through kinematical and dynamical investigation. We introduced the formulae for centripetal acceleration of a mass point in uniform circular motion through the first and second Newtown's laws combined with experimental demonstrations. Based on D'Alembert's principle, an inertial force was introduced to the noninertial system which is attached to the skater. We then carried out analysis on the necessity for body inclination during the curve motion. The mathematical formula for the body inclination angle was provided. The corresponding results were verified by a mini track experiment. We discussed the related parameters which depend on the linear velocity and radius of curvature that vary with the established tactics and the real match situation. Finally, the key factors were provided for the short-track speeding skaters while passing through the curves.

INCLINED BODY AND FLEXED BOARD—MECHANICS IN CARVING TURN OF ALPINE SKIING1)
WAN Chao, YI Kaijun, HU Jing, LUO Kai, WANG Ning
2022, 44(2):  479-483.  DOI: 10.6052/1000-0879-22-094
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Alpine skiing is a traditional Winter Olympics event. The carving turn technique in this competition contains a lot of mechanics phenomena, which is very suitable as a practical case to be used in mechanics course and in the introduction of popular science. In this paper, D'Alembert's principle and beam's bending theory are used to study the two key characteristics of alpine skiers' body inclination and snowboard deformation during carving slalom, respectively. The results indicate close relationship between the carving outcome and body inclination/snowboard deformation. The mechanics model in this paper is simple and easy to understand, and it contains clear knowledge points. It can be used as a teaching case for mechanics courses to cultivate students' practical ability for solving practical problems, and it can also be used as a popular science case to demonstrate the application of mechanics in people's daily life.

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