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2016 Vol. 48, No. 3

Display Method:
Kang Houjun, Guo Tieding, Zhao Yueyu
As a hot research topic, the nonlinear dynamics of cable-stayed bridge has been highlighted in the field of mechanics, structure and bridge. With the development of new materials, such as CFRP cable, and new construction technology, the main span of cable-stayed bridge has been enlarged, which make this kind of bridge more competitive in bridge engineering. However, the increase of its span and the application of new materials make the bridge structure become lighter and more flexible, so that the nonlinear vibration of the structure becomes more prominent than ever, which may endanger the safety of the bridge. Based on our research of the nonlinear dynamics of cable-stayed bridge in recent years, the research progress in recent 10 years on the nonlinear dynamics and modeling of large span cable-stayed bridge is reviewed in detail. The nonlinear dynamic models, theories and modeling and solving methods for cable, beam, cable-beam and cable-stayed bridges are discussed. The results show that the cable stayed bridge has rich nonlinear dynamic behavior due to the coupling problem of the multi flexible cable, the large span beam, and the complexity of the environmental load. At the same time, due to the bottleneck problem of high dimensional nonlinear system, the study on the nonlinear dynamic behavior of the whole cable-stayed bridge becomes very complicated. Finally, based on the future development trend of cable-stayed bridge, some new ideas on nonlinear dynamics of cable-stayed bridge is proposed and future research directions are discussed.
2016, 48(3): 519-535. doi: 10.6052/0459-1879-15-436
Zhang Hao, Zheng Xiaobo, Jiang Nan
Open-loop active control of a turbulent boundary layer has been achieved in skin-friction reduction and suppression of coherent structure bursting process by means of periodic oscillating of a piezoelectric oscillator embedded on the surface of a flat plate wall. Ten experimental cases were carried out under variable input voltage amplitudes and frequencies. At 2 mm downstream of the piezoelectric oscillator, the simultaneous time series of streamwise velocity component at di erent wall-normal positions in the turbulent boundary layer were finely measured by hot-wire anemometer and a mini single-sensor probe. The e ects of piezoelectric oscillation on the mean velocity profile, drag-reduction rate and conditional phase-average waveform of coherent structure burst were investigated at Reθ=2 183. An upward shift in the log-law of mean velocity profile is observed, which indicates the reduction of skin-friction. With the larger amplitude of vibration, the higher drag reduction rate is achieved. Furthermore, a maximum rate of 25% can be reached when the vibration frequency is very close to the burst frequency of maximum-energy scale, which indicates that the manipulation of energetic-scale coherent structure burst is the key of wall-bounded turbulence drag reduction. In addition, by comparing the conditional phase-average waveforms of manipulated and unmanipulated cases, the waveform for manipulated conditions has more decreased amplitude with its wave crest damping rapidly in the later stage of high-speed sweep event and the sweep process of high-speed fluids are shorten. The vibration of piezoelectric oscillator can suppress the coherent structure sweep process of high-speed fluids, weaken the shear process of the high-speed fluids with the surface of the wall, bate the amplitude of coherent structure burst in the near-wall region, and as a result, reduce the skin-friction drag.
2016, 48(3): 536-544. doi: 10.6052/0459-1879-15-020
Wang Yunpeng, Liu Yunfeng, Yuan Chaokai, Luo Changtong, Wang Chun, Hu Zongmin, Han Guilai, Zhao Wei, Jiang Zonglin
JF12 hypersonic shock tunnel has been designed and built in Institute of Mechanics, CAS. The performance tests demonstrated that this facility is capable of reproducing the pure airflow with Mach numbers from 5-9 at altitude of 25-50 km with at least 100 ms test duration. Therefore, a sti construction balance, that is the traditional internal strain-gaged balance (SGB), was considered to use in this long-test duration impulse facility due to its mature technology, simple structure and low cost. However, when the force test is carried out in shock tunnel, the inertial forces lead to low frequency vibrations of the model and its motion cannot be addressed through digital filtering since enough cycles cannot be found during a shock tunnel run. This implies restriction on the model's size and mass as its natural frequencies are inversely proportional the length scale of the model. Therefore, there are still many problems for the force measurement in a shock tunnel, especially for the large and heavy model. In order to overcome the technical di culties, JF12 series SGBs were designed and fabricated. The maximum loads are from 1 kN to 30 kN for the test models with di erent scale. The di erent structures were proposed and optimized for two types of balance, i.e., the sting and cassette balances. The finite element method was employed for the analyses of vibration characteristic of the model-balance-support system in order to ensure enough cycles, especially axial force signal during 100 ms test duration. In addition, the force tests were carried out using several large-scale test models. JF12 series pulse-type SGBs show good performances and the frequency of the model-balance-support system increases due to the sti construction of the SGB.
2016, 48(3): 545-556. doi: 10.6052/0459-1879-15-295
Ding Jue, Li Jiahua, Qiu Xiao, Weng Peifen
During the burst reinforcement period of fog, air pollution and low visibility are very serious, which is closely related to the turbulence characteristics of the atmospheric boundary layer, the dynamics and scattering properties of suspended particles. Based on the particle population balance equation and Mie theory, a program is self-developed. The computed particle size distribution function and particle scattering property are consistent with the experimental and theoretical data, which verify the correctness of models and numerical method. Numerical study on the mechanism of droplet spectrum broadening, visibility reducing during the fog burst-enhanced phase is conducted, and the e ects of turbulent transport and particle local aggregation on the coagulation of particles are discussed. Combining with particles scattering nature, the influence of particle turbulent dissipation rates on the radial relative velocity and the transmissivity of system in the fog development are analyzed numerically. Relation between the radial relative velocity, the transmissivity of system and the particle size are discussed. The computed results suggest that the radial relative velocity of particles increases slowly and then increases rapidly with the rise of turbulent dissipation rate. At 1 000 s, the turbulent dissipation rate is 1.0×10-2m2/s3, and the dimensionless radial relative velocity of particle is 0.096 9. For 0.6 μm wavelength of visible light, the transmissivity of fog is 0.47. Furthermore, aerosols are coagulated with fog droplets in the development region of fog to decrease atmosphere visibility, which radiation properties are di erent from pure droplets.
2016, 48(3): 557-565. doi: 10.6052/0459-1879-15-118
Yan Kai, Ning Zhi, Lü Ming, Sun Chunhua, Fu Juan, Li Yuanxu
Pressure swirling atomizers are wildly used in the fuel injection systems of aero-engines, marine engines, vehicle gasoline direct injection engines and gas turbines, et al. Considering about a pressure swirling atomizer liquid jet, the correlation of breakup droplet size and velocity distributions of an annular swirling viscous liquid sheet is studied. Joint probability density function of droplet size and velocity distribution of an annular swirling viscous liquid sheet is reduced based on maximum entropy method. The correlation between droplet size distribution and droplet velocity distribution are then discussed. Results show that with the right form of joint probability density function, the conservation laws of mass, momentum and energy must be included together as the constraint conditions. Droplet size and velocity distributions are closely related. And the liquid sheet swirling strength does not affect the structure of joint probability density function a lot, but the liquid swirling strength affect the distribution region to some extent.
2016, 48(3): 566-575. doi: 10.6052/0459-1879-15-084
Li Guangli, Cui Kai, Xiao Yao, Xu Yingzhou
The high pressure capturing wing configuration is a novel conceptual form which improves lift and lift-todrag ratios by the coupling relationship between the body and HCW. Based on the deign principle, the HCW position is determined by body compression shock and its own compression shock, while it is hard to obtain directly using theory. In order to solve this problem, this paper developed a kind of e ective method to design the position of capturing wing. The method is based on the analysis of parameters which determine the position of capturing wing, combined with the uniform experimental design method, the computational fluid dynamics analysis and surrogate model building to obtain the relationship between the position of capturing wing and the design parameters. The results from the validation case of cone body combination with capturing wing showed that this method can determine the optimal position of capturing wing in a large design space. In addition, cone compression angle, free field Mach number and blunted radius of capturing wing are the key parameters a ecting the position of capturing wing for the cone combination capturing wing. The results from the analysis of surrogate model showed that the position of capturing wing is in monotonic proportional relationship among the above three parameters.
2016, 48(3): 576-584. doi: 10.6052/0459-1879-15-391
Wang Shuai, Yu Wenhao, Chen Juhui, Zhang Tianyu, Sun Liyan, Lu Huilin
Bubbling fluidized beds have been widely applied to various industrial processes owing to superior inter-phase contact and high heat transfer characteristics. Fundamental knowledge of the hydrodynamic characteristics is essential for the design of such reactors. In bubbling fluidized bed systems,the non-uniform flow structure in the form of bubbleemulsion phases makes the accuracy of numerical model limited. Bubbles are the typical meso-scale structures in bubbling fluidized beds. To describe the e ects of such meso-scale structures, a bubble structure-dependent (BSD) drag model is developed with one extremum condition of energy dissipation consumed by the drag force, which is incorporated into the two fluid model. The simulations of gas-solid flow behavior in bubbling fluidized beds with with Geldart A and B particles are performed and some parameters including bubble velocity and bubble diameter are analyzed. The results indicate that the present model with consideration of bubble e ects obtains a zonal distribution of the drag coe cient with solid concentration, which establishes a relationship between the drag coe cient and the local structural parameters. Comparisons with experimental data, the BSD drag model can obtain a better prediction than the conventional drag model. Meanwhile, the simulation reveals that the BSD drag model has a more significant impact on the predition of bubbling fluidization with Geldart A particles.
2016, 48(3): 585-592. doi: 10.6052/0459-1879-15-089
Zhang Xinshu, Hu Xiaofeng, You Yunxiang, Fu Huiping, Duan Jinlong
Based on the impressible Navier-Stokes equation, the characteristics of the vortex induced motion (VIM) for a multi-column floating platform are investigated by employing an improved delayed detached eddy simulation method (IDDES). Mesh parameters and boundary condition developed in the numerical method are extensively studied. The transverse and yaw motion responses of a Tension Leg Platform (TLP) with four columns are computed in three current headings including 0°, 22.5°and 45°, for di erent reduced velocities. In addition, the behavior of vortex induced motion response, the frequency ratio and the displacement power spectral density are computed and systematically analyzed. The numerical results show that the present predictions of VIM agree well with those from experiments. The lock-in frequency occurs in the transverse reduced velocities ranging from 7.0 to 14.0 and the transverse motion amplitudes vary between 0.2D and 0.4D (where D denotes the width of the column). The yaw motion amplitudes increase with the reduced velocities, which may be related with galloping. It is also found that the transverse motion frequency is the same as the yaw response frequency as the excited yaw moment is governed by the lift force acting on the vertical columns. The frequencies of motion in 22.5° and 45° incidences are higher than that in 0° . However, the yaw motion energy in 22.5° and 45° are around 10% of that in 0 incidences. Further, the three-dimensional fluid pattern is analyzed in the process of VIM for a multi-column floating platform.
2016, 48(3): 593-598. doi: 10.6052/0459-1879-15-446
Ma Tianbao, Ren Huilan, Li Jian, Ning Jianguo
Numerical simulations of explosion and impact problems have important engineering application value in the fields of national defense and civil security. Numerical simulations for these problems have a lot of di culties because the explosion and impact problems are strongly nonlinear transient dynamic problems in which multi-material mechanical behaviors are involved under the condition of high strain rate, high temperature and high pressure. Therefore, in this paper , the pseudo arc-length method for 3D nonlinear hyperbolic conservation system is proposed and the process of the algorithm realization is analyzed. The numerical results show that the algorithm improves the resolution of the shock wave strong discontinuity e ectively. The additive Runge-Kutta method for gaseous detonation numerical simulation is developed. In this method, the nonlinear convection part is solved implicitly while the chemical reaction source part is handled explicitly. The results show that the additive Runge-Kutta method can well capture and accurately describe the complex structure and typical characteristics. The parallel Eulerian numerical method of 3D multi-material hydrodynamics is investigated for the requirement of large-scale computation in engineering practical physical problems. The 3D explosion and impact problem parallel computation hydrocode is developed and the test method for this parallel hydrocode is proposed. According to the above works, some problems of large-scale and high-precision calculations for explosion and impact problems are solved. Finally, the experimental and numerical investigations on heavy-caliber shaped-charge penetration in thick concrete target are carried out, and the e ectiveness of the proposed numerical method is demonstrated by typical explosion and impact engineering problems.
2016, 48(3): 599-608. doi: 10.6052/0459-1879-15-382
Xia Wei, Feng Haocheng
Functionally graded materials (FGMs) with continuously varied composition e ectively reduce the mismatch at bonding surface between di erent constituents. As thermal protection structures, functionally graded panels (FGPs) eliminate the internal thermal stress concentration which arises from aerodynamic heating. The aeroelastic flutter boundary of an FGP is analyzed considering the structural geometric nonlinearity due to thermal post-buckling deflection. The e ective FGM properties are calculated using the rule of mixture homogenization with the power law distribution assumption. The first-order shear deformable plate theory, von Karman strain-displacement relations and the first-order piston theory are adopted to formulate the nonlinear aeroelastic finite element equations of FGPs in supersonic flow according to the principle of virtual work. The numerical simulation results of thermal post-buckling response are obtained using the Newton-Raphson iterative method, and the mechanism of post-buckling deflection a ected by the airflow is discussed. The panel flutter boundary is determined by analyzing the stability of post-buckling equilibriums. It is concluded that the symmetry of a ceramic-metal FGP is destroyed by through-the-thickness material distribution, and the panel tends to buckle to the metal side under in-plane thermal stresses. The position of maximum post-buckling deflection moves to the down-stream in the supersonic airflow, and the post-buckling deflection decreases with the increase of flow dynamic pressure. The geometric nonlinearity increases the flutter critical dynamic pressure of post-buckled FGPs when the large post-buckling deflection is occurred at relative high temperature and low non-dimensional dynamic pressure flow. However, the geometric nonlinearity is not so important at high non-dimensional dynamic pressure flow because the post-buckling deflection is restrained to a small one by the supersonic airflow.
2016, 48(3): 609-614. doi: 10.6052/0459-1879-15-361
Xue Binghan, Lin Gao, Hu Zhiqiang, Pang Lin
Frictional contact analysis is one of the most challenging problems in computational mechanics. The functional system of the contact problem is not only nonlinear, but also non-smooth, so in general the convergence and accuracy of contact algorithms are di cult to be guaranteed. For 2D elastic frictional contact problem, the scaled boundary isogeometric analysis combined with B di erential equation method (SBIGA-BDE method) is developed. Based on the scaled boundary isogeometric transformation, the contact equilibrium equation is derived by using virtual principle. The contact conditions are formulated as B di erential equation and satisfied rigorously. The convergence of the algorithm to solve the B di erential equation is guaranteed by the theory of mathematical programming. In the proposed method, only the outer boundary including the contact boundary need to be discretized isogeometrically, which reduces the spatial dimension by one and the boundary are represented accurately. The real contact length can be detected by the knot insertion algorithm. In addition, as the interpolatory functions used in geometry modeling and numerical analyzing are the same, the time costs in mesh generation is saved. The numerical examples, including Hertz contact problem and cantilever beams frictional contact problem, are presented and compared with analytic solution and ANSYS results. It validates the e ectiveness and accuracy of the proposed method in solving 2D elastic frictional contact problem.
2016, 48(3): 615-623. doi: 10.6052/0459-1879-15-329
Zhang Caigui, Cao Fu, Li Lian, Zhou Yan, Huang Runqiu, Wang Qizhi
Inevitable man-made or natural disasters, such as explosion, impact, earthquake, etc, often cause the failure of a large number of civil engineering facilities, and hence rock behavior under dynamic loading has become the focus of special attention. Dynamic fracture toughness of rock is the material parameter for characterizing its resistance to dynamic crack initiation, propagation and arrest. Rock dynamic fracture toughness is then classified into three kinds: dynamic initiation, dynamic propagation and dynamic arrest. Although there were some achievements for studying rock dynamic initiation and propagation, the study on rock dynamic arrest, being a puzzling problem, has been so far almost ignored. In the present research, the single cleavage drilled compression (SCDC) specimen of rock was impacted by split Hopkinson pressure bar in the model-I dynamic fracture test, where a crack propagation gauge (CPG) was glued on the SCDC specimen to monitor the whole fracture process, including dynamic initiation, propagation, and arrest. An experimental-numerical-analytical approach was adopted to determine the dynamic initiation, propagation, and arrest toughness of rock material. The CPG signal indicated that after the arrest of the crack in the SCDC, the stopped crack will be reinitiated, and propagated out of the CPG monitoring range. This dynamic arrest process is analyzed from an energy perspective, and some required attentions in determining the dynamic arrest toughness are pointed out. The results show that the rock dynamic initiation toughness and propagation toughness increase with the increasing dynamic loading rate and crack propagation velocity, respectively, and the dynamic initiation toughness is larger than the arrest toughness.
2016, 48(3): 624-635. doi: 10.6052/0459-1879-15-349
Wang Guosheng, Lu Dechun, Du Xiuli, Li Meng
The classical strength criteria including Mohr-Coulomb strength theory, Drucker-Prager strength theory, Matsuoka-Nakai strength theory, Lade-Duncan strength criterion and Hoek-Brown strength criterion are developed into dynamic multiaxial strength criteria by deriving the strain-rate-dependent strength parameters of each static strength criterion. The strain-rate-dependent strength parameters are obtained by combing the dynamic uniaxial S criterion and the basic strength indexes of concrete material. Then the change rules and value range of strength parameters of five strength criteria are analyzed by these statistical material parameters of S criterion. On the basis of the uniaxial compressive and tensile strengths of concrete, the strength curves of each dynamic multiaxial strength criterion at plane stress condition, meridian plane, deviatoric plane and the strength surfaces in the principle stress space are given. Then the di erences of the five dynamic multiaxial strength criteria are analyzed. The five dynamic strength criteria are evaluated by the dynamic biaxial and true triaxial strength tests. The dynamic Drucker-Prager criterion shows big di erences with the dynamic test results of concrete. All of the dynamic strength criteria are able to reasonably describe the strength rules under biaxial compression-tension. It also shows big di erence among these dynamic strength criteria, while the dynamic Matsuoka-Nakai criteria describes the strength rules better than others. Under dynamic true triaxial proportional loading, Mohr-Coulomb and Hoek-Brown strength criteria cannot consider the influences of intermediate principal stress, while the Matsuoka-Nakai and Lade-Duncan strength criteria are capable of well describing the test results of concrete.
2016, 48(3): 636-653. doi: 10.6052/0459-1879-15-433
Xu Zejian, Ding Xiaoyan, Zhang Weiqi, Huang Fenglei
Impact shearing loading technique at high strain rates is an important foundation for studying of dynamic behaviors and micromechanism of materials. Using the split Hopkinson pressure bar (SHPB) technique, material behaviours can usually be investigated under strain rates up to 104s-1. To obtain strain rates that exceed 104s-1 under dynamic shearing, however, pressure-shear plate impact technique or direct impact method with an air-gun launched projectile has to be used. In this paper, a new double-shear specimen that can be used under the SHPB technique is proposed. With this method, dynamic shear properties of materials can be tested precisely under strain rates ranging from 103 to 105s-1. Complex interfaces or connectors are not needed between the specimen and the bars. With the aid of a simple fixture, the specimen can contact with the bars directly, with its transverse movements limited. In this work, shear stress-shear strain curves of pure copper were acquired at strain rates between 1 400 and 75 000 s-1. The dynamic loading processes were modeled by ABAQUS/Explicit to check the validity of this testing method. The results show that the shear component dominates the stress and strain fields, which are distributed uniformly in the central part of the shear zone. The measured shear stress-shear strain curves agree very well with the simulation results. It shows that the new double-shear specimen provides a convenient and e ective way to test dynamic shear properties of materials under high strain rates.
2016, 48(3): 654-659. doi: 10.6052/0459-1879-15-445
Meng Lingkai, Zhou Changdong, Guo Kunpeng, Zhang Xiaoyang
Until now, a number of classical hyperelastic-finite plasticity constitutive models have been proposed. However, most of them are based on the classical Armstrong-Frederick kinematic hardening rule in consideration of the complexity brought by the introduction of the intermediate configuration in the hyperelasticity theory. Hence, based on the existed constitutive theories, the methodology of Lion decomposition theory was extended utilizing the notion of the multi-mechanism process and clearly put forward the conception of the multi-intermediate configuration. Furthermore, the classical concept of the objectivity in the continuum mechanics for better application to the hyperelasticity theory was generalized and then a new hyperelastic-finite plastic constitutive model was proposed. The new constitutive model not only meets the thermal dynamic laws but also can incorporate several classical kinematic hardening rules which were usually adopted in cyclic plasticity of infinitesimal deformation theory (e.g. the A-F model, Chaboche model, O-W model and the K-O model, etc.). Therefore, this model corresponding to finite deformation problems contains two typical characteristics adopted by infinitesimal deformation theory: the additive decomposition property and step mutation feature of the backstress on the critical surface. Thus, the present model can be treated as parallel to the corresponding form in the small deformation case. Finally, the situation accounting for Karim-Ohno kinematic hardening rule is under specific consideration and compared with the hypoelasticity constitutive model.
2016, 48(3): 660-674. doi: 10.6052/0459-1879-15-333
Xu Zhaodong, Xu Chao, Xu Yeshou
It is of great importance to reduce micro-vibration e ect on precision instrument, and employing viscoelastic damper to reduce micro-vibration is an innovative and challenging issue. In this paper, the molecular chain network model is employed to analyze the viscoelastic material microstructures, and the e ect of the network chains and free chains on the viscoelastic properties of viscoelastic material is comprehensively considered, and then a mechanical model of VE damper under micro-vibration is proposed based on molecular chain structures. The standard linear solid model and Maxwell model are adopted to describe the mechanical behaviors of the single network chain and single free chain, respectively. Moreover, eight-chain network model and three-chain network model are then employed. Additionally, temperature-frequency equivalent theory is adopted to reflect the temperature e ect. The proposed model is able to describe the mechanical properties of viscoelastic damper at di erent frequencies and temperatures, and this model can reflect the material microstructure e ect on its viscoelastic properties. To verify the proposed model and reveal the mechanical behavior of viscoelastic damper under micro-vibration excitations, tests on viscoelastic damper are carried out. The results show that viscoelastic damper has good energy dissipation capacity; the dynamic properties are significantly influenced by frequency and temperature, and the proposed model can accurately describe the dynamic properties of viscoelastic damper at di erent temperatures and frequencies under micro-vibration excitations.
2016, 48(3): 675-683. doi: 10.6052/0459-1879-15-394
Chen Xiangwei, Cao Qiupeng, Mei Fengxiang
It is an important and di cult problem to study the stability of the non-steady and nonholonomic mechanical systems, and it is di cult to construct the Lyapunov function directly from the di erential equation. This paper gives an indirect method. The ten kinds of generalized gradient systems are proposed and the di erential equations of the ten kinds of generalized gradient systems are given respectively. Furthermore, the generalized gradient representations of a nonholonomic system of Chetaev's type are studied. The condition under which a nonholonomic system can be considered as a generalized gradient system is obtained, so the nonholonomic system of Chetaev's type is transformed into each generalized gradient systems. The characteristic of the generalized gradient systems can be used to study the stability of the nonholonomic system. This method appears to be more e ective when it is di cult to construct the Lyapunov function directly. Some examples are given to illustrate the application of the result.
2016, 48(3): 684-691. doi: 10.6052/0459-1879-15-268
Zhang Xiaoshun, Zhang Dingguo, Hong Jiazheny
The dynamics of a flexible beam which is rotating in a plane is further studied in this paper. The dynamic model of the rigid-flexible coupling system under large deformation is established and here by the dynamic simulation is also carried out. In this dynamic model not only the transversal bending deformation and the longitudinal deformation (including the axial stretching deformation and the longitudinal shortening term caused by the transversal bending deformation) of the flexible beam are considered, but also the curvature e ect induced by the longitudinal deformation is included. In the previous studies the bending deformation energy of the beam is usually expressed in terms of the bending deformation directly without considering the longitudinal deformation e ect. To take into account the influence due to the longitudinal deformation on the bending deformation energy of the beam the precise curvature formula in the form of parametric equation expressed in the floating frame of reference is used to calculate the bending deformation energy. And consequently the rigid-flexible coupling dynamic model of the system with the said the longitudinal deformation induced curvature e ect (LDICE) model is obtained. To validate the algorithm presented in this paper, several dynamic simulation examples are given. The results show that the dynamic model presented in this paper can not only be used in the analysis of the small deformation dynamics, but also in the large deformation dynamics, and indicate that the dynamic model with the curvature e ect obtained in this paper is more suitable to solve for the large deformation dynamic problems than the high-order coupled (HOC) model presented in the existing literature. The results obtained using the proposed model are compared with the results obtained using the absolute nodal coordinate formulation (ANCF) which is suitable for large deformation problems, and consequently validate the dynamic model proposed in this paper.
2016, 48(3): 692-701. doi: 10.6052/0459-1879-15-385
Li Jie, Xu Jun
A quantitative approach is proposed for stochastic dynamic stability analysis of structures. The classical concept of stochastic dynamic stability is firstly revisited. It is pointed that the dynamic stability of structures not only depends on structural parameters, but also relates to the applied external excitations. A new criterion for identifying dynamic stability of structures is introduced and the definition of stochastic dynamic stability of structures is therefore formulated based on the criterion. According to the principle of preservation of probability, the generalized density evolution equation for probability-preserved system is introduced firstly and then the equation for probability-dissipated system is derived. On the basis, the probability of stability/instability can be obtained via solving the equation for probability-dissipated system by introducing the physical mechanism of dynamic instability of structures as the triggering force of probability dissipation. Numerical algorithms for solving the generalized density evolution equation for probability-dissipated system are provided. According to the obtained probability, it is readily applicable to quantitatively evaluate stochastic dynamic stability of structures in the sense of stability in probability 1 or a given probability. Stochastic dynamic stability analyses of typical structural dynamic systems are carried out by the proposed approach, where the results by Monte Carlo simulations are employed for comparisons. The numerical results verify the e ectiveness of the proposed approach.
2016, 48(3): 702-713. doi: 10.6052/0459-1879-15-304
Luo Qingqun, Yang Jieming
The phenomenon that the gas dissolved in water can be adsorbed and accumulated on the hydrophobic surface is discovered by condensed matter physics. Thus, when the distance between the hydrophobic objects is small enough, the gases adsorbed on the hydrophobic surfaces will connect each other and form nanobubble bridges. The nanobubble bridges cause hydrophobic attraction. However, the formation/disappearance process and the morphology of nanobubble bridges have not been provided in academe of mechanics. In this article, the interactions of a pair of graphene in liquid water with and without gas phase are studied using molecular simulation. The changes of structural phase diagram and potential of mean force of the system, the density distribution of gas and water were analyzed. The results show that the hydrophobic attraction of two pieces of graphenes is really caused by the nanobubble bridge. When the distance between the pair of graphenes is less then approximate 0.5 nm, hydrophobic attraction is led by vacuum nanobubble bridge no matter with or without gas phase in water. When their distance is greater than approximate 0.5 nm, the hydrophobic attraction without gas in water is led by the nanobubble bridge of vapor, and the hydrophobic attraction with gas in water is led by the nanobubble bridge of the gas.
2016, 48(3): 714-719. doi: 10.6052/0459-1879-15-426
Liu Lele, Zhang Xuhui, Liu Changling, Ye Yuguang
Any perturbation to the thermodynamic equilibrium by exploitation may push out hydrate-bearing sediments (HBS) out of the stability zone, thus inducing hydrate dissociation, loss of cementation, which, in turn, can cause submarine landslides and loss of platform foundations during gas extraction operations. Therefore, a thorough understanding of mechanical properties of HBS is of great importance for stability analyses under di erent environmental conditions. A series of drained triaxial shear tests were carried out on a self-developed apparatus with the samples prepared by gas diffusion method, in which the time domain reflectometry technique was used in measurement of hydrate saturations in real time. A meso-mechanical and mixed model for the elastic modulus of HBS was proposed based on the classical series and parallel models, including the parameter of statistical force transfer paths between particles in HBS. A constitutive model of HBS was improved by coupling the statistical damage theory and the Mohr-Coulomb failure criterion. It is shown that the stress-strain curve changes from strain-hardening into strain-softening with the increase of hydrate saturation and the decrease of e ective confining pressure; the secant modulus and the peak strength of HBS increase when the hydrate saturation and the e ective confining pressure increase; the cohesion of HBS increases obviously with the increase of hydrate content, and the internal friction angle changed little with the increase of hydrate amount; the proposed mixed model for elastic modulus and the constitutive model of HBS are both reasonable and feasible.
2016, 48(3): 720-729. doi: 10.6052/0459-1879-15-400
Kang Yili, She Jiping, Lin Chong, You Lijun
Brittleness is one key parameter during drilling and fracturing in shale formations, however, influencing mechanism of soaking by drilling & completion fluid on shale brittleness has not been focused. In this paper, triaxial mechanical tests of shales soaked by fluid were performed; and optimized brittleness evaluation model was employed to evaluate brittleness of shale. Results indicate that the brittleness of Yanchang shale is larger than that of Longmaxi shale, and two kinds of drilling & completion fluid can decrease shale brittleness, in which oil-based drilling & completion fluid reduces shale brittleness more evidently. The reduction of brittleness Longmaxi shale is greater than that of Yanchang shale. It is demonstrated that five factors can weaken the brittleness of shale as follows: (1) Higher strength of bedding plane can lead to lower brittleness; (2) Spontaneous imbibition causes high pore pressure and great stress intensity factor of fractures; (3) Alkali erosion can cause mineral grains crack and dissolution pore; (4) Hydrate swelling of shale results in swelling stress; (5) Lubrication of drilling & completion filtrate results in lower friction coe cient. Yanchang shale has lower bedding plane strength. So the brittleness of Yanchang shale is larger than that of Longmaxi shale; Oil-based drilling & completion fluid has greater imbibition amount, higher pH and stronger lubrication; therefore, brittleness of shales soaked by oil-based drilling & completion fluid decreases more evidently. Besides, Longmaxi shale has smaller contact angle, greater imbibition amount, and stronger alkali erosion, the reduction of brittleness is greater than that of Yanchang shale under the same condition. This work can provide some evidence for drilling and hydraulic fracturing in shale formation.
2016, 48(3): 730-738. doi: 10.6052/0459-1879-15-286
Zhan Shige, Zhang Panfeng, Sun Zhongkui, Wang Jianshan
The paper introduced the applications for NSFC programs on mechanics in 2016. The statistics of application projects for General Programs, Young Scientists Fund, Fund for Less Developed Regions, Key Programs, Excellent Young Scientists Fund, National Science Fund for Distinguished Young Scholars, and Joint Research Fund for Overseas Chinese Scholars and Scholars in Hong Kong and Macao are presented and compared with applications in 2015.
2016, 48(3): 739-740. doi: 10.6052/0459-1879-16-112