力学学报

AN EXPERIMENTAL INVESTIGATION OF THE COHERENT STRUCTURES OF THE FLOW BEHIND A BACKWARD FACING STEP

The coherent structures of the flows in the recirculate region, reattaching region and tht boundary layer redeveloping region behind a backward facing step were investigated experimentally. The step height is 2cm, and the Reynolds number based on step height is 1900 to 3400. The flow structures were visualized by hydrogen bubble technique. The large vortical structures in the outer region and the streaky structures in the wall region were investigated.

1993, 25(2): 129-133. doi: 10.6052/0459-1879-1993-2-1995-623

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CALCULATION OF THE SURFACE TENSION OF LIQUID METAL LI AND ITS TEMPERATURE DERIVATIVE

The surface tension of liquid metal Li and its temperature derivative are calculated using the density-functional theory within the gradient approximation[1]. A new method for the determination of four parameters is proposed. In the method the pseudopotential parameters rc and rco, the hard-sphere diameter σ, and the hard-sphere packing fraction η are evaluated simultaneously using the experimental data of the isothermal compressibility and density of the liquid at meltingpoint and zero pressure, the defini...

1993, 25(2): 134-139. doi: 10.6052/0459-1879-1993-2-1995-624

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SHORT FATIGUE CRACK GROWTH UNDER MEAN STRESS, UNIAXIAL LOADING

An investigation of the effects of mean and alternating stresses on short fatigue crack growth under uniaxial loading is reported. Tests under stresscontrolled conditions with different combinations of mean and alternating stresses were conducted at room temperature using a 1.99% NiCrMo steel. The fatigue process was divided into three phases: Plasticity Localization, Microstructural Short Crack (MSC) growth, and Physically Short Crack (PSC) growth. The effect of mean stress on the above three phases is stu...

1993, 25(2): 140-151. doi: 10.6052/0459-1879-1993-2-1995-625

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A NEW KIND OF OBJECTIVE STRESS RATE

The paper deals with the definition and representation of objective stress rates. At first the requirement in symmetry to the form of the objective stress rates is given up and a family of generalized stress rates is defined. From the Cauchy fundamental principle of stress, two new unsyrnmetric objective stress rates are derived through objective rates of stress vectors, these rates are in form independent of deformation and deformation rate. The new objective stress rates represent the true change of the s...

1993, 25(2): 152-158. doi: 10.6052/0459-1879-1993-2-1995-626

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DYNAMIC CRACK-TIP FIELD IN AN ELASTIC-VISCOPLASTIC MATERIAL

The asymptotic analysis of he stress and strain field at a propagating crack tip is given by means of an elastic-viscoplastic model. It is assumed that the viscosity effect can be neglected for elasticity strain, and only for plasticity strain, the viscosity is significant. By use of this model, the constitutive relations of a rate sensitive material and the dynamic equations of the stress and strain field a crack-tip are derived. The stress and strain singularity exponent of crack-tip field is given by mea...

1993, 25(2): 159-168. doi: 10.6052/0459-1879-1993-2-1995-627

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INVESTIGATION ON DYNAMICAL BIFURCATIONS OF A NONLINEAR PARAMETRIC EXCITATION SYSTEM

In order to study dynamical bucking and bifurcations of an elastic beam, we establish nonlinear Mathieu equation of this beam and discuss thoroughly the characteristic of 3/2 subharmonic bifurcation in parametrically excited system. Our studies show that researchers only got bifurcating characteristic of a degenerate case when they investigated these problems. We analyse influence of truncation in bifurcating equation on bifurcation set, and obtain some new results. Finally, we give result of experiment on ...

1993, 25(2): 169-175. doi: 10.6052/0459-1879-1993-2-1995-628

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RESPONSE OF A CATAMARAN WITH ASYMMETRIC CROSS SECTION HALF-BODY IN SEA WAVE

This paper studies the dynamic response (including the ship motion andvibration of the half-body) of a catamaran with asymmetric cross section half-body in head wave condition. Since the principal inertia axis of the cross section does not coincide with the direction of the ship motion, the two half-bodies are restrained by each other through two fixed pcints, studying the response by BISHOP'S THEORY of hydroelasticity of ships [1] is dif-ficul. In this paper, the hydroelasticity motion differential equatio...

1993, 25(2): 176-184. doi: 10.6052/0459-1879-1993-2-1995-629

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A 3-D LASER DOPPLER VELOCIMETRY SYSTEM WITH THE SINGLE COLOR LASER

A 3-D LDV system for measuring three velocity components by using a He-Ne laser has been presented. Four incident beams were arranged to be an isosceles triangle with a right angle and focused by a single front lens. Such a beam arrangement has been examined as a most efficient form of the light energy utilization. It is also well suited to measure the velocity profiles close to a wall even in the situations in existing two different layers which the incident beams have to penetrate through, In order to sep...

1993, 25(2): 185-192. doi: 10.6052/0459-1879-1993-2-1995-630

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MULTIPLICATION OF MOIRE INTERFEROMETRY FRINGES

A ingenious combination of moire interferometry, carrier method and multiplication techniques is first proposed for multiplying moire interferometry fringes, including displacement fields and strain fields. The measuring sensitivity of moire interferometry method may be raised hy more than 4 times, by this multiplication method. Rigorous theoretical analysis is given. Experimental demonstration is conducted successfully in this paper.

1993, 25(2): 193-200. doi: 10.6052/0459-1879-1993-2-1995-631

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CHARACTERISTICS OF FLOW AROUND A Y-SHAPE PRISM AND ITS WIND LOADING

The characteristics of flow around Y-shape prisms under various wind directions and its wind loading were studied by means of wind tunnel test. The results show that as the wind is along one leg of the model, the vibrations which are mainly caused by turbulence of oncoming flow are weak, and as the wind is along the angular bisector of two legs, the vibrations induced by vortex are strong. Increasing turbulence intensity in oncoming flow will cause both fluctuating lift and drag forces to increase significa...

1993, 25(2): 201-206. doi: 10.6052/0459-1879-1993-2-1995-632

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NUMERICAL SIMULATION OF THE AFTERBODY FLOWFIELD OF A SPHERE AT LOW SUPERSONIC SPEED

Based on the flow pattern stniccure obtained from free flight experiments, a method of solving the low supersonic, high Revnolds numbers separated flow problem is proposed in this paper by which the inviscid region of the afterbody flowfield of a sphere has been simulated numerically. Flow characteristics, such as separated shock, wake shock, separation line and wake structure, obtained from computation are in good agreement with the experimental results. The detailed flow structure in the after body flowfi...

1993, 25(2): 207-212. doi: 10.6052/0459-1879-1993-2-1995-633

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DISTURBED CONSTITUTIVE EQUATION AND INSTABILITY OF EXTENSIONAL FLOW OF VISCOELASTIC FLUIDS

In the present paper a concept of "quasi-short memory" is introduced instead of "short memory" in order to develop a theory for disturbed constitutive equation for the basic flow of non-Newtonian fluids. The constitutive equation is derived for general basic flow and rheological models. The constitutive equation of second order for disturbed extensional flow is used to study the instability of stretching sheet of viscoelastic fluid. The instability criterion is given for general rheological model and the ap...

1993, 25(2): 213-217. doi: 10.6052/0459-1879-1993-2-1995-634

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THE EXPLORATION OF REDUCING 3-D WAVE REFLECTION IN HIGH SPEED 2-D FLEXIBLE WALL ADAPTIVE WIND TUNNEL

This paper describes the development of reducing shock wave reflectionduring 3-D model testing in 2-D transonic flexible wall adaptive wind tunnel of late years in Northwestern Polytechnical University. It includes the present status of research, testing method, results and analyses. According to the preliminary research, it is considered that transonic 2-D flexible wall adaptive wind tunnel has all its own potential (latent capacity) and the future prospect of application.

1993, 25(2): 218-225. doi: 10.6052/0459-1879-1993-2-1995-635

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THE RATE-TEMPERATURE EQUIVALENCE OF VISCOELAS-TIC BEHAVIOR FOR GLASSY POLYMER PMMA

The law of the strain-rate-temperature equivalence of PMMA up to yielding is discussed based on the viscoelastic/plastic constitutive model and experimental data of stress-strain curves at the range of T = 233-343K and ε=1.0×10-4-1.0×10-1s-1. The existence of the shifting factor ar and normalized stress-strain master curve shows the existence of strain-rate-tem. perature correspondence of PMMA before yielding.

1993, 25(2): 226-231. doi: 10.6052/0459-1879-1993-2-1995-636

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THE EFFECT OF UNDERLOADING ON Al-2024 FATIGUE CRACK GROWTH

Fatigue test with a large number of underloading were performed. Fatigue crack growth delay after the action of underloading was found. The delay was correlated to the development of shear lips on the crack surface. It was found that in stage one (during underloading) and in stage two (after underloading) the effect of shear lips on fatigue crack growth was different. The effect of the shear lips was much stronger in stage two than in stage one. A calculation model of fatigue crack growth including the effe...

1993, 25(2): 232-236. doi: 10.6052/0459-1879-1993-2-1995-637

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THE EQUIVALENCE BETWEEN BOUSSINESQ-GALERKIN SOLUTION AND HU HAICHANG SOLUTION IN ELASTICITY

The purpose of this paper is to discuss the restriction of the potential functions of Boussinesq-Galerkin. We have proved that Boussinesq-Galerkin solution and Hu Hai-chang solution are equivalent if the elastic region is convex in the z-direction.

1993, 25(2): 237-241. doi: 10.6052/0459-1879-1993-2-1995-638

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THE STABILITY OF A TIPPE TOP ON A SLIGHTLY ROUGH HORIZONTAL PLANE

The stability of a tippe top rotating on a slightly rough horizontal plane is considered in this paper. The stability criterion of the permanent rotation about the vertical polar axis not only with low viscous friction but also with low dry friction is derived by using Rumiantsev's theorem of partial stability. This criterion is reasonable because it is not expressed in terms of the precessional velocity, but the angular velocity of the permanent rotation.

1993, 25(2): 242-248. doi: 10.6052/0459-1879-1993-2-1995-639

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EXPERIMENTAL INVESTIGATION ON DYNAMIC BUCKLING AND PLASTIC COLLAPSING OF RECTANGULAR PLATES SUBJECTED TO SOLID-FLUID IN-PLANE IMPACT

Dynamic buckling and plastic collapsing of rectangular plates under solid-fluid in-plane impact are in vestigated experimentally. The plates exhibit three different critical conditions: bucklin'g, plastic incipience and plastic collapsing. The conditions are defined by three kinds of critical equivalent membrane strains. Based on the definitions, the critical values of each plate are determined. The modes of flexural motion and plastic collapsing of the plates are observed in the experiment. Moreover, the e...

1993, 25(2): 249-256. doi: 10.6052/0459-1879-1993-2-1995-640

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1993 Vol. 25, No. 2

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