Experimental investigation of creep-crack-tip deformation using moiré interferometry

High-temperature moiré interferometry was applied to obtain full-field creep-crack-tip displacements of a three-point bend Al 2024-T4 specimen uner constant temperature of 200°C up to 720 hr.C ^* was evaluated by the moiré data obtained at discrete time intervals. Test results indicate that under steady-state creep condition, the creepcrack-tipv-displacement rate agrees with the asymptotic solution based on theC ^* integral. However, no creeping behavior was observed for the crack-tipu-displacement field aftert=276 hr. This discrepancy may be due to the initial large creep-crack-tip blunting and cavitation damage which alter the creep-crack-tip singular field such that theC ^* integral is no longer applicable to characterize the steady-state creep-crack-tip field. Postmortem studies of the tested specimen also revealed transgranular fracture path with the ordered grain boundaries perpendicular to the crack line, which may relate to the existence of the substantial crack-tip blunting and noncreeping behavior of theu-displacement field, i.e., the size and shape of material grain boundaries play an important role in the crack-tip-creeping behavior of the material.Paper was preseted at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–13.     

Acoustic resonance associated with vibrations of an array of plates in a subsonic flow

The acoustic resonance effect occurring in the vibrations of an array of profiles (cascade) in a gas flow has been studied by a number of authors [1–8]. Relying on assumptions of a heuristic nature [1, 2, 7, 8] and using rather crude models [4, 6], they have derived criteria governing the acoustic resonance regimes and given the effect a certain physical interpretation. However, many problems of a physical bearing with regard to the quantitative and qualitative principles of the effect have been left unresolved. For a more complete and rigorous solution of the problem the author has previously [9, 10] analyzed the natural modes of a gas flowing past an array of plates It was determined that in the array domain the vibrational modes of the gas are localized in the vicinity, of the array and the eigenvalues are determined by the characteristic dimensions of the interstitial channel (as an open resonator). Also, the eigenfrequences were determined for the gas in the flow plane with the array absent [9]. Under spatial periodic conditions, such that the flow in the plane can be considered as a certain model of flow in an annular duct, these eigenfrequencies concurred with those obtained earlier in [1, 2, 4–6]. The results of [9, 10] are used in the present study as a basis for investigating certain laws and relations governing the unsteady aerodynamic characteristics of arrayed profiles in or close to regimes such that the gas can execute natural vibrations in the array domain and in an annular duct in the absence of the array.Novosibirsk. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 139–144, September–October, 1972.     

Propagation of a flush wave in a prismatic channel

By a water flush there is generally understood an unsteady-state flow of water, arising in millraces, with the breaching (rupture) of a dam. The special characteristics of a flush wave are also possessed by a flow in a lower millrace at some distance from the dam, with the overflow of water arising in the reservoir over the crest of the dam. Usually, the necessary information on the parameters of a flush wave with its motion in natural channels is obtained by numerical solution, in a digital computer, of the equations of not fully established one-dimensional flow [1–3]. These calculations are very labor-consuming and require rather detailed information on the channel. Therefore, it is of practical importance to clarify the overall laws governing the propagation of flush waves in schematized, in particular, in prismatic channels. In some cases, on the basis of such laws, it is possible to make a preliminary diagnosis of the expected scales of the phenomenon.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 39–44, January–February, 1975.     

Theory of a calorimeter at a permeable surface

The calorimetric method for determining the heat flux at a permeable (or sublimating) surface requires the solution of several specific heat-transfer problems, since the calorimeter, bringing about discontinuities in the boundary conditions at the wall (the cessation of blowing, a jump in the temperature of the wall and of its catalytic properties, etc.), introduces perturbations into the boundary layer and measures a heat flux differing from the flux in the absence of a calorimeter. Within the framework of the boundary layer, schematization of such problems is usually based on the isolation of an internal boundary layer (sublayer), which is the region of the effect of the new phenomena at the wall, and develops in the main boundary layer [1–5]. To take account of the effect of the inhomogeneity of the flow in the main boundary layer on heat transfer through the sublayer, here the method of mean-mass values is used, which, as has been demonstrated using various examples in [4] and in the present work, has a good degree of accuracy (even in the neighborhood of the breakaway point) and is suitable for the profiles of an inhomogeneous flow of rather general form. Based on this, for a laminar boundary layer finite formulas are obtained below for the heat flux to a calorimeter of relatively small size at a permeable wall, which can be used for the analysis of experiments.     

Interaction between the effects of local and overall imperfections on the buckling of elastic columns

A. van derNeut has studied the buckling of elastic columns made from thin-walled members, with particular reference to the effects of imperfections. In this paper, a graphical method is used to extend his work and to make an exhaustive study of the combined effects of both ‘local’ and ‘overall’ imperfections. The resulting picture is remarkably simple, and the effects of imperfections are well described by the celebrated Perry formula in conjunction with a single imperfection parameter compounding simply the local and overall imperfections. Experiments on small-scale rubber model columns substantiate the main results of the theoretical analysis.     

Drag of porous cylinders in a viscous fluid at low reynolds numbers

The problem of flow past a permeable cylinder at low Reynolds numbers is of interest for the solution of a number of problems in chemical technology in, for example, the design of porous electrodes and porous catalysts and in the calculation of nonstationary filtration of aerosols by fibrous filters. In the present paper, we solve the problem of transverse flow of a viscous fluid past a continuous cylinder in a porous shell and, in particular, in the case of a porous cylinder under conditions of constrained flow (system of cylinders) and an isolated cylinder at arbitrary permeability. The analogous problem of Stokes flow past permeable spheres has been solved in a number of papers [1–3].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 122–124, November–December, 1979.     

Gas flow in cylindrical capillaries under intermediate conditions

At present, there are sufficient solutions of the problem of free-molecular gas flow through a short cylindrical channel, for example, [1–3]. In intermediate flow conditions, for Knudsen number Kn 1, solutions have been obtained for the limiting cases: an infinitely long channel [4] and a channel of zero length (an aperture) [5]. However, no solution is known for short channels for Kn 1. The present work reports a calculation by the Monte Carlo method of the macroscopic characteristics of the gas flow through a short cylindrical channel (for various length—radius ratios), taking into account intermolecular collisions.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 187–190, January–February, 1977.     

Recoil in macromolecular solutions according to rigid dumbbell kinetic theory

A macromolecular solution is represented by the simple model of rigid dumbbells suspended in a Newtonian fluid with Brownian motion included. Hydrodynamic interaction is not taken into account. It is found that for this model there will be recoil after the cessation of steady shearing flow. The ultimate shear recovery S _∞ is developed as a power series in κ^?, the shear rate prior to the cessation of the steady shear flow: $$S_\infty = (\theta _0 /2\eta _0 ) \kappa ^\user1{ - } + O(\kappa ^\user1{ - } )^3$$ where η₀ and θ₀ values of the viscosity and primary normal stress functions respectively at zero-shear rate. The coefficient of the term in (κ^?)³ is calculated. In addition, the behavior of the normal stresses during the recoil process is found; during recoil τ₂₂?τ₃₃ has the opposite sign from τ₁₁?τ₂₂.     

Relaxation and none quilibrium radiation behind shock waves in air

This article discusses relaxation behind shock waves in air at velocities from 8 to 12 km/sec. Profiles are obtained for the parameters of the gas behind the front. The populations of the radiating states of the atoms and the molecules are calculated. In a number of spectral ranges the intensity of the radiation passes through a maximum exceeding the equilibrium level. A comparison is made with experimental data obtained in shock tubes. The radiant heat fluxes from the relaxation zone are calculated. The contribution of this radiation to the radiative heating of blunt bodies with flow around them at hypersonic velocities is evaluated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza., No. 4, pp. 161–174, July–August, 1970.The authors thank their co-workers in the High Temperature Institute of the Academy of Sciences of the USSR, above all, L. M. Biberman, V. S. Vorob'ev, A. N. Larar'kov, and G. É. Norman for their valuable evaluations and for their interest in the work.     

MHD flow in the vicinity of the stagnation point in a purely azimuthal magnetic field

Axisymmetric MHD flow in the vicinity of the stagnation point in the presence of a purely azimuthal nonhomogeneous magnetic field B {0, B, 0} is studied. This problem belongs to the class of MHD problems whose solutions are known as solutions of the layer type [1]. This class also includes, in particular, the classical exact solutions of the Navier-Stokes equations.The approximate solutions of the analogous MHD problems for the limiting cases of large and small values of the diffusion number ==/ have been considered in [2–5]. In this case it is possible to divide the flow into the so-called viscous and current layers, for each of which the approximate equations, simpler than the exact equations, are solved numerically or in quadratures. Using this technique it is possible to avoid the basic mathematical difficulty, which is that the sought solution of the boundary-value problem must be selected from a family of two-parameter solutions. The approximate method permits dividing the problem into two stages (corresponding to the two boundary layers) in each of which one unknown parameter is determined (in place of their simultaneous determination by direct integration of the basic equations).The drawback of the approximate methods [2–5] is their nonapplicability in the most interesting case, when the thicknesses of the current and viscous layers are of comparable magnitude, i. e., when the kinematic and magnetic viscosities ( and ) are quantities of the same order. We should also note the poor accuracy of the methods in the framework of the considered approximations for a comparatively large volume of the calculations required, which, in turn, prevents obtaining more exact solutions.The present paper presents a numerical integration of the equations describing MHD flow in the vicinity of the stagnation point over a wide range of S and numbers (Alfvén and diffusion numbers), without the assumption of their smallness, with preliminary determination of the unknowns at the zero of the derivatives of the sought functions with the aid of the method of asymptotic integration.A critical value of the Alfvén number is found, for which the retardation of the fluid by the magnetic field (for the first considered configuration of the magnetic field) at the wall is so intense that the friction vanishes everywhere on the surface of the solid body. It is also found that with further increase of the number S a region of reverse flow appears near the wall, which is separated from the remaining flow by a plane on which the z-component of the velocity is equal to zero.