Stress analysis for a rectangular thick plate of a composite material with a spatially locally curved structure under forced vibration

Natural and forced vibrations of a thick rectangular plate fabricated from a composite material with a spatially locally curved structure are investigated with the use of exact three-dimensional equations of motion of the theory of elastic anisotropic bodies. The investigations are carried out within the framework of the continuum approach developed by Akbarov and Guz. It is supposed that the plate is clamped at all its edges and is loaded on the upper face with uniformly distributed normal forces periodically changing with time. The influence of the parameters of local curving on the fundamental frequency of the plate and on the distribution of the normal stress acting in the thickness direction under forced vibration is studied. The corresponding boundary-value problems are solved numerically by employing the three-dimensional FEM modelling.Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 40, No. 6, pp. 779–790, November–December, 2004.     

Natural vibration of the beam-strip fabricated from a composite material with small-scale curvings in the structure

The frequency and forms of natural vibrations of a hinged beam-strip fabricated from a composite material with small-scale curved structures have been investigated using the plate theory in the framework of the Timoshenko hypothesis. The mechanical relationships of the composites are described by the Akbarov and Guz continuum theory. Using the corresponding variational principle, a method of solution of the considered problem is presented. Specific numerical investigations have been carried out for the case when the strip material consists of alternating equidistantly located and curved isotropic layers. The effect of existence of the curving in the beam-strip structure on its frequencies and forms of natural vibrations is studied. Cases of periodic and local curvings in the beam-strip material structure are considered separately.Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Yilziz Technical University, Civil Engineering Faculty, 80750 Yildiz-Istanbul, Turkey. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 4, pp. 502–512, July–August, 1996.     

The Discretized Generalized Korteweg-de Vries Equation with Fourth Order Nonlinearity

In this paper, we study a discretized version of the (generalized) Korteweg-de Vries equation, _t u + _x ³ u + u ⁴ _x u = 0. After a number of estimates, we utilize the Contraction Mapping Principle to prove the global well-posedness of this equation in a certain discrete Banach space. Our results are analogous to those of Kenig, Ponce, and Vega in the continuous setting. However, due to the nature of the Fourier multipliers, the proofs of several of these estimates in the discrete setting require new techniques. Our results yield a numerical procedure for computing the solution. We present a numerical algorithm which is based on successive iterations to obtain a fixed point guaranteed by the Contraction Mapping Principle. This fixed point is the desired solution to the discrete equation.     

Stress Intensity Factors near the Cracks on the Edges of Openings in Composite Plates

Based on the photoelasticity method, the behavior of stress intensity factors (SIFs) near cracks propagating from the edges of openings in plates made of elastic and linearly viscoelastic fibrous composite materials is studied. It is found that the relative value of the SIF, K ₁/K ₁ ⁰ (K ₁ ⁰=₀c), near the crack tips on the edges of openings in composite plates is a function of the ratio c/R, whose numerical values depend on the mechanical properties of materials of the plates. Using the quasi-elastic method for solving the viscoelastic problems, the effect of viscoelastic properties of the plate material on the value of K ₁/K ₁ ⁰ is estimated. It is shown that the values of the function K ₁(t)/>/K ₁ ⁰ near the cracks on the opening edges in plates made of linearly viscoelastic fibrous composites grow under creep.     

Disturbed critical surface waves in a channel of arbitrary cross section

Long waves in a current of an inviscid fluid of constant density flowing through a channel of arbitrary cross section under disturbances of pressure distribution on free surface and obstructors on the wall of the channel are considered. The first order asymptotic approximation of the elevation of the free surface satisfies a forced Korteweg-de Vries equation when the current is near its critical state. To determine the coefficients of the forced Korteweg-de Vries equation, we need to solve a linear Neumann problem of an elliptic partial differential equation, of which analytical solutions are found for constant current and rectangular or triangular cross section of the channel. It is proved that the forced Korteweg-de Vries equation has at least two solutions when the current is supercritical and the parameter is greater than a critical value _c >0. It is also proved that there do not exist solitary waves in a current exactly at its critical state. Numerical solutions of steady state are obtained for both supercritical and subcritical currents.     

A Class of Global Optimization Problems as Models of the Phase Unwrapping Problem

Let I={(i,j) i=1, 2,..., N ₁, j=1, 2,..., N ₂} and let U=U_i,j, (i,j)I be a discrete real function defined on I. Let []₂ be modulus 2, we define W:I , ) as follows W=[U]₂. The function U will be called phase function and the function W will be called wrapped phase function. The phase unwrapping problem consists in recovering U from some knowledge of W. This problem is not well defined, that is infinitely many functions U correspond to the same function W, and must be `regularized' to be satisfactorily solvable. We propose several formulations of the phase unwrapping problem as an integer nonlinear minimum cost flow problem on a network. Numerical algorithms to solve the minimum cost flow problems obtained are proposed. The phase unwrapping problem is the key problem in interferometry, we restrict our attention to the SAR (Synthetic Aperture Radar) interferometry problem. We compare the different formulations of the phase unwrapping problem proposed starting from the analysis of the numerical experience obtained with the numerical algorithms proposed on synthetic and real SAR interferometry data. The real data are taken from the ERS missions of the European Space Agency (ESA).     

The mixed three-dimensional problem for steady vibrations of plates

Using the symbolic method of homogeneous solutions, we study the problem of the steady-state vibrations of an isotropic plate whose upper face is stress-free, and whose lower face is rigidly restrained. We carry out a numerical study of the dispersion equation. We exhibit the barrier frequencies at which a penetrating harmonic or biharmonic solution arises. Theories of applied type are proposed. Three figures, 2 tables. Bibliography: 5 titles. Translated fromTeoreticheskaya i Prikladnaya Mekhanika, No. 27, 1997, pp. 10–17.     

微极热弹性无限板的轴对称自由振动

研究了在应力自由和刚性固定边界条件下,无能量耗散的均匀、各向同性微极热弹性无限板的轴对称自由振动波的传播,导出了相应的对称和斜对称模态波传播的闭合式特征方程和不同区域的特征方程.对短波的情况,应力自由热绝缘和等温板中对称和斜对称模态波传播的特征方程退化为Rayleigh表面波频率方程.根据导出的特征方程得到了热弹性、微极弹性和弹性板的结果.在对称和斜对称运动中计算了板的位移分量幅值、微转动幅值和温度分布,给出了对称和斜对称模式的频散曲线,并示出了位移分量和微转动幅值和温度分布的曲线.能够发现理论分析和数值结论是非常一致的.     

On sets of integers with prescribed gaps

For a fixed setI of positive integers we consider the set of paths (p _o,...,p _k ) of arbitrary length satisfyingp _l –p _l–1I forl=2,...,k andp ₀=1,p _k =n. Equipping it with the uniform distribution, the random path lengthT _n is studied. Asymptotic expansions of the moments ofT _n are derived and its asymptotic normality is proved. The step lengthsp _l –p _l–1 are seen to follow asymptotically a restricted geometrical distribution. Analogous results are given for the free boundary case in which the values ofp ₀ andp _k are not specified. In the special caseI={m+1,m+2,...} (for some fixed m) we derive the exact distribution of a random m-gap subset of {1,...,n} and exhibit some connections to the theory of representations of natural numbers. A simple mechanism for generating a randomm-gap subset is also presented.     

Error estimates for the numerical solution of a time-periodic linear parabolic problem

In this paper we consider the numerical solution of a time-periodic linear parabolic problem. We derive optimal order error estimates inL ₂() for approximate solutions obtained by discretizing in space by a Galerkin finite-element method and in time by single-step finite difference methods, using known estimates for the associated initial value problem. We generalize this approach and obtain error estimates for more general discretization methods in the norm of a Banach spaceB L ₂(), e.g.,B=H ₀ ¹ () orL (). Finally, we consider some computational aspects and give a numerical example.     

Stress Analysis for a Thick Rectangular Plate of a Composite Material with a Spatially Periodically Curved Structure under Forced Vibration

Natural and forced vibrations of a thick rectangular plate fabricated from a composite material with a spatially periodically curved structure are investigated with the use of exact three-dimensional equations of motion of the theory of elastic anisotropic bodies. The investigations are carried out within the framework of the continuum approach developed by Akbarov and Guz'. It is supposed that the plate is clamped at all its edges and is loaded on the upper face with uniformly distributed normal forces periodically changing with time. The influence of curving parameters on the fundamental frequency of the plate and on the distribution of the normal stress acting in the thickness direction under forced vibration is studied. The corresponding boundary-value problems are solved numerically by employing the three- dimensional FEM modeling.     

Various methods of determining the natural frequencies and damping of composite cantilever plates. 2. Approximate solution by Galerkin's method for the trinomial model of damping

The application of Kantorovich's method to a trinomial model of deformation taking into account transverse bending of a plate leads to a connected system of three ordinary differential equations of fourth order with respect to three unknown functions of the longitudinal coordinate and to the coresponding boundary conditions for them at the fixed end and on the free edge. For the approximate calculation of the frequencies and forms of natural vibrations Galerkin's method is used, and as coordinate functions we chose orthogonal Jacobi polynomials with weight function. The dimensionless frequencies depend on the magnitude of the four dimensionless complexes, three of which characterize the anisotropy of the elastic properties of the composite. For the fibrous composites used at present we determined the possible range of change of the dimensionless complexes d₁₆ and d₂₆ attained by oblique placement. The article examines the influence of the angle of reinforcement on some first dimensionless frequencies of a plate made of unidirectional carbon reinforced plastic. It also analyzes the asymptotics of the frequencies when the length of the plate is increased, and it shows that for strongly anisotropic material with the structure []_T the frequencies of the flexural as well as of the torsional vibrations may be substantially lower when flexural-torsional interaction is taken into account.For Communication 1 see [4].Institute of Engineering Science of the Russian Academy of Sciences, St. Petersburg, Russia. St. Petersburg State University, Russia. Translated from Mekhanika Kompozitnykh Materialov, No. 1. pp. 23–33, January–February, 1997.     

Rigid-plastic model of a fibrous composite: Problems involving axial symmetry

A rigid-plastic transversally isotropic medium with the additional kinematic assumption of inextensibility along the symmetry axis was examined as a model of a fibrous composite with a metal matrix. A complete system of equations for the axially symmetric deformation of such a body was obtained. In contrast to planar deformation examined in our previous work [5, 6], the axial symmetry problem is not locally statically determined, which prevents the separate analysis of the stress and velocity fields in the general case. The principal simplifications in formulating the equation system may be obtained for special cases of the stress state. One such simplifying assumption is extension of the axial conditions _r= or _rz=0 over the entire body. This hypothesis yields a separate system of equations for stresses. On the other hand, the common assumption of complete plasticity for an isotropic body when two principal stresses are equal does not provide fundamental simplification due to the lack of coincidence between the directions of the major stresses and the symmetry axes. An analytical solution was obtained for the axially symmetric problem of a plate with a round hole. A special feature of this problem is that the type of system of equations changes in the plastic region. The stress distribution in the circular hole was found to depend significantly on the type of equation system.M. V. Lomonosov Moscow State University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 3, pp. 300–311, May–June, 1997.     

AnL 1 smoothing spline algorithm with cross validation

We propose an algorithm for the computation ofL ₁ (LAD) smoothing splines in the spacesW _M (D), with . We assume one is given data of the formy _i =(f(t _i ) + _i , i=1,...,N with {itt_i} _i=1 ^{N D} , the _i are errors withE( _i )=0, andf is assumed to be inW _M . The LAD smoothing spline, for fixed smoothing parameter0, is defined as the solution,s , of the optimization problem (1/N) _i=1 ^N ¦y _i –g(t _i ¦+J _M (g), whereJ _M (g) is the seminorm consisting of the sum of the squaredL ₂ norms of theMth partial derivatives ofg. Such an LAD smoothing spline,s , would be expected to give robust smoothed estimates off in situations where the _i are from a distribution with heavy tails. The solution to such a problem is a thin plate spline of known form. An algorithm for computings is given which is based on considering a sequence of quadratic programming problems whose structure is guided by the optimality conditions for the above convex minimization problem, and which are solved readily, if a good initial point is available. The data driven selection of the smoothing parameter is achieved by minimizing aCV() score of the form .The combined LAD-CV smoothing spline algorithm is a continuation scheme in 0 taken on the above SQPs parametrized in, with the optimal smoothing parameter taken to be that value of at which theCV() score first begins to increase. The feasibility of constructing the LAD-CV smoothing spline is illustrated by an application to a problem in environment data interpretation.     

A conformal mapping algorithm for the Bernoulli free boundary value problem

We propose a new numerical method for the solution of the Bernoulli free boundary value problem for harmonic functions in a doubly connected domain D in where an unknown free boundary Γ₀ is determined by prescribed Cauchy data on Γ₀ in addition to a Dirichlet condition on the known boundary Γ₁. Our main idea is to involve the conformal mapping method as proposed and analyzed by Akduman, Haddar, and Kress for the solution of a related inverse boundary value problem. For this, we interpret the free boundary Γ₀ as the unknown boundary in the inverse problem to construct Γ₀ from the Dirichlet condition on Γ₀ and Cauchy data on the known boundary Γ₁. Our method for the Bernoulli problem iterates on the missing normal derivative on Γ₁ by alternating between the application of the conformal mapping method for the inverse problem and solving a mixed Dirichlet–Neumann boundary value problem in D. We present the mathematical foundations of our algorithm and prove a convergence result. Some numerical examples will serve as proof of concept of our approach. Copyright © 2015 John Wiley & Sons, Ltd.     

Cost benefit analysis of series systems with warm standby components

This paper deals with the cost benefit analysis of series systems with warm standby components. The time-to-repair and the time-to-failure for each of the primary and warm standby components is assumed to have the negative exponential distribution. We develop the explicit expressions for the mean time-to-failure, MTTF, and the steady-state availability, A _T () for three configurations and perform a comparative analysis. Under the cost/benefit (C/B) criterion, comparisons are made based on assumed numerical values given to the distribution parameters, and to the cost of the components. The configurations are ranked based on: MTTF, A _T (), and C/B where B is either MTTF or A _T ().     

Spannungsfeld einer auf den Rand einer Halbebene wirkenden Einzellast bei elastischer Anisotropie

Summary The solution of the two-dimensional stress problem in an anisotropic medium is given in closed form for the boundary conditions of a concentrated load at a point on the straight boundary of a semi-infinite plate. The properties of a periodic repetition of such concentrated loads are discussed and compared with an existing observation of a non-uniform stress distribution in -iron.     

On the construction and calculation of optimal nonlifting critical airfoils

Numerical calculations are carried out in the hodograph plane to construct optimal critical airfoil shapes and the flow about them. These optimal airfoil shapes give the highest free-stream Mach numberM for a given thickness ratio and tail angle _t (nonlifting) for which the flow is nowhere supersonic. A relationship betweenM and for various _t is given. Analytical and numerical solutions to the same problem are found on the basis of transonic small-disturbance theory. These results provide a limiting case asM 1, 0 and agree well with the calculations of the full problem. Using a numerical method to calculate the flow about general (subsonic) airfoils, a comparison is made between the critical free-stream Mach numbers for some standard airfoil shapes and the optimal free stream Mach number of the corresponding and _t . A significant increase in the critical free-stream Mach number is found for the optimal airfoils.     

On the Number of Appearances of a Word in a Sequence of I.I.D. Trials

Let X ₁,...,X _n be a sequence of i.i.d. random variables taking values in an alphabet =₁,...,_q,q 2, with probabilities P(X _a=_i)=p _i,a=1,...,n,i=1,...,q. We consider a fixed h-letter word W=w₁...w_h which is produced under the above scheme. We define by R(W) the number of appearances of W as Renewal (which is equal with the maximum number of non-overlapping appearances) and by N(W) the number of total appearances of W (overlapping ones) in the sequence X _{a 1} a1n under the i.i.d. hypothesis. We derive a bound on the total variation distance between the distribution (R(W)) of the r.v. R(W) and that of a Poisson with parameter E(R(W)). We use the Stein-Chen method and related results from Barbour et al. (1992), as well as, combinatorial results from Schbath (1995b) concerning the periodic structure of the word W. Analogous results are obtained for the total variation distance between the distribution of the r.v. N(W) and that of an appropriate Compound Poisson r.v. Related limit theorems are obtained and via numerical computations our bounds are presented in tables.     

Interaction of Plane Stress Waves in a Three-Layer Structure. 1. Degenerate Solutions in Terms of Characteristics for a Homogeneous Structure

Exact expressions in terms of characteristics for calculating the normal-stress waves propagating across the layers of different materials are deduced. A one-dimensional boundary-value problem is considered for a three-layer structure of sandwich type. The faces of the layered structure are free from loads or one of them is rigidly fixed (variant 1), or one face is rigidly fixed and the other is subjected to an impact of a mass M with a speedV₀ (variant 2). For the boundary conditions of variant 1, relationships are obtained which allow one to reduce the analytical continuation of a solution in time to a periodic procedure if solely the initial disturbances of the strain field in the layers are given. It is shown that, in this case, the Cauchy problem with the initial strain field is reduced to graphoanalytically constructing the superposition patterns of the forward and backward waves. The fundamental features of the construction are demonstrated for a uniform bar with a piecewise constant distribution of strains along its length. To solve the problem of impact loading in variant 2, analytical results for a uniform plate are used, which allows us to account for the direction of mass forces in collision. In the latter case, the possibility of mass recoil is revealed in the first and second time cycles. The analytical constructions presented are focused on an exact calculation of stresses upon response of a layered plate to initial disturbances within its layers, as well as to an external dynamic action. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 5, pp. 585–606, September–October, 2005.