Second-Order Nonlinear Cauchy Problems in a Four-Dimensional Space

We consider a general second-order Cauchy problem in a four-dimensional space and establish sufficient conditions for its integrability by a generalized Lie-series method.     

On Constitutive Equations of Cauchy Elastic Solids: All Kinds of Crystals and Quasicrystals

Our concern is with the problem of determining general reduced forms of constitutive equations of Cauchy elastic solids under all kinds of material symmetries, including well-known crystal classes and newly discovered quasicrystal classes. By means of Tschebysheff polynomials we present in unified forms simple irreducible representations for elastic constitutive equations under the infinitely many subgroup classes C _2m+1, C _2m+2, D _2m+1 and D _2m+2 for all m = 1, 2,... Moreover, we provide a simple representation for elastic constitutive equations under the most complicated point group, i.e. the icosahedral group. Each presented representation is expressed in terms of not more than nine polynomial tensor generators only. This revised version was published online in August 2006 with corrections to the Cover Date.     

SOME PROBLEMS IN THE Z-C-X SPACE

ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｉｓｐａｐｅｒｉｓａｃｏｎｔｉｎｕａｔｉｏｎｏｆＲｅｆ.[1 ]ｂｙｔｈｅａｕｔｈｏｒ,ｉｔｉｓｏｎｅｏｆｔｈｅｍａｉｎｃｏｎｔｅｎｔｓａｂｏｕｔｉｎｖｅｓｔｉｇａｔｉｎｇｒａｎｄｏｍｓｏｌｕｔｉｏｎｓｏｆｒａｎｄｏｍｏｐｅｒａｔｏｒｅｑｕａｔｉｏｎｓ.Ｔｈｒｏｕｇｈｏｕｔｔｈｉｓｐａｐｅｒｌｅｔ(Ｅ ,Ｂ)ｂｅａｍｅａｓｕｒａｂｌｅｓｐａｃｅ ,ｗｈｅｒｅＥｉｓａｓｅｐａｒａｂｌｅｒｅａｌＢａｎａｃｈｓｐａｃｅ ,Ｂｄｅｎｏｔｅｓｔｈｅσ＿ａｌｇｅｂｒａｏｆｇｅｎｅｒａｔｉｎｇｂ…     

Cauchy–Born Rule and the Stability of Crystalline Solids: Static Problems

We study the connection between atomistic and continuum models for the elastic deformation of crystalline solids at zero temperature. We prove, under certain sharp stability conditions, that the correct nonlinear elasticity model is given by the classical Cauchy–Born rule in the sense that elastically deformed states of the atomistic model are closely approximated by solutions of the continuum model with stored energy functionals obtained from the Cauchy–Born rule. The analysis is carried out for both simple and complex lattices, and for this purpose, we develop the necessary tools for performing asymptotic analysis on such lattices. Our results are sharp and they also suggest criteria for the onset of instabilities of crystalline solids.     

The Geometry of the Solution Set of Nonlinear Optimal Control Problems

In an optimal control problem one seeks a time-varying input to a dynamical systems in order to stabilize a given target trajectory, such that a particular cost function is minimized. That is, for any initial condition, one tries to find a control that drives the point to this target trajectory in the cheapest way. We consider the inverted pendulum on a moving cart as an ideal example to investigate the solution structure of a nonlinear optimal control problem. Since the dimension of the pendulum system is small, it is possible to use illustrations that enhance the understanding of the geometry of the solution set. We are interested in the value function, that is, the optimal cost associated with each initial condition, as well as the control input that achieves this optimum. We consider different representations of the value function by including both globally and locally optimal solutions. Via Pontryagin’s maximum principle, we can relate the optimal control inputs to trajectories on the smooth stable manifold of a Hamiltonian system. By combining the results we can make some firm statements regarding the existence and smoothness of the solution set.     

On Cauchy singular integrals and stress intensity factors for 2D mode I cracks in an infinite solid

This paper comments on some of the different numerical techniques commonly employed in evaluating Cauchy singular integrals of the first kind; e.g. as pertaining to 2D through cracks in a brittle material undergoing Mode I loading. In addition, a different more direct method is proposed here. Also, two different ways to calculate the stress intensity factor (K_I) are contrasted. The accuracy attained by the different methods in calculating K_I, and the factors affecting the calculation, are compared. Finally, comments on calculating the stress field of a 2D crack and important considerations are presented.     

Some Specific Features of Integral Equations with the Cauchy Kernel on a Closed Contour in Hydrodynamic Problems

Singular integral equations of the first and second kind with the Cauchy kernel on a limiting narrow closed contour are theoretically considered. The initial equations are found to become different on the limiting contour. This singularity of integral equations with the Cauchy kernel does not allow boundary-value problems of the flow around thin airfoils to be solved correctly; therefore, a system consisting of integral equations of the first and second kind is proposed for solving such problems. The results of the present study are tested against an exact solution of the problem of the flow past a flat plate.     

On the Fréchet Differentiability of an Invariant Torus of a Nonlinear Countable System of Difference Equations Defined on an Infinite-Dimensional Torus and Containing Deviations of a Discrete Argument

We give sufficient conditions for the Fréchet differentiability of an invariant torus of a nonlinear system of difference equations in the space of bounded numerical sequences. The system under consideration contains countably many angular and normal variables and also independent deviations of a discrete argument.     

On the impossibility of linear Cauchy and Piola-Kirchhoff constitutive theories for stress in solids

We investigate the possibility of linear elasticity as an infinitesimal theory based on a genuinely linear response function which retains its validity even for finite deformations. Careful consideration of the domain of definition of the stress response function, the definition of linearity and the notion of material frame-indifference leads to our main result that an exact linear constitutive theory for elastic solids is impossible. We then generalize our result to viscoelasticity theory where the stress response is dependent on deformation gradient histories.

---

Zusammenfassung Verf. betrachten die Möglichkeit linearer Elastizität als infinitesimale Theorie begründet auf einer echt linearen Reaktionsfunktion die ihre Gültigkeit sogar für endliche Deformationen behält. Genaue Betrachtung des Definitionsbereiches der Spannungsreaktionsfunktion, der Definition von Linearität, und des Objektivitätsbegriffes führen zum Hauptresultat dass eine echt lineare Theorie für elastische Körper unmöglich ist. Das Resultat wird dann auf viskoelastische Theorie verallgemeinert, wobei die Spannungsreaktion von der Vorgeschichte des Deformationsgradienten abhängt.

---

---

Department of Aerospace Engineering and Mechanics

---

Department of Mathematics     

On the Ill-Posedness of the Prandtl Equations in Three-Dimensional Space

In this paper, we give an instability criterion for the Prandtl equations in three-dimensional space, which shows that the monotonicity condition on tangential velocity fields is not sufficient for the well-posedness of the three-dimensional Prandtl equations, in contrast to the classical well-posedness theory of the two-dimensional Prandtl equations under the Oleinik monotonicity assumption. Both linear stability and nonlinear stability are considered. This criterion shows that the monotonic shear flow is linearly stable for the three-dimensional Prandtl equations if and only if the tangential velocity field direction is invariant with respect to the normal variable, and this result is an exact complement to our recent work (A well-posedness theory for the Prandtl equations in three space variables. arXiv:1405.5308, 2014) on the well-posedness theory for the three-dimensional Prandtl equations with a special structure.     

On Obtaining a Solution to Optimization Problems for Solid,Elastic Plates by Restriction of the Design Space*

The problem of minimizing the static compliance of a solid elastic plate, for given total plate volume, is considered. Nonexistence of solutions in the case of a design space consisting of thickness functions for an isotropic, homogeneous plate is identified as being caused by the nonclosedness of the corresponding set of responses (deflections). It is shown that existence can be obtained if a bound on the gradient of the thickness function is imposed. It is also shown that this restriction of the design space ensures the existence of solutions to a broad class of plate optimization problems, static as well as dynamic.     

On Korn's Inequality in the Second Case

A simple proof of Korn's inequality in the second case is given. Technicalities are minimized by using both the compatibility condition between the symmetric and skew parts of the gradient of a vector field and a result of Magenes and Stampacchia [10] and Necăs [11] asserting the closure of the range of the gradient operator on L²-functions. This revised version was published online in August 2006 with corrections to the Cover Date.     

A unified evolution equation for the Cauchy stress tensor of an isotropic elasto-visco-plastic material

In the present study an evolution equation for the Cauchy stress tensor is proposed for an isotropic elasto-visco-plastic continuum. The proposed stress model takes effects of elasticity, viscosity and plasticity of the material simultaneously into account. It is ascribed with some scalar coefficient functions and, in particular, with an unspecified tensor-valued function N, which is handled as an independent constitutive quantity. It is demonstrated that by varying the values and the specific functional forms of these coefficients and N, different known models in non-Newtonian rheology can be reproduced. A thermodynamic analysis, based on the Müller–Liu entropy principle, is performed. The results show that these coefficients and N are not allowed to vary arbitrarily, but should satisfy certain restrictions. Simple postulates are made to further simplify the deduced general results of the thermodynamic analysis. They yield justification and thermodynamic consistency of the existing models for a class of materials embracing thermoelasticity, hypoelasticity and in particular hypoplasticity, of which the thermodynamic foundation is established successively for the first time in literature. The study points at the wide applicability and practical usefulness of the present model in different fields from non-Newtonian fluid to solid mechanics. In this paper the thermodynamic analysis of the proposed evolution-type stress model is discussed, its applications are reported later.      

Construction of an asymptotic solution of the Cauchy problem for a degenerate linear system in the singular case

We propose an algorithm for the construction of an asymptotic solution of the Cauchy problem for a singularly perturbed linear system of differential equations with degenerate matrix of coefficients of derivatives in the case where the limit pencil of matrices is singular. __________ Translated from Neliniini Kolyvannya, Vol. 11, No. 2, pp. 271–288, April–June, 2008.     

On the Cauchy problem for two-dimensional systems of linear functional differential equations with monotone operators

We establish new efficient conditions sufficient for the unique solvability of the Cauchy problem for two-dimensional systems of linear functional differential equations with monotone operators. Published in Neliniini Kolyvannya, Vol. 10, No. 4, pp. 560–573, October–December, 2007.     

Representation of a solution of the Cauchy problem for an oscillating system with pure delay

We obtain a representation of a solution of the Cauchy problem for a linear inhomogeneous differential equation with constant coefficients and pure delay. We use special matrix functions called a delayed matrix sine and a delayed matrix cosine. They have the form of matrix polynomials of degree dependent on the value of delay. __________ Translated from Neliniini Kolyvannya, Vol. 11, No. 2, pp. 261–270, April–June, 2008.