Finite Matrix Methods

Non-relativistic quantum theory of atoms and molecules is built upon wave-functions constructed from antisymmetrized products of single particle wave-functions. The same scheme has been adopted for relativistic theories, the main difference now being that the single particle functions are 4-component spinors (bispinors). The finite matrix method approximates such 4-spinors by writing... 

Note that in equation system (2.64) the coefficients matrix is symmetric, sparse (i.e. a significant number of its members are zero) and banded. The symmetry of the coefficients matrix in the global finite element equations is not guaranteed for all applications (in particular, in most fluid flow problems this matrix will not be symmetric). However, the finite element method always yields sparse and banded sets of equations. This property should be utilized to minimize computing costs in complex problems. 

A square matrix is one in which the number of columns is equal to the number of rows. An important type of square matrix which arises quite often in the finite element method is a symmetric matrix. Such matrices possess the property that aij = aji- An example of such a matrix is given below ... 

With the availabihty of computers, the transfer matrix method [14] emerged as an alternative and powerful technique for the study of cooperative phenomena of adsorbates resulting from interactions [15-17]. Quantities are calculated exactly on a semi-infinite lattice. Coupled with finite-size scaling towards the infinite lattice, the technique has proved popular for the determination of phase diagrams and critical-point properties of adsorbates [18-23] and magnetic spin systems [24—26], and further references therein. Application to other aspects of adsorbates, e.g., the calculation of desorption rates and heats of adsorption, has been more recent [27-30]. Sufficient accuracy can usually be obtained for the latter without scaling and essentially exact results are possible. In the following, we summarize the elementary but important aspects of the method to emphasize the ease of application. Further details can be found in the above references. 

Finite element methods [20,21] have replaced finite difference methods in many fields, especially in the area of partial differential equations. With the finite element approach, the continuum is divided into a number of finite elements that are assumed to be joined by a discrete number of points along their boundaries. A function is chosen to represent the variation of the quantity over each element in terms of the value of the quantity at the boundary points. Therefore a set of simultaneous equations can be obtained that will produce a large, banded matrix. 

The modeling of complex solids has greatly advanced since the advent, around 1960, of the finite element method [196], Here the material is divided into a number of subdomains, termed elements, with associated nodes. The elements are considered to consist of materials, the constitutive equations of which are well known, and, upon change of the system, the nodes suffer nodal displacements and concomitant generalized nodal forces. The method involves construction of a global stiffness matrix that comprises the contributions from all elements, the relevant boundary conditions and body and thermal forces a typical problem is then to compute the nodal displacements (i. e., the local strains) by solving the system K u = F, where K is the stiffness matrix, u the... 

Fig. 6.22. Comparisons of the longitudinal splitting length, Z,p, between analysis and finite element method for graphite fiber-epoxy matrix orthotropic laminates. After Tirosh (1973).

Remark. Apart from the question whether the set of all eigenfunctions is complete, one is in practice often faced with the following problem. Suppose for a certain operator W one has been able to determine a set of solutions of (7.1). Are they all solutions For a finite matrix W this question can be answered by counting the number of linearly independent vectors one has found. For some problems with a Hilbert space of infinite dimensions it is possible to show directly that the solutions are a complete set, see, e.g., VI.8. Ordinarily one assumes that any reasonably systematic method for calculating the eigenfunctions will give all of them, but some problems have one or more unsuspected exceptional eigenfunctions. 

The first group of methods then manipulate a very small subset of vector elements Vi at a time, and a direct method continually updates the affected elements rj. Such methods are collectively known as relaxation methods, and they are primarily used in situations where, for each elements >< to be changed, the set of affected rjt and the matrix elements Ay, are immediately known. This applies in particular to difference approximations and also to the so-called Finite Element Method for obtaining tabular descriptions of the wave function, i.e. a list of values of the wave function at a set of electron positions. (Far some reason, such a description is commonly referred to as a numerical solution to the Schrodinger equation). Relaxation methods have also been applied to the Cl problem in the past, but due to their slow convergence they have been replaced by analytical methods. Even for numerical problems, the relaxation methods are slowly yielding to analytical methods. 

Fano [96]. The K-matrix method has been successfully applied to a number of problems in atomic physics [167,193], particularly in conjunction with 13-splines [75,194-200]. B-splines are a convenient tool to represent accurately in finite boxes the radial component of continuum atomic orbitals [82], their major feature being a property known as effective completeness [201]. 

Continuum wavefunctions can also be generated by solving the partial differential equation (3.3) directly without first transforming it into a set of ordinary differential equations. One possible scheme is the finite elements method (Askar and Rabitz 1984 Jaquet 1987). Another method, which has been applied for the calculation of multi-dimensional scattering wavefunctions, is the 5-matrix version of the Kohn variational principle (Zhang and Miller 1990). 

The finite element method, as applied to an engineering structure, consists of dividing the structure into distinct nonoverlaping regions known as elements the elements are connected at a discrete number of points along the periphery, known as nodal points. For each element the stiffness matrix and load vector are calculated, by assembling the calculated stiffness matrix and load vector of the elements, one obtains the overall stiffness and vector of the system or structure the resulting simultaneous equations for the unknown displacement components of the structure (unknown nodal variables) are solved and the stress components are evaluated for the elements. 

Keywords Boundary Finite Element Method, composite laminates, stress localization, laminate free-edge effect, free-edge stresses, crack problems, transverse matrix crack, numerical methods, boundary discretization... 

For numerical investigations of stress localizations in laminates, the discretizational effort can be reduced significantly if only the boundary needs to be discretized, as it is for e -ample the case in the classical boundary element method (BEM). But in this method a fundamental solution is needed which is in many cases difficult to achieve or even unknown. The Boundary Finite Element Method (BFEM) to be presented here does not require such a fundamental solution, because the element formulation is based on the finite element method (FEM), Thus the BFEM can be characterized to be a finite element based boundary discretization method. This method was originally developed from Wolf and Song [10] under the name Consistent Finite Element Cell Method for time-dependent problems in soil-mechanics. The basic assumption of this method is that a stiffness matrix describing the force-displacement relation at discrete degrees of freedom at the boundary of the continuum is scalable with respect to one point in three-dimensional space, the so-called similarity center, if similar contours within the continuum are considered. In contrast to this, the current work deals with the case of equivalent cross-sectional properties, i.e., that cross-sections parallel to the boundary can be described by the same stiffness matrix, which is the appropriate formulation for the case of the free-edge effect and the matrix crack problem. The boundary stiffness matrix results from a Matrix-Riccati equation. The field quantities inside of the continuum can be calculated from an ordinary differential equation. 

In the following results are presented for the application of the Boundary Finite Element Method both for the case of the laminate free-edge effect and for the case of a single transverse matrix crack in the framework of linear elasticity theory. 

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