Analysis of fracture processes in structures of quasi-brittle concrete-like materials is here discussed on the basis of discrete cohesive crack models and of a nontraditional boundary element method. This method, called 'symmetric Galerkin BEM', is characterized by the combined use of static and kinematic sources (i.e. traction and displacement discontinuities) to generate a symmetric integral operator by its space discretization in the Galerkin weighted-residual sense. Consistently, the discrete crack model is enforced in a weak sense and expressed in terms of Prager's generalized variables. On this basis, some of the main aspects of a computational theory of quasi-brittle fracture mechanics are presented and discussed.

N. Mahomed and M. Kleiber. An aspect ratio condition for the buckling and folding of polymeric jets in injection mould filling. CAMES 1998 (5)

The behaviour of planar Newtonian and non-Newtonian polymeric jets is investigated in the context of injection mould filling. The incompressible Stokes flow model consistent with the application of injection mould filling is described together with the shear rate dependent fluid viscosity for a typical polymeric melt. The numerical procedure for the solution of the nonlinear system is briefly discussed as well as the mesh generation and the melt front tracking algorithms employed.
In the analysis, the buckling behaviour of Newtonian and non-Newtonian jets are firstly compared. Thereafter the behaviour of Newtonian jets are analysed for various values of the aspect ratio in an attempt to study the validity of the Cruikshank buckling conditions for planar Newtonian jets. It is argued that the Reynolds number of highly viscous polymeric melts is relatively low and the aspect ratio condition is the critical condition dictating the buckling behaviour. Finally, an aspect ratio design criteria is established for buckling-free and folding-free flow of polymeric jets.
*Keywords:* injection moulding, free surface flows, unsteady non-Newtonian flow.

Y.M. Pochtman, V.V. Skalozub and D.V. Nagornyy Axiomatic approach to the multicriteria structural optimization. CAMES 1998 (5)

The paper presents the axiomatic approach for solving the multicriteria optimization of thin-walled structure such as vertical cylindrical reservoirs subject to pitting corrosion. The probabilities of derivation of the compromise optimal project based on MaxMin principle are investigated. The analytic dependencies for estimation of the partial criteria weighting coefficients are obtained. The project consists of the optimal thickness of reservoir shell along its height

Zh. Yu and L.T. Meng. On-line identification of squeeze-film dynamics of multi-mode rotor-bearing system. CAMES 1998 (5)

This paper presents a practical algorithm for on-line parameter identification of squeeze-film bearing of multi-mode rotor-bearing system. The identification procedure is based on modeling each of the bearing pedestal by applying a multi-frequency excitation force on, the rotor and frequency transfer function data are used. It suggested that accurate identification coefficients with reduced standard errors can be achieved without resource to full or reduced-order rotor system measurements. The approach can be applied to rotor-bearing system with any degree of complexity and other types of bearing. Simulation and experimental investigation show that the identification algorithm developed in the paper will considerably simplify the measurement and calculation task for testing work in laboratory and industrial environment without any lost of identification accuracy. The experimental results of stiffness and damping characteristics of the squeeze-film bearings for different rotating speed are also presented.

C.I. Bajer. The space-time approach to rail/wheel contact and corrugations problem. CAMES 1998 (5)

The paper presents a space-time discrete modeling of the dynamic rail-wheel contaet problem and an analysis of the induced corrugations. First, the space-time approach to simple contact problems is presented: Then, the resulting differential equation of motion is solved by discrete time integration. An arbitrary mesh modification, both in time and space, enables an easy modeling of rapidly varying contact zone. The velocity formulation is used and the discontinuity of the velocity in the contact is removed by a special algorithm. Finally the discussed technique is used to simulate interaction of the elastic wheel and rigid rail. It is shown that the contact force oscillates and the material of the wheel rotates oscillatory.

A. Zochowski. Implementation of the harmonic transformation method in shape optimization. CAMES 1998 (5)

The paper describes an implementation of the variant of the speed method in shape optimization for plane elastic structures, based on harmonic transformations. It is coupled with special method for solving the singular elliptic problems resulting from geometric features like e.g. reentrant corners. Both approaches are based on the works of the author. The interactive system has been built, based on MATLAB environment, and the examples showing the robustness of the algorithms were solved.

A. Jarzebowski and J. Maciejewski. Some remarks on the applicability of rectangular elements to plane strain boundary value problems. CAMES 1998 (5)

The aim of this paper is the discussion on the applicability of some rectangular elements to plane strain boundary value problems. Four different elements were considered: 4-node, 5-node, Serendipity 8-node and Lagrangian 9-node. Two cases: the material layer loaded by a concentrated vertical force and the same layer loaded by a symmetrical rigid punch were discussed. An elastic material was used to avoid the influence of the constitutive model on solutions. To model interface behaviour on the contact surface a Coulomb friction condition was applied. The use of the 4- and 5-node elements resulted in the prediction of the `island' pattern of stress and strain tensors distributions and their non-applicability was proved independently from the boundary condition. The 8-node element predicted erroneous distributions of nodal forces and should be avoided in the case of contact problems. Among the discussed group of elements only the 9-node element turned out to be applicable for boundary value problems under plane strain condition.

E. Kita and H. Tanie. 2D shape optimization using genetic algorithm. CAMES 1998 (5)

This paper presents an optimal design method of continuum structures by genetic algorithm. Profiles of the objects under consideration are represented by the spline functions and tnen, the chromosomes for the profiles are defined by the coordinates of the control points of the functions and the material code of the structures. The profiles and the material code are optimized by the genetic operations in order to determine the object satisfying the design objectives. The minimum weight design of the plate is considered as a typical example. The present method is applied to the problem in which the profile and the material of the objects are unknown.
*Keywords:* genetic algorithms (GAs), boundary element methods (BEMs), shape optimization, material selection, Riesenfeld spline functions.