A state of the art of the problem of buckling in sandwich structures is discussed and the shortcomings of some existing theories shown. A specified classification of the forms of stability is given and, in accordance with it, a refined theory for the study of the mixed forms of stability is formulated. Different models of the fillers are classified according to their stress–strain state. For the transversely soft model of the filler a set of geometrically nonlinear refined relations is derived. These relations are used to describe the subcritical instantaneous equilibrium of the sandwich plates in the case of both large and small changes in the shear stresses.

Attempts to model the degradation of polymer composites have been restricted to modelling the effects of selected degradation mechanisms. No comprehensive model has yet been accepted to predict the effect of the natural environment on the strength of polymer composites. From a review of available literature, it appears that the matrix of a polymer composite is most affected by exposure to the natural environment. Further, the damage appears to progress from the surface into the interior of the laminate. An approach has been developed to determine the properties of the damaged layer and combine the properties of the damaged and undamaged layer to obtain bulk material properties of the laminate.

Due to the lack of durability data for polymer composites in South Africa, a study has been initiated to determine the effect of the South African climate on polymer composites. Laminates exposed and tested were fabricated from two different epoxy resins with woven glass reinforcement. Environmental exposure was conducted at locations with significantly different climates. Compression test results show that while epoxy 1 experienced a negligible change in strength, epoxy 2 appears to have benefited the most from possible post curing during exposure. Cracks on the surface of epoxy 2 have progressed into the laminate but have not as yet reached the first layer of woven fibre after 20 weeks of exposure.

A dislocation-disclination model is proposed, describing the heterogeneous nucleation of an embryo of hcp martensite at a tilt grain-boundary segment containing some extrinsic dislocations. The total energy gain due to hcp embryo nucleation is analyzed in detail, and the existence of both the equilibrium and critical embryo sizes under varying external conditions (temperature and shear stress) is shown. Depending on the external conditions, these characteristic embryo sizes may vary in wide ranges. So, the equilibrium size increases while the critical size decreases as the external shear stress increases and the temperature decreases. It is also demonstrated that a critical external stress exists which induces athermal embryo nucleation when the nucleation-energy barrier disappears and the terms of equilibrium and critical embryo sizes lose their significance. The critical external stress has been studied, depending on the temperature and characteristic parameters of the grain boundary where the fcc-to-hcp martensite transformation takes place. We have shown, in particular, that the critical external stress increases in direct proportion to both the grain-boundary misorientation angle and temperature.

A new approach is developed for analysis of lattice shells of revolution. This approach shows to be more efficient in application to structural analysis and optimisation. Constitutive equations are developed and the expressions for components of stress and strain tensors are derived for the shells of revolution with different lattice patterns. The model for cylindrical lattice shell was realised in a computer code using symbolic computation. Numeric verification of the mathematical models is performed. The advantage of a new homogenisation approach is based on the ability to calculate structural stress resultants without performing finite element analysis which allows to achieve higher computational efficiency. Also this approach allows more design variables to be assigned in the further optimisation analysis.

A theoretical model is proposed which describes heterogeneous nucleation of an embryo of hcp-martensite near a free surface at a tilt grain boundary segment containing some extrinsic dislocations. The total energy gain due to the hcp-embryo nucleation is analysed in detail depending on internal (the distance from free surface and orientation of the grain boundary plane) and external (the temperature and shear stress) conditions. It is shown that the main characteristics of the hcp-embryo nucleation (the energy barrier and critical size) depend strongly on the distance from the free surface.

A dislocation–disclination model is proposed describing heterogeneous nucleation of an embryo of hcp-martensite at a tilt grain boundary segment containing some extrinsic dislocations. The corresponding energy gain is analysed in detail. The equilibrium and critical embryo sizes under external conditions of temperature and stress are examined and discussed.

The design of composite structures against buckling presents two major challenges to the designer. First, the problem of laminate stacking sequence design is discrete in nature, involving a small set of fiber orientations, which complicates the solution process. Therefore, the design of the stacking sequence is a combinatorial optimization problem which is suitable for genetic algorithms. Second, many local optima with comparable performance may be found. Most optimization algorithms find only a single optimum, while often a designer would want to obtain all the local optima with performance close to the global optimum. Genetic algorithms can easily find many near optimal solutions. However, they usually require very large computational costs. Previous work by the authors on the use of genetic algorithms for designing stiffened composite panels revealed both the above strength and weakness of the genetic algorithm. The present paper suggests several changes to the basic genetic algorithm developed previously, and demonstrates reduced computational cost and increased reliability of the algorithm due to these changes. Additionally, for a stiffened composite panel considered in this study, we present designs lighter by about 4% compared to previously obtained results.

A rational transverse shear deformation higher-order theory of multilayered anisotropic plates and shallow shells is developed for the solution of statical problems for two possible cases: cross-ply and angle-ply laminates. The theory developed differs from existing ones by three features. Firstly, it is based on the hypotheses which are fully tied to the physical and mechanical characteristics of the anisotropic layers. Secondly, the theory is built on a rational level of difficulty, i.e. it does not add complexity in comparison with other known theories developed for more simple laminated structure. Thirdly, the hypotheses take directly into account the influence of external subject to both normal and tangential loads.Relying on the specific approach for the derivation of hypotheses all the relations of the stress-strain state of anisotropic laminated shells are obtained. Using the variational approach the system of governing differential equations and corresponding boundary conditions are derived.The analytical solution for this system is given, and both special cases are stated, namely, cross-ply and angle-ply laminates, for which such solution exists, The results of the calculations are given and compared with exact three-dimensional and some approximate solutions available in the literature. The influence of the laminated structure upon the exactness of results and the characteristics of stress-strain state is studied and discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.

The design, FE analysis, deflection and strength optimisation, manufacture and physical testing of an all-composite camera mounting gimbal for airborne use was discussed. The existing magnesium design is considered too heavy and its metallic structure is both prone to harmonic oscillations and radar detection, thus a new composite design has been proposed. The use of finite element packages allowed the complex fibre angle optimisation to be performed. A number of prototype gimbals were manufactured using a new process of a closed mould technique, with expanding silicone inserts and a pre-impregnated carbon fibre/epoxy resin woven material. Finally, these prototypes were physically tested to check that they met the strength criteria and to confirm the FEA results.

Stress analysis of multilayered pressure vessels possessing cylindrical anisotropy and under internal, external and interlaminar pressures is given. The special case when the axis of anisotropy coincides with the axis of symmetry Oz and the stresses do not vary along the generator is investigated. In this case there exists a plane of elastic symmetry normal to this axis at every point of the cylinder so that each layer may be considered as orthotropic. However, elastic properties can vary through the thickness of a layer. Exact elasticity solutions are obtained for both open-ended and closed-ended cylinders using a stress function approach. The method of solution allows the forces on the layer interfaces to be taken into account with relative ease. Numerical results are presented for thick cylinders with isotropic and orthotropic layers, and stress distributions across the thickness are shown.

A laminated cylindrical shell of finite length under combined loads is optimized for minimum sensitivity of buckling load to variations in ply angles subject to a constraint on buckling load. The design variable is taken as the fiber orientation of individual layers. The general theory of laminated plates is employed to determine the buckling loads. The formulation includes the contribution of the shear deformation and the variation of the radius over the thickness of the shell. Numerical results are given for both thin and thick shells. The results are given for various values of the external pressure and different shell aspect ratios. It is shown that the minimum sensitivity design depends on the constraint on buckling load.

The design of hybrid symmetric laminated plates consisting of high-stiffness surface and low-stiffness core layers is presented. The maximisation of the fundamental frequency and frequency separation is performed over a discrete set of available ply angles. Minimum cost design using a hybrid construction is determined subject to a constraint on the fundamental frequencies or frequency separation. Boolean variables are introduced to specify stacking sequences and to obtain an expression for the frequency which is linear in terms of the design variables. Solution of the linear optimisation problem yields an optimal stacking sequence for the specified objective function. The effect of hybridisation is investigated for various parameters of the laminate such as the aspect ratio and the number of plies. Results are given for hybrid graphite-epoxy/glass-epoxy laminates.

A new method for determining the optimal direction and volume fraction of fibers at each point of a structure has been developed. A finite-element discretization is used. The fiber orientation and the fiber volume fraction are assumed to be constant within each element of the model, but they vary from element to element. An algorithm is obtained using variational formulations. It consists of two alternating minimizations: a local one and a global one. The algorithm is shown to be convergent. Results are presented for various laminates subjected to different kinds of edge loads.

The problem of constructing effective constitutive laws is solved for matrix composites with a physically non-linear discrete phase. The technique employed is a variant of the self-consistent method in which interaction between inclusions is described by disturbance of external field. The approach is based on generalisation of the Eshelby’s theorem. Use of a thermodynamic time scale leads to a reduction of this problem to the solution of four linear integral equations. This procedure presents the possibility of applying this averaging scheme to engineering problems of materials design. As an example the problem of designing artificial bio-composites is discussed. The material is modelled as a two-phase matrix composite with a brittle elastic matrix and visco-plastic inclusions. Dense hydroxyapatite is considered as the matrix material and recommended properties of the plastificator are derived under the condition that material should possess the mechanical properties as close to the human cortical bone as possible. The elastic constants and the yield stress for plastificator are obtained as functions of the volume concentration of phases. The application of the method to the design of artificial bio-composites is discussed.