Structural integrity of engineering components made of functionally graded materials

Abstract

Functionally graded materials (FGM) are composite materials with microstructure gradation optimized for the functioning of engineering components. For the case of fibrous composites, the fibre density is varied spatially, leading to variable material properties tailored to specific optimization requirements. There is an increasing demand for the use of such intelligent materials in space and aircraft industries. The current preferred methods to study engineering components made of FGM are mainly modelling particularly those that are finite element (FE) based as experimental methods have not yet sufficiently matured. Hence this thesis reports the development of a new Mindlin-type element and new Reissner-type element for the FE modelling of functionally graded composite (FGC) structures subjected to various loadings such as tensile loading, in-plane bending and out-of-plane bending, buckling and free vibration. The Mindlin-type element formulation is based on averaging of transverse shear distribution over plate thickness using Lagrangian interpolation. Two types of Mindlintype element were developed in this report. The properties of the first Mindlin-type element (i.e. Average Mindlin-type element) are computed by using an average fibre distribution technique which averages the macro-mechanical properties over each element. The properties of the second Mindlin-type element (i.e. Smooth Mindlin-type element) are computed by using a smooth fibre distribution technique, which directly uses the macro-mechanical properties at Gaussian quadrature points of each element. The Reissner-type element formulation is based on parabolic transverse shear distribution over plate thickness using Lagrangian and Hermitian interpolation. Two types of Reissner-type element were developed in this report, which include the Average and Smooth Reissner-type elements. There were two types of non-linearity considered in the modelling of the composite structures, which include finite strain and material degradation. The composite structures considered in this paper are functionally graded in a single direction only, but the FE code developed is capable of analysing composite structures with multidirectional functional gradation. This study was able to show that the structural integrity enhancement and strength maximisation of composite structures are achievable through functional gradation of material properties over the composite structures.