ABSTRACT of the Doctoral Thesis

Finite Element Modelling of Discontinuously Reinforced Metal Matrix Composites

by Emmerich Weissenbek (1994)

Within the family of metal-matrix composites (MMCs), discontinuously reinforced MMCs are currently used as a good compromise between high performance and low processing costs (They are expensive in absolute terms, but less expensive than continuously reinforced MMCs). In this work, numerical modelling of inclusion based effects on the material behaviour for such MMCs is carried out.

Special attention is given to the reinforcement shape and arrangement effects, where mainly two influences are possible. The first one is due to the reinforcement particles in real MMCs and the second aspect is based on the influence of different micromechanical models simulating the behaviour of regular arrangements. The first aspect considers the material whereas the latter gives rise to artificial effects, which can lead to a decreased quality of the computed results.

Both overall behaviour and microstress fields are considered for tensile and thermal loading.

The simulation of the overall behaviour of periodic materials is briefly discussed and it is shown that a multiphase material represented by a periodic 3/D model cannot have an isotropic behaviour. Some meanfield approaches (which cannot capture the arrangement influence investigated here) are considered. Good agreement between the periodic microfield approaches (PMAs) and the meanfield methods is found for the overall elastic behaviour, but the onset of yielding is predicted at lower loads by PMA.

Different PMAs, 2/D (plane and axisymmetric) and 3/D geometries are presented. A good agreement between results from 3/D and 2/D axisymmetric models is found (except for the thermal loading of particle reinforced materials, where axial and transversal directions are represented differently), whereas plane models do not adequately represent a discontinuously reinforced material.

In the case of 3/D models for particle reinforcement, it is found that the anisotropy of face and body centered cubic arrangements is less pronounced than for simple cubic models. The inclusion shape also influences the overall behaviour. Cubes and cylinders lead to a stiffer response than spheres. In the thermal loading case, the influence of both arrangement and shape (except cylinders) is negligible.

For aligned short fiber reinforced MMCs, it is found that staggered models give more realistic results than unstaggered ones because the unstaggered models represent a composite with a layerwise arrangement (reinforced and unreinforced layers), which primarily leads to major differences in the thermal behaviour.

The influence of thermal residual stresses resulting from the production process and cyclic loading are also taken into account.

revised 951214 (hjb)