Prediction of the fatigue of metal matrix composites using theory of cells
Discontinuous and particulate metal matrix composites have emerged as a set of materials which has found increasing niche areas of use. They are now widely used in both diesel and petrol internal combustion engines, as well as in sports bicycles and other areas where their combination of unique properties can be exploited to advantage. The inclusion of fibres into a base matrix produces a complex material both in its make up and mechanical properties and it would be an advantage to be able to predict a candidate metal matrix composite material's mechanical and thermal properties prior to that material's development. One such approach, the so called Theory of Cells, is a micromechanical approach which uses the analysis of repeating cells within the composite to make prediction of the composite's mechanical properties. In the present study, this approach has been employed to predict the fatigue life of a series of different metal matrix composites at ambient temperature. These composites include some materials with SiC fibres and some with Al203 fibres. Using data obtained from the monolithic matrix material and the individual fibres theoretical S/N and Strain/N curves were produced. This was possible by assuming that the matrix material in the composite fails at the same fatigue stress level as does the monolithic matrix material or, if fibres fail, this will be at the failure level of the individual fibres. These curves were then compared to experimental data for all metal matrix composites and good agreement was obtained for all but the low cycle fatigue regime. A finite element programme was employed to predict fatigue life in the low cycle fatigue regime and the results were compared to the predictions made by the Theory of Cells. It was found that the finite element was no better at predicting the fatigue life of the composite than the Theory of Cells. Both systems however predicted an area of high stress in front of the fibre in the direction of loading. Fatigue tests were carried out on one particular material at both 200°C and 300°C and the fatigue life was compared to that predicted by the Theory of Cells. It was found that the predictions became increasingly inaccurate with increasing temperature.