Fracture and Fatigue Behaviour of Mullite/Molybdenum Composites

The fracture and fatigue behaviour of a mullite/molybdenum composite is investigated. The attention is focused on the measurement of fracture toughness, KIc, on long through the thickness cracks by using SENB specimens, and on the growth of indentation cracks under static, monotonic and cyclic loads. Molybdenum was chosen to reinforce the mullite matrix because of the similar thermal expansion coefficients for both phases. It is essential to know and take into account the shape of the initial indentation cracks as well as the eccentricity change after extension under monotonic and cyclic stress. This study shows that, in mullite/molybdenum composites, static fatigue effects are negligible, but these composites are susceptible to mechanical degradation under cyclic loads. It is shown that the fatigue crack growth rate exhibits a high dependence on Kmax and that the fatigue sensitivity, defined as the ratio between fatigue crack growth rate threshold and KIc, is much lower than for other materials processed by powder metallurgy.