Fracture Resistance of Mullite Under Static and Cyclic Loads

Abstract

The issue of whether ceramic materials might be sensitive to mechanical degradation by fluctuating loads has been addressed in many investigations in the last ten years. From these studies it is known that in many polycrystalline ceramics subcritical crack extension takes place under cyclic loads, leading to complete failure at stresses which are considerably smaller than their fracture resistance under monotonic loading. However, the question of how much of the observed crack growth under fluctuating loads is either just a consequence of slow crack growth induced by the influence of the environment or an intrinsic fatigue effect (cyclic fatigue) remains to be elucidated for some specific ceramic materials. Mullite (3Al{sub 2}O{sub 3}-2SiO{sub 2}) is one of the most common components of traditional ceramic materials. It is also considered an appealing material for high temperature applications because of its very good chemical, thermal and mechanical properties over a broad temperature range. However, mullite exhibits a relatively low fracture toughness at room temperature (about 2.3 MPa m{sup 1/2}) mainly because no significant toughening mechanism appears to be present in this material. In this investigation the mechanical behavior of mullite under static and cyclic loading for four different microstructures is studied, in termsmore » of grain size distribution, with specimens produced from two different commercial powders. The work was addressed to document the existence of cyclic fatigue by measuring fatigue life under static and cyclic loading, since no information was available on this topic for this particular material.« less