SUBCRITICAL CRACK-PROPAGATION UNDER CYCLIC AND STATIC LOADING IN MULLITE AND MULLITE-ZIRCONIA

The subcritical crack growth behaviour, under both static and cyclic l oads, is examined in pure mullite and in a mullite-zirconia composite. It is found that in both materials the crack growth is influenced by the humidity of the testing environment. The crack growth rate depende nce on the maximum stress intensity factor in the pure mullite is the same for static and cyclic loading. The presence of monoclinic zirconi a particles in the mullite matrix in a ceramic composite enhances the crack growth resistance and produces faster crack growth rates occur u nder cyclic loading compared to static loading. These results, support ed by the microstructural examination of crack profiles and fracture s urfaces, are explained in terms of the interaction between the stress field of the zirconia particles and the cracks. (C) 1997 Elsevier Scie nce Limited.