FRACTURE RESISTANCE OF MULLITE UNDER STATIC AND CYCLIC LOADS D. Casellas,* C. Baudin,** M. Osendi,** L. Llanes* and M. Anglada* *Universitat Polite `cnica de Catalunya, E.T.S.E.I.B. Dept. de Cie `ncia dels Materials i Enginyeria Metal.lu ´rgica, Avda. Diagonal 647, 08028 Barcelona, Spain **Instituto de Cera ´mica y Vidrio, CSIC, 28500 Arganda de Rey, Madrid, Spain (Received August 12, 1997) (Accepted September 25, 1997) Introduction 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 (see reference (1) for a review). 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. Crack growth rates under static and cyclic loads can be analysed in ceramics by comparing the values obtained under the same maximum stress intensity factor per cycle but with different amounts of unloading, i.e., different values of R (ratio between the minimum and maximum stress per cycle). Then, if cyclic crack growth rates for small R values are much larger than those obtained under static loading (R value of 1), there is a clear indication of the existence of a cyclic fatigue effect. However, when the differences are small, cyclic fatigue effects are not evident and a more detailed examination is needed in order to assess its importance. Furthermore, since it is well known that in ceramics with R-curve behaviour long cracks do not show the same behaviour as small natural cracks, it may be more useful to compare fatigue lives of specimens with controlled small cracks under static and cyclic loading than to do it on the basis of crack growth rates of long cracks. The real mechanism of cyclic fatigue for most advanced ceramics seems to be the degradation of their toughening mechanisms (2). In ceramics that exhibit toughening by extrinsic mechanisms, the increase in toughness is achieved through a rising resistance curve, i.e., the driving force for crack growth decreases with crack extension as closure tractions develop in the crack wake and locally shield the crack. Cyclic fatigue effects are then associated with the degradation of the wake toughening mechanisms under fluctuating stresses, which allows the crack to extend under smaller crack driving forces (2). However, the details of the mechanisms of degradation are still unclear in some materials. Mullite (3Al 2 O 3 -2SiO 2 ) is one of the most common components of traditional ceramic materials (3). 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 (4 –7). However, mullite Pergamon Scripta Materialia, Vol. 38, No. 1, pp. 39 – 44, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 + .00 PII S1359-6462(97)00434-X 39