SUBCRITICAL FATIGUE-CRACK GROWTH IN ALUMINA .1. EFFECTS OF GRAIN-SIZE, SPECIMEN SIZE AND LOADING MODE

The effect of both grain size and specimen size upon the slow fatigue crack growth characteristics in alumina is investigated using notched double cantilever beam type specimens. The fatigue crack growth rate can be described by the usual power law, da/dN = A (K-max)(n), with a value of n between 13 and 20. Both threshold stress intensity factor and crack growth rate, for a given value of the maximum applied stress intensity factor, are influenced by the load ratio R. An anomalous ''short crack behaviour'' was observed and is discussed. From the recorded load-notch opening displacement curves, it is shown that the fatigue cracks are bridged by both frictional ligaments, which give rise to a hysteresis loop, and elastic ligaments which decrease the specimen compliance. The equivalent bridging load (the load measured at loading pins) which is required to overcome the bridging resistance is measured as a function of crack length. It is shown that the elastic ligaments make the largest contribution to the bridging forces, which are much larger for the large grain size alumina. An effect of specimen size upon the crack growth resistance was also observed.