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 c an be described by the usual power law, da/dN = A (K-max)(n), with a v alue of n between 13 and 20. Both threshold stress intensity factor an d crack growth rate, for a given value of the maximum applied stress i ntensity factor, are influenced by the load ratio R. An anomalous ''sh ort 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 com pliance. The equivalent bridging load (the load measured at loading pi ns) 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 l arger for the large grain size alumina. An effect of specimen size upo n the crack growth resistance was also observed.