THE EFFECTS OF GAMMA-IRRADIATION ON SUBCRITICAL CRACK-GROWTH IN ALUMINA

A constant load test rig is described which allowed the time to fractu re to be measured for ceramic bars loaded in 4-point bending while exp osed to 1.5 Gy/s of Co-60 gamma-rays. Two grades of alumina, 97.5% and 99.5%, were used to compare subcritical crack growth (SCCG) under con stant load with and without exposure to gamma-radiation. Dynamic fract ure tests on 30 samples of each material were used to determine the We ibull modulus from which the critical failure stress values for the ga mma-irradiated and non-irradiated samples fractured under constant loa d could be determined and used to compare failure times. The time to f ailure for a given ratio of applied stress to critical stress was foun d to increase by a factor approximate to 9 for gamma-irradiated 97.5% alumina, but the same dose rate reduced the time to failure of the 99. 5% alumina by approximate to 2. Measurement of the length of cracks ex tending from the tensile surface of test samples showed a much higher proportion of short cracks in a gamma-irradiated, 97.5% alumina sample compared to a non-irradiated sample which had predominantly longer cr acks. The crack size distribution in gamma-irradiated 99.5% alumina sh owed a significant increase in the number of large cracks leading to a shorter time to failure. It was concluded that ionising radiation inh ibits crack growth in aluminas with silica-rich grain boundary phases. (C) 1998 Elsevier Science B.V. All rights reserved.