The effects of γ-irradiation on subcritical crack growth in alumina

Abstract

A constant load test rig is described which allowed the time to fracture to be measured for ceramic bars loaded in 4-point bending while exposed to 1.5 Gy/s of 60Co γ-rays. Two grades of alumina, 97.5% and 99.5%, were used to compare subcritical crack growth (SCCG) under constant load with and without exposure to γ-radiation. Dynamic fracture tests on 30 samples of each material were used to determine the Weibull modulus from which the critical failure stress values for the γ-irradiated and non-irradiated samples fractured under constant load could be determined and used to compare failure times. The time to failure for a given ratio of applied stress to critical stress was found to increase by a factor ≈9 for γ-irradiated 97.5% alumina, but the same dose rate reduced the time to failure of the 99.5% alumina by ≈2. Measurement of the length of cracks extending from the tensile surface of test samples showed a much higher proportion of short cracks in a γ-irradiated, 97.5% alumina sample compared to a non-irradiated sample which had predominantly longer cracks. The crack size distribution in γ-irradiated 99.5% alumina showed a significant increase in the number of large cracks leading to a shorter time to failure. It was concluded that ionising radiation inhibits crack growth in aluminas with silica-rich grain boundary phases.