High Temperature Impact Fracture Toughness Testing of Alumina and Alumina-Zirconia Composite

Impact fracture toughness KId tests were carried out on a alumina and alumina-zirconia composite at temperatures up to 1200°C. The one-point-bend impact tester was used and the specimens were pre-cracked by the bridge indentation method. A simple formula was employed for evaluating the dynamic stress intensity factor KI(t) from the measured impact force. In order to examine the accuracy of this KId measuring system, the natural angular frequency of the pre-cracked specimen was measured by using a strain gage and a FFT analyzer, since it has a significant effect on evaluating KI(t). The measured values were found to be in good agreement with those calculated from the simple formula based on the Euler-Bernoulli beam theory. It is concluded that the natural angular frequency can be precisely estimated by the formula. The existence of the contact force acting on the crack surfaces was confirmed by comparing the static load-strain curves of the pre-cracked specimens with those of the notched specimens. The effect of contact force on the measured KId value was discussed in relation to the test conditions such as loading rate, specimen size and time-to-fracture. It was experimentally demonstrated that the effect of contact force was negligible in the present test conditions. It was found that the fracture toughness values of alumina and alumina-zirconia composite increased with the loading rate, and the KId of alumina was almost constant at temperatures up to 1200°C, while the KId of alumina-zirconia composite decreased with test temperature.