Viscoelastic effects in testing of an Al2O3 ceramic containing a glassy phase

An alumina with 3 wt% glassy phase was tested at different loading rates at two temperatures (900 and 1000 °C). It was found that an increase in fracture toughness was accompanied by a decrease of the bending strength at the same loading rates. A model is given, which describes the experimental results by linear viscoelasticity of the second phase. Whereas the bulk properties are mainly due to the alumina grains and, therefore, remain nearly unchanged, the crack growth and the fracture behaviour in the intergranular regions is dominated by the viscosity of the glassy phase. This leads to a non-unique value ofK _Ic, which is dependent on the temperature and the loading rate.