The double-torsion technique has been used to study subcritical crack growth in mullite at room temperature (RT) and above. The crack velocity was evaluated by measuring a change in crack length during a prescribed time under a constant applied load. Threshold values of stress intensity factor, below which no propagation occurs, of 1.5 and 1.6 Mpa m(12) were found at RT and high temperature, respectively. Considering a power law, measured stress exponents of 24, 35, 18 and 10 at RT, 1100, 1200 and 1300 °C were obtained. A change of crack growth mechanisms is observed around 1200 °C, with cleavage crack growth occurring below 1200 °C and viscous intergranular crack growth dominating at higher temperatures.

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