CRACK-PROPAGATION BEHAVIOR IN MULLITE AT HIGH-TEMPERATURES BY DOUBLE-TORSION TECHNIQUE

The double-torsion technique has been used to study subcritical crack growth in mullite at room temperature (RT) and above. The crack veloci ty was evaluated by measuring a change in crack length during a prescr ibed time under a constant applied load. Threshold values of stress in tensity factor, below which no propagation occurs, of 1.5 and 1.6 MPa m(1/2) were found at RT and high temperature, respectively. Considerin g a power law, measured stress exponents of 24, 35, 18 and 10 at RT, 1 100, 1200 and 1300 degrees C were obtained. A change of crack growth m echanisms is observed around 1200 degrees C, with cleavage crack growt h occurring below 1200 degrees C and viscous intergranular crack growt h dominating at higher temperatures. (C) 1996 Elsevier Science Limited .