Fracture phenomenology of a lithium-aluminium-silicate glass-ceramic

Crack propagation mechanisms in a lithium-aluminium-silicate glass-ceramic have been studied as a function of both initial flaw size and temperature. Using controlled surface cracks, the fracture stress at room temperature was found to conform to the Griffith flaw size dependence. Extrapolation suggests that processing flaws of the order of 6 to 8 m are strength-controlling in the material investigated. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. Up to 900° C, catastrophic transgranular crack propagation occurred from emplaced cracks. At 1000° C and above, subcritical crack growth occurred intergranularly and the extent of slow crack growth prior to catastrophic failure increased with increasing temperature. The influence of loading rate on slow crack growth and fracture stress was explored at 950 and 1100° C. Generally, the extent of slow crack growth decreased with increased loading rate until at a sufficiently high rate, catastrophic fracture occurred directly with no slow crack extension. These results are discussed in terms of the role of plastic accommodation in the crack extension process, a phenomenon which seems mechanistically dependent upon remnant amorphous (uncrystallized) phase at grain boundaries.