Thermal activation effects in crack propagation and reliability of fused silica

The thermal activation energy u(1) characterizing changes in crack velocity with temperature for fused silica in water is determined. The determination is based on a new analysis that incorporates the familiar Arrhenius term for thermally activated processes and a term characterizing the departure of the propagating crack system from fracture equilibrium. In determining u(1) from experimental data, the Arrhenius term and the nonequilibrium term are of approximately equal magnitude. The analysis is applied to an extensive compilation of crack velocity measurements in fused silica to arrive at an estimate of u(1) = (69 +/- 11) kJ mol(-1), where the value represents the multi-laboratory mean +/- variability. This value is greater than that assumed in some earlier works and characterizes a typical within-laboratory increase in crack velocity of a factor of approximately 10(2) in fused silica between freezing and boiling water conditions and a decrease of a factor of 10(3) in component time to failure for the same change in conditions. The multi-laboratory variation in velocity at fixed temperature is comparable to the within-laboratory maximum increase, perhaps obscuring temperature effects.