A fracture mechanics study of subcritical tensile cracking of quartz in wet environments

Load relaxation and cross-head displacement rate-change experiments have been used to establish log10 stress intensity factor (K) versus log10 crack velocity (v) diagrams for double torsion specimens, of synthetic quartz cracked on thea plane in liquid water and moist air.For crack propagation normal toz and normal tor at 20°C,KIc (the critical stress intensity factor) was found to be 0.852±0.045 MN·m−3/2 and 1.002±0.048 MN·m−3/2, respectively.Subcritical crack growth at velocities from 10−3 m·s−1 to 10−9 m·s−1 at temperatures from 20°C to 80°C is believed to be facilitated by chemical reaction between the siloxane bonds of the quartz and the water or water vapour of the environment (stress corrosion). The slopes, of isotherms in theK-v diagrams are dependent upon crystallographic orientation. The isotherms have a slope of 12±0.6 for cracking normal tor and 19.9±1.7 for cracking normal toz. The activation enthalpy for crack propagation in the former orientation in liquid water at temperatures from 20°C to 80°C is 52.5±3.8 kJ·mole−1.A discussion is presented of the characteristics of theK-v diagrams for quartz.