Effects of OH content, water vapor pressure, and temperature on sub-critical crack growth in phosphate glass

The effects of temperature and water vapor pressure on the rate of sub-critical crack growth in meta-phosphate laser glasses containing different OH concentrations (128 and 773 ppmw) are reported. The crack velocity was measured using the double-cleavage-drilled-compression method. When plotted as a function of stress intensity, the samples have the classic region I, II and III crack growth properties similar to that reported for silicate glasses. The glass containing the larger OH content has a 10-fold greater crack velocity in region I; crack velocities is region II are the nearly the same for both glasses. The crack velocities are analyzed using a chemical kinetic and mass-transport limited reaction rate model. At temperatures >150°C and water vapor pressures >10 mmHg, crack tip blunting is observed and the glass containing the larger OH content is more prone to blunting.