Slow crack growth and fracture toughness of sapphire for a window application

The slow crack growth parameters, fracture toughness, and inert strength of the r- and a-planes of sapphire were measured in water in order to perform a life prediction on a pressurized sapphire window. The window is being considered for use in a combustion chamber on the International Space Station. Sapphire is relatively susceptible to stress corrosion in water despite a large strength in the absence of humidity. Two life prediction approaches were considered: a deterministic fracture mechanics approach and a Weibull based reliability approach. Preliminary results indicate that the window is feasible if a "short" finish is avoided. Fractography and additional predictions are being performed.