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The fracture properties of an ultralow-expansion glass intended for use in windows for the Space Shuttle were characterized by strength and fracture-mechanics techniques to provide reliable design data. Proof-test diagrams for predicting minimum times-to-failure under specified service loads were developed from measurements of subcritical crack growth in water and air. Failure predictions were confirmed from strength measurements in water. In vacuum (<10−4 torr), the fracture behavior was similar to that of other high-SiO2 glasses, as evidenced by the absence of subcritical crack growth and by insensitivity of the critical stress intensity factor to temperature.