Thermal Considerations

Shielding window glass is a non-crystalline, super cooled liquid in its normal state, and is very thermal sensitive, especially to rapid temperature changes. It has a linear stress-strain curve, indicating its brittle nature. As a brittle material, it will theoretically fracture at approximately 10,000 lb/in² under tensile stress, and experience compressive fracture at approximately 5 times that value. However, surface imperfections such as non-visible micro cracks, visible scratches or annealing thermal stresses, considerably reduce the theoretical surface strength of the glass. 

Thermal stress is caused by a differential in glass temperature from one point to another, within the glass. With a moderate linear thermal expansion coefficient and a very modest heat transfer coefficient, most shielding glass types experience thermal stress if the temperature change rate of the glass is greater than 2^oF/hr, and may fracture in a tension mode at higher temperature change rates.  Glass can operate at low temperatures, or very high temperatures, if the thermal change is gradual and uniform. Glasses designed specifically for rapid thermal changes or severe thermal gradients are not manufactured in the cerium stabilized condition, and will therefore "brown" in a high radiation field environment.