Mechanically prestressed windows

A method was developed for mechanically prestressing glass used in spacecraft windows so that surface imperfections of the glass will not propagate under stress. A metal ring, in tension, is placed around the periphery of the glass where the interference fit between the ring and glass fixes the glass in a state of uniform compression. When the glass is subjected to a pressure loading, the compressive prestress prevents tension loading of the outer surface and, therefore, eliminates the tendency for propagation of surface imperfections.