The Sandwich Seal In The Development And Control Of Sealing Glasses

The sandwich seal is discussed as a precise indicator of expansion mismatch between glass and other sealing materials. The basic mismatch equation is experimentally verified. Factors that are responsible for high precision in mismatch evaluation are (a) uniform stress in the centre glass member used for photoelastic analysis and (b) large retardation per unit strain. Effects due to annealing variations are discussed with experimental examples. It is felt that certain characteristics of the butt seal and the cylindrical bead seal present both practical and experimental difficulties, which the sandwich seal overcomes.