The magnetophotoelastic analysis of residual stresses in thermally toughened glass

The determination of individual stress values in the multiaxial residual st ress distributions found in toughened glass is a problem of major practical importance in the glass industry. In spite of many years of development, c onventional experimental stress analysis techniques have not been able to p rovide a generally applicable means of obtaining this information. However, a unique method has now been developed for this purpose, based on the prin ciples of magnetophotoelasticity, in which the combined optical retardation effects due to (i) the stress-birefringence of the material and (ii) a sup erimposed magnetic field, are utilized. The paper summarizes the basic theory of magnetophotoelasticity and describ es experimental verification work using simple uniaxial systems of constant and linearly varying stress. Two important developments follow. First, a s olution technique is developed whereby principal stress differences can be determined at any position through the thickness of a toughened glass plate , for an assumed biaxial residual stress distribution consisting of a combi nation of parabolic and linear stress variations. Second, an oblique incide nce technique is employed to obtain individual stresses from the previously determined principal stress differences. A polariscope, designed and built for the implementation of the technique a t any position over the area of an automotive windscreen, is described. Typ ical experimental results are presented and shown to compare well with the limited information available from conventional techniques.