Gradient scattered light method for non-destructive stress profile determination in chemically strengthened glass

A new non-destructive gradient scattered light method is presented for micron-scale stress profile measurement in chemically strengthened (chemically tempered, ion exchanged) glass. Direct non-destructive stress measurement in the surface layer (<100 µm) of chemically strengthened glass is reported for the first time. This is accomplished by passing a narrow laser beam through the surface layer of the glass at a considerably large incidence angle of 81.9°. The theory of gradient scattered light method is based on the ray tracing of ordinary and extraordinary rays in chemically strengthened glass and calculating the optical retardation distribution along the curved ray path. The experimental approach relies on recording the scattered light intensity and calculating the optical retardation distribution from it. The stress profile is measured in a chemically strengthened (8 h at 480 °C in a salt mixture of 80 mol% KNO3 and 20 mol% NaNO3) lithium aluminosilicate glass plate to illustrate the capability of the method. Measured surface compressive stress was −1053 MPa and case depth 365 µm. Method is validated with transmission photoelasticity. The method could also be used for stress profile measurement in all transparent flat materials (such as very thin thermally tempered glass slabs or polymers). Additional new applications could be: (1) enhanced version of Bradshaw’s surface layer etching method for stress profile measurement in case of ultra-thin case depths <20 µm; (2) micron-scale non-destructive tomography of layered polymeric gradient-refractive-index materials. The experimental procedure is developed to the level of full automation and the measurement time is less than 10 s.