Siim Hödemann will defend his PhD thesis "Residual stress determination in chemically strengthened and thermally tempered glass plates using scattered light method" (materials science) on 1 March 2019.
Prof. Jaak Kikas, University of Tartu, Estonia
Assistant Prof. Eric R. Tkaczyk, Vanderbilt University Medical Center and Vanderbilt University, USA
Prof. Vincenzo M. Sglavo, Department of Materials Science and Technology, University of Trento, Italy
A new optical method for evaluation of the stress profile in chemically strengthened glass based on confocal detection of the scattered laser beam is presented. Confocal microscopy allows scanning within the light beam and collecting Rayleigh-scattered light from a microscopic volume. Estonian company GlasStress Ltd produces SCAttered Light Polariscope SCALP-05 which is a unique device in the world that determines the strength of tempered glass (for example, Gorilla glass used on phone screens, tempered window glasses, sauna doors) very quickly and without damage to the glass. For this purpose, the apparatus is placed on a glass with a super-thin layer of oil, the strength of which is known. SCALP, which uses photoelasticity of scattered light, responds to glass strength in four seconds. The developed algorithm eliminates the influence of light ray bending in the makes the measurement result up to ~25% more accurate. A linear correlation was found between surface compression measured with oblique incidence scattered light method and breaking strength measured with traditional four-point bending tests. Stress inhomogeneity was also investigated. It has been shown that surface compression of thermally tempered glass panels may vary both locally (at a distance equal to the distance between the cooling jets) and globally, i.e., stresses near the edges and corners of the panels differ significantly from the stresses in the middle part of the panels.