Digital photoelasticity of glass: A comprehensive review

Surface stress meters utilising the optical waveguide effect of chemically tempered glasses

An apparatus used directly on a flat sheet of transparent material, especially tempered glass, for simultaneous measurement of sign and amplitude of stress across the section of said glass sheet using scattered light. The apparatus allows reliable and repetitive measurements independently of the operator's subjective observations, since the data is read through an automatic system. The apparatus has a Helium-Neon (He-Ne) laser source of 1 to 5 mW, whose light beam is linearly polarized, then modulated with a series of wave plates (2,2', 2"), then passed through a collimator (3) and subsequently made to strike a prism (4) optically coupled with the glass sheet (V) being tested. Variation in intensity of the interference fringes is revealed by a TV camera (5) having a macrophotographic lens system (5') which focusses the scattered light on a solid-state array photodetector (5'). The photosensitive zone of the detector is positioned to detect intensity of the scattered light along path (BC) of the laser beam through the section of the glass sheet (V). An electronic circuit can thus convert the video signals transmitted by TV camera ( 5).

Apparatus for automatic measurement of stress in a transparent body by means of scattered light

The introduction of gas lasers has greatly increased the number of biascope applications. It has been found that optical waveguide effects caused by the refractive index gradient affected the biascope pattern but, provided that stress birefringence was distinguished, by analyser rotation, from intrinsic refractive index differences between guided modes, and that the birefringence and refractive index values were dissimilar, surface stress determination was possible. Birefringence and refraction patterns were recorded and compared for a variety of glasses.

Laser biascope for surface stress measurement of tempered glasses

We performed depth analysis of a compression layer in Corning Gorilla Glass 3, one of commercialized chemically-strengthened glasses, using depth-resolved micro-Raman spectroscopy. We obtained a depth variation of Raman spectra and an ion-exchange rate of Na for K was determined by energy dispersive X-ray spectroscopy. We found that a peak position around 1100 cm11 in the Raman spectra is shifted to higher wavenumber and the ion-exchange rate of Na for K increases with a decreasing depth when shallower than 330 ̄m. This correlation can be qualitatively explained as follows: compression of a TO2 tetrahedra network (T = Si or Al) induced by the ion exchange gives rise to an increase in frequency of vibrational modes of the TO4 tetrahedra. ©2016 The Ceramic Society of Japan. All rights reserved.

https://www.jstage.jst.go.jp/article/jcersj2/124/10/124_16138/_pdf

Depth analysis of a compression layer in chemically strengthened glass using depth-resolved micro-Raman spectroscopy

A critical ray was excited along the glass surface by laser beam incidence. Part of the critical ray was refracted during propagation. The angle of refraction depended on the refractive index of the glass sirface. When the glass surface was birefringent due to the photoelastic effect, the angles of refraction differed from each other between two linearly polarized beams. This difference allows surface stress determination.

Thermally tempered glass surface stress measurement by critical ray

Sheet and plate glasses have refractive index inhomogeneity parallel with their surfaces. Refractive index maxima near the surfaces behave as optical waveguides. Light waves can be excited in the guides from outside and can be extracted non-destructively, using the technique popular in optical integrated circuits. Stress birefringence, caused by surface stress in thermally tempered glasses, gives different effective indices for two linearly polarized light beams which vibrate in directions vertical to and in parallel with the surfaces. Effective index observation by two polarized light beams allows non-destructive surface stress determination.

Optical waveguide effect of thermally tampered sheet and plate glasses—application to surface stress measurements

The gas laser-Senarmont compensator-biascope combination has enabled non-destructive measurement of residual surface stresses in float glasses using the optical waveguide effect in tin-diffused surface layers. The biascope can detect surface stress as low as 10 kg cm -2 . Annealing and elastic deformation effects in glasses are clearly identified.

Laser Sénarmont biascope for residual surface stress measurement of float glasses

Graded-index planar optical waveguides and surface compression layers were formed simultaneously in the surface layer of glass plates by ion exchange. A change in the refractometer patterns was observed. Before ion exchange, only a critical ray fringe was observed, but with ion exchange a guided wave fringe appeared on the high effective refractive index side of the ‘critical ray’ fringe, and the number of guided wave fringes increased. The guided wave fringe or fringes were birefringent, whereas the ‘critical ray’ fringe was kept nonbirefringent. It was concluded that the ‘critical ray’ propagated along the bottom of a waveguide, ie at the foot of the refractive index distribution.

T. Kishii

Papers

Thermally tempered glass surface stress measurement by critical ray

Laser biascope for surface stress measurement of tempered glasses

Optical waveguide effect of thermally tampered sheet and plate glasses—application to surface stress measurements

Laser Sénarmont biascope for residual surface stress measurement of float glasses

Critical ray-like propagation modes along graded-index planar optical waveguides