A portable scattered light polariscope SCALP has been developed, which permits measurement of the residual stress profile through the thickness of glass panels. At a glass factory strength assessment of glass panels of different thermal treatment was carried out using both residual stress measurement with SCALP and the traditional four-point bending tests. Linear correlation between the residual surface stress and the bending strength was observed. At another glass factory residual stress in glass panels was measured before performing the traditional fragmentation test. The results of the fragmentation test were extremely scattered and had almost no correlation with the values of the residual stress. It is concluded that sufficiently reliable assessment of the strength of glass panels is obtained by measuring the residual stress at the surface.

No abstract

The paper shows that the residual stress at the surface of 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 may be considerably different from the stresses in the middle part of the panels. That should be borne in mind while assessing the degree of temper by non-destructive measuring of the residual surface stress.

A new non-destructive gradient scattered light method is presented for micronscale stress profile measurement in chemically strengthened (chemically tempered, ion exchanged) glass. Direct non-destructive stress measurement in the surface layer (\100 lm) 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% KNO 3 and 20 mol% NaNO 3 ) lithium aluminosilicate glass plate to illustrate the capability of the method. Measured surface compressive stress was -1053 MPa and case depth 365 lm. 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 lm;(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.

A new optical method is presented for evaluation of the stress profile in chemically tempered (chemically strengthened) glass based on confocal detection of scattered laser beam. Theoretically, a lateral resolution of 0.2 μm and a depth resolution of 0.6 μm could be achieved by using a confocal microscope with high-NA immersion objective. The stress profile in the 250 μm thick surface layer of chemically tempered lithium aluminosilicate glass was measured with a high spatial resolution to illustrate the capability of the method. The confocal method is validated using transmission photoelastic and Na+ ion concentration profile measurement. Compositional influence on the stress-optic coefficient is calculated and discussed. Our method opens up new possibilities for three-dimensional scattered light tomography of mechanical imaging in birefringent materials.

No abstract

A recently developed nondestructive method was used to investigate the stress buildup in chemically strengthened lithium aluminosilicate glass. We utilized an updated version of the gradient scattered light method, which now enables more precise determination of the depth coordinates, recovering a more detailed stress profile around the knee. The main motivation of the work was to characterize and optimize the development of the knee-shaped breaking point in stress profile in lithium aluminosilicate glass prepared by the Saunders-Kubichan method of one-step strengthening in a mixture of KNO 3 +NaNO 3 molten salt bath. In the industry, a two-step process is still commonly used to build such a stress profile; the one-step strengthening will simplify the process as well as save the cost. Compared to previous studies, which used a destructive method based on transmitted light photoelasticity, we found that in the samples ion exchanged for 24 hours, the knee-shaped breaking points were situated two times deeper whereas the case depths were 28% shallower. The measured stress profiles were validated by stress equilibrium and by comparison to Na + ion concentration profiles. K E Y W O R D S glass, ion exchange, polarization, refractive index, scattering 2408 | HÖDEMANN Et Al. How to cite this article: Hödemann S, Valdmann A, Paemurru M, et al. Measurement of stress build-up of ion exchange strengthened lithium aluminosilicate glass.

Residual stresses in tempered glass plates can be measured with the scattered light method. Usually a rectilinear propagation of light is assumed. However, due to the gradients of the refractive index, the light rays curve. We have experimentally demonstrated this for tempered (surface stress 120 MPa and more) and annealed glass plates. For a laser beam of finite width this also results in broadening and loss of photoelastic modulation. While some features of the experimental picture (dependence of the period of modulation on depth), can be captured in the simple model of rectilinear propagation, the other observations need development of more elaborated models.