Abstract The birefringent properties of glass can be well utilized for non-destructive testing of thermally tempered glass. The surface compression stress as well as the compression zone depth of thermally tempered glass is commonly measured with a scattered light polariscope. The measurement with scattered light polariscope provides information about stresses acting perpendicular to the measurement direction. Therefore, the measurement depends on the direction. In order to make a statement about the rough level of the prestress, a measurement in one direction can be sufficient, assuming an isotropic stress state and a relatively homogeneous distribution of the surface compression stress. However, in order to be able to make a more detailed statement about a non-isotropic stress state, measurements in up to four directions are necessary. Information about the direction of principal stresses and differences of principal stresses can then be evaluated. In the present work, it is investigated which deviations can be expected by measurements performed in only one direction or by measurements using up to four directions. For this purpose, five methods are introduced, which deviate in the assumptions made for the current stress state. The methods are compared using measurements on thermally tempered glass in a four-point bending test and on thermally tempered glass specimens without any external load. 

https://doi.org/10.1007/s40940-022-00184-y

Evaluation methods for surface compression stress measurements with unknown principle stress directions

In der Praxis wird der Abstand vom Bohrungsrand zur Glaskante nach DIN 18008‐3 bei punktgehaltenen Verglasungen häufig unterschritten, sodass nach Norm nur die Festigkeit des Basisglases angesetzt werden darf. Gemäß aktuellen Untersuchungen werden die Verteilung der Eigenspannungen und die charakteristische Biegefestigkeit bei Verringerung des Randabstandes zur Bohrung nicht relevant beeinträchtigt. Dies kann auch anhand eines Praxisbeispiels einer Aufzugverglasung aus ESG mit randnahen Bohrungen verifiziert werden. Hier wird mit spannungsoptischen Messungen die Vorspannqualität des Glases untersucht. Abschließend werden Bruchspannungen aus verschiedenen zerstörenden Versuchen an randnahen Bohrungen miteinander verglichen.

Punktgehaltene Gläser mit geringem Bohrungsrandabstand

In the present paper, optical anisotropy effects in architectural glass are evaluated using digital image processing. Hereby, thermally toughened glass panes were analyzed quantitatively using a circular polariscope. Glass subjected to externally applied stresses or residual stresses becomes birefringent. Polarized light on birefringent materials causes interference colors (iridescence), referred to as anisotropies, which affect the optical appearance of glass panes in building envelopes. Thermally toughened glass, such as toughened safety glass or heat strengthened glass, show these iridescences due to thermally induced residual stress differences. RGB-photoelastic full-field methods allow the quantitative measurement of anisotropies, since the occurring interference colors are related to the measured retardation values. By calibrating the circular polariscope, retardation images of thermally toughened glass panes are generated from non-directional isochromatic images using computer algorithms. The analysis of the retardation images and the evaluation of the anisotropy quality of the glass is of great interest in order to detect and sort out very low quality glass panes directly in the production process. Therefore, in this paper retardation images are acquired from different thermally toughened glass panes then different image processing methods are presented and applied. It is shown that a general definition of exclusion zones, e.g. near edges is required prior to the evaluation. In parallel, the limitations in the application of first-order statistical and threshold methods are presented. The intend of the investigation is the extension of the texture analysis based on the generation of Grey Level Co-occurrence Matrices, where the spatial arrangement of the retardation values is considered in the evaluation. For the first time, the results of textural features of different glass pane formats could be compared using reference areas and geometry factors. By reduction of the original image size, the computation time of textural analysis algorithms could be remarkably speeded up, while the textural features remained the same. Finally, the knowledge gained from these investigations is used to determine uniform texture features, which also includes the pattern of anisotropy effects in the evaluation of thermally toughened glass. Together with a global evaluation criterion this can now be implemented in commercial anisotropy measurement systems for quality control of tempered architectural glass.

/pdf/digital-image-processing-methods-for-the-evaluation-of-54jj6gleji.pdf

C. Schuler

Papers

Digital image processing methods for the evaluation of optical anisotropy effects in tempered architectural glass using photoelastic measurements