Method and device for monitoring residual stress in glass products

Abstract

Stress in glass products arises from poor thermal conductivity and different cooling conditions of the outer and inner layers. Excessive stress levels and their uneven distribution adversely affect the mechanical and thermal strength, as well as the safety of glass products. To remove stresses, glass products undergo a controlled thermal process called annealing. After the process, it is important to determine the magnitude and distribution of remaining residual stresses. The photoelastic method based on the phenomenon of forced birefringence is used to study stresses. Measurement methods using a handheld polarimeter are sensitive to the influence of the human factor. For this reason, when the same product is inspected by different quality control employees, it is possible to obtain significantly different results. Another disadvantage is the need to break the product each time to measure the bottom thickness. The aim of this work was to replace the entirely manual stress monitoring method still used in some glassworks. This article presents an automated, non-destructive method and device for measuring residual stresses at the bottom of glass products. The developed method allows for fully automatic measurement of unit stresses (normalized to thickness) in round, open-top glass products. Automation involves performing measurements using a four-axis manipulator, a commercially available polarimeter with a polarizing camera, and a dedicated measurement system for determining the bottom thickness using the contact method. The authors describe the developed method, the selection of hardware solutions, and the method of implementation in an automatic stress monitoring device. The article presents sample measurement results in the form of color maps of stress distribution at the bottom of a glass product.