GB/T 30020-2023Test method for determing defects of glass - Photoelastic scanning method (English PDF)

Foreword

This document was issued on 17 March 2023 by the State Administration for Market Regulation; Standardization Administration of the PRC and takes effect on 1 October 2023.

It is a GB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

1 Scope

GB/T 30020-2023 sets out how defects in glass are found by scanning the glass with polarized light and reading the stress field around them, covering the testing principle, the devices, the steps, the records and the report. It applies to defects that cause stress concentration in the glass. In transmission-type testing the polarized light passes through the sheet from one side and reaches the analyser on the other, so an inclusion declares itself not by its own contrast but by the pattern it produces in the glass around it. The record clause asks for the classification and the position of each defect in the scanned image and for a count of the point defects in the piece under test, and the report clause fixes what has to be stated. An informative annex applies the result to assessing the risk of spontaneous breakage in tempered glass, referring to JG/T 455. That is the reason the method exists: an inclusion a fraction of a millimetre across is invisible to inspection by eye, survives tempering, and can break a façade pane years after glazing, when replacing it means access equipment and a closed street. For float glass and tempering plants, façade contractors, and testing laboratories.

This document describes the testing principle, devices, steps, records and reports of determining

glass defects through the photoelastic scanning method.

This document is applicable to the testing of defects that cause stress concentration in

transparent glass and its products, and provides a reference for assessing the possible self-

explosion risk of tempered glass in service.

2 Normative References

This document does not have normative references.

3 Terms and Definitions

The following terms and definitions are applicable to this document.

3.1 photoelastic scanning method

Photoelastic scanning method is a method of scanning the glass under test with polarized light

and determining the location of glass defects through the stress concentration spots.

3.2 tensile stress zone of tempered glass

Tensile stress zone of tempered glass refers to a zone distributed within the range of 0.2 h ~ 0.5

h from the upper and lower glass surfaces, and the tempering stress is the tensile stress in the

thickness direction of the tempered glass.

NOTE: h is the thickness of tempered glass.

3.3 compressive stress zone of tempered glass

Compressive stress zone of tempered glass refers to a zone distributed within the range of 0.2

h from the upper and lower glass surfaces, and the tempering stress is the compressive stress in

the thickness direction of the tempered glass.

5.4 Image Analysis System

It consists of a computer, buzzer and analysis software, and shall be able to automatically

identify the image with a sudden change of stress concentration light spot caused by defects

and send out an alarm.

5.5 Digital Magnifying Glass

The magnification shall be higher than 60 times, and the depth of field shall be larger than 12

mm. It shall be able to read and identify the color and size of defects on the glass surface and

inside the glass, and the depth from the glass surface and take pictures.