Overview
Self Break of Temper Glass
Introduction
Temper Glass Sometimes Self-Breaks
A tempered glass sheet is usually made of thermally tempering process. It is a process by rapidly cooling the surface of a glass sheet heated to a temperature near its softening point. However, this temper glass sometimes breaks spontaneously even without any external force, although this is very rare. This phenomenon is called "self-destruction", "self blasting", "spontaneous break", or "self break" of the tempered glass sheet.
Principle
Self-Destruction Occurs Presumably Owing To Tiny Foreign Matter Present In A Tensile Stress Layer Existing In The Internal Portion Of The Tempered Glass Sheet
It has been reported that especially when spherical fine particles of nickel sulfide are present, the crystalline transition from α-NiS to β-NiS occurs during the use of the tempered glass sheet, and the destruction of the glass occurs because of a volume change during this transition.
It has now been found that a very small proportion of tiny foreign matter present in the tensile stress layer of a tempered glass sheet has such defects as strain or fine cracks. These defective particles become the cause of self-destruction. It is discovered that minute foreign matter of nickel sulfide, even when having no strain nor cracks at an early stage of use, produces strain or cracking in the course of use by the transition from α-NiS to β-NiS, and becomes the cause of self-destruction. It has been confirmed, on the other hand, that chromite, gas bubbles, salt bubbles, or tiny foreign matter not containing strain and cracks, which are usually contained in the sheet glass, do not lead to the self-destruction.
Solutions
No Solutions So Far Can 100% Eliminate Glass Self-Break
The tiny foreign matters which become the cause of self-destruction are very fine with a diameter of about 0.3 mm to 0.05 mm and occur in glass product with a very low probability. Therefore, it is difficult to discover it.
Since it is extremely difficult to detect a microscopic foreign matter by the inspection of all glass sheets and to remove a glass sheet containing such specific foreign matter which become the cause of self-destruction, such an inspection step cannot be incorporated in the production of tempered glass sheet.
To check the strength of tempered sheet glass, a test has been made by letting a steel ball fall from a certain height onto a glass sheet to bring it into collision with the glass sheet. This test can determine the strength of the tempered glass sheet, but does not prevent glass from future self-destruction.
After temper glass is installed as windows and doors, curtain wall or any glazing areas, it may lead to catastrophic events if self destruction occurs. Many researches have been done trying to get rid of the annoying self break, but no perfect solutions have arrived so far. Instead, a process is developed to minimize the chances of self break. It is known as heat soak test.
