The automatic explosion of tempered glass under the action of no direct mechanical external force is called self-explosion of tempered glass. According to industry experience, the self-explosion rate of ordinary tempered glass is about 1~3‰. Self-explosion is one of the inherent characteristics of tempered glass.
There are many reasons for expanding the self-explosion, and simply summarize the following:
1 glass quality defects
A. There are stones and impurities in the glass. Bubbles: Impurities in the glass are the weak points of the tempered glass, and also the stress concentration. In particular, if the stone is in the tensile stress zone of the tempered glass, it is an important factor leading to the cracking.
The stone is present in the glass and has a different coefficient of expansion than the glass body. The stress concentration in the crack region around the stone after the tempering is multiplied. When the coefficient of expansion of the stone is smaller than that of the glass, the tangential stress around the stone is in a tension state. Crack propagation that occurs with stones is highly prone to occur.
B. The glass contains nickel sulfide crystals.
Nickel sulfide inclusions are generally present as crystalline small spheres having a diameter of from 0.1 to 2 mm. The appearance is metallic, and these inclusions are NI3S2, NI7S6 and NI-XS, where X=0-0.07. Only the NI1-XS phase is the main cause of spontaneous smashing of tempered glass.
The theoretical NIS is known at 379. At C, there is a phase transition process, which is accompanied by a volume expansion of 2.38% from the a-NIS hexagonal system in the high temperature state to the B-NI trigonal system in the low temperature state. This structure is preserved at room temperature. If the glass is heated in the future, the a-B state transition may occur rapidly. If these impurities are inside the tensile stress of the tempered glass, the volume expansion causes spontaneous bursting. If a-NIS exists at room temperature, it will slowly change to the B state after several years and months. The slow increase in volume during this phase change does not necessarily cause internal cracking.
C. The surface of the glass is scratched, burred, deep blasted, etc. due to improper processing or operation, which may cause stress concentration or self-explosion of tempered glass.
2 The stress distribution in the tempered glass is uneven and offset.
The temperature gradient generated by the glass in the direction of the thickness of the glass when heated or cooled is uneven and asymmetrical. The tempered products have a tendency to self-explosion, and some produce "wind explosion" when chilled. The tempered glass forms a self-explosion if the tensile stress zone is offset to one side of the article or offset to the surface.
3 The influence of the degree of tempering, the experiment proves that when the degree of tempering is increased to 1 level/cm, the self-explosion number is 20-25%. It can be seen that the greater the stress, the higher the degree of tempering and the larger the self-explosion.