Cause of Tempered Glass Explosion

The self-exploding rate of domestic self-exploding rate varies from manufacturer to manufacturer, ranging from 3% to 0.3%. Generally, the self-exploding rate is calculated based on the number of sheets. It does not consider the area size and the glass thickness of the single glass, so it is not accurate enough and can not be compared with each other more scientifically. 

In order to unify the calculation of the blow-up rate, a unified assumption must be established. A uniform condition was set: Each 5 to 8 tons of glass contained a nickel sulfide that was sufficient to cause self-detonation; the average area of each piece of tempered glass was 1.8 mm; nickel sulfide was evenly distributed. Then calculate the 6mm thick tempered glass calculation of self-explode rate of 0.64% to 0.54%, that is 6mm tempered glass self-explosion rate of about 3 ~ 5 ‰. This is basically in line with the actual value of domestic high-level processing companies. Even if it is completely produced according to the standard, it cannot completely avoid the self-explosion of the tempered glass. Large buildings will easily use hundreds of tons of glass, which means that the presence of nickel sulphide and foreign phase impurities in the glass is very high. Therefore, despite the hot dip treatment of tempered glass, self-explosion is still inevitable.

Causes of uncontrollable self-detonation of tempered glass - The source of uncontrollable self-detonation of nickel sulphide (NiS) and heterogeneous phase grained tempered glass is not only the nis particles traditionally recognized but also many other heterogeneous phase particles. The crack initiation and propagation in the glass is mainly due to the residual stress generated near the particles. This kind of stress can be divided into two types, one is the phase change stress in the phase change expansion process, and the other is the residual stress caused by the mismatch in the thermal expansion coefficient. Nickel sulphide (nis) and heterogeneous phase particles. The interior of the glass contains nickel sulphide impurities, which exist in a state of small crystals. Under normal circumstances, it does not cause glass breakage. However, due to the reheating of the tempered glass, the phase state of nickel sulphide impurities is changed, and the high-temperature α state of nickel sulphide is quenched when the glass is cooled. When frozen, it may take up to a year before they return to beta state. Because the nickel sulfide impurity in the low temperature beta state will produce an increase in volume and local stress concentration in the glass interior, the self-detonation of the tempered glass will occur. 

However, only relatively large impurities will cause self-detonation, and tempered glass self-detonation occurs only when the impurities are in the core of tensile stress. Nis is a kind of crystal, there are two crystal phases: high temperature phase α-nis and low temperature phase β-nis, phase transition temperature is 379°C. When the glass is heated in the tempering furnace, the heating temperature is much higher than the phase transition temperature, nis All changed to alpha phase. However, in the subsequent quenching process, α-nis is too late to convert to β-nis, which is frozen in the tempered glass. At room temperature, α-nis is unstable and has a tendency to gradually change to β-nis. This transition is accompanied by a volume expansion of about 2 to 4%, which causes the glass to undergo a large phase change tensile stress, thereby causing self-explosion. Scanning electron micrographs of the nis stones extracted from the fragmented glass fragments show that the surface is undulating and very rough. Heterogeneous particles cause the self-detonation of the tempered glass, which can be seen in cross-sectional photographs of the broken glass fragments at the source. The first cracking traces caused by a spherical microparticle initiate the secondary cracked boundary region.

How to identify the self-detonation of tempered glass First of all, to see if the explosion point (the crack of the tempered glass is radial and all have the starting point) is in the middle of the glass, such as the edge of the glass, generally because the glass has not been chamfered or the edge of the glass has been damaged. Concentration of stress, cracks caused by the gradual development; such as the initiation point in the middle of the glass, see if the explosion point is composed of two small polygons similar to the two butterfly wing like pattern (butterfly spot), if carefully observe the two small polygon common side (The butterfly's torso) should have small black particles (nickel sulfide stones) that are visible to the naked eye, and it can be judged to be self-destructive; otherwise it should be destroyed by external forces. The typical characteristic of glass explosion is the butterfly spot. The glass fragments are distributed radially. The radiation center has two glass blocks shaped like butterfly wings, commonly known as "butterfly spots." The nis stones are located at the interface of two "butterfly spots".

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