Self-explosion and its classification Tempered glass self-explosion can be expressed as the phenomenon that the tempered glass breaks automatically without external direct action. In the process of tempering processing, storage, transportation, installation, use, etc., the tempered glass may explode. The self-detonation can be divided into two types according to different causes: one: it is caused by visible defects in the glass, such as stones, sand, bubbles, inclusions, notches, scratches, bursts, etc.; two: it is caused by nickel sulfide in the glass (NiS) Self-detonation caused by the expansion of impurities. These are two different types of self-exploitation, which should be clearly classified, treated differently, and treated and handled in different ways. The former is generally visible and relatively easy to detect, so it can be controlled in production. The latter is mainly caused by the volume expansion of tiny nickel sulfide particles in the glass, which cannot be visually inspected, so it is uncontrollable. In terms of actual operation and handling, the former can generally be removed before installation, and the latter continues to exist because it cannot be inspected, becoming the main factor for the self-detonation of the tempered glass in use. The replacement of nickel sulfide after self-detonation is difficult, and the processing cost is high. At the same time, it will be accompanied by large-quality complaints and economic losses, causing dissatisfaction and even more serious consequences for the owner. Therefore, the self-detonation caused by nickel sulfide is the focus of our discussion.
2. Self-explosion mechanism of tempered glass
The expansion of nickel sulfide inside the tempered glass is the main cause of the self-explosion of the tempered glass. After the glass is tempered, the surface layer forms compressive stress. The inner core layer exhibits tensile stress, and compressive stress and tensile stress together form a balanced body.
The glass itself is a brittle material, which is resistant to compression but not tensile, so most of the glass breakage is caused by tensile stress. When the phase change of the nickel sulfide crystal in the tempered glass occurs, its volume expands. The expansion of the nickel sulfide in the tensile stress layer of the glass core causes greater tensile stress inside the tempered glass. When the tensile stress exceeds the limit of the glass itself, This will cause the tempered glass to explode.
Therefore, the glass processing industry at home and abroad unanimously believes that nickel sulfide is the main reason for the self-explosion of tempered glass. There are some other factors for the self-explosion of tempered glass: unreasonable glass slotting and drilling, poor quality of the original glass, uneven thickness such as embossed glass, and uneven stress distribution such as bent tempered glass and regional tempered glass.
Commissioned by the Ministry of Construction, the Curtain Wall Engineering Committee of the China Building Decoration Association conducted an investigation on the safety of existing curtain walls in 10 cities including Beijing, Shanghai, Tianjin, Chongqing, Xi’an, Wuhan, Shenzhen, Harbin, Xiamen, and Wenzhou. On the basis of self-inspection and self-inspection in 10 cities, 120 existing building curtain wall projects recommended by the urban construction administrative department were provided. In this investigation, 437 pieces of curtain wall glass were damaged. There were 17 investigations on the all-glass curtain wall, 10 of which found that large glass shattered, a total of 68 pieces, 3 pieces of glass ribs were broken, and many glass curtain walls were found without rib glass. There are 180 air leaks in insulating glass, and the phenomenon of peeling off of coated glass is also relatively high in some cities. Nine curtain wall projects with important hidden dangers were found in the investigation, accounting for 9.38% of the total number of investigation projects. If the tempered glass is removed by self-explosion, the proportion drops to 2.3%. The use of tempered glass for curtain walls, doors, and windows has resulted in a high number of glass breakage accidents in glass curtain walls and doors and windows. It is urgent to change this situation.
(2) Characteristics of uncontrollable self-explosion of tempered glass
The problem of self-explosion of toughened glass is unclear, and the responsibility is difficult to understand. There is no certainty about the time of self-detonation. It may be just after it is released, or it maybe 1-2 months after leaving the factory, and it may be 1-2 years after leaving the factory. It may cause 4-5 years after the production of the tempered glass.
The domestic explosion rate varies from manufacturer to manufacturer, ranging from 3% to 0.3%. Generally, the self-detonation rate is calculated in units of pieces. The area and thickness of a single piece of glass are not taken into account, so it is not accurate enough, and a more scientific comparison can not be made. In order to measure the self-detonation rate uniformly, a unified assumption must be determined. Uniform conditions are determined: every 5 to 8 tons of glass contains enough nickel sulfide to initiate self-detonation; the area of each piece of tempered glass is 1.8m2 on average; nickel sulfide is evenly distributed. Then the calculated self-detonation rate of 6mm thick tempered glass is 0.64% to 0.54%, that is, the self-detonation rate of 6mm tempered glass is about 3‰ to 5‰. This is basically consistent with the actual value of domestic high-level processing enterprises.
Even if it is completely produced according to the standard, it cannot completely prevent the tempered glass from exploding. Large buildings can easily use hundreds of tons of glass, which means that there is a high probability of nickel sulfide and heterogeneous phase impurities in the glass, so although the tempered glass is hot-dipped, self-explosion is still inevitable. 4. The tempered glass is not controllable Causes of self-explosion The source of uncontrolled self-explosion of tempered glass is not only nis particles in traditional understanding but also many other heterogeneous particles. The crack initiation and propagation in the glass are mainly due to the residual stress generated near the particles. This type of stress can be divided into two types, one is the phase change stress during the phase expansion and the other is the residual stress caused by the mismatch of the thermal expansion coefficients. Nickel sulfide (nis) and heterogeneous particles. The glass contains nickel sulfide impurities and exists in the state of small crystals. Under normal circumstances, it will not cause glass damage. However, due to the reheating of the tempered glass, the phase state of nickel sulfide impurities is changed. The high-temperature alpha state of nickel sulfide is when the glass is rapidly cooled. If they are frozen, it may take several years for them to return to the beta state. Due to the low-temperature beta state nickel sulfide impurities, the volume will increase, and local stress concentration will occur inside the glass. At this time, the tempered glass will explode. However, only relatively large impurities will cause self-detonation, and the self-detonation of tempered glass can only occur when the impurities are at the core of the tensile stress.
The heterogeneous particles cause the tempered glass to explode. It can be seen in the cross-sectional photo of the glass fragments at the source of the fracture. The boundary area between the first cracking trace and the second fragmentation caused by a spherical fine particle.
(3) How to identify the self-explosion of tempered glass
First look at whether the initiation point (the tempered glass crack is radial and has a starting point) is in the middle of the glass, such as at the edge of the glass, generally because the glass has not been chamfered or damaged or the edge of the glass is damaged, causing stress concentration and cracks gradually developing Caused by; if the initiation point is in the middle of the glass, see if the initiation point has two small polygons similar to the pattern of two butterfly wings (butterfly spots), if you carefully observe the common edges of the two small polygons (the body of the butterfly) There should be small black particles (nickel sulfide stones) visible to the naked eye, which can be judged to be self-explosive; otherwise, it should be destroyed by an external force. The typical characteristic of glass self-explosion is the butterfly spot. The glass fragments are distributed in a radial pattern. There are two glass blocks in the center of the radiation shaped like butterfly wings, commonly known as “butterfly spots”. The self-explosion of tempered glass is an important issue that urgently needs to be solved for the safety of glass curtain walls. But for the concept of safety glass, the traditional concept is that (full) tempered glass belongs to safety glass. In addition to its high strength, the main reason is that when the (full) tempered glass is broken, the whole glass will be broken into small honeycomb obtuse angle particles, which is not easy to hurt people. Through this investigation and numerous accident practices, this concept was questioned. It is necessary to discuss the use of safety glass on the glass curtain wall of high-rise buildings. For the safety glass used in the glass curtain wall of high-rise buildings, the main concern of safety is that the glass breaks and falls and hurts people. There should be three requirements here:
First, the glass has sufficient strength so that it does not break under the design load.
Secondly, if the glass breaks, it should have anti-crushing and scattering properties, so that it will not fall and fly when it is in a broken state.
The third is sufficient fracture toughness k1c. 2. The (full) tempered glass has two safety factors: high strength and its destruction form are obtuse small particles. However, it does not have anti-shattering and scatters ability, which is a key safety factor for high-rise building glass curtain walls, and the resulting unsafe consequences. (Full) The tempered glass shattered large groups of fragments with obtuse angles scattered from high altitude Even if the particles are small, the speed is already very large, which can also hurt people. One of the culprits is the acceleration of gravity in free fall. Regardless of the shape of the glass fragments on the glass curtain wall of high-rise buildings, such as the high-rise buildings scattered down, it is dangerous or even fatal. In addition, (full) tempered glass self-detonation damage has no aura, and there is currently no effective method for complete prevention. It is a cancer of glass curtain walls, glass self-detonation breaks up and scattered at high altitudes. It is not safe to use (full) tempered glass for high-rise building glass curtain walls.
Security is a relative concept and is conditional; it is not absolute and unconditional. Without the use conditions, it is not comprehensive to define the glass curtain wall safety glass only from the shape of its fragments. Toughened glass is not broken, but the particles of glass are small, but the fragments are easy to fall and splash and cause accidents. Therefore, In many foreign glass curtain wall technical standards and specifications, it is clear that a single piece of tempered glass should not be used for glass curtain walls, and anti-shatter glass should be used. (Full) tempered glass is used on the glass curtain wall of high-rise buildings in Japan, and a layer of the anti-scatter film must be added to ensure safety.
“Strong but not broken, broken and not scattered”, anti-scattering glass is the safety glass used in the glass curtain wall.
Post time: 2020-06-30