Research status of self ignition of tempered glass in China
With the development of society and the progress of science and technology, glass is more and more widely used in people's daily work and life. In order to improve the safety and strength of glass, tempered glass is widely used in automobile windshield, building curtain wall, furniture and stoves and other products. However, as glass itself is a brittle material, its tensile strength is far lower than the compressive strength, there is almost no plastic deformation in the process of fracture, failure is often sudden and catastrophic. So far, there are no effective ways to prevent sudden breakage of glass products in the world. The explosion of automotive glass, the breakage of building curtain wall glass, the sudden burst of bathroom glass, the breakage of tempered glass coffee table and cooking utensils are still being reported. In busy urban areas, tempered glass has become a "time bomb", especially for glass curtain walls suspended in high-rise buildings, any glass wall rupture or fall accident can cause catastrophic consequences. In recent years, there are more and more hidden dangers of curtain wall glass, which has attracted the attention of experts and governments at home and abroad [1-5].
China has the largest number of glass curtain walls in the world (more than half of the world total), and the safety of glass curtain walls can not be ignored. By the end of 2004, about 110 million square meters of curtain walls (including lighting roofs) have been built in China, accounting for more than 50% of the world's total, of which open and hidden frames account for about 60% of all curtain walls. In recent years, glass curtain wall ruptures have occurred frequently. For example, Shanghai Daily reported on August 2, 2006 that a piece of glass suddenly fell on the 36-storey glass curtain wall outside Shanghai Jinjiang International Shopping Mall, splashing glass fragments onto the sidewalks and roads about 40 square meters in front of the building. On June 20, 2006, Liaoning Evening News reported that a glass curtain wall on the 20th floor of a tall building in Dalian fell and 16 people were injured by debris. In June 2006, another tragic incident occurred in Shenzhen, where a glass curtain wall crashed and killed a primary school boy. On September 8, 2006, a glass curtain wall of a building in Shanxi Road, Nanjing, crashed from the 13 floor and injured 6 pedestrians. On the night of July 31, 2006, the glass curtain wall of CITIC Taifu Building in Shanghai burst and a 75-minute "glass rain" broke out. Two people were injured.
Obviously, understanding the real cause and mechanism of tempered glass self-explosion is very important to reduce and prevent accidents. It is generally believed that the initiation of glass spontaneous explosion can be divided into two types: one is caused by visible defects in glass, such as the development of surface scratches or edge defects. The two is self detonate caused by phase change expansion of nickel sulfide (NiS) in glass. The former is relatively easy to detect, so it is controllable in production. The latter is mainly caused by the volume expansion of tiny nickel sulfide particles in the glass, which can not be easily tested and therefore can not be controlled. In practice, the former can be removed before installation, and the latter can not be tested and continue to exist, become the main factor of tempered glass self-explosion in use, generally referred to as the latter case. Because of the unpredictable spontaneous explosion caused by nickel sulfide and the large economic losses caused by the spontaneous explosion in service, it is called "glass cancer". Therefore, the purpose of this work is to study the type and mechanism of the self-explosion source of tempered glass, so as to reduce and prevent the occurrence of the self-explosion accident of tempered glass.
2 research methods and processes
Generally speaking, the spontaneous explosion of tempered glass is caused by the stress concentration caused by the impurity particles in the tensile stress layer. Its typical failure morphology is shown in Figure 1. The common feature of spontaneous explosion is that there is a pair of butterfly-shaped fragments (butterfly patches) at the source of the damage. The interface between the butterfly patches is usually the point where the damage occurs, and the impurity particles causing the damage can be found in the tensile layer of the interface. In order to clarify the mechanism of self-explosion of tempered glass, 9 pieces of glass fragments were collected and the source of self-explosion was analyzed. Small particles were found on the cross section of butterfly-like glass fragments at the source of self-explosion. The local particle composition was analyzed by scanning electron microscopy (LEO, Oberkochen, Germany), and the mechanism of self-explosion was analyzed by finite element method (FEM).
3 research results on self ignition mechanism of tempered glass
Mechanism of self ignition of tempered glass caused by 3.1 nickel sulfide particles
The mechanism of the self ignition of toughened glass caused by nickel sulfide particles has been widely studied [6-9]. Fig. 2 is a typical morphology of nickel sulfide particles. Fig. 3 is the result of corresponding energy spectrum analysis. Nickel sulfide is a kind of crystal with high temperature and low temperature. The phase transition temperature is 379 degrees. When glass is heated in a tempering furnace, nickel sulfide is completely converted into a high temperature phase because the heating temperature is higher than the phase transformation temperature. In the subsequent quenching process, the high temperature phase can not be transformed into the low temperature phase, thus being frozen in the tempered glass. At room temperature, the high temperature phase is unstable, with the trend of gradually changing to low temperature phase. This transformation is accompanied by about 2%~4% volume expansion, which causes the glass to withstand tremendous phase transformation tensile stress, resulting in spontaneous explosion. In order to reduce the self ignition caused by nickel sulfide particles, the toughened glass can be homogenized by [10,11]. Many tempered glass factories at home and abroad have adopted homogenization treatment process to prevent spontaneous explosion, but the practical application results show that the homogenization treatment of tempered glass, in the service process will still occur spontaneous explosion phenomenon.
3.2 self ignition mechanism of toughened glass caused by heterogeneous particles
We collected 9 pieces of butterfly patch fragments from different occasions, and found the small particles at the interface with the diameter of 0.1-0.4 mm. These small particles are tensile stress layers with a certain depth from the surface of the glass, as shown in Figure 4. The traces in Fig. 4 clearly show the failure process. First, the local primary cracking is caused by the expansion of the particles in the tensile stress zone of the glass, and then the secondary cracking and the overall fracture occur.
In order to further analyze the morphology and composition of these small particles, the fragments were observed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) under the same conditions. The results show that the impurity particles which lead to the spontaneous explosion of tempered glass are not only nickel sulfide particles, but also other heterogeneous particles such as pure polysilicon, alumina and sodium aluminate. Of the nine samples we found, five were pure polysilicon, two were NiS, one was Al2O3, and one was Na2Al2Si5O10. It is precisely because many self-explosion phenomena are caused by these heterogeneous particles, so domestic and foreign tempered glass enterprises for nickel sulfide homogenization treatment, can not completely avoid the tempered glass service process of self-explosion.
Because the mechanism of self-explosion of tempered glass caused by single-crystal polysilicon, alumina and sodium aluminate particles is very similar, we take typical heterogeneous single-crystal polysilicon as an example to explain the mechanism of self-explosion of tempered glass caused by such particles. Fig. 5 is a cross-sectional diagram of a typical heterogenous granular elementary polysilicon and an energy spectrum analysis along the white line in the diagram.
Most of the particles are spherical, and the surface is very smooth, as shown in Figure 5. The microhardness value is 6.5GPa, which is higher than that of the surrounding glass (5.4GPa). The expansion coefficient of silicon is about 3-5 *10-6K-1, and that of soda-lime silica glass is about twice that of the ordinary silica glass. In the process of cooling, the surrounding glass produces more and more compressive stress on the spherical particles of silicon, whereas the radial compressive stress and tangential tensile stress are the same for the surrounding glass. For physically toughened glass, the compressive stress on the surface is balanced with the compressive stress on the surface. The tangential tensile stress around the silicon particles is superimposed on the tensile stress of the tempered glass, so that the plane tensile stress around the silicon particles perpendicular to the glass surface reaches the maximum. When the local tensile stress reaches a certain extent, the glass will break down. At the same time, when the maximum tensile stress is close to the fracture strength of glass, a dangerous unstable system will be formed. Once there is temperature change or external force, the peak value of local stress may exceed the strength value and destroy. From Figure 4, we can see that the glass at the edge of the particle is extruded and there are cracks in the tangential direction. The local stress in glass is mainly due to the difference of expansion coefficient between glass and silicon particles. According to the elastic theory, the extrusion stress is mainly determined by the temperature difference, the difference between the expansion coefficients of the two materials and the elastic coefficients. The stress state in the glass around the particle is spherically symmetrical and decays rapidly with distance. The absolute value of radial and tangential stresses is twice that of the same tangential stress. They can be expressed as: (omitted)
The particles will be subjected to hydrostatic pressure and vice versa. For silicon particles in glass substrate, the temperature difference during cooling is
Through the analysis of the cross section of glass fragments, it is found that the self-explosion of tempered glass is caused not only by nickel sulfide particles in traditional knowledge, but also by many other heterogeneous particles such as silicon, alumina and sodium aluminate. In a total of nine cases, five of the small spontaneous explosive particles were found to be pure polysilicon, two were NiS, one was Al2O3, and one was Na2Al2Si5O10. The crack initiation and propagation in glass are mainly caused by the combined action of residual tensile stress caused by heterogeneous particles and residual stress of glass itself. There are two main types of stresses that cause the spontaneous explosion of tempered glass: one is the stress produced by the phase transformation process, the other is the residual stress caused by the mismatch of the thermal expansion coefficient between the heterogeneous particles and the glass.
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