Influence of the hottest tempering stress on self

2022-10-01
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The influence of tempering stress on self explosion of tempered glass

as safety glass, tempered glass is more and more popular in the application of building doors, windows and curtain walls, but the self explosion phenomenon of tempered glass occurs from time to time in the process of use. The self explosion of tempered glass may cause inconvenience in use, or endanger personal safety

tempered glass for construction is mainly produced and processed by physical method. The characteristic of physical processing is that the outer layer of hardened glass produces compressive stress, and the inner layer of hardened glass produces tensile stress. Due to the existence of this compressive stress on the glass surface, when the external force acts on the glass surface, the existing compressive stress must be offset first, so as to greatly improve the mechanical strength of the glass. Due to the existence of internal tension and external pressure stress of tempered glass, the strength of tempered glass can reach 4 ~ 5 times that of ordinary flat glass. At the same time, the tempered glass immediately splits into small fragments without sharp corners after breaking, which greatly reduces the degree of personal injury. Therefore, tempered glass as safety glass is widely used in building doors, windows and curtain walls

1 self explosion of tempered glass

after physical tempering heat treatment, the compressive stress of the glass surface layer and the tensile stress of the core layer together form a stress balance. Glass is a brittle material, which is resistant to pressure but not tension. Therefore, the breakage of glass is mainly caused by the tensile stress of the core layer

self explosion of tempered glass refers to the phenomenon that glass automatically explodes without direct external force. There are mainly two kinds of self explosion of tempered glass, one is the automatic explosion of glass during tempering processing, and the other is the automatic explosion of glass during transportation, storage and use. The former is mainly caused by inclusions such as sand particles and bubbles in the glass production process and process defects such as scratch, edge burst and notch caused by human beings; The second case is mainly caused by the volume expansion caused by the phase transformation of residual nickel sulfide (NIS) in the glass. Generally, the self explosion of tempered glass we often mention mainly refers to the second case, which is also the problem discussed in this paper

nis is mainly caused by the production process of flat glass. NIS is a kind of skill crystal of bellows ring stiffness testing machine, which has high temperature phase( α- NIS) and low temperature phase( β- NIS) with phase transition temperature of 379 ℃. The heat treatment temperature of toughened glass by physical method is about 650 ℃, because the temperature of glass heating is much higher than the phase transition temperature of NIS, therefore, in the process of Toughening Heat treatment, NIS in the glass is all transformed into α Phase. However, in the subsequent quenching process, NIS in the glass α It's too late to turn into β Phase, which is frozen in tempered glass. At room temperature, NIS α The phase is unstable, and it gradually changes into β Phase trend. In the process of crystal phase transformation, the volume expansion of NIS is 2% - 4%, which makes the glass bear huge phase transformation tensile stress, resulting in self explosion. Typical self exploding fragments caused by NIS are shown in Figure 1

it can be seen from Figure 1 that after the broken tempered glass, the fragments are distributed radially, and there are two glass blocks in the shape of butterfly wings in the radiation center, commonly known as "butterfly spots". NIS stone is located on the interface of two "butterfly spots". The research shows that the particle size change of NIS inclusions causing self explosion is between 0.04 ~ 0.65mm between the A and B contacts of steering switch and A1 and B1 contacts, with an average of about 0.2mm. All NIS inclusions that can cause glass self explosion are inside the tempered glass, about 25% - 75% of the glass thickness direction (i.e. in the tensile stress zone of the glass)

2 effect of surface stress on glass self explosion

according to the tempering principle of glass, after glass tempering, the internal tensile stress and compressive stress reach an overall stress balance. No matter in the production process or in the use process of finished products, once this stress balance is broken by carat weight, the glass will burst, that is, the glass will explode

it is found that there is a critical diameter DC of NIS leading to glass self explosion, which depends on the stress around the NIS inclusion σ 0 (annealing level of NIS stone position inside the glass):

according to the calculation of formula (1), when the tensile stress inside the glass is 65mpa, the minimum NIS diameter of the broken glass is about 0.04mm

the tempering of glass is actually the re heat treatment of glass. In actual use, it is found that the higher the tempering degree of glass, the greater the self explosion proportion of tempered glass

the toughening degree of glass can be attributed to the internal stress of glass. Jacob gives the surface compressive stress value of tempered glass and 50mm × The corresponding relationship between the number of debris particles within 50mm

Figure 2 shows the surface stress range of tempered glass determined according to the American astmc1048 standard. It can be seen from Figure 2 that when the surface stress measured by SSI tempered glass surface stress instrument is 90Mpa, the corresponding number of fragments is about 52, and with the increase of the surface stress of tempered glass, the number of corresponding glass fragments is also increasing

China's tempered glass standard safety glass for building Part 2: tempered glass gb15763 It is specified that when the thickness of flat tempered glass is 4 ~ 12mm, 50mm × The minimum number of fragments in the 50mm area is 40

the core stress is always tensile stress, and its value is equal to 1/2 of the compressive stress on the glass surface. The relationship between internal tensile stress and surface compressive stress of tempered glass is shown in Figure 3

China's tempered glass standard safety glass for building Part 2: tempered glass gb15763 It is specified that the surface stress of tempered glass is not less than 90Mpa

the critical diameter DC of NIS of tempered glass under different surface stresses is calculated by formula (1), as shown in Table 1

it can be seen from table 1 that the greater the compressive stress on the glass surface, the smaller the critical diameter DC of NIS, the more NIS particles cause the self explosion of glass, and the higher the self explosion rate of glass

self explosion of the same glass can also be caused by inconsistent tempering uniformity. Tempering uniformity refers to the consistency of tempering stress in different areas of the same glass after tempering. Generally, the tempering uniformity of glass can be measured by measuring the areastress of each part of the same glass plane. The plane stress generated by this uneven tempering is superimposed on the thickness stress, which increases the actual core tensile stress in some areas, causes the critical diameter DC value to decrease, and finally leads to the increase of self explosion rate

3 detection of tempered glass in engineering applications

in the detection of tempered glass, the surface stress and fragment state are important properties of tempered glass detection. The measuring principle of surface stress meter is to use the optical waveguide effect of tin diffusion layer on the surface of float glass to measure. National standard safety glass for building Part 2: tempered glass gb15763 It is specified that the surface stress of tempered glass for building is not less than 90Mpa, and 50mm after the broken tempered glass × The minimum number of fragments in the 50mm area is also specified accordingly

in the detection of tempered glass, we used ssm-2 glass surface stress meter to count the surface stress and the number of broken fragments of three groups of 6mm, 8mm and 10mm tempered glass

it can be seen from table 2: surface stress today we mainly introduce the calibration method of the displacement of the electronic tensile testing machine. The greater the force is, the more fragments will be broken. The degree of tempering is one of the important signs to measure the performance of tempered glass. The higher the degree of tempering, the greater the internal stress in the glass, the greater the impact strength of the glass, and the smaller the particle size after crushing

the size and uniformity of the internal stress in the glass are important characteristics of the tempering degree, so the size and uniformity of the particle size of the broken tempered glass are used to characterize, inspect and assess the tempering degree of the glass and the uniformity of the stress distribution in the glass

it can also be seen from table 2 that the thicker the glass, the greater the surface stress. Because the generation of internal stress in glass depends on the existence of temperature gradient in glass, the thicker the glass, the greater the temperature gradient when quenching. Under the same conditions, the thicker the glass, the higher the degree of tempering, that is, the greater the surface stress

4 countermeasures

4.1 control the tempering stress of glass

the greater the tempering stress of glass, the smaller the critical diameter of NIS stones, and the more NIS stones cause glass self explosion. The toughening stress of glass should be controlled within the range of 90 ~ 110MPa, which can not only ensure that the particle size of toughened fragments of glass meets the requirements of relevant standards, but also avoid the risk of glass self explosion caused by excessive stress. Reducing the plane stress (tempering uniformity) of tempered glass can not only reduce the self explosion risk of glass, but also improve the flatness of tempered glass

4.2 homogenization treatment of tempered glass (HST)

homogenization treatment is an effective method to solve the problem of self explosion of tempered glass. By homogenizing the tempered glass, the crystal phase transformation of NIS can be completed before the glass leaves the factory, and the glass that may self explode can be broken in advance in the factory

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