[China Glass Net] Due to its unique permeability, glass has not been replaced by its products in the field of construction and transportation. In addition to the characteristics of sheltering from the wind and rain, with the advancement and improvement of science and technology, it also provides beautiful views, excellent performance and comfortable indoor environment. However, due to the lack of understanding of the characteristics of the people, when the glass is damaged, it is considered that the glass has "self-explosion". However, if the glass is not subjected to external stress, it will not break, that is, the so-called "self-explosion" phenomenon. In fact, we should call it: glass breakage.
Due to its crystal structure, glass has no yielding process before it suddenly ruptures. Therefore, people generally think that glass has produced "self-destruction." Since glass is a non-durable item and fragile, all glass breaks are caused by tensile stresses greater than compressive stress and surface defects, and are never the cause of pure compressive stress. The glass will be damaged after the surface compressive stress layer is penetrated. Although the glass edge and surface scratches and microcracks and other defects do not completely penetrate the compressive stress layer, these small damages change with temperature over time. Under the repeated action of stress such as wind load, it will slowly become effective and become larger, which will lead to the "self-explosion" of the glass. Because there is no obvious reason before the glass breaks, it often makes people feel unexpected. It is the reason of the glass manufacturer. It should be compensated by the glass manufacturer. This is not the case.
Since glass is a poor conductor of heat, dangerous pressure is created when the glass encounters sudden changes in temperature. At this time, the temperature difference between the glass surface and the inside of the glass may cause the glass to break. For very obvious reasons, the risk of sudden heating is less than sudden cooling. Sudden heating increases the compressive stress on the glass surface, and sudden cooling increases the tensile stress on the glass surface, causing the glass to break. The thermal conductivity of the glass gradually decreases as the temperature decreases. At temperatures above 39 ° C, the radiant conductivity of the glass increases significantly, and at 30 noon in the summer, the surface temperature will reach about 80 ° C. The heat transfer of thick glass to radiation is more obvious than that of thin glass, so thick glass is prone to thermal cracking.
The tempered glass is heated to a softening point range of about 650 ° C, and then rapidly cooled on the surface of the glass to form a high compressive stress on the surface. The strength of the tempered glass is generally 4 to 5 times higher than that of ordinary glass. Due to the high strength of tempered glass, many people mistakenly believe that tempered glass can withstand rough handling during transportation and can be installed at will. In fact, this is not the case. Like tempered glass, tempered glass should be carefully protected from damage to the edges and surfaces of the glass during handling and installation. If large edges and scratches appear on the edges and surfaces of the glass, the strength of the glass will drop sharply and the tempered glass will be damaged.
Semi-tempered glass heats ordinary glass to about 621 ° C. The heating temperature of semi-tempered glass is low, and the cooling rate is also slow compared with tempered glass. The strength of the semi-tempered glass is about twice that of ordinary glass. The properties of semi-tempered glass, such as color, purity, chemical composition and light transmission properties, have not changed. Hardness, specific gravity, coefficient of thermal expansion, softening point, thermal conductivity, solar transmittance and rigidity have not changed. The physical properties of the change are curved. strength. Under uniform load conditions, semi-tempered glass is 2 times stronger than the same ordinary glass. Therefore, it has better resistance to thermal shock, wind load and impact from external objects such as hail. More importantly, semi-tempered glass does not have the so-called "self-explosion" phenomenon like tempered glass.
Analysis of the causes of glass breakage 1. Design reasons
The design strength of the glass is lower than the allowable strength value, which can cause damage to the glass. The glass structure design is unreasonable, resulting in damage caused by excessive local stress of the glass, such as: triangular, convex and concave, digging holes and other structures, which will cause local stress concentration of the glass.
The thermal cracking of the glass during design is not considered or considered insufficient. Glass, especially heat-absorbing glass and heat-reflecting glass, when exposed to sunlight, due to factors such as curtain effect, the glass surface will form a non-uniform temperature field, thereby forming thermal stress in the glass, when this thermal stress exceeds the strength of the glass. In the range, thermal cracking occurs, and the glass appears in a curved state when it is thermally exploded.
When the glass thickness is too thin or the deflection is large during design, after the glass is subjected to the wind load or the wind pressure, the deflection of the glass sealant is caused by the deflection, and the glass in contact with the frame is damaged.
2. Influence on glass during production, transportation and handling
In the production, transportation and handling of glass, if the glass is improperly protected, serious scratches, bursting, abrasion and other defects will occur on the surface or edge of the glass, which will also cause local stress concentration on the glass. Causes damage to the glass.
3, the stress generated during installation causes damage to the glass
When the glass is installed, due to the difference in the error range between the building and the glass, the glass may be distorted and deformed, causing additional stress on the glass, which may cause damage to the glass. Due to the hardness of the glass block and the improper selection of the position and quantity, the stress on the edge of the glass may be concentrated or excessively large, causing damage to the glass. The broken state is a straight shape.
4. Uneven settlement of the building, causing damage to the glass
A large curtain wall enterprise in China once measured the installation of a building curtain wall before and after installation, and the building settled up to 12mm. Exceeding the allowable flexural strength of the glass, it is bound to cause damage to the glass. The photograph below shows the amount of deformation of the point support member in the horizontal direction, which is also sufficient to cause damage to the glass.
5, the cause of man-made
When the glass is in use, people intentionally or unintentionally damage the glass. When the glass is mounted, the smashing and knocking of the edge of the glass may inadvertently cause damage to the glass. Or people are unintentionally damaged by the impact of the glass when they are active. Intentional: When manufacturing ice cracked glass, artificially damage the glass. Or people's active impact on the glass is broken.
6. Reasons for natural disasters
Typhoons, hail, sandstorms, etc. caused damage to the glass.
7, the reason for the "self-explosion" of glass
The national standard "Safety glass for construction Part II: Tempered glass" GB15763.2-2005 Appendix A.2 has a clear description of the self-explosion of tempered glass: due to the presence of tiny nickel sulphide stones in the glass, a part of the stones after heat treatment During the time, the crystalline state changes, the volume increases, and microcracks are induced inside the glass, which may cause the tempered glass to self-explosion. There are three common ways to reduce this self-explosion: 1. Use original tablets with less nickel sulfide stones, that is, use high-quality original tablets. 2, to avoid excessive steel stress. 3, hot dip / detonation / homogenization. Special attention: avoid the furnace temperature exceeding 320 degrees during hot dip / detonation / homogenization, and the surface temperature of the glass exceeds 300 degrees.
In fact, the tempered glass causes a "self-explosion" phenomenon, in addition to nickel sulfide, there are also single polycrystalline silicon, aluminum oxide, Na2Al2Si5O10 and the like. However, the hot dip/detonation/homogeneous process only acts on the "self-explosion" caused by nickel sulfide, but has no effect on the "self-explosion" caused by other substances. Therefore, the "self-explosive" profile of tempered glass: three thousandths of compensation. However, a famous foreign processing enterprise promised that after the hot dip of tempered glass, the “self-explosion†rate should not exceed five thousandths. In fact, Australian technicians tracked 12 buildings for 12 years. In a total of 17,760 pieces of glass, a total of 306 pieces of glass "self-explosive" occurred. The statistics showed that the glass self-explosion rate was 1.72%.
The "self-explosion" of tempered glass has its own unique shape, namely the "butterfly spot" phenomenon. Only in the presence of glass: nickel sulfide, elemental polycrystalline silicon, aluminum oxide, Na2Al2Si5O10, etc., and the external stress exceeds the compressive stress on the glass surface, the so-called "self-explosion" phenomenon occurs in the tempered glass.
In summary, there are many reasons for glass breakage, and the phenomenon of "self-explosion" is only one of them. If the glass is not subjected to external stress, there is no glass breakage. Therefore, when the glass is damaged, it is necessary to analyze it from many aspects in order to confirm the cause and determine the responsibility. If the glass is not damaged, it is claimed that the glass is “self-destructiveâ€. It is necessary for glass manufacturers to take responsibility.
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