The causes and preventive methods of thermal crack

Reasons and prevention methods of thermal explosion of coated glass

with the rise of glass curtain walls in modern buildings, the decoration of coated glass makes buildings colorful, bright and generous, and has the effect of energy conservation and consumption reduction. These advantages are widely recognized, but it has attracted a series of problems such as "light pollution" and safety of curtain wall glass, of which the safety has attracted much attention

the safety of unqualified curtain walls mainly focuses on the falling off of glass and the falling after glass fracture. The main reasons for glass falling off are the poor quality of glue, double-sided stickers and other materials, the insufficient curing time, the incompatibility between glue and glass, the poor quality of gluing, the non-standard construction, wind and earthquake effects, etc. Therefore, it is necessary to check the design, procurement, construction and supervision to prevent and eliminate the falling off of glass

but the glass used in curtain walls, especially the coated glass, has been used. Let the technicians lead us to have a detailed understanding! In the process, the phenomenon of cracking or cracking is more common. People are not clear about this phenomenon, and often simply blame it on the poor quality of the glass, and call it the so-called "air time bomb". Therefore, it is necessary to analyze and explore the characteristics, causes and prevention of this phenomenon, so that the public can better understand and use the coated glass of the curtain wall

there is no cracking of coated glass under normal use, and most of the cracking of coated glass belongs to thermal stress cracking. Under the strong sunlight, the glass absorbs the radiant energy in the sunlight and converts it into heat energy in the glass body, so that the temperature of the exposed part of the glass is relatively increased and is in a state of thermal expansion. However, the glass edge area inside the aluminum frame structure cannot be affected by the same solar radiation or shadow, and the heat dissipation is uneven. Therefore, the overall temperature distribution of the glass is uneven, resulting in internal thermal stress, and the thermal expansion of the middle area of the glass produces tensile stress on the glass edge area, If this tensile stress exceeds the tensile strength of the border area, it will lead to glass fracture. This phenomenon is called thermal stress fracture of glass

due to the more significant heat absorption of coated glass, the difference in absorbed solar radiation energy causes greater temperature difference of glass and more thermal stress cracking. In contrast, this phenomenon occurs more in coated glass used for windows and exposed frame curtain walls than in hidden frame curtain walls. The characteristics, causes and prevention of thermal stress cracking of coated glass are further discussed below

1. Characteristics of thermal stress fracture: the fracture line is at right angles to the edge of the glass, starts from the edge or corner of the glass, and splits into two or more at a distance of about 50mm from the glass or corner. The shape is irregular zigzag single line, extends to the middle, and usually forms an arc line in the middle of the glass

2. Bulk absorption of coated glass: after coating, the chemical properties of the glass are basically unchanged, but its physical properties are greatly changed, especially the optical and thermal properties. The solar radiant energy absorption rate of coated glass is much higher than that of ordinary transparent glass and body colored glass. Generally, the solar radiant energy absorption rate of coated glass is several times that of ordinary transparent glass and body colored glass. Therefore, under the same service conditions, the heat absorption of coated glass is much more than that of ordinary glass, and the temperature is much higher, that is to say, the temperature difference in different parts of glass is much larger. Therefore, in view of these advantages of coated glass, in the process of installation and use, corresponding measures different from ordinary glass should be taken to ensure the safe use of coated glass

3. Orientation of buildings: in view of the sensitivity of coated glass to light and heat energy, the orientation of coated glass is very important, especially for glass with large absorption. The heat absorption of glass used in the East, Southeast, South and southwest is much more than that of other surfaces. The thermal stress fracture of glass should be considered as an important factor in the design. It is recommended to use light colored glass (such as white glass) coating products to reduce the heat absorption of glass, The safer way is to strengthen the glass and process it into tempered and semi tempered coated glass

4. Climatic conditions: in areas or seasons where the temperature difference changes greatly in the morning, coated glass is a high absorber of solar radiation heat energy, and the temperature rise rate of glass is very fast. The aluminum frame is an excellent radiator. After cooling day and night, the temperature of the glass and aluminum frame has been reduced to the lowest. The solar radiation heat energy in the morning soon heats up the glass, while the temperature rise of the aluminum frame is relatively small. For example, the contact between the glass and the aluminum frame causes uneven distribution of the temperature difference of the glass, and the temperature difference of the local climate, the orientation of the glass, sudden sunny showers, and the air conditioning system are easy to make the temperature difference of the glass exceed the limit and break. The selection of glass determines whether the glass is strengthened according to the above situation. The color and size of the selected glass, as well as the coating of the film glass of the impact tester of Jinan new era Gold Testing Instrument Co., Ltd. under the conventional manual pendulum impact tester, should be constructed in strict accordance with the relevant specifications

5. Thermal insulation between glass and the surrounding environment: glass is a poor conductor of heat, and the temperature difference caused by uneven heat absorption and dissipation is difficult to balance by itself. Therefore, when installing glass, try to use elastic materials with low thermal conductivity as gaskets to isolate the glass from aluminum frames with good thermal conductivity and cool walls, so as to reduce the formation of uneven temperature difference of glass

generally, the cracking of the hidden frame curtain wall is much less, and the main reason is that the elastic materials with poor heat conduction around the glass are in soft contact with the aluminum frame, so there is no heat exchange between the glass and the aluminum frame, reducing the temperature difference in various parts of the glass caused by the heat dissipation of the glass; On the contrary, for coated glass used in exposed frames, fixed windows, sliding windows, floor glass and other places, heat absorption in the middle leads to temperature rise, while the periphery is clamped and cannot absorb heat energy to form a low-temperature zone. If the glass contacts the aluminum frame, the glass conducts heat to the aluminum frame, which not only makes the glass in the aluminum frame, but also keeps the glass around the aluminum frame low temperature. In many cases, the temperature in the middle of the glass reaches ℃, while the temperature at the edge is only ℃, with a difference of dozens of degrees, It is obvious to feel it by hand, so a great thermal stress is generated in the glass, which is easy to cause the glass to crack

this kind of coated glass is the most common thermal stress fracture at present. To avoid the above problems, try to make the glass thermally insulated from the aluminum frame. When the glass is installed, the lower balance pad two rubber blocks, which not only ensure the elastic contact between the glass and the lower part and avoid local stress, but also make them thermally insulated. The two sides and the upper part of the glass and the aluminum frame maintain sufficient clearance. After the rubber strips are sandwiched between the two sides of the glass and the aluminum frame, glass glue can be applied, The purpose is to ensure their absolute separation, elastic contact and thermal insulation

6. Edge quality of glass: glass is a brittle material, and its allowable edge stress is closely related to the edge quality of glass. The edge is the most fragile part of the glass. There are many microcracks and defects (such as edge collapse, angle collapse, uneven clamp edge, etc.) that will reduce the allowable stress at the edge by more than ten times. It is easy to break from this point when encountering a large annual profit of 200million yuan. Therefore, it is best to cut the coated glass with a cutting machine, and the cutting quality of the glass edge must be strictly checked during manual cutting, A more effective method is to machine grind the edge of the glass after cutting, which can greatly reduce the edge microcracks

7. Size and shape of glass: for coated glass, the greater the thickness of the glass, the greater the absorption of solar radiation energy, so that the thermal stress of the glass increases, and the glass is easier to crack; Similarly, the larger the area of glass, the easier it is to crack; The larger the ratio of long and short sides of glass, the easier it is to form bending stress, which will increase the probability of thermal stress cracking; In addition, the more irregular the shape of the glass is, and the unbalanced forces on various parts and directions will also increase the probability of thermal stress cracking. Reasonable design is a crucial factor to reduce the impact in this regard

8. Influence of Shadow: outdoor objects or building structures themselves may leave shadows on the glass, resulting in different heat absorption and temperature differences in different parts of the glass. The more irregular the shadow shape, the easier the glass is to crack

9. Influence of indoor shading devices: dark curtains, shutters and indoor objects close to glass (including stickers and coatings on glass) have high absorption of solar radiation energy and generally have high emissivity. Therefore, in addition to the direct radiation of the sun, the middle area of the glass is also subject to the re radiation of indoor objects after heat absorption, which also reduces the heat dissipation of the glass. Therefore, the indoor sun shading device increases the non-uniformity of glass temperature and the probability of self explosion of thermal stress. The above devices should be avoided as far as possible

10. Influence of glass reprocessing: reprocessing of coated glass increases the factor of glass fracture. After hot bending, the annealing quality of coated glass is poor, which increases the uneven stress of glass; The coated glass is processed into hollow products, and the gas in the middle expands with the heat absorption of the glass, and puts pressure on the edge of the glass to increase the probability of glass self explosion. Therefore, it is best to grind or strengthen the coated glass into hollow glass

coated glass has its inherent disadvantages, but it has a series of advantages that cannot be replaced by other materials. In order to carry forward the dazzling position of coated glass in the construction industry, coated glass suppliers provide users with more product performance parameters, cooperate with users to provide more reasonable design and construction schemes, clarify people's misunderstanding of coated glass, and promote the use of coated glass. Zhonghua glass () Department