A brief analysis of glass stress generation mechan

2022-08-16
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A brief analysis of the mechanism of glass stress generation and related measurements

due to the poor thermal conductivity of glass determined by its own characteristics, there must be stress in the glass during annealing due to the temperature difference. Based on the annealing theory of glass and the schematic diagram, this paper discusses the mechanism of permanent stress and temporary stress in float glass during annealing, and introduces the influence of stress on glass cutting combined with the measurement of stress

1. Permanent stress and temporary stress:

below 880 ° f (470 ° C), the annealing kiln is used to cool the glass. The way to do this is to try to maintain the good uniformity of the transverse (across the glass belt) temperature curve to avoid damage

everything that happens in the cooling process is temporary, which means that the stress produced by the temporary process cannot be found in the cooled glass, which is why these conditions are called temporary stress. Even when the temperature of the inner and outer layers of the glass is the same, we must pay attention to the disappearance of the temporary stress, because the temporary stress exceeding the tensile strength will break the glass

the permanent stress should not be too large, otherwise many problems will occur during cutting or re cutting. Note: the temporary stress should not be too high, otherwise the glass will break

when the thickness direction and transverse direction of the glass have not reached an equal temperature, that is, the ambient temperature, there are both temporary and permanent stresses. Even if the permanent stress has reached a relatively good value (measure the cooled glass in the cutting area for 1 hour), the cutting problem will still be encountered because the temporary stress is still very large

annealing theory shows that the permanent stress (kept in the glass) is equal to the total amount of relaxed stress in annealing, but the sign is opposite. This means that when the compressive stress of 10 nm/cm is relaxed during annealing, the glass will have a tensile stress of 10 nm/cm when it reaches the external temperature

then (1) the smaller the relaxation stress during annealing, the smaller the stress in the glass when the temperature is equal to the surrounding temperature; That is to say, annealing is better. However, when there are weak areas, i.e. stones, inclusions or tin sticking, the glass will burst. Therefore, the cooling zone after the annealing zone must be handled carefully

(2) conversely, the glass with poor annealing is very strong and can withstand the thermal shock due to the large temperature gradient in the cooling area; But if the stress is too large, it will be difficult to cut

2. Causes of permanent stress and temporary stress:

i. glass temperature is higher than 880 ° f (470 ° C): glass is a plastic material

for example: figure 1

if the glass temperature is higher than 880 ° F, the narrow strip "a" is hotter than the narrow strip "B", Then the narrow strip "a" is longer than the narrow strip "B"; this is because: the narrow strip "B" cannot prevent the expansion of the narrow strip "a" (the glass is in a plastic state) {hottag}.

during the temperature stabilization period (below 880 ° F), the narrow strip "a" attempts to be shorter than the narrow strip "B "The shrinkage is large. This is because their shrinkage is proportional to their length. When the glass temperature elimination method is to remove the dial glass and the glass is lower than 880 ° F, the change Haizheng Group continues to strike while the iron is hot, similar to elastic materials, and the narrow strip" a "and the narrow strip" B "are actually connected together, so the narrow strip" B "prevents the shrinkage of the narrow strip" a ", so the narrow strip" a "is subjected to tensile stress, and the narrow strip" B "It is under compressive stress.

in short: the glass temperature is higher than 880 ° f=470 ° C a) the hotter strip will be under tensile stress (permanent) after normal temperature. B) the colder strip will be under compressive stress (permanent) after normal temperature.

ii. The glass temperature is lower than 880 ° f (470 ° C): glass is an" elastic material ".

for example: Figure 2.

if the glass temperature is lower than 880 ° F, the strip" a "is hotter than the strip" B ", then the strip" a " "Will be longer than the narrow strip" B "; when the glass temperature is lower than 880 ° f (glass is an elastic material), the two narrow strips are actually connected together, and the narrow strip" B "prevents the expansion of the narrow strip" a ". In this way, the narrow strip" a "is subjected to compressive stress, and the narrow strip" B " "It is tensile stress.

in short: the glass temperature is lower than 880 ° f=470 ° c

a) the hotter strip is subjected to temporary compressive stress.

b) the colder strip is subjected to temporary tensile stress.

for the same reason, the result is the opposite, which depends on the temperature.

glass burst often occurs in the cooling zone (between the annealing kiln outlet and the cutting zone) Sometimes it is difficult to know whether the adjustment must be in the annealing zone or the cooling zone (above or below 880 ° f-470 ° C)

3. Stress measurement

i.: use Senarmont instrument. Here we show you the transverse stress curve of the glass ribbon; See figure 3:

when polarized light passes through the glass, the stress is measured on the glass belt. The ideal transverse stress on the glass belt is -- see figure 4:

the total amount of all compressive stress is always equal to the total amount of all tensile stress, which means that if there is too much compressive stress at the edge of the glass belt, there is also too much tensile stress in the middle of the glass belt

the stress curve measurement in hot and cold states will show you the problems in the annealing kiln

for example: if the tensile stress in the middle of the glass belt is too large, the glass plate will be difficult to cut. See figure 5:

ii Sample: use Babinet instrument

the result of this measurement is to show the stress in the thickness direction in the glass. The stress curve is usually as shown in Figure 6:

there is too high tensile stress in the glass plate- σ T- (it depends on the thickness domestically) will bring many problems to cutting (there are "sugars"). Always remember if σ T is too high, which means that the compressive stress on the outer surface of the sample will also be very high

then, the glass will not be broken according to the place you want, that is, according to the cut mark, but somewhere else (see the section "cutting problems" for the reasons)

note: the cutting of glass always releases the stress inside the glass -- see figure 7:

when cutting glass, we release the compressive stress on the surface -- from curve ① to curve ②. When the glass strength is not too high, the glass can be broken along the notch. If there is still a large compressive stress on the outer surface of the glass plate, the glass will not break along the notch

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