# Measurement method of the hottest Glass Stress

2022-08-09
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Glass stress measurement method

stress control is an extremely important part of glass production process. The method of applying appropriate heat treatment to control stress has been well known to glass technicians. However, how to accurately measure the glass stress is still one of the difficult problems for the majority of glass factories. The traditional empirical estimation has become increasingly unsuitable for the requirements of today's society for the quality of glass products. This paper comprehensively introduces the commonly used stress measurement methods, hoping to be helpful and enlightening to everyone

1. Theoretical basis of stress measurement

1.1 polarized light

as we all know, light is an electromagnetic wave whose vibration direction is perpendicular to the forward direction and vibrates on all vibration surfaces perpendicular to the forward direction. For example, polarized light can be obtained by introducing a polarization filter that only allows light in a certain vibration direction to pass through the optical path

1.2 birefringence

glass is an isotropic body with the same refractive index in all directions. If there is stress in the glass, the isotropic property is destroyed, causing the refractive index to change, and the refractive index in the two principal stress directions is no longer the same, that is, birefringence. The relationship between refractive index and stress value is determined by the following formula:

nx - NY = CB（ σ x – σ y)

where: NX and NY are the refractive indices in X and Y directions respectively. σ x 、 σ Y is the stress in X and Y directions respectively. CB is the stress optical constant, which is physical property. 5) overload protection: when the load exceeds 3 (5)% of the maximum value of each gear, the constant is only related to the variety of glass

1.3 optical path difference

when polarized light passes through a stressed glass with a thickness of T, the light vector will split into two components that vibrate in the X and Y stress directions respectively. If VX and vy are the velocities of the two vector components respectively, the time required to pass through the glass is t/vx and t/vy respectively, and the two components are no longer synchronized, but there is an optical path difference δ：

δ = C (t/vx - t/vy) = t (NX - NY)

where C is the speed of light in vacuum

combined with the above two formulas, the following formula is obtained:（ σ x – σ y) = δ/ (TCB)

that is, there is a certain relationship between stress and optical path difference. Generally, the optical path difference is measured with the help of optical interference principle, so as to calculate the stress value. It should be emphasized that what is obtained is not the absolute value of stress, but the difference between the two principal stresses. Sometimes, although the measured stress is zero, in fact, both principal stresses exist, but they are equal. A typical example is flat glass. From the plane, there are equal surface compressive stress and core tensile stress. The surface compressive stress is numerically equal to twice the core tensile stress, but the stress cannot be measured by using plane transmitted light. The reason is σ x = σ y 。 Samples must be taken so that light can pass through the end face of the glass before measurement. Therefore, it is extremely important to design appropriate stress testing methods for different products according to the process conditions

1.4 interference color

after the two vector components pass through the polarizer, they vibrate in the same plane, and there is a certain optical path difference. If the coherence conditions are met, interference will occur. The light intensity I produced by interference is determined by the following formula:

i = a2sin22（ β – α) Sin2 (p δ/λ)

The meaning of each symbol in the formula is shown in Figure 1. The following conclusions can be drawn from this formula:

a) when β = α When the directions of the two principal stresses are consistent with the directions of the polarizer and the polarizer respectively, I = 0. This black stripe is the "isoclinic line", and the stress at all points on the line has the same direction. This principle is often used to determine the direction of stress

b) when β – α = At 45o, that is, the principal stress direction and polarization direction are 450, at δ = 0、1 λ、 two λ、 three λ…… N λ At, I = 0. That is, when the optical path difference is an integral multiple of the wavelength, black stripes appear

c) when β – α = At 45o, the following wavelengths of light can be transmitted better: sin2 (P δ/λ) = 1, i.e λ = two δ、 two δ/3、2 δ/5、2 δ/7、……。 The following wavelengths of light are blocked: sin2 (P δ/λ) = 0, i.e λ = δ、δ/2、 δ/3、 δ/4、……。 White light is a mixture of light waves of various colors with wavelengths ranging from 400 to 700nm, and the effective wavelength is generally calculated as 565 nm

therefore, when white light is used as the light source, the glass will show the measurement benchmark. According to the shape and size of the specimen, the colorful interference color can be used to estimate the stress value. The color bands formed by the same interference colors are called "isochromatic lines", and the stress values on the lines are equal

2. Commonly used stress measurement methods

2.1 qualitative and semi quantitative measurement methods

the method of observing residual stress in glass with orthogonal polarization is well known, and this method is widely used to determine the stress in glass qualitatively or semi quantitatively

the simplest stress meter is usually composed of a white light source and two polarizers. The optical axes of the polarizers are perpendicular to each other. The glass sample is placed between the two polarizers, and the direction of the principal stress is 450 with the polarization axis. If there is non-uniform stress perpendicular to the direction of light propagation in the glass, black, gray and white interference bands can be observed. When the stress is higher, colored interference fringes such as yellow, red and blue can be seen. Stress free glass can only observe a uniform dark field

for annealed glass products, generally only gray and white interference color appears. At this time, in order to improve the resolution, it is necessary to add a sensitive color chip. Sensitive color chip is actually an artificial birefringent with an optical path difference of 565nm, which is equivalent to artificially increasing or decreasing the total optical path difference by 565nm, so that the color interference color appears in the field of view and improves the discrimination ability of the naked eye to the interference color

below is the comparison table of interference color and optical path difference

interference color o: P optical path difference without sensitive color chip nm optical path difference with sensitive color chip nm

black 0565

gray 150415

light yellow 250315

yellow 300265

Orange 450115

Red 50065

purple 5650

Blue 600

Blue Green 650

green yellow.750

yellow 850

Orange 950

Red 1050

purple 1130

Green 1300

green yellow 1400

pink 1500

purple 16 95

another more accurate color comparison method is to use a set of standard optical path plates including at least 6 pieces to compare the interference color of the tested glass sample with the standard plate under polarized light, so as to judge the stress

the standard optical path sheet is a uniform birefringent sheet. The optical path difference of each sheet is artificially controlled between 21.8 – 23.8 nm, with a diameter of at least 30mm, which is basically the same as that of each sheet in the group

by increasing or decreasing the number of standard optical path pieces, the interference color of the glass sample is the same as that of the standard piece group. According to the number of standard pieces and the optical path data of each piece, the stress value in the glass can be calculated

2.2 Senarmont quantitative stress measurement method

the polarization direction of polarizer and polarizer must be 45o with the horizontal line, and they must be perpendicular to each other. The direction of one of the principal stresses of the tested sample must be consistent with the horizontal line, that is, the direction of the principal stress must be 45o with the polarization direction. If the sample is a cylindrical product such as a bottle, place the bottle horizontally so that the axis of the bottle coincides with the horizontal line

the polarizer can be rotated, and the rotation angle is indicated by the scale. When using, first turn the polarizer to the 0 scale; Then place the tested sample and adjust the direction of the sample so that the direction of the principal stress at the tested point is 45o with the polarization direction; Then turn the polarizer until the measured point becomes the darkest; Write down the corner reading, and each degree is equivalent to 3.14nm optical path difference

according to the rotation direction, it can be judged whether it is compressive stress or tensile stress. For example, what should we do if the noise of high and low temperature test chamber is too loud? If turning the polarizer clockwise can darken the measured point, it is tensile stress, otherwise it is compressive stress. It should be pointed out that if the quarter wave plate is installed by rotating 90o, the stress property represented by the rotation direction of the polarizer is exactly the opposite, and the absolute value of the reading remains unchanged. If there is any doubt about the instrument, take 25X 200mm flat glass to measure its core stress. It is known that the core stress is tensile stress, so it can be used to verify the stress test direction of the instrument

The accuracy of a quarter wave plate has a great impact on the measurement accuracy of this method. Generally, the optical path error of the wave plate is required to be within +/- 2nm. Senarmont method is applicable to glass products with known stress direction, such as flat glass, bottle, glass tube, etc. For products with complex stress direction, tardy method is more convenient

2.3 the tardy quantitative stress test method

is different from the Senarmont method: the tardy method adds a quarter wave plate, the optical axis of the two quarter wave plates is 45o with the polarization direction, and both wave plates can be removed from the optical path; The principal stress direction in the glass sample coincides with the polarization direction. The rest is similar to the Senarmont method

when the comprehensive strength of the test is low, first remove the two quarter wave plates from the optical path; Then put in the tested sample. At this time, the black stress isoclinic line on the sample can be seen from the polarizer, that is, on this line, the stress direction is the same and consistent with the polarization direction; Then adjust the placement direction of the sample so that the isoclinic line passes through the measured point; Push two quarter wave plates into the optical path, and the isoclinic line disappears; At this time, the polarizer can be rotated until the light at the measured point is the weakest; The following steps are the same as Senarmont method

since tardy method requires the stress direction to be consistent with the polarization direction, the isoclinic property can be used to realize the relative adjustment of the direction, and it is not necessary to accurately determine the actual direction of the stress

the optical axes of the two quarter wave plates are perpendicular to each other, and the effect on the optical path is mutually compensated, so the accuracy requirements of the wave plates can be lower, and only the relative error between the two wave plates needs to be controlled. Therefore, the measurement accuracy of this method is better than that of Senarmont method

2.4 Babinet compensator method

babinet compensator is a birefringent element with adjustable optical path difference, which is equivalent to adding an artificial stress sheet with adjustable stress value in the stress meter. Its direction is opposite to the stress direction in the measured glass sample. When the two values are equal, the stresses cancel each other, and the extinction black stripes are observed under orthogonal polarization

The Babinet compensator is generally composed of two quartz wedges with the same size and perpendicular optical axes. One wedge is fixed, and the other one can slide. The sliding position is converted into a reading by the micrometer screw. The optical path difference is linearly related to the sliding distance of the wedge

this method is relatively simple. First, determine the main stress direction of the measured point, rotate the compensator micrometer screw until the measured point is covered by black stripes, record the reading of the micrometer screw and multiply it by the compensator constant to obtain the stress value of the glass. The direction of stress is also determined according to the rotation direction of the micrometer screw

this method is simple and accurate. The disadvantage is that the compensator is expensive

3. Several problems needing attention

3.1 all methods measure the difference between the two principal stresses that are perpendicular to each other. If the two principal stresses are equal, even if the stress value is large, the measured stress is also

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