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Measurement technique of surface stress of flat glass

  • Source: Unknown
  • Date: April 16, 2020
  • Number of views:
Flat glass is generally divided into tempered glass, semi tempered glass and annealed glass according to the size of surface compressive stress. According to the American ASTM C1048-1997b standard, the surface compressive stress range of various glasses is: tempered glass>69MPa (10000psi), semi tempered glass: 24MPa (3500psi)~51MPa (7500psi). The newly issued national standard of ">has also made clear requirements for stress, semi tempered glass is 24-69MPa, and tempered glass is more than 95MPa.
 
For a long time, domestic manufacturers only rely on a large number of impact tests and fragment tests to determine the tempering production process parameters and control the glass quality due to the lack of detection means and understanding of international technology for a long time. It is not only expensive, but also time-consuming and laborious, with slow feedback and poor operability. The inspection work is often a formality, and it is impossible to really put the quality of batch products under effective control. For semi tempered glass, impact resistance and fragment test have no effect. To achieve comprehensive quality control quickly and conveniently at low cost, stress detection methods must be used to stabilize production and product quality.
 
The surface stress nondestructive testing technology is conducive to stabilizing the production and quality of tempered glass, and more importantly, it provides a practical test means for inspecting the quality of semi tempered glass.
 
　　1、 Glass stress detection principle
 
After the glass is cooled by heat treatment, a certain pressure is usually formed on the surface, and its direction is parallel to the glass surface. Due to the balance between the compressive stress on the surface of the glass and the internal tensile stress, and the isotropy of the stress, it is impossible to measure the stress with the transmitted light perpendicular to the glass plane.
 
When polarized light passes through the stressed glass, it forms two beams perpendicular to each other and with different propagation speeds. This phenomenon is commonly called "birefringence". The birefringence index is: Dn=na nb, Dn is proportional to the stress s in the glass, that is, Dn=C * s. Assuming that the propagation distance of the light in the glass is t, the optical path difference of the two beams R=Dn * t, that is, the stress can be calculated by the following formula:
 
S=Dn/C or s=R/t * c
 
Where C is called the stress optical constant, and for float glass, C=2. 65。
 
At present, there are two methods to measure surface stress: differential surface refraction method and surface glancing angle polarization method
 
2. ORIHARA original surface stress meter
 
This instrument was developed by ORIHARA of Japan, and China Building Materials Institute has also successfully developed similar instruments. ORIHARA Refractory Surface Stress Instrument is generally composed of light source, prism, filter, telescope objective, micrometer eyepiece, etc.
 
The working principle of DSR is shown in Figure 2. When the light of a certain incidence angle reaches the interface between the glass and the prism, due to the different birefringence, the beam will be divided into two beams reflecting back to the prism at different critical angles a and b. By measuring the distance between the two beams with a micrometer eyepiece, Dn data can be obtained and the stress s value can be calculated according to the formula s=Dn/C.
 
If the stress s=100MPa, C of float glass=2. 65 ′ 10-12m2/N, according to the formula Dn=C * s=0. 000265。 This data is very small, and it is difficult to measure accurately. Therefore, the precision of DSR surface stress meter is not high, and it can only be used to measure high stress glass, not semi tempered glass. ORIHARA Ziyuan clearly stated in its instrument operation manual that "this instrument is not intended and cannot be used to measure glass products with stress values less than 10000 psi (69 MPa)." Even if this instrument is well done, it cannot be used for semi tempered glass.
 
ORIHARA converts the original working principle diagram. The incident polarized beam enters the thin layer on the glass surface, runs a distance t parallel to the glass surface and leaves the glass. Due to the compressive stress on the glass surface, the beam produces birefringence, and its optical path difference can be accurately measured with the help of a quartz wedge. Quartz wedges can be made into a variety of precision grades, so ORIHARA refraction can be accurately measured regardless of the stress.
 
3. Instrument calibration method
 
ORIHARA provides a method to calibrate the surface stress gauge. A mechanical mechanism is used to apply a constant mechanical stress on the glass strip. The calibration result can be obtained by comparing the calculated mechanical stress value with the measured data of the instrument.
 
Simply support the two ends of the glass strip with width of W and thickness of t, and place it at L away from both ends. If F force is applied at the distance, the surface stress of the middle section of glass S=F ′ 6L0/wt2.
 
The calibration accuracy of this method mainly depends on the geometric dimensions of each part of the device. The error of L0 value must be less than 1%, so that the calibration result with 1% accuracy can be obtained. The shape of glass cross section is also very important. If the glass with a cross section of 10mm ′ 75mm is ground at 2mm ′ 45 °, the section modulus will be reduced by about 1%, and the actual stress ratio will also increase by about 1%.
 
4. Application
 
(1) Tempered glass
 
The traditional method of inspecting tempered glass is impact test. GB9963 and GB9656 stipulate that the number of fragments within the range of 50mm ′ 50mm must be more than 40. The test shows that there is a good corresponding relationship between the surface stress value and the number of fragments within the stress range of tempered glass. According to the chart given by Jacobs, the surface stress value corresponding to the number of 40 fragments is about 82MPa.
 
(2) Thermally reinforced glass
 
Heat strengthened glass is often called "semi tempered glass" in China. It is widely used in buildings abroad and has good wind pressure resistance. This kind of glass tends to stay in the window frame after being damaged by natural disasters, and at the same time, it can ensure that it will not self explode.
 
It should be pointed out that heat strengthened glass and tempered glass are two different types of glass, and it cannot be simply considered as a kind of glass whose performance is inferior to that of tempered glass because it is commonly known as "semi tempered glass". Generally speaking, tempered glass is applicable to the parts that can be touched by human body and low rise buildings. Since the self explosion characteristic of ordinary tempered glass is difficult to completely avoid (self explosion caused by nickel sulfide impurities in the glass), there must be some self explosion in a large number of applications, and the falling debris is easy to hurt people. Therefore, the use of heat strengthened glass in high-rise buildings should be avoided as far as possible. If tempered glass must be used for other reasons, it shall be aged tempered glass.
 
Since the fragmentation state is similar to that of annealed glass, semi tempered glass can only be inspected by stress measurement. It should be noted that the stress range of heat strengthened glass specified in ASTM is 24-51Mpa, and the stress of tempered glass must be greater than 69MPa. Glass with a stress of 51-69MPa is a waste product, which can neither be used as tempered glass nor as heat strengthened glass.
 
(3) Tempered safety glass
 
A lot of experience shows that when tempered glass is used for safety glass, its surface stress value is usually greater than 103 MPa. A common example is automotive glass. If the stress is less than 103MPa, although the number of fragments can still meet the requirements of relevant standards, the glass strength is slightly insufficient. In the process of installation and use, due to insufficient stress to ensure that the glass edge is not damaged, glass fragmentation often occurs.
 
(4) Hot bending glass
 
Hot bending glass is a kind of annealed glass. The stress in the glass shall be as small as possible to ensure that the glass will not break due to excessive uneven stress. The typical use is fishbowl glass. The stress in the bending part is relatively concentrated, and the measured surface stress is generally about 15MPa, which is easy to cause glass fragmentation. By detecting the stress value and adjusting the cooling mode of the glass in a targeted way, the local stress can finally be reduced to below 7MPa, basically solving the problem of corner cracking.
 

The above is the sharing of Dongyi Seiko on "the measurement technology of surface stress of flat glass". Shenzhen Dongyi Seiko Equipment Co., Ltd. specializes in the supply of stress testers, glass stress testers, Japanese original stress testers, fsm-6000le surface stress testers and other stress detection equipment, and provides professional laboratory solutions and equipment support services. Consulting hotline: 400-992-8117
 

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