1. Finite element simulation analysis
According to the principle of LCD display, the transmittance of LCD is T, such as formula (1):
In formula (1), n represents the birefringence of LC, D said the thickness of the liquid crystal layer, a representation of the wavelength of light, azimuth angle of the liquid crystal molecules for psi. It can be seen from the formula that the light transmittance is directly related to the thickness of liquid crystal layer D. In this paper, we try to obtain the change trend of liquid crystal layer thickness in Mura display area of LCD screen under different temperature and force load by thermodynamic simulation and experiment, in order to further study the relationship between Mura display phenomenon and liquid crystal layer thickness change. The reinforcement LCD is obtained by binding the LCD screen with the strengthened glass, and the finite element simulation model is shown in figure 1:
It mainly includes strengthening glass, optical glue, upper polarizer, CF substrate, liquid crystal layer, TFT substrate and lower polarizer. Because the thermodynamic parameters of each component are difficult to be measured independently, the components are equivalent, and the thermodynamic parameters of each part are obtained by experimental measurement and finite element simulation analysis. Table 1 gives the thermodynamic parameters of each element in the simulation model, in which LCD1 and LCD2 are the same performance parameters, the liquid crystal screen from different manufacturers, Ex, Ey, Ez are the elastic modulus of X, y and Z respectively. The shear modulus of XY, Gxz and Gyz is Gxy, XZ and YZ, and the coefficient of thermal expansion is the coefficient of thermal expansion.
The finite element simulation software is used to simulate the LCD screen and the LCD screen respectively, and the loading and binding methods are shown in figure 2:
Take LCD 1 as an example. Figure 3 shows at a temperature of 22 DEG C when tension is 12N LCD original LCD screen LCD screen and reinforcement layer stress (Mpa) reinforcement distribution map and 70 degrees under the environment of tension is 12N LCD liquid crystal layer stress (Mpa) distribution, found in the surrounding liquid crystal screen original ring and reinforced glass edge area the stress concentration and reinforcement screen, under high temperature stress increased.
It can be seen from formula (1) that the thickness of liquid crystal layer has a direct influence on the transmittance of light at different wavelengths. The thickness change value (mm) of the LCD layer 1 and the LCD layer 1 of the LCD screen under the action of 12N pulling force at 70 degrees of temperature are shown in figure 4:
According to the simulation results, it can be seen that the stress concentration of the liquid crystal screen and the strengthening liquid crystal screen around the reinforcement ring and the edge of the strengthening glass, and the thickness of the liquid crystal layer has obvious change, resulting in the change of transmittance of each wavelength is not consistent, so as to appear Mura. The thickness of the liquid crystal layer at the edge of the reinforced glass is obtained at the normal and high temperature with the increase of the tensile force by the finite element simulation, as shown in figures 5 and 6:
According to the above simulation results show that the liquid crystal layer thickness increases with the increase of tension, and through the above two picture can be seen, the same external conditions, the liquid crystal layer thickness of liquid crystal layer thickness is greater than the original LCD screen 1 LCD screen of the original 2, and reinforcement treatment and high temperature environment will make the change of liquid crystal the thickness increased significantly. In order to explore the difference of the influence of the same temperature change on different liquid crystal screens, the liquid crystal thickness changes at 70 degrees and the liquid crystal layer thickness change at 22 degrees are subtracted to get the data shown in figure 7:
The difference of the liquid crystal layer thickness between the liquid crystal screen and the high temperature environment is between 14nm and 58nm, while the difference between the thickness of the LCD layer in the room temperature and the high temperature environment is between 80nm and 223nm. Therefore, compared with the LCD screen, the reinforcement screen is more sensitive to the changes of the external environment, and the sensitivity of the different LCD screen to the external environment is also different.