What are these black spots on the edge of bus glass used for?

Image source: Tuchong Creative

What are these black spots on the edge of bus glass used for?

The answer is to prevent the glass from exploding.

Because the glass window is fixed on the metal window frame, the edge of the glass is tightly connected to the metal frame. In the hot summer, the metal frame and glass exposed to the scorching sun will produce different degrees of temperature rise. The glass absorbs less heat, while the metal frame absorbs more heat. The temperature in the middle area of ​​the window glass is lower, while the temperature of the edge and the metal frame is higher, forming a temperature gradient in the glass.

In addition, due to thermal expansion and contraction, the glass and metal frame will expand to varying degrees under warming conditions. Although the glass does not expand as much as the surrounding metal frame, the glass in the area where the metal frame overlaps will expand due to heat and be pulled by the expansion of the metal frame.

In this way, the glass is deformed. Note that this is a local deformation. In the vicinity of the deformation, the glass from the edge to the inside generates interactive internal stresses. That is, the edge glass exerts force inwards, and the middle glass exerts force outwards. The smaller the range of action of this internal stress, the stronger the stress effect. When the interaction force inside the glass reaches a certain level, it will explode.

To avoid this, technicians put this layer of black spots on the edge of the car window. (Old sliding windows rarely have black spots.) Because black absorbs heat, when exposed to the sun, the area with more black elements has a higher temperature. The black spots on the edge of the car window are of different sizes and density distribution. The closer the black spots are to the edge of the glass, the larger the black spots are and the higher the temperature is; conversely, the closer the black spots are to the center of the glass, the smaller the black spots are and the lower the temperature is.

With this transition layer, the temperature gradient distribution area inside the glass window becomes larger, the scope of internal stress becomes larger, and the resulting effect is weakened accordingly.

You don't have to worry too much. In recent years, the quality and output of tempered glass in China have increased simultaneously. Bus windows are greatly affected by vibrations, and the surrounding environment is also bad. But after taking so many buses, only a few "lucky people" have encountered the window exploding. And even if the glass explodes, it will not shatter into slag like in the movies, at most it will crack.

In addition, the tempered glass of bus windows is close to passengers, and the driving of the vehicle often causes rapid changes in light and temperature conditions, so the risk of self-explosion needs to be considered. However, the glass of home decoration and buildings generally does not face frequent and rapid changes in temperature environment, and is far away from people, so there is generally no need to worry about its self-explosion.

Source: China Science and Technology Press