vetro schegge auto

Why don’t any shards form when car glass breaks?

The windshield and side window glass of our cars does not create splinters in the event of an accident because it is produced using particular mechanical and thermal processes. These, as we will see, avoid the formation of dangerous splinters following a possible breakage or in any case drastically limit the quantity produced. That is, the breakage mode is very different from that of a glass falling to the ground. The greater safety obtained is the result of specific engineering that leads to distinguishing common products from more specific ones. safety glass, often characterized by an intermediate layer between two levels of glass, capable of retaining the fragments produced. In this article we focus on the latter by describing the processes and physical reasons which lead to this technological solution.

Common types of safety glass

The glass used as a closing element of a vehicle has specific characteristics, such as to guarantee a certain resistance to impact. Not only that, they must also be able, once their resistance has been reached following the application of high forces on them, to break in such a way as to limit damage and safety to people as much as possible: in these cases we speak of Safety GlassFrom a technical point of view they are distinguished three main typologies.

Armored glass

These are glasses that are made by inserting a layer of glass during the lamination process. wire mesh with the function of retaining the fragments following impact and fracture (i.e. reaching the mechanical resistance of the element). These therefore allow the separation function offered by the glass itself to be kept intact from a macroscopic point of view. Furthermore, they greatly delay the propagation of heat, for example generated following fires.

Laminated or laminated glass

Similarly to the case of reinforced glass, laminated glass is obtained by interposing a layer of glass between two sheets of glass. third sheet of plastic material. This package thus prepared is heated and hot pressed. The final result is that the glass, if subjected to a violent impact, shows a radial break and the plastic material inserted during the processing retains the fragments that form following the numerous fractures caused.

Tempered glass

These are glasses that guarantee greater resistance to impacts thanks to all the manufacturing processes they undergo up to the use phase. Their particularity lies in the fact that their breakage evolves towards a widespread fragmentationin which each small piece of glass forming the whole takes on a shape without sharp edges. Although they greatly limit the risks following breakage, tempered glass has the defect of being difficult to work with after the tempering process, as well as having the tendency to break completely even if only a small part of the sheet is stressed beyond the resistance limit.

Among those previously examined, tempered and laminated glass I’m practically always used in the world of cars. They are used to make the windshield and all the windows in the vehicle. In particular, the windshield is usually made of laminated glass (in many countries by law) so that it can guarantee its integrity even after a strong impact. In fact, the road safety problem does not only concern the limitation of glass fracture and the creation of strong sharp edges, but also the maintenance of integrity in the moments following the impact, so as to guarantee safety in the driving phases immediately after.


The glass tempering process

From what has been described above, the glass processing process that takes it from its initial composition to its actual use appears crucial. In this context, the tempering process is the one that plays the fundamental role in the production of sheets used for windshields and car windows, for example. Tempering is a thermal processing of simple glass plates which consists of two distinct phases:

  • In a first phase, the plate under examination undergoes a strong heating up to approximately 700°C;
  • In a second phase, the same plate that has reached these high temperatures is quickly cooledso as to generate a significant thermal shock on the surface of the plate.

The contraction and expansion of glass under thermal action are the cause of the birth of a state of internal tension which improves the mechanical characteristics of the plate as a whole: that is, a more resistant product than the starting one. This is possible because the surface layers of the glass cool down first, because they are directly exposed to cold sources. Instead, the internal core of the sheet has a gradual temperature variation from the initial condition of 700°C, because it is not directly exposed to the outside but constrained by the surface layers.

However, tempering does not only allow for more resistant glass to be obtained: as mentioned above, if the resistances are reached (even if higher than those of common glass), the breakage now occurs in such a way as not to generate fragments with sharp edgestherefore dangerous for the safety of people. Rather, the glass that has undergone the tempering process will break forming many small fragments that are almost spherical, or in any case with a rounded outer surfacealthough not necessarily regular!