How Automotive Safety Glass Works

#1
Modern vehicles have two main types of glass on the body, the front, laminated windshield, and the side tempered glass.
These are both considered "safety" glass, and work in different ways to protect occupants from getting hurt when they break.


The front windshield is made of laminated glass. That is, two layers of regular glass sandwiched around a thin laminate layer.
The glass is prepared per DOT standards as marked on the bottom right:



The thin laminate layer helps to keep the glass together when an object strikes and cracks the glass.
Here I attempt to break the windshield using a toothbrush I borrowed from my brother:



After a few blows, you can see how the glass has held together. The glass has broken into relatively large pieces surrounding the impact zone but is held together.



Breaking through the second layer of glass gives us an idea of the gooey laminate layer in between. This is meant to held the pieces of glass together when broken, and form a barrier against object penetration.
That's how it protects occupants.



The side glass on most (regular) vehicles is tempered. Tempering is a heat treatment process that makes the glass much stronger. It is also less expensive than laminated glass, due to its simple construction.
Tempering is also considered a safety glass.



Again, I demonstrate how the glass will break by striking it with my brother's toothbrush. This glass was noticably harder to bust than the front windshield.



Here we see that the glass breaks into millions of small pieces. It made quite a mess.



Smaller tiny pieces of glass aren't as sharp and won't cut or harm you as easily as big shards. I also noticed the glass bits were breaking up on its own and jumping around, as if the heat treatment had it under stress and it was releasing that built up energy.



And that's pretty much how the glass on your vehicle is made to keep you safe!
 
#2
Speedkar9,
Thanks for posting this excellent series of photos. It's a great teaching point for my kids learning to drive. I believe it was up until the very early 1960s that laminated glass was used in all of the car windows but then auto makers changed to tempered glass except for the windshield. As you mention, tempered glass is cheaper to produce and the change was made for economic reasons - it did not improve safety. In fact, flying fragments of tempered glass in an accident can still induce injury.

You used your brother's toothbrush for this demonstration, rather than your own. Was that your brother's car, too? :)
 
#3
The tempered glass in the side windows is designed to save you from drowning if your car ever goes into the water. If you drive into the water you can't open the doors because of the pressure difference, and you may not be able to open the power windows if they shorted out because of the water. This means that you must wait until the entire door is under water and the front half of the interior is full. What to do? Pull your headrest out of the seat, the prong or prongs that support and adjust it are designed to fit into any one of the corners of a side window . Use it like a lever and snap shatter the window (side windows are designed to be easier to break from the inside). You will be able to crawl out way before the door is fully submerged and not be struggling under water to open said door.
 
#4
I'm glad you enjoy my post. Please check out my channel for more videos of me tearing down a Corolla.

The tempered glass is still classified as a safety glass. While small bits and fragments may still bruise you, its much safer than having untreated glass that can break into giant shards that may decapitate someone.
 
#5
speedkar9

Great job and I learned something. I didn't think that windshields were still laminated but were now tempered safety glass. The theory was that in a collision the windshield would shatter and continue to move forward over the hood away from the front seat passengers. Is the Laminated glass designed to pop out in a head-on? Nobody seems to know. Once again good article and great pictures. Oh speaking of pictures how do I get a picture onto my profile and/or my post?
 
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