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Automotive Safety Glass Strength - Amount of Force to Shatter Safety Glass


Phil Fields

I have searched the net for information about automotive safety glass, but could not find the information I was looking for. Does anyone have information concerning the amount of force it would take to shatter safety glass?
Here is the issue. My son (9th grade) had an issue on the bus last week, another child pushed his head into the window of the rear emergency exit door, shattering the window. The school principle is considering this a shared responsibility. I have no doubt that my son my have started the issue, basic teenage boy B. S., but for another child to react with such force, I do think is appropriate and does not warrant shared responsibility.
I am planning on discussing this with the school principle, this information will enable to discuss the issue with facts.

p.s. My son is fine, he did not get injured, he said he had a headache for half a day.

Cordon - 2007

Re: Question, Automotive Safety Glass Strength

I googled "tempered glass" and came across this:

Annealed Glass Tempered Glass
Typical Breaking Stress (large light 60 sec. load) 6,000 psi 24,000 psi

Typical Impact Velocity Causing Fracture (1/4" light 5 gm missile, impact normal to surface 30 ft/sec 60 ft/sec

Steve McQuality

Quality Engineer
Just food for thought... I used to work for an independent test lab in the construction industry, and we used to certify tempered safety glass to an ASTM standard. It's been too many years for me to remember the specification/test standard and I don't recall if automotive safety glass was included - or just tempered glass for doors/windows, etc.

Try doing a search for ASTM standards. I'll try to do a quick look on my own at the same time...

... As I was looking, ANSI rings a "louder bell" in my mind. Try looking for ANSI standard for safety glass or safety "glazing", as installed glass is often referred to...
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I don't know that you could figure this out without having the detailed specifications for the piece of glass in question. It has been awhile since I've been on a school bus, but I seem to recall that the bus windows were not nearly as thick as what I see in the windshield of my car today. I would think a windshield would be much more impact resistant since it has a greater possiblity of being struck by rocks, birds, etc.

Can you take a look at another intact piece of glass to see if there are any markings that might indicate a specification, manufacturer or anything else? I know there are on my windshield, but can't tell you what they are without being in my car right now.


Automotive windshields are a lamination of a plastic film between 2 pieces of glass and are designed so that the glass remains attached to the plastic in the event of impact, thus reducing the potential injury from flying shards.

Side lights and bus door windows are made from toughened or tempered glass. During processing the glass is heated then blasted with cold air which creates a stress “sandwich” with compression on the surface and tension in the middle. When tempered glass breaks, the stress is released instantaneously which causes the glass to shatter into small, relatively safe fragments.

Typically, it requires a high force in the centre of the panel to cause it to break, however the edge is the weakest area and a small sharp impact on or near the edge can cause it to shatter.

If the bus window had a small edge defect or there was a small sharp object in the seal (E.g. a small stone, a fixing screw) touching the edge of the glass, then the force would be concentrated on the edge causing the glass to shatter more easily.

I’m sorry if I can’t be specific about the forces required to break the glass, but it depends on many factors such as the thickness, the quality of the tempering as well as the edge condition and the seal condition.


You got some great responses. I tested this more times than I care to imagine. U.S. Federal Motor Vehicle Safety Standard called for dropping a five pound sttel sphere (approx 4.5 inches in dia as I remember) from 12 feet high onto a 12" X 12" sample cut from a windshield. It must withstand this force. Then we would see how far above the standard we were performing by doing the same test from 19 feet of drop. Many times this held too, but wnough broke to say this was the acid test.

Hope that helps. Its a field test type of answer versus a lab answer which was so well done in :applause:a previous post.
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