# Tempered glass physics

I have two related questions

Why is a thicker tempered glass stronger than thinner? For example, I have to hit much harder at 3/4” tempered glass panel surface than I hit 1/4” panel in order for glass to shatter. My understanding is that tempering pre stresses and creates a tension in a surface layer. Why does the glass thickness matter if the “shield” is about the same.

Did anybody study possibility of tempering the glass panel all the way through as opposed to just the surface?

• When you hit glass, you're bending it, and putting one side of the glass in tension. A thicker pane of glass will not bend as much for the same input force – Jonathan R Swift Jun 21 '19 at 9:43

Tempered glass resists fracture because it is manufactured in such a way as to place its surfaces into a state of residual compressive stress. To cause it to experience brittle failure in tension (which is normally how glass breaks) requires overcoming those frozen-in surface stresses, which normally act to squeeze shut any surface pits or scratches at which brittle failure would begin.

It's going to take more work to shatter a thick piece of tempered glass simply because the thicker a piece of material is, the better it is at distributing applied stresses so that they do not cause it to fail.

Since the greatest stresses in a material being bent by a load are at its surfaces, a strengthening treatment which is applied to those surfaces will yield significant improvement in its performance even though the interior of the part hasn't been strengthened. This approach has been satisfactory for things like sliding glass doors and car windshields, but the invention of smart phone touchscreens has required the development of highly-strengthened glass called "gorilla glass", which I recommend you run a search on for the details of its manufacture.

By definition, "tempering the glass panel all the way through" is not possible. The interior and exterior parts of the glass must have different properties.

Tempered glass is made by cooling the exterior and interior at different times:

• The glass panel is heated to a uniform temperature hot enough to soften it.
• The exterior surface is cooled by air until it becomes hard.
• The interior part remains soft enough to flow.
• The entire panel is allowed to slowly cool back to room temperature.

The essential part happens during the final cooling period. As it cools, the glass shrinks, but the outer surface is already cooler, so shrinks less than the interior. By the time the entire panel has cooled, the outer surface is pulled tightly together by the interior, which has shrunk more than it has. At the same time, the interior is pulled outward by the exterior layer.

This stress (compression of the surface layer and tension of the interior) is what gives the glass its strength.

On the other hand, while the compressed outer layer is extremely resistant to damage, when it does experience damage, it is catastrophic. A hard blow by a sharp object can create a crack in the outer layer. This will relieve some of the internal stress and cause the crack to spread rapidly. This will distort that part of the glass, which will create extra stress on the surrounding areas, which will themselves crack and propagate the destruction throughout the entire panel.

This explosive shattering into small pieces is what makes tempered glass much safer, as it does not leave any large jagged pieces that could cause severe injuries. Side windows on cars are made of tempered glass for this reason.

More interestingly, it is possible to break the interior of tempered glass without breaking the exterior layers, such as by a single hammer blow to a nail directed into the edge of a glass table top. Though it can look beautiful, the result should not be used, as it will no longer have its original strength nor its safety features. It should be discarded or safely framed as "art".

The "Time Warp" TV show featured an example of interior shattering:

• Also, if you want to break a piece of tempered glass you go at an edge or corner (of the outside glass) with a hard sharp punch and a hammer. That makes the surface divot that @RayButterworth talks about here, and breaks the whole panel. (Info picked up from family members who are firefighters). – TimWescott Jun 21 '19 at 18:48