Re: What caused tempered glass to shatter?

An interesting question. The short answer is that the tempered glass breaks by two different mechanisms, and the difference in the mechanisms causes the glass to appear to react differently. The problem is that tempered glass has a built-in self-destruct mechanism that obscures the results.

Tempered glass is produced by uniformly heating a pane of glass to a high temperature (near the glass softening point), then cooling the outside surface of the glass with jets of cool air. The outside of the glass contracts and solidifies while the center of the glass is still very hot. As the center of the glass pane gradually cools and shrinks, it puts the outside surface of the glass in compression, while the center of the glass in in tension. These residual stresses are strong enough to cause the entire pane of glass to shatter if there is a point of initiation, causing the glass to fracture into many fine particles. It is also this tempering process that is responsible for making tempered glass much stronger than regular glass.

Your question deals with what mechanism is responsible for causing a localized failure of the pane of glass. For example, if you were to try and cut a pane of tempered glass with a glass cutter, generally the glass will fracture as soon as the glass cutter scores the glass surface (this experiment is not recommended; I tried this experiment not knowing the glass was tempered and ended up with a nasty cut). In the case of a sharp metal object pressing against the tempered glass, there may be sufficient force to cause the glass to fail at the surface, immediately under the sharp metal object. If the localized stress exceeds the strength of the glass, the glass will begin to fracture at that point, and the fracture will spread throughout the tempered glass due to the residual stress built into the glass. The equation for stress is force/area. According to http://www.alumaxbath.com/tech/tgp.htm, the stress required to break tempered glass is about 24,000 psi.

Let's say that I am using a screwdriver to apply force to the glass, and the tip measures 0.25 inches by 0.01, then the area of the tip is 0.0025 square inches. The force necessary to exceed the 24,000 psi strength of the glass is 25,000*0.0025 or 60 pounds of force. If the object is much smaller, obviously a much smaller force is needed to break the glass. So, with a sharp metal object, I really don't need to apply much force to make the glass fracture.

For the sake of illustration, let's say that instead of a pane of glass, you are pressing against a sheet of aluminum metal. If you press against the sheet with a sharp steel object, you will see a dent develop in the aluminum sheet, caused by the local failure of the aluminum. Aluminum is much tougher than tempered glass, so even if I push hard enough to produce a hole, the aluminum wouldn't shatter like the tempered glass will if I start to put a hole through it.

In the second case, you are pushing against the tempered glass with your hand to cause it to fail. If I wanted to exceed the local strength of the glass with my hand, as in the case of the pointed object, I would have to apply a lot of force, certainly a lot more force than is needed in the case of the pointed object. If the area of my hand is perhaps 5 inches by 5 inches or a total of 25 square inches, the force from my hand is spread over a much larger area of glass. If the same 24,000 psi strength must be exceeded for the glass to fail by the same mechanism, then the force is equal to 24,000 psi * 25 square inches or 600,000 pounds to get the glass to fail at the interface between my hand and the glass. Clearly my hand cannot apply that much force.

When a load is spread relatively evenly over the glass, the whole piece of glass will bend away from the direction where the force is applied. If you think about a sheet of aluminum replacing the glass again, as I apply more force with my hand, you can perhaps envision eventually bending the aluminum away from my hand. As my hand presses against the glass or aluminum, the surface in contact with my hand builds up compressive forces as it is bent, while the surface on the opposite side of the plate starts building up tensile forces. As I push harder, the tensile forces opposite my hand may eventually build to a point that the glass (or aluminum) will fail. Again, because of the residual stresses in the tempered glass, the glass will break into thousands of tiny pieces. However, in this second case, because the force is distributed over a much larger area, much more force is required to break the pane of glass than is necessary using a sharp metal object. Up until the point of fracture, the pane of glass has been resisting this force. When the glass fractures, it is the greater force that causes the remnants of the pane of glass to move away from your hand.

I hope that gives you a bit of insight into what is happening in the two cases.