I have noticed that in very thick glass (>1"), cracks don’t seem to spread-a piece can chip out, but the cracks don’t spread . In thinner glass(like a bottle or window pane), you hit it, and the entire piece will shatter. Why is this?
My understanding is that thick panes of glass are usually tempered, If you hit them hard enough, they will make little redneck diamonds. Thinner sheets either can’t be tempered or it is not economical to do so. Tempering glass makes it tougher, being thicker also helps here. The chip that comes out is the part of the glass deformed by the impact which separates, expending the energy of the impact and leaving the rest of the mass relatively untouched.
Regular thin glass propagates the impact all the way through, and out to the edges, deforming the entire sheet. Multiple fractures start and run through with the shockwave.