I read an article a while back that stated certain properties of gorilla glass and it's strengths and one sort of flaw. The flaw had to do with drops or pressures applied to the edges. This is the weak area and can cause the entire piece to shatter. A drop that hits it more or less centered on the glass, away from edges is usually not going to break as a drop or pressure on the edge would be much more likely to shatter the piece of glass. I've dropped mine a few times before I got a case and cringed each time as I picked it up to examine. Lucky I guess, no breakage. If it hits just right tho, it shatters. Glad you got it replaced. Sometimes Verizon comes thru as they should.
I would confer with this information. The weakest part of any glass that's been molded, free-flowed, and especially cut is the edges. Since there are microscopic stress fractures that accompany the cutting process, it's nearly impossible to make the edge as stress-free as the uncompromised middle of the glass. Even heating the edge to melting point doesn't remove all stress since there will be temperature differences at the time of edge treating which remain as stress once cooled. In all drop tests you see where they demonstrate the Gorilla Glass against a "normal" glass screen, the ball drops square in the center, and in a bend test, it's across the entire surface - again with the fulcrum squarely in the center of the screen.
Believe it or not, it's much more difficult to fracture the center of glass than you might think, and in terms of impact pressure it's many times higher than the same type of hit to an edge. The stresses mentioned above play a part, but even more important is that when hit in the middle, the glass would have to fracture in many directions at once to break so the surrounding glass around the impact point acts as a vice to keep the glass from cracking and vibrations centered there, and then as a buffer to allow the vibrations to travel in all directions evenly. Conversely if the glass is hit on an edge, there is really only one direction the fracture must travel to relieve the stress - any direction that is perpendicular to the edge, and there's a "free plane" (the edge) that allows the stress to be relieved immediately.
When hit with a BB, for instance, the stress must "fan out" from the point of impact and find its way to the opposite side of the glass quickly. This is why if your windshield is hit by a stray stone, you most often get that circular, cone-shaped mark on the glass. It"s the vice-like properties of the glass around it which can't expand to distribute the stress that causes the impact to move through the face of the glass and to the back of the glass in a widening pattern.
Still, there are times when those micro-fractures along the edge can be under such extreme pressure - albeit extreme in relation to the lack of stress further on in the glass, that can suddenly and without apparent influence and warning just decide to release that stress and you have a crack. There have been many posts of people who went to bed at night and their phone was fine, but the next morning it had a crack on it. The same story plays out with phones in pockets, on desks, virtually anywhere.
Things that can contribute to setting the crack off (the catalyst), are changes in temperature (especially if different across the face of the glass or if changing from two extremes quickly), vibration of any kind, bumps, drops or even the slightest pressure. In the case of the pocketbook fracture, it could have been even a soft item resting against the screen that triggered the failure and due to a relatively low amount of pressure, but simply enough to deflect the glass and enhance the stress at an edge.
Finally, twisting flat pane glass is even more stressful than simply bending, and so a phone that is for instance sitting on a coin on the desk and a downward pressure causes one or two corners to dip further than the other three or two, will cause torsion stress that will "tear" at the edge, setting off the fracture. This can happen very easily in a front or rear pocket while bending or sitting down. Many have reported their phones cracking in their pockets after bending down to pick up something.
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