What would cause a glass outdoor table top to explode spontaneously?

Question

This happened a couple of weeks ago and when we found it the glass was shattered into hundreds of tiny pieces and was making a crackling noise. There were no sudden changes in temperature and it was not struck by any type of missile.

Very strange!!

Answer 1

heat man

Answer 2

Even without a sudden change in temperature a glass table top could "explode", especially if it was fitted or held in place with screws or other structures that ran through holes in the glass.

Assuming it was a glass/metal table, heating (by sunlight) could have caused differential expansion of the glass and metal. This could have caused the metal to expand in the s***w-holes and crack the glass.

Another process that may have happened (and both may have happed simultaneously) would be for the table or table top to expand in a way that put tension on the glass table top.

Glass is a "fragile" material (this is the technical term, not the common one). A fragile material cannot deform, so if the tension exceeds the material resistance it cracks (sometimes in a spectacular way, as in your example).

The opposite of "fragile" is "ductil", a material that is able to deform under tension (like metal). Within limits, the material is able to deform and return to its original shape (like a rubber band). Above that limit the material deforms permanently, like a crushed Coke can.

The fact that the glass pieces were "crackling" suggests termal expansion caused tension in the glass past its resistance limit and it cracked. Because there were no previous cracks or defects in the glass (i.e., weak spots) the whole piece suffered sudden "catastrophic failure" (another technical term from materials science). Once the tension released, the broken pieces were returning to their original configuration macking the crackling noise.

This is a bit like the sounds you can ear from ice when you drop it on your juice glass. Sometimes you also get the ice cubes to crack.

Answer 3

Glass outdoor table tops, by law, must be tempered glass.  Tempered glass is prestressed glass that is very difficult to break because it is tensioned in a direction opposite of which is needed to break it.  When it does break, however, it breaks catastrophically, exploding.  This is also part of the design and the reason for legally requiring tempered glass.  Instead of having large shards of dangerous glass outside on your deck, you are left with thousands of harmless little cubes of glass.  Lots of things can start your glass cracking--heat stress, stress around the attachment points, a manufacturing flaw, falling acorns, etc.  All it takes is one minescule crack to make a piece of tempered glass explode.