What happens when a bullet hits an 'unbreakable' Prince Rupert's drop? - PHYSICS

For those of you who aren't familiar with the Prince Rupert's drop, this weird, scientific enigma is a glass object that's created by dripping molten glass into very cold water.
Is a teardrop-shaped piece of glass that's pretty much unbreakable at its bulbous 'drop' end, but which shatters from the slightest pressure at the elongated tail end. Scientists have been obsessed with them since the 1600s. But what happens if you shoot one with a bullet?

In glorious slow motion, you can watch as the bullet bounces right off the wide end of the drop, sending out shock waves that then rattle the rest of the structure and cause the thin end to break, resulting in the entire thing exploding.

When the Prince Rupert's drop is made, molten glass is poured into extremely cold water, causing the outside of the drop to cool and solidify almost instantly, while the inside remains molten and cools more slowly.

Because of thermal expansion, glass wants to expand while it's hot, and contract while it's cool.

That means that as the molten inside of the glass gradually cools down, it wants to contract and pull the solid outer layer inwards. But because the outer layer is already solidified, this just makes the whole thing tighter, making that bulbous end of the Prince Rupert's drop pretty much indestructible, and, as we now know, bullet-proof.

But because the outside of the glass is in extremely high compressive stress, and the inside is in extremely high tensile stress, if one link is ever broken, then the whole thing explodes, feeding off its stored internal energy.

This is what happens when the fragile thin end at the back of the drop gets broken - it releases all that pent-up energy and causes the whole thing to shatter.

Video source via Smarter Every Day:https://www.youtube.com/watch?v=24q80ReMyq0

Story via ScienceAlerthttp://www.sciencealert.com/watch-what-happens-when-a-bullet-hits-an-unbreakable-prince-rupert-s-drop

What is Prince Rupert's Drop anyway?:http://www.businessinsider.com/the-physics-of-prince-ruperts-drop-glass-smashing-2015-6