Mechanics: Prince Rupert's Drops

PIRA Class: 1R20.70

Purpose:
To demonstrate how the stress distribution within an object can result in unusual strength and fragility characteristics.

Description:
You can strike the bulb of a Prince Rupert's drop with a hammer and it will not break. However, a small nick to tail will cause the entire drop to explode.

Drip molten glass into cool water. The outside quickly cools and begins to contract while the inside is still molten. When the inside begins to cool the outside is already hardened. The cooling inner glass pulls in on the already hardened outside glass. The result is a Prince Rupert’s Drop in which the surface is in high compressive stress (being compressed) and the inside is in high tensile stress (being stretched). The cooled drop is in an unstable equilibrium.

The high compressive stress in the surface of the bulb (up to 7,000 times atmospheric pressure) makes the surface fracture resistant. The bulb can be hit with a hammer or even struck with a fired bullet without breaking. However, if you disturb the tail of the drop creating a crack that reaches the inside of the drop which is under tension, the entire drop will explode with the explosion propagating from the tail to the bulb.

We have a very small box with a few Prince Rupert's drops. When held between crossed polarizers the stress in the glass becomes visible. The stressed glass rotates the angle of polarization and the amount of rotation id wavelength dependent resulting in a rainbow of colors.

Equipment:

Prince Rupert's drop, two large polarizers

Location:
Shelf 164 (Prince Rupert's drop), shelf  362 (polarizers)

Videos
Smarter Every Day
Smarter Every Day Part 2
Phys.org article