Why is an edge of broken glass so sharp?

[Nepsesh]

A simple question that has always had me wondering.

 

A broken pane invariably presents shards with smooth, perpendicular edges. A steel edge of 90 deg, even when precision ground and then honed, will not cut an unwary finger so readily as the bluntest profile in glass. The shape seems almost irrelevant to its keenness.

 

Is it "molecular" or what? Does anybody know please?

[Phi for All]

A steel edge needs to have the right balance between toughness, which it gets from the iron, and hardness, which it gets from other elements like carbon or chromium. It needs to have an edge that's sharp enough to cut (which is more like 15-20 degrees for a straight razor; 90 degrees wouldn't be good even for a wedge or an axe), but not so brittle that the edge is destroyed the first time you use it.

 

The glass is under no such restrictions. The only job it has at that point is to cut you. It doesn't have to preserve it's razor edge, and it breaks in such a way that the razor edge has smaller razor edges, so a piece of broken glass is like a serrated knife, with tiny little saws along its length. It's extremely efficient at parting skin with little pressure. It won't hold up to much abuse, though.

 

Glass doesn't have a crystal structure the way you might think, iirc. It's an amorphous solid, and it behaves in some interesting ways.

[swansont]

I would venture a guess that cleaving along a sharp edge is a lower energy configuration than leaving a rounded edge, but you'll get a spectrum of sharpness. You get a lot of edges, so some will invariably be sharper than others.

[Nepsesh]

Thanks for your most interesting and prompt replies. Much appreciated. M.

[studiot]

First off let me point out that most glass surfaces/interfaces with the rest of the world are not sharp they are smooth.

For example the surface of a window pane or light bulb.

 

However glass also has significant locked in residual stresses.

These make it quite unlike a metal lattice which is basically in equilibrium.

The glass structure is more like a taught bow, with significant energy available if the string is released.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&biw=&bih=&q=residual+stresses+in+glass&gbv=2&oq=residual+stresses+in+glass&gs_l=heirloom-hp.3...688.11641.0.11938.26.15.0.9.9.0.265.1718.0j9j2.11.0....0...1ac.1.34.heirloom-hp..11.15.1922.ucBFGoQkFeY

 

Traditional stress relieving techniques like anealing do not work fully with glass.

 

Glass is subject to a particular form of brittle failure known as fast fracture.

This is exacerbated by the residual stress energy, which promotes rapid and catastophic crack propagation once initiated, because of the available residual stress energy.

The shortest distance between two points in the engineering world is a straight line, so straight cracks with angular (sharp) edges form preferentially over other shapes.

 

http://ocw.mit.edu/courses/materials-science-and-engineering/3-11-mechanics-of-materials-fall-1999/modules/frac.pdf

 

Does this help?

[Nepsesh]

Yes, very helpful. Now you mention it there's a lot of info online about this subject.

 

I was previously aware of the awsome Rupert's Drops phenomenom yet did not connect the two. Thanks for your reply.