# Why can one bend glass fiber?

Why can one bend glass fibers without breaking it, whereas glasses one comes across in real life is usually solid?

Is there also a good high-school level explanation of this?

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I think it depends a lot on how the glass is formed. en.wikipedia.org/wiki/Prince_Rupert's_Drop youtube.com/watch?v=6V2eCFsDkK0 –  Dan Brumleve Mar 13 '11 at 22:35

The critical parameter for materials under stress is the strain, defined in a general way{*} as the fraction change some length over that same length: $$\lambda = \frac{\Delta l}{l}$$

So take a given a fiber of diameter $d$, and bent around a radius of curvature $r$, the strain of either the inside or outside edge is: $$\lambda = \frac{((r \pm d) - r) \theta}{r \theta} = \pm \frac{d}{r}$$

Now you need only look up the breaking strain for the glass in question.

{*} Or fractional change in volume over volume, or displacement as a fraction of length, or... It really depends on the details of the situation you're trying to measure, but the above definition will work for tension and compression.

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In addition to the question of bend radius - there is also an effect of surface scratches.

Most materials are very strong - they fail because a surface flaw allows a stress concentration - ie a crack to form. glass fibre has a very smooth surface because of the way it is made and can be put under high stress without cracking.

You can show this with a thick glass rod (or you could before we were banned from doing anything interesting by 'safety'). Clean the surface with acid, or heating and annealing, so any defects are removed you can then bend the rod like rubber. But touch the bent part with a metal ruler to create scratches and it will shatter

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D'oh! I've never see this demo, and now and want to. Bad. –  dmckee Mar 13 '11 at 18:56
@dmckee: me too. Maybe when I'm back in the classroom... –  Jerry Schirmer Mar 13 '11 at 20:00
Thanks, I will try this. Nevertheless, do you have a video link for it. –  student Mar 14 '11 at 8:20

Having spent a great deal of time working with "normal" glass (one of my many hobbies), I can assure you that, in fact, all glasses can be bent. When cutting large sheets of glass, I always see the sheet bend before it breaks. Every now and then you have to strike it twice; the first time you bend it, it fails to break. Fiber optics differ from the usual glass partly in that it is very thin and so bends much more easily. After you've cut a few thousand pieces of glass you come to recognize differences between different compositions. Some glasses feel very smooth when they bend, others, like the stained art glass shown below, are rather ragged and difficult to bend. (And all breaks or cuts in glass begin with a bend.)

To get a more familiar metaphor, think about wood. It doesn't surprise you when a slat of wood that is 1mm in thickness is bent in a circle. Now imagine doing the same thing with a much thicker piece of wood. You can't do it with the thicker wood because it's too thick.

In addition, regular glass differs from fiber optic glass in that fiber optic glass is very pure and this makes it very strong. In the art glass community, thin glass rods is called "stringer". Let me see if I can find some photos of cold stringer being bent.

Okay, I found some clear stringer. I put it on a sheet of stained glass supported by two pennies. I took two photos, before and after pressing the stringer in the middle. You can see the bending. Straight:

Bent:

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I should add that the stringer shown above is Bullseye 1.5mm clear stringer: bullseyeglass.com/products/accessory-glass/rods-stringer.html and the background glass is either an old style no longer sold or is: bullseyeglass.com/products/sheet-glass/streakies/… –  Carl Brannen Mar 14 '11 at 2:20
Hmmm. It seems likely that the 1.5mm stringer is probably Uroboros system 90 clear 1.5mm, a type that isn't sold anymore that I could find. –  Carl Brannen Mar 14 '11 at 2:29