Geology Experiments

Snap bracelets from tape measures - Bistable structures

Sun, 21st Nov 2010

Part of the show Smart Pills: Drugs to Boost Brain Power

What you Need

An old tape measure you no longer want

Some tape

Some old scissors or tin snips

What to do

Cut a length of tape measure about 15-20cm long.

Warning - the cut edges will be very very sharp

Cover the cut edges with some kind of fairly tough tape, to make them a bit safer, though still be careful.

Curve the tape around a tight radius of around 6-10mm so it just starts to deform, you can either do this by rolling it around a pencil or I found it easier to just bend it between my hands until you can feel it starting to deform.

Try bending some tape with the inside of the curve and on the outside, and see if it behaves differently.

If you over do it you may need to bend the tape into its original curve a little.

What may happen

You should find that the tape has two stable states rolled and straight. Depending how you rolled it the tape will behave slightly differently.

 

Making a bistable structure from a tape measure

Why does it happen?

The tape measure starts out with a curve across the tape, which gives the extended tape some depth making it much stiffer than it would be otherwise. This is done by deforming the steel tape in the factory so that the bottom is just slightly longer than the top, making it curve.

 

Tape

Straight Tape

A tape measure is slightly curved which makes it much stiffer when unrolled.

It is curved because it has been bent so that the bottom is slightly longer than the top across the tape.

When you deform the tape, you do something very similar, but this time bending it in the other direction into a coil.

 

Rolled Tape

In the rolled tape the outer side is slightly longer than the inner one.

This gives two stable states, but to get from one to the other without distorting the tape even more it has to go through a flat stage. And this stage is very unstable, (particularly for tapes which have been bent backwards) giving the structure two stable states so it is called bistable.

 

Stresses if rolled inwards

Stresses if rolled outwards

If you have bent the tape upwards a flat tape can roll longways of crossways, but remaining flat is very unstable.

Similarly if rolled downwards the tape has 2 states, but the transition between them is more violent, making a better snap bracelet.


These properties can be used in a variety of ways, as snap bracelets or bicycle clips of course, but also for remote handling in nuclear power stations, and for making poles to mount cameras on for the military. To find out more see the interview with Keith Seffen in Naked Engineering

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