Monday, 22 July 2013

Post 110 Why annealed copper wire?

Just before we get onto that I found a better illustration of what happens when we bend wire - illustrated on a stress strain curve.

We start by winding the wire onto the bonsai branch. Depending on the winding pitch and the size of the branch etc some parts of the wire will still be in the elastic range and some parts may have been bent far enough to reach the plastic range.

As we start to reposition/reshape the branch by bending it and the wire we take the wire into the plastic range between A and B. If you drop a vertical line down to the Strain axis that will show the distance moved.

As we continue to bend in the plastic range more effort is required because the wire hardens with the work being applied - it work hardens.

If we keep bending to C and then release the pressure, because we are no longer in the elastic range the wire does not retrace its path but follows the line down to F. So going to C meant that we bend the branch a distance of G and then when we take off the pressure it returns to F. That is without any consideration of the additional recovery forces applied by the branch which remains elastic, and wants to go back to where it started
That's why when you use wire to bend a branch of any maturity you ALWAYS have to bend it further than you want to reach a certain position.

That analysis is based on the generic ductile metal curve. It gets really interesting when you start to look at the curves of the real data for Drawn Copper, Annealed Copper and Aluminium.

Annealing of copper is done to change its properties to become softer and more maleable, by heating to a high temperature and then cooling to change the crystalline structure of the material.

Thanks to Cambridge Uni for this chart.

The RED line is for Work Hardened copper - and with wire being made by a die drawn process it will be work hardened.
The BLUE line is for annealed copper.
The PURPLE line is for pure aluminium.

 So what does this tell us?

You can see that the work hardened copper has a short elastic displacement but with high effort. This says that relatively it takes much more effort to change its shape and unless you take it far enough it will come straight back. Once it yields however it doesn't work harden much and will continue to deform with reducing effort.

It gets more interesting when you look at the annealed copper curve. It basically has no elastic range, is easier to bend, deforms straight away and the more you bend it the harder it gets, all good characteristics for bonsai work and exactly why some would say that if it's not annealed it's not worth using. To put some cream on the cake the fact that it has no elastic behaviour means it is more likely to hold its position when load is released.

Now the aluminium. It has a short elastic range and then after that is like the annealed copper only easier to bend (and thus has less holding strength). But you can only take it so far before it will weaken and break. This data also shows that the peak stress of aluminium is about half that of annealed copper. In the numbers I was using in the last post I assumed it was 70% higher. If it is 100% higher you are going to have to use an aluminium wire of around 20% increase in diameter to match the annealed copper, so it's not such a big difference, but you still have the overbending/recovery to deal with.

 So today's conclusion are:
  • Annealed copper is much much better than aluminium, in all regards.
  • If you are going to use copper if it's not annealed you might be better off with aluminium.
  • If you are going to use aluminium use a heavier wire than you would for copper and be prepared to have to overbend to get a branch to stay where you want it.
 Isn't it great when the science confirms what you already know.

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