The challenge presented here is to take a piece of ordinary copper
tubing (purchased at any hardware, plumbing or refrigeration supply
shop) and bend it back and forth. You will notice that the material
becomes harder the more you work it. Bend a piece back and forth
in several places, to make it as uniformly hard as possible. Bare
hands are the only tools allowed.
1. Your copper tube must be straight enough to pass through a tube
of 25 mm inside diameter.
2. Your hardened tube will then be clamped to a table, and a weight
hung from it at 20 cm from the table edge.
3. When a deformation of 30 mm at the 20 cm position is recorded,
the tube will be considered to have failed.
4. The winner will be the team which produces the hardest tube,
i.e., the tube which will support the greatest weight before failure.
If you work the tube too much you risk breaking it. Should this
occur, use the second piece of tube supplied. If both pieces of
tubing are broken, you are disqualified.
Metals are crystalline materials (a curiosity in our popular nomenclature:
fine "crystal" glass is not crystalline). That is, metals
are made up of atoms arranged on a lattice, in a regular array.
The sketch below is schematic.
If we try to deform a metal (or any other crystalline material),
calculations will show that it is unreasonable to expect the metal
to deform by having one crystal plane slip over the adjacent plane
as a unit. Rather, slip or deformation takes place a little at a
time, as indicated in the sketch. Note the curved line A ... B in
the figures. This marks the boundary between the region A-B-C of
the slip plane which has slipped, i.e., on which deformation has
already taken place, and the remainder of the sample which has not
yet slipped, in response to a shear applied to the top surface of
the block in the sketch. Such a line is called a dislocation, and
you will notice that the material in the immediate vicinity of the
dislocation is deformed or strained, anti highly stressed. As deformation
continues, dislocations such as the one shown will move. That is,
as regions on which slip has occurred expand, dislocations such
as A .. B move to A'.. B'. As slip takes place on a great many atomic
planes to produce an observable effect, many dislocations are produced,
and these interact through their stress fields to form tangles,
as if they were so many pieces of string. One result of such interactions
is that metals become progressively harder as they are deformed,
and eventually they may fail, when stresses in the vicinity of dislocation
tangles become sufficient to rupture the bonds holding the metal