western
Physics and Astronomy

                                   
Hard Work - Work Hard
Objective Harden a piece of copper tubing by bending it back and forth. This is called "work-hardening".
Participants Teams of up to six.
Materials Two pieces of copper tubing approximately 30 cm long will be provided.
Rules

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.

Judging

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.

Background

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 together.

Source Technical University of Nova Scotia.

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