You can view the following 'speeded up' QuickTime video clips of the tensile tests:
- Full tensile test of annealed copper speeded up 20 times (1.2 MB) ... in separate window ... video alone
- Necking of annealed copper speeded up 5 times (330 KB) ... in separate window ... video alone
- Full tensile test of work hardened copper speeded up 20 times (560 KB) ... in separate window ... video alone
- Necking of work hardened copper speeded up 5 times (490 KB) ... in separate window ... video alone
You can also view image sequences of the tensile tests which also display the growth of the stress-strain graph:
- Image sequence with growth of stress-displacement graph for an annealed copper rod ... in separate window
- Image sequence with growth of stress-displacement graph for a work hardened copper rod ... in separate window
As with aluminium, the tensile test graphs for the copper specimens exhibit the shape as the typical stress against strain graph in Theory 1, although the difference in properties can clearly be seen between the two specimens. The annealed specimen has been heat treated at 800°C for two hours, so recovery, recrystallisation and grain growth all take place.
While the annealed specimen yields almost immediately, the work hardened specimen yields at a stress many times higher, as predicted in the theory. The difference in ductility between the two specimens is apparent, with the annealed specimen straining to almost three times that of the work hardened.
Once the annealed specimen begins to yield, work hardening occurs, which is why the stress required to deform the specimen continues to increase for a long time. The full extent of this can be seen in the photo below, However, the other specimen has already been work hardened, so once yielding starts no further work hardening can occur, so the stress does not increase.
The fracture mechanism for both of the copper samples is similar to that of duralumin discussed in Results 1, in that there is microvoid formation and coalescence as seen here:
There is also a less pronounced cup and cone profile:
There is also the formation of shear bands , which causes the change in colour seen in the photograph of unstretched and full stretched specimens above, as the oxide layer on the surface mixes with fresh layers from the shear bands, resulting in the brighter but matt finish seen once deformation has occurred.
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