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Fracture
When
materials are subjected to a large enough strain they fail by fracture.
The photograph shows a series of cylindrical samples that have been
tested in tension at different temperatures or strain-rates. Sample (a) is
a glass tested at 800 K and the material has formed a neck due to viscous flow.
The same material (b) tested at 273 K has a brittle behavior and fractures
before any plastic flow has occurred. Samples (c) and (d) are polypropylene;
(c) was tested at room temperature and a low strain-rate and behaves in a
ductile manner, (d) was tested at the same temperature but a high strain-rate
and behaves as a brittle material. The last two samples (e) and (f) are a low
carbon steel alloy. When tested at 273 K (e) the material shows a ductile
behavior, necking and forming a "cup and cone" ductile fracture surface.
When tested at 200 K the same alloy behaves in a brittle manner.
If
the work done to cause failure is measured in these experiments it is found
that brittle fracture is associated with low work to failure, whereas viscous
flow or ductile behavior exhibits high work to failure. In each case,
the work per unit volume of the material under test is just the area under the
stress-strain curve. |
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