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A
neck is formed in a tensile test specimen when the UTS of the engineering
stress/strain curve is reached. Prior to this stress-strain condition the
plastic deformation occurs throughout the sample and the material deforms
under constant volume conditions. At the UTS deformation becomes localized
in the neck region and the cross-sectional area of the sample in that region
continues to decrease until ductile fracture occurs.
The
deformation in the neck region is unstable as the true stress continues
to increase as the area decreases. This region of the tensile stress/strain
curve can only be realized under constant strain-rate test conditions.
For a constant load-rate test, the sample fails once the neck forms. |
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