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In
a well annealed low carbon steel alloy the initial stage of plastic deformation
starts at the upper yield point shown in the diagram. The material can
then be deformed at the same strain rate by a lower applied stress, the
lower yield point. During this period of plastic deformation the sample
exhibits a region of stable local deformation that increases in size as
the deformation continues. This region in the polycrystalline sample is
known as a Luders band. The region propagates until it fills the gauge
region of the test sample and may result in a sample deformation of several
percent. Once the gauge region is fully Luders deformed, global plastic
deformation of the sample starts and the material undergoes conventional
plastic deformation until ductile fracture occurs.
Several
slip systems are active in the sample during the propagation of the Luders
bands and generate an approximately uniform dislocation distribution throughout
the sample. |
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