The document discusses the double yield point phenomenon seen in certain materials under tensile stress. It defines the double yield point as the point where a material begins to deform permanently beyond the stress level where Hooke's law no longer applies. For some low carbon steels and grades of polyethylene, small atoms clustered around dislocations initially interfere with slip at the first yield point but slip begins at a higher second yield point as the dislocations move away from the clusters. Structural measurements were carried out before and after deformation to study the double yield point in polyethylene samples.

1. Double Yield Point

2. Muhammad Umair Bukhari
Engr.umair.bukhari@gmail.com
www.bzuiam.webs.com
03136050151

3. Definition:
Stress beyond which a material deforms by a relatively large
amount for a small increase in stretching force.
Beyond this stress, the material no longer obeys Hook, s law .
OR
It can be defined as the point where a tensile test piece begins
to extend permanently. If the load is reduced to zero , the test
piece will not return to its original length.

4.  The stress-strain curve for certain low carbon
steels displays a double yield point.
 The material is expected to plastically deform at
stress δ1.
 However, small interstitial atoms clustered
around the dislocations interfere with slip and
raise the yield point to δ2.
 Only after we apply the higher stress δ2 does the
dislocation slip.

5.  After slip begins at δ2 , the dislocations move away
from cluster of small atoms and continues to move
very rapidly at the lower stress δ1 .
 We can easily determined the lower yield strengths for
metals having this type of stress-strain behavior

6.  We can also see the double yield point phenomenon in
polythene.
 Under mechanical deformations, three grades of
polythene which differ mainly in terms of branch
content exhibit two yield points.
 The first yield point occurs at low applied strains and
marks the onset of temporary plastic deformation;
 The second yield point occurs at higher strains and
marks the onset of permanent plastic deformation,
and is associated with the development of a neck in
tension

7.  The present work describes structural measurements
carried out on these three polythene samples before
and subsequent to different levels of tensile
deformation
 X-Ray diffraction, transmission electron microscopy
and optical microscopy were carried out on the
isotropic materials and after the first and yield points
under tensile deformation.