Abstract_Thesis - Theo.chemie.tu-darmstadt.de

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Nov 29, 2013 (3 years and 10 months ago)

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SUPER
-
STRENGTH AND MORPHOLOGICAL ALTERATION OF POLYAMIDE 6
(PA 6) SUBJECTED TO HIGH PLANE
-
STRAIN COMPRESSION


ABSTRACT

Two plane
-
strain deformation methods, compression in the channel die and rolling
with the side constraints, have been studied in details
for PA 6 material. The main similarity
in these methods is cavity
-
free deformation. Such plane
-
strain deformations lead to the
higher strength of oriented PA 6 than that for the unoriented materials. For the channel die
compressed PA 6, the tensile strengt
h approaches 420 MPa. In comparison, low carbon
steel reaches only 220 MPa. Rolling with the side constraints greatly suppresses the
widening and edge cracks, such process is the fundamental plane
-
strain compression,
although
some other deformation modes c
an also take place.

The experimental studies on PA
6 subjected to the compression in a channel die and rolling with the side constraints were
designed to find out the differences in structure and mechanical properties of materials
deformed at various condi
tions. It was documented that channel
-
die compression leads to
strong orientation (texture) of both crystalline and amorphous components comparable to
that obtained by rolling. It was demonstrated that PA 6 rolled in the channel shows a
significant increas
e in the tensile strength. The stress

strain curves of channel die
compressed and rolled samples exhibit the initial elastic region followed by the region of
plastic deformation. The ultimate tensile strength of rolled samples at higher deformation
ratio i
s lower than for the compressed one.

In highly deformed PA 6 rods the fraction of

-
crystals is oriented with macromolecular axes parallel to the flow direction while the fraction
of

-
crystals, which appeared in the rolled samples at the compression ratio

of 1.6
-
1.8, is
tilted by approx.20
o

with respect to

-
crystal orientation. It results from the generation at a
lower strain of 1.6~1.8 of large number of shear bands, separated from each other by 100
-
200

m and initially tilted by

45
o
.

The unique fractur
e behaviour of highly oriented PA 6 rods
leading to tilted flat and nearly smooth surfaces, is connected with shear bands generated
earlier at the compression around 1.6~1.8. The shear bands produce a fraction of γ
-
form
crystals with macromolecules aligned

in the shear planes and in shear direction.

The
crystalline texture and lamellar structure of PA 6 samples after the deformation by the two
mentioned methods are analogous at the corresponding strains. The resultant lattice texture
determined by WAXD as f
ollows
: the molecular chains in α
-
crystals are highly aligned in the
free direction; the (002) plane normals are distributed quite closely around the load direction;
and the (200) plane normals are equally divided and orientated in the direction from ±20


to
±26


away from the CD, giving the dual lattice structure. Simultaneously, the γ
-
form being a
minor component (2
-
3 % only), exhibits cross
-
hatched structure with the macromolecular
chains aligned at acute angles to the texture axis of the α
-
crystals. The α
-
cry
stalline domains
in the oriented materials can be described, according to the SAXS patterns, as perpendicular
to the FD chevrons shaped crystalline slabs when viewed from CD direction, with
macromolecules aligned in FD.