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Fabrication and Mechanical Response
of
Commingled
GF/PET
Composites
NIKLAS SVENSSON* and ROSHAN SHISHOO
Swedish Ins t i tu tefor Fiber
and Polymer
Research
P.O. B m
04,
SE43122
Mlilndal, Sweden
and
MICHAEL
GILCHRIST
Lkpcutment of Mechanical Engineering
University
College
Dublin
Belfild, Dublin 4 , Ireland
The mechanical properties and the response to mechanical load of continuous
glass
fiber reinforced polyethylene terephthalate (GF/PET) laminates have been
characterized. The laminates were manufactured by compression molding stacks of
novel woven and warp knitted fabrics produced from commingled yarns. The lami-
nate quality was examined by means of optical and scanning electron microscopy.
Few
voids were found and the laminate quality
was
good. Resin pockets occurred in
the woven laminates, originating from th e architecture of the woven fabric. The
strength of the fiber/matrix interface was poor. Some problems were encountered
while manufacturing the laminates. These led to fiber misalignment and conse-
quently resulted in tensile mechanical properties that were slightly lower than ex-
pected. Flexural failures
all
initiated
as
a
result of compression, an d
it
is
possible
that the compression
strength
of the matrixmaterial, rather th n its tensile strength,
might limit the ultimate mechanid performance of the composites. Flexural failures
for both materials were very gradual. The warp knitted laminates were stronger and
stiffer than the woven laminates. The impact behavior was also investigated; the
woven laminates exhibited superior damage tolerance compared
with
the warp
knitted laminates.
INTRODUCTION Hybrid
yams
containing both reinforcing fibers and
lass fiber reinforced polyethylene terephthalate
G
GF/PET) ha s excellent potential for future struc-
turd
applications of composite materials. Compres-
sion molding and diaphragm forming are currently the
most widely used methods for manufacturing com-
posite components, and the research that has been
carried out in these areas,
m a d y
experimental, has
studied impregnation, consolidation, and processing
parameters and their influence on the mechanical
properties of the composite. Some reasons for the low
market acceptance of thermoplastic composites are
the high price of prepreg, their high processing tem-
peratures, and high melt viscosities, which impose
severe requirements on tooling and manufacturing
equipment, and also the fact tha t very little
is
known
about their long term behavior.
Corresponding uthor.
the-thermoplastic
matrkin
the form of fibers, split-
flm, or powder are a fairly recent development that
make it quicker and easier to manufacture thermo-
plastic composites. Commingling offers the most inti-
mate blend of reinforcement and
matrix fibers,
which
can subsequently be converted into fabrics or pre-
forms. In commingled yams the reinforcement and
the matrix are mixed intimately at the filament level.
Fabrics are mostly produced from the yarns, an d the
fabrics ar e then molded into composite components or
laminates. Full impregnation and wet-out require
a
significant decrease in viscosity, which, in practice,
means heating the matrix polymer above
its
melting
temperature. Overheating will degrade the material
and consequently will reduce the composite mechani-
cal
properties.
A
commingled fabric
is
generally non-extendable and
cannot flow to
fill
a
mold
1).
Preforms
with
a
near
net-
shape facilitate manufacturing,
and
well-designed tool-
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Fabrication
and Mechanical
Response
of Commingled
GF/PET
Composites
ing
is
important. Low consolidation pressures are de-
sirable as they reduce the stringency of the require-
ments on the tooling material and avoid excessive
wear of machines and tools. Pressure should also be
applied during cooling in order to prevent deconsoli-
dation, which results in reduced mechanical proper-
ties of the composites
(2).
The hybrid yams are also
used
in
processes such as filament
winding
and pul-
trusion. Here again,well-controlled pressure and tem-
perature at the mandrel/die are important. In theory,
it is
faster
to pultrude thermoplastic composites than
thermoset composites. An additional advantage is that
the pultruded thermoplastic profiles may also be post
formed and welded 3).By using hybrid yams in pul-
trusion, an unlimited variety of material combinations
may be produced, and combinations with fabrics are
possible
(4).
A complex relationship exists between the process-
ing conditions, the morphology
in
the composites, the
crystallinity, and the mechanical properties
in
semi-
crystalline composites:
this
relationship
has
previously
been studied for GF/PET by Ye and Friedrich 5).The
degree of crystallinity is often not the sole reason for
variations in mechanical properties. Rather, these are
due to the large differences
in
morphology that result
from
Merent
thermal histories during manufacturing
of composites.
In
commingled polypropylene PP) com-
posites a low cooling rate gives a morphology with
la ge voids and coarse spherulites.
During
fracture of
these composites the cracks tend to propagate along
the spherulite boundaries, resulting in a lower fi-ac-
ture toughness than for the composites manufactured
with
a
high cooling rate
(6).
he
case
of fabric based
composites
is
more complex since large
resin
pockets
are present and the size,shape, distriiution, and num-
ber of these depend on the type of fabric used.
Wakeman
et
aL
7)
reported that laminates manu-
factured from commingled GF/PP fabrics had a non-
uniform fiber distribution with s t r e a k s of dry glass
fibers.
This
could be due to separation of the Me rent
fiber types during the weaving process because of the
large difference in stiffness between the reinforcing
fibers and the matrix
fibers.
Widespread fingering, i.e.,
a phenomenon where the molten matrix rushes ahead
locally within the dry fiber bed, also gives
a
nonuni-
form impregnation of the fibers (8).hese factors might
explain some of the variability in experimental results
that have been reported
in
the literature. Shrinkage
may occur when heating commingled yams and fab-
rics. A high draw ratio is used in the spinning of ther-
moplastic fibers and these will hence be highly orient-
ed. On heating, the fibers
will
relax and distort the
fabric or fiber architecture 9).
Ye and Friedrich
(5)
concluded that it is important
to avoid slow cooling in order to prevent the formation
of spherulites, microcracks and voids, which all con-
tribute to a large decrease in crack propagation ener-
gies in both Modes I and II. Shonaike
et
aL (10) found
that the strength of commingled GF/PET laminates
increased with
an
increased holding time. This was
attributed to
an
increased adhesion between fibers
and matrix. The mechanical properties of 0 and
O /90° GF/PET laminates consolidated in an auto-
clave at pressures of 0, 0.3, .7MPa were determined
by Andersen and Lystrup (1
1).
As
can
be seen from
Table 1
the laminates consolidated in vacuum were of
the same quality
as
those manufactured at
a
higher
pressure: Table 1 . The effect of yam sizing was stud-
ied by Krucinska and Krucinski (12), who manufac-
tured co-woven fabrics of glass fiber and polybutylene
terephthalate (GF/PBT) where the glass fibers were
sized with either a plastic size suitable for the PBT
matrix or a traditional textile sizing The latter was
used to protect the yams from abrasion during the
weaving process and contained mainly starch and lu-
bricants. The bendmg strength for laminates with the
same fiber volume fraction bu t with M er en t
sizes
dif-
fered by
a
factor of 3.2 and
the
interlaminar shear
strength by a factor of 2.4: Table 1. Shonaike et aL
(13) observed that gradual coolug gave
a
higher flex-
ur l modulus
in
the fiber direction bu t
a
lower modu-
lus in the transverse direction when compared with
rapid cooling of compression molded unidirectional
GF/PET composites: Table 1 . Very low values were
seen for the transverse strengths
owing
to a poor ad-
hesion between the PET matrix and the glass fibers.
The higher bending modulus was attributed to the
presence of larger spherulites in the gradually cooled
specimens. On the other hand, a hgher f rsl ture tough-
ness was seen for the rapidly cooled specimens. A
Table
1.
Mechanical Propertiesof Commingled GF/P€l Composites Found in the Literature.
Fiber Flexural Strength Flexural
Modulus
lnterlaminar
Volume
Fraction
0 0
Shear Strength
Material ( I (MPa) ( G W (MPa)
Ref.
GF/PET, UD, 0.7MPa consolidation
GF/PET, twill, 0.7MPa consolidation
GF/PET, twill, vacuum consolidated
GF/PBT, UD, co-woven, plastic size
GF/PBT, UD, co-woven, textile size
GF/PBT, UD, co-woven, plastic size
GF/PET, UD, gradual cooling
GF/PET. UD. auenched
~
45
45
45
56.0
55.8
66.6
40
40
842
387
395
81
0
250
820
1081
1098
33
19
20
40.5
30.2
43.4
38.5
36.0
POLYMER CWOS f lES AUGUST lssS,Vol. 19 No.
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Niklas Svensson, Roshan Shishoo,
and
Michae l
Gilchrist
more extensive review on manufacturing and mechan-
ical properties of commingled thermoplastic compos-
ites
can
be found in Svensson et aZ. 14).
Hollow structural thermoplastic beams (commingled
GF/PET) and thermoset sandwich beams (GF/epoxy,
GF/polyester, GF/vinylester) were manufactured using
compression molding and resin transfer molding, re-
spectively, by Svensson
et
aL
1
5).The beam preforms
were produced
using
woven and
warp
knitted fabrics
together with braiding. The beams were characterized
in three-point bending under both static and impact
load conditions. In both instances the failures initiated
at
the compression side of the beams, more noticeably
so
for the thermoplastic beams.
As
mentioned previously, textile technology
can
be
used advantageously in the composites industry. In
this work, laminates manufactured from two novel fab-
rics and comrmngled GF/P m yams have been charac-
terized with respect to tension, in-plane shear, and
flexure. The behavior under impact load
was
also
in-
vestigated.
An
extensive scanning electron microscopy
SEW
analysis has been carried out, and several mi-
crographs are used to gain an understanding of the
fracture processes and to illustrate some of the ad-
vantages and problems with these new materials.
EXPERIMENTAL
The commingled yam used for production of the
laminates in this work contained glass fibers and
PET
fibers with a
glass
fiber volume fraction of
5@ .
The
yam s were produced by means of air-jet texturizing
11).
Laminates were manufactured from two different
fabrics, i.e., one woven and one warp knitted. The
woven fabric was tailored with the main fraction of the
fibers in the warp direction (100 warp yams and
20
weft yams per 1OOmm ) as seen
in RLJ.
The warp
knitted fabric was unidirectional and the commingled
yam rovings were held together by a thin PET binding
yarn.
Hgwe 2 shows the warp knitted fabric, and Q. 3
is an
optical micrograph of the binding yarn. The
binding yam spacing was 19.7 mm and there were 94
warp yam s per 1OOmm.
The formability, i.e., bending and shear properties,
and the compressional behavior of the two fabrics
were examined by means of the Kawabata Evaluation
System (KES)
16).
which
is a
well-known system for
the characterization of mechanical properties and sur-
face properties of fabrics and nonwoven materials.
The
two
fabrics were cut and stacked in the appro-
priate number of layers and fiber angles to produce
unidirectional and cross-ply laminates with a target
thickness of
3
mm.
The
laminate dimensions were
350 X 350mm. teel guide b rs were used to obtain
a
uniform laminate thickness. The consolidation pres-
sure was hence provided by the actual compaction
of
the fabric stack to a thickness of
3
mm. Twenty layers
were used for the warp knitted fabric and 12 layers
for the woven fabric. The laminates were compression
molded
in
a
hydraulic press between steel platens
coated with
a
polytetrafluoroethylene (PTFE) release
FYg 1 .
h woven
fabric
used for manufmtwing laminateS.
The
mainfraction
5/6)
f
t h e w s
run
in
the warp direction,
which
is
lefr/right
in
thephotograph
FYg
2. 7he
warp
knitted u n i d k c bnal
fxbrlk.
The commin-
gled yams
are
held
together by a hinPET binding ya m
The
warp direction
is
iefr/right in thephotograph
FYg 3.
The thin
PET binding
yam -jiorn top to
h t -
t o 4
holding
the
commingled
warp
bundles
@J-
M a g @ -
cation X50.
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Fabrication and Mechanical Response of Commingled GF/PET Composites
agent. The mold was heated to 210°C within 20 min
and the consolidation time was 20 min. Water was
used for cooling and the cooling rate was 21°C/min.
Pressure
was
applied during heating aswell as during
cooling. Some slight warpage was seen for some of the
laminates.
The glass fiber volume fraction for all the laminates
was determined by matrix bum-off in an oven and
was 48.1% on average with a standard deviation of
1.2%. The quality of the laminates was examined by
observing polished cross sections
in an
optical micro-
scope.
The tensile properties in the
warp
and weft direc-
tions of the two different kinds of laminates were de-
termined in accordance with
ASTM
D3039. The in-
plane shea r behavior was determined by means of
tensile testing of cross-ply specimens with fiber angles
of ? 45
as
described in the
ASTM
D3518
standard.
The flexural properties of the laminates were deter-
mined in the
warp
and weft directions using a three-
point bend test, ASTM D790M. with a span to thick-
ness ratio of 16:
1.
RESULTS AND DISCUSSION
The particular
fiber
architectures made the fabrics
very drapable
in
the weft direction but
also
thereby
difficult to handle and align, especially the
warp
knit-
ted fabric. Variations
in
fiber angles between the plies
in the laminates were almost inevitable.
The results from the characterization of the fabrics
(for clarity the nomenclature h m he Kawabata Stand-
ard
is
used throughout t h i s paper) showed that the
woven material had
a
higher shear modulus,
G,
as
well as a higher shear hysteresis, 2HG5, than the
warp knitted material. The bending stifhess, B, of the
warp knitted fabric was greater in the warp direction.
The values for the weft direction were several orders of
magnitudes lower: this was due to the thin PET bind-
ing yams. The same observations were made for
bending hysteresis, 2HB. The shear and bending hys-
teresis in a fabric depends on factors such as the
fiber-to-fiber friction, the fiber architecture, and the
mechanical properties of the fibers. The elongation
under a tensile load of 491 N/m, EMT, was slightly
higher for the woven material; this was due to the
crimp in the fiber architecture which gave a lower stiff-
ness for the fabric. The compression energy,
WC,
is a
measure of the amount of energy required to reach
a
certain compactional force
in
the material and , among
others, is dependent upon the compression stiffness
of the fabric, the fiber-to-fiber friction, and the fiber
architecture. This energy was 65% higher for the
warp
knitted fabric than for the woven. The thickness of the
fabric
at
maximum compression,
lM,
as 30% lower
for the warp knitted than for the woven fabric: test
values for the two fabrics are seen in Table 2. The val-
ues for a typical woven wool suit fabric are included
as
a
reference
17).
The Kawabata Evaluation System
has previously been used by Ramasamy and Wang
18)
or the characterization of powder coated and com-
mingled carbon fiber/nylon tows.
Shishoo and Choroszy 19)developed a measure of
the fabric formability using three of the material pa-
rameters as determined by means of KES:
B X E M T
2HG5
ormability
=
The formability was calculated for the woven GF/
Pm material, the warp knitted GF/PET material and
the reference wool fabric, see
Table
2. For the two
m e n materials an average of the
warp
and
weft
prop-
erties were used, whereas for the warp knitted fabric
only the warp EMT value was used due to difficulties
in measuring the elongation of only the thin
binding
y a m s .
The
warp
knitted fabric had
a
marginally h a -
er formability than the woven GF/PET fabric. Both
the commingled yarn fabrics were more formable
th n
the reference wool material, mainly because of the low
shear losses and the hgh bendrng sti he ss
glass
fibres.
In
the optical microscopy examination the crimp in
the woven laminates was clearly seen Fig. ).
This
particular reinforcement microstructure
was
also re-
sponsible for the large resin pockets that form in the
material, which also can be seen in Fig.
4
and in
an
SEM
micrograph of
a
mixed mode fracture surface,
Flg. 5.
The
fiber
distribution was good outside the
resin pockets. The reinforcing fibers in the
warp
knit-
ted fabrics were non-crimped and hence the number
Table 2. The Results From the Formabili ty and Compressibi lity Characterization of the Woven and
the
Warp Knitted GFlPET Fabrics
Using the Kawabeta Evaluation System (KES). A Typical
Suit
Meterial, i.e., a Woven Wool Fabric, is Given as a Reference
(17).
Woven Warp Knitted Wool
GFlPET GFlPET Fabric
Shear
modulus,
G,
(N/mx )
Shear hysteresis 2HG5, (N/m)
Bending
modulus,
warp B, Nm%)
Bending
modulus,
weft B,
Nm2/m)
Bending hysteresis warp 2HB, (10-2 Nm/m)
Bending hysteresis weft 2HB, ( lo+ NWm)
Tensile extension warp EMT (%)
Tensile extension weft EMT,
(%)
Compression energy WC (102 Nm/rn2)
Compression thickness TM
(mm)
Formabili ty, (BxEMT)/2HG5,
(lo- )
0.80
1.73
1.71
0.33
3.25
0.77
0.69
0.65
0.28
1
oo
0.40
0.54
0.87
1.53
0.002
3.68
0.55
0.47
0.70
0.48
2.34
6.19
0.24
0.24
0.10
0.10
4.99
4.99
0.21
0.61
0.19
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Niklas
Svensson,
Roshan Shishoo,and
M ic h ae l
Gilchrist
Flg
4. An optic l
micrographfrom
a polished
c ro s s
section
of
a
muen
laminate.
The
crimp of the
glass
j er yams
is clearly
seen
(ie. the
curued
yarns running
from left
to
right). In the center
of
the micro-
graph is a large resinpocket which
architecture.
has ormeddue to
uleactualj er
FYg
5.
An
SEM micrographfrom
a mized
modefracture
SUT-
face
showing a b e esin
pocket
in a woven
laminate.
and size of resin pockets were smaller. Cracks were
occasionally seen in and between the fiber bundle in
both types of laminates; Rg. 6. Very clean glass fibers
were seen on the fracture surfaces, here in pure Mode
II,
indicating poor fiber/rnatrix adhesion;
Rg. 7.
More
details on the mixed mode fracture behavior of the
two materials
can
be found in Svensson
et
aL
(20).
Voids in the resin rich regions were seen only occa-
sionally in the examined laminates.
The results from the tensile tests are given in Table
3. For the tests in the
warp
direction no major matrix
cracking or fiber fractures could be heard prior to
fail-
ure of the test specimens, which occurred very sud-
denly. A small stiffening of the materials was apparent
as the load increased. This
can
be explained by reori-
entation of the misaligned glass fibers during loading.
The warp knitted laminates were
slightly
stiffer than
the woven laminates and both materials were equally
strong. In the weft direction the woven laminates were
stiffer and stronger due to the reinforcing
glass fibers
in the weft yams. The edge view
in Rg.
shows that
FYg
6.
Fine
cracks
were occasionally
obserued in
and
be-
tween
th mb u n d le s.
Magnification X
100.
extensive delaminations and fiber pull-out have taken
place during the fracture of a woven tensile specimen.
Rgure
9
shows the surface from
a
woven tensile speci-
men, and
the
architecture of the fabric
with
distinct
warp and weft ya ms can still be seen after the failure.
In the warp knitted laminates some longitudinal split-
Table 3. The Mechanical Properties
of
the Two Different GF/P€ Laminates.
The Standard Deviation Is Given in Brackets
Woven Warp Kni tted
Tensile modulus,
OD
(GPa)
Tensile strength,
O ,
(MPa)
Tensile modulus,
go ,
(GPa)
Tensile strength, go , (MPa)
In-plane shear modulus (GPa)
In-plane shear strength (MPa)
Flexural modulus, 0 , (GPa)
Flexural strength, 0 , (MPa)
Flexural modulus, go , (GPa)
Flexural strength,
go ,
(MPa)
~
22.9 (2.1)
510 (28)
6.9 (1.4)
131
(6)
4.4 (1.3)
99
(1)
29.0 (1.3)
494 (36)
10.7 (0.2)
214
(9)
28.2 (1.4)
487 (26)
3.5 (0.6)
6.6 (1)
4.3 (0.9)
88
(15)
35.0 (1.3)
747 (20)
4.6 (1.4)
25 (4)
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Fabrication and
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GF/PET
Composites
Q. 7. Very cleanfibers
were
seen
on hefracture surfaces,
here
a
pure Mode Iffracture of
a woven lamina&,
indicating
prfiber//matriwadheswn
Q. 8.
An
edge
view
of a woven
tensile specimen
showing
Out.
extensive
interlaminar
damage ClehmiMtio~ ndjzmpul l -
ting occurred at failure. The transverse tensile failures
of the warp knitted laminates resembled those of a
pure thermoplastic while the woven laminates be-
haved
in a similar
elastic manner in both the warp
and weft directions.
Assuming
a
glass
fiber modulus of
72.0
GPa,
a
PET
modulus of 3.0 GPa, and a fiber volume fraction of
48.loh,
he rule of mixtures predicts a tensile modu-
lus of 36.2 GPa for a unidirectional laminate. The
transverse modulus is predicted to be
6.3
GPa if the
Poisson contraction effect is taken into account (21).
The experimental values of the tensile properties,
T le 3, were sllghtly lower than expected and than
those predicted by the rule of mixtures. For the warp
knitted laminates,which were considered as unidirec-
tional, the longitudinal and transverse moduli were
28.2 GPa and
3.5
GPa. The large deviations were
probably due to the poor fiber/matrix adhesion and
misalignment during stacking.
An
additional distor-
tion of the fiber architecture may also have taken
place during heating owing to the relaxation and con-
traction of the PET fibers.The woven material cannot
be easily modeled by the rule of mixtures, but the
properties in the warp direction should be lower than
for the warp knitted because of the smaller fraction of
glass
fibers in this direction and
also
because of the
crimp
in
the
fiber
architecture. Correspondingly, the
properties in the weft direction should be higher,
which agrees well with the experimental results. A
good agreement between the predictions using the
rule of mixtures and the mechanical properties of
braided commingled GF/nylon composites was ob-
served by Fujita et
aL
(22).
The shear modulus for the
two
materials were simi-
lar and the woven laminates were mar- stronger
than the warp knitted laminates; Table
3.
The scatter
in shear properties was larger for the warp knitted
laminates.
In flexure the warp knitted laminates were stronger
an d stiffer
than
the woven laminates in the warp
direction while
the
woven laminates were stronger
and stiffer when tested in the weft direction.
Again,
this was due to the reinforcing glass fibers
in
the
weft yam s of the woven laminates. The stiffness and
strength in the weft direction were very low for the
warp knitted laminates. The values of the moduli were
significantly higher
in
flexure
than
in tension, and
these are in good agreement with the values predicted
by the rule of mixtures equation. Similar observations
have previously been made for textile composites by,
for example, Miider et
aL
(23), ho determined the
tensile modulus for warp knitted biaxial GF/PP to be
19.2
GPa and the
flexural
modulus to be
23.8
GPa.
The flexural modulus is dependent upon the stackmg
sequence and may also be less sensitive to the proper-
ties of the fiber/matrix adhesion and to fiber misalign-
ment. For both materials the failures initiated on the
compressive side under the loadmg pin. Cracks and
limited delaminations propagated from this initial fail-
ure until
fin l
rupture. In the woven laminates cracks
FYg. 9.
The
surfae
of a
woven tensile specimen
The
abric
architedure
with
distinct
warp
and
we@ yams
is
st i l l
appar-
ent
ajter
f-e.
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Niklas Svensson, R o s h n Shishoo,
and M ic h ae l
Gilchrist
Flg.
10.
7he Compression
damage under the
oading pin
that
initiated
jinal failure in
a
woven three-point bending
speci-
men
FYg. 1 1 . A micrograph
showing
the macrocrack
on the
ensile
surface of a wovenjlexural
specimen
FYg
12. Thedamageon
hetensilesw-jkeof
a
warp knitted
bundles.
jl.exuralspecimen The*fmctures OcCLvred in the
distinct
were also seen to propagate along adjacent weft bun-
dles. The compression damage from
a
woven speci-
men
can
be seen in
Rg.
10.
racture took place on the
tensile side within the
glass
fiber yarns, as can be
seen from the micrographs of the tensile damage of a
woven and a warp knitted specimen,
Rgs.
1 1 and
12,
respectively. Typical load/deflection curves of the
woven and warp knitted specimens can be seen in Rg.
13.
The warp knitted laminates had a higher failure
load but exhibited a larger drop in load bearing capa-
bilities. In all cases the tests were stopped manually
after this load drop since the toughness of the materi-
als
prevented the specimens from completely fractur-
ing
before the specimens folded in between the sup-
ports of the bending
jig i.e., the graphs in Rg.13 do
not show the ultimate flexuralstrain.
The results from the mechanical characterization
are summarized in
Table
3.The standard deviation for
the different tests are also given. Large scatter was
seen, especially
so
for the shear, transverse tension,
and transverse flexure of the warp knitted laminates.
This can be explained by the fiber/matrix interfacial
properties and the variations in fiber angles in the
laminates.
St.
John
(1) reported that the poor compressional
strength of the matrix limited the flexural strength of
GF/PP laminates that could be obtained. Preliminary
results for a
similar
GF/PET yarn showed a linear re-
lation between strength and fiber content due to the
superior shear strength of the
PETmatrix.
Hamada et
aL
(24)
examined the flexural properties of compres-
sion molded commingled GF/nylon composites. At
short consolidation times, buckling and cracks ap-
peared on the compression side during three-point
bend tests. At longer consolidation times the failures
initiated as fiber fractures on the tensile side of the
specimen. Choi
et
aL
(25)
observed scatter in the re-
sults in three point bending tests of unidirectional
glass fiber reinforced polyamide (GF/PA6) laminates
and attributed this to poor fiber/matrix interfaces,
misaligned fiber bundles, and resin rich regions. A
large plastic energy absorption
was
seen, and
this
coin-
cided with crushing-like failures on the compression
face of the specimens. The span to thickness ratio used
was
33.3:
1. The matrix had undergone large plastic
2000
500
*
..
.._...
atp
knitted
. .
.
500 t
0
Deflection
mm)
0 2
4
6
Rg.
13.
Typical
load
deflection
curues
rom the
three-point
exhibited
a
larger
drop in load
carrying
capabilities
after
ini-
bending tests. The
warp
knitted laminates were stronger
but
tialfracture.
366
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Response of Commingled GF / P E T
Composites
FXg
14. Extensine delamination seen
in
a back-lit woven
impacted
crossply
laminate.
deformation during crack propagation. Initially it was
believed that the flexural failures were a combination
of tension and shear, even though a span to thickness
ratio of
1 6 1 was
used. However, the majority of the
results available in the literature do describe failures
as crushmg damage on the compression face or ma-
trix initiated even at span to thickness ratios higher
than 16: . Hence it
is
believed that the compressional
strength of the matrix limits the flexural performance
of thermoplastic matrix laminates.
The material used in
this
work showed a poor fiber/
matrix adhesion, which might partly explain he lower
than expected mechanical properties. Observations of
the fiber/matrix bonding quality in commingled g l a s s
fiber composites vary. Ye and Friedrich (5)examined
GF/PET
laminates
in Mode I and Mode crack propa-
gation. In
all
cases matrixwas
seen
on the fi ers on the
fracture surfaces, indicating a goo interfacial bonding.
Shonaike et aL (10)reported that the three-point bend-
ing fractures in GF/PET initiated in the matrix and
not along the fiber/matrix interface, which was in-
dicative of a good bonding. Jang and
Kim
(26)m-
proved the flexural
strength
and the interlaminar shear
strength of co-woven carbon fiber reinforced poly-
etheretherketone (CF/PEEK) laminates by 52% and
16%.
respectively, by means of a
3 min
low tempera-
ture oxygen plasma treatment. The effect of plasma
treatment in the present material system
will
be
eval-
uated.
Drop weight impact tests were
carried
out
in
order
to investigate the material behavior a t higher defoma-
tion rates. The impactor was a hemisphere with a
radius of
5
mm. The weight of the impactor was 14.26
kg and the drop height 0.97 m resulted in an impact
energy of 136 J and an impact velocity of 4.4m/s.
Both the woven and the
warp
knitted laminates used
for these experiments were symmetric cross ply lami-
Fig 15.
A warp
knitted
impacted
~ o s s - p l y
pecimen. The
main
cracks were in all cases
ormed
in thejiberdiredions.
nates. The woven laminates were
3
mm
thick while
the warp knitted laminates had
a
thickness of 4 mm.
The size of the specimens were 60 X 60 mm. The
specimens were freely supported by a stiff metal ring
with an inner diameter of 40 mm. Extensive delami-
nations and cracking occurred in both materials as
can be seen in the back-lit woven Sample in
Rg.
14.
The warp knitted sample, Fig. 15, shows the main
cracks in the 0 and 90" irection which were present
in
all
Samples.
Rgures
I6 through 19 show SEM micrographs from
different areas of
an
impacted woven sample. Large
plastic deformation of the m trix and
a
considerable
amount of
fiber
fracture occurred during the impact.
If the area of delamination is taken
as
a measure of
impact resistance, the woven laminates were superior
to the warp knitted laminates, which in some cases
almost split up along the mid-plane; Rg.20.
Plastic deformation is possible in the
PET
matrix
even a t fairly high deformation rates,
as
was shown in
the impact tests. The flexural failures were very grad-
ual, which indicated the ability of the material to ab-
sorb large amounts of energy. The fiber architecture of
the woven laminates tended to limit th e extent of
delamination efficiently because of the interlacement
Cracking
\
Fig. 17
Delamination
ig. 19 \
Fig. 18
Fig
16.
A
sch e m a t ic di gmm of
a
sectioned
impact specimen
and
the
locations
where
the
micrographs
of Figures 17-19
were taken he top
face of
the pecimenwas impacted
POLYMER
COMPOSITESy
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Niklas Svensson, Roshan Shishoo,
and
M i ch a e l Gilchrist
Fig 17. Fiberfracturesandmatriwcrmkingatthetopfme
where the impactor
hit the specimen
Fig. 18. Large plastic
deformation of
the matrix,
which has
been sheared by the impactor.
of the warp and weft yams. The slight fiber misalign-
ment
in
the fabrics together, the formation of resin
pockets during manufacturing, and the poor fiber/
matrix interfacegiving extensive fiber pull-out all con-
tribute to a high apparent fracture toughness of the
two materials. Little is known about the
fatigue
per-
formance of textile thermoplastic composites, but work
has been initiated by the authors to investigate the
degradation of mechanical properties resulting from
cyclic loadings.
From the literature survey conducted it is obvious
that there is
a
lack of complete material
data
for com-
mingled composites. Usually, only the flexural modu-
lus and strength and interlaminar shear strength are
reported, and
this
is of course due to the simplicity
of
canying
out these tests. Processing optimization has
not yet been obtained for commingled materials, and
problems with voids, fiber misalignments, microcracks,
and fiber/matrix adhesion persist.
CONCLUSIONS
By means of textile technologies such
as
fiber inter-
mingling, weaving, braiding, and knitting, advanced
preforms giving excellent composite mechanical prop-
erties
c n
be produced. The tensile, in-plane shear, and
flexural properties of GF/PET composites has been
determined experimentally. The laminates were com-
pression molded from novel warp knitted and woven
fabrics produced by commingled GF/PET yams. The
Kawabata Evaluation System was successfully em-
ployed to estimate the formability and compressibility
of the
two
fabrics. The laminate quality was examined
by means of optical and scanning electron micros-
copy. Few voids were found and
the
laminate quality
was good. Resin pockets appeared in the woven lami-
nates, and these were originated from the architecture
of the woven fabric. The strength of the fiber/matrix
interface
was
poor. The tensile properties were slightly
lower than predicted, and this
was
attributed to a poor
fiber/matrix adhesion and fiber misalignment. The
flexural performance of the laminates was limited by
the compressional strength of the PET matrix. How-
ever, the materials are believed to have large energy
absorption capabilities both in static loading and under
impact. This is thought to be due to fiber misalign-
ments, the toughness of the matrix, and the extensive
fiber pull-out present because of the poor fiber/matrix
adhesion.
Fig 19. M w f i m m m s
were seen
in the impact speci
mens where the impactor
hndpeneirated
the back me.
Fig 20. Delamination o an impacted warp knitted specimen
th t
almost
split up along the mid-plane.
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Fabrication and MechanicalResponse of
Commingled
G F / P E T
Composites
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