gilchrist 1998d

8/19/2019 Gilchrist 1998d

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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-

360

POLYMER

COMPOSITES

AUGUST 7 9 9 8 ,

Vol.

79 No. 4


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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.

4

361


3/10

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.

362

POLYMER

COMPOSITES

AUGUST 1998 Vol.

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No. 4


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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

POLYMER

COMPOSITES

AUGUST ISM, Vol. 19 No. 4

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5/10

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)

364

POLYMER

COMPOSITES

AUGUST

1998 Vol.

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No.4


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Fabrication and

Mechanical

Response of Commingled

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.

POLYMER

COMPOSITES

AUGUST

W SS, Voi.

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No.

4

365


7/10

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

POLYMER COMPOSITES AUGUST 1998 Vol. 19 No.

4


8/10

Fabrication

and

Mechanical

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

UGUST 19MyVol. 19 No.

4

367


9/10

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.

368

POLYMER

COMPOSITES

AUGUST

1998 Vol.

19

No. 4


10/10

Fabrication and MechanicalResponse of

Commingled

G F / P E T

Composites

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