PEEL TEST FOR THE STUDY OF

THE FIBRE POLYMER INTERFACE

JIANAN SUN

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science,

Department of Chernical Engineering and Applied Chemistry, in the University of Toronto

O Copyright by Jianan Sun 2001

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ABSTRACT

PEEL TEST FOR THE STUDY OF THE FIBRE POLYMER INTERFACE

Master of Applied Science

Deprrtment of Chernical Engineering and Applied Chemistry

University of Toronto

Nine fibre-resin s'stems have been studid using perl test to measurr. rhe

adhesion betwen tibrrs and polymers. SEM photos have b e n used to provide additional

cvidence on the failure of the interface. Four types of peelings werc obsen*t.d inçluding

poor adhesion. btittlr fracture. ductile fracture and fibre damage. The "wings" in the o o d

adhesion cases proved that the resistancr to shear failure of the interface w s very strong.

The Iength of proccss zone. lp. was estimated. and the t ende displacements in the

rnatris undcrneath the fibre were also estimatrd. The displacements invol\-d sherir. and

the iiacture proccss \vas complex. involving different amounts of the three modes of

fracture. ai diffrrent points of the fibre circumferencc.

Funher work. more drtailed analysis should be attempted. not requiring the

assumption of linear change in E],.~.

ACKNOWLEDGEMENTS

1 am very grateful to Professor EV1.R. Piggott for his supervision. guidance. and

patience in completing this work. Vrry special thanks to Jenny Clifford for her truly

frirndship. hrlp and encouragement.

1 am thankful to all members in -4dvanccd Composite Physics and Chemistp

Group for their suppon and frirndship. thank you. Jim. Mçhael and Vicky. Thanks to Xlr.

Shu-ren Dai for the technical assistance.

Special thanks go to my husband. Yongli. for his understanding and technical

suppon in cornputer. Lots of thanks estend to my parents. brother who supponcd me in

rvery aspect of my lire and rspecially during the study in the University of Toronto. 1 am

grateful to mp mother-in-law. coming from China and taking care of my newbom baby.

Trace?. thus 1 can concentrate on my work. Dear Trace'.. h m loves yoii!

TABLE OF CONTENTS

..................................................................................... 1 . INTRODUCTION 1

......................................................................................... 1 - 1 Fibre composite materials 1

1.2 Fibres ......................................................................................................................... 3

1.3 Polymriric Matris ...................................................................................................... 7 1.3.1 Low-density Polyrthylene (LDPE) ................................................................. 7 1.3.2 PEEK .................................................................................................................. 8 'I I I . J . 3 Epo'ry ................................................................................................................. 9

1.4 The Interface ........................................................................................................... 1 1 1.4. I Interface efkct on composite properties .......................................................... 12 1.4.2 Theorics of adhrsion and types of bonding ................................................... 13 1.4.3 Surface treatments of tibrcis and effrcts on composite properties ................... 14 I A.4 Environmental effect on the interface .............................................................. 17 1 4 . 5 bleasuremrnt of interface proprnirs ............................................................... 18

1.5 Peel test ................................................................................................................... 31

1.6 Objectives of this rcsearch ...................................................................................... 76

2 . EXPERIMENTAL METHOD 8 8 . 8 8 ~ ~ ~ o ~ ~ ~ 8 e ~ ~ 8 ~ ~ ~ ~ ~ 8 e ~ ~ 8 0 . ~ 8 ~ 8 ~ 8 m 8 8 m 8 8 8 0 m 8 . 27

2.1 Materials .............................................................................................................. 27

2.2 Sample preparation ................................................................................................. 77 2.2.1 Fibre embedding equipmrnt ........................................................................... 17 2.2.2 Fibre embedding .............................................................................................. 29

2.3 Sample selrction .................................................................................................... 30

2.4 Peel tests .................................................................................................................. 3 1

. . . 4 4

2.3 Ettects of temperature ............................................................................................. 3 3

2.6 itlicrostructure observation ..................................................................................... 34

3 . EXPERIMENTAL RESULTS ........ .. ..................................................... 35 .) . 3.1 Peel Cunes ............................................................................................................. J ?

3.1 . 1 Typical Peei Cumes ........................................................................................ 35 .................................................................................. 3 . 1 2 Exceptional Peel Curves 40

................................................................... 3.2 Peel Strengths of Fibre-Resin Spstems 43

* * . . ....................................................................................................... 3.3 Thermal Eftects 46

............................................................................................................ 3.4 SEM Photos 1 7 ....................................................................................... 3.41 Fibre-LDPE Systems 47 ....................................................................................... 3.4.2 Fibre-PEEK Systems 53 ........................................................................................ 3 - 4 3 Fibre-çpoxy Systems 57

................. ............... 4 . DISCUSSION ................................................. 63

4.1 Depth o f fibre rmbrdded ........................................................................................ 63

4.2 Types of peelings .................................................................................................. 66 4.2.1 Poor adhesion ................................................................................................... 68 4.2.2 Brittle failure .................................................................................................... 68 4.2.3 Ductile hilure .................................................................................................. 71 4.2.1 Fibre damage ............................................................................................. 7 3

1.3 Peel rinalysis ............................................................................................................ 75

4.4 Modes o f M u r e ................................................................................................. ..... 77

5 . CONCLUSIONS ...................... ........................ .......a.......................... 8 2

6 . RECOMMENDATIONS ... .................................................................... 83

7 . REFERENCE ............ ............... .,. .................................................. 84

8 . LIST OF NOMENCLATURE ..................... ..... ..................................... 87

9 . APPENDIX ...................................................................................... 8 9

TABLE OF FIGURES

3 Figure 1.1 Section of a composite ...................................................................................... - Figure 1 2 Esample of repeating units in polymer molecules (a) .4 polypropylene

7 molecule (b) X nylon 6.6 molecule ............................................................................. - Figure 1.3 .Arrangement of carbon atoms in a graphite crystal .......................................... 6

Figure 1 .-i Chernical structure of Kevlar 49 fibre ............................................................... 6

Figure 1.5 Scanning electron microphotograph of a fibrillated Kevlar 49 fibre ................ 6

Figure 1.6 Chain structure of LDPE .................................................................................. 8

Figure 1.7 Chemical structure of (a) eposide (b) rpoxy resin (DGEBA) before curing .. 10

Figure 1.8 Fracture e n e r g (Mm-2) for some common materials .................................... 1 1

Figure 1.9 Schematic illustration of the componrnts of the thrre-dimensional interphase

betwen tibre and matris .......................................................................................... 12

Figure 1.1 O Fracture toughness vrrsus shear strrngth for a carbonipolyester composite 1 3

Figure 1.1 1 Interface bonds formrd by ( a ) molecular entanglemcnt: ( b electrostatic

attraction: ( c ) interdiffusion of elrments: ( d ) chrmical reaction betwern groups .-1 on

one surface and groups B on the other surface: (e) chemical reaction following

forming a new compounds: (t) mechanical interlocking .......................................... 1-1

Figure 1.12 Function of a silane coupling agent: (a) hpdrolysis of organosilane to silanol:

(b) hydrogen bonding brtween hydrosyl groups of silanol and glass surface: (c)

polysilosane bonded to glass surface:(d) organofuctional R-group reactrd with

..................................................................................................................... polymer 1 5

...... Figure 1 . 1 3 The four methods currentl y used to measure the properties of interface 1 8

.................. Figure 1.14 3 typical load profile obtained from the microcompression test 20

................................. Figure 1. l 5 Typical pull-out cune. fibre debonds when force=F., 20

33 Figure 1.16 Esperimsntal arrangement for the basic peel test ..................................... -- Figure 1.17 Peeling around a steel roller P is the peel force and W is the weighr of the

-77 roller. per unit width of tape ..................................................................................... -- Figure 1.18 Modified peel test set-up ............................................................................. 3

Figure 1.19 A typical load vs displacement curve in the peel test .................................... 24

Figure 1.70 Force displacement plot from single fibre peel test ...................................... 25

Figure 1. I The induction heater ................................................................................... 28

Figure Z.Z(a) The embedding setup in the induction heatrr. (b) Cross section of the

stainless steel capsule ............................................................................................. 29

Figure 2.3 Sample selection by microscope .................................................................. 2 1

Figure 7.4 Peel test perfomed on the CHAN 1000 Automatic Balance .......................... 3 2

Figure 2.5 Schernatic plot of peel test performrd on the CHAN 1000 Automatic Balance

.................................................................................,..........,.......,......................... 3 2 9 9 Figure 1.6 The cartridgr heater ....... . ...................... . . ..... . . .. . ....... . . .......... ............................ J J

Figure 2.7 Sprcialiy designed holder for Scanning .............................................. ........... 34

Figirr 3.1 Typical pccl çiirws for Fibre-LDPE systrms ( a ) E-glass-LDPE ( b) Carbon-

LDPE (c) Kevlar-LDPE ........................................................................................... 36

Figure 3 .lTypical peel ciirves for Fibre-PE EK systems (a ) E-glass-PEEK I b ) NCiF glass-

PEEK (c) Carbon-PEEK ........................................................................................ 37

Fig 3.3Typicai peel cunres for Fibre-epoxy systems (a) E-glass-epoxy (b) Carbon-epoxy

( C ) Krvlar-eposy ....................................................................................................... 3 8

Figure 3 .4 Typical peel çunes for Fibre - eposy sustems at 50DC ( a E-glass-cposy < b

Kevlar-epoxy ........................................................................................................ 39

Figure 3.5 Tvpical peel Cumes for Fibre - epoxy systcms at 100°C (a) E-glass-eposy ( b )

Kevlar-eposy ................................ . . ........................ . . . . . , . , . ,. , . , 39

Figure 3.6 Tupical peel Cumes t'or Fibre - eposy systerns at 1 50°C (a) E-glass-eposy (b )

Kdar-cpos!- ................................................................. . . . . . . . . 40

Figure 5.7 Typical peel curves For Fibre - cpoxy systems at 200°C (a) E-glass-epos! ( b )

Kevlar-epoxy ....................................................................................................... 40

Figure 3.8 Schematic drawings of two types of the fibre embedding in the resin (a) Fiber

cmbedded too deep (b) Fibre embedded too shallow d- Diameter of the fibre w-

Esposed width of fibre observrd by the microscope .................... .... ...---......... 42

Figure 3.9 Peel çun9es for tihrr broke (a) E-glass- rposy (b) Carbon-LDPE .................. 42

Fig 3.10 Peel curves for fiber came out with low values (a) E-glriss- LDPE (b) Carbon-

PEEK (c) Kevlar-epoxy ............................................................................................ 43

Figurr 3.1 1 Schematic drawing of fiber embedding in the resin with a dope ( a ) E-glas

PEEK (b) E-glass-LDPE ........................................................................................... 44

Figure 3.12 Peel curws for fiber smbeddsd in the resin with a dope (a) E-plass PEEK

(b) E-glass-LDPE ...................................................................................................... 44

Figure 3.13 Peel Cumes o f fiber defibrillation (a) Kcvlar LDPE (b ) Kevlar-epoxy ........ 45

Figure 3.14 The peel strength of E-glass-sposp at difkrent temperature ........................ 47

Figure 3.15 The pccl strrngth of Kevlar-epoxy at different temperature ......................... 47

Figure 3.16 Fibres partly peeled from LDPE ................................................................... 49

Figure 3.17 The hollow lefl by the Rbrc a k r peeiing from LDPE .................................. 50

Figure 3.18 (a) The peeling point of E-glass-LDPE (b) The end point of carbon-LDPE (c)

The end point of Kcvlar-LDPE ................................................................................. 51

Figure 3.19 Kevlar fibre peeled off from LDPE (a) Kcvlar fibre de fibrillation and a littlr

- 3 bit kinking ( b ) Kevlar fibre kinking .......................................................................... 1-

Figure 3.20 Fibres partly preled from PEEK ................................................................... 54 - -

Figure 3.21 The fibres after peeling from PEEK ................................. .. ........................... ss

Figure 3.22 The hollon Ieft by the fibre afisr peeling from PEEK .................................. 56

Figure 3.23 Fibre partlj . perled from spoxy ..................................................................... 58

Figure 3.24 The hollow left bp the fibre aftcr peeling from eposy ................................. 59

Figure 3.25 The fibre on its socket ................................................................................ 60

............................................................. Figure 5.26 The fibres after peeling from epoxy 61

........................................................... Figure 3.17 Krvlûr fibre a% er peeling from epoxy 6 1

(a) defibrillation and a crack in the fibre (b) more detail of the crack (c) detibrillation and

kinking of the fibre ................................................................................................... 62

Figure 4.1 Schematic drawing of a fibre embedded in the resin ...................................... 63

Fisure 4.2 Esamples of 3Omm diameter fibres cmbedded at axial depths of 2.5. 5 . 7.5mm

(thrse are acceptable) and 10 and 12.5mm which are unacceptably deep ................ 64

Figure 1.3 Artists impression of hollow lrft uhen (a) carbon fibre \\.as prrlcd from epoq .

and (b ) glass fibre kvas peeled from eposy ............................................................... 66

Figure 4.4

Figure 4.5

Figure 4.6

....................................................... Poor adhesion of E-glass-PEEK interface 68

Brittle failure of fibre-epoxy interface ......................................................... 70

Brittle failure of NGF glass-PEEK interface ................................................. 71

..................................................................... Figure 1.7 Sçhcmatic formations of ribbons 7 1

Figure 1.8 Ductile failure of E-glass-LDPE interface ................................................. 71

Figure 1.9 Schematic drawing of ductile failure during the peeling process ................... 72

Figure 4.10 SEM photos for LDPE in punch test ............................................................. 73

Figure 4.1 I Fibre damagr in Ksvlar-resin systems .......................................................... 7 1

Figure 4 . I > Schernatic of fibre tilting and kinking mrchanism ..................................... 74

Figure 4.13 Elastica curie of the peeling fibre ................................................................. 73

Figure 4.11 Schematic drawing of fibre movement and deformations ( a ) shows the

segment of fibre lcngth r in relation to c u n e radius R (b) shows the tibrr bcing

partly liftrd out of the resin and ( c ) shows more drtail of the modes of fracture at

.......................................................................................... emtxping end of the fibre 78

LIST OF TABLES

Table 1 . 1 Trnsilr properties of some metallic and composite materials ........................... 3

Table I . 2 Propenies of somr commercial reinforcing fibres ............................................. 4

< Table 1.3 Typical composition of glass fibre ( in u-eight percent) ......................................

Table 1 .-I Some mcchanical propertics of these three polymrrs ........................................ 8

Table 2.1 Somr propenirs of polymers used .................................................................... 17

Table 2.2 The smbedding temperature for di fferent fibre ................................................ 30 . -

Table 3 . l Peel strength ot tibre-resin systems tested ........................................................ 46

Table 4.1 The axial drpths of the fibres embeddrd in the resin s h o w in the SEM photos

................................................................................................................................... 65

.................................. Table 4.2 Toughness and SEM obsen.ations of the systems testcd 67

Table 4.3 l p for fibre-resin system trsted .......................................................................... 76

............................................... Tûhle 4.4 Displact.mt.nts nt the end of process zone (pm) 80

Table 9.1 Thermal effect on peel strength of E-glass-rposy and Kevlar-epoxy M " u u * . 89

1. INTRODUCTION

1.1 Fibre composite materials

-4 fibre composite material has at leasr two cornponents of different natures. and

the performance characteristics of the composite are designed to be greater than that of

eaçh cornponent. In the most ysnrral case. the composite material consists of one or more

discontinuous phases distri buted in one continuous phase. The continuous phase is çalled

the matris and the other one is called the reinforcement (ligure 1 . 1 ). hlistures of straw

i reinforcement) and mud (matris) for building bricks are the tirst knoan synthstic

cornpositc matsrials.

Modern fibre composite materials consist of fibres of high stren~th and modiilus

embedded in and bonded ta a matris with distinct interfaces between thsm. In this hm.

both tibres and matris rrtain their physical and chrmical identities. .et the). producc a

cornhination of proprrtirs that cannot be achieved with either of the constituent alonr. In

ysnerüi. tihres art. the principal load-cap. iny members. the surrounding matris kzeps

them iri the desircd location. transfers the load to them. and protects them from

mechanical ahrasion and environmental damagr.

The principal fibers in commercial use are wrious types of glass and carbon as

w l l as K d a r tibers. Othsr fibers. such as boron. silicon carbide and aluminum oside.

are used in limited quantities. The tibres usrd are eithsr continuous or discontinuous in

form. chopprd fibres. short fibres. etc.

The matrix material may be a polymer. a mrtal. or a cerarnic. A polymer is

defined as a long-chain molecule containing one or more repeating units of atoms (Figure

1.2). joined together by strong covalent bonds. A polymeric material is a collection of a

large number of polymrr molecules of similar cliemical structure.

Figure 1.1 Section of a composite [ I ]

(b)

Figure 1 .> Esample of repeating units in polymer molecules

(a) .A polypropylene molrcule ( b ) A nylon 6.6 molecule

Fibre-reinforced polymer composite materials are uscd extensively for their low

density and potential high strength. Because of their Iow density. the specific strength and

specific modulus of these composite mate ria!^ are remarkably superior to those of

metallic materials (Table 1.1 1. Thus. fibrr-reintorced polymer composites have rrnrrged

as a major class of structural material and are used as substitutions for mrtals in many

weight-critical components in aerospace. automotive. sporting goods. marine engineering

and other industries.

Table 1 . 1 Tensile properties of somr metallic and composite materials [ 2 ]

Specitk Modulus. Tensile Specific Spscitic Tensile

grav iîy GPa strength. modulus. strength.

MPa 1 OOm" 1 0'm"

SAE I O 1 O steel

AL 7 178-T6 aluminurn a l l q

7-7 PH stainless steel

INCO 718 nickel a11oy

High-strengtli carbon t1bt.r-epos~

High-modulus rsarbon fiber-cposy

E-~lass t? ber-epoq

Kevlar 49 fiber-epoxy

" The modi~liis-neight ratio and ttir strenyli-~veight ratio are obtained by dit iding the absolute

values uith the specitïc weight of the respective material. Specific ueight is detined as weight

per unit volume. It is obtained by rnuItiplying densih; by the acceleration due to gravity.

1.2 Fibres

Fibres are the principal component of fibre composite materials. They occupy the

largest volume fraction in composite laminate and sustain the major portion of the load

acting on the composite. A number of commercial fibres and thrir properties are listed in

Table 1.2.

Table 1.1 Properties of somr commercial reinforcing fibres [3]

Fibre Typiçal Speci tic Modulus. Tensile Spec itic Specific

diamrter gravity GPa strength. modulus. Tensi le

um ?4 pa l Ohm strrngth.

. -. - - .- - - - - - - - . . .

Glass

E-glas

S-glass

PAX carbon

,A S-4 a

Pitch carbon

P-5 j

Aramici

Kdar-49

Ksvlar 149

Boron

Sic

~lonofilament

:\1:03

a Hercules. ' Amoco. c Dupont.

Glass in bulk form is very bnttle and easy to crack. But when made in the form of

fibers. it can b r quite strong. Glass fibres are the most common reinforcing fibres because

of their Ion cost and high tensile strength. The commonly used glass fibres are E-glass.

Another type. knoun as C-glass is used in chemical applications requiring greater

corrosion resistance to an alkaline environment than E-glass: D-plass. with hi@ dielrctric

proprrties. is used for the construction of clectronic materials for telrcornmunication: S-

elass. with high strength and higher modulus than E-giass. is used for making structures - with high mechanical performances. The chemical composition of E-glas and S-glas are

s h o w in Table 1.3.

Table 1.3 fipical composition of glass fibre (in weight percent)

Glass fibre SiOl Al7O3 Ca0 Mg0 BiO; Na10

type

The advantage of carbon fibres consists of high specitic strength and high specitic

modulus (Table 1.2). Because of thtir high cost. thry are usrd mostl!. in the acrospace

industry. uhrrr ~vright swing is much more crucial than cost. The high trnsilr modulus

of the carbon fibre results from the graphitic crystal. in which carbon atoms are arranged

as in Figure 1.3. The planes are held together by weak van der LC'aals forces. and the

carhon iitoms in n plmc are hrld by strong covalent bonds. This results in tiighl)

anisotropic physical and mechanical propsrties for the fibre. Carbon fibres are

manuiàctured from polyacrylonitrile (PAN). rayon and pitch. High strength carbon tibres

w r e initially produced from P.AN. such as . 4S4 from Hercules.

Revlar -1') and 1-19 belong to a group of aramid tibres that ha1.r the lotvest specific

gravit? and the highrst specitic tende strength among the current widely ussd

reinforcing fibres (Table 1.2). The chernical structure of K d a r 49 is illustraid in Figure

1 .4. The polymer chain direction corresponds roughly to the fibre direction. In the

transverse direction. neighboring çhains are held togrther by weak hydrogen bonds and

van der Waals forces. This structure gives the fibre a v e p high tensile strength in the

longitudinal direction. But the fibre is rrlativcly w a k in the transverse direction. When

the fibre is bent into a loop. it bucklrs on the insidr of the loop and splits longitudinally

on the outside of the loop. Furthemore. when the load increases to the breaking point. the

fibre cracks longitudinally or fibrillates. nther than breaking transversely. as evidenced

by the scanning electron microphotognph shown in figure 1.5.

Figure 1.3 Arrangement of carbon atoms in a graphite crystal [3]

<- Fibre direction ->

Figure I .-i Chernical structure of Krvlar 19 tibrr [J 1

Figure 1.5 Scanning electron microphotograph of a fibrillated Kevlar 49 fibre [.i 1

1.3 Polymeric Matrix

The matrices used in fibre composites transfer the mechanical load to the fibres

and protect them from the outsidc environment. The matrix material must wet the fibres

and offer good compatibility with them.

Csually. polymcrs are divided into two large groups: thermoplastics and

thrrmosets. In a thermoplastic polymer. individual molecules are linear or branched in

structure with no chernical linking between them. The. are held in place b!. van der

Waals bonds or hydrogrn bonds. With the application of heat and pressure. these

intrrmolrcular bonds çan be temporad- broken. and the niolecules can be mowd to neu.

positions. Aftcr cooling down. the molecules tieezt. in their new positions. restoring the

secondary bonds betwrn thcm and resulting in a new solid shape. Thus. a thcrmoplastiç

polymer can be heat softened. rnelted. and reshaped as many times as drsired. Anlong

therrnoplastic resins are: polyethylene (PE). polppropylrne (PP). polyvinyl chloride

(PVC). polyrther ether krtone (PEEK). polyamide (Nylon) and polycarbonate (PC ). etc.

In a thermoset polymer. the molecules are chemically joined together by cross-

links. fonning a rigid. three-dimensional network structure. Once thrse cross-links are

forrnrid durin9 ihc curing reaction. the thermoset polymer cannot be meltttd and reshaped.

The following are some common thermoset matrices For composites: polyesters and vinyl

esters. cposirs. polyirnidrs. etc.

Ttvo thermoplastic resins. low-drnsity polyethylene (LDPE) and PEEK. and one

thermoset. rposy. are discussed in drtail. The propcnies of these polymers are s h o w in

Table 1.4.

1 .X I Low-density Polyethylene (LDPE)

LDPE. as a general purpose thenno plastic material. was first manufactured frorn

sthylene in the early 1940s and used as a wire coating. It contains man)- short branches.

mostly two to four carbon atoms long and a few long branches (Figure 1 . 6). It is a wax-

like thermoplastic sofiening at 80-130UC. It is tough but has lou. tensile strength. one of

the lowest tensile moduli (Table 1.4). and it is di fficult to stick things to it.

Table 1.4 Some mechanical propertiss of these threr polymers

Material LDPE PEEK Epoxy

Density 0.92" l.3* 1 .O*

Young's Modulus. GPa 0.2 3 .Z* 3.5

Tensile Strength. MPa 1 O 100" 60

O O Elongntion 350" 40* j*

Work of Fracture. k ~ m " 6.6" * O. 1

Glass Transition Temperature. "C -pz*** I43*

* Data obtained from Seymour [hl

* * Data obtained from Mallick [3]

*** Data obtained frorn Liu and Piggott ["] and the rest of data obtainrd from Pigon [XI

Figure 1.6 Chain structure of LDPE [ Q I

LDPE is a semicrystalline solid. its crystallinity is nomially 30--!0°,~. lncreasing

cpstallinity in LDPE increases its stiffness. chernical resistance. tensile strength. heat

rcsistance and stress-crack resistance.

The distinctive properties of toughness and flesibilitp. ease of processine and low

pricr of LDPE. makr it a major component in a variety of packaging. constniction.

agricul tural. indusrrial and consumer markets. The limitations of the pol'mer are: the lou.

softsning point and t hr low tensik strength.

1.3.2 PEEK

PEEK. a high performance thtirnoplastic material. was introduced intv the

commercial market bp ICI in 1981. It is a linear aromatic thermoplastic basrld on the

following repeating unit:

ether ether

PEEK is also a sernicrystalline polymer. Amorphous PEEK is produced if the

melt is qurnched. Increasing crystallinity increases both the modulus and the +Id

strength of PEEL but reduces its strain to failure [ l O]. Because of its çrystallinr structure

and the riromatic chernical structure. PEEK exhibits excellent environmental resistanctt

compared with other thennoplastic resins.

The outstanding propertp of PEEK is its high fracture toughness. which is 50-100

times higher than eposirs (Table 1 A). PEEK has a g las transition temperature ut' 143°C

and a çrystalline mrlting point of 335°C. The melting temperature of PEEK ranges from

570-400uC. The maximum continiious use temperature is 250°C. Another important

advantaçe of PEEK is its low water absorption. which is less than 0.jo6 at 23°C

compared to - 1 - 5 O 6 for conventional aerospace epoxies [;]. in addition. it does not

dissolw in common solwnts. Thesc propertirs have caused it to be used to replace

rposies in some aerospace composites. PEEK is not onl:. ussd in conditions \\hich

rrquirr high toughnrss.

1.3.3 Epoxy

E p o s ~ resins. first devrlopèd in the 1940s. are one of the major thermoset matris

materials. .An rposy is a polymer that contains an sposide group (one «s)gr.n atom and

two carbon atoms) in its chernical structure (Figure 1.73). Diglycidyi ether of bisphenol A

(DGEBA) contains two rposide groups. one at each end of the molecule (Figure 1.7 b).

The epoxide groups are very reactive. so when a hardener such as amine with active

hydrogen atoms is added. it copolymerizes to tom a three-dimensional crosslinked

stntcture.

Figure 1.7 Chernical structure of (a) eposide (b) epoxy resin (DGEBA) before curing [3]

The propsnies of a cured rposy resin depend mainly on the cross-link density

(spacinp between successivr cross-link si tes). In grnrral. the tensile modulus. g l a s

transition temperature. and chernical resistanct: are improved with inçreasing cross-link

densir). but the strain-to-tiaçturc and hacture toughness are reducrd. Eposy resins are

brittle. which causes restriction in their use in sewral applications.

The matris fracture characteristics (strain to tàilure. work of fracture. or fracture

toughnrss) are as important as liphtncss. stiffness. and strength propenies. Figure l .S

compares somr common materials in terms of their fracture touglmess as mensurcd by

the fracture surface enrrgy in hm'. Thermoseting resins have values that are only slightly

higher than those of glasses. Thennoplastic resins such as PMMA have fracture e n e r g of

about 1 k ~ r n " . while PEEK resins have srveral k~rn-?. alrnost approaching those of 7075-

T6 aluminum alloy . Elastomer-modi lied or rubber-modi fied cpoxies enhance the

toughness of a hard. bnttlr e p o y matris bu adding a small amount of elastomsr or rubbttr

to f o m a second phase in the cured matri?; and prevent it microcracking.

PEEK resins

Etastomer modified epoxies

Polymethyl methacrylute

Inorganic glasses

Figure 1.8 Fracture energy (kJm-2) for somr common materials ( Adapted from Krishan [ I 1 1)

Since spoxy exhibits valuable properties. ix.. good adhesion to fibres. !ou.

shrinkage. high clectriçal resistance and good corrosion rrsistûnce. it is widely used in

man? areas such as glass fibre reinforcrd rposy in pipe in oil field. as uell as pressure

vrssels. tanks and rocket motor casing. In aerospace industry. more and more çarbon

fibre reinforcrd epos!. is being usrd.

1.4 The Interface

In a fibre composite. the mechanical properties of the combination of tibre and

matris cannot be achieved by rither of the component with the presence of an interface

between these two components. The growing amount of fibre-reinforced composites in

many engineering areas has made many people focus on the study of interface (or

interphase).

The lnirrfirce is a surface fomrd by a common boundary of reinforcing fibre and

rnatrix. and transfers loads between them. Its mechanical properties are different tiom

those of the fibre and matrix. In contrat. the inferphme is the region between the tibre

1 1

and the matns. which includes a11 the volume altered during the fabrication process.

Figure 1.9 schematically shows the concept of interphase according to Drzal et al [ I 21.

Interphase

mat erial

Bulk fiber

Figure 1.9 Schttmatic illustration of the componcnts of the three-dimensional interphase betwen fibre and matris

1.4.1 Interface effect on composite properties

The intemal surlàce area occupicd by the interface is quitr cstensiw. It c m rasily

20 as high as 16500 rnm2;mm:' in a composite containine 50% af fibre u-ith a fibre

diameter of O.0076 mm [13]. It is possible that a small change of the intcrhçe will greatl?

affect the composite properties. For example. it has bren kno1r.n for a long tirne that the

shear strength and toughness are hiçhly dependent on the strength of the interiàcr. The

shear strength is increased while the roughness is reduced when the adhesion of the fibre

and matrix is o o d (Figure 1.10). Good adhcsion also increasrs the tcnsilç strength.

stiffness. fatigue and environmental resistancs of the composites [ 141.

Figure I . 1 O Fracture toughness vcrsus shear strength for a carbodpolyester composite [ I il

1.4.2 Theories of adhesion and types of bonding

T h m are many theorics used to rsplain the interfacial adhesion including:

adsorption and wetting. electrostatic attraction. chemical bonding. reaction bonding and

mechanical interiocking. which are schematically shown in Figure I . I 1 . In addition to

abow major theories. hydrogen bonding. van der Waals forces and other low cnergy

forces ma). also bc involvrd. .-Ill these theories take place at the interface rcgion eithcr in

isolation. or. in combination to producr the final bond.

.-lti.~orprion uncl irening is based on chcmical bond formation across the i nterhce.

Moreover. good wetting of fibres by matris material is necessary for proper consolidation

of a cornposi te during the impregnation stages of preparation. EZrcrrusrtrris ~trrrttcriort

bonding is dur to differrnt rlrctrostatic charges between cornponrnts at the interface.

Interdijkion is based on the bond strengrh in polymrr matris composites. which

depends on the amount of moiecular entanglement. the number of molrcules involvrd

and the strength of the bonding between the moiccules. Chrmicd bontiing is the oldcst

and best known of al1 bonding theories. Reuctbn bonciing relates to a new compound at

the interface region formed between the matris and reinforcing elements. .Ilechrrniccrl

hontling States the strength of joints due to pits. corrugations of the fibre at the surface.

Fip re 1 . 1 1 Interface bonds formed by (a) molecular entanglrment: (h) rlrctrostatic attraction: ic) interdiffusion of elements: (d) chemical reaction b r twrn groups .A on

one surface and groups B on the other surface: (e) chrmicol reaction Follouing forming a new compounds: ( t ) mechanical interlocking ( Adapted from Jang-Ky

Kim et al [ I h l )

1.4.3 Surface treatments of fibres and effects on composite properties

The interaction of ii fiber with a matrix material depends stron& on the fibre

surface layer. For fibre polyner composite. fibre surfaces are trecitrd to snhancr the

interface bonding and presene it in a service environmeni. particularly in the presrnce of

moisture and at moderate temperame.

The major problcm of the p l a s fibre is its hydrophilic nature. The fibre surface

attracts water. resulting in loss of the strength for the early composites 1171. With the

development of silane coupling agents. the problem was solved. Silane agents are applied

as a protective coating for g l a s fibre surfaces. makinç it hydrophobie. and as a coupling

agent to promote the adhesion with the polymer matrix. Chernical bonding is the main

theop that explain the interfacial bonding mechanism of silane coupling agents [SI.

Basrd on the chemical bonding throry. the bifunctional silane molecules act as a link

between the resin and the glass by forming a chtmical bond with the surface of the glnss

through a silosane bridge. while its organofunctional group bonds to the polymer resin.

The fiinction of the silane coupling agents is schematically illustrated in Figure 1.12.

Figure 1.11 Function of a silane coupling agent: (a) hydrolysis of oqanosilant: to silanol: ( b ) h!.drogrn bonding bctween hydrosyl groups of silanol and glass surface: ( c ) polysilosanr bonded to g l a s surfacdd) organohctional R-group reactcd with

polymer [Si

The chemical bonding theory is \+eV important to the themioset matris

composites. On the othsr hand. the compatibilit? betwrn the silane and the matris rcsin

is more important than chemical bonding in thermoplastic matris composite due ro the

unreactive molecules in the thermoplastic. The reactivity ma) br improved by including

chemicals in the size or using coatings. for example. mbber irnpregnated glass fibre

bundles.

AI-Xloussawi et al. [ l X I indicatrd that the interphase produced by the interaction

of size with eposy is cornpletely different to the bulk matrix material. The interphase

material has a lower glass transition temperature. TE. higher modulus and greater tensile

strength but lower fracture toughness than the bulk matrix. On the contrary. Chua and

Piggon [IL)] reported that. the presence of large arnount of siloxane MPS in a polyster

resin reduces the modulus and compressive strength. while increasing the Fracture of

toughness.

Surface treatment of carbon tibres c m be classified into two groups: oxidative and

non-osidatiïe treatmrnts. Oxidative treatrnents include: dry osidation. plasma etching

and iwt usidtition. U'hiskcrization. plasma deposition and polymer coating are among the

non-o'iidatiw treatrnents of carbon fibre surfaces. The effect of the surface treatments on

the carbon tiber is summarized as follow:

( 1 ) A weak layer ma- be removed;

( 3 Pitting of the fibre surface results in high surface roughness:

( 3 ) Rcactiw groups such as carboxyl. hydroxyl and carbonyl are formcd on the

tibre surface.

The shear. flesiiral and tende strengths of cartion-rrinhrced composites are

increrised due to the surface treatment. The compressive strength is increased slightly.

and the fracture toughnw Gi, is alrnost doublrd with inçreasing drgrce of treatment [3 11. L i n g fragmentation tests [ I 21 and tibre pull oui tests 151. i t has bren shoirn that tibre

surface trcatments improve the interface bond strength. rd. or fracture ioughness. G : c,.

There are three surface treatment rnethods h r Kevlar fibre: chèmical

etchiny'graliing. plasma treatment and application of coupling agents. These enhance the

chernical interaction between the fibres and the polymer resins by introducing reactive

hnctional groups on the tibre surface. However. these approaches are not so effective.

for example. Kalanta and Drzal [ Y ] reported that a strong interface bond does not

guarantre the best mechanical performance because the fibres are easily damaged during

the surface treatment process due to skin-to-core inhornogeneity. low transverse and

compressive strengths of the fibre. Piggott [ l il indicated that this fibre is otien used

without an! surface treatment.

1.4.4 Environmental effect on the interface

Composite materials corne across different environmental conditions such as

temperature. moisture. solvents. acids etc. in their senice period. These elrments have a

negative effect on the performance of composite materials. For example. 48% of boats

and an rven higher proportion of swimming pools built with glass composites are

affrcted by blistering due to continuous contact with water [2]. Brittle fracture of

insulators made from glass fibre composites was caused by acidic environment [3].

Composite material iised in arrospacr and other industries fails due to harsh

environmental conditions.

Sloisture affects noi only the fibre and matris of composite material. but also the

interface. tspecially in the case of glass. Thus many studies have been focusrd on the

coütirig of the glass fibre to promote good retention of adhesion in the prescnccl of water.

U'oo anci Piggott [14 1 reponed that water absorption \vas present in the tibn-matris

interface through microscopic examination. rspecially in the composites of giass and

DER 3 3 1 eposy resins.

Chua etc. [ 2 5 ] resrarched the effect of hot watrr on the glass-polyester interphase

b> pull out test. The fibre-polymer bond strength was reduced almost to zero aftcr 10Oh

in 7 5 " ~ water. but at 6 0 " ~ the reduction was only -!O-5oo6. The strtrngth reduced b>- 20%

aftcr l3OOOh immersion at 2 2 ' ) ~ . and recovered when the samples wcre drisd out at 8 0 " ~

for 72h.

.Alimuddin and Piggott [IO 1 invsstigatcd the r ffect on three tibre-rposy interfaces

of various liquids immersion at different temperature and various time by the perl-test.

The fracture toughness of glass-epoxy was reduced by 90°6 in 2 5 " ~ u-ater after 1Oh. At

elevated temperature the pmcess was much faster and needed a shorter time. High

temperature water uas required for significant loss for both carbon-epoxy and Kevlar-

epoxy interfaces. Sulfuric and acetic acids at 1 5 ' ~ had about the sarne effect as water at

9 0 ' ~ . Organic tluids such as Kerosene had very little effrct.

Piggott and Wang [27] found that in the range of 10-1 loOc. the interfacial shear

strength of carbon in rposy. as measured using pull out test, was relatively insensitive to

temperature.

1.4.5 Measurement of interface properties

Thcre are man- rxperirnent techniques to measure the mechanical propcnies of

the fibre-matris interfaces in composites. In general. there are two different niethods

depending on the nature of spccimens smployed and the scalc of trsting [2S 1: one

intuives the tcsling of single tibre microcompositrs in which indit idual tibrrs rire

embeddsd in matris blocks: and the other uses bulk laminate composites to measure the

inttxlaminar propenirs. Microcomposite methods providr direct measurements of

interface propçrties. whilr methods based on bulk composites. inevitably produce results

that are affectrd by the failurr of the surrounding matrix.

Four methods. currrntly cvidrly used are: the pull out test. the microtcnsion test.

the tibrt. miçroçonipression test. and the single fibre fragmentation test ( Figure 1.13 1. h

these tests. the bond at the interface is measured in rems of the interface shrar strrnpth.

~ h . or interfacc fracture toughnrss. G. usually g iv in some mean values.

U

Micro teosioa

Microcorngrenrion Fragmentation

Figure 1.1 3 The four methods currently used to measure the propenies OF interface

The fragmentation test is one of the most popular methods. In this test. a single

fibre is embedded in a dog-bone shaped tensile sarnple of matrix. When the specimen is

subjectrd to a tsnsils load. the load is transferred to the fiber via shear strains and stresses

through the interface. Whcn the tensile stress in the tlbre reachcs its ultimatr strength. it

fragments into two parts. If loading continues. the fibre breaks into smaller lengths until

the fibre fragment length becomes too small to be broken. This fibre length is called the

criricd lrngtli. /,. The average shear strength at the interface. ra. c m be rstimated from

eqi~ation 1 . 1 [YI:

wlicrt. a.,, is th.: fibre rrnsile strcngth. d is the fibre diamrter. and 1, is rnean tibrc ti-a, urnent

Iength tl. is npprosimatel~ 3,A). oh is calculatcd b~ mcasiiring a lonpcr Irnpth. Le..

3mm. and thrn extrapolatinp to li

The tiagrnriitation mrthod is a simple technique that gives us a qualitative

measurtt of th.: fibrc:rnatrix interfacial strength. On the other hand. this technique will

work only if the tibre lailure strain is lrss than that of the rnatrix. blorrover. Giti in

equation 1 . 1 is not easy to obtain. since 1, can be very small. such as O 3 m m [Y]. In

addition. for sirnplicity. it is assumed that the interfacial shear stress is constant ovrr the

ti bre

actua

ength. actually this assumption is a poor one.

The microcompression test (or microindentation test) is a msthod that uses an

composite. .An indcnter is used to displacc a tibrr aligned perpcndicular to the

composite surface ( Fisure 1.1 3). By measuring the applied force and the displacrment.

the interface stress can be obtained. A typical force vs. time profile is shonn in figure

1.14.

Figure 1.14 .A typical load profile obtained from the microcompression test [;O]

This technique has been ussd sxtensively for polyrncr composites. and

instnimrnts for this test have been commrrcializrd. The advantagt: includes the abil i ty to

test on real composites and simplicity of the test. However. the force applird on the tibrc

causes a Poisson's expansion that increases the pressure at the interface. and then higher

values arc produced. In addition. the test cannot be utilized with polyrner fibres such as

Kevlar [ 15 1.

In the fibre pull out test. a fibre is panially embedded in a matris block. When the

fibre is loadcd under tension. the force is rccorded as a function of time or displacement

(Figure 1.15). In this curvr. thrre are two regions. initial fibre stressin- Ieading to

interfacial hilure. and post-drbonding friction. In the tirst region. the dope of the forcc-

displacement plot w s detsrmined mainly bu the stretching of the frer length of the tibre.

Debonciing occurred when the force. F. reached a critical value. F;+ In the second region.

the force decreased suddenly. and re-establishcd itsslf as frictional sliding ocçurrrd.

Figure 1.15 T>pical pull-out cune. tibrr debonds when force+., [ 1 (11

This technique has bern wideiy used for polymsr composites. I t providrs

information about what is really happening at the interface. The maximum force Fa,, is

considrred as the drbonding force and is used in different computations. The pull out

method also providrs independent information about the friction following the

debonding. However. it is very difficult to prrform. The debonding is accompanied by

tnction with two unknowns. the coefficient of friction. p. and the pressure across the

interface. P. bloreover. there is a large stress concentration at the fibre entry point. which

forms an elevated meniscus around the fibre during cunng of the matrix material. and the

point at the tibre end inside the polymer. Thus it makrs the test results inaccurate.

The microtension test Follo\vs the same principle as pull out test. i.e.. a single fibre

is pulled out from a sample of rrsin. It has almost the same advantages and disadvantagtts

3s pull out test. but it provides less information on the friction aRer debondinp.

.Ali the testins methods just mentioncd: pull out. microtension. miçrocomprcssion

and fragmentation. involve centrosymmetric strcssing. or conditions close thcrcio. Thest.

kinds of tests do not provide much uscful information about the fibrematris adhesion

sensitive propertirs of reinforced polymers. Alternative tests. such as the peel test. should

be more sensitive to the interface in composites and should be studied tùnher.

1.5 Peel test

Peel tests provide a usrfui means of assessing the strength of adhesivr. In

industry. such tests are otien used for pressure srnsitiw adhesive. panicularly tapes

which are difficult to test in other ways. I t has three distinct advantages compared to

other test mcthods: bond Mure proceeds ai a controllrd rate. the pcrl force can be used

to mrasure the work of fracture. and also it represents a mode of failure under senice

conditions. The results can be analyzed in terrn of the work or energy of adhesion.

rspecially when at lrast one of the adhering members is flexible. üsually. tests are carried

out at angles of 90" or 180'. but the analyses can be made for any angle [3 i l . The basic

sxperimental method is illustrated in Figure 1.16.

Gent and Kaang [XI reponed on work involving three commercial adhesive tapes

which were applied to two flat rigid substrates. and peeled off at various angles at a

constant rate. In order to reduce the amount of bending at the line of detachment. some

csperiments were camsd out as shown schematically in tigure 1.17. with the tape bring

peeled off around a steel roller. The- found that in ordsr to rninirnizr the bending energy

loss. the peel angle should br small. However. the peeling strip rrnded to stretch

significrintly whcn the peel angle approached 0". The bsst peel angle was 15" to minirnize

both t'ffccts.

Figure 1.16 Esperimental arrangement for the basic peel test

Figure 1.17 Peeling around a steel roller P is the peel force and W is the weight of the roller. per unit width of tape

Karbhari & Enginecr [3?. 311 and Xie Br Karbhari [3] modified the peel test to

characterize the interfacial bond strength between polyner composite and çoncrete. -4

strip of 3 . 4 mm wide. 330 mm long unidirectional carbon-epoxy or glass-epoq-

composite was attached to a block of concrete through a layer of resin adhesive. The peel

test fisture is illustrared in figure 1.1 8. The debonding force was calculated by taking an

average of al1 the praks appearing in the force vs displacement cL1n.e s h o w in tigure

1.19. The interfacial fract~irc enrrgy. G. is given bu:

where P is the pcel load. E is the strain in the peel m. u is the peel angle. and t are the

\\.idth and thicknrss of the perl strip respectively. and Ii' is the stnin snrrgy in the pwl

a m corresponding to a strain. E.

Base plate

Figure 1.18 Modified peel test set-up

When the peel tests were conducted at different peel angles. it was found that the

peel force decreased with increasing peel angle. which agreed with the findings of Lake

on flat tapes [... Il. On the other hand. the total interfacial tiacture energy increased with

increasing perl angle. The same conclusion aas obtained from peel test on tapes [?].

Figure 1.19 A typical load vs

The interfacial energy. G. can be divided

G r . i.e.

displacement cunre in the pecl test

into the opening mode. G ! . and a shrar mode.

G : decreases with increasing peel angle. u. whrreas G!i increases nith increasing u. Nie

and Karbhari [VI discussed four possible crack propagation paths nt the polymer

composite concrete interface. The crack propagation dong the interface betwen the strip

and the adhesiïe laysr pielded the lowest fracture energy. and thrrrforc. was the most

likely failurc paih. Karbhari and Engineer ['.-Il also investigated the environmental effects

on adhesion of glassiepoxy and carbodepoxy composite with concrete.

Alimuddin and Piggott [26] used single fibre peel tests to estimate the fracture

toughnrss of fibre-polymer interface. Single fibres of carbon. glass. and Kevlar were

embeddcd to about half their diameters in an rpoxy resin and thrn peeled off. .A peél

force-displacement curve (Figure 1.20) could be obtained during the proccss of the

peeling. and the perl ioad LUS calculated through taking an average of al1 the peaks.

O 1 2 Drspbcement (mm)

Figure 1.20 Force displacement plot from single fibre peel test

The work of fracture. Gt. is obtained &om:

whsrt. P is the prel load. u is the peel angle and d is the diameter of the fibre. When the

pcrl angle is 90". the equation 1.4 can be sirnplified to:

They found that G i for the glass-epoxy system was 1 JO ~m". which for carbon-eposy it

was 60 ~m". Fibre fibrillation affected the result of Krvlar-rposy systern. Research on

the environmental effrct on the peel test was also carricd out. as detailed in section 1 A.4

hrrein. In thttir stud?. the? assumed only mode 1 fracture was important during the

process of peeling. tlowcver. this assumption is not enough to intrrpret the s-hole process.

1.6 Objectives of this research

The purpose of this research is to provide more information about interfacial

proprnies of fibre-polymer composites through perl test based on the work of

.-\limmuddin. Various fibre-polymer resin systems are to be examined. including E-plass-

LDPE. Kevlnr-LDPE. carbon-LDPE. E-glass-PEEK. XGF glass-PEEK and carbon-PEEK.

E-glass-eposy. Krvlar-eposy rind carbon-epoxy. The effect of heating on the interfacial

proprnies for E-glass-epoxy and Kevlar-eposy systcms is to be investigatsd. .A nru

model fur the peel test is <O be set up.

2. EXPERIMENTAL METHOD

2.1 Materials

E-glass fibres (diameter. d=llpm) from Fibreglass Canada. glass fibres (d=l O pm)

from NGF Canada. Krtvlar 49 fibres (d=12pm) h m Du Pont. and carbon . - S I fibres

(ci-8 ym ) ti-orn Hescel w r e tised.

Low-density polyethylene (LDPE) from Dow Coming. polyetheretherketonr

(PEEK) from ICI Victrex (Grade I j lG) . and rposy from Hexccl Composites (Hercules

8552) . were usrd as rnatrix. The properties of these polymers are shown in Table 2.1.

Table 2.1 Some properties of polynicrs uscd

Material LDPE* PEEK** Eposy* * *

Densi t)- 0.92 1.3 1 1.30

Young's 4lodulus. GPn 0.3 3 -6 4.7

Tensilc: Strength. MPa 14 92 ! 2 1

4.0 Elongation >-CO0 4. O 1.7 i

Work of Fracture. Hm'* >60 6.6 0.7

Glass Transition Temperature. *C - 122 143 200

* Data obtnined from Liu and Piegott [ 7 ]

* * Data obtainrd from Zhang [ '"1 *** Data obtained from Hescel composites

Togetlier u ith ndditional data from Modern Plastic Enc)clopedi;i [ L I 1

2.2 Sample preparation

2.2.1 Fibre embedding equipment

Due to the high processing temperature of LDPE. PEEK and rposy. a specially

drsigned induction heater (Figure 2.1) was used. It consisrcd of three pans. uhich w r e

the p o w r unit s h o w on the right. the fibre embedding set-up in the center. and the

control unit on the left. The power unit provided about jOw at 30-1 ZOWz. A copper tube.

which formed a coi1 in the centre embedding set-up part. was used and water could run

through i t to cool down the system whcn the heat uas provided. The control unit kvas

designcd to control the heating rate. holding temperature. holding timc and the cooling

rate by programming. The center embedding set-up was where the resin \vas hratrd and

the single fibre was rmbedded in the resin.

Figure 2.1 The induction heater

The center embrdding set-up is shown in more dstailrd in Figure 2.2(3) . .-- thrrmocouple rising tiom the base. ~vas used to support the capsule that hdd the resin as

w l l as rnsasuring the temperature of the capsule and the resin. The glass cowr

sunounding the tht.rmoçoupie. was used to kecp a nitrogen atmosphtre around the

polymer to prevsnr an). undesirrd osidization of the rrsin under high temperature. I t \vas

bottle-shaped so that the big pan fittrd the base and the small pan tittcd in the çopper mil.

.A capsule made of stainlrss steel was used to hold the rrsin. sec Figure l.2( b 1.

The scrru. made the removal of the resin v e q ras.. by just screwing out the scrsw with

the resin rittached.

Figure I .?(a) The cmbedding sctup in the induction hratrr. (b ) Cross section of the stainless steel capsule

2.2.2 Fibre embedding

a. Samplrs of LDPE

.A capsule was put on the thermocouple. so that its tip was located just under the

screw of the capsule. Power was switched on. and the control unit was programmed. The

hrating rate \vas set rit I°C 1s. The holding tempenture was set at I?O°C for polyethylene.

The holding time u-as 100s. so that the rrsin was definitelu meited. And the cooling rate

was I°C . 'S. One polysthylene pellet was put into the capsule for each sample prrparation.

Afier the resin melted and held for 100s. under an Olyrnpus binocular microscope. a fibre

kvas carcfully placed on the surface of the resin. During cooling. it sank into the resin to

about one half of its diameter. hfter the capsule kvas cooled doun to the room

temperature. it Kas put into a special sample stand.

It was very difficult to control the embedded depth of the fibre in the resin. When

the resin cooled down. the volume of the resin changed. and the resin shrank. At first. the

fibre 'floated" on the resin. and then with the temperature going dow-n. it submerged in

the min . The E-giass fibre. trndrd to sink into the resin more quickly than carbon fibre.

with Kevlar fibre in brtween. So different Fibres a-err cmbcdded at different tempriratures.

( sre Table 2 .2) . Only one polyethylene pellet was ussd. becauss more resin caused more

shrinkage.

b. Sarnplrs of PEEK

The proçessinp temperature of PEEK was 400°C. and the procedure was almost

the same as that of LDPE. Samplçs of E-glass-PEEK. NGF glass-PEEK and carbon-

PEEK w r e préparcd. The ernbedding temperatures of these fibres are s h o w in Table 2.2.

I'hc epos). LI hiçh. as rcceivrtd. had an appropriate amount o t' amine curiny agent

in it. \vas kspt nt -20°C until required. Before use it u.as put into the Lah-Line vacuum

oven and kcpt rit 1 20°C and vacuum jOmmHg for 1 Ominutcs to remove an! cntrapped air.

U'hrn it i w s çooled down to the room temperature. it solidificd and was cut to small

pieces. The holding trrnperature for the epoxy was 700°C. the holding time was 360s.

and the embeddins temperatures of the fibres are listed in Table 2 .2 . After the capsule

u-as çooled d o w to lZO°C. it was removrd to the prrhrated own and krpt üt 1 10°C for

30 minutes so that the resin ws cured.

Table 2.2 The embedding temperature for ditferent tibrc 1

Resin LDPE PEEK

1 I I

; Fibre E- Kevlar Carbon

l 1

I l c rrlass ! 1 !

2.3 Sample selection

The samples were selected by an Olympus universal research microscope. 'rlodel

VANOX. Those samples. which had more than 80% diameter showing. were kcpt for

peel tests. Figure 2 -3.

hl icroscopr - Esposctd dirimeter of tibrr

Embedded Portion of fiber /

Figure 2.3 Samplr selection by microscope

2.4 Peel tests

The tests urre performed on the Cahn 1000 .-\utornatic Balance s h o w in Figure

2.4. .-\ Sçhrmatic plor ut' Peel test is shown in Figure 2.5. .A capsule [cas put into a sample

holdrr that was placed on the platform of the set-up. .A rectangular copper sheet uas

attachrd to the suspendrd hook from the balance. and the position was adjustrd so that its

lower edge was jmm away from the resin surface. One frec end of the Abrc was glued to

the copper shert with Krazy glue. The angle between the loading direction and the resin

surface was 90". After the glue was dried. the peel test was performed at the speed of

Immimin. The load range was 0-70mN. The peel loads were recorded on a chart recorder

at a chart specd of 3crn'min.

- m. ! Fibre

Figure 7.1 Peel test performed on the Cahn 1000 Automatic Balance

1:cpper sheet 1 1 Fibre -

Capsule ,,In ?

Figure 2.5 Schematic plot of peel test performed on the Cahn 1000 Automatic Balance

32

2.5 Effects of temperature

A specially designed cartridge heater (Figure 2.6) was used to examine thermal

effects on the peel test. It was made up of two parts. the cartridge heater and tcmpsrature

controller. The heater consisted two parts. an aluminum stand which was usrd as a

sarnple holder and an aluminum block in which two çartridge hratrr eiernents were

plupgcd in to provide the heat needrd. On top of the s~and. thcre uas a hole to tit in the

capsule. Under the capsule. there nas thermocouple insidr the stand. irhich uas

connected to the temperaturc controllrr that measured the temperaturc of tlis samplr: and

controlled it to a preset value. The block was fixed to the stand by a screw. Once its test

trmprrature was reachrd. the peel test kvas performed usine the procedure described in

section 2.4.

Figure 1.6 The cartridg heater

Sampltts of E-glass-rposy. Kevlar-eposy were used to test the thermal effrcts at

50. 100. 150. 180 and 300°C respecti~ely.

2.6 Microstructure observation

Panly pssled fibre-polymer samples werr mounted into the spcicially drsigned

holder for scanning shown in figure 2.7. The screw for capsule and the screw for bar uere

used to fis the capsule and the bar. respectively. And there is a screw inside the capsule

under the bar to adjust the hright of the bar. One free end of the fibre was glued to the bar

with Krazy glue. -4ftrr the glue was dried. the screw undrr the bar was adjusted to raise

the bar and tighten the fibre. so that the angle betwern the loading direction and the m i n

surface was 90".

H I T X H I scanning èlectron microscope (SEM ). bfodsl S-2500. was used to

record the microstnicture of -pical samples for e w r y fibre-resin systrm trsted.

Fibre

Res in

Capsule

Screw - *- Screw t'or for Bar Capsule

- Figure 1.7 Specially designed holder for Scanning

3. EXPERIMENTAL RESULTS

3.1 Peel Curves

3.1.1 Typical Peel Curves

The direct rcsults obtained frorn the peel test were the peel Cumes. which are the

force versus displacement cunes.

Typical c u n w for samples of E-glass-LDPE. Kevlar- LDPE. carbon-L. DP E are

s h o w in figure 3 . l ( n ) . ( b ) and ( c ) separatelu. Figure 3.3a). ( b ) and ( c ) show tvpicnl perl

cnnes obtained with samples of E-glass-PEEK. NGF glass-PEEK and carbon-PEEK.

Those of sarnples of E-glass-epoxy. Kevlar-epoxy and carbon-epoxy are shown in figure

3.3(a). ( b ) and (cl. In addition. those of samples of E-glass-epo'ry and Ksvlar-eposy at

50. 100. 150 and 200°C are shown in figure 3.4 - Figure 3.7. resprcii\.ely.

.Althouyh the forces for al1 fibre-matris systems tested are qiiite difkrent. t1it.r~. is

something in similar in the peel cuncs. For esample. in the curw of tigure 5.1 ( a ) frorn

point -4 to B. the force incrrased monotonously with the displacement up to a prak at

point B. followd b! n drop to point C. where fibre matris interface failed and debondiny

of tibre oççurred. .\fttrr that. the force recoverrd and tomrd the zigzag shapc of the

cune. represents succrssivr debonds of the fibre tiom the rnatrix. .At Inst. it got to point

D and decreased to zero load sharply as the fibre came out frorn the rnatrix. In the

followinp figures. same situation happened and more than 4 drbonds were obtained. The

force peaks w r e usuaily sharp. although somr were a littlc bit blunt. And in the final

part. sornetirne the force gradually decreased to zero as the fibre peel off \tas complcted.

ser tigure 3.l(b). In figure ;.4(b). the force increased lineariy to the peak and the

displacrment for the first peak was O.Smm. The displacements for the tirst prak are

usuall> betueen 0.5- l . h m . The rmbedded length of the fibre \vas estimated to be the

distance betwen the staning point and the end ~vhere force becaine zero again. For a

succrssîùl peel cun-r. the distance is usually brtwren 3 m m .

O 1 2 3

Dis placement, mm

O 1 2 3 4

Displacement, mm

Displacement, mm Displacement, mm

Figure 3.5 Typical perl cunes for Fibre - rpox! systems at 100°C (a) E-glas-epoxy (b ) Kcvlar-epoq-

Displacement, mm Displacement, mm

Figure 3.6 Typicûl ped curies for Fibre - èposy s>.strms cit 150°C (a) E-glass-eposy ( b ) Kevlar-cposy

Displacement, mm

(4 (b)

Fisure 3.7 Typical peel curves for Fibre - epoxy systems at 200°C (a) E-plass-epoxy (b) Kevlar-epoq

3.1.2 Exceptional Peel Curves

As descnbed in Section 1. afier selection by the microscope. samples. u-hich had

more than 80% diameter of the fibre showing. were kept for peel tests. Some samples of

elass-resin and carbon-resin systems. which had only about 80% diameter shorving (see w

figure 3.8(a)). gave the peel cunes s h o w as figure 3.9 and fibre broke in the peel test.

The force had one. two. or even thrre peaks. which represents the debondinys of the fibre

from the matris. And the iast prak was very high. followed by a sharp drop. When the

fibre broke. the load Ml to zero.

For somr sarnples. which had the sntire diarnctrr of the tibre shoa i n ( s r r figure

3.Qih)). the tïbre carne out tiom the resin with peel cun-e shown as ligure 3.10. Less

dsbonds uith low praks and a sharp drop w r s obsened. When the fibre came out from

the resin. the load f d l to zero.

For somr fibres. which could be seen not embedded at ri fixed drpth (sçe figure

3.1 h a ) ) . psel cunses as shown in figure X I ? @ ) were obtained. In the figure. two low

peaks can be sçen ai first ~ i i t h highcr peaks later. presumably due to the fibre bring

embedded much dreprr in the rrsin. In addition. for the case shou n in ligure 3. l l t b ).

pecl curies likr Bgure l l 2 ( b j were obtained. :\ wry high pecik apprnred at tirst and

followd by somr lowcr peaks prrisurnablp due to the fibre being rmbcddrd much Iess

deepl! in the m i n . These high peaks were not includcd when the peel strength was

çalculatsd.

For Kedar-resin systems. sometime fibre de fibrillation 11 as obsen ed. and it can

31~0 bc noted tiom the pcrl cunre. see figure 3-13. Here. the largrst \.durs rit points .A

markrd were recorded when the defibrillation of the fibre was observed. And they were

not includrd when perl strength was calcuiated.

1 r I Fibre

I

1- Fibre I

Figure 3.8 Sçhcmatic draaings of two types of the fibre embedding in the m i n 4 (i Fiber embsddrd too derp ( b ) Fibre rmbcddsd too shallon,

J- Dinmrtcr of the fibre w-Esposed width of fibre obsrn-ed bg the microscope

O 1 2 3 4

Displacement, mm

O 1

Dûplacement, mm

Figure 3.9 Peel Cumes for fiber broke (a) E-glass- epoxy (b) Carbon-LDPE

1

Fibre ;

Figure 3.1 1 Sçhematiç drauing of tibsr embçdding in the m i n with a slope ( a ) E-glas PEEK ( b ) E-glas-LDPE

Displacement, mm

(a

Figure 3.12

Displacement, mm

Peel curves for fiber embedded in the resin with a slope ( a ) E-glass PEEK (b) E-glass-LDPE

Displacement, mm Dis placement, mm

Figure 3.1 5 Peel cunqrs of tiber defibrillaiion ( a ) Krvlar LDPE ( b ) Krvlar-epos!

3.2 Peel Strengths of Fibre-Resin Systems

I t \vas rissurneci that rach peak in the peel cun-es represents a fresh debondinp.

thrreforr rin average of al1 the peaks escept the large one caused by iht: fibre M u r e

(break or detihrillation) from inchidual succrsstiil peel test. \vas counted as the peel

strcngth of the fibre-resin systern. The perl strengths of difirent Iibre-resin systcms

tested are s h o w in Table 3.1.

For carbon fibre. the peel strengths of carbon-LDPE. carbon-PEEK and carbon-

eposy are about the same. The perl strengths of Kevlar-LDPE and Kevlar-epoq- and art:

almost same. The peel strengths of E-&Lss-LDPE. E-glass-PEEM and E-glas-rpos) are

quitr difkrent. The peel strength of E-giass-PEEK was much loiver than the other two

systems with the same tibre. NGF glass fibre. which \vas impregnatrd with rubber. was

used to do the peel test. The diameter of NGF glass fibre is less than one half of that of E-

dass. the perl strength of the former is a little bit louer than the latter. C

Table 3.1 Peel strength of fibre-rrsin systrms tested

1

LDPE Kevlar I

PEEK

PEEK ' NGF g las

P E E i i Carbon S

Eposy

3.3 Thermal Effects

Fig 3.14 and Figure 3.15 show the thermal rffect of the peel tesi on E-glass-epoq

and Kwlar-eposy resprçtivrl y.

For both systems. thrre was no significant efkct on peel strenpth at differrnt

temperature. The mean value at 100°C of Kevlar-eposy systrm. is a littltr l o w r than

those at the other temperature. however. this doesn't affect the trend.

Temperature, OC

Figure 3.14 The peel strength of E-glass-rpoxy at differrnt temperature

Figure 3.15 The p w l strength of Kcvlar-cposy at diffcrent trmperaturr:

3.4 SEM Photos

In order to show both fibre and polymer. partly peeled samplrs were used. The!

wrre mounted in the fixture described in section 3.6 and vacuum coated with gold before

mounting in the SEM.

3.4.1 Çibre-LDPE Systems

Panly peeled fibres are shoun in Figure 3.16. The adhrsion in al1 cases appears to

have been remarkablj good. with s'oti wings' drawn out by the tibre aftrr peeling at the

cdnc of the hollow. shoning also that the polymer is still ductile. There is some resin

clinging io the edge of the fibre aker peeling. This is particularly noticeable in the case of

carbon as shown in figure 3.16(b).

The hollows l d t bg the pulled out fibres suggrst that the polymer has yielded and

tlowrd on a fine scak in the case of glass and Kevlar. see figure 3.1 7(a) and cc). With

çarbon. rhç hoilon has large pits indicating severe working in figure 3.17(b).

The peeling point of E-glass-LDPE looks similar to the end points of the carbon-

LDPE and Kevlar-LDPE sholvn in figure 3.18. the extension of the rrsin by the fibre çan

bs observsd. which also show the ductile propet-ty of LDPE.

In addition. the defibrillation and kinking of Krvlar Fibre can br çlearly obsen-cd

in figure 3.19.

3.4.2 Fibre-PEEK Systems

In the case of PEEK. the partly peeled tïbres show little èvidrncr of strong

adhesion. since there is no significant amount of polymer adhrring <O the fibres. see

figure 3 2 0 and 3.2 1.

The hollows. howcver. do suggest some difference. The g l a s and carbon fibres

nppear to have prelrd without damage to the hollow. showing in the figure 3.12(a) and

(c). While there is rvidcnce of some work in the case of the rubbrr coated glass. see

figure 3.22(b).

3.4.3 Fibreepoxy Systems

Panly perled fibres from rpoxy are s h o w in Figure 3 .23 . Three fibre-cpoq-

systems are almost the same. The adhesion looks very good. There are still resin 4ings'

attached to the fibre which appear to have brokrn away from the matrix without an)

significant extension of the resin. This resin appears to be much more brittle than LDPE.

The holiows left by the fibres atier peeling look similar too. (see figure 3.24). The

cdges of the hollou-s are badly darnaged. They show a tcnsile failurc a-ith failure

approximatcly normal to the tensile stress. This can be scrn in much detail in figure 3 . 3 .

D o w into the hollow. there is not so much darnage to the surface. In particular in the

case of carbon-riposy. the hollow is almost smooth.

In tigure 3 . 3 . the procrss of the fracture c m be sern çlearly: the fibre was

embedded just half of its diameter to the resin. and rhc: cracks can bc sern ro be

dewloping close to the fibre and about holf a diameter or more a w y from its edge.

The fibres aftrr peeling rue shown in figure 3.26. The embedded lowçr half

surfaces of carbon-eposy are more or less clean. There is a little bit resin clinging to the

l o w r half of E-glas-cposy and Ktvlar-rpoxy. But the resin. broken away from the body

rcsin. t oms wings on the peeled off fibre. Detibrillation of the K r h r fibre is not 50 clear

in figure 3.26(c ). i t çan be srrn in figure 3.27 in detail.

The de fibrillation and kinking of Kevlar fibre can be clearly obsen.ed from tigure

3.27. Besidc the kinking and defibrillation. there is a crack in the fibre s h o w in figure

3.17(a) and much detailed in figure 3.27(b). actually the crack in figure 3.27(b) is the

continuation of that in figure 3.27(a). The crack drveloped from the rniddlr of the tibre

and the opening of the crack in figure 3.27(b) is almost half of fibre diarnrtre.

4. DISCUSSION

4.1 Depth of fibre embedded

As mentioned in Section 2.3. aftsr selsction by a microscope. the samplss which

h d more than 80% diarneter showing. were kept for peel tests. In Section 3.1.2. peel

cunes for the samplcs. which had only about 80%0 diarneter showing. have been pro\.idçd

(tigire 3.9). So we will tirst csaminr how deep the fibre will have bcrn embttddrd in the

resin to have between 8096 and the entire diametrr showing.

Figure 4.1 Schrmatic drawing of ri fibre embedded in the resin

Figure 1.1 is a schrmatic drawing of an embedded fibre. Here. d is the diamrtrr of

the fibre. t is the depth frorn the fibre a i s to the surface of the resin and w is the width of

the fibre showing. w can be obtained through:

For d=30mm. for exampie. we have the axial drpths and corresponding widths

sliown in Agure 4.2. For the limiting case. olSOO/o diameter showing. w is 2-l.Omrn and t

is 9.0mm.

Figure 1.2 Examples of 3Omm diarneter fibres rrnbcdded at axial depths of 2.5. 5. 73mm (these are acceptable) and 10 and 1 Zjmm which are unacceptably deep

In table 4.1. we review the pictures of section 3.4. and how deeply the fibres

shown appear to bave been ernbedded. Figure 3.16(b). figure 3.1 7 (b) and (cl. figure

3.18(b). figure 3.25(a). (b) and (c). figure 3 . 3 ( a ) and (b) and tigure 3.26(a) and (b )

brlong to the case (>O. In figure ;.8(b). the fibre mas rrnbrdded to lrss than one haif of its

diameter. Thus fibres. such as shown in figure 3. 10(a) and (b). figure 3.2 1 (b ) and tigure

322(a) and (b). may have givrn results that uere slightly lotver than the average rttsults

for this type of samples.

Table 4.1 The axial drpths of the fibres rmbrdded in the resin shoum in the SE41

photos

(a) E-glass-LDPE

Fisure 3.16 1 i tzO

i (b) Carbon-LDPE , 1 (c) Kevlai-LDPE ,

l

Figure 3.17 t =O l t>O t>O 1

1

, Figure 3.18 l t 4 t>O t=O i

Systems (a ) E-glass-PEEK ( b ) KGF-slass-PEEK ( c ) Carbon-PEEK 1 I

Figure 3.20 t <O 1 t<O t=O

Figure 3.2 l * t<O t t O I

1 Figure 3.22 1 t <O 1 t<O I t =O 1

1 1 i

i i i 1

/ Systems i (a) E-glass-epoxy (b) NGF-glass-epoxy i (c) Carbon-epoxy ; Figure 3.23 , t>O

' Figure 3.25 t>O

; Figure 3 2 6 I

I P O

* HolIow not visible

Figurr 4.3 shows very rough profiles of hollows lrfi afier a carbon fibre has been

perled. ligure 5.24 (b). and glas fibres. figure 3.24 (a) and figure 3.26(a). Thesr appear

to have bern embedded to about the maximum arnount permissiblc. The adhesion appears

to have been very good in these cases. since the fibres plucked out a ereat deal of epoxy

from the mrniscus area. Ir seems probable that ~po , , in these cases. (Here r s ~ is the

tensile strength of the interface bond. O,. is the ultimatc strength of the matris.)

Figure 4.3 Xnists impression of hollow left when (a) carbon tibrc \vas peelcd from rposy and (b) glass tibrr \\as peeled tiom eposy

4.2 Types of peelings

Nine tibre-resin systems including three fibres. one. glass. having tneo different

surface coatings. and thrçe polymer resins haw been trsted in this research. Thcire are

several different types of peeling process during the peel test. They can be çlassified into

poor adhesion. brittle Failure. ductile faiiure and fibre damage. and will be discussed

separately. Table 4.7 revirws the works of fracture estimated from the peei load/ ?d

(Equation 1.5) and the SEM observations. The systems are Iisted in the order of

increasing toughness.

4.2.1 Poor adhesion

This kind of peeling was found in the case of E-glass-PEEK (Figure 4.4). The

toughness (70 ~m") was rather low cornpared with other E-glass-resin systerns ( 1-10 ~rn"

for E-glass- epoxy and 200 ~ r n " for E-glass- LDPE). There was no darnage to the hoilow

and the edges of the hollow after the fibre was perlrd off(f1gurr 4.-l(b)) and there was no

resin clinging to it (figure -l.l(c)). This means the adhrsion bctween the fibre and resin

\\.as rather weak. In this case. high ternpenturr (-100~)~) required for the mbedding

process m q have pyrolysed or weakrncd the silane coating on the g l a s tibre surface.

(a) E-glass fibre peeled from PEEK (b) hollow left by the fibre (c ) clcan fibre al-ier peeling

Figure 4.4 Poor adhesion of E-glass-PECK interface

4.2.2 Brittle failure

Brittle failure is usually associated with little plastic deformation during the

fracture process. It was very noticeable in fibre-epoxy systems. and the failure in NGF-

glas-PEEK and carbon-PEEK systems can also be counted as briale ( figure 4.5 and 4.6).

Frorn figure -l .j(c) and (d). brittle fractures such as river markinp. scarps. cracks

and ribbons c m be seen very clearly at the rdge of the hollow lefi by the fibre. The crack

fronts may initiatr and progress on slirhtly differrnt planes. the convergence of thrstt

produces a scarp or a ribbon or river markings. For example. the formation of a ribbon

can be schematically illustnred as figure 4.7 [37]. DOLW in the hollow. there was also

some damage caused by the peeling fibre. From figure .l.j(b). the crack propagates with

no plastic deformation at all. Again. the resin kings" still adhering to the fibre appeared

to have brokrn away from the body of the polymer without any siynificant extension of

ihs resin. These b a h prow that the adhesion was rathrr yod. and the failure \ras brirtlr.

On the othrr band. thrre was no significant resin adhrring in the case of NGF

ulass-PEEK. rvhile there was evidence of some sork d o w in the hollou lrft b'. the fibre. C

sec figure 4.6t b). Ribbons can be sern ver). clearly down in the hollow and also nrar to

the edge of the hollow.

( a > çarbon fibre peeled tiom cpoxy

. - - C

( b ) crack propagation

Scarp

Crack

(c) holiow lefi by the carbon fibre (d) hollow lefi b~ the E-glass fibre

Figure 4.5 Brittle failure of fibre-epoxy interface

(a)NGF glass tibrr peelsd from PEEK (b) hollow left by the fibre

Figure 4.6 Bnttle Mure of NGF glass-PEEK interface

+ Fibre -+

+-- Resin --,

Figure 4.7 Schrmatic formations of ribbons

4.2.3 Ductile failure

In this research. ductile failure is used to descnbc fractures where a large amount

of plastic deformation and drawing took place prior to failure. The failure in fibre-LDPE

systems belongs to this type. Figure 4.8 shows the amount of drawing which may occur.

These features are illusrrated schematically in figure 4.9. When the fibre was subjected to

the perl force. the fracture of the interface happened at the Iower side of the fibre first.

and the resin at the rdge of the hollow \vas extended by the fibre. This can br seen in

detail in figure 4.8(c). At the tip of the advancing crack. the front has a meniscus on both

sdgrs of the hollow: see figure 1.8(b). With the continuation of the peel force. thc

interface failed with resin drawn away from the bulk matrix. Down into the hollow

(figure 1.8(c)) therc is also plastic de formation with resin drawn by the fibre. Punch shear

failure in LDPE had a very similar appearance to this [XI: see figure 4.10.

( a ) E-glass fibre peeled from LDPE (b) crach propagation (c) holloic le t i b' the fibre

Figure 1.8 Ductile failure of E-glass-LDPE interface

Figure 4.9 Schematic drawing of ductile failure during the peeling process

72

Figure 4.10 SEM photos for LDPE in punch test

4.2.4 Fibre damage

In the case of Keviar. the works of fracture were rather high. panly due to the

défibrillation. tilting and kinking of the fibre (figure 4.1 I ). .A kink. a double turn in

opposits directions. ma!. be represrntrd by an N-band [3L)] as shown schematically in

t i r 4 . 1 Figure 4.12 summarizes the possible rnrchanism of fibre tilting and

kinking. When the fibre was pecled from the resin. there was a bending of the fibre first.

so that the fibre \vas siibjeçted to a compressive stress in the innrr pan of thé cun-e and a

tensilr: stress in the outer part. Failure originatrd by çornpressiv<: yirlding of the tibre

( tigurc 4.1 :(a)). followd by the formation of a yield N-band in the tibrc: ( figure l . l ? ( b)).

Funher shrar led to an incrensr in the angle of fibre tilting ( figure 4. I I( c I ). The nest stcp

was the growth of a fibre kink. Cracks c m be observed in the fibre kink. This is in accord

with the property of Kevlar fibre which has becin discussed in section 1.2.

( a ) kinking and defibrillation in Kevlar fibre

o f Kevlar-LDPE s>.stem

(b) tilting and defibrillation in Kevlar tÏbre

of Kevlar-eposy systern

Figure -1.1 I Fibre damaçe in Kwlar-resin systems

formation of Y-band - fibre tilting

1_11_+

fibre kinking

d

Figure 4.12 Schematic of fibre tilting and kinking mechanism [-VI

4.3 Peel analysis

Ln the test. the peeling tibre assumes a curve very similar to the elastica curve [-IO].

Khatibzadeh & Piggott [-LI ] gave a numencal analysis of this curve for the pull out test.

Apart from the still adhering region of the peeling fibre. the curve is the same as the

elastica cunee and can be illustrated as in figure 4.l3.Here we have indicated some

pol~mer displaccrncnt benrath the tibre in the process zone. (This process zone is

cquivalrnt to the process zone at the tip of a crack [-QI.) W r assume the process zone has

lcngth (;) dong the fibre.

i!) çan b s cstimated if ws knou. the strength of the fibre-matri'c bond. ~ d . whcn the

tibrr force P acts pcrpendiculorly to the polymer surface. If we assume o~ acts al1 round

the st.micircumfert.ncr. ;rt;t':! (d= tibre ciiametre). then:

2 P O,, = -

,dl,.

Figure 4.13 Elastica curve of the peeling fibre

thus. liJ can be obtained t'rom:

f hs I!, for each systsni is listsd in table 4.3. for o: =a,,.

I t crin be seen tiorn the table that ip c m bt. \.en. srnaII. It is probably iinrcalistic to

cspecr O: =O,,:, for the E-$riss and XGF ghss fibre smbeddsd in PEEL. In the case of

carbon-PEEK. hows~w. where the rrsult is the same within rxperimentnl srror as those

of cposy and LDPE matrices. it may be assumed that xihrsion is good: probably with the

bond strength grrater rlian the matrix strength. Such a strong bond was obsenrd in

trans\ms tests dn unidirecrional carbon-PEEK [43 Tlius h r carbon-PEEL Lie ma! be

jiistitird in assuminp 0, =a,,:,.

I , Kev lar-eposy 2

*q,.,,, values for the pol>.mers corne tiom Table 2.1

In all the other cases. the microscopie svidence stronyly supports the case for a,

=om,,. On this basis. the lp values in table 4.3 are realistic for al1 cases except the two

glasses in PEEK. However. they are not vcry accurate with carbon-eposy. For rxample.

the epoxy fractured over a much wider zone than given by the fibre diameter. With the

LDPE. the Qings" involve a slightly grrater width. Thus l p is probably overestimated. In

the case cf Kevlar. which is not elastic under these conditions. lr, estimate may

nevttnhclsss have somc validity.

4.4 Modes of failure

Fracture can occur in thrse modes. mode 9.. oprning rnodr. and modes 10- and 1 1.

a-hich are shear modes [J-l]. Figure 4.14 is a schttmatic drawing showing a small amount

of fibre being peeled i a length r=d?) with the rrst. brhind and at the left. stiii undisturbed.

The fibre fracture piws rise ro sliding dong the bottom of the slot. i.e.. mode 10.

fracture. At the samr tirne. the tibre is being liftrd off by the prcl force and giving mode

9.. Thesr movements are shown. somewhat rnagnitied for clarity. at point C in figure

4.l-h b ) and (c). At point E. on the other hand. we have pure shear. rnodr 11. only.

Halfwy betwen thesr points. point D. the movement parallei to the tibre risis. mode 10..

is one hallof what it is at C. bteanwhile. the lifting displacernent resolves into mode 9..

directrd towards the tibre mis. and mode 11.. which is tnngential.

Wc c m estimatr the nrnounts of these. roughly. We assume that the displacernent

t r in the x direction. is zero at point A. where .r=O. ,I,, is assumed to be zero for sc0.

and approximately constant at the rna~imum value. ,lm,,,. o w r the distance K. Since

&4 - = s, . we can estimate ~i from ?Y

which for constant E, gkes

The y displacement at C can be estimated since. over the short distance involved.

the arc and the chord are almost the same length: ses figure 414(a). Let this displacement

be 1.. then

but rjincr sin+$=r, R=qr,,. we have

This analysis can br improvrd by assuming the wholr procrss takes place o w

the proçrss zone. Icngth lp. and that zt~,, is O at x=O and c,?~,,,,, at.r=l!] with c;!., increasing

linearl! lvith x. So equation (-1.4) becomes:

1 I l = - 1 c . , , , .cm-~d~

I;,

which g i ~ e s

Using the approximate cquation from the elastica curve [?'Il

Substituting qm tiom equation (4.12) and l p from equation (4.3) with cl =a,,, 2 into

equation (4.9) and (4. IO). we have

(WC ilse O,,,,, 2 for since the tibrç displacement and hrncc the restraining forcc in the

polymer is assumrid to increase linearly from .-\ to B.)

Table 4.4 Displacements at the end of proccss zone (pm)

1 PEEK 1 0.063 1 1

Table 4.4. which lists the estimated displacemrnts. indicatrs that for eposy. the

displacements for carbon and glass are the same within esperimcntal rrror. (This was

k50°io). This is hardly tnir with the LDPE. We do not rstimate the displacements for the

poor adhesion cases. since the value of ad is unhown. For Kevlar also. the displacement

calculate ma. not be very meaningful because this fibre has a Ion. modulus in

compression and it kinks. so the smooth rlastica cun-e would not be obtained. and

moreover the value of Ef is uncertain.

Epoxy

LDPE

0.053

0.63

0.068

1.5

Finally. WC should note that the kings" in the good üdhssion cases contirni the

rrsistancr to failure in pure shear with ceramic-polymer interface. In pull out tests. mean

shear stresses of up to ten times the trnsile strength of the polymer wrre required btfore

shear hilure could be induced when giass fibres wrre pullcd out of an rpoxy resin [45].

Clearly. resistance to shear failure is strong here. too.

5. CONCLUSIONS

1 . The psrl tests shon- that fibre-polymrr adhesion can be very strong: probably s t rongr

than the polymsr itself.

1. S h c x failiirr of the bond is strongly rrsisted as witnrssed by the bings " of polymer

still adhrring to the peelsd tibrrs.

3. The pcol tailurr procrss zone c m br vrry small. i.r.. lrss than 20% of the fibre

diameter. Howevsr. with \ e n ductile pol+hylene. it can be up to t~vo fibre dirimeters.

4. The perl failure is ven complrx involving tensilr (mode 9. ) and shear ( modes Il and

I I I i in di f i rent amounts. at dit'ferent locations around the fibre circumstance.

5 . Openhg modc displacements of the polymer at the bottom of the hollow arc prohabl!.

\Cr! irnrill in the case of the eposy. .-Ilthough apparsntl! niuch higher wirh the

pol!,eth!,lene. the formation of ductile wings show that lar-e extensions. up to a tibrr

Jicimctcr or mort.. c m bc sern in the rneniscus region.

1. Further work using Laser Raman technique <O check the elastica c u n r in process zone.

is nesdsd.

2 . More detailed analysis should bs attempted. not requiring the assumption of a linriar

change in q,,.

7. REFERENCE:

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[2] P.K. blallick. Fibçr-Reinforced Compositrs: Materials. Manufacturing. and Design. Marcel Dekker. Inc.. i 1 993 ). Chapter 1

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[hl R.B. Seymour. Polvmeric Composites. VSP. ( 1990). Chapter 7

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[ 1 1 ] K.C. Krishan. Composite blatrrials Science and Encinesrinp. Springer. ( 1998 1. Chapter 3

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[Yi ] T. Yorita. J. Matsui and H. S. Matsuda. Effects of surface treatment of carobon tibre on mechanical pro~crties CFRP. Proc. [C'CI-I, composiic. i11terjiicr.s (H. lshida and J.L. Koenig. cds. ). Elsevier. ( 1986) 123- 133

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8. LIST OF NOMENCLATURE critical length of fibre (mm)

diameter of fibre (mm)

tibre displacement at .Y and direction respectively ( m m )

fibre trnsile strenyth ( blpa)

tlesure fibre strain at the surtàce

maximum tlexure Libre main at the surtàce

interfacial bond strength ( MPa)

interfacial tiact~ire cnrrgy (k~rn")

interfacial frictional shear strcngth ( MPa 1

intcrhcial sherir strength (MPa)

lenyth of the process zone ( mm)

mean ti hrc: fragment kngtli ( mm )

peel anplr. ( " 1

peel load (5 )

radius of tibre (mm)

radius of peel cunrtïbre (mm)

axial drpth of the fibre embedded in the resin (mm)

strain in the peel a m

strain enerpy density

tcnsils strength of fibre-matris interface bond ( MPa)

transition temperature ( O C )

ultirnate tensile strength of the matris (MPa)

width of the fibre showing under the microscope (mm)

work of fracture for opening mode(Nm or .JmW'))

Young's Modulus of fibre (GPa)

Table 9.1 Thermal cffect on peel strength of E-glass-epoxy and Kevlar-cpoq

* not tested