Parte della lezione è stata tratta dalla lezione svolta · Schema della lezione - Introduzione - Effetto dell’interfaccia sulle proprietà meccaniche dei compositi - Micromeccanica

Interfacce e adesione

5a Scuola AIMAT: I Materiali Compositi

Ischia Porto (NA) 15-19 Aprile 2002

Parte della lezione è stata tratta dalla lezione svolta dal prof. Alessandro Pegoretti alla

Schema della lezione

- Introduzione

- Effetto dell’interfaccia sulle proprietà meccaniche dei compositi

- Micromeccanica all’interfaccia: meccanismi di trasferimento degli sforzi

- Misura dell’adesione fibra-matrice: metodi diretti ed indiretti

- Meccanica della frattura all’interfaccia fibra-matrice

- Come migliorare l’adesione fibra-matrice? Cenni sui trattamenti superficiali

The fibre-matrix interface • The interface between fibre and matrix is crucial to

the performance of the composite - in particular fracture toughness; corrosion; moisture resistance.

• Weak interfaces provide a good energy absorption mechanism - composites have low strength and stiffness, but high fracture toughness.

• Strong interface results in a strong and stiff, but brittle composite.

The fibre-matrix interface Adhesion between fibre and matrix is due to one (or more) of 5 main mechanisms:

1. Adsorption and wetting - depending on the surface energies or surface tensions of the two surfaces. Glass and carbon are readily wetted by epoxy and polyester resins, which have lower surface energies.

The fibre-matrix interface - adhesion mechanisms

2. Interdiffusion (autohesion) - diffusion and entanglement of molecules:


3. Electrostatic attraction - important in the application of coupling agents. Glass fibre surface may be ionic due to oxide composition:


4. Chemical bonding - between chemical group in the matrix and a compatible chemical on the fibre surface:


5. Mechanical adhesion - depending on degree of roughness of fibre surface.

Larger surface area may also increase strength of chemical bond.

An interface (2D) or interphase (3D)

is the region of significantly changed chemical composition that constitutes the bond between the matrix

and reinforcement (Metcalfe - 1974)

bulk matrix

bulk fiber

modified matrix

interphase

surface layer

adsorbed material

(after Drzal et al. 1983)

Perche’ interfaccia o interfase?

‘ differential thermal contraction (residual stresses)

‘ different cooling conditions (e.g. carbon fibre is thermally conductive)

‘ matrix contracts during cure (thermosets) ‘ different Poisson´s ratio of fibre and matrix ‘ fibre surface influences cross-link density ‘ crystals can nucleate at fibre contact sizing and binder can be present ‘ water in-take along fibre interface

Why are interfaces in composites important? Surface area !

Fiber filled

df = 10 µm Φf = 0.6

Vc = 1 m3 Vf = 0.6 m3

vf =π4

df2 = 7.854x10−11 m3

fiber

Nf = Vfv f

= 7.639x109

Af = af Nf = π d f (1) Nf = 240.000 m 2

m3

Particulate filled

dp = 5 µm Φp = 0.4

Vc = 1 m3 Vp = 0.4 m3

vp =π6 dp

3 =6.545x10−17 m3

part.

Np = Vpv p

= 6.112x1015

Ap = ap Np = π d p2 N p = 480.000 m 2

m3

Techniques for studying surface structures and composition

- Scanning electron microscopy (SEM)

- Transmission electron microscopy (TEM)

- Scanning tunneling microscopy (STM)

- Atomic force microscopy (AFM)

Microscopy

Spectroscopy

- Auger electron spectroscopy (AES)

- X-ray photoelectron spectroscopy (XPS)

- Secondary ion mass spectroscopy (SIMS)

- Ion scattering spectroscopy (ISS)

- Fourier transformed infrared (FTIR) spectroscopy

- Raman spectroscopy (RS)

- Nuclear magnetic resonance (NMR) spectroscopy

S.Incardona, C.Migliaresi, H.D.Wagner, A.H.Gilbert, G.Marom, Comp.Sci.&Techn. 47, (1993), 43

Photomicrographs of isothermal crystallization of J-Polymer® (DuPont) on a HM pitch based carbon fiber.

30µm

N.Klein, G.Marom, A.Pegoretti, and C.Migliaresi , Composites, 26(10) (1995) 707

Thermo-mechanical properties of transcrystalline layers in PA66 - Kevlar composites by dynamic mechanical thermal analysis (DMTA)

0

1000

2000

3000

4000

5000

6000

-150 -100 -50 0 50 100 150 200

stora

ge m

odul

us (M

Pa)

temperature (°C)

quenched matrix

crystallized matrix

tc matrix

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

-150 -100 -50 0 50 100 150 200

tanδ

temperature (°C)

quenched matrix crystallizedmatrix

tc matrix

Influence of fiber-matrix adhesion on mechanical properties: case of graphite/epoxy composites

M.S.Madhukar, L.T.Drzal, Fiber-matrix adhesion and its effect on composite mechanical properties:

I. Inplane and interlaminar shear behaviour of graphite/epoxy composites, J. Comp. Mater., 25 (1991) 932

II. Longitudinal (0°) and transverse (90°) tensile and flexure behaviour of graphite/epoxy composites, Comp. Mater., 25 (1991) 958

III. Longitudinal (0°) compressive properties of graphite/epoxy composites, J. Comp. Mater., 26 (1992) 310

IV. Mode I and mode II fracture toughness of graphite/epoxy composites, J. Comp. Mater., 26 (1992) 936

Material Tensile Modulus(GPa)

Tensile Strength(MPa)

Interfacial ShearStrength – ISS (MPa)[Fragmentation test]

Interfacial FailureMechanism

AU-4(untreated)

234 3585 37.2 Friction

Fibers(Hercules)

AS-4(surface treated)

234 3585 68.3 Interfacial

AS-4C(epoxy coated AS-4)

234 3585 81.4 Matrix

Matrix Epon 828(DGEBA + mPDA)

3.6 89.6 -- --

Effect of fiber-matrix adhesion on the longitudinal tensile behavior in graphite/epoxy composites fiber

matrix

σL σL

M.S.Madhukar, L.T.Drzal, J. Comp. Mater., 25 (1991) 958

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0 1 2 3

Longitudinal Tensile Modulus

Longitudinal Tensile Strength

NO

RMA

LIZE

D L

ON

GIT

UD

INA

L D

ATA

NORMALIZED INTERFACIAL SHEAR STRENGTH


fibermatrix

τ

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3

Data 1

Inplane Shear Modulus (45° Tension Test)Inplane Shear Modulus (Iosipescu Shear Test)Inplane Shear Strength (45° Tension Test)Inplane Shear Strength (Iosipescu Shear Test)Interlaminar Shear Strength (Short-Beam Shear Test)

NO

RMA

LIZE

D S

HEA

R D

ATA


Effect of fiber-matrix adhesion on the inplane and interlaminar shear behavior in graphite/epoxy composites

fibermatrix

σT

σT

Effect of fiber-matrix adhesion on the transverse tensile and flexural behavior in graphite/epoxy composites


0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 1 2 3

Transverse Tensile ModulusTransverse Flexural ModulusTransverse Tensile StrengthTransverse Flexural Strength

NO

RMA

LIZE

D T

RAN

SVER

SE D

ATA


starter crack

ENF specimen

Effect of fiber-matrix adhesion on Mode II fracture toughness in graphite/epoxy composites


0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3

NO

RMA

LIZE

D G

IIC


End Notched Flexure(Mode II Fracture Toughness)

‘Short’ fibres don’t reinforce as effectively as ‘long’ or continuous fibres

• The load transfer mechanism results in end effects which may reduce the fibre stress.

• Difficult to control the alignment of short fibres.

• Randomly-oriented short fibres cannot be packed at such high volume fractions as continuous fibres.

Short fibres are more often used with thermoplastic resins. Processes like injection moulding lead to considerable fibre damage and reduction in length:

Load transfer between matrix and fibre

Under applied tension, load is transferred by shear at the matrix/fibre interface.

At fibre ends, the strain in the matrix is higher than in the fibre:

(Matthews & Rawlings, ‘Composite Materials’, Woodhead)

Micromechanics of stress transfer across the interface

fibermatrix

unloaded case

loadload

loaded case

complex stress field often described by rough models, like

shear-lag model (Cox, 1952)

simplified physical model (Kelly-Tyson, 1965)

Stress variation in a short fibre


Stress variation in a short fibre - experimental evidence


When a stiff fibre is embedded in a relatively flexible matrix, shear stress and strain are a maximum at the fibre ends.

The tensile stress in the fibre, on the other hand, is zero at the fibre end and increases towards the centre. Note low stress at fibre end, increasing to maximum value about 7 radii towards the centre.

• The tensile stress in the fibre thus increases from zero at the ends to a maximum value of σf

max = Ef ε, where e is the strain applied to the composite.

• A fibre is said to be of the critical length if it is just long enough for the tensile stress to reach its maximum value.

L > Lc

L < Lc L = Lc

Lc / 2

fibre

tens

ile s

tress

fib

re te

nsile

stre

ss

σfmax = Ef ε

σfmax = Ef ε

Simple model for critical length Consider a half fibre. The maximum tensile force in the fibre, diameter D, is balanced by the shear force at the fibre/matrix interface, so:

τσ

λ

τσ

τππσ

2

2

24

max

max

2max

fcc

fc

cf

DL

DL

DLD

==

=∴

=

Lc / 2

σfmax

τ centre line

‘λ’ is fibre aspect ratio

Critical fibre length Critical fibre length thus depends on τ, the interfacial or matrix shear strength, and varies according to both fibre and matrix:

matrix fibre Lc (mm) Lc / DAg alumina 0.4 190Cu tungsten 38 20Al boron 1.8 20epoxy boron 3.5 35epoxy carbon 0.2 35polycarbonate carbon 0.7 105polyester glass 0.5 40polypropylene glass 1.8 140alumina SiC 0.005 10

Fibre end effects • Because of the low stress at fibre ends, the average

stress in the fibre will be lower than that in a continuous fibre, even if it is longer than the critical length.

• For fibre length L = 5 Lc, the average fibre stress (σfav)

is about 90% of the maximum stress (σfmax) .

• It can be shown that (for L > Lc):

−=LLc

favf 2

1maxσσ

Stiffness of short fibre composites

For aligned short fibre composites (difficult to achieve in polymers!), the rule of mixtures for modulus in the fibre direction is:

€

E =ηLE fVf + Em(1−Vf )

The length correction factor (ηL) can be derived theoretically. Provided L > 1 mm, ηL > 0.9

For composites in which fibres are not perfectly aligned the full rule of mixtures expression is used, incorporating both ηL and ηo.

Theoretical length correction factor

( )( )2/L

2/Ltanh1L β

β−=η

( )DR2lnDEG8

2f

m=β

00.10.2

0.30.40.50.60.7

0.80.91

0 0.5 1 1.5 2

fibre length (mm)

leng

th c

orre

ctio

n fa

ctor

Theoretical length correction factor for glass fibre/epoxy, assuming inter-fibre separation of 20 D.

Strength of short fibre composites The micromechanics of strength are more complicated than for stiffness. Strength depends on the relative failure strains of fibre and matrix (amongst other things).

Essentially, there is little difference between short and continuous fibre composite strengths once L ≥ 10 Lc.

H.L.Cox “The elasticity and strength of paper and other fibrous materials” Br.J.Appl.Phys. 3, (1952) 72

hp: - linear elastic behavior for matrix and fiber; - perfect adhesion (no debonding).

σf (x) = Ef ε c 1 − cosh [β (L/2−x)]cosh (β L/2)

τf (x) = Ef εc β

rf2

sinh [β (L/2−x)]

cosh (β L/2)

0

200

400

600

800

1000

0 200 400 600 800 1000

σf (M

Pa)

x (µm)

-40

-20

0

20

40

0 200 400 600 800 1000

τ f (MPa

)

x (µm)

A.Kelly, and W.R. Tyson, “Tensile properties of fiber-reinforced metals copper/tungsten and copper/molybdenum” J. Mech. Phys. Solids 13, (1965) 329

hp: - linear elastic behavior for fiber; - elasto-plastic behavior for matrix; - debonding may occur.

L < L t L = L t L > L t

t t

fiber/matrix stress stress,

L < L t L = L L > L

axial fiber stress, σ f

( σ f )* max = E f /E c σ c

τ y τ

(σf )max =τy L

r

Stress transfer at fiber-matrix interface: Kelly-Tyson model

Consideriamo una fibra corta immersa in un cilindro di matrice. Ipotizziamo che:

• La fibra abbia un comportamento perfettamente elastico fino a rottura

• La matrice abbia comportamento elasto-plastico con carico di snervamento uguale a sy

Da un bilancio di forze, si ricava che lo sforzo che agisce sulla fibra all’ascissa x è:

2

0

2)( 2)( rrdxxr

x

mx πσπτπσ ∫ +=

Forza tramessa sulla superficie laterale della

fibra dalla matrice

Forza trasmessa sulla superficie

normale

1 2

o anche:

σf

τ

τ

σf + dσf

( ) ( ) ( ) 044

22

=πτ−π

σ−π

σ+σ=∑ dxddddF fffx

L’ e q u a z i o n e p u o ’ e s s e r e risolta se e’ nota la funzione t(x). Nel caso in esame, a v e n d o l a m a t r i c e c o m p o r t a m e n t o e l a s t o -plastico, t è costante ed u g u a l e a l c a r i c o d i snervamento a taglio ty

ty rappresenta la resistenza al t a g l i o d e l l a m a t r i c e a l l ’ i n t e r f a c c i a . E s s o è approssimativamente uguale a ½ smy

xr yx τσ2

)( =

Lo sforzo applicato alla fibra aumenta linearmente con x

• Per fibre corte, per motivi di simmetria, lo sforzo raggiunge il suo massimo valore, sfmax al centro della fibra, quando cioè x è uguale ad L/2.

rLy

f

τσ =max

L

σf τy

Ma lo sforzo massimo che può accumularsi sulla fibra ha un limite, quello massimo che la fibra vedrebbe se fosse continua. In questo

caso infatti fibra e matrice si deformano delle stesse quantita’

mfc εεε ==f

f

c

c

EEσσ

=

cc

ffcont E

Eσσ =

σf

• Mentre per fibre continue la condizione di sforzo massimo e’ presente su tutta la fibra, per fibre corte essa si verifica soltanto se la fibra è più lunga di una quantità che definiamo “lunghezza di trasferimento di carico (load transfer length), Lt, definibile da:

y

fcontt

dL

τ

σ

2=

L<Lt L=Lt L>Lt

σfcont corrisponde pertanto al piu’ alto dei possibili

valori di σfmax

σ

σfmax

Ma Lt dipende dal carico applicato al composito. Per ogni

valore di sforzo applicato sc esiste un valore corrispondente

di Lt

cc

ff E

Eσσ =max

y

ft

dL

τ

σ

2max=

Lt non e’ un valore unico, ma dipende dal carico applicato al composito: Per ogni valore di sc, esiste un valore di Lt

Ma: aumentando σc, aumenta il valore di Lt, e pertanto il valore di σfmax.Il massimo valore che Lt puo’ assumere è quello che rende il carico applicato sulla fibra uguale al suo carico di rottura, σfb. Quando aumento il carico applicato al composito, aumenta l’σfmax applicato alla fibra, finchè non viene raggiunta la sua resistenza σfb. In queste condizioni la lunghezza della fibra viene definita Lunghezza critica di fibra, Lc. Essa è uguale a:

σc

Lt Lt Lc

y

fbc

dL

τ

σ

2=

• All’aumentare del carico applicato al composito, aumenta il carico trasmesso alla fibra. Il limite del carico che si puo’ raggiungere sulla fibra e’ pero’ rappresentato dalla resistenza della fibra stessa, σfb.

• La lunghezza di fibra che permette il trasferimento da parte della matrice alla fibra di un carico pari alla sua resistenza viene definita Lunghezza critica di fibra, Lc Se la fibra e’ piu’

lunga della lunghezza critica,

raggiunto un determinato carico

sul composito essa si rompe. Se e’ piu’

corta viene espulsa (cede l’interfaccia).

Stress transfer at fiber-matrix interface: Kelly-Tyson model

The “transfer length” Lt is given by: Lt =r Efτy Ec

σc

Lc/2 Lc/2

σc

(σf )max=σfb

and hence: τy = ISS = rLc

σfb

The “critical length” Lc is given by:

Lc =rτy

σfb

Quantitative measurement of fiber-matrix interfacial adhesion: state of the art

HISTORICALLY, TWO GENERAL METHODOLOGIES, BASED ON:

INDIRECT TESTING of collective behaviour of fibers in a matrix (real composites)

- Interface strength interpreted via simplistic model

- Fast but questionable results are obtained

DIRECT TESTING probes interfacial behavior of individual fibers in a matrix (microcomposites)

- More fundamental and accurate information

- Variability within and between techniques

- Issue of relevance to macrocomposites

INDIRECT TEST METHODS - (I)

[±45°] tensile test ASTM D 3518

τ 12 =σ x

2

0

[10°] off-axis tensile test

INDIRECT TEST METHODS - (II) Rail shear test ASTM D 4255

Two rails - tension Three rails - compression

INDIRECT TEST METHODS - (III)

In-plane lap-shear test ASTM D 3518

Transverse tensile test ASTM D 3039

INDIRECT TEST METHODS - (IV)

Short beam interlaminar shear test ASTM D 2344

σxMAX=

3 F L2 B h2

τxyMAX=

3 F4 B h

τxyMAX

σxMAX

=h

2 L

Experimental problems with the short beam interlaminar shear test

Iosipescu shear test ASTM D 5379 INDIRECT TEST METHODS - (V)

Matrix cracking in a 90° specimen

Matrix cracking in a 0° specimen

INDIRECT TEST METHODS - (VI): delamination tests

Modes of interlaminar crack propagation

a) Mode I opening mode b) Mode II sliding shear mode c) Mode III tearing mode

a) b) c)

Mode I Interlaminar fracture toughness Double Cantilever Beam ASTM D 5528

G Ic =P 2

2BdCda

C =δP

C =2 a 3

3 E1 I

where

for the classical beam theory:

INDIRECT TEST METHODS - (VII): delamination tests (cont.)

Mode II Interlaminar fracture toughness

End Notched Flexure specimen

G IIc =9 a 2 P δ

2 B (2 L3 + 3 a 3 )

End Loaded Split specimen

G IIc =9 a 2 P δ

2 B (L3 + 3 a 3 )

DIRECT TEST METHODS - (I) Fiber microdebonding test

ISS = Fp

π d L

Fiber pull-out test

ISS = Fp

π d L

SEM micrograph of PCL droplet on Kevlar 149 fiber.

fiber microdebonding

a)

b)

SEM micrographs of PCL droplet before (a) and after (b) debonding.

A.Gati, M. Sc.Thesis, The Weizmann Institute of Science, Israel (1996)

DIRECT TEST METHODS - (III)

Multi-fiber pull-out test

Y.Qiu and P.Schwartz, Comp.Sci.&Techn. 48 (1993) 5

Microindentation test

t < 3 ÷ 4 d

ISS =Fpπ d t

DIRECT TEST METHODS - (IV): Fiber fragmentation test

load

load

matrix

fiber

ISS =d σ fb(Lc )

2 Lc

σ fb (L) = α LL0

−1/βΓ 1+ 1

β

the “saturation length” Ls is related to the critical length Lc: Lc = 4/3 Ls

average fiber strength, σ fb(L), depends on the fiber length

(generally this dependence follows the Weibull statistics),i.e. :

Fiber fragmentation: test apparatus

Fiber fragmentation observed under polarized light

Glass fibers in PA6 matrix

Carbon fiber in epoxy matrix

Fiber fragmentation in carbon/epoxy composites: effect of temperature

For a “soft” epoxy matrix (Tg = 40°C)

0

10

20

30

40

0 10 20 30 40 50 60

ISS sized fibersISS desized fibersmatrix shear strength

shea

r stre

ss (M

Pa)

temperature (°C)

strain rate = 0.008min -1

M.Detassis, A.Pegoretti, and C.Migliaresi, Comp. Sci. & Techn., 53 (1995) 39.

Fiber fragmentation in carbon/epoxy composites: effect of temperature

For a stiff epoxy matrix (Tg = 150°C)

0

10

20

30

40

50

0 50 100 150 200


shea

r stre

ss (M

Pa)

temperature (°C)

strain rate = 0.008min -1

A.Pegoretti, C.DellaVolpe, M.Detassis, C.Migliaresi, and H.D.Wagner CompositesPartA, 27 (1996) 1067.

Fiber fragmentation in carbon/epoxy composites: effect of strain rate

For a “soft” epoxy matrix (Tg = 40°C)

0

10

20

30

40

50

0 0.005 0.01 0.015


shea

r stre

ss (M

Pa)

strain rate (min -1)

temperature = 20 °C

M.Detassis, A.Pegoretti, and C.Migliaresi, Comp. Sci. & Techn., 53 (1995) 39.

Fiber fragmentation in thermoplastic matrix composites: nylon6/glass fibers

Impossibile visualizzare l'immagine. La memoria del computer potrebbe essere insufficiente per aprire l'immagine oppure l'immagine potrebbe essere danneggiata. Riavviare il computer e aprire di nuovo il file. Se viene visualizzata di nuovo la x rossa, potrebbe essere necessario eliminare l'immagine e inserirla di nuovo.



aluminum plate PTFE sheet

nylon-6 film

E-glass fibers


Impossibile visualizzare l'immagine. La memoria del computer potrebbe essere insufficiente per aprire l'immagine oppure l'immagine potrebbe essere danneggiata. Riavviare il computer e aprire di nuovo il file. Se viene visualizzata di nuovo la x rossa, potrebbe essere necessario eliminare l'immagine e inserirla di nuovo.Impossibile visualizzare l'immagine. La memoria del computer potrebbe essere insufficiente per aprire l'immagine oppure l'immagine potrebbe essere danneggiata. Riavviare il computer e aprire di nuovo il file. Se viene visualizzata di nuovo la x rossa, potrebbe essere necessario eliminare l'immagine e inserirla di nuovo.

fiber 45 mm

4 mm 90 µ m

Temperature = 300 °C Pressure = 10 kPa (under vacuum) Time = 50 min

load load

Fiber fragmentation in thermoplastic matrix composites: nylon6/glass fibers: effect of temperature

A.Pegoretti, L.Fambri and C.Migliaresi, Polymer Composites, 21 (2000) 466.

0

5

10

15

20

25

30

20 40 60 80 100 120 140 160 180

ISS unsizedISS polyamide sizedISS epoxy sizedmatrix shear strength

shea

r stre

ss (M

Pa)

temperature (°C)

strain rate = 0.008min-1

Fiber fragmentation in thermoplastic matrix composites: nylon6/glass fibers: effect of strain rate

A.Pegoretti, L.Fambri and C.Migliaresi, Polymer Composites, 21 (2000) 466.

0

5

10

15

20

25

30

35

40

10-3 10-2 10-1 100 101

ISS unsizedISS polyamide sizedISS epoxy sizedmatrix shear strength

shea

r stre

ss (M

Pa)

strain rate (min-1)

temperature = 25 °C

What is happening when a fiber breaks in a polymer matrix ?

When a fiber filament breaks, cracks will propagate from the broken fiber end either by: - interfacial debonding; - trasverse matrix cracks; - conical matrix cracks, or combinations of the three modes.

Example: fracture patterns at a broken fiber end depend on fiber/matrix adhesion. glass/epoxy system: a) interfacial debonding); b) radial matrix crack; c) conical matrix crack; d) mixed matrix crack.

A.Pegoretti, M.L.Accorsi and A.T.DiBenedetto, Journal of Materials Science, 31 (1996) 6145.

a) b)

c) d)

Recent trends in fiber/matrix load transfer: a fracture mechanics approach for fiber/matrix debonding.

E-glass fiber in nylon-6 matrix: debonding when fiber fails

load load

10 mm

fiber

debonding

T=25°C

matrix

load load

10 µm

T=100°C

fiber

matrix

debonding

Finite elements modeling of the fiber/matrix debonding

fiber

matrix

fiber fracture point

fiber/matrix debonding zone

Ld

fiber

matrix


FEM mesh

A BC

DE

F

z

r

0

5

10

15

20

0 2 4 6 8 10 12 14 16 18 20

strai

n en

ergy

rele

ase

rate

(J/m

2 )

debonding length, Ld (µm)

matrix = epoxy (E=2.9 GPa)fiber = S-glass (E=86.9 GPa)strain = 1%

A.Pegoretti, M.L.Accorsi and A.T.DiBenedetto, Journal of Materials Science, 31 (1996) 6145.

Strain energy release rate for the fiber/matrix debonding - I

fiber

matrix


fiber/matrix debonding

Ld

Strain energy release rate for the fiber/matrix debonding - II

0

100

200

300

400

500

0 1 2 3 4 5 6

G (J

/m2 )

strain (%)

elastic

elastic-plastic

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8

stre

ss (M

Pa)

strain (%)

epoxy matrix

A.Pegoretti and A.T.DiBenedetto, Composites Part A, 29(9-10) (1998) 1063.

Strain energy release rate for the fiber/matrix debonding - III

A.Pegoretti, M.Fidanza, C.Migliaresi and A.T.DiBenedetto, Composites Part A, 29 (1998) 283.

0

50

100

150

200

250

300

350

unsized polyamide sized epoxy sized

T = 20°CT = 100 °C

Gar

rest (J

/m2 )

fiber surface treatment

Example: E-glass fibers in nylon-6 matrix

Can microcomposites (low volume fraction) be considered as representative for interfaces in macrocomposites (high volume

fraction) ? Problem n° 1: thermal

stresses H.D.Wagner, J.Adhesion, 52 (1995) 131.

Thermal stresses may induce fiber buckling in high modulus fiber embedded in termosetting matrices

Carbon fiber in epoxy matrix

Thermal stresses may induce fiber fracture for high modulus fiber embedded in termoplastic matrices

Carbon fiber in J-polymer

S.Incardona, C.Migliaresi, H.D.Wagner, A.H.Gilbert, G.Marom, Comp.Sci.&Techn. 47, (1993), 43

M.Detassis, A.Pegoretti, C.Migliaresi, H.D.Wagner, J.Mater.Sci, 31 (1996) 2385.

Experimental evaluation of thermal stresses

How can fiber-matrix adhesion be improved ?

Fracture surfaces of epoxy composites after 72 hr in boiling water

Matrix modifications

Surface treatments on fibers

Fibers surface treatments - (I): glass fibers

Typical component of a glass fiber size

• Film-forming resin ... 1-5 %wt

• Antistatic agent……. 0.1 - 0.2 %wt

• Lubricant ………….. 0.1 - 0.2 %wt

• Coupling agent……...0.1 - 0.5 %wt

- SILANE - TITANATE - ZIRCONATE

Fibers surface treatments - (II): silane coupling agents

R-SiX3

R is a group which can react with the resin

X is a group which can hydrolyze to form a silanol group in aqueous solution

a) hydrogen bonding between hydroxyl groups of silanol and glass surface; b) polysiloxane bonded to glass surface; c) organofunctional R-group reacted with polymer

+ H2O → R-Si(OH)3 + 3 HX

Fibers surface treatments - (III):

commercial coupling agents

Fibers surface treatments - (IV): effect of silane coupling agents on the mechanical properties of glass fiber

composites

E.P.Plueddemann, H.A.Clark, L.F.Nelson, and K.R.Hofman Mod.Plast, 39 (1962) 136.

0

100

200

300

400

500

600

700

None Vynil silane Methacrylate silane

DryAfter 2-hr water boil

Flex

ure

stren

gth

of c

ompo

site

(MPa

)

E-glass surface treatment

Fiberglass reinforced polyester composites

Fibers surface treatments - (V) carbon fibers

SURFACE TREATMENT forms chemical bonds to the carbon surface, to give a better cohesion to the resin system of the composite

SIZING is a neutral finishing agent (usually epoxy) to protect the fibers during further processing (eg prepregging) and to act as an interface to the resin system of the composite

Fibers surface treatments - (VI) carbon fibers

Whiskerization Polymer grafting

Pyrolitic carbon deposition

OXIDATIVE NON-OXIDATIVE

Gaseous oxidation

Oxidation in air

Oxidation in oxygen and oxygen containing gases (O3 , CO2)

Catalytic oxidation

Liquid phase oxidation

Chemical (HNO3 , H2O2 KMnO4, NaClO chromic acid)

Electrochemical (HNO3 , NaOH)

Fibers surface treatments - (VII) carbon fibers Chemical groups produced by surface treatments on carbon fibers

J.C.Goan, T.W.Martin, R.Prescott 28th SPI Conf., (1973) Paper 21B.

Fibers surface treatments - (VIII) polymeric fibers

ARAMID FIBERS

- chemical etching/grafting (HCl, H2SO4, NaOH → reactive amino groups fiber damage may occur!) - plasma treatment (in ammonia or argon → 50-400% adhesion increase)

- application of coupling agent (not particularly successfull)

Ultra High Modulus Polyethylene Fibers (UHMPE)

- chemical etching (KMnO2, H2O2, K2Cr2O7 → 6-fold ISS increase in epoxy)

- plasma treatment (in oxygen or air)

M.S.Silverstein, O.Breuer J.Mater.Sci., 28 (1993) 4718.

Fibers surface treatments - (IX): plasma treatment of UHMWPE fibers

C.Della Volpe, L.Fambri, R.Fenner, C.Migliaresi, and A.Pegoretti, J. Mater. Sci., 29 (1994) 3919.

untreated UHMWPE fibers

plasma treated UHMWPE fibers (air, 20 W, 30 min, 10-5 bar)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4 5

load

(N)

displacement (mm)

untreated fiber

treated fiber

Fp

2 µm

2 µm

Fibers surface treatments - (X): plasma treatment of UHMWPE fibers

Fibers surface treatments - (XI): plasma treatment of UHMWPE fibers

Effect of time Effect of temperature

Stability of plasma treatments

C.Della Volpe, L.Fambri, R.Fenner, C.Migliaresi, and A.Pegoretti, J. Mater. Sci., 29 (1994) 3919.

Matrix modifications

Example: maleic anhydride or acrylic acid grafted onto polypropylene (Polybond™)

J.M.H.Daemen and J. den Besten, Eng. Plastics, 4 (1991) 82.

Books:

• J-K. Kim and Y-W. Mai “Engineered Interfaces in Fiber Reinforced Composites”, Elsevier Oxford (1998)

• E.P. Plueddemann “Silane Coupling Agents” Plenum Press NY 2nd Edition (1991)

• J-P.Donnet and R.C.Bansal “Carbon Fibers” Marcel Dekker NY 2nd Edition (1990)

Conferences:

• IPCM, Interfacial Phenomena in Composite Materials - biennal (next 2003)

• ECCM, European Conference on Composite Materials, biennal (next Brugge – Belgium June 3-7, 2002)

• IPC, Interfaces in Polymer Composites biennal (next, Orlando, FL; December 9-11, 2002)

• ICCI International Conference on Composite Interfaces

Journals:

• Composites Interfaces, VSP.

• Composites Science and Technology and Composites Part A,, Elsevier

• Polymer Composites, Society of Plastics Engineers SPE

Documents

Parte della lezione è stata tratta dalla lezione svolta · Schema della lezione - Introduzione - Effetto dell’interfaccia sulle proprietà meccaniche dei compositi - Micromeccanica