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Proceedings of he FIrSt Pacific/AsIa Offshore Me hamcs SymposIum

Seoul, Korea,

2428

June, 1990

COPYright @1990 by The InternatIOnal SocIety of OffshOle and Polar Engme s

ANALYSIS ON FATIGUE FRACTURE OF PITCH TYPE CFRP BY E METHOD

R I Murakami

Fuilt'l iry 01

Tol,u,/iillla

Toku5lJ1l11a JAP 1S

T.

Adachi

HIJalp

('OI'POI

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3. EXPERIMENTAL RESULTS AND DISCUSSION

3-1

S-N

Curve of

CFRP

Fig.2 shows

S-N curves

of p i t ch

type

CFRP

for

the

s t r e s s r a t i o s

of

R=O

and 0 .5 .

F ig.2

a l so inc ludes t he

r e su l t

o f

epoxide

r e s i n for

,R=O. For R=O, though t he re a re a few s ca t t e r ed

fa t igue s t r eng th o f CFRP, t he

S-N curve

1S d i

v1ded

i n to th ree

regions ;

Stage

a) ,

b)

and

c ) .

Such a

div i s ion agrees we l l w1th

a gener

a l

fea ture of S-N

curve

for composite mater -

1als Shiwa e t

aI ,

1986). When t he s t r e s s r a

t io

inc reases , the

decrease in

fa t igue

s t reng th i s grea te r in high cycle region than

in low

cycles region.

The same

dependence

of

fa t igue s t r eng th on s t r e s s r a t i o

has

been

s tud ied

by

the o the r re sea rche rs Fu j i i

and

Zako,

1985).

7

The

fa t igue s t r eng th a t 10

cyc les

for

CFRP

i s about 5.6 t imes grea ter

than

t h a t fo r

epoxy. This sugges ts t h a t t he o r i en t a t i o n of

pi t ch

type carbon

f ibe r

in r e s i n matr ix con

t r ibu te s

to 1ncrease i n f a t igue s t r eng th .

Slnce

t he r a t i o of fa t igue s t r eng th to u l t i

mate

t e n s i l e

s t reng th

i s

72 for CFRP

but i s

about

10

fo r

epoxy,

t he

decrease

in

fa t igue

s t reng th due

to

CyCl1C s t re s s

i s l e s s

for CFRP

than

for epoxy.

S

2

C r R P . 4

__ _ _ _ _ _ _ _ _ C f . ~ P , ~ R ~ 4 ~ ~ ~

_ _ _ _ _ _

_ _

Fig .2 S-N curves

for CFRP

3-2 Fractographic Resul t s

We have observed

the

corne r c rack ing

on

fa t1gue

f rac tu re

sur face

and have measured

i t s

shape

and dimension. Then,

the

c r i t i c a l s t r e s s in

t ens i ty ,

K

fc

'

a t

f i n a l fa t igue

f rac tu re was

decided

Murakami , 1987). Fig .3 shows t he r e

l a : i o n

between

t he K

f

value and s t r e s s am

pl1tude for

R=O.

The k

fc

value i s

independent

5

20

30

G . MPa)

Fig.3 Fat igue f rac tu re toughness

for CFRP

150

of t he s t r e s s ~ l i t u d e and i s a cons tant of

about 5.4MPa*m . This sugges ts t h a t as the

fa t1gue

damage

for CFRP has approached a con

s t an t

value , the

fa t igue f rac tu re

would be

happened.

Fig.4 shows t y p i ca l frac tographs of CFRP

for R=O.

For

R=O,

t he re

a re the p l a i n

and

m1r

ro r l i k e p a t t e rn in epoxide r e s i n . Never th less

the

carbon

f ibe r

i s randomly

or i en ted

in the

r e s i n

matr ix , 1 t

f a i l s

v e r t i c a l l y to

the load

ax i s . The

sur face

of

carbon

f ibe r

i s

smooth

and does not adhere to epoxide res in . As shown

in

Fig .5 , the

roughness of t he f i n a l

f rac tu re

sur face

i s grea t and

the

epoxide

r e s i n

adheres

t o t he

carbon f iber .

Then, t he cyc l i c s t r e s s

gives

r i s e

to

the

damage

of r e s in mat r ix -ca r

bon f ibe r

i n t e r f ace

and

may

promote

to an

in

t e r fa c i a l

decohesion

for CFRP.

When

the

s t r e s s r a t i o increases from R=O

to

R=0.5,

the morphology of fa t igue

damage

plane i s almost

the

same

as

tha t for R=O, as

shown

in

Fig .6 . Thereis a mirror l i k e a rea

Kunio,

1982)

on the

fa t igue f rac tu re su r face ,

as

shown in

Fig .7 .

This

p a t t e rn

i s very

Slm1-

l a r to t he f rac tu re

appearance

of r e s i n matrix

1n CFRP, as

shown in Fig .4 .

3-3 Rela t ion between AE

Pro te r t i e s

and Fat igue

Damage

Fig.8

shows

the

r e l a t ion

between

AE cumu

l a t ive

event

count or AE

energy

and

number

,of

Fig.4 Fractograph of fa tg iue

damage

plane for CFRP, R=O, c(L=23MPa

Fig .5 Fractograph of

fa ina l f rac tu re

sur face

for CFRP, R=O, c ( ~ = 2 3 M P a

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Fig.6

Frac tograph of fa t igue damage

plane for CFRP, R=O,

c ~ = 1 5 . 7 M P a

cyc les for CFRP, R=O and s t r e s s ampl i tude , 0 0- =

30MPa. The fa t igue damage process i s div ided

i n to th ree regions according

to

AE

cumula t ive

event count as fo l lows; Region 1 means

the

p r o ~ e s s t h a t AE

cumula t ive event

count

l i n ea r -

ly

I n c r e a s e ~

with inc reas ing

number

of

cyc les

N

f

12.5*10 cyc les) . Region2

means

the pro-

cess

t h a t

AE

cumula t ive event

count s c ~ r e c e l y

changes w ~ t h number

of

cyc les 12.5*10 cyc les

N

f

17*10

cyc les) . Region 3

means

the

process

tha t AE cumulat ive event count rap id ly in -

creases

~ i t h l ~ c r e a s i n g number 0 cyc les and

the

specImen f Ina l ly fa i ls 17*10

cyc les

N ) .

. When

t he s t r e s s ampl i tude

decreases ,

glon 2 expands and t r an s f e r s to Region 3 . t

before a

f ina l

f rac ture as

shown

in

Fig 9 J U ~ f _

t e r an

enfurance l imi t

of specimen, R e g i o ~ 3

Fig 8

Rela t ion between

AE cumula t ive event

count

or AE

energy

and number

of cyc les ,

CFRP, R=O, ~ ~ = 3 0 M P a

I

I

I I

~ ~ ~ ~

I I I I

I f I I

t

I I I

~ ~ ~ ~

I I

I I I I

I I 1 I

~ ~ ~ ~

r :

: . 11 ;

:

----- ------ ----- --

T

No of eye les

25000.00

Fig 9 Rela t ion between

AE cumula t ive event

count

or AE

energy

and

number

of cycl les ,

CFRP, R=O. 5, cr 1l. =25MPa

151

Fig.7

Frac tograph

of fa t igue f rac ture

sur f ace for epoxy, R=0,

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I

I

I

I I I I

- - - - - - - - - - - ~ - - - - - - r - - - - - - - - - - -

I I I I

I I I I

, I I I

I I I

-----r-----'------

I

I I

I I

I I

---r-----'------

AE

AlI1llitude

I

I

I

I

mY)

10.00

F i g . l l Rela t ion between

AE

ampl i tude

and AE event count for CFRP

shown in Fig .12 .

But, Region 2 i s

small

and

the AE

does not

happen on

unloading

process .

This

suggests

t ha t the re i s a c r i t i c a l

s t r e s s

for

an occurrence

of AE

on

the

unloading

pro

cess . Then, when the

s t r e s s

r a t i o inc reases ,

it i s

considered

t h a t

for

R=O.5, the AE

would

not

happen on

the

unloading

process s ince the

mlnimum cyc l i c s t r e s s

i s

grea te r t han a

criti-

ca l s t r e s s .

For

epoxide

r e s i n , the

AE

even t

has a low ampl i tude

below 1.6mV as

shown in

Flg .13 .

These

AE even t s

are

a

very

s imi la r to

tha t

in

Region 1 for CFRP.

Shiwa

e t

al 1986) have shown t he same AE

events for GFRP wi th epoxide matr ix as

the

present s tudy .

According

to

t h e i r

s tudy , the

AE events

in Region

1 r e su l t from

the

f a i l u r e

of epoxide

matr ix: I t

has

a l s o been r epor ted

t ha t the

microcracking

i n i t i a t e s in the

r e s in

matr ix under a cons iderab le

low s t r e s s S a t o

e t a I , 1986).

As

mentioned above, t he AE

events

in Region 1

for

CFRP were the same as

t h a t for

epoxode r e s i n . However

for

CFRP, it

happens a t an i n i t i a l s tage of number of

cyc les

whi le

fo r

epoxide

r e s i n ,

the

AE even t

occurs a f t e r a ce r t a in of s t r e s s cyc le . I t i s

then considered t h a t the AE event in an

i n i

tlal s tage of Region 1 r e su l t s from a damage

of i n t e r face between

r e s in matr ix

and carbon

f ibe r due to the cyc l i c

s t r e s s

and t h a t Region 1 means theoprocess which such

a f a t igue damage extends to the r e s in matr ix .

This i s co inClden t wi th the f r ac tog raph ic

r e

s u l t s for

CFRP.

I t i s

considered

t ha t

for

CFRP, the r eS ln

matr ix t r an s f e r s

the

load

to a carbon f ibe r

which

sus ta ins an appl ied

load.

When the

crack

caused by

an i n t e r f ac i a l

decohesion would i n i

t l a t e in the

r e s in matr ix ,

the f ibe r p lays a

ro le in

a crack

br idging.

In

Region

2,

because

the

f ibe r

which

i s p lay ing as a crack b r idg ing

can be

f a i l e d

through

the f a t igue

damage,

a

few AE

even t

of hlgh

ampl i tude would

sporad

i c a l l y happen. Such sporadic AE event was ob

served in Region 2 a s shown i n F ig . l0 b ) . Also,

t h i s

r e su l t ag rees wi th

the

f ractographic ob

se rva t ion where

many carbon f i b e r s

were

per

pendicu la r ly

f a i l e d

to

the

load

a x i s on

the

fa t igue

f rac ture

sur f ace .

When the

fa t igue

damage approaches a

c r i t i c a l value,

the

crack w i l l

s t a r t to pr o

pagate and then

the

reSln matr ix

and the

ca r

bon f ibe r

w i l l be f a i led .

Such

a

f a t igue dam

age

process

i s Reion 3 in

which

many

AE even t s

b u r s t l y happen.

Therefore , for

CFRP, the fa -

152

I

I

I I I I

I I I I

-----,------,------r-----'------

I I I

I I I I

I I I ,

I I

-----,------,----

I I

I I

I I

I I I I

.. . . . . . . . . . .

1-

--r

.........

- -

..........

I I I

I I I ,

:

J

: :

-----,---- -,------r-----'------

I I I I

I I I I

I I I I

I I I I

No of yeles

1111866.00

Fig.12

Rela t ion between

AE cumulat ive event

count or AE energy and

number

of

cyc les

for

CFRP, R=O.

5,

0 110 =16. 7MPa

I

I

, ,

, ,

-----,------ ----

..

-,------

,_--rr-II

,

,

,

,

..

- -

..........

,

,

:

I:

\I

III:

.......... - - .......... , .... -- -r ... - _ .... _

I I

t

I

I I I I

I I I

I , I I

-----,------,------r-----'------

I I I

I I

I I I I

t

I I I

11 of eye

les

1000000 00

Fig.13 Rela t ion between AE cumulat ive event

count or

AE energy and number

of

cyc les

fo r epoxy, R=O, Jo. =4. 5MPa

t i gue s t reng th

in

Stage a ) of S-N curve de

pends

on the

adhesion

of i n t e r face between

the

carbon f ibe r and the r e s in matr ix . In Stage c)

o f S-N curve ,

the

carbon

f ibe r c on t r i bu t e s to

the inc rease

in

f a t igue

s t reng th

because

of

low appl ied loading . Stage b) i s

the

t r a n s i

t i o n process from Stage a ) to Stage c ) .

4. CONCLUSIONS

The

f a t igue s t r ength

for CFRP in

which

c o a l t a r p i t c h

type carbon f ibe r were

randomly

combined

in epoxide r es in with a f ibe r content

of about 33 has s tud ied by means of acous t ic

emission

method and

f ractography.

The

conclu

s ions

obtained

are

as fol lows.

1)

For

CFRP used in t h i s

s tudy ,

the

decrease

in f a t igue

s t r e ng t h

due to

s t r e s s

cyc le

was l e s s

than

t ha t for epoxy. The o r i en t a

t i o n of p i t c h

type

carbon f ibe r to epox ide

matr ix cont r ibu ted

to

r e inforce the

com

pos i t e . The f a t lgue

s t reng th decreased wi th

inc reas ing s t r e s s

r a t i o

and

showed an obv i

ous

mean

s t r e s s

dependence.

2) On the

fa t igue

f rac ture sur face for CFRP,

t he re was a mirror

l i k e appearance in the

r e s in matr ix . While, many carbon

f i b e r s

were f a i l e d through a

cleavage

p a t t e r n and

were perpend icu la r ly to the t en s i l e ax i s .

3)

The occurrence

of AE

events

c l o s e l y r e l a t

ed to the

f rac ture

appearance and

were

d i

vided

i n t o

t h ree reg ions according

to the

AE cumulat ive

even t

count .

When the

s t r e s s

ampl i tude decreased, Region 2 in

which

t..

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there are a few

sporadic

AE events re la ted

to the fa i lu re

of

carbon

f iber and i t

be

came greater .

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T.Fuj i i and M.Zako(1985); Fracture and Mech

anics for

Composite Mater ia l , Jikkyo

Press,

pp. 142.

G.Maiser,

H.Ott , A.Protzner

and

B.

Protz(1986)

; Composites, Vol.17,

No.2,

pp.111.

A.Poursar t ip M.F.Ashby and P.W.R.Beaumount(

1986) ,Compos. Sci .

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K.Suzuki, H.Nakanishi,

M.Iwamoto,

K.Kyou

K.Koike,

M.Imura

and E.Jinen(1987), Fatigue

Fracture

Mechanism of

ClassA-SMC by Acoustic

Emission

Method ,

J . Soc. Mat. Sci . ,

Japan,

Vol.36, No.411,

pp.1402.

M.Shiwa, S.Yuyama and T.Kishi(1986),

Acoustic

Emission

Signal Analysis during Fatigue

Damage

of

GFRP ,

Proc. of Progress in

Acoustic

Emis

sion 3, Jap.

SOil:. o.fNDI pp,.55,4.

T . F u j i i ~ 1 9 7 6 ) , 'Composite MateJi ia,1(8), , J . Soc.

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275

pp. 78'5.

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Per 1;aJIUIll h Press . pp . T1 4

T.

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T'empei:a1 1llJi"e'"

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~ '

J .

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'Sase and:Appliea

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N.Sato,

T.Kurauchi and

O.Ka.migaito(1986),

Detect ion of

Damage

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Jap.

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