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JO U R N A L O F MA T E R I A L S SC I E N C E 4 ( 1969) 625- 633

T h e M e c h a n i c a l P r o p e r tie s o f A s b e s to s

d . A V E S T O NNationa/ Physical Laboratory, Teddington, U K

Received 4 Ja nuary 1969

Ste reoscan e lec tron m ic roscope s tud ies and measuremen ts o f t ens i l e s t reng th and Yo ung 's

modu lus have been made on ch ryso t i l e and f i ve na tu ra l l y -occu r r i ng f i b rous amph ibo les .

The e f fec t o f hea t on the modu lus and f r ac tu re mechan ism has a l so been examined .

W i th the excep t ion o f t r emo l i t e w h ich has a d i f fe ren t m ic ros t ruc tu re f r om the rema in ing

amph ibo les , t he modu l i and s t reng ths were s im i la r . The loss o f s t reng th wh ich occu rs

when the f ib rou s amp h ibo les , and to a l esse r ex ten t, ch ryso t i l e a re hea ted be low th e i rdecompos i t i on tempera tu re i s shown to be due to an inc rease in i n te r f i b r i l l a r bond ing

caus ing the ma te r ia l t o become no tch -sens i t i ve

1 . I n t r o d u c t i o n

T he c om m e r c i a l l y i m po r t a n t f i b r ous s il ic a te s ar e

c h r y s o t il e w h i c h a c c o u n t s f o r 9 0 ~ o f t o t a l

a s b e st o s p r o d u c t i o n a n d t h e a m p h i b o l e s c r o c id o -

l it e a n d a m o s i t e w h i c h m a k e u p m o s t o f th e

rem a ind e r . Ch ryso t i l e , Mg3(Si~O~) (OH)4 has a

c o m p l e x t u b u l a r s t r u c t u r e w h i c h h a s n o t b e e nf u l l y r e s o lv e d w h e r e a s t h e a m p h i b o l e s , a l t h o u g h

s us c e p t ib l e t o e x t e ns ive c a t i on s ubs t i t u t i on , a l s o

o c c u r i n a n o n - f i b r o u s f o r m w h i c h e n a b l e d

W a r r e n t o d e t e r m i n e t h e s t r u c t u r e o f tr e m o l i t e a s

ea r ly a s 1930 [1] . I t i s base d o n inf in i t e do ub le

c ha i n s o f s i li c on oxyge n t e t r a he d r a , h e l d t oge t he r

by f i ve 6 - c o - o r d i na t e d a nd t w o 8 - c o - o r d i na t e d

c a t io n s p e r u n i t c el l. T w o h y d r o x y l g r o u p s a r e

a t t a c h e d t o t h e c e n t r a l c a t io n o f e a c h u n i t c e ll

a n d t h e re i s a s m a l l a m o u n t o f a d d i ti o n a l

h y d r o g e n , p r o b a b l y a s S i O H g r o u p s o n t h epe r i phe r a l s i l i c a t e c ha i n s . T he a s be s t i f o r m

va r i e ti e s a pp r ox i m a t e t o t he f o l l ow i ng i de a l is e d

f o r m u l a e b u t w i t h e x t e n s i v e i n t e r c h a n g e o f F e "

a n d M g " :

C r o c i d o l i t e N a 2 F e a " F%'" SisO2z(OH)2

Am os i te M g2F es" S isO22(OH)zAc t ino l i t e Ca2 F% " SisO2~(OH)~

Tr em ol i te Ca2Mg5 SisO~,2(OH)2

A n t hop hy l l i t e M g7 S is O ~2(O H )2

T h e y oc c u r a s e x t r e m e l y c lo s e pa c k e d l a t h - l i ke

c rys ta l s , which we sha l l ca l l f ib r i l s , a l igned wi th

t he i r c - a xes pa r al l e l. A bund l e c on t a i n i ng s e ve r al

hundred such f ibr i l s w i l l be ca l l ed a f ibre . The

f i ne st f i b re w h i c h c a n be u s e d f o r X - r a y d i f fr a c -

t i o n ( a b o u t 1 0 F m t h ic k ) g iv e s a p h o t o g r a p h w i t h

f u l l r o t a t i on a l s y m m e t r y so t ha t t he f i b ri ls c a nno t

b e m u c h m o r e t h a n 0 .1 F m t h ic k , n o r c a n t h e y be

m uc h l e s s t ha n t h i s a s qu i t e s ha r p s po t s a r eob t a i n e d [ 2] . E l e c t r on m i c r og r a ph s o f c h r ys o t i l e

f ib r e s s how t he m t o a ve r a ge a bou t 0 .03 F m

d i a m e t e r [2 ] a n d t o b e f il le d a n d s u r r o u n d e d b y

a m o r p h o u s m a t e r ia l o f t h e s a m e c o m p o s i t i o n .

T h e c o m m o n s t r u c tu r e a n d c l o se l y s i m i la r c el l

pa r a m e t e r s o f the a m ph i bo l e s s hou l d g i ve r is e t o

f a i r l y s i m i l a r m e c ha n i c a l p r ope r t i e s , bu t t he

pub l i s he d da t a , a l t hough c on f l i c t i ng , s ugge s t s

o t he r w i s e . T hus H odgs on [ 3 ] r e po r t s t e n s i l e

s t re n g t h s o f 3 5 0 0 0 a n d 2 5 0 0 0 k g / c m 2 f o r

c r oc i do l i t e a nd a m o s i t e bu t l e ss t ha n 5000 kg / c m 2f o r t h e r e m a i n i n g a m p h i b o l e s . N a d g o r n y i et al

[ 4] how e ve r f oun d t ha t t he s t r e ng t h s o f c r oc i do -

l i t e a nd a n t hophy l l i t e ( a nd c h r ys o t i l e ) w e r e

s i m i l a r bu t de c r e a s e d i nve r s e l y w i t h f i b r e

d i a m e t e r , i n a hype r bo l i c m a nne r c ha r a c t e r i s t i c

o f w h i s ke r s, f r om 25 000 k g / c m 2 a t a d i a m e t e r

d = 2 / x m t o 5000 kg / c m 2 a t d = 25 / z m . O n t he

o t h e r h a n d Z u k o w s k i a n d G a z e [5 ] f o u n d a

l e n g t h / s t r e n g t h re l a ti o n s h i p b u t n o d i a m e t e r /

s t r e ng t h e f fe c t s o t ha t s t r e ng t h s up t o 60 000

k g / c m ~ ( d e p e n d in g o n l e n g th ) w e r e m a i n t a i n e d

a t l e a st up t o d = 30 F m . A no t h e r puz z l i ngf e a t u r e i s t he d r a s t i c l o s s o f s t r e ng t h w h i c h ha s

b e e n f o u n d [3 ] w h e n t h e a m p h i b o l e s a r e h e a t e d

625



J . A V E ST O N

t o a m o d e r a t e t e m p e r a t u r e w h i c h i s i n s u f f i c i e n t

t o c a u s e a n y c h a n g e i n c r y s t a l s t r u c t u r e .

T h e o b j e c t o f t h e p r e s e n t w o r k w a s t o m e a s u r e

t h e s t r e n g t h a n d m o d u l u s o f c hr y s o t il e , t h e

n a t u r a l l y o c c u r r i n g a m p h i b o l e s a n d s y n t h e t i c

a m p h i b o l e s o f c o n t r o l l e d c a t i o n s u b s t i t u ti o n , t o

d e t e r m i n e w h e t h e r t h e r e i s a n y c o r r e l a t i o n

b e t w e e n s t r e n g t h a n d c o m p o s i t i o n , a n d a l s o t o

c o m p a r e t h e f r a c t u re m e c h a n i s m s o f th e

n a t u r a l a n d h e a t - t r e a t e d f i b r e s . T h e r e i s a l s o

n e e d f o r w o r k o n m o d e s o f fa i lu r e o f b u n d le s o f

i n t e rac t i n g f i b r i l s .

2 . E x p e r i m e n t a l

T h e a s b e s t o s f i b r e s , s u p p l i e d b y C a p e A s b e s t o s

F i b r e s L t d , w e r e a s f o l l o w s : c h r y s o t i l e , T h e t f o r d ,

C a n a d a ; c r o c i d o li t e , K o e g a s , C a p e P r o v i n c e ;a m o s i t e , P e n g e, T r a n s v a a l ; a n t h o p h y l l i te , F i n -

l a n d ; t r e m o l i t e , P a k i s t a n a n d a c t i n o l i t e , K o e g a s .

T h e y w e r e a s m i n e d , a n d c o n s i s te d o f b l o c k s o f

a d h e r e n t , c l o s e l y p a c k e d f i b r i l s v a r y i n g i n

l e n g t h f r o m 0 . 6 c m ( a c t i n o l it e ) t o 1 2 c m ( a m o s i t e )

S t a n d a rd T e m - P r e s h y d r o t h e r m a l e q u i p m e n t

w a s u s e d t o s y n t h e s i s e t h e s o d i u m m a g n e s i u m

a m p h i b o l e a c c o r d i n g to t h e m e t h o d o f G i e r e t a l

[6].

T e n s i l e s t r e n g t h s a n d Y o u n g ' s m o d u l i w e r e

m e a s u r e d w i t h a n " I n s t r o n " t e s ti n g m a c h i n e . T h e

s t r ai n r a t e w a s k e p t r o u g h l y c o n s t a n t b y v a r y -i n g th e c r o s s h e a d s p e e d b e t w e e n 0 . 01 a n d 0 . 1 c m /

m i n a c c o r d i n g to t h e l e n g t h o f f i br e , a l t h o u g h

p r e l i m i n a r y t e s t s w i t h 3 c m l o n g c r o c i d o l i t e a t

t h e s e t w o s p e e d s s h o w e d t h a t t h e s t r e n g t h w a s

i n d e p e n d e n t o f s t r a i n r a t e w i t h i n t h is r a n g e . E a c h

f ib r e w a s f i rs t w e i g h e d o n a " C a h n " e l e c tr o -

b a l a n c e [ 7 ] a n d i t s c r o s s - s e c t i o n o b t a i n e d f r o m

t h e m e a s u r e d d e n s i ty o f t h e m a t e r i a l i n th e c a s e

o f t h e A m p h i b o l e s , a n d t h e l i t e ra t u r e v a l u e o f

2 .5 5 [ 3] f o r c h r y s o t i l e . T h e f i b r e s w e r e t h e n g l u e d

w i t h A r a l d i t e to t h e c e n t r e s o f f la t s c u t i n t h ee n d s o f a p a i r o f 3/16 i n . d i a m e t e r b r a s s r o d s

w h i c h w e r e a l i g n e d i n a j i g a t t h e r e q u i r e d g a u g e

l e n g t h . W h e n t h e g l u e h a d s e t , t h e r o d s w e r e

i n s e r t e d i n t o s p ec ia l m o u n t i n g s o n t h e " I n s t r o n " ,

f i x e d i n p l a c e w i t h s e t s c r e w s a n d t h e j i g f i n a l l y

r e m o v e d . S p e c i m e n s f o r s h o r t t e r m s h e a r t e s t s

w e r e s e t i n c a s t i n g A r a l d i t e t o p r e v e n t s p l i t t i n g ,

c u t t o l e n g t h w i t h a d i a m o n d s a w a n d t h e n

c l e a v e d t o r e m o v e t h e A r a l d it e . T h e h e a t -

t r e a t e d c r o c i d o l i t e d i d n o t c l e a v e e a s i l y , s o

f u r t h e r m a c h i n i n g w i t h a d i a m o n d s u r f a ce

g r i n d e r w a s n e c e s s a r y .S t e r e o s c a n s a m p le s w e r e c o a t e d w i t h 5 0 0 A o f

g o l d - p a l l a d i u m a l l o y .

6 26

3 . R e s u l t s

3 .1 . T e n s i l e S t r e n g t h a n d M o d u l u s

T h e m e a n t e n s i l e s t r e n g t h s ( m a x i m u m l o a d

d i v i d e d b y t h e o r i g i n a l c r o s s - s e c t i o n ) a n d

m o d u l i a r e g i v e n i n th e t h i r d a n d s i xt h c o l u m n s

o f t a b le I . A p a r t f r o m t r e m o l i t e th e s t r en g t h s a r e

o f t h e s a m e o r d e r , a n d t h e m o d u l i t h e s a m e

w i t h i n t h e s c a t t e r o f t h e r e s u lt s , a g a i n w i t h th e

e x c e p t i o n o f t r e m o l i te . T h e m o d u l i o f a c t i n o li t e

a n d a n t h o p h y l l it e a ls o a p p e a r s o m e w h a t l ow e r ,

a l t h o u g h t h e s e a r e l e ss r e l i a b le d u e t o t h e s h o r t

s p e c i m e n l e n g t h a n d s m a l l n u m b e r ( a b o u t t e n

c o m p a r e d w i t h s i x ty t o s e v e n t y f o r t h e o t h e r s ) o f

s a m p l e s t e s t e d . C h r y s o t i l e l o s e s a b o u t o n e - t h i r d

o f i t s s t r e n g t h w h e n h e a t e d a t 5 0 0 ~ C ( 1 h , N ~

a t m o s p h e r e ) w h e r e a s c r o c i d o l i te u n d e r t h e s a m e

c o n d i t i o n s b e c o m e s s o b r i t t l e t h a t s a m p l e si n v a r i a b l y b r e a k a t t h e g l u e . E v e n a t 3 8 0 ~ C t h e y

r e t a i n o n l y a b o u t o n e - t h i r d o f t h e i r f o r m e r

s t r e n g t h a l t h o u g h t h e m o d u l u s i s u n a f f e c t e d .

F i b r e c r o s s - s e c t i o n a l a r e a s w e r e i n g e n e r a l

v a r i e d f r o m 5 t o 1 0 0 • 1 0 - 6 c m 2 ( d = 2 5 t o

1 14 / xm ) , b u t w e r e e x t e n d e d d o w n t o 5 • 1 0 7

c m z ( d ~ 7 / z m ) f o r c h r y s o t i l e a n d c r o c i d o l i t e ,

a n d u p t o 5 • 10 - ~ c m z ( d N 2 5 0 F m ) f o r

c h r y s o ti l e a n d t r e m o l i t e . T o t e s t f o r a n y d e p e n d -

e n c e o f s t r e n g th o r m o d u l u s o n c r o s s - s e c t i o n t h e

s l o p e o f t h e l i n e w h i c h g a v e t h e l e a s t s q u a r e s f i t

t o t h e d a t a , a n d i t s s t a n d a r d d e v i a t i o n w e r ec o m p u t e d . B r o a d l y s p e a k in g i f t h e r e s u l ts f o r a ll

l e n g t h s o f a g i v e n fi b r e a r e t a k e n t o g e t h e r t h e

o n ly ca ses w h ere t h e r e i s a s i g n i f i can t (b y t h e

c r i t e r i o n t h a t t h e s l o p e s h o u l d b e a t l e a s t t w i c e

i t s s t a n d a r d d e v i a t i o n ) s i z e d e p e n d e n c e a r e t h e

s t r e n g t h o f h e a t e d c r o c i d ol i te a n d t h e m o d u l u s o f

n a t u r a l c h r y s o t i l e . F o r a l l f i b r e s t h e s m a l l s i z e

d e p e n d e n c e c o u p l e d w i t h th e s c a t te r o f t h e

r e s u l t s p r e c l u d e a n y t h i n g o t h e r t h a n a l i n e a r f i t

t o t h e d a t a d o w n t o 7 / z m .

T h e s t r e s s / s t r a i n c u r v e s f o r t h e a m p h i b o l e sw e r e e s s e n t i a l l y l i n e a r u p t o f r a c t u r e , w h i c h o n

s o m e o c c a s i o n s o c c u r r e d s u d d e n l y a n d o n o t h e r s

a s a se r i e s o f s t ep s a s i n f ig . 1 . As t h e r ed u c ed

s l o p e B C c o r r e s p o n d s t o t h e r e d u c e d c r o s s -

s e c t io n o f t h e f i br e r e m a i n i n g u n b r o k e n , t h e

f i n a l b r e a k i n g s t r e s s c a n b e o b t a i n e d f r o m t h e

p r o d u c t o f t h e s tr a i n a n d m o d u l u s . C o m p a r i s o n

w i t h t h e s t r e n g t h s a s o r i g i n a l l y d e f i n e d s h o w e d

t h a t t h e s c a t te r w a s r e d u c e d a n d o f c o u r s e t h e

m e a n s t r e n g th i n c r e a s ed .

Th e s t re s s / s t ra in cu rv e fo r ch ry so t i l e ( f i g . 2 ) i s

m o r e c o m p l e x : a f t e r a n i n i t i a l l i n e a r r e g i o n( w h i c h d e p e n d s o n t h e r a t i o o f f ib r e le n g t h t o

d i a m e t e r ) t h e c u r v e b e c o m e s c o n v e x , a l t h o u g h



MECHANICAL PROPERTIES OF ASBESTOS

T A B L E I

Material Length Strength Youn g's modul us

cm

M ean :t : one Extrap olated Slope of s t rength M ean 4- one E xtrapolated Slope of

s tandard to zero cross- versus cross- s tandard to zero cross- mo dulusdeviation of sectional sectional area devia tion of sectional area versus cross-points fro m area ~ e* 4- ~* points from s ~* sectional area

me a n k g / c m ~ 10-4kg/cm~ 10 7 me an kg/cm 2x 10 ~ 4- ~*kg/cm 2 • 10 4 kg/cm ~ 10 6 kg/cm ~ 10-8

Croci dolit e 0.3 2.79 4- 0.66 3.03 t 0.14 6.02 4- 3.0 - - - - --

3.0 2.04 4- 0.46 2.10 • 0.08 - 2.04 • 2.0 1.69 4- 0.19 1.70 :k 0.03 - 1.0 i 1.5

5.0 1.68 4- 0.37 1.67 4- 0.07 0.49 4- 2.1 1.78 4- 0.09 1.81 • 0.02 - 73 ~_ 5 3

Crocidoli teaf ter 1 .0 1.104-0.25 1.294-0.07 -5. 1 • 1 .9 1.6 5• 1.69:t :0 .03 -11 .5 4- 9.7

3 h a t 380 ~ C 3.0 0.83 4- 0.21 0.94 4- 0.05 - 3.1 i 1.1 1.77 § 1.77 :k 0.02 - 2.3 i 5.9

5.0 0.65 4- 0.28 0.76 ~ 0.06 - 4.1 4- 1.9 1.76 • 0.13 1.77 :k 0.03 - 4.1 • 9.4

Am os it e 0.3 1.55 4- 0,50 1.59 • 0.18 - 0.55 4- 3.2

3.0 1.63 4- 0.40 1.74 4- 0.13 , - 2.2 ~ 2.4 1.71 4- 0.14 1.77 ~ 0.04 - 12.2 4- 7.8

5.0 1.714-0. 42 1.804-0.11 -2 .2 4- 2.3 1.72=k0.08 1.76=k0.02 - 10 .1 4- 4.1

9.0 1.16 4- 0.41 1.08 4- 0.10 1.9 4- 2.2 1.72 • 0.07 1.72 :k 0. 02 - 1.2 4- 4.1

Tr emo li te 3.0 0.31 4- 0.13 0.42 4- 0.04 - 0.36 4- 0.11 1.52 4- 0.15 1.49 :k 0.10 1.03 4- 3.8

Act ino lit e 0.6 1.89 4- 0.47 1.94 4- 0.31 - 1.58 4- 9.03 1.46 4- 0.20 1.59 :k 0.15 - 36.8 ~: 40.2

An th op hy ll it e 0.6 1.35 ~ 0.50 1.22 4- 0.46 3.15 ~: 4.55 1.54 :k 0.09 1.53 • 0.09 - 0.31 4- 10.8

Ch rys oti le 0.3 4.48 :k 1.02 4.59 4- 0.20 - 31.1 4- 24.7

3.0 3.06 ~ 0 . 8 3 3.13 4- 0.18 2.86 4- 5.7 1.60 =k 0.13 1,69 :k 0.04 - 20.5 ~: 8.1

Ch rys oti le aft er 3.0 2.31 :k 0.45 2.40 • 0.09 0.87 • 5.8 1.65 4- 0.06 1.64 4- 0.03 3.1 • 7.3

1 h a t 500 ~ C

e'is the standard deviation of the line or of its intercept on the strength or modulus axis.

t h e r e is n o a p p a r e n t b r e a k a g e o f t h e f ib r il s, a n do n r e m o v i n g t h e s tr e s s i t i s f o u n d t h a t a p e r m a n -

e n t s t r a in h a s d e v e l o p e d . O n f u r t h e r s t r e s s in g

o v e r t h e r a n g e A B , t h e o u t e r f i b r i l s w e r e s e e n t o

b e b r e a k i n g , a n d a t B t h e f i b r e e i t h e r b r o k e

c o m p l e t e l y o r a s i n t h e c a s e i l l u s t r a t e d i n f i g . 2 ,

t h e b u l k o f t h e f ib r e p u l l e d o u t f r o m a s h e a t h o f

b r o k e n f i b r i l s w h i c h r e m a i n e d i n t h e g l u e . A s t h e

s t a t i c f r i c t i o n e x c e e d s t h e s l i d i n g f r i c t i o n a p o i n t

C ( w h i c h i s a t s t r e s s z e r o f o r s o m e s p e c i m e n s ) i s

r e a c h e d , w h e r e t h e in t e r fi b r il l a r a d h e s i o n i s

r e c o v e r e d a n d t h e c u r v e r e s u m e s i t s o r i g i n a l

s l op e f r o m C t o D . A t D p u l l - o u t o c c u r s a n d t h e

s t r e s s f a l l s t o z e r o a s t h i s i s c o m p l e t e d .

S p e c i m e n s w h i c h f a i l e d i n t h i s w a y w e r e n o t

i n c l u d e d i n th e d a t a o n w h i c h t a b l e I a n d f ig . l 3

a r e b a s e d b u t a r e p l o t t e d i n f i g . 3 a s a v e r s u s 1 / r .

W i t h t h e a s s u m p t i o n t h a t t h e r e m a i n i n g f i b r e

c r o s s - s e c t i o n i s c i r c u l a r , t h a t i t s c r o s s - s e c t i o n a l

a r e a is n o t s i g n i fi c a n t ly le s s t h a n t h e o r i g i n a l

f ib r e , a n d t h a t t h e l e n g t h b o n d e d t o t h e o u t s i d e

f ib r il s a t p u l l - o u t is e q u a l t o t h e l e n g t h l i m m e r s e d

i n th e g l u e ; t h e n a r o u g h e s t i m a t e o f t h e i n t e r -

f i b ri ll a r s h e a r s t r e n g t h r c a n b e m a d e f r o m t h ep u l l - o u t c o n d i t i o n :

2~rrlr ~ 7rr2~

2 r lor o- ~ -- "

r

T h e p u l l - o u t f a i lu r e s s h o u l d l ie o n a li n e o f s l o p e

2I t , a n d t h e t e n s i l e f a i l u r e s i n a r e g i o n b o u n d e d

b y t hi s li n e a n d t h e m a x i m u m t e n s il e s t re n g t h .

P u l l - o u t f a i l u re s t o t h e r i g h t o f t h e l in e w i ll o c c u r

w h e n t h e r e i s a s i g n i f i c a n t r e d u c t i o n i n c r o s s -

s e c t i o n a l a r e a o n p u l l - o u t .

T h e p o i n t s i n t h e r e g i o n s o f in t e r e st a r e r a t h e r

s p a r s e b u t i t d o e s a p p e a r t h a t t w o l i n e s t h r o u g h

t h e o r i g i n w i t h s i g n i f i c a n t l y d i f f e r e n t s l o p e s c a n

b e d r a w n f o r th e h e a t e d a n d u n h e a t e d f ib re s. T h e

l e n g t h o f f i b re e m b e d d e d i n t h e g l u e l, w a s 0 .8

c m i n e a c h c as e. T h e c o r r e s p o n d i n g v a l ue s o f r

a r e c a 1 7 0 a n d 1 00 k g / c m u r e s p e c ti v e ly .

3 .2 . S h o r t B e a m S h e a r T e s t s

W h e n a n i s o t r o p ic b r it t l e m a t e r i a l i n th e f o r m o f

a b e a m o f s p a n l, b r e a d t h b a n d d e p t h d i s

s u b j e c t e d t o a c e n t r a l l y a p p l i e d l o a d F , i t w i l l

f a i l w h e n t h e t e n s i l e s t r e s s a t t h e l o w e r s u r f a c e ( r ,

g i v e n a p p r o x i m a t e l y b y

3 F l

6 27



J . A V E S T O N

7o

4Eo

b

2 .0

1 .0

D

A C

0 1 .0

~ strain

2 . 0

F i g u r e 7 S t r e s s / s t r a i n c u r v e fo r c r o c i d o li te f ib r e w i th I =

3 . 0 c m , d = 3 8 # m .

8 "0

4 " 0

3 . 0

1s

x

'Eo 2 . 0"a

b

1 . 0

/ l o o i x o

o~ Ix

J / . ~ , ~ , 9

A

A

A

lx

Ix

o

A A

A

A

&

0 1 0 1 I 0 [ 0 3 I 0 [ 0 5 I 0 " 0 7

~'r,u

F i g u r e 3 S t r e n g th o f c h r y s o ti le a s a fu n c t io n o f 1 / r . O p e n

s y mb o l s r e p r e s e n t u n h e a te d a n d s o l i d p o i n ts h e a te d

f ibres. Ci rc les corresp ond to pu l l -out and tr iangles to

tensi le fa i lure .

B

3 - 0 - v e r /R e

2 . 0 -

I E

o

1 " 0 I r

0 1 " O 2 " 0

~ s tra in

E

I8 . 0

F i g u r e 2 S t r e s s / s t r a i n c u r v e fo r c h r y s o t il e fi b r e w i th I = 3 . 0

c m, d = 4 1 F m .

e q u a l s t h e s t r e n g t h o f t h e m a t e r ia l . A t t h a t p o i n t

t h e s h e a r s t r e s s r i s a p p r o x i m a t e l y

3 F d e

r=~-d=~l"

H o w e v e r , i f t h e m a t e r i a l i s h i g h l y a n i s o t r o p i c

w i t h r ~ e , r m a y e x c e e d t h e s h e a r s t r e n g t h r F

6 2 8

b e f o r e e = e F a n d t h e m a t e r i a l w i l l t h e n f a i l

r e p e a t e d l y i n s h e a r a t c o n s t a n t F t o g i v e t w o

b e a m s o f d e p th d / 2 , a n d s o o n .

I n o r d e r t o e s t a b l i s h t h e e f fe c t o f h e a t o n t h e

s h e a r s t r e n g t h o f c r o c i d o l i t e , a s e r ie s o f s h o r t

b e a m s h e a r t e s t s o n t h e h e a t e d a n d n a t u r a l

m a t e r i a l w a s m a d e . T h e I n s t r o n l o a d t r a c es f o r

t y p ic a l u n h e a t e d a n d h e a t e d s p e c i m e n s a r e

i l lu s t r a t e d i n fi g s. 4 a n d 5 . T h e u n h e a t e d m a t e r i a l,

i n v a r i a b l y f a i l e d b y s h e a r w h e r e a s t h e s p e c i m e n s

h e a t - t r e a t e d a t 3 8 0 a n d 5 0 0 ~ C a l l f a i l e d i n

t e n s i o n , u s u a l l y a f t e r t h e s h e a r s t r e s s h a d

e x c e e d e d t h a t a t w h i c h t h e u n h e a t e d s p e c i m e n s

f a i le d . A l t h o u g h i n p r i n c i p le e a c h p e a k i n f ig . 4y i e l d s a s e p a r a t e v a l u e o f r F , in p r a c t i c e t h e

c o m p u t e d s tr es s a n d g e o m e t r y o f t h e sp e c im e n s

b e c o m e l e s s w e l l - d e f i n e d a s t h e d e f l e c t i o n a n d

n u m b e r o f c r a c k s i n c re a s e . A s i n g le v a l u e o f r F

w a s t h e r e f o r e c o m p u t e d f r o m e a c h s p e c i m e n

u s i n g t h e f i r s t m a x i m u m i n F a n d t h e o r i g i n a l

s p e c i m e n d i m e n s i o n . T h e r e s u l t s a r e s u m m a r i s e d

i n t a b l e I I w h e r e t h e f i g u r e s i n p a r e n t h e s e s a r e t h e

c o m p u t e d s tr es se s f o r t h e m o d e o t h e r th a n t h a t

i n w h i c h t h e s p e c i m e n f a i l e d .

3 .3 . S t e r e o s c a n M i c r o g r a p h sT h e S t e r e o sc a n e l e c t ro n m i c r o g r a p h o f c r o c i d o -

l i t e ( f ig . 6 ) is ty p i c a l o f a l l t h e s t r o n g a m p h i b o l e s



M E C H A N I C A L P R O P E R T I E S O F A S B E S T O S

SO -

2C

o,

10

/I I ~ I I

0 0"01 0 "02 0"0 3 0"0 4 0"015

Flexure rnm

Figure 4 Thr ee po in t l oad ing t es t f o r na tu r a l c r oc i do l i t e

specimen 0.30 cm wide X 0.265 cm deep x 2.25 cm span.

Sam ple f a i l ed i n shear a t T = t 55 kg / cm 2.

40

30

:~ 20

o

1C

I I

0 0"01 0"02 0"03 0-04

F lexure mr&

Figure 5 T h r e e p o i n t l o a d i n g t e s t f o r c r o c i d o l it e a f t e r

hea t i ng f o r 4 .8 h a t 500 ~ cm i n a N 2 a tmo sphe r e . Spe c ime n

0.40 cm w ide = 0.35 cm deep X 2.0 cm span fa i led in

t ens i on a t cr = 2230 kg /em 2 a t wh i ch po in t t he shear s t r ess

wa s 195 kg/c m ~.

Figure 6 S t e r e o s c a n m i c r o g r a p h o f n a t u ra l c r o c i d o l it e .

Figure 7 S t e r e o s c a n m i c r o g r a p h o f t r e m o l it e .

T A B L E I I

Material Mode of failure Tensile stress at failure Shear stress at failure

kg/cm 2 kg/cm ~

Natur al crocidoli te shear (2600) 155

she ar (2860) 153

shear (2200) 176

Croc idol ite after 4 h at 280 ~ C shear (1850) 168

shear (1280) 168

Croc idol ite afte r 2 h at 380 ~ C tensile 2870 (193)

Croc idol ite after 5 h at 500 ~ C tensi le 2230 (195)

tensile 1840 (175)tensile 1330 (106)

tensile 1360 (221)

6 2 9



J . A V E ST O N

to the principal tensile stress over large areas ofthe fractu re surface.

The surface o f chryso t i le which had been

carefully freed from fine materia l appeared

similar to crocidolite , except that the fibrils werefiner. However, some of the fine tangled ma teria l

which had been pee led f rom the main b lock and

bent through an acute angle in the process , or

any fibre which had been drawn over a sharp

edge such as a razor blade, assumed the curious

kinked or spira l configuration shown in fig . 10.

Figure 8 Stereoscan mic rograph o f f rac tured natura l

crocidol i te f ibre.

Figure 10 Stereoscan mic rograph o f "k i nked" chrysot i l e

f ibre.

Figure 9 Stereoscan mic rograph o f frac tured sur face o f

crocid ol i te f ibre which had been h eated for 1.5 h at 500 ~

in a N 2 atmosp here.

in revealing a c lose-packed bundle of well-

aligned fibrils 0.1 to 0.2 Fm across which

appeared to be con t inuous a long the whole

length of the specimen. The correspo nding

micro graphs of the h eated fibres are identical.

In contras t , tremolite (fig. 7) contains short , less

well-a ligned fibrils . Although the micrographs

of the s ides o f the na tura l and hea t- t rea ted

crocidolites are indis tinguishable from each

other, the fracture surfaces are s trikingly differ-

ent. The na tur al fibre (fig. 8) is alm ost com -

plete ly divided at the extreme ends into thecomponent fibrils , whereas in the heat-treatedmateria l (fig . 9) the crack has travelled normal

630

3 .4 . S y n th e t i c Amp h ib o le s

Gie r et al [6] have shown that under extreme

hyd roth erm al condi tions (3000 arm , 700 ~ C) i t is

possible to synthesise fibrous amphiboles of

con tro l led cat ion con ten t . We have been unab le

to do more than conf i rm the i r resu l ts : theprod uct a lways consis ted of a wadding-like mass

of den sely intertan gled fibres (fig. 11) whic h was

quite unsuitable for tensile measurements .

However, i t was c lear from the tes ts on the

na tura l f ib res repor ted above tha t the re was

unlikely to be any direct correla tion between

mechanic al properties and chem ical compo sition.

4. Discussions

According to Hodgson [3] asbestos fibres are

built up from a series of overlapping fibrils of

various lengths which are held together in thecrys ta l by hydrogen bonding . He showed tha tover the temp erature range (200 to 500 ~ C) in



MECHANICAL PROPERTIES OF ASBESTOS

Figure 11 Stereoscan micrograph of sodium magnesium

amph ibole synthesised at 700 ~ C/3000 atm.

which virtually a ll the s trength is los t , the

amphibo le f ib res lose on ly a smal l amount o f

so-called physically combined water, which is not

associated with a ny crysta llographic change in the

fibrils , and suggested that the corresponding

decrease in hydrogen bonding so weakens the

adhesion between the fibrils that they are pulled

apar t on f rac ture .The Stereoscan microg raph of trem olite (fig. 7)

is consis tent with this mod el and the low s trength

of trernolite could well be du e to some of the

fibrils pulling out on fracture . Howeve r, the

micrographs o f a ll the s t rong amphibo les, o f

which crocidolite (fig. 6) is typical, show that the

fibrils are essentia lly continuous. I f this were not

so- for example if some fibrils were less than

1 mm - then the chance o f see ing the end of a

fibril in fig. 6 would be about 1/20, whereasapart from some surface debris a ll the 200 or so

fibrils appear continuous.Moreover , the shear s t reng ths repor ted in

sec t ion 3 .2 show tha t fa r f rom there be ing a

decrease in interfibril lar adhesion on heating

there is actually an increase. Admittedly if the

heat-tr eated (380 a nd 500 ~ C) specimens had

failed in shear a t the s tresses given in parenth eses

in table II then the increase in shear s trength

wou ld be ins ignificant, but i t must be em phasised

tha t a l l o f them fa i led in tens ion and th a t

judg ing f rom the difficulty experienced in c leav-ing these specimens the actual shear s trengths

were greatly in excess of these values . The shear

s trength of chrysotile on the oth er ha nd was so

low that i t was not possible to prepare three-

po in t load ing spec imens o f adequa te d imens ions

and with the ends free from splits .

We therefore propose a s tructure for asbestoswhich is based on bundles of continuous fibrils

in which the interfibril lar adhesion is low for

chrysotile , somewhat higher for the amphiboles ,

a n d much higher for the heat-treat ed amphiboles .

As the s trongest f ibres break at a s tress of ca E/30

compared to the theoretical value for a perfect

brit t le materia l of ca E/10 the component fibrils

are assumed to contain s tress-concentrating

flaws, such as growth s teps , dis tributed ran do ml y

throughout the fibrils . I t then follows that the

strength o f the bundle or fibre will be the average

of a la rge number o f un i ts a r ranged in pa ra l le land thus independent o f f ib re d iamete r . How-

ever, unless the fibre has completely uniform

properties a length-dependent s trength must

arise. In practice, fibres of length l/n with

strength dis tribution f(x) were cut from fibres of

leng th I . The combined probab i l i ty tha t any

one of the n fibres will have a s treng th between

x and (x ~- dx) and that the remaining (n -- 1)

will have a strength greater than x (i.e. that the

fibre of length l will fa il between x an d (x + dx)

is given byoo

n f ( x ) [ I f ( x ) d x l ( ~ - l ) d xx

and hence the s trength dis tribution for fibres of

leng th I i f they break a t the weakes t po in t i s

given by

oo

x

This func t ion , computed by assuming tha t the

under ly ing s t reng th d is t r ibu t ion of the shor tes t

(0.3 cm) fibres is normal , is draw n o n the s tr ength

his tograms of crocidolite and chrysotile in figs .

12 and 13. If the component fibrils were com-

plete ly indepe ndent over their entire length, then

each would b reak a t i t s weakes t po in t and the

strength of the fibres would be less tha n t hat

computed on the weakest l ink basis . Therefore

as the s trengths o f the chrysotile and crocidolite

fibres fi t the com pute d curves reasonably well i t

appears tha t the fibrils are coupled to the extentthat the fracture region is res tric ted to less than0.3 cm, a lthou gh over shor ter dis tances the fibre

6al



J . A V E S T O N

m u s t e v e n t u a l l y b e h a v e m o r e a s a p a r a l le l b u n d l e

i n o r d e r t o a c c o u n t f o r t h e a p p e a r a n c e o f t h e

f r a c t u r e s u r f a c e i n f i g . 8 a n d t h e a b s e n c e o f a

s t r e n g t h d i a m e t e r e f f e c t .

2C ~ I= 5.0 cm16

12

4 I

o

1 1 ~ 3 ' 0 c m

i l 1= 0 . 3 c m

4 ;0 2 .0 3 " 0

(7 kgf cm-2x 10 4

Figure 12 H i s t o g r a m o f c r o c i d o l i t e s t r e n g t h . C u r v e f o r 3 m m

f i b re i s n o r m a l w i t h t h e v a l u e s o f th e m e a n a n d s t a n d a r d

d e v i a t i o n g i v e n i n ta b l e I . O t h e r c u r v e s a r e c o m p u t e d f r o m

t h is o n a " w e a k e s t l in k " h y p o t h e s i s .

16

14

12 1=3.0cm

10

8

l2k~

~6 0

0 i

1 2 3 4 "5 46 7 8o ' k g f c m - 2 x l 0 -

Figure 13 A s f i g . 1 2 b u t f o r e h r y s o t i l e .

O n h e a t i n g c r o c i d o l i t e fi b r e s it i s s u g g e s t e dt h a t t h e a d h e s i o n b e t w e e n t h e f ib r il s r e a c h e s th e

p o i n t w h e r e t h e s t r e s s c o n c e n t r a t i o n a t t h e t i p

6 3 2

o f a s u r f a c e c r a c k c a n n o l o n g e r b e r e l ie v e d b y a

s e c o n d a r y c r a c k p r o p a g a t i n g p e r p e n d i c u l a r l y i n

a p l a n e o f w e a k n e s s a c c o r d i n g t o t h e m e c h a n i s m

s u g g es t ed b y C o o k a n d G o r d o n [ 8] . T h e o n s e t o f

a s t r e n g t h / d i a m e t e r r e l a t i o n s h i p f o r c r o c i d o l i t e

w h i c h h a d b e e n h e a t e d a t 3 80 ~ C a n d t h e

f r a c t u r e s u r f a c e o f t h e h e a t - t r e a t e d ( 5 0 0 ~ C )

c ro c id o l i t e ( f i g . 9 ) a re co n s i s t en t wi th t h i s

i n c r e a s e i n n o t c h s e n s i t i v i t y a s t h e i n t e r f i b r i l l a r

a d h e s i o n i n c r e a s e s .

M a n y o f th e p r o p e r t i e s o f c h r y s o ti l e m a y b e

s i m i la r ly i n t e r p r e t e d i n t e r m s o f a d h e s i o n b e t w e e n

th e f i b r i l s . Th e p u l l -o u t d a t a i n f i g . 3 su g g es t a

m o d e r a t e i n c r e a s e i n i n t e r f i b r i l l a r a d h e s i o n o n

h e a t i n g w h i c h i s c o n s i s t e n t w i t h t h e l o w ( r e l a t i v e

t o c r o c i d o l i t e ) l o s s o f s t r e n g t h a n d a l s o w i t h t h e

d e c r e a s e d c u r v a t u r e o f th e s t r e s s / s tr a i n c u r v e sf o r t h e h e a t e d f i b r e s . T h e p e r m a n e n t s t r a i n

d ev e lo p ed i n t h e t h i ck e r ch ry so t i l e f i b re s ( f i g . 2 )

r e s u l t s f r o m t h e m o r e h i g h l y s t r e s s e d o u t e r

f i b r i l s u n d e r g o i n g a p e r m a n e n t d i s p l a c e m e n t

r e l a t i v e t o t h o s e a t t h e c e n t r e , s o t h a t t h e y

r e m a i n i n t e n s i o n w h e n t h e s t r a i n i s r e m o v e d .

S i m i l a r l y , w h e n a f i b r e i s b e n t t h r o u g h a n a c u t e

a n g l e t h e o u t e r f i br i ls a r e d i s p l a c e d , a n d i f t h e

a d h e s i o n c a n b e r e c o v e r e d - a n d t h e s t r e s s / s t r a in

c u r v e ( f i g . 2 ) p r o v i d e s s o m e e v i d e n c e f o r t h i s -

t h e n t h e f i b r e w i l l a s s u m e t h e s p i r a l c o n f i g u r a t i o n

sh o wn in f i g . 1 0 .T h e s t r e n g t h o f a s b e s to s , w h i c h i s i n f e r i o r o n l y

t o p r i s t i n e g l a s s f i b r e s a n d w h i s k e r s i s n o t

e x p l o i te d c o m m e r c i a l ly i n u n r e i n f o r c e d p r o d u c t s

s u c h a s c l o t h a n d p a p e r , w h i c h f a i l b y t h e s h o r t

f i b re s p u l l i n g a p a r t , o r i n r e i n f o r c e d p l a s t ic s

w h e r e t h e p r o b l e m is o n e o f a l ig n m e n t a n d

w h e r e t h e a b o v e r e s u l t s c o u l d b e s i g n i f i c a n t .

C u r r e n t a l i g n m e n t m e t h o d s i n v o lv e t h e e x t ru s i o n

o f a s u s p e n s i o n o f t h e d i s p e r s e d f ib r e t h r o u g h a

h o l e o r s l o t . W i t h t h e a m p h i b o l e s t h e p r o c e s s i s

s t r a i g h t f o r w a r d , b u t t h e y h a v e o t h e r d i s -a d v a n t a g e s a s a r e i n f o r c i n g m a t e r i a l . C h r y s o t i l e ,

o n t h e o t h e r h a n d , d i s p e rs e s t o a ta n g l e d m a s s o f

f i n e f i b r e s a n d i t i s n e c e s s a r y t o p r e - t r e a t t h e

m a t e r i a l w i t h h o t p h o s p h o r i c a c i d [ 9 ] . I t w a s

f o u n d i n t h e p r e s e n t w o r k t h a t t h e i n c r e a s e i n

i n t e r f i b r i l l a r a d h e s i o n s i n t h e h e a t - t r e a t e d

m a t e r i a l w a s s u f f i c ie n t t o p r e v e n t c o m p l e t e

d i s p e r si o n a n d t h a t a p r o d u c t o f fi ne w h i s k e r-

l i k e f i b re s re su l t ed .

I n c o n c l u s i o n i t m u s t b e a d m i t t e d t h a t n o

s i n g l e p i e c e o f e v i d e n c e f o r t h e h y p o t h e s i s t h a t

t h e s t r e n g t h o f a s b e s t o s i s d e t e r m i n e d l a r g e l y b yth e i n t e r f i b r i l l a r ad h es io n i s su f f i c i en t i n i t s e l f .

T o p r o v e a d i r e c t c o r r e l a t i o n i t w o u l d b e



MECHANICAL PROPERTIE S OF ASBESTOS

n e c e ss a r y t o m a k e m o r e a c c u r a t e m e a s u r e m e n t s

o f s h e a r s t r e n g t h e n c o m p a s s i n g s e v e r al v a ri e t ie s

o f a s b e s t o s b o t h b e f o r e a n d a f t e r h e a t i n g , b u t t h e

m a t e r i a l i s s u c h t h a t i t is d i f fi c u lt t o d e v i s e s u c h

e x p e r i m e n t s to g i ve a n u n a m b i g u o u s a n sw e r .

T h e d i s c r e p a n c i e s b e t w e e n t h e s t r e n g t h s o f th e

le ss c o m m o n a m p h i b o l e s r e p o r t e d h e re a n d t h o s e

i n th e l i te r a t u r e , f u r th e r e m p h a s i s e t h e p r o b l e m s

a s s o c i a t e d w i th o b t a i n i n g a r e p r e s e n t a t i v e s a m p l e

o f a n a t u r a l p r o d u c t . H o w e v e r , w h e n t a k e n

t o g e t h e r , t h e e v i d e n c e f r o m t h e s t e r e o s c a n m i c r o -

g r a p h s t h e s h e a r t e s t s, th e s h a p e o f t h e s t r e s s /

s t r a i n c u rv e s , th e s t r e n g t h / d i a m e t e r r e l a t i o n s h i p

f o r th e h e a t e d ( b u t n o t t h e u n h e a t e d ) c r o c i d o l i t e

f ib r e s a n d t h e p u l l - o u t d a t a f o r c h r y s o t i le , a r e

c o n s i d e r e d r e a s o n a b l y c o n v in c i ng .

R e f e r e n c e s

I . B . E . WARREN, Z . K r i s t . 72 (1930) 42.

2 . E . S . W . W H I TT A K E R " F i b r e S t r uc t u r e " , e d i t e d by

J . W . S . H e a r l e a nd R . H . P o t e n ( B u t t e r w or th s ,

L on don , 1963 ) p . 594 .

3 . a . A . H O D G S O N , R oy a l I n s t i t u t e o f C he mi s t r yLec tu re Ser ies (1965) No . 4 .

4. E. M. NADGOR NYI~ L. F. GRIGOREVA~ a nd A . P .

IVANOR, I z v e st A k a d . N a u k S S S R N e o r y . M a t h s .

1 (1965) 1117.

5 . R . ZUKOWSKI and R . GAZE, N a t u r e 183 (1959) 35.

6. T. E. GIER, N. L. COX, and H. S . YOUNG~ Inorg.

C h e m . 3 (1964) 100.

7 . L . CAHN and H . R . SCHULTZ, V a c u u m M i c r o b a la n c e

T e c h n i q u e s 3 (1963) 29.

8 . J . CO OK and s . E . GORDON, P r o c . R o y . S o c . A282

(1964) 508.

9. J . GOLDEN, N a t u r e 220 (1968) 64.

633

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