the properties of coupling agents in improving ultrasonic transmition

7/17/2019 The Properties of Coupling Agents in Improving Ultrasonic Transmition

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Pergamon

I n t . J . R o c k M e c h . M i n . S c i . G e o m e c h . A b s t r . Vol. 33, No. 4, pp. 417-424, 1996

Copyright © 1996 Elsevier Science Ltd

01 48 -9 06 2(9 5 )0 00 74 -7 P r in ted in G rea t Br it a in . A l l r igh ts r e se rved

0148-9062/96 15.00 + 0.00

T e c h n i c a l o t e

The Pro perties of Coup ling gents in Improving

Ultrasonic Transmission

J.-F. COUVREURt

J.-F. THIMUSt

INTRODUCTION

Ultrasonic wave propagation is often used in rock

mechanics and mining sciences. It is applied in the field

for geophysical investigations and in the laboratory for

material characterization and non destructive evaluation

[1]. Wave propagation velocity, then attenuation have

been used to determine dynamic properties of rocks

(Edyn, Vdy . . . ) [2]. Both these parameters seem to be

correlated [3, 4] but the attenuation appears to be more

sensitive to weathering condition [5], anisotropy [6],

grain dimensions [5, 6] or porosity [3, 7] of a rock.

Ultrasonics in the laboratory can be used during

mechanical or thermal tests in order to find correlations

with cracking, compaction and pore collapse [4, 5, 8-12].

The wave is generally recorded by transducers put in

contact with a specimen. At the interface between the

transducers end platens and the specimen, one places a

coupling agent to improve wave transmission [13, 14]. A

coupling agent can thus be defined as a product placed

between transducer and specimen in order to accomplish

a good coupling by filling the voids and irregularities at

the interface. But perfect coupling, as postulated by the

impedance theory, can not be completely satisfied and

depends both on the kind of coupling agent and on the

applied stress. Standards specify how the energy

transmission between the transducer elements and test

specimen can be improved. They especially recommend

the use of a coupling agent: ASTM [13] proposes a thin

layer of an electrically conductive adhesive like epoxy

while ISRM [14] suggests a thin film of grease, Vaseline,

glycerin, putty or oil but it prefers an epoxy adhesive or

phenyl salicylate if hard coupling is required. Other

compounds have also been used by many researchers

(Table 1).

The choice of coupling agents appears relatively

broad. A study of the effects of coupling agents on

ultrasonic propagation should consider the type of

application, based on the stress level and the kind of

recorded waves. Li and Nordlund [l 5] have studied such

effects at low stress levels (up to 5 MPa) for compression

(P) waves by means of delays and maximum

t U n i t 6 d e G 6 n i e C i v i l , U n i v e r s i t 6 C a t h o l i q u e d e L o u v a i n , P l a c e d u

L e v a n t , 1 , B - 1 3 4 8 , L o u v a i n - l a - N e u v e , B e l g i u m .

peak-to-peak amplitudes. In these conditions they have

observed that liquids (grease, epoxy and water) are

better than metal foils (aluminum), that no difference is

noted between these liquid coupling agents and that the

P wave transmissibility of aluminum foil is strongly

dependent on the contact pressure, contrary to liquid

compounds. Considering the technical difficulty in

obtaining a perfect coupling condition with metal foils,

they recommend to use visco-liquid coupling agents as

acoustic coupling agents in P wave measurements. In

this paper, the effects of coupling agents on ultrasonic

propagation o f compression (P) and shear (S) waves are

analyzed, and this for a large stress range (1-30 MPa).

Common tools (delays and amplitude attenuation) as

well as frequency and energy analysis are considered.

EXPERIMENT L DESIGN

Ultrasonic device

The equipment for the ultrasonic experiments is

composed of a PUNDIT unit (Portable Ultrasonic

Non-destructive Digital Indicating Tester), a P S waves

selecting adapter and two 50 mm diameter transducers

(500 kHz fundamental rated frequency, 100 V pulse

amplitude, 40 pulses per second and 3/~sec pulse width).

They are disposed at the top and bottom of the

specimen, so that the wave propagates mainly in the

loading direction. The output signals are recorded by a

digital oscilloscope and a PC computer (sampling

frequency of 20 MHz) and saved at regular periods

during the test to permit further analysis (Fig. 1). The

expected values of the propagation delays of P waves

(tp = 8.0 sec) and of S waves (ts = 12.4 psec) are given

by the equipment manufacturer. These values are based

on regression o f arrival times of waves which propagate

through different aluminum alloy calibration bars.

Testing procedure

The emitter is placed against the receiver via the

coupling agent to be tested (no sample in between). The

compression load is applied with a strain rate of

0.096 mm/min. From about 1 to 30 MPa and back to

1 MPa, P and S waves and the corresponding load are

regularly recorded, at about every 2 min. An example of

recorded waves is given in Fig. 2.

417



4 18 C O U V R E U R a n d T H I M U S : T E C H N I C A L N O T E

Tab l e 1 . L i t er a t u re r e v i e w o f u se d c ou p l i n g age n t s

L e a d f o i l

A l u m i n u m f o il

T a o a n d

King [6]

M o u s t a c h i e t al.

[9]

C o u v r e u r

a n d Thi m us [ 11 ] A z e e m u d d i n e t

al.

[16] P y r a k -

N o l t e et al.

[17]

S c ot t e t al.

[18]

Li and N ordlund [15[

G r e a s e

Oil

V a s e l i n e

H o n e y

W a t e r

R a o a n d R a m a n a [ 1 2 ] I S R M

N o r d l u n d

[15]

S i gg i n s [2] ISR M [14]

I SRM [ 14 ]

K l i m i s e t al.

[19]

S i g g i n s

[2] Gui l l aume

et al.

[20]

L i

a n d N o r d l u n d

[15]

[14] Li a n d

E p o x y a d h e s i v e

P h e n y l sa l i c y l a t e

L o c k n e r

e t al.

[ 4 ] A STM [ 13 ] I SRM [ 14] L i a n d

N o r d l u n d [ 1 5] A z e e m u d d i n e t al. [16] S c ot t e t al.

[18] B l a i r a n d S p a t h i s [ 2 1 ] W a t a n a b e a n d S a s s a

[22]

S i g g i n s [2] ISRM [14]

T h e r e p r o d u c i b i l i t y o f t h e r e c o r d s h a s b e e n v e r i f i e d .

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

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

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

h a v e b e e n e x e c u t e d a t t h r e e d i f f e r e n t d a t e s w i t h o u t a n y

n o t a b l e d i f f e r e n c e s a t s a m e s t r e s s e s ( F i g . 3 ) . A n d a s c a n

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

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

d i ff e re n t c o u p l i n g a g e n t s ( a l t h o u g h t h e s e t - u p h a s t o b e

p a r t l y r e m o v e d b e t w e e n e a c h t e s t ) .

I n te rp r eta t ion too ls

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

v a l u e c a l c u l a t e d o n t h e n o i s e p e r i o d , i .e . t h e d e l a y t a k e n

b y t h e w a v e t o a r r i v e t o t h e r ec ei ve r. O n t h i s w a v e , s o m e

p a r a m e t e r s a r e a n a l y z e d :

• T h e d e l a y o r ar r iv a l t i m e o f t h e w a v e t h r o u g h t h e

s p e c i m e n ( t , a n d t , ). T h e P w a v e i s c o n s i d e r e d t o

h a v e a r r i v e d w h e n i t s a m p l i t u d e r e a c h e s 1 0 % o f t h e

f i r s t peak va lue . F or the S wave , i t i s eas ier to

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

t h e s h e a r w a v e a r r i v a l m a y b e o b s c u r e d b y m o d e

c o n v e r s i o n o f S w a v e i n t o P w a v e , r e f l e c t i o n s o f P

w a v e a n d r i n g i n g o f t h e t r a n s d u c e r s [ 2 , 1 3, 1 4 ] . T h e

m e a s u r e o f d e l a y s ( s e e ) i s d i r e c t l y r e l a t e d t o t h e

ve loc i t i e s .

T h e f ir st p e a k ( A t a n d A , ) o r t h e f i rs t p e a k - t o - p e a k

a m p l i t u d e ( A A p a n d A A , ) g i v e a n e v a l u a t i o n o f t h e

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

t h e s p e c i m e n ( m V ) .

T h e d o m i n a n t f r e q u e n c y o f t h e f i r s t p e r i o d o f t h e

s i g n a l ( f p a n d f ~ ) i n d i c a t e s a p o t e n t i a l c h a n g e o f t h e

s p e c t r a l c o n t e n t o f t h e w a v e ( k H z ) .

T h e m a x i m a l a m p l i t u d e o f t h e p o w e r s p e c t r a l

d e n s i t y ( i . e . t h e n o r m o f t h e F a s t F o u r i e r

T r a n s f o r m ) o f t h e f i r s t p e r i o d

PSDp

a n d

PSD, )

i s

a n o t h e r m e a s u r e o f t h e a t t e n u a t i o n .

oupling agents used

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

c o u p l i n g a g e n t s , r e s p e c ti v e ly , o f 4 0 a n d p m t h i c k n e s s.

F o u r v i s c o u s o r l i q u i d c o m p o u n d s a r e a l s o c o n s i d e r e d :

g r e a s e , h o n e y , g e l f o r m e d i c a l s c a n n i n g a n d c o m m o n

water .

R E S U L T S A N D D I S C U S S I O N S

N o c o u p l i n g a g e n t s, m e t a l f o i l s a n d v i s c o - li q u i d

o m p o u n d s

F i g u r e s 4 a n d 5 d i s p l a y t h e e v o l u t i o n o f t h e d e l a y ( t p

a n d t ,) a n d t h e f ir st p e a k (A p a n d A s ) i n f u n c t i o n o f t h e

P u n d i t

o a d i n g

R e c e i v e r

C o u p l i n g a g e n t

E m i t t e r

~1~Ill~Ill~Ill~IliA

16 chann els I t

a t a a c q u i s i t i o n

c a r d

P C

G PI B I

i n t e r f a c e

b o a r d

TRI G O s c i l l o

I i

PI B

P/ S

w a v e s e l e c ti o n

F i g . 1 . A c q u i s i t i on s e t - u p .

L P T

R e c e p t i o n E m i s s i o n



C O U V R E U R a nd T H I M U S : T E C H N I C A L N O T E 4 19

<

3 0 0 0

o o o t A

o O a / I

? :

w v e

V l r i ' l l

l r ' r l i l l - - v ' ~

V 2 ° ° v V v V - v V V o

800 -7

o A A l ^ A o s _

A A a A n ^ A

1 , ~ . hA A /~_

i u o i V

o o M \ o W o

- 4 0 0

- 60 0 t I T S)

i

i

- 8 0 0 •

F i g . 2 . E x a m p l e o f P a n d S r e c o r d e d w a v e s 2 9 M P a , w i t h g r e a s e , t i m e w in d o w : 0 - 3 5 0 g s e c ).

s t r e s s , a n d t h i s f o r P a n d S w a v e s . T h e y a r e su b d iv id e d

b e tw e e n th e me ta l f o i l s a n d t h e v i s c o -l i q u id c o m p o u n d s .

T h e f i r s t p e a k s a s me a su r e d i n r e V o l t s a r e n e g a t i v e

because the f i r s t a r r iva ls a re nega t ive (a s seen a t F ig . 3 ) .

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

a t t h e i n t e r f a c e b e tw e e n t h e t r a n sd u c e r s i s o b v io u s o n

F ig s 4 a n d 5 . D e l a y s a n d a mp l i t u d e s a r e a lw a y s w o r se

w i t h o u t t h a n w i t h a n y c o u p l i n g a g e n t .

F r o m F i g s 4 a n d 5 , t w o g r o u p s o f c o u p l in g a g e n t s

a p p e a r l o g i c a l l y : me ta l f o i l s a n d v i s c o - l i q u id

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

so me sc o r e b e t t e r i n e a c h g r o u p : l e a d f o i l f o r t h e f i r s t

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

s e e ms l es s a t t e n u a t in g t h a n a lu m in u m f o il , e sp e c ia l l y A p

a n d A s [ F ig . 5 ( a ) a n d ( c) ]. T h i s su r e ly c a n n o t b e im p u te d

to t h e c o u p l in g a g e n t s t h i c k n e s s ( d e l a y s o f a b o u t

0 .01 ~ tsec for 40 ~ tm) . Th e d i f fe rence be twee n grease an d

h o n e y r e l a t i v e t o w a te r a n d g e l i s mo r e n o t i c e a b l e . G e l

i s mo r e d e f i c i e n t i n c a se o f S w a v e p r o p a g a t io n [ F ig . 4 (d )

and 5(d) ] . I t i s in te res t ing to no te tha t wa te r i s r ea l ly a

p o o r c o u p l in g a g e n t . F o r S w a v e , i t b r i n g s n o

i m p r o v e m e n t r e l a t i v e t o n o c o u p l i n g a g e n t a t a l l

[ F ig . 4 ( d ) a n d 5 ( d ) ] b u t f o r P w a v e i t i s n o t q u i t e t h a t

bad [F ig . 4 (b) and 5(b) ] . I t conf i rms two ideas : the use

o f a c o u p l i n g a g e n t , e ve n a n o t v e r y a d a p t e d c o u p l i n g

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

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

u s e d r a t h e r f o r P w a v e t h a n f o r S w a v e p r o p a g a t i o n .

Lead foil, grease and honey

Figu re 6 i s a synthe s is of F ig s 4 a nd 5 , i. e. tp , t s, Ap an d

A s f o r t h e t h r e e b e s t c o u p l in g a g e n t s : l e a d f o i l , g r e a se a n d

h o n e y . F i g u r e 7 c o n s i d e r s t h e d o m i n a n t f r e q u e n c y ( f p

a n d f~ ) a n d t h e m a x i m a l a m p l i t u d e o f t h e p o w e r s p e c tr a l

d e n s i t y PSDp a n d PSDs).

I f o n e c o n s id e r s th e P w a v e p r o p a g a t io n , b o th v i s c o u s

c o u p l in g a g e n t s a r e c l e a r ly r e c o mme n d e d b e c a u se t p i s

c o n s t a n t a n d i n d e p e n d e n t o f t h e s t re s s [ F ig . 6( a) ]. T h e

a m p l i t u d e Ap begins to inc rease wi th the s t r e ss be fore

s tab i l iz ing [F ig . 6 (b) ]. I t con f i rms the re su l t s f rom Li and

N o r d l u n d [ 1 5 ] . N e v e r th e l e s s a t h ig h e r s t r es se s ( f r o m

a b o u t 1 5 MP a ) , l e a d f o i l b e c o me s n e a r ly a s g o o d a s t h e

o th e r tw o a g e n t s . T h i s c o u ld b e r e l a t e d t o t h e f r e q u e n c y

c o n te n t o f t h e P w a v e t h r o u g h l e a d f o i l [F ig . 7( a) ]: u p

to 1 0 - 15 M P a , t h e d o m in a n t f r e q u e n c y in c r e a se s t o

r e a c h t h e c o n s t a n t v a l u e o f t h e o n e o f g re a s e a n d h o n e y .

I n d e e d t h e h ig h e r t h e f r e q u e n c y , t h e sma l l e r t h e

a t t e n u a t io n a n d t h e h ig h e r t h e v e lo c i t y [ 1 ] .

F o r t h e S w a v e p r o p a g a t i o n , t h e v i s c o u s c o u p l i n g

a g e n t s a r e su r p r i s i n g ly n o t so p o o r : t s r e ma in s s t a b l e

[F ig . 6 (c )] an d As fo l low s a r eas ona ble t r end [F ig . 6 (d)] .

M a y b e t h e y p o s s e s s e n o u g h v i s c o s i t y t o b e a b l e t o

p r o p a g a t e S w a v e s . T h e f a c t t h a t h o n e y i s a b i t b e t t e r

t h a n g r e a se [ F ig . 6 ( d ) ] c o u ld c o n f i r m th i s e x p l a n a t i o n .

A t h ig h e r s t r e s se s , l e a d f o i l b e c o me s c l e a r ly t h e

b e s t c h o i c e , e sp e c i a l l y w h e n a t t e n u a t io n i s c o n s id e r e d

[Fig. 6(d)].

I f o n ly t h e d e l a y is me a su r e d , o n e c a n b e l ie v e t h a t

g r e a se o r h o n e y a r e w e l l a d a p t e d [ F ig . 6 ( b ) ] . O n e mu s t

k e e p i n min d t h a t d e l a y s a r e i n t h i s c a se t h e a r r i v a l t ime s

o f t h e f i r s t p e a k o f t h e S w a v e ; i t is i n d e e d t o o d i f f i c u lt

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

th e S w a v e s t a r t s b e c a u se o f P w a v e i n t e r f e r e n c e

[F ig . 8 (a ) ] . On the cont ra ry , for lead fo i l , the de lay



4 2 0 C O U V R E U R a n d T H I M U S : T E C H N I C A L N O T E

1500

1000

500

5 0 0

lOOO

1500

' P w a v e

0 A p

z - d * I

I I r I

l / /

, ,

6 0 0

4 0 0

2 0 0

- 2 0 0

4 0 0

6 0 0

i

S w a v c

1'0 ~ _ ~ ~ 1 5 ~ ~ 0 t 25 A / / ~ # / 3 0

A , ~ t ( I t s ) V Y

t s

Fig. 3. P and S waves recorded at three differentdates (with grease, 5 MPa).

8 . 2

8 .1

8 . 0

7 . 9

- (a)

P w a v e Me t a l f o i l s

X N o c o u p l a n t

L o a o L

I I I I I I I

5 10 15 20 25 30 35

1 3 . 6 ; ( C ) S w a v e

Me t a l f o i l s

N o c o u p l a n t

1 3 . 4 L e a d f o i l

, -- , . . . . . . A l u m i n u m

~L 13.2

1 3 . 0

. . . . . ~ t . . . . . . L

2 .8

0 5 10 15 20 25 30

I

35

8 . 2

8 .1

g g

8 . 0

7 . 9

- < ) - W a t e r

~g~ m ~ . . . . . . . .

- - .

x . Z . ~x . . . . . . . . .

5 10 15 20 25 30

V i s c o u s c o m p o u n d s

N o c o u p l a n t

G r e a s e

- - × - - H o n e y

- - + - - G e l

I

35

1 3 . 6

1 3 . 4

1 3 . 2

1 3 . 0

1 2 . 8

0

V i s c o u s c o m p o u n d s

- - ~ x ( d ) N o c o u p l a n t

G r e a s e

_ . - x - - H o n e y

\ x ~ - - + - - G e l

\ x - ~ - W a t e r

- - x . + ~ - . x . . . . . . .

x ~ x

.

.

+ ~ O+~

× x ~ I L I i ' × ' r . . . . × ' 1 . ×

5 10 15 2O 25 3O

o ( M P a )

o ( M P a )

Fig. 4. Delaysvs stress--metal foils and visco-liquidcoupling agents.

I

35



COUV REUR and THIMUS: TECH NICAL NOTE 421

>

E

<=

o (MPa)

a )

0 5 10 15 20 25 30

I I I I I I

-250 ~

-500

Metal foils

-750 -- ~ Nocouplant

Lead foil

...... Aluminum

-1000 - -

35

I

-250

-500

>' -750

E

-1000

<

-1250

-1500

-1750

(C) o (MPa )

5 10 15 20 25 30 35

I I I I I I I

Metal foils

- - No couplant

- - Lead foil

inum

>

E

<=

0

-250

-500

-750

-1000

b )

5 10 15 20 25 30

_ M . . . . I I I I I I

_ , _ , _ - , - - - - - - - , - - - , - - , ,

x . . X : . : . , + b m ~ x - + x - + . +

- - o - , ~ - - + ~

.. . x,~+ Viscous compounds

x+x - - No couplant

- - - Grease

-- ×-- Honey

--+-- Gel

- <)- Water

35

I

d )

-250

-500

> 750

-1000

<

-1250

-1500

-1750

5 10 15 20 25 30

~.) .

I I I I I I

×.

• . × . × . .

×

. . ~ ~

Viscous c°mp°unds ~ x ~

• No couplant -• x'-7- ~'-

-- Grease

• - ×' Honey

--+-- Gel

- <)- Water

Fig. 5. First peaks vs stress- -metal foils and viscous coupling agents.

35

I

i s e a s i l y d e t e r min e d . T h e su d d e n i n c r e a se o f a

m e a s u r e m e n t o f t h e w a v e e n e r g y

~ 0

= A2dt (V2s)

af t er the exp ecte d de lay (t~ = 1 2.4 Ixsec) for lead foil ,

con t ra r i l y to th e grease , conf i rm s th is ana lys is [F ig . 8 (b)] .

I n c id e n t a l l y n o f r e q u e n c y c h a n g e s a r e n o t e d f o r t h e S

waves [Fig. 7(c)] .

T h e

P S D

va lues [F ig . 7 (b) and (d) ] conf i rm the

e v o lu t i o n p o in t e d o u t b y t h e A v a lu e s .

First pea k a nd peak to p eak amplitudes

B o th p a r a me te r s a r e u se d i n t h e l i t e r a tu r e . F ig u r e 9

d i sp l a y s t h e p e a k - to - p e a k a mp l i t u d e A A in f u n c t i o n o f

8 . 2

8 . 1

8 . 0

-250

-500

-750

-I000

7.9

0

P wave

I I I 1 I I

5 10 15 20 25 30

tr [MPa]

5 10 15 20 25 30

I I I I I

B )

13.6-- (C)

13.4 -

=lr~ 13 2

13.0[

I 12 81

35 0

35 0

I

-250

~ ~ -500

-75C

,~ -1000

. . - , . . . . . . . . . . . . . . . • . . . . . . . . . -1250

-- - 1500

-1750

S W a V e

I I I

5 10 15 20 25 30

[MPa]

5 10 15 20 25 30

I I I I I 1

(D) Lead foil

G r e a s e

Fig. 6. Delays and first peaks vs stress--c ompari son of lead foil with grease and honey.

I

35

35

I




P w a v e

6OO

500

4OO

300

0

A )

I I i I I I I I

5 10 15 20 25 30

150

125

75

5o

35

c )

S w a v e

--...+ . . . y

I I I I I I I

5 10 15 20 25 30 35

6OO

500

4OO

3O0

B )

I I I I

5 I O 15 20 25 30

t r [ M P a ]

lO0O

755

250 /

I 5 o ;

35 0

(D)

. o . . . ~ . . . . . . . . . . . . . . . . . . -

/ i Le a d foal

i I

G r e a s e

/ . . . . . . H o n e y

I I I I

5 10 15 20 25

[ M P a ]

F i g . 7 . D o m i n a n t f r e q u e n c i e s a n d m a x i m a l a m p l i t u d e s o f PSD comparison of l e a d fo i l w i th gre a se a nd hone y.

I

3o

I

35

P~

<

m

400 - Man ufacturer 's ra ted

delay:t ,=12.4 I ts S wave

I

P w av e e ffec ts i / f ' " ~ " ~ ~

2 o o - ; , I f ' +

, . r , j + . .. .. . • . . . . .. . . . .. . . . . " . , /

o _ _ _ x _ . ~ - - - . - - - - - ~ - , : - ~ .-+ -,, , I F " . - , \ ,

, ,o , i , ; ~ , , ) ] , + . , . - - , , ~ 1 2 o

. I ~ t / / 1 , ~ k ]

+ . , 1

-2 00' .. -1 ~ / / +-. 7

+-, ' . , ,1'

~ . 2 J k . /

",Y '

_4 00 i "....

I 0 0 0

8 0 0

6 0 0

4 0 0

2 0 0

0

6

Lead foi l

. . . . . . G r e a s e

. . . . . . . . . .

/

. . . . . . . . . . . . . . y . . . . . . . .. . . . . . . . t

10 12 14 16 18 20

t ( I t s )

F ig . 8 . Ar r iva l t im e s of S wa ve s for l e a d fo i l a nd gre a se .



C O U V R E U R a n d T H I M U S : T E C H N I C A L N O T E 4 23

the stress. It brings no more information than from the

first peak analysis. It is not surprising: Klimis

et al

[19]

for several rocks and Thimus [8] for frozen soils have

observed a linear relation between the two first peaks o f

waves i.e. between the first minimum and the first

maximum.

Loading and unloading

Nothing special is observed during unloading: the trans-

missibility of the coupling agents particularly of the metal

foils decreases approximately along the loading path.

This indicates that the coupling capability is strongly de-

pendent on the pressure. Li and Nordlund have observed

the same evolution for P waves at low stresses [15].

Pract ical aspects

Some practical considerations can be taken into

account to choose the coupling agent. When the

coupling agent also has to act as a glue a conductive

epoxy can be successfully used. One should

moreover avoid the use of a visco-liquid coupling agent

on too porous media since it can penetrate into the

material induce additional stresses against the grains

and damage the specimen ends. Probably for these

reasons Tao and King [6] have used lead foil for their

rock specimens and a viscoelastic coupling agent for non

rock specimens. On the other hand the friction risk on

the platen ends is maybe greater with metal foils. Finally

the ease of use can lead to choose the viscous

compounds for example when point transducers are

used.

All these considerations have to be relativized when

applied on transducers-coupling agents-rock specimen

system but can anyway give useful clues. One should also

be cautious when encountering great velocity or

amplitude increase at the beginning of a mechanical test:

1 8 0 0 - -

x + x - , - v - - - -x ~ +

' . . . . .

1 50 0 - - ~ x ' ~ ' ~ '' + ' ~ , + ~ x ~ + ~ x ~ ,. . + ~ = z ~ : r x ~_,.,.,.,.._+ ~

1 20 0 - - ~ m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 0 0

<

6 0 0

3 0 0

l

5 1 0 1 5 2 0 2 5 3 0 3 5

g

E

<

2 1 0 0 - -

1 8 0 0

1 5 0 0

1 2 0 0

9 0 0

6 0 0

3 0 0

N o c o u p l a n t . . . . . x . . . . Hon e y

~ a - - L e a d f o i l + -- G el

. . . . . . . . . . A l u m i n u m - - 0 - - Wat e r

r e a s e a ~ a . . ~ . ~ _ . _ . . _ ~ . . m ~ . . . . . . . . . . . . . . . m

x ' ' ' ' ' ' ' ' '

. ' x '

t

5 1 0 1 5 2 0 2 5 3 0

O ( M P a )

F i g . 9 . P e a k - t o - p e a k a m p l i t u d e s v s s t r es s .

3 5




it can very well be related to coupling agent set-up. The

use of amplitude ratio [22], quality factor by spectral

ratio technique [1, 6, 7, 8, 19, 20] or transfer function [9]

should enable to clear up such trouble by making a ratio

between the output and the input waves, i.e. the wave

received after propagat ing through the specimen and the

wave recorded with emitter and receiver placed against

one another.

C O N C L U S I O N S

Some commonly used coupling agents were experi-

mented and compared in function of their ability to

improve the propagation of ultrasonic waves. Compres-

sion P) as well as shear S) waves have been considered.

The authors have analyzed the wave velocity and the

attenuation as functions of stress and coupling agent.

Differences are more notable on attenuations, which are

equally evaluated in time and frequency domains.

These coupling agents can be classified into two sorts:

metal foils and visco-liquid compounds. Between them,

lead foil, grease and honey score the best. The interest

of using a coupling agent should become obvious.

For P wave propagation, grease and honey are

globally the best coupling agents although at high

stresses more than 15 MPa) the abilities of lead foil

come close to their performance. It could be related to

a stabilization of the wave frequency through lead foil

which at this stress level, reaches the constant frequency

value of the viscous compounds. For S wave

propagation, lead foil scores globally the best but at low

stresses before 5 MPa), grease and especially honey

seem not so bad although one should not forget that the

determination of shear delays is quite inaccurate for

those visco-liquid coupling agents. When only the

measurement of the delays is required, the differences

between coupling agents in the same class are generally

not sensitive, except for S waves through water.

Consequently if only P waves are required, viscous

coupling agents such as grease and honey are

recommended. They can also be applied for P and S

wave propagation at low stresses, e.g. when just a

contact pressure is applied to the transducers. At high

stresses, lead foil can be confidently used for P waves

only and it is strictly recommended when P and S waves

are recorded together.

Some practical reasons can sometimes come into

account to decide the choice of a coupling agent. They

can be related to the facility of use, the need of glue for

the transducers or the kind of material. These

considerations should be relativized for applications on

rock specimens but they can give useful clues.

Acknowledgements--The a u t h o r s w o u l d l i k e t o t h a n k t h e L a b o r a t o i r e

d u G 6 n i e C i v i l f o r t h e i r a s s i s t a n c e w i t h t h e e x p e r i m e n t a l a n d

a c quis i t ion se t -up , a nd pa r t i c u la r ly B . S ine , i t s d i r e c tor , B . Ga lopin ,

a n d E . B o u c h o n v i l l e . D i sc u s s i o n a n d c o r r e c t i o n s a b o u t t h i s p a p e r w i t h

A . v a n H a u w a e r t h a v e b e e n r e a l ly p r o f i t ab l e . T h i s r e se a r c h w o r k w a s

f u n d e d b y t h e F o n d s d e D ~ v e l o p p e m e n t S c i en t i fi q u e f r o m t h e

U n i v e r s it ~ C a t h o l i q u e d e L o u v a i n .

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the properties of coupling agents in improving ultrasonic transmition

Documents

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properties of coupling

hard coupling

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acoustic coupling agents

choice of coupling agents

kind of coupling agent

viscoliquid coupling