The Properties of Coupling Agents in Improving Ultrasonic Transmition
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Transcript of 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
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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
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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
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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
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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
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4 22 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
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 .
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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
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4 24 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
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 .
Accepted fo r publication 18 October 1995.
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