A NEW TECHNIQUE OF MEASUREMENT BY COUPLING OF A CYCLIC LOW FREQUENCY STRESS WITH AN ULTRASONIC WAVE

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A NEW TECHNIQUE OF MEASUREMENT BY

COUPLING OF A CYCLIC LOW FREQUENCY

STRESS WITH AN ULTRASONIC WAVE

G. Gremaud

To cite this version:

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A

NEW TECHNIQUE OF MEASUREMENT BY COUPLING OF A CYCLIC LOW FREQUENCY

STRESS WITH AN ULTRASONIC WAVE

G. Gremaud

I n s t i t u t de ge'nie atomique, Swiss Federal I n s t i t u t e o f Technology, 1 01 5 Lausanne, Switzer Zand

Abstract.- By measurement o f t h e low frequency i n t e r n a l f r i c t i o n spectrum, t h e d i f f e r e n t mechanisms which c o n t r o l t h e d i s l o c a t i o n m o b i l i t y , p r e s e n t very s i m i l a r peaks. I n t h e same way t h e microdeformation curves measured by t h e u l t r a s o n i c methods do n o t present g r e a t d i f f e r e n c e s between d i f f e r e n t mechanisms.

To a v o i d t h i s i n d e t e r m i n a t i o n on the mechanisms, a new technique has been developed (1)

.

I t c o n s i s t s i n measuring t h e a t t e n u a t i o n Aa and t h e r e l a t i v e change A t / t o f propagation time o f u l t r a s o n i c waves i n a sample, w i t h a permanent s i n u s o i d a l low frequency a p p l i e d s t r e s s ( 0 ) . Curves Aa(o) and A t / t ( o ) , each one measured d u r i n g one c y c l e o f t h e s t r e s s , are drawn as a f u n c t i o n o f t i m e o r temperature. These curves present d i f f e r e n t shapes and d i f f e r e n t e v o l u t i o n s depending on the i n v o l v e d mechanism. Some experimental r e s u l t s obtained by t h i s technique a r e presented a t t h i s conference i n two o t h e r papers (2,3).

I n t h i s paper, t h e measuring device w i l l be presented.

1. I n t r o d u c t i o n . - I n o r d e r t o make t h e above-described measurements, a s p e c i a l device has been developed. This device i s composed o f f o u r d i s t i n c t p a r t s :

a) a h i g h p r e c i s i o n u l t r a s o n i c echometer which i s a b l e t o measure a u t o m a t i c a l l y t h e a t t e n u a t i o n and t h e r e l a t i v e change o f propagation t i n e w i t h a s e n s i t i v i t y o f dB/vs and r e s p e c t i v e l y , and w i t h a measurement r e p e t i t i o n r a t e o f 10 kHz. b ) a mechanical system which allows t o generate,a compression s t r e s s o f any waveform

t o the sample. Sinusoidal stresses can be generated between .001 and 2 Hz w i t h ampli-

tudes from

2

t o 50 kg/cm2.

ultrasounds sample C ) a cryogenic system which a l l o w s t o r e g u l a t e t h e

temperature o f t h e sample between 4K and 300K. d ) an analog system o f data a c q u i s i t i o n which draws

quartz

t h e curves Aa(a) and A t / t ( o ) , and a l s o t h e curves

t

I sm a ( t ) , A t / t ( t ) and T ( t ) . The curves Aa(u) and At/t(cr)

,I---- - -

'

_N

,

a r e then t r e a t e d on a Hewlett-Packard 9835 desktop

/

*.@'

-1cm- 1'

@

0 computer. T h i s i n s t a l l a t i o n i s constructed i n such a way

t h a t i t can be used d u r i n g i r r a d i a t i o n experiments. Fig.1: P r i n c i p l e o f t h e A magnetic f i e l d can a l s o be a p p l i e d t o t h e sample. measurements

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JOURNAL DE PHYSIQUE

temps obsolu

de porcours echantillon

'

osci~~oscope

4

Fig.

2:

Principle of the generator of ul trasounds

Horloge

20MHz

ultra - stable

2. The Ultrasonic Echometer.- An ultrasonic echometer working w i t h a technique of echces has been developed. The principle of the generator i s presented .in f i g u r e 2. A burst of ultrasounds (frequency f u S between 4 and 40 MHz and duration

tt

from .5 t o 2.5 v s ) i s emitted by a quartz transducer i n t o the sample with a r e p e t i t i o n r a t e from 1 t o 10 kHz ( f i g u r e 1 ) . The same transducer receives the successive echoes coming from the sample. For each t r a i n of echoes, the attenuation a and the r e l a t i v e variat- ion of propagation time A t / t are measured.

The generator ( f i g . 2 ) allows also t o measure the absolute velocity of the u l t r a - sounds with a high precision by the following technique: one or two bursts of u l t r a - sounds a r e a l t e r n a t i v e l y emitted i n t o the sample. The time

tabs

between the two bursts i s manually adjusted t o obtain the exact superposition of the echoes coming from the two emitted b u r s t s . This time

tabs

represents then the absolute propagation time of the ul trasounds.

To measure the attenuation, a system of synchronized electronic gates ( c ) allows t o s e l e c t manually two d i f f e r e n t echoes i and j ( f i g u r e 3: tland

t2

adjustable from 2.5 t o 100 ps, t adjustable from - 3 t o 5 ~ s ) . The peak amplitude ( d ) of the two

P

selected echoes i s measured and memorized i n analog form. Then a logarithmic d i f f e r - e n t i a l amplifier measures the attenuation in decibels. This signal i s f i l t e r e d by a low-pass f i l t e r and automatically calibrated i n dB/us (1 v o l t per dB/ps). Two separated c i r c u i t s s h i f t automatically the analog zero in order t o p l o t very small variations of the attenuation.

The maximum s e n s i t i v i t y of the attenuation measurements i s about 10-"B/ps.

Generateur Oscilloteur

+

d'un ou de --) relaxe deux trains (a) (logique

MECL 1

To measure the r e l a t i v e variation of the propagation time, a one-shot pulse ( b )

20 M H z

7

I I Cchos

I +tabs il-

-

Trep

-

I l I

-

Trep- 1 I capocite compensation de

'0'

--"

_inn

l I I niart--

nn

nnn

1

UUU I I I I 8 ,

\ I I

fus

lUUU BUU

-train 2

ic)

n

emission de 1 train d'ondes

-

1-

bmission de 2 troins d'ondes

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j-t,-;+t J

'

, , 2 1 1 (b) _I I ) I I k I I I ( c )

n

mernorisation (d) I I

Fig. 3 : Principle of the attenuation measurement

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JOURNAL DE PHYSIQUE F i g . 5: P r i n c i p l e o f t h e s t r e s s device

0

of w i d t h tcr (between 0 and @ 100 u s ) can be generated by

@ _{steps of one nanosecond ( f i g . 4 ) .}

T h i s p u l s e h a s h i g h s t a b i l i t y ,

0

b e t t e r than 20 ps.

0

_{The emission o f t h e b u r s t}

0

o f ultrasounds ( a ) i s synchro-

n i z e d on t h e posi t i v e - g o i n g edge o f t h e pulse ( b )

.

The w i d t h tcr o f t h e pulse ( b ) i s a d j u s t e d manually

-0

a t t h e beginning of t h e experiment, so t h e

negative-going edge of the p u l s e i s e x a c t l y centered i n t h e middle o f t h e n t h echo. A second one-shot p u l s e ( c ) i s then generated, which begins a t t h e negative-going edge o f t h e f i r s t p u l s e and ends a t t h e f o l l o w i n g zero-crossing o f t h e o s c i l - l a t i o n s o f t h e nth echo. The w i d t h A t o f t h i s second p u l s e i s p r o p o r t i o n n a l t o t h e r e l a t i v e

0

v a r i a t i o n o f t h e propagation time o f t h e u l t r a -

sounds and i s measured by a time-amplitude con-

@ v e r t e r ( d ) . The s i g n a l o f t h e c o n v e r t e r i s memo-

I '

, ,

I ' r i z e d i n analog form, then c a l i b r a t e d t o o b t a i n

I

4 '

10 mV p e r ns. I t i s then f i l t e r e d by a low-pass

A

f i l t e r and transformed i n t o t h e value o f r e l a t i v e

v a r i a t i o n of t i m e ( 1 v o l t f o r a v a r i a t i o n o f o f A t / t c r ) . Two separated c i r c u i t s s h i f t automa- t i c a l l y t h e analog zero i n o r d e r t o p l o t v e r y

0

@ small v a r i a t i o n s o f t h e r e l a t i v e change o f pro-

@

pagation time.

The maximum s e n s i t i v i t y obtained on A t / t i s about

l o - = .

To o b t a i n such p r e c i s i o n , i t was necessary t o s t a b i l i z e t h e temperature o f t h e e l e c t r o n i c c i r c u i t s a t 70°c, w i t h a s t a b i l i t y b e t t e r than

.lOc.

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steps t o the one-shot pulse ( b ) when the time-amplitude converter reaches one of the

ends of i t s range.

3 .

The S t r e s s Device.- The mechanical system i s represented i n figure 5.

A

compression

s t r e s s can be applied t o the sample ( 2 ) by two s t e e l pieces ( 1 ) .

A

motor of very low

i n e r t i a (10) controls, through a t r a i n of gear-wheels

( 9 )

and micrometric screws

(8),

the displacement of a p l a t e ( 6 ) . This displacement i s transformed i n a strength on

the sample by a l i n e a r calibrated spring ( 4 ) . A t the mobile extremity of the spring,

a strength-cell ( 5 ) measures the s t r e s s on the sample. The mobile p l a t e (6) i s bound

t o the fixed plates by two s t e e l bellows

(7),

which allows t o make the preliminary

vacuum inside the i n s t a l l a t i o n before work i n a helium or nitrogen atmosphere

(cryogenic f l u i d s ) .

The e l e c t r o n i c system of s t r e s s regulation allows t o generate two types of s t r e s s

programs

:

a ) l i n e a r s t r e s s ramps with stages ( f i g . 6 a ) by a closed-loop regulation of the motor

velocity through a tachymetric dynamo.

b) s t r e s s e s of any wave form by a second close-loop regulation allowing t o follow

an analog s t r e s s reference, f o r example sinusoidal s t r e s s e s ( f i g . 6 b ) .

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JOURNAL DE PHYSIQUE

due to the f a c t t h a t the absolute attenuation and velocity of the ultrasounds can

vary quickly during an increase or a decrease of the temperature, which induces an

opening of the Aa(o) and A t / t ( o ) cycles ( f i g . 7 ) .

This treatment i s made by a program developed on a Hewlett-Packard 9835 calcul-

a t o r .

The experimental curves a r e d i g i t i z e d , which allows t o eliminate the electronic

noise. Then the program calculates the necessary corrections t o close the Aa(a) and

A t / t

( 0 ) cycles, on the assumption t h a t the d r i f t due t o the temperature i s l i n e a r

with the time on one cycle of the s t r e s s . Then the curves, automatically scaled,

a r e drawn on a d i g i t a l p l o t t e r . Examples of such results can be found in two other

papers [2,3)

.

5. Conclusion.-

A