E Q U I P M E N T F O R S T U D Y I N G T H E C R E E P

L O N G - R U N S T R E N G T H O F F I L M S A T

L O W T E M P E R A T U R E S

E. S. Umanskii, V. V. Kryuchkov, I. E. Debrivnyi, V. I. II'chenko, and V. G. Tinyakov

AND

UDC 539.3/5:678

The p rob l e m of producing low t e m p e r a t u r e s in long-durat ion t e s t s is quite complex. Liquid ni t rogen is usual ly employed as a cooling agent for these pu rposes under l abora to ry conditions; this agent r equ i re s ut i l izat ion of complex and c u m b e r s o m e equipment . Regulation and s tabi l iza t ion of the t e m p e r a t u r e p re sen t additional diff icult ies [1-4].

Bear ing in mind the shor tcomings of such units, we built a s imple type of equipment cha rac t e r i zed by the use of t h e r m o e l e c t r i c a l semiconduc tor ba t t e r i e s for cooling the effect ive pa r t of the cryogenic cell , The opera t ion of these ba t t e r i e s is based on the Pe l t i e r effect [5, 6]. The expe r imen ta l equipment was de- s igned for studying c reep and r e c o v e r y a f te r r emova l of the load, the long- run strength, and re laxa t ion of the s t r e s s e s of magnet ic c a r r i e r s in the t e m p e r a t u r e range f r o m 0 to -8O~ The equipment cons is t s of the cryogenic cell and the s y s t e m for producing a negat ive t e m p e r a t u r e in this cel l , the loading device, a s y s - t e m for measu r ing and record ing the fo rces and deformat ions , a s y s t e m for record ing and automat ica l ly control l ing the t empe ra tu r e , and a control panel.

The cryogenic cel l I is a compact a luminum beake r of r ec tangu la r c r o s s sect ion and c o m p r i s e s the sample 2 held in the c l amps 3, 4 and the r e s i s t a n c e t h e r m o m e t e r 5,

N 22C

Fig. 1. Electromechanical scheme of the equipment.

Kiev Polyteehnical Institfite, Kiev. T rans l a t ed f r o m l~roblemy Prochnost i , No. 9, pp. 107-111, September , 1973o Original a r t ic le submit ted April 10, 1971.

�9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 ~'est i7th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. ,4 copy of this article is available from the publisher for $15.00.

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~220V

I o

Exits to the rectifiers of the thermobatteries

~ _ Contacts of r ~- ] thehydrorelay

Contacts of the I r i = ~ : ~ - ' i ~ f i I ternperatttre ~ _ ~ ~ indicator

t_ ~ L A A

aXScale in ~

,~Tir~, Regime Ii~ I~I L.O__iLoJ ~ I Lc~_.iL__~rA Regime !II r~egime II

Fig. 2. E lec t r ica l scheme of the device for controlling the tempera ture .

The effective volume of the cryogenic cell is cooled by three thermoelec t r ica l two-cascade semi- conductor batteries 6. The two-cascade thermobat tery connected in ser ies with the voltage suppIy com- p r i ses twenty thermoelements in the lower cascade, and four in the upper cascade. A thermcbat te ry is a simple cooling device which pumps heat f rom a lower (cold junction) to a higher tempera ture level (hot junction).

The hot junctions of the thermobat tery are fixed in the water-cooled heat exchanger 7, and the c ryo- genic cell is mounted on the cold junctions. The thermobat ter ies were so arranged as to ensure a mind- mum of heat res is tance between the cryogenic coil, the heat exchanger and the thermobat te r ies . The space between them was thermal ly insulated, and the entire sys tem was shielded by the plastic foam casing 8.

The temperature in the cell is automatically controlled by the recording MSR1-02 bridge with a th ree - posit ion control ler and kept within :~1~ by two the rmomete r s . The scale of the device is cal ibrated f rom +30 to -120~ The desi red temperature is set by two MSR1-02 indicators, which are mechanically con- nected to the contacts of the position control ler . The lat ter contacts s teer the operation of the executive device whose electr ical scheme is shown in Fig. 2.

The line vol tage of 220 V is supplied via a safety fuse and a switch to the t r ans fo rmer T r ! . The bulb L 1 indicates the presence of the line voltage. The voltage on the VS1~-33 rect i f ier which feeds the thermo- bat ter ies is supplied via the contacts K 1 and K 2. Bulb L 2 indicates the presence of voltage on the inputs of the rec t i f ier . The contacts K 1 and K 2 belong to the force re lays R t and R 2. The windings of re lays R 1 and R 2 are fed by the line voltage of 220 V via the contacts of the hydrore lay and the temperature indicators

Kmi n and Kma x.

When the water p ressu re in the main line suffices for cooling the hot junctions of the thermobat ter ies , the contact of the hydrore lay is closed and the voltage is supplied to the windings of the MKU-48 relay~ If the water p ressu re is insufficient, or water is lacking, the a la rm bulb L 3 ignites and the executive rel~tys are shut off. If the pointer of the tempera ture indicator is located between the pointers Tmin and Tmax, re lay R 1 is shut off (Kt in the right-hand position), and voltage is supplied to R 2. The thermobat ter ies are fed f rom the slide of an au to t ransformer (nominal regime I).

When the temperature in the cell r ises, the pointer moves to the right towards indicator Tmax, and closes the contact Kmi n (regime II). Relay R~ is thus put in operation, the full line voltage is supplied to the thermobat ter ies via the normal ly open contact Kt, and the thermobat ter ies operate at maximum output as long as the temperature in the cell does not attain its previous value (regime I).

Under regime III ( temperature below the fixed value) relay R 2 is shut off, contact K 2 is broken, and the supply of voltage to the thermobat ter ies is stopped. After that, the operation regimes of the control ler are repeated. Adjustment of the temperature control ler reduces to choosing the pa ramete r s of the current through the thermobat ter ies under the nominal operation regime I, which is achieved by appropr ia te r egu la - tion of the t r an s f o rm er Tr I and the step commutator in the VSP-33 rec t i f ie rs . Setting of the indicators to the

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0 ,25 /A- - , A--A --,--, / , - 2 0

f

o,z

0 12 24 J6 Zr 50 72 t,h

Fig. 3. Creep and recovery plots of the Pyral magnetic carrier at various tem- peratures.

minimum difference Tmax-Tmi n allows the temperature in the cell to be kept constant to an accuracy of :kl ~ in the range mentioned.

When the thermobatteries change over to regime I, the stability of the temperature control system is quite high. To attain lower temperatures (down to -80~ a liquid with a temperature of-30~ which was cooled in an autonomous system was used as cooling agent, instead of water.

To avoid condensation of moisture on the sample dur- ing prolonged tests, provision is made for purging the effec- tive volume of the cell with an inert gas.

The experimental procedure at low temperature goes as follows:sample 2 (see Fig. i) is fixed in the mobile 3 and immobile 4 clamps provided with the rigid rods 9, 10. Rod I0 of the immobile clamp projects from the cryogenic cell through the seal 11 and is connected with the dynamometer 12. The latter is rigidly fixed in the holder 13. The mobile

clamp is freed by means of the active bar 9 on the ca r r i age 14 which can be easi ly displaced s t r ic t ly hor i - zontally in ball bearing.s 15.

Sample 2 is loaded via the flexible element (rope) 16 which at one end is connected to ca r r i age 14 and at the other end hinged to the loads 18 via block 17. The shift of the ca r r i age is recorded by the deforma- tion indicator 19, which is a beam of uniform res i s tance ; two semiconductor tension gauges included in the bridge sys tem are glued onto the beam. The device, which consis ts of the m i c r o m e t e r sc rew 20, the guide rod 21, and two indicators 22 of the watch type (1 p scale divisions), war ran ts sufficiently accura te r e co rd - ing of the ca r r i age displacement, and permi t s cal ibrat ion of the deformation indicators .

To avoid breakage of the sys tem for measur ing the deformations during rupture of the sample, and also to c a r r y out experiments on the relaxation of s t r e s ses , the ca r r i age bar is provided with the stop 23 which is set in dependence on the maximum possible deformation.

In the sys tem chosen for measur ing deformations f rom the displacement of the active ca r r i age bar, the deformations of the coupling links and those produced by the tempera ture change are superposed on the sample. Since the rigidity of the loading sys tem is much higher (by a factor 103-104) than the rigidity of the mater ia ls examined, and the deformation produced by the change of the tempera ture can easily be taken into account during calibration, the displacement of the active bar ref lects the deformation of the tested par t of the sample to a high accuracy .

By numerous cal ibrat ions (a covar str ip fixed in the clamps, i.e., a str ip of mater ia l with known elast ic proper t ies) it was established that the sample-deformat ion indicator chain cor rec ted for a constant t empera tu re component leads to deformations measur ing 2-10 p (at effective loads f rom 0.2 to 10 kg and sample lengths between 50 and 200 mm).

Check tests with a ICM-7 cathetometer (1 # scale) yielded values almost equal to the indications of the indicator and the recorded data (the sca t te r did not exceed 1-3%)o

The creep and r ecove ry curves were recorded and the time during the long-run strength tests de te r - mined on the e lectronic one-second KSP-4 potent iometer by means of the e lec t r ic signals emitted by the semiconducting deformation indicators.

Visual inspection of the deformation was performed also by means of watch type indicators with refe- rence to the displacement of the active bar of the mobile clamp.

To record the forces acting on the sample, four semiconducting tensoresistors included in a bridge system were glued onto the ring-shaped dynamometer 12.

In studies on the relaxation of stresses and in those on the long-run strength, creep, and recovery of magnetic carriers, the signal produced by the change of the force is recorded on a KSP-4 potentiometer. The bridge systems used for recording the deformations and forces were fed by the source BSP-24/I of stabilized voltage via a voltage divider. To study the creep and recovery of magnetic carriers exposed to load pulses and the creep produced by vibrations, the test equipment was provided with an eleetromechani- cal system producing load pulses and containing an electronic control block.

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*,%

o.2o

0 , 1 2 . • 1 7 7 1 7 7 1 7 7 • , .-_

i �9

T-O~ ~ ,.t~ --r-- --a--

-gO

, -40

"~- ~ 0 ~ I

t f f

u~.,. . . . . . . . . I ' [ p = Pst =0.15 kg P=Pst +Pdyn

A

- j

l

{ P=Pst = 0.15 :!

i l l g! !i

O dO 160 240 ff2Ot, min 0 4 8 35 t ,h

Fig. 4. Plots represent ing the creep and recovery of the PE-31 mag- netic c a r r i e r under a static and, subsequently, under a pulsed load at var ious tempera tures (pulse of rectangular shape; duration 0.2 sec; 10,000 pulses).

The sys tem for producing load pulses consists of interchangeable weights, a plunger electromagnet , dampers , load counter, and a load pulse genera tor (multivibrator).

The magnitude of the static and pulsating loads is determined by the masses m 1 and m 2 of the weights. A fixed p rog ram of load pulses is executed as follows. The moment the voltage is applied to the winding of the electromagnet , rod 24 is moved downwards, and mass m 2 is abruptly brought to act on the sample which was previously loaded by the mass m 1. The next moment the rod ra ises mass m2, and the force on the sample is determined by the mass ms.

A load pulse genera tor operating under the 'autovibration regime executes the fixed p rog ram of load- ing-unloading cycles . The required frequency range of pulses and the time of loading and unloading can be fixed by means of the electronic control block. To avoid the appearance of vibration p rocesses in the s y s - tem exposed to load pulses, we employed the symmet r i ca l two-band liquid damper 25. Prac t ica l ly every des i red shape of the load pulse can be fixed by varying the viscosi ty of the liquid, or the rat io between the d iameters of disc and vesse l .

The signals emitted by the deformation and force indicators during pulse-wise loading were recorded on two ENO-I type oscillographs with prolonged afterglow. The pulse shape and the onset of vibrational processes were also recorded on an oscillograph.

The reported pulse loading system and the recording of the forces and deformations are quite uni- versal, for they permit the total (elastic, viscoelastic and residual) deformation to be determined not only during a cycle but also after a definite number of cycles. The program of pulse-wise loading was so set up as to warrant that the test would bear the closest possible resemblance to actual operation of magnetic carriers during starting and stopping of apparatus in which they are employed. The program of testing under pulse-wise loading includes the following four stages:

I) creep under the action of a constant load P = Psi = 0.15 kg during 48 h;

II) the action of a load pulse Pdyn = 0.3 kg; the total load acting on the sample then equals P = Pst + Pdyn = 0.15 + 0.3 = 0.45 kg. The duration of a pulse equals 0.2 see; time between pulses: 2.2 sec; rate of loading: 2 m/sec, I0,000 pulses of rectangular shape;

Ill) after 10,000 pulses the sample is exposed for 48 h to the load P = Psi = 0.15 kg;

IV) recovery of the sample during 48 h after the entire load has been relieved (P = 0).

The creep and recovery of magnetic carriers exposed at a constant temperature to a static load, or to a static load followed by load pulses, were studied in the equipment described.

1146

At a given load (0.05-0.5 kg) and t e m p e r a t u r e each magnet ic c a r r i e r type was r ep re sen t ed by 15 s a m - ples , the length of the tes ted pa r t s being 200 ram. Loading and unloading of the sample during the study on c reep and r e c o v e r y was done at a constant de fo rmat ion ra te equal to 2 m / s e c . The s amp le s were kept in the cell at the tes t t e m p e r a t u r e for 1-2 h. Typical c reep and r e c o v e r y curves for the magnet ic c a r r i e r P y r a l at the constant s t r e s s o" = 1.5 k g / m m 2 and the t e m p e r a t u r e s 0; - 2 0 ; - 4 0 ; - 6 0 ; - 80~ a re shown in Fig. 3.

The effect pu l se -wi se loading has on the c reep and r e c o v e r y of the magnet ic c a r r i e r PE-31 at t e m - p e r a t u r e s ranging f r o m 0 to -80~ is shown in Fig. 4. The c reep and r e c o v e r y cu rves taken during s tat ic and subsequent pu l se -wise loading made it poss ib le to invest igate the dependence of instantaneous, v i s c o - e las t ic , and res idual de fo rmat ions on the shape and nature of the acting load, t ime, and t e m p e r a t u r e . It was es tabl i shed that during the act ion of a s ta t ic load followed by pu l se -wise loading, the deformabi l i ty of the magnet ic c a r r i e r s examined i n c r e a s e s not iceably.

Io

2.

3.

4.

5. 6.

LITERATURE CITED

A. P. Klimenko, N. V. Novikov, et al., Cold in Engineering [in Russian], Mashinostroenie, Moscow (1969). N. V. Novikov, A. A. Lebedev, and F. F. Giginyak, Equipment for Mechanical Tests on Construc- tion Materials at Low Temperatures [in Russian], Naukova Dumka, Kiev (1968). Structural Properties of Plastics, edited by R. M. Shneiderovich [in Russian], Mashinostroenie, Moscow (1968). I. Hugo, I. I<abelka and others , P las t i c Construct ion Mater ia l s [in Russian], Mashinost roenie , Mos- cow (1969). M. S. Sominskii , Semiconductors [in Russian], F izmatgiz , Moscow (1961). P. S. Kireev, Phys ic s of Semiconductors [in Russian], Vysshay a Shkola, Moscow (1969).

1147