A S T A N D F O R I N V E S T I G A T I N G T H E C R E E P AND L O N G

T I M E S T R E N G T H OF M A G N E T I C T A P E T Y P E C O M P O S I T I O N

F I L M S AT H I G H T E M P E R A T U R E S

E . S. U m a n s k i i , V. V. K r y u c h k o v , UDC539.3/5 :678 I . E . D e b r i v n y i , V. I . I I ' c h e n k o , a n d V. I . T i m y a k o v

Fig. 1. General view of the twelve section experimental stand.

In various equipment of the new technology, including computers , p rogramming control lers , and sound and video r eco rde r s , polyethylene terephthalate magnetic tape has found wide use as a mater ia l for r eco rd - ing, storing, and supplying information [1-3]. Theopera t ingcondi t ions of magnetic tape are charac ter ized by heavy loads over a wide time and tempera ture range which causes significant deformation and therefore the distort ion of less of the recorded information.

As a result the problem of experimental ly studying the creep and long time strength of these mater ia ls at the operating loads over a wide range of t empera tures is especial ly press ing.

It is well known that c reep and long time strength tests differ in the work required and the length of the test . To save time and costs mult isect ion equipment is used. However, in test ing both soft and stiff polymer mate r ia l s the a l ready available mult isect ion equipment is not direct ly applicable to testing the creep and long time strength of magnetic tapes because of the peculiar i t ies of the mate r ia l being tested [4].

In the Department of the Strength of Materials of Kiev Polytechnic Institute we have built a twelve section stand and developed a method for creep and long time strength testing of magnetic tapes over a wide t ime and temperature range.

As a result of the purposes and methods of the tests , in building the stand it was assumed that it must provide the following:

1) the possibili ty of testing direct ly samples of mag- netic tape 3.8 to 50 mm wide and 25 to 250 ram long;

2) reliabil i ty and accuracy in fastening the samples in the clamps;

3) uniform heating over the length of the gauge length of the sample;

4) quick and smooth application and removal of the load on the sample;

5) automatic maintenance and control of the specified positive tempera ture with an accuracy of • 1~ for long period of time;

The Kiev Polytechnic Institute. Transla ted f rom Prob lemy Prochnost i , No. 5, pp. 103-107, May, 1973. Original ar t ic le submitted November 30, 1972.

�9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 Test 17th 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 ~tny means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

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Fig. 2. E l e c t r o m e c h a n i c a l p l an of one of the s e c t i o n s of the s t a n d .

r a i s i n g the load i s r e m o v e d in Studying r e c o v e r y .

6) reliability and quite high accuracy in mea-

suring the deformation and load.

The experimental stand (Fig. 1) consists of

twelve vertical similar sections in a common thermal

chamber, a mechanical system providing smooth

application and removal of the load, a system for con-

trolling and automatically maintaining the specified

temperature, and a multichannel system for measuring

and recording the deformation. The electromechanieal plan of one of the sections of the stand is shown in

Fig. 2. The sample 5 is fastened in the upper 9 and

lower 3 clamps, the design of which mades it possible to reliably fasten very thin samples with thicknesses

of 0. 003 to 0.07 mm and also to control the clamping f o r c e .

With the help of the guides 8 and the flexible rod 12 the upper moveable clamp 9 may move in a vertical

direction. The lower stationary clamp is seated on

the rod 2, which is rigidly connected with the ring

dynamometer i. which measures the load acting on

the sample. The load is applied to the sample 5

through the flexible cable 12, one end of which is fastened to the clamp 9 and the other of which is

fastened in a hinged manner through the block 14 to the balance rod 29 and the weights 27. which are located

on the moveable rod 26. The rod is lowered with the

help of the four entry screw 25. which quite quickly

and smoothly applies the load in studying creep and by

Such a system of applying the load makes possible creep

testing with a constant load. In this case the stress on the sample increases constantly as a result of the

decrease in the cross section of the sample.

For creep testing with constant stresses special interchangeable paired blocks 17 are used. One of

the blocks, the shaped one, automatically decreases the load on the sample proportionally to its change in

cross section. Assuming that the volume of the sample during deformation remains practiaal[y constant

and that the sample elongates uniformly over its length, the profile of the shaped block is designed based

on the relationship [5]:

R R0 T -- r(1 +8) '

w h e r e e i s the r e l a t i v e e longa t i on of the s a m p l e , r i s the r a d i u s of the round b lock to which the u p p e r c l a m p is f a s t e n e d t h rough the f l ex ib l e e l e m e n t , and R 0 and R a r e the p r o j e c t i o n s of the r a d i u s v e c t o r on the h o r i - zon ta l ax i s a t the s t a r t i n g and a s s i g n e d p o s i t i o n s of the s h a p e d b lock .

The c a s i n g of the t h e r m a l c h a m b e r 11 i s a r i g i d r e c t a n g n l a r desig-a which r e s t s on the heavy s u p p o r t s 24 and serves as a supporting element for mounting each section. The thermal chamber I0 is made of heat

resistant material with internal thermal insulation. In the working area of the chamber, divided in the

vertical direction in two halves by the heaters, are located the twelve samples with the clamps, six samples on each side. The heating elements 6 are spirals wound on porcelain insulators. A thin screen of copper foil 7 protects the sample from thermal radiation and aids in creating a uniform temperature field around

the sample. The fan 19 is used to circulate the heated air in the chamber. On both sides of the chamber

there are the covers 18. For observation of the samples during testing the covers are equipped with the

observation windows 20. The temperature is maintained constant over the whole volume of the chamber

and is regulated with an automatic servo system. The temperature sensor is the copper resistance thermo-

meter 4, which has twelve coils located uniformly over the volume of the chamber. The temperature is recorded and regulated by an EMP-109IMZ electronic bridge operating together with the electromechanical

servo system (Fig. 3).

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Thermal resistance

j j J J ~

f

'R3 �84

ltage.

I rap.an@ circuil

J /

t

. ? 7 v

[ Sw s

Fig. 3.

heater

Sw 2

Functional plan for tempera ture regulation.

The slide of the potentiometer I~ is connected with the mechanism for movement of the ar row of the EMP-109IMZ potent iometer and the winding with the mechanism for movement of the temperature sensor . The potent iometer P~ is connected to a bridge consisting of the r e s i s to r s R 2 and R 3. The voltage taken from the slides of these r e s i s to r s is fed to the voltage compar i - son circui t . The slide of the r e s i s to r R 2 serves for manual control and R 3 is connected with the SD-2 motor of the p rogramming unit.

The voltage compar ison c i rcui t supplies to the motor control c i rcui t a signal to the amount and the sign of the difference between the specified and actual t empera tures in the working a rea of the chamber.

The control c ircui t determines the operation of the RD-09 motor , which moves the slide of the auto- t r a n s f o r m e r supplying the hea ter with a rate p ropor - tional to the value and sign of the difference signal.

If for any reason the tempera ture in the thermal chamber changes with constant power in the heater the RD-09 motor is turned on, which changes the voltage in the heater to res to re the previous tempera ture .

In case of a change in the circuit voltage with a constant tempera ture the tempera ture regulator operates in a s imi la r manner . The average power in the heater is set by the potent iometer tt 2 or R 3 (with p rog rammed control). Long use of the tempera ture regulator has shown that it provides reliable continuous operation for 1000 to 1500 h. The accuracy in maintaining tempera ture is • I~ for a 20 to 200~ scale and may be increased by choosing a na r rower tempera ture range such as 50 to 100~ In addition to study- ing c reep and long time strength, this experimental stand may be used for making short time strength and relaxation tes ts . For this purpose each section of the stand is equipped with special clamps which are located on the movable screw stage 25. The screw is rotated by a direct cur rent moto r and reducer .

To r eco rd the force t ransmi t ted to the sample in studying short time strength and relaxation and also for determining the s ta r t of operat ing conditions in creep and recovery tests there are cemented to the ring dynamometer 1, which is in the load network, four s imi la r semiconductor s t ra in gauges which are in a bridge circui t . The high coefficient of s t rain sensit ivity of the gauges makes it possible to record a change in load directly on the recording device without an intermediate amplif ier , which increases the accuracy and reliabil i ty of the measuremen t s . The bridge circuit is supplied f rom a BSP-24/1 source of stabilized voltage. To change the range of forces measured f rom 0.5 to 25 kg and consequently the value of a division and the recording scale the voltage divider 15 is used (Fig. 2).

For each section the load is recorded on an N-700 osci l lograph and simultaneously on an EPP-09M3 electronic potentiometer .

Fo r high accuracy and reliabil i ty the deformation of the samples is r ecorded in the following ways.

1. The initial portion of the c reep and recovery diagram (t = 0 .1-30 see) is r ecorded on the N-700 loop osci l lograph. The signal for the change in load f rom the ring dynamometer is fed to the input of one of the loops of the osci l lograph and the signal for deformation f rom the elast ic elements 21 to the input of the other loop. One end of the elast ic element is fastened stat ionari ly and the deformation from the head of the ICh-10 indicator is t ransmi t ted to the other .

The t imer establ ishes conformity of the load and the deformation at the given moment of the test . The sensi t ivi ty of the loop osci l lograph is chosen so that the initial portion of the c reep diagram is recorded with high accuracy and to the neces sa ry scale in applying and ' removing the load. The instantaneous de- formation in applying and removing the load is a very important charac te r i s t i c of the tested mater ia l since this makes it possible to judge the elast ic proper t ies of magnetic tapes operat ing under various load condi- tions.

2. To obtain the full c reep and r ecove ry diagram in the case of large deformation use is made of the slide wire sensor 13, which is a constantan wire wound on an insulated rod which is fastened on the per imete r of the disc.

627

% i 7=8t)~ 0 - -

= :

o - - 1

o,+! , i

L7 12 Z~ 35 O0 60 72 r r, h

Fig. 4. Creep and recovery diagram for Pyra l magnetic tape (cr = 1.2 kg/mm2).

The disc rotates as a result of the movement of the moveable clamp. The signal of the amount of de- formation is received by a s i lver contact which slides on the slide wire.

The sensor is connected in a bridge circui t in the diagonal of which is connected an EPP-09M3 electronic potentiometer . Balancing of the bridge circui t (establishment of the zero reading) is done with the res is tance R b. This method makes is possible to reliably record the accumulation and drop in defor- mation af ter 5 sec of loading with an accuracy up to 5#x. It is especial ly sensitive with large deformations and significant ra tes of creep. These two methods automatically r ecord the initial portion and the total c reep and recovery diagram of the investigated m a t e r - ials.

3. For visual measurement of the deformation of the sample and calibration of the semiconductor deformation s t ra in gauges mechanical movement gauges, dial gauges, are used.

Movement of the upper clamp is recorded by two type ICh-10 or ICh-50 dial gauges depending upon the amount of deformation. These are fastened in the special holders 28. The force on the jaw of the indi- cator is t ransmit ted f rom the balancing weight. The indicator has coarse (movement of the rod 23) and fine (using the m i c r o m e t e r screw 22) adjustment and is located so that it may measure the elongation of the sample (creep) during application of the load and recovery during removal of the loado

4. The deformation of the sample may be measured on the basis of the distance between reference points on the sample using the cathetometer 30 sighted through the quartz observation windows 20. The agreement of the deformation measurements by these methods is checked by the cathetometer 20 sec af ter application o r removal of the load.

These methods of measur ing deformation provide high accuracy and reliabili ty in measur ing both in the initial moment of application and removal of the load and during a long time interval.

It should be mentioned that the dimensions of the thermal chamber make it possible to make tests on large samples . This is very important for magnetic tape, the proper t ies of which cannot be studied on mic rosamples . The comparat ively great length of the samples, up to 500 ram, pract ical ly eliminates the influence on the total deformation of the end effect, the elongation of the mater ia l in the clamps, and also increases the accuracy in measur ing relative deformation.

This unit has been used to study the creep and recovery of different types of magnetic tapes [4] with constant loads ( s t resses) . In choosing the types of magnetic tapes it is neces sa ry to study in detail the mater ia ls of pract ical interest .

Since the magnetic tapes themselves are used as test samples , the resul ts obtained are direct ly charac te r i s t ic of the proper t ies of the tested mate r ia l s .

The magnetic tapes were tested with loads of 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 kg and temperatures of 20, 40, 60, and 80~ for 48 h. The recovery af ter removal of the load was observed for the same time and tempera tures . The test p rogram was set up taking into account the operating tempera tures and loads most commonly encountered under actual service conditions.

Thirty samples of each type of magnetic tape were tested for a given load and tempera ture . The gauge length of the samples was 200 mm. The load was applied to and removed f rom the samples with a constant rate of deformation of 0.25 m / s e c .

The samples were f i rs t held in the thermal chamber at a constant tempera ture for 1-2 h. The r e - sults of a pre l iminary experiment showed that in this time the samples are completely heated to the speci- fied t e m p e r a t u r e . Figure 4 shows typical creep and recovery- curves for Pyra l magnetic tape in a tem- perature range of 20 to 80~

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The stand built and the method developed make it possible to study the rules of accumulation and de- crease not only of total but also reversible (elastic and highly elastic) defomation which for the investigated materials has great value.

1.

2.

3.

4. 5.

LITERATURE CITED

i. E. Goron (editor), Problems in the Magnetic Recording of Electrical Signals [in Russian], "Svyaz" (1970). V. G. Korol'kova (editor), The Theory and Techniques of Magnetic Recording [in Russian], "Mir ," Moscow (1968). V. G. Makurochkin, Magnetic Recording in Computer Technology [in Russian], "Sovetskoe Radio," Moscow (1968). E. S. Umanskii, I. E. Debrivnyi, and V. V. Kryuchkov, Probl. Prochnosti, No. 5 (1972). V. E. Gul', The Structure and Strength of Polymers [in Russian]," Khimiya," Moscow (1971).

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