EFFECT OF GEOMETRIC THE MECHANICAL

CHARACTERISTICS

PROPERTIES

OF A SPECIMEN

OF POLYARAMIDE

ON

FIBRES

S. E. Kudryavtseva, V. V. Kovriga, and E. G. Lur'e

UDC 677.017.42:539.412

The possibility of evaluating the mechanical properties of polymer fibres from the results of testing complex yarns presents considerable interest, both for the use of fibres and yarns and also to characterize polymers in the oriented state. The effect of various geometric characteristics of yarns on deformation-strength properties has been examined in a number of researches [1-5]; however, the principles described in these relate only to the behavior of oriented rigid-chain polymers under a regime of active loading. In the present article, we discuss the features of the mechanical behavior of geometrically different specimens of complex yarns and monofilaments from polyaramide fibre SVM-K, prepared by spinning from solution with subsequent heat-treatment. Special attention was paid to the stress-relaxation regime. Complex yarns with a twist of 100 twists/m containing various numbers of monofilaments and SVM-K monofilaments were tested on an Instron-ll21 breaking machine under stretching regime at a deformation rate of 10%/min and stress-relaxation in the deformation range 1-3% at room temperature. The intensity of stress-relaxation was estimated from the coefficient of relaxation decay 1/15, defined as the ratio of the initial stress to the difference between the initial stress and the stress after 103 sec from the start of relaxation. Points on the curves correspond to average values from 5 tests in experiments on relaxation and 10 tests in continuous loading. Extension tests of SVM-K complex yarns having various linear densities showed that an increase in linear density leads only to a slight decrease in modulus and essentially does not affect fibre strength (Table 1). The elongation at break rises somewhat for yarns with a large linear density. Failure of elementary filaments in complex SVM-K yarns takes place almost simultaneously, and the extension curve usually does not have a sawtooth section or stress-decay at the end. A characteristic feature of the deformation dependences of the coefficient of relaxation decay (Fig. 1) of complex yarns is a departure of the dependences onto a plateau at deformations greater than 2% after a sharp decrease at smaller deformations. While this feature is a consequence of a specific deformation behavior of the material and not of the complex yarn as a bundle of fibres, a similar dependence should also be observed for a monofilament. To predict changes in stress during the relaxation process, one can use a very simple geometric model (Fig. 2). A difference in length of the monofilaments in a bundle should lead both to a decrease in degree of realization of strength and to a certain decrease in stress in the twisted complex yarn. Difference in length is essentially not inherent to a continuous yarn; however, it does arise in a specimen of finite length. The test procedure used in the present work assumes the use of specimens with a rather large base, which eliminates the generation of differences in length because of a "barrel" effect. Fixing the ends of the fibres in flat pneumatic damps with a preliminary small tension (since the filaments have a very high elastic modulus, a small tension does not make a marked contribution to the overall assigned deformation) to even out the specimen after fastening in the upper clamp does not lead to any additional variation in length. It may be considered that the difference in length ~o is connected basically with the geometric characteristics of the specimen and is defined as the ratio of the increase in length of fibre 2 (12) located along a spiral determined by the twist on the outside of the specimen as compared with the length of fibre 1 (11) located in the center of the bundle to the length of fibre 1: 12--11 11

l~ 11

1.

From the geometric model, it is evident that the ratio of the lengths 12 and 11 is proportional to the ratio of segments AB and CD, which may be defined as follows: AB = ~/d ~y~q- | / 4 K '~

CD = 1/2K

where do/is the diameter of the complex yarn and K is twist.

NII Plastmass, Moscow. Translated from Khimicheskie Volokna, No. 5, pp. 41-43, September-October, 1992. 394

0015-0541/92/2405-0394512.50 ©1993 Plenum Publishing Corporation

Then, q~=2K3/d~y+ I/4KZ--I - - - - ~ - - 1 or, if we express the diameter of the complex yarn via the diameter of a monofilament df

~=4~-

~,

(1)

where n is the number of monofilaments in the complex yarn. In [6] the experimentally determined difference in length of high modulus yarns was 0.1-0.3%. For the investigated SVM-58.8, SVM-29.4, and SVM-6 yarns, relationship (I) gives the values 0.15, 0.08, and 0.01%, respectively. In view of the fact that the stretching diagram of SVM-K yarns is approximately a straight line, the decrease in stress because of difference in length at an assigned deformation e0 is Ao=E (eo--4),

(2)

where E is the elastic modulus of the fibre. If the relaxation intensity at a deformation e 0 for a monofilament or monofilaments stretched to the maximum extent in a bundle is ~n ~

frO lOs max - - ~max

o~,,,

(3)

'

for the complex yarn, we have 0

10z

-- c'mia--Crmin 13~~ =

l + ~,

m/n'

O~f- i),

(4)

10 a

EO

~=~o-~

,ffml-

10° • "

ff:nlax~

(5)

As is evident from Fig. 2, at deformations greater than 2%, the intensity of relaxation of SVM-K yarns essentially does not change, and it may be considered that a = 1. In the deformation region of approximately 1%, already a small increase in deformation leads to a significant increase in intensity of relaxation and to an approach to the v a l u e cr103. F o r amin t03 -~- Crmax103, we obtain ~ =~o1(~o-~)

(6)

or

~°~='~ ~ o - V ~

+ *i

(7)

Equations (4) and (6) permit one to recalculate the deformation dependence of the coefficientof relaxation decay of a monofilament as a function of the coefficientof relaxation decay for twisted complex yarns. The calculated curves shown in Fig. 1 by the dotted line describe well the experimental data. However, one must bear in mind that the increase in frictional force in the twisted yarn exerts a definite effect on d e f o r m a t i o n - s t r e n g t h characteristics. In the general case, the frictional force which arises on contact and catching of fibres in a yarn is proportional to the contact area and to the normal pressure [1]. The contact area is determined by a function of the applied load, the twist, and the number of monofilaments. In this case, the total stress which arises in the complex yarn is given

by 'r

% = O ~ y + o r ( K , . , p),

(8)

where P is the applied load and a F is the stress which arises in the fibre because of frictional force. The intensity of relaxation allowing for friction is given by 13e~y=

0

o~d

,0, q - A f t F "

o~

(9)

395

T A B L E 1. D e f o r m a t i o n - S t r e n g t h Characteristics of S V M - K Yarns • Linear Number [v.mre ~., [[ Elastic •density, 'of filamentsluiameter' GPa tex I in yarn / #m i moddlus, 6,0 29,4 58,8

50 200 300

I0 15 15

Elongation at break, %

Strength, GPa

106±8 2,7±0,13 99_+_3 ' 2,5±0,25 89-+-4 2,5~--+-0, II

3,0___0,41 2,9~0,24 3,6+0,I 5

x[] /8

/0

¢

2

C0 , °/o

Fig. 1. Dependences of coefficient of relaxation decay on deformation: Experimental for SVM-58.8 (1) and SVM-6 yarns (2); for tows of 10 monofilaments (3); for tows of 5 monofilaments (4); for a monofilament (5); and calculated for SVM-58.8 yarns (6) and SVM-6 (7).

/////)/

, ////,/

f \

.

-I

.~2 •

.8 D • Fig. 2. Scheme of disposition of minimally and maximally stretched monofilaments in fastening a specimen of complex yarn in breaking machine: 1) length of fibre located in center of bundle; 2) length of fibre located along spiral.

396

1o 9 8

7 6

20

60

Zo ,ram

Fig. 3. Dependence of coefficient of relaxation decay 1/fl on clamped length of specimens 10.

Considering that ActF is small, we obtain

I

[~eY= t~ f'-- E (8o~ f - - q / ~

(lo)

From the proposed model, one can conclude that the action of the frictional force should lead to an increase in the coefficient flcy" But judging from the dependences given in Fig. 1, the effect of frictional force on the intensity of relaxation in polyaramide fibres is small. In the model, no account was taken of differences in modulus, which are the consequence of different conditions for precipitation in the inner and peripheral layers of the spun yarns. However, as is seen from comparing the experimental dependences (see Fig. 1), on the whole the total effect of various factors on the mechanical behavior of the complex yarn does not lead to significant changes in the relaxation dependences. A study of the dependence of strength on the length of specimen between clamps [3, 4] indicates the existence of a dimensional effect of strength for technical yarns. Because of this effect, full realization of fibre strength is possible only in specimens having a length greater than 200 mm. The reasons for the decrease in strength may be either a decrease in frictional and adhesive force, or also, defectiveness in the yarn. Obviously, both factors should also show up in changes in the intensity of relaxation on changing the sample length. In the case of overcoming frictional forces, a decrease in clamped length should lead to a decrease in their total magnitude and to an increase in intensity of relaxation in the specimen. As is evident from Fig. 3, for polyaramide yarns the defectiveness over the sample length turns out to be more important, and it may be considered that behavior during the relaxation process is determined by the features of the polymer itself and not by the complex yarn as a bundle of fibres. The relationship given shows that both for reliable determination of strength and also for determining values of the coefficient of relaxation decay, which practically do not depend on sample length, one should use specimens no shorter than 100 mm.

CONCLUSIONS -- The features of the deformation behavior of complex yarns from polyaramide fibres with different geometrical characteristics have been examined. -- It has been shown that the composition of the complex yarn essentially does not affect its deformation-strength or relaxation properties. -- The effect of specimen characteristics on the intensity of stress relaxation in a yarn has been determined.

397

REFERENCES

.

3. 4. 5. 6.

398

t~. A~ Nemchenko, N. A. Novikov, et al., Properties of Man-Made Fibres and Methods of Determining Them [in Russian], Khimiya, Moscow (1973). E. V. Lugert, M. N. Ivanov, and K. E. Perepelkin, Khim. Volokna, No. 3, 52-53 (1989). E. V. Lugert, M. N. Ivanov, et al., Khim. Volokna, No. 6, 26 (1989). E. Ya. Sorokin and A. D. Koldobskii, Khim. Volokna, No. 4, 44-45 (1984). K. E. Perepelkin, M. S. Shkolyar, et al., Khim. Volokna, No. 1, 47-49 (1987). L. V. Kompaniets, Author's Abstract of Candidate's Dissertation, [in Russian], Inst Khim. Fiz. Akad. Nauk SSSR, Moscow (1984).