(33) Vol. 45, No. 8 (1989) 337

Transaction

(Received January 9, 1989)

FORMATION OF KINK BAND IN FATIGUE PROCESS OF NYLON 6 FIBER*

By Toshihiko Ohta

(Toyobo Co., Ltd., Research Center, 2-1-1, Katata, Otsu, Shiga, 520-02. Japan)

ABSTRACT

From the results of structural analysis on the formation of the deformation bands found in Nylon 6 fiber subjected

to appreciable cyclic stress in fatigue process, it is concluded that they are formed by kinking, so called, the kink

bands having the sharp line boundary at which melecular orientation changes sharply. Furthermore, the formation

and growth mechanism of the kink band have been studied based on the relationship between the fatigue behavior and

the formation of the kink band. The fine structure of highly drawn Nylon 6 fiber has been discussed from orderly

formation mechanism of the kink band.

1. INTRODUCTION

The fatigue in textile fibers is a particularly important

factor in tire cord, where the fibers are subjected in

their end uses to appreciable cyclic stress. Although

many investigations on fatigue in textile fibers have been

carried out, most of theme-s is confined to phenomenolo-

gical analyses. In order to analyze fatigue process of

textile fibers from the viewpoint of internal fiber struc-

ture, microscopic observations have been carried out for

a filament in nylon 6 tire cord fatigued by a running test.

Many deformation bands were found in the bending re-

gions of the filaments as shown in Fig. 1, whose growth

was inferred to cause the breakage of nylon 6 tire cords

in fatigue processes . Such deformation bands were also

observed in highly drawn fibers subjected to bending de-

formation by a disc tester and a tube tester for fatigue

tests The growth of this band is taken up as a mea-

sure of the fatigue phenomena in tire cord .

In this study, nylon 6 bristles is used as a model mate-

rial for a filament in tire cord , and the structural analy-

sis of the deformation bands formed during fatigue pro-

cess of the nylon 6 bristle is carried out with using of

Fig. 1 Microscopic photograph of a filament taken from

nylon 6 tire cord fatigued by running test.

optical and electron microscopies, and X-ray diffrac-

tometry.

The relationship between the growth of the deforma-

tion bands and the reduction of breaking strength by

fatigue is clarified.

2. EXPERIMENT

2.1 Materials

In order to analyze the changes in the fiber structure

induced by deformation, the highly drawn nylon 6 bris-

tles with diameter of 0.37 mm and 0.7 mm are used as

the specimen for experiments in stead of a filament in

tire cord. The deformation bands are formed in these

specimens by the following ways.

This paper had been reported at the 15th SPSJ

Annual Meeting (Nagoya , 1966) and the 3rd Internation-

al Symposium on Man Made Fibers (Kalinin , USSR,

1981).

338 SEN-IGAK1{AISHI(報 文) (34)

1) Nylon 6 bristles (0.37 mm diameter) were fatigued

with a disk tester at room temperature for 60 mi-

nutes under the conditions of the controlled strain

range from 10 to 15 % and the frequency of 1720

cycle/min.

2) Nylon 6 bristles (0.70 mm diameter) were inset in

rubber in close-fitting way, heated at the controlled

temperatures from 40 to 200 °C and compressed

along the bristle axis with a vise. By this method

the specimen is practically subjected to the bending

deformation.

2.2 Observations with optical microscope

Observations with an optical microscope were carried

out for obtaining the relationship between the macrosco-

pic deformation of the specimen and the formation of de-

formation bands. The angle between the propagating

direction of the deformation bands and the fiber axis was

obtained by measuring the rotation angle of a sample hol-

der set in the optical microscope. The change in molecu-

lar orientation at the boundary of deformation bands was

observed by a polarized microscope.

2.3 Observations with an electron microscope

Morphological chnages in the bristle surface induced

by deformation were observed with an electron micro-

scope using the surface replica method.

2.4 X-ray diffraction measurement

In order to obtain the bending angle of polymer chain

axis at the boundary of the deformation bands, X-ray dif-

fraction patterns were taken in the region where de-

formation bands were formed.

2.5 Measurement of breaking strength

The breaking strength of bristles fatigued by a disc

tester was measured at the controlled atmosphere of 65

% RH and 20•Ž with a tensile tester.

3. RESULTS AND DISCUSSION

3.1 Formation of kink bands

Fig. 2 shows the micrograph of nylon 6 bristle fati-

gued by a disc tester for 30 minutes at the controlled

strain ratio of 15 %. Similar deformation bands as

shown in Fig. 1 are obviously formed over several hun-

dred microns. The propagating direction of these bands

makes the angle of 35 degrees to bristle axis. A pola-

rized micrograph of this specimen is shown in Fig. 3.

Only the deformation bands are brightened and the light

intensity increases sharply at the one end but decreases

gradually at the other end of boundary in deformation

bands. This is understandable as the projection of these

bands to the bristle surface, which are considerably

propagating to the inside of the specimen. From this

fact it is considered that the molecular chain axis para-

llel to the bristle axis before deformation is bending

sharply at the boundary of these bands. It would be

concluded that these deformation bands are formed by

kinking, so cal led, kink band. Fig. 4 shows a polarized

micrograph of the cross section of this specimen. It is

found that kink bands are formed at the compressed side

in bending region and considerably propagating to the

inside of the bristle, which direction is nearly perpendic-

ular to the neutral plane on bending deformation.

The temperature dependenceof the formation of kink

band was investigated by compressing along the bristle

axis at the various controlled temperatures from 40 to

200•Ž. This result is summarized in Table 1. The

following informations are obtained from this experiment.

Fig. 2 Microscopic photograph of nylon 6 bristle fati-

gued by disc tester.

Fig. 3 Polarized microscopic photograph of nylon 6

bristle shown in Fig. 2.

(35) Vol. 45, No. 8 (1989) 339

1) Kink bands are easily formed at higher temperatures

in the same compression ratio.

2) Kink bands are difficult to be formed at lower

temperatures than 50 °C in the all compression

ratio, which would be considered as the critical

temperature for the formation of the kink bands.

3) The angles between kink band and bristle axis are

widely distributing over the angle range from 32 to

46 degrees and may be independent on the tempera-

ture of compression. The histogram of the prop-

agating direction of kink bands based on the Table 1

is shown in Fig. 5. There is a maximum peak at

the narrow angle range from 35 to 37 degrees

which is considerably different from the direction of

the maximum shear stress. From the above men-

tioned facts it would be suggested that the deforma-

tion mechanism of kink band is relating to the char-

acteristic fiber structure of nylon 6.

Fig 4 Polarized microscopic photograph of cross sec-

tion of nylon 6 bristle shown in Fig. 2.

Table I Temperature dependence on formation of kink band

340 SEN-IGAKKAISHI(報 文) (36)

Fig. 5 Histogram on the propagating direction of kink

band.

Fig. 6 Formation of crack along the propagating line of

kink band.

Fig. 7 Microscopic photographs on the broken end of

bristle at second stage (a) and third stage (b) in

fatigue process.

3.2 Propagation of kink band in fatigue process

The fatigue process of nylon 6 bristle during 60 mi-

nutes was investigated by the disc tester with the con-

trolled strain of ± 10 %. It is classified in four prop-

agating state of the kink band. Breaking strength of

bristles in each stage was measured by the tensile tester.

These results are summarized in Table II . In the first

stage up to 10 minutes of fatigue time the kink bands

can not be observed, but in the second stage from 10 to

20 minutes of fatigue time they are )rarely observed with

the short length. In the third stage from 20 to 40 mi-

nutes of fatigue time they are easily found with the

length of several hundred microns as shown in Fig. 2.

In the fourth stage from 40 to 60 minutes of fatigue time

the formation of crack along the propagating line of the

kink band is sometimes recognized, as shown in Fig. 6,

which may cause the remarkable reduction of breaking

strength of bristle. Fig. 7 shows two types of micro-

graphs of the broken ends of bristles ruptured in the ten-

sile test. In the first and second stage of fatigue proces-

ses the broken ends of bristles show the plane normal to

the bristle axis and the breakage of bristles does not al-

ways occur in the region where many kink bands are

formed. In the third and fourth stages of fatigue proces-

ses the broken ends of bristles begin to show the broken

plane along the propagating line of kink bands. From

these facts, it would be concluded that the remarkable re-

duction of breaking strength in fatigue process is not

caused by the formation of kink bands, but by growth of

kink bands up to the critical size for formation of crack.

The relationship between the fatigued time and the re-

sidual strength in Table II is given by the curve shown

in Fig. 8, which is similar to that of nylon 6 tire cords.

Considering the correlation of this fatigue curve with the

propagation of kink bands based on the above mentioned

facts, it would be deduced that the plateau region in the

fatigue curve corressponds to the propagating processes

on the formation and growth of kink bands until some of

kink bands grow up to the critical size for formation of

crack.

Table II Classification of fatigue process by propagation of kink band.

(37) Vol. 45, No. 8 (1989) 341

Fig. 8 Fatigue curve of nylon 6 bristle obtained from

Table II.

3.3 Growth mechanism of kink band

Fig. 9 shows an electron microscopic image of the sur-

face replica of bristle in the third stage of fatigue proces-

ses, in which the shadowing direction is parallel to the

fiber axis. There are some kink bands with a narrow

width of less than 0.5 K . Each of them has the sharp

line boundary, in which the streaks parallel to the fiber

axis are sharply kinking at the line boundary of them

(ref., Fig. 10). Although two larger kink bands, are

observed as if they intersect each other in the center of

this photograph, they are composed of four independent

kink bands having a different deformation (ref. , Fig. 10).

Because the step of two pairs of the large kink bands

propagated toward up and down has the opposite sign

(shade and sunny for the shadowing direction in Fig. 9)

and the direction of kinking is opposite in a pair of the

large kink bands propagated toward up (or down) as can

be seen in Fig. 9 (ref. Fig. 10). Therefore, it is consi-

dered that they are independently growing to the four

directions from both ends of the hollow normal to the fi -

ber axis. This is also supported from the fact that each

of small kink bands around the large kink bands has a

hollow and is growing from the one or both ends of a

hollow.

Moreover, the following informations are obtained from this photograph.1) The sign of step in every kink band coincidess with

that of the hollow.

2) The dimension of width in every kink band is suit-able for that of the step in the hollow.

From these facts it is considered that the width of kink

bands is determined at the first stage in the formation of

them and the growth of them mainly progresses along the line boundary of them.

Fig. 9 Surface replica of nylon 6 bristle fatigued by

disc test. (bristle axis t ).

Fig. 10 Model on the growth mechanism of kink band

in fatique process.

(a) hollow

(b) formation and growth of kink band from

both ends of hollow.

Considering the growth mechanism of kink bands

based on the above mentioned facts, the model as shown

in Fig. 10 is proposed. In the first stage of growth, a

hollow is formed in the normal direction to the fiber axis

by a cyclic compressing stress, which becomes to the

342　 SEN-1.GAKKAISHI(報 文)　 (38)

embryo for kinking. Then, the kink bands with the

same step as a hollow are formed from the one or both

ends of the hollow and grow along the line boundary of

them. Therefore,- it would be considered that the large

hollow shown in Fig. 9 is formed by gathering of two

neighbor hollows after the formation of them. Moreover,

the growing directions of them would be affected by the

shape of the hollow's ends as shown in Fig. 10.

3.4 Deformation mechanism of kink band

The surface replica of a kink band is shown in Fig.

11. The sharp line boundary is observed, which makes

the angle of about 35 degrees to the fiber axis. The

streaks parallel to the fiber axis are bent sharply at the

line boundary of the kink band with the bending angle of

about 45 degrees, but they are parallel each other within

the kink band. This fact is consistent with the change

in the crystalline orientation obtained from the X-ray dif-

fraction pattern in the same region of this specimen.

The arrow in Fig. 12, shows the diffraction spot from

(002) plane within the kink bands which makes the angle

of 45 degrees to that before kinking. The shape of this

spot indicates that the degree of crystalline orientation is

scarecely changed within or without them.

Fig. 11 Surface replica on a kink band.

(bristle axis 1 )

Fig. 12 X-ray diffraction pattern from kink band, which

is shown with the arrow. (btistle axis t )

By Orowan'> the deformation mechanism of kink bands

formed in the single crystal of zinc or cadomium has

been explained from the relationship between the slip

plane in single crystal and the propagating direction of

kink bands as given by the following equation (1).

(1)

where C is the period of crystal lattice, d is the spacing

of slip plane and 6 is the angle between the slip plane

and the axis normal to the propagating line,

Equation (1) is applicable only when the bending de-

formation of the slip plane holds the mirror of symmetry

element for the boundary plane of kink band. Then,

equation (1) can be extended to equation (2) for applica-

tion to the deformation mechanism of the kink band

formed in nylon 6 bristle.

(2)

n, m : integer

where w is the bending angle of slip plane and equation

(2) becomes equation (1) at ƒÃ=2ƒÆ.

In the case of nylon 6 the hydrogen bonded sheets are

adopted as the slip plane, so that the value of d equals

3.70 A. The values of C and Į , equal 17.24 A (fiber

period) and 55 degrees, respectively. Substituting these

values into equation (2), each value of w calculated by

Table M Relation between ƒÃ and n. m, in equation(2).(at ƒÆ =55•Ž)

(39) Vol. 45,No. 8 (1989) 343

the various combination of m and n is shown in Table Hi.

Since the observed value of ƒÖ is about 45 degrees as

mentiooeA; above, it is found that the deformation

mechanism of the kink band in nylon 6 bristle corres-

ponds to that of n=1, m=4 in Table III. In other word, it

can be indicated that the four hydrogen bonded sheets

participate in one unit of deformation with formation of the kink band. The systematic deformation occurs ev-

ery four sheets. as shown in Fig. 13. It is reasonably

explained by this :mechanism that the degree of crystal-

line orientation within the kink bands is scarcely dis-

turbed and the line boundary of the kink bands is sharp-

ly formed.

Fig. 14 shows a macroscopic model of the formation of

the kink band in nylon 6 bristle. The slip of hydrogen

bonded sheets gradually progresses owing to the cyclic

compression in the bending region of bristle, so that the

hollow normal to bristle axis is formed and then kink

bands are formed to the direction as expressed in equa-

tion (2) with the same step as that of hollow.

3.5 Fine structure of nylon 6 fiber

As can be seen in Fig. 9 the kink bands are formed

with the orderly deformation having a sharp line bound-

ary and a specific sign of step. Further, their propagat-

ing line can be explained by equation (2). These facts

are difficult to be explained by the two phase model for

the fine structure of nylon 6 fiber having crystallinity of

30-40 %, crystallite size of several 10 A and long

period of about 100 A. Therefore, it would be consi-

dered that amorphous phase of highly drawn nylon 6 fi-

ber is composed of . highly oriented molecular chain,

which is scarcely distinguished from the regularity of

molecular orientation in crystalline phase, i. e., the fine

structure of highly drawn nylon 6 fiber for tire cord can

be treated as the monophase structure.

Fig. 15 shows the striped pattern observed in the

forming region of kink band, which may be Moire

fringes. If the range of that magnification is about 10-

20, it is suggested that the periodic structure from 0.5 -to

1.0 u exists in the fiber structure of nylon 6. However,

the further investigation on this structure is necessary.

Fig. 13 Deformation mechanism of kink band.

Four hydrogen bonded sheets participate as one

unit of deformation.

Fig. 14 Macroscopic model on the formation of kink

band.

4. CONCLUSION

Structural analyses on the formation of deformation

bands in fatigue process of nylon 6 fiber are carried out

by using the bristle as a model material for a filament in

tire cord.

Fig. 15 Striped pattern in the forming region of kink

band.

344　 SEN-IGAKKAISHI(報 文)　(40)

The following results are obtained.

1) Deformation bands are formed by kinking, so called,

kink bands. Each of them has the sharp line

boundary at which molecular orientation changes

sharply.

2) Remarkable reduction of breaking strength in fatigue

process is mainly caused by the formation of crack along the propagating line of the kink band, which

has grown up to critical size for cracking.

3) Each of the kink bands has a hollow normal to the

fiber axis which becomes the embryo for kinking.

The growth of them arises from the edge of hollow

with the same step as that of hollow.

4) Deformation mechanism of the kink bands is similar

to that of twin crystal, which can be explained on

the bases of Orowan's equation.

From the orderly formation mechanism of kink band it

would be considered that the fine structure of highly

drawn nylon 6 fiber can be treated as monophase struc-

ture.

REFERENCES

1) W. G. Klein, M. M. Pott, W. J. Hamburger; Text. Res,

J. 32, 393, (1962)

2) W. J. Lyons; Text, Res. J. Res. J.,32, 750, (1962)

3) W. J. Lyons; ibid., 32, 448 (1962)

4) I. Okamura, S. Fujimoto; Zairyo Shiken, 7, 63 (1958)

5) H. Noshi, H. Fujimoto, T. Yoshida; Senikikai Ga.l-

kaishi, 15, 46 (1961)

6) T. Ohta; unpublished

7) E. Orowan; Nature, 149, 643 (1942)

ナ イ ロ ン6繊 維 の疲 労過 程 にお け る

kind bandの 形 成

東洋紡 ・総合研究所 太田利彦

疲労過程で くりかえ し応力 を受 けたナイロ ン6繊 維に

発見 された変形帶の形成に関する構造解析の結果から,

変形帯がkinkingに よって形域 されており,分 子配向が

鋭 く変化 している鋭いline boundaryを もつkink band

で あ る ことが結 論 された。 さらに,疲 労挙動 とkink

bandの 形成 との蘭係, kink bandの 生成～成長機講につ

いて考察 し,そ の秩序正 しい彩成機構か ら,高度 に延伸

されたナイロン6繊 維 の微細構造についても推察した。