Dynamic properties of nylon monofilament on Instron tensile strength tester

Transcription

1 Indian Journal of Fibre & Textile Research Vol. 23, March 1998, pp. 1-7 Dynamic properties of nylon monofilament on Instron tensile strength tester Someshwar Bhattacharya,Prakash Khatwani, Vijay Diwankatera, Mankodi Hireni & Deepa Changlani Textile Engineering Department, Faculty oftechnology & Engineering, M S University of Baroda, Vadodara , India Received 25 January 1996; revised received and accepted 4 August The effects of repeated load cycles, stress and rate of strain on dynamic properties of nylon monofilament have been studied using Instron tensile strength tester. It is observed that the load-extension hysteresis loop changes its shape with the change in number of cycles but tends to reach a limiting value after a certain number of cycles. Dynamic modulus, imaginary part of dynamic modulus and loss factor are more at a higher stress,the values increase sinusoidally but do not follow any definite trend. At higher number of cycles, the dynamic properties remain almost constant. Keywords: Complex modulus, Hysteresis loop, Loss angle, Nylon monofilament,strain amplitude, Stress amplitude 1 Introduction Dynamic properties of fibre/filament form an important aspect of mechanical behaviour. There are several methods employed for measuring the dynamic behaviour using different frequency range (Table I). Out of these methods, direct observation of stress-strain loop is one of the easiest and simplest methods. In the present study, an attempt has been made to assess the dynamic behaviour of nylon monofilament using cyclic loading method on Instron tensile stre~gth tester. Also, an attempt has been made to know the effects of repeated load cycles, stress and rate of strain on dynamic properties of nylon monofilament. 2.2 Methods The cyclic loading was imposed to the specimen and the stress-strain loop was recorded directly on the Instron tensile strength tester. The stress amplitude (am),strain amplitude(em) and the energy loss were simply found from the loop as shown in Fig.I. The energy loss in oscillation can be 2 Materials and Methods 2.1 Material Nylon monofilament of 20 denier was used for the study. Table I-Frequency Method Direct observation of stress-strain loop Free vibration Forced resonant vibration Direct observation offorced vibration Flexural resonance of specimen Velocity of sound waye, continuous Pulse velocity range for dynamic test Frequency range 1-10 Hz I-50Hz I-300Hz I-200Hz 20 Hz-IO khz 500 Hz-30 khz to-100khz RUIII,m mr mm Fig. l-idea1ised hysteresis loop in stress-strain test under repeated loading (a,.,-stress 8mplitude and em-strain amplitude)

2 2 INDIANJ. FIBRETEXT.RES.,MARCH1998 expressed by several inter-related parameters, like imaginary parts (E) of the complex modulus (E*), loss angle (0) and loss factor (tan 0). The complex modulus can be defined as Energy loss per radian= 1/2 am em sin ".. -CI.; 60 I/) UJ l% l I/) 40 STRAIN,1m m I min o 0 040, , )2 0'36 O'~O 4S 18 E*= I + ig/ tan 0 = Ff/g or g/ = g tan Stress-Strain/Load-Elongation Curve The stress-strainlload-elongation curve (Fig.2) of 20 denier nylon monofilament yam was plotted from twenty-five readings, which were taken on Instron tensile strength tester at 100 mm gauge length and 100 mm traverse speed Repeated Loading Test Instron tensile strength tester was used for repeated loading test. The minimlulj load was set at 5g (2.25 g1tex). The maximum load and the traverse speed were set according to Table 2. Fig.2 shows the points of loads/stresses for cyclic loading tests Energy Loss per Radian The hysteresis loops of stress-strain were obtained by running the chart at the respective cycle. The area ofthe loop was measured by digital planimeter and then converted into g1tex. (mm/min) of 20 o 4 B ' ELONGATION l m m Fig. 2-Load-~longation/stress-straincurveof 20 deniernylon monofilament;the points indicatethe differentstress values appliedduringrepeatedloadingtests [ g/tex; g/tex; g/tex; g/tex; g/tex; g/tex; g/tex; g/tex; g/tex; g/tex; g/texand g/tex] Stress and Strain Amplitudes From the hysteresis loop, the stress amplitude (am) and the strain amplitude (em) were also found out (Fig. I). The tests were carried out under standard atmospheric 3 Results and Discussion condition (65% RH and 27 ± ie). When loading and unloading cycles were repeated under some controlled standard conditions, the load-extension loop changed its shape, more and more slowly as the number of Table Upper limit set on the (mm/min) instrument Yeildduring of stress(%) repeated traverse loadingrate test at different traverse rates '\" The values are calculated from the breaking stress and yeild stress of the parent material,"1'1' ""1""'1" ", II~I'I'I'~!!"'"''I''"'' I" I'~I" 1'"'1'1' 1'''1''1 I '"I",. ""1 1;1"

3 BHA IT ACHAR YA et aj.: DYNAMIC PROPERTIES OF NYLON MONOFILAMENT 3 cycles increased, tending to a limiting form, which was reproducible in successive cycle. This was also observed by Leaderman2 These continuing effects are due to the viscoelastic behaviour of the filament. 3.1 Effect of Cycles on Dynamic Properties Tables 3-5 show the calculated values of E,E1 and tan () and Figs 3-5 show the graphical representation of these values. The characteristic sigmoidal variation of the real part of the modulus and the peak in the imaginary part are clearly apparent. It may be noted that similar effects are also observed in di-electrical properties. The way in which the changes in If, If' and tan () were taken place is shown by the representative curves in Figs 3-5. There are many structural features present in filaments. The filament deformation can broadly be divided' into following two mechanisms: (i) inherently elastic and tend to recover, and (ii) plastic and tend to lead the permanent deformation. When both types of mechanism act together, the actual extent of recovery will depend on how effectively the elastic deformation is pulling back the structure against the resistance of the plastic deformation. At room temperature, the whole system would be in rigid glassy chain but some of the chain Table 3-Effect of loading-unloading cycles on dynamic properties at 100 mm I min traverse speed No.of Stress Strain Loss Loss Dynamic modulus (E:) Imaginary part of dynamic modulus cycles amplitude amplitude angle factor (a.,} (e,,) (0) (lano) ( I') deg en/lex gllex en/lex gllex en/tex gllex SO {)J '

4 4 INDIAN J. FIBRE TEXT. RES., MARCH 1998 Table \ \ \ eN/tex factorangle \ gltex dynamic (0) Imaginary Effect ( cn/tex E' ( y -,Iii \ (tano)(in LossDynamic \ \ \ \ \ \ \ amplitude g/tex (0) g/lex Stress en/tex modulusdynamic \ \ angle Strain f1.23 g/tex amplitude (em) en/lex II) deg ) Imaginary of modulus ( PI) part loading-unloading modulus deg ofpart of cycles on dynamic properties at No. of No.of 200 mm I min traverse 500 mmspeed I ruin traverse speed ycles on dynamic properties at lill ~'lltlj 1111 ih'11,ii ii' " I ""II" """ I "I'! 'I~ I" 1'1'" I' 'I' 'nl'i',1"""'!"l"'", '''I'~'I' 1" 'I 'II II! r"';~ ''1'

5 BHAITACHARY A et at.: DYNAMIC PROPERTIES OF NYLON MONOFILAMENT no 500 Cdl ~50 ".. ::: G VI '3 150 :> o % i 300 «z,.. o 250 lbl ".-.!ZOOI... ~,- III :> oj i:: '50 o z Col 200 ISO 5 ~~~wso~~~~~~~=~~~ NO.OFCIClES 0""""",~ 510~~~50~~m~~~~~~*~ NO.OF CYCLES Fig. 3--Effect of number of loading-unloading cycles on dynamic modulus at 100 mmlmin traverse rate at different stress levels [a-a I-At gltex; b-bl gltex; C-CI gltex and d-dl gltex] Fig. 5--Effect of number of loading-unloading cycles on imaginary part of dynamic modulus at 100 mmlmin traverse rate at different stress levels [a-al gltex; b-bl gltex; c-ci gltex and d-d gltex] 0,' ; 01 Cll: o... v « III III ọ. 0 1 lal Cdl Ie I NO. OF titles " CCiI o I I I I I I I I I I 5 to 20 JO W IJO 200 ) SOO ' too IClOO Fig. 4-Effect of number of loading-unloading cycles on loss factor at 100 mm /min traverse rate at different stress levels [a-al-i1.93 gltex; b-bl gltex; c-cl gltex and d -dl g ftex] segments will become free and the material will act like a highly crosslinked material where the deformation is possible only by changes in bond lengths, bond angles, bond rotations and intermolecular spacings. The resistance to these deformations will be mainly from the internal energy i.e. entropy of the material. All these modes of deformation exist in the filament and play an important role in dynamic properties. If the crosslinking is dense enough, the short connecting chains will have very small effect and the change wm appear as minor transition within the glassy region. On the other hand, if the connecting chains are long, there may be a major transition. The loss tangent seemed to follow a little sinusoidal variation with the cycle, but no definite trend with frequency was observed Effect of Stress on Dynamic Properties From Tables 6 and 7, it is clear that the dynamic properties (E~ E" and tan 0) are high when the maximum load (stress) is adjusted at hi~er side. When higher stresses are applied, the disturbance of the structure may be such that,

6 6 INDIAN 1. FIBRE TEXT. RES., MARCH 1998 instead of a mere displacement to a new position in occur at the long chain molecules without loss in a minimum free energy trough, there is a passage continuity, whereas short chain of molecules over a free energy maximum to a new state. This is would lead to fracture. The crystal morphology a plastic deformation. One of the special features may be altered by disruption and recrystallisation of polymeric material is the presence of long chain or by disturbance of chain folds. Crystal defect and short chain molecules. Drastic change can may move under stress. In disordered regions, roperties Table cn/tex en/tex gttex Effect gttex Dynamic Stress of amplitude stress Loss (e) (tano factor Loss amplitude g/tex dynamic Imaginary Stress ( (am) II) Dynamic ( modulus(tan I) 'I, part of (am) on (E') 0) ) dynamic properties I at 100 mm I min No. of traverse cn/tex speed No.of at 200 mm I min factor "ili"i"llilll I' 'I'HII "' '" 'I I' 'IIf'1 I' 't I "I"~ II~I"''I''I I' '11"'11 'I"I"""!" '1""1'1'1 '"11'1"1"'1", 1"1' ~I '1"1'1' I,,I I; II I r ""~;.~ '! '1

7 BHA ITACHARV A et al.: DYNAMIC PROPERTIES OF NYLON MONOFILAMENT 7 glassy chains may lie forced into new conformations by bond rotation, chain may slip, break and reform. Furthermore, in considering how all these possible modes of deformation will operate in dynamic properties, there are three other complicating features. Firstly, many of these modes are time dependent. Secondly, fibre structures are anisotropic and so the direction of stress and strain must be considered. Thirdly, fibre structures are inhomogeneous and so the distribution of stress and strain must be considered. The explanation of the mechanical properties of a filament is a problem where the thermal fluctuation and the individuality of the molecules can take place. Where an elastic deformation involves a long-range movement of molecular segments past one another, there will be viscous drag (very similar to liquid viscous drag) slowing down the deformation. Similarly, a long chain crystal transition from a coiled chain to an extended chain, although dominated by reversible change between states at different energy levels, will be slowed down by viscous drag. All these possible modes of deformation playa major role in dynamic properties. 3.3 Effect of Rate of Strain It has been observed that the changes in dynamic properties are more dominant at a lower traverse speed where more time is involved. For all kinds of deformation, time is required and more deformation can be expected with more time. Therefore, the dynamic properties would be effected more at lower traverse speed compared to higher speed. 4 Conclusions In repeated loading-unloading cycles test under controlled condition, the load-extension hysteresis loop changes its shape, more and more slowly as the number of cycles increases, tending to a limiting form. Similarly, E: En and tan 0 remain almost constant at higher cycles. Therefore, it can be said that the dynamic properties are independent at higher frequency level. At higher stress, the molecular structural network would tend to plastic deformation due to the presence of long chains of molecules in polymeric material. The material would not lead to any loss of continuity due to the presence of long chains of molecules. However, the short chains of molecules would lead to fracture. These may be the reasons for the higher values of E', En and tan o. The loss factor follows a sigmoidal pattern, but no definite trend is observed. Dynamic properties change with the change in stress sigmoidally and also in increasing order. At lower traverse'speed, where more time is involved, dynamic properties are more dominant. Acknowledgement The authors would like to thank Dr D K De, Professor in Textile Engineering and Pro-Vice Chancellor of M S University, an~ Prof. V H Kapadia, Head, Textile Engineering Department; MS University of Baroda, for constant encouragement during the study: They also wish to. thank Dr R J Mistry, Director, TAIRO, for his valuable guidance. The dedication shown by the staff of the department and in particular by Mr. B B Shah and Mr. Dilipbhai Solanki for skilled analytical and graphical support is also acknowledged. References I Morton W E & Hearle J W S, Physical properties oftexti/e fibres (The Textile Institute,Heinemann,London),I975, Leaderman H, Elastic and creep properties of filamentous materials and other high performance polymers (The Textile Foundation, Washington D. C.), 1943, Hamburger W J, Text Res J. 18. (1948) Chaikin M & Chamberlain N H. J Text lnst, 46 (1955) T-25.