Fibers and Polymers 2014, Vol.15, No.3, 605-613

DOI 10.1007/s12221-014-0605-1

On-line Interferometric Study on the Mechanical Fracture Behaviour by Crazing Observed in Stretched Polypropylene Fibres T. Z. N. Sokkar, K. A. El-Farahaty*, and A. A. S. Azzam Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt (Received December 29, 2011; Revised April 25, 2012; Accepted May 28, 2012) Abstract: Two-beam polarizing light interference microscope devised by Pluta is used for in situ investigation of fracture mechanism of as-spun isotactic polypropylene (iPP) fibres by crazing during cold drawing process. The study includes characterization of crazing of polypropylene fibres (involving craze initiation, craze propagation and craze breakdown) as a function of crazing strain, crazing temperature and stretching speed. The investigation of craze damage showed that it is increased rapidly with stretching speed and then increased slowly to level off. Also it was found that, stretching iPP fibre at relatively low speed (lower than 0.015 cm/sec) at constant temperature 19 oC would reduce the effect of fracture on the stretched filaments. The time to crazing, the time to failure and the areal craze density of iPP fibres during cold drawing process are estimated. Finally, observing the craze formation at different temperatures showed that there was a critical value of stretching temperature for the formation of crazes in iPP fibre, and it was found to be 40 oC. The obtained results are correlated to the corresponding variations in some optical and structural properties of iPP fibres due to stretching. Keywords: Crazing, Isotactic polypropylene fibre, Cold drawing, Stretching speed

The next stage is the formation of cracks, which is termed nucleation or initiation. This phase consists of the material gradually deforming, until such times as the molecules of the material have become sufficiently dislocated to the extent that recognizable defect or crack has formed. Rates of crazes initiation and growth depend strongly upon the applied stress conditions [10-12]. Drawing conditions are important variables in the crazing process as reported by Muzzy and Hansen [13]. The crazing behavior of polymers has been studied via many different experimental techniques. The most often used are the scanning electron microscope (SEM), transmission electron microscope (TEM) [14], optical interferometric measurement [15,16], low angle electron diffraction (LAED) and small angle X-ray scattering (SAXS) [17,18]. Normally, these experimental techniques are used to investigate the craze microstructure. The interferometric methods have been used to study the opto-thermal and opto-mechanical properties of natural, synthetic and optical fibres. Two-beam interferometric technique is common technique applied to study the optical properties of polymeric fibres during extension and also be used to detect the growth of microcracks deformation in iPP fibres stretched at different draw ratios [19-23]. Determination of the optical properties, e.g. birefringence of iPP fibre, gives information about the variation of structural properties; e.g. the orientation of the molecular chains enables one to characterize the behaviour of iPP fibres during different stretching processes. How material deforms before break is one of its most important characteristics. This work aims to use two-beam polarizing light interference microscope devised and applied by Pluta [24-26] for the investigation of the of mechanical fracture behaviour of iPP fibres, a semi-crystalline polymer,

Introduction Fracture of polymeric fibres is a consequence of the accumulation of several cracks. The initiation of crack occurs across the fibre’s cross section instead of being restricted to the near surface regions. This would cause the crack to initiate mainly at the fibre core [1]. On the other hand, microcracks are observed during the drawing process of oriented polymeric fibres. It was noted that, when the microcracks reach a critical length they propagate catastrophically the structure of the drawn samples [2]. It has been found that, in semicrystalline materials, cracks are always preceded by crazes which are caused by voids nucleation during the stretching of these materials [3]. Crazes are observed as thin planar defects that strongly reflect and scatter the light and they grow along the plane of maximum principle stress and runs ahead of crack during the stretching process of polymeric fibres [4]. Crazing phenomenon is an important fracture mechanism of polymers and is associated with the crack propagation, which represents the first stage of the fracture process of drawn fibres [5,6]. The craze initiation, craze growth, breakdown of craze fibrils, the initiation and propagation of microcracks and ends in macroscopic failure induced by microcrack cascades and macrocrack propagations. Crazing can, therefore, be considered as precursors of cracks and, if stress becomes sufficiently high, a craze is able to transform into a growing crack through the progressive breakdown of craze fibrils, leading ultimately to failure of the material [7-9]. There are a number of stages of crack growth. Initially, structure may contain no defects and this is called the dormant phase. During this stage the crack does not exist. *Corresponding author: [email protected] 605

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by crazing during cold drawing process in order to prevent or control crazing in the sample. The study includes the following procedures: a. Characterize the crazing of iPP fibres (involving craze initiation, craze propagation and craze breakdown) as a function of crazing temperature and stretching speed. b. Estimate the time to crazing and the time to failure (denoting the degree of safety) of iPP fibres during cold drawing process. c. Estimate the normalised areal craze density which is a measurement of the crazing damage. d. Correlate the obtained results to the corresponding variations in some optical properties and deformation structures observed by means of interferometry during straining of iPP fibres.

Material The tested sample was as-spun isotactic polypropylene (iPP) fibres manufactured in Leeds University. The raw material is isotactic polypropyelene (iPP). The as-spun fibres/ filaments were extruded at take up speed=100 m/min, spinning temperature profiles (5 zones extruder) = 230, 240, 245, 245, 245 oC, pump speed=15 rpm, spinneret (D/L)=3.2, spinneret hole = 20, and diameter of orifice (hole) = 0.8 mm.

Experimental Techniques The investigated sample material was as spun isotactic polypropylene (iPP) fibres. A single fibre of iPP sample was strained until crazing initiation, and then craze propagation was observed and recorded in real time using a CCD camera. Capturing images from the same locus of iPP fibre as a function of strain up to about rupture allows in situ observation of the mechanical fracture process. Pluta polarizing interference microscope in transmission sited in its uniform field mode [26], (where the tested sample of iPP is viewed in cross polarized light), was used to investigate the fibre morphology during stretching process up to rupture. The optical parameters were determined using Pluta polarising microscope in the uniform field mode connected with an automatic stretching device [27] designed for this purpose. In this work the design of the stretching device was modified. The stretching device consists of two step motors, M, rotating in reverse direction as demonstrated in Figure 1. In this version of the device we used stepper motors with resolution = 200 step/revolution and smaller diamaters (1.2 cm for each), to increase the spatial resolution of the original version of the stretching device. The iPP fibre sample, F, was fixed between the two stepper motors, M. Stretching of the fibre takes place when the two motors, M, rotate with the same speed in opposite rotation directions. The motors, M, were controlled using a suitable interfacing circuit, I, connected to the parallel port of the PC and driven

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Figure 1. Schematic diagram of the Automatic stretching device: (M) Two Stepper-motors rotating in reverse direction, (F) Fibre sample, (H) Heater, (T) Thermocouple, (C) Temperature Controller, (S) Stage device, (K) Breading board, (I) Stepper-motors controlling, (P) Captured Pattern, and (D) Controlling software, cf. ref. [27].

using software, D, designed for the purpose of controlling the two stepper motors, M, and recording the various strain values in their real time during the stretching process up to about fracture. Also a temperature controller, C, connected to a heater, H, using a thermocouple, T, was attached to the system to control the temperature of the fibre specimen during the stretching process. On the other hand, Pluta polarizing interference microscope in its non-duplicated image mode connected with the automatic stretching device was used to investigate the effect of crazing on the optical properties of iPP fibres during stretching process up to fracture.

Results and Discussion Samples of iPP fibres having initial length equals 14.8 cm were stretched under the effect of different values of constant stretching speed, 0.045, 0.135, 0.225, 0.315, 0.405, 0.495 and 0.585 cm/sec, respectively, at constant temperature T = 19 oC. The field of view was adjusted; the length and the diameter of the fibre were measured in real time using a designed software and CCD camera attached to the computer linked to the Pluta polarizing microscope. Figure 2(A) show the non-duplicated microinterferograms of iPP fibers obtained with Pluta interference polarising microscope. These microinterferograms were captured during the stretching process for different strain values 0, 1, 1.1, 1.2, 1.3, 1.6, 1.7 and 2, respectively, at constant stretching speed value 0.405 cm/sec. Monochromatic light of wavelength λ = 546.1 nm was used to illuminate the fibre in transmission. Figures 2(B) show optical micrographs of longitudinal views for a tested sample of iPP fibres viewed in cross polarisation using Pluta microscope. These micrographs were captured for the same different strain values and the same value of constant speed listed above. White light was used to illuminate the fibre. With advanced stretching process,

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Table 1. Values of birefringence obtained according to the relation (1) Strain values 0 1 1.1 1.2 1.3 1.6 1.7 2

Birefringence values given in Figure 2(A) -0.001 0.007 0.010 0.010 0.010 0.011 -

subtractive set up of Pluta microscope [24] was used to give non-duplicated images for the direct measurement of the birefringence (∆n) of the tested samples polypropylene fibres. The value of the (∆n) of the fibre can be obtained directly from the following equation [36]: Zλ ∆n = -----dh Figure 2. (2A) microinterferograms of iPP fibre obtained via Pluta polarising microscope, (2B) optical micrographs of longitudinal views of iPP fibre observed between two-cross polarisers, respectively, at different strain values: 0, 1, 1.1, 1.2, 1.3, 1.6, 1.7 and 2, respectively for constant stretching speed V = 0.405 cm/sec and temperature T = 19 oC.

different phenomena related to the fracture behaviour of iPP fibres, i.e. necking, craze initiation and craze propagation up to about fracture, were observed and recorded in real time. Necking took place in association with a reduction of the cross-section, and the material was stretched significantly at the same time. The formation of necking phenomena was observed at strain value = 1, (Figure 2(A,B.b)), where the specimen thickness varied between 123 and 142 µm. After neck formation, it was noticed that the craze initiation strain value was equal to 1.6; (Figure 2(A,B.f)), while the craze growth propagated in the strain interval (1.7 to 2), (Figure 2(A,B.g,h)), till the occurrence of fibre fracture. At strain values 1.7 and 2, it was noticed that the crazes arise as a result of stretching the oriented fibre material. The fibrillar/ void structure of the crazes is of sufficient size to scatter light; hence the fringe shift inside the fibre was deformed, which causes difficulty to detect it in the image (Figure 2(A,B.g,h)) [28]. Generally, Figure (2A,B) reveal that the crazes were extended perpendicular with respect to the stretching direction. It is also noticed that several crazes have run through the entire cross section without failure of the specimen, indicating the load-bearing nature of the crazes, which agrees with that reported by Meyers and Chawla [29]. In this work the

(1)

Where, Z is the fringe shift displacement inside the fibre, λ is the wavelength of the light used, d is the diameter of the fibre sample and h is the interfringe spacing. Owing to the fact that some birefringent samples acquire a spectrum of colour when observed in white light through crossed polarizers [24-26], Figure 2(B) illustrate that, the original sample (strain = 0) is nearly isotropic material, i.e. the material is randomly oriented, therefore it is colourless. To confirm this conclusion, the birefringence of iPP fibres having the values listed in Table 1, was calculated according to equation (1) and it was found to be −0.001 at strain = 0. The birefringence values increased with strain until it reaches 0.0113 at strain 1.6. The increase of birefringence as a function of strain indicates that, the molecular chains become more oriented during the stretching process. Increasing the stretching value leads to more molecular chains orientation, i.e. the material exhibited “anisotropic properties” which increases as a function of strain. This is clear from the different colour image of the fibre with different strain values. Also birefringence cannot be measured at strain values 1.7 and 2 because the craze deformation causes difficulty to detect the fringe shift inside the fibre [28]. To through light on the phenomenon of craze deformation in iPP fibre, we will investigate the craze formation in the interval between craze initiations up to just before fracture of the fibre. Figure 3 shows optical micrographs of longitudinal views of a sample of iPP fibres which reveal the formation of different stages of crazes from its initiation up to about fracture at strain values 1.6, 1.7, 1.81, 1.84, 1.87, 1.9, 1.95, 1.97, 1.98, and 2, respectively, captured at constant stretching speed value 0.405 cm/sec and constant temperature T=19 oC.

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Figure 4. Image processing for craze density measurement of iPP fibre, shows images of the initial crazes and crazes precursor the fracture point at strain value (1.6 and 2) respectively. (i) original image; (ii) thresholded image; (iii) binary image.

Figure 3. Images of different stages crazes appear on the surface of iPP fibre until the fracture occurs observed between two-cross polarisers at strain values (1.6, 1.7, 1.81, 1.84, 1.87, 1.9, 1.95, 1.97, 1.98, and 2), respectively at constant stretching speed V = 0.405 cm/sec and temperature T = 19 oC.

The areal craze density, which represents the craze damage variable, was calculated from the optical micrographs given in Figure 3 according to the procedures given by Luo and Liu [31]. The collected images of different stages of crazes (micro-cracks) were analysed, via suitable software designed for this purpose and calculate the areal craze density as a function of strain. Figure 4(A,B) shows optical micrographs of the crosssections of iPP fibres at the initial appearance of crazes and at crazes precursor the fracture of the material at the strain values 1.6 and 2, respectively, from the beginning of stretching process. Each image was firstly converted to 8 bits-per-pixel mono image, and then the pixels with a certain intensity range (e.g. pixel intensity from 0 to 128 shown in (Figure 4(A,B.(i,ii)) were selected by thresholding. Finally, the thresholded image was further masked to be a binary image as shown in (Figure 4(A,B.(iii)). The designed software was used to measure the areal craze density, simply by dividing the area of the crazed region, which is presented in black in (Figure 4(A,B.(iii)), and the whole image area [31]. The function of this programme is described by the flow chart shown in Figure 5. The normalised areal craze density (d) as a function of normalised strain (ε) of iPP fibres for different values of constant stretching speed was presented in Figures 6. This

Figure 5. The flow chart of the designed software programme for areal craze density measurement of iPP fibre.

Figure 6. Normalised areal craze density of iPP fibre against normalised strain during craze growth at stretching speed values (0.045, 0.135 and 0.585 cm/sec) and constant temperature T=19 oC.

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density is rapidly increases with increasing the normalized strain (ε) and then increases slowly to level off. The experimental

relation between the normalised areal craze density d and the normalised strain (ε) for iPP fibres at constant stretching speed value was fitted by the suggested mathematical relation:

Table 2. Values of the fitting parameters obtained according to the empirical relation (2)

a1 (2) d = a0 + -------------------------( ε – b /b ) 1+e 0 1 Where, a0, a1, b0 and b1 are constants having the values listed in Table 2. The elapsed time for craze initiation, craze propagation and craze breakdown was determined experimentally for iPP fibres at different values of stretching speed and at constant temperature value. Figures 7(A,B) show non-duplicated microinterferograms, (Figure 7(A)), and optical micrographs of longitudinal views (Figure 7(B)) at the craze initiation of iPP fibre. These images were captured at different stretching speed values 0.045, 0.135, 0.225, 0.315, 0.405, 0.495 and 0.585 cm/sec,

Stretching Fitting parameters of the experimental data given in Figure 6 speed (cm/sec) a0 a1 b0 b1 0.045 0.0117 1.0333 0.6323 0.07625 0.135 -0.0755 1.0144 0.33281 0.13631 0.585 -0.0809 0.9935 0.33825 0.11956

Figure 7. Initial stages of craze phenomenon iPP fibre observed using, (a) microinterferograms obtained using Pluta polarising microscope, (b) optical micrographs of longitudinal views of iPP fibre observed between two-crossed polarisers at different stretching speeds: V = 0.045 , 0.135, 0.225, 0.315, 0.405, 0.495 and 0.585 cm/sec, respectively at temperature T = 19 oC.

Figure 8. The effect of drawing speed on the elapsed time at which the following fracture mechanisms of iPP fibre were observed: craze initiation, propagation and fracture occurs at temperature T = 19 oC, (a) via Pluta polarising microscope in transmission and (b) uniform field mode where the fibre was observed between twocrossed polarisers.

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respectively, and at constant temperature T = 19 oC. Figure 8(a,b) represents the elapsed time of craze initiation, craze propagation and fracture of iPP fibres depending on stretching speed, at constant temperature T = 19oC. This Figure shows a rapid decrease followed by a slower decrease in these times with increasing stretching speed. That is to say, increasing the stretching speed causes the fracture to occur more rapidly, which agrees with the behaviour reported by Ward [32]. This agreement is confirmed by the two different methods using Pluta polarising microscope in transmission: a) non-duplicated image mode and b) uniform field mode where the fibre was observed between twocrossed polarisers. The birefringence profile refers to the variation of birefringence across the fibre radius. This profile can be determined by using modified profiling software, (taking into account the refraction of the incident beam caused by the fibre layers) on the basis of the following equation [33]: λ ∆Z 1 ∆nQ = --------------------------- ------------Q- r R – ( Q – 1 )r 2b

j = Q–1

∑

∆n J

details, the observed behavior of the birefringence profile at different strains showed a slight increase in the values of the birefringence for the outer layers. On the other hand, for the inner layers, the increase in the birefringence values occurred at a higher rate. As a result, the inner layers become more oriented than the outer layers during the uniaxial stretching

(3)

j=1

Where, r is the layer thickness (r = R/N), R is the fibre radius, N is the suggested number of layers, Q is the layer number, ∆ZQ is the fringe shift displacement corresponding to the Qth layer, ∆nj is the birefringence of the jth layer, and b is the interfringe spacings. Birefringence profile can be obtained, taking into account the refraction of the incident beam by the fibre layers [33]. The birefringence profile was determined according to equation (3). Figure 9 shows the distribution of the birefringence values across the crosssectional layers of the stretched iPP samples at temperature value 19 oC and stretching speed value 0.405 cm/sec. An increment in the birefringence value was noted during the stretching process which implies more orientation of the molecular chains in the direction of stretching. Also, in more

Figure 9. Birefringence profiles of iPP fibres at different strains, constant stretching speed value 0.135 cm/sec and constant temperature T=19 oC using automated Pluta microscope.

Figure 10. Initial of craze phenomenon observed using, (A) microinterferograms obtained using Pluta polarising microscope, (B) optical micrographs of longitudinal views of iPP fibre observed between two-cross polarisers at different temperatures: T = 19, 25, 30, 35, and 40 oC respectively for stretching speed V = 0.135 cm/sec.

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Table 3. Values of the craze width at different temperatures given from Figure 10(A) Temp (oC) values 19 25 30 35 40 45 50 55 60 65

Craze width (micron) values given in Figure 10(A) 32 28 26 17 9 -

of iPP fibres. Thus we may conclude that, the inner layers of the stretched samples were more affected by the stretching process than the outer layers. The effect of temperatures on the craze initiation is studied. This study is carried out for constant value of stretching speed 0.135 cm/sec. Figure 10(A,B) shows the differentially sheared images (Figure 10(A)) and optical micrographs of longitudinal views (Figure 10(B)) of iPP fibres at craze initiation for five different values of stretching temperatures, 19, 25, 30, 35 and 40 oC. It is noticed that there is a transverse spread of craze initiation at low temperature (19 oC), i.e., the outer layers as well as the inner layers of the fibre were affected by the crazes. On the other hand, at higher temperatures, it was noticed that, the inner layers of the fibres were more affected by the craze initiation. This was confirmed by the observed decreasing of the transverse width of the crazed regions as shown in Table 3. Figures 11, 12, show that the observations of the fracture mechanism at different temperatures. The observed behaviours for the time and strain, at which the craze initiation, propagation and fracture. Moreover, increasing the temperature at stretching speed (0.135 cm/sec) had the following effects on the fracture mechanism. According to (Figure 11(a)) and (Figure 12(a)), it was time-to-fracture showed an initial decreasing trend as a function of the temperature. At temperature T = 40 oC, the time where the craze initiation or propagation occurred, showed remarkable behaviour that, after temperature T = 40 oC the formation of craze and craze propagation was no longer observed during the stretching process. This may indicate that the temperature T = 40 oC is the temperature after which the iPP fibre would fracture without being crazed or propagated. Thus T = 40 oC can be considered as critical temperature of craze initiation (or propagation) at stretching speed value 0.135 cm/sec. Moreover, the investigation of the strain values, at which, the craze, its propagation or fracture is observed, followed the same trend, see (Figure 11(b)) and (Figure 12(b)). It was found that the

Figure 11. The effect of temperature on (a) the elapsed time (b) The strain, at which the following fracture mechanisms were observed: craze initiation, propagation and fracture occurrence of iPP fibre, via Pluta polarising microscope at constant drawing speed V = 0.135 cm/sec.

results obtained using Pluta polarising microscope followed the same behaviour when the sample was observed between two cross polarisers, which confirms the obtained behaviour. The critical temperature of initial craze or craze propagation observation was found to be T = 40 oC, which agrees with the behaviour reported by Ward [32]. The fracture mechanics and its relation to both the temperature and speed were studied during the stretching of iPP samples. The change of the craze initiation time as a function of the surrounding temperature at different stretching speeds, is clear from the Figure 13, it was noticed that the stretching speed caused the craze to initiate earlier as the speed increased. On the other hand, the influence of temperature was observed at lower speed more obvious than the higher speeds. Since at stretching speed value (0.135 cm/ sec) the craze initiation varied from ~160-100 seconds, while for stretching speed value (0.495 cm/sec) the craze initiation varied in the range ~55-30 seconds. Finally, we

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may conclude that the temperature had less effect on the fracture mechanics at higher stretching speed. Also increasing the stretching speed accelerates the fracture mechanics of the fibre under study [32].

Conclusion

Figure 12. The effect of temperature on (a) the elapsed time (b) The strain, at which the following fracture mechanisms of iPP fibre were observed: craze initiation, propagation and fracture occurrence, observed between two-cross polarisers at drawing speed V = 0.135 cm/sec.

Fracture mechanics of polypropylene fibres were investigated by crazing during cold drawing using Pluta polarising microscope. The experimental results reveal that: 1. Craze damage, which is represented by a normalized areal craze density, increases rapidly with stretching speed and then increases slowly to level off. 2. Elapsed craze initiation time, elapsed craze propagation time and craze breakdown time decrease exponentially with stretching speed. This behaviour shows a rapid decrease followed by a slower decrease in these times with increasing stretching speed. That is to say, increasing the stretching speed causes the fracture to occur more rapidly, therefore, it is recommended to stretch the iPP fibre at relatively low stretching speed (lower than 0.015 cm/sec) at constant temperature 19 oC. 3. Craze initiation time, craze propagation time and craze breakdown time decrease exponentially with stretching temperature at constant drawing speed value 0.135 cm/ sec. 4. Craze initiation strain, craze propagation strain and craze breakdown strain decrease exponentially with stretching temperature at constant speed value 0.135 cm/sec. 5. The critical value of stretching temperature for the formation of crazes in iPP fibre was found to be 40 oC. After this value, crazes were not observed during the drawing process at constant stretching speed value 0.135 cm/sec. To avoid the crazes formation in iPP fibres, it is recommended that the iPP fibre must be stretched at temperature higher than 40 oC.

References

Figure 13. The time at which the craze in iPP fibre is initiated as a function of the temperature for different stretching speeds, obtained using Pluta polarising microscope.

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