MECHANICAL FAILURE OF MULBERRY AND TASAR SILK

This article investigates the tensile and twist failure of mulberry and tasar silk fi laments. The mulberry and tasar fi laments were subjected to uniaxial loading on Instron tensile tester at diff erent rate of extensions and gauge lengths. Furthermore, the number of turns to rupture the silk fi laments was tested using a twist tester. The results showed that the mulberry fi lament has higher tensile and twist strength than that of tasar fi lament. The SEM photomicrographs of the region of fracture divulged that the tensile and twist failure of mulberry and tasar fi laments take place in catastrophic and non-catastrophic modes, respectively.


INTRODUCTION
Silk fi ber is recognized as queen among the textile fi bres. The history of silk fi bre dates back to roughly 5000 years, since then it remains an important luxurious textile material for its elegant aesthetic look [1]. Moreover, silk fi bre has outstanding potentiality as a structural material because of its unique combination of strength and ductility compared to any other natural fi bres. Despite of this fact, the reported work on the tensile failure mechanism of silk fi lament is not adequate [2,3].
The strength of any textile material is largely governed by two important testing parameters viz., extension rate and gauge length [4]. To the authors' knowledge, no information is available on the eff ect of extension rate and gauge length on silk fi lament.
High twisted silk fi laments are often used to produce crepe fabric to have crinkle eff ect for specific end uses. But if a silk fi lament is twisted beyond a certain level, it will eventually break [5]. Therefore, it is interesting to see the maximum value of twist a silk fi lament can withstand before its rupture. Literature hardly reports anything with reference to the rupture of silk caused by twist.
So, the present investigation was mainly focused with a view to studying the aforementioned aspects of silk fi laments produced by two common silkworms, namely, mulberry (Bombyx mori) and tasar (Antheraea mylitta D.).

EXPERIMENTAL 2.1. Materials and preparation
Mulberry and tasar silk cocoons were collected from Central Silk Technological Research Institute, Central Silk Board, Bangalore, India and Raw Material Bank, Chaibasa, Bihar, India, respectively. Cocoons were softened and reeled by following diff erent procedures for mulberry and tasar silk as described elsewhere [6]. This was required to obtain mulberry and tasar silk fi laments.
The mulberry cocoons were taken in a wire mesh cage and softened in the fi rst pan at 65 0 C for one minute, then in the second pan at boiling for about one and half minutes, and fi nally at 65 0 C for one minute. The softened cocoons were hand-brushed at boiling in order to obtain the true end of the fi lament. Due to the hard and compact nature of the tasar cocoons, softening with 10% ethylenediamine was required at 80 0 C for 50 min. The cocoons were defl ossed manually to obtain the true end of the fi lament. Both mulberry and tasar fi laments were degummed with 25% Marseilles soap (on the weight of the material) at boil for 90 min at a liquor ratio of 50:1. The degummed fi laments were washed, dried and conditioned at tropical atmosphere of 27 o C and 65% relative humidity (according to ISO 139-1973 IE, s.2.3.1) for 48 hours. The deniers of the mulberry and tasar fi laments were obtained 9.5 and 10, respectively. Subsequently, these fi laments were evaluated for tensile and twist failure.
An Instron tensile tester was used to study the tensile behavior of mulberry and tasar fi laments. The fi laments were tested at diff erent rate of extensions, viz., 50, 100, 200, 300 and 500 mm/min at a constant gauge length of 100 mm. The fi laments were also tested at diff erent gauge lengths (20, 50, 100, 250 and 500 mm) at a constant extension rate of 200 mm/min. For each set of experiment, 50 tests were conducted.
The number of turns to rupture the silk fi laments was studied using a twist tester. A nominal gauge length of 250 mm was used in the twist tester. The counter was set at zero. The sample was mounted between the clamps under a tension of 2.5 cN/tex. One end of the specimen was secured in the nonrotatable clamp and the other end of the specimen was inserted through the rotatable clamp. The fi lament was slowly extended through the open clamp until the pointer attached to the nonrotatable clamp reached the re-quired tension. The rotatable clamp was revolved either in clockwise or anticlockwise directions until the fi lament breaks. The number of turns per inch (TPI) to rupture the silk fi laments was measured from the counter. The average number of TPI to break was calculated based on 30 readings in each case.
After the tensile and twist failure of the fi laments, the broken ends were collected and characterized by scanning electron microscope (SEM).

RESULTS AND DISCUSSIONS
Tenacity of both mulberry and tasar fi laments tested at diff erent extension rates are given in Table 1. It is clearly demonstrated that the strength for both the fi laments is an increasing function of extension rates. A 10-fold increase in extension rate, i.e., from 50 mm/min to 500 mm/min, produces 18.4 % and 14.6 % increase of strength values for mulberry and tasar fi laments, respectively. An increase in the fi lament strength with the extension rate can be substantiated on the basis of the fact that the tensile behavior of silk fi lament is a time dependent phenomenon due to its visco-elastic nature. Therefore, when a fi lament is subjected to tensile load, the relaxation time will be higher for lower extension rate than that of higher extension rate. A higher relaxation time obviously allows more stress relaxation, which is responsible for the decrease in stress at certain level of extension for fi lament tested at lower extension rate as compared to one tested at higher extension rate. On the contrary, less time available for fi lament rupture allows less stress relaxation at higher extension rate and thereby yields more tenacity [7,8].
The tenacity values of mulberry and tasar silk tested at various gauge lengths are shown in Table 2. It is noted that the tenacity of both the silk fi laments diminishes with an increase in gauge length. This can be advocated to the presence of fl aw in the silk fi lament, which leads to the localization of stress in excess of theoretical strength, whereby the rupture process is initiated. It thus follows that the fall in strength of silk fi lament with increasing test length is due to the presence of a distribution of fl aw of wide-ranging magnitude, since the probability of encountering a large fatal fl aw increases with test length [9.8]. Table 3 shows the number of TPI to break the mulberry and tasar fi laments tested at both clockwise and anticlockwise direction of twist. It can be observed that the number of twists to break the fi lament in clockwise direction is signifi cantly higher than that of anticlockwise direction for both mulberry and tasar silk. This can be ascribed to the presence of natural convolution of silk in Z direction (i.e., anticlockwise direction), as a result it takes higher turns in clockwise direction to break. The phenomena of untwist as well as twist occurred while rotating the silk fi lament in clockwise direction in comparison to the only twist phenomenon happened in anticlockwise direction. Therefore, by clockwise rotation the natural convolution of silk fi lament will open up fi rst and then by further clockwise rotation it would break. In contrast, when a silk fi lament is rotated in anticlockwise direction, the twist will continually increase and eventually the fi lament breaks, so the number of twists to break will be less in this case.
It is clearly observed from Table 1 and 2 that the mulberry fi lament is signifi cantly stronger than that of tasar fi lament. Furthermore, it is appreciated that the number of twists to cause rupture is again higher for mulberry fi lament compared to tasar fi lament ( Table  3). The supremacy of mulberry silk in terms of strength and twist to break can be ascribed to its more ordered structure at molecular and morphological levels in comparison to the tasar silk as illustrated in Table 4 [6]. Poor orientation in tasar silk is related to the higher percent of bulky groups present in fi broin [6]. Lucas et al. [10] also showed that the ratio of number of amino acids with bulky side groups to the number with short side chains in tasar is almost twice that of mulberry silk.
The fi lament fracture morphology has been analyzed with the SEM photomicrographs. Some selected tensile and twist fracture morphologies of mulberry and tasar fi laments are depicted in Figures 1 and 2, respectively.
It is evident from Figure 1[a] that the tensile fractography of mulberry fi lament shows a sharp edge as if the fi lament is cut by a scissor. This can be attributed to the fact that the tensile failure of mulberry fi lament is possibly propagated by catastrophic mode of rupture. The photomicrograph of tensile failure of tasar fi lament shows granular surface running across the transverse direction as well as longitudinal cracks on the fi lament surface (Fig. 2[a]). This could probably be due to the occurrence of partial crack growth in the tasar fi lament preceding to its catastrophic failure under tensile loading. [a] [b] The twist fractography of mulberry fi lament shows a wedge shape end ( Fig. 1[b]). The fi lament segments of mulberry fi lament are separated apart to form wedge shape cross-section due to the bending and shearing while twisting. Furthermore, it is apparent from Figure 1[b] that the twist fracture occurs in catastrophic manner in mulberry fi lament. It is observed from Figure 2[b] that tasar fi lament shows fraying type of section. In this case the failure is perhaps initiated by breaking of few fi brils owing to shearing and bending and then it is transmitted transversely from one fibrillar component to other. Thus, twist fracture of tasar fi lament occurs in non-catastrophic mode.
From the foregoing discussion it is obvious that the modes of failure for mulberry and tasar fi laments are diff erent. For e.g., the tensile and twist rupture of mulberry fi lament occur in catastrophic mode as compared to the non-catastrophic mode of tasar fi lament. This could be due to the better structural integrity of mulberry fi lament than that of tasar fi lament as explained in the preceding paragraph.
It can be also noticed from Figures 1 and 2 that there are defi nite dissimilarities in the mechanisms of tensile and twist failure for silk. The tensile failure occurs when the axial stretch of the fi lament is higher than its breaking extension, whereas, twist failure is caused by an increasing amount of bending and shearing stretch due to the twisting of fi lament until it can able to withstand.

CONCLUSIONS
The tenacity of mulberry and tasar silk increases with the increase in extension rate. A drop in tenacity is noticed for both the fi laments as the gauge length increases. The number of TPI required to break the fi lament in clockwise direction is appreciably higher than that of anticlockwise direction for both the silk fi laments considered in this study. Mulberry fi lament shows higher strength and it needs more TPI to break as compared to tasar fi lament. The tensile photomicrograph of mulberry fi lament displays a sharp edge, but for tasar fi lament it displays granular surface in the transverse direction in addition to longitudinal cracks on the fi lament surface. The twist fractography of mulberry fi lament illustrates a wedge shape, however tasar fi lament demonstrates fraying type of section. Mulberry silk has diff erent failure mechanism than that of tasar silk.