PSI - Issue 14

Ramesh Babu Adusumalli et al. / Procedia Structural Integrity 14 (2019) 150–157 R.B. Adusumalli / Structural Integrity Procedia 00 (2018) 000 – 000

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existence of strong nanocomposite structure. The step and spilt fractures were not seen in flax and pulp fibres because of the collapsed lumen. SEM images shown in Fig. 6 indicate that typical fracture occurred in step or split mode resulted in a crack propagation along the length side to a distance of 300-1000 µm. Further images also indicate that cracks were initiated at medulla or at vicinity of medulla. This clearly indicates that the post-yield incremental modulus change found in hair (around 32% strain in Fig. 3.) is due to strain induced alignment of fibrils which are embedded in a stable fashion in an amorphous matrix possessing good mechanical integrity. The maximum strengths of individual fibres were recorded and probability of survival is plotted against this maximum strength in Fig. 7 to find the weibull modulus β or weibull shape factor. β was found to be low for natural fibres (1 for pulp and 2.1 for flax and 3.4 for hair) compared to synthetic fibres (6 for lyocell, 7 for viscose and 12 for kevlar fibre) as shown in Fig. 7 and also as explained in Revol et al. (2016) and in Chawla (1998). It is clear that lower values of weibull modulus correspond to broader distribution. Since natural fibres have defects at nanostructural level in the composite structure of their cell wall or cortex and also have lumen or medulla in their central axis, their strength values gave a broader strength distribution. It is clear that higher tensile modulus mostly results in lower weibull modulus which can be seen from Table 1 and Fig. 7. Flax fibre has higher tensile modulus and lower weibull modulus than hair. Similar comparison also exists between viscose and lyocell. The above statement is true in case of highly anisotropic fibres, but isotropic fibre such as glass also showed lower value of weibull modulus (3.5) indicating the influence of defects exist in amorphous and high modulus glass fibre. Indentation modulus values (Table 1) are compared between anisotropic fibres and also between anisotropic and isotropic fibres (Glass). For flax as given in Bourmaud et al. (2012) and for lyocell fibres as reported by Gindl et al. (2006), the indentation modulus is lower than tensile modulus, this is because during the tensile stretching the microfibrils in the fibres are pulled and are forcibly oriented in the fibre direction resulting in a reduced MFA, but during the nanoindentation testing, the MFA either remains the same or even increases due to compression. This increase of MFA results in smaller values of indentation modulus as the microfibrils are being compressed part laterally and part longitudinally with the force of compression acting at an angle. The difference in modulus could also be due to the widely reported explanation that covalent bonds are broken in tensile testing whereas in nanoindentation the bonds that are broken are mostly hydrogen bonds as given in Gindl et al. (2008) and in Varughese et al. (2013) because nanoindentation test is done in <2 µm diameter and <1 µm depth. This variation in modulus could also be due to their microstructural defects that exist in small scales /regions because these molecular solids have crystalline and amorphous regions intertwined and intricate in nature and molecules are oriented in the direction of fibre length. In fact this difference is high for fibres with high tensile modulus, which is why kevlar revealed lower values of indentation modulus as given in Bencom-Cisneros et al. (2012). The difference becomes minimal for low modulus fibres which is why viscose fibres indentation modulus is same as that of tensile modulus. The longitudinal bonds in kevlar are strong covalent bonds which can resist tensile force to a great extent but the lateral bonds, i.e. the bonds in between the chains of repeating units of para-phenylene terephthalamide are either hydrogen bonds or other weak forces of attraction. These bonds are easy to break and reform, even if hydrogen bonds with the neighboring molecules are broken during the downward sliding of a chain, a new set of bonds can be formed immediately after the sliding chain comes in to alignment with the bottom molecules. This makes it relatively easy to compress the fibre in the direction along the chains. So kevlar presents great resistance to tensile force but presents little resistance to compression. The odd man out in the set of seven fibres is glass as this is the only inorganic fibre in this study. The values of indentation modulus and tensile modulus are almost similar for glass, this is because E-glass fibres are isotropic due to their amorphous, three dimensional network structure. Unlike quartz fibres, E-glass fibres are made of SiO 2 , Cao and Al 2 O 3 . In silica, silicon is covalently bonded to oxygen giving three dimensional structure, but this network structure is highly altered due to the presence of Cao and Al 2 O 3 , thus making E-glass fibre to be highly amorphous i.e. devoid of any long range orders. Not only elastic modulus, but thermal expansion coefficient and strength values are also same along the fibre axis and perpendicular to it as given in Chawla (1998). The high strength of E-glass fibres is attributed to a comparatively defect-free structure, whereas aramid, lyocell, flax, hair, viscose fibres attain their strength as a result of improved orientation of their atomic or molecular structure in the fibre direction as explained in Agarwal et al. (2006). As shown in Table 1, indentation modulus is higher than tensile modulus for hair, this is because hair is a composite