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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Fig. 4. (a) Orientation of microfibrils in S2 cell wall layer of wood fibre. P represents the primary wall, CML represents compound middle lamella, BP represents bordered pit, S1 and S2 represents secondary wall layers, Fb represents fibril. (b) Bunch of misaligned viscose fibres after embedding in epoxy resin, whereas inset picture (rectangular box) shows the aligned fibre selected for nanoindentation.

Fig. 5. Load-depth curves and corresponding post indent images from nanoindentation tests (note the five indents). Curve represents the unbleached and unrefined (UB_UR) pulp fibre which is used to make newspaper.

4. Results and discussion It is clear from fibres cross sectional images shown in Fig. 1 that natural fibres flax, pulp and hair have empty space or loose network of microfibrils at the middle of the fibre cross sections. This empty space (lumen or medulla) is the reason for usage of these natural fibres in heat and sound insulation as reported in Buksnowitz et al. (2010). Tensile stress-strain plots of all fibres are shown in Fig. 3 and average values are compared in Table 1. Even though kevlar fibre was not tested in this study, its properties are mentioned in Table 1 and the data was taken from Chawla (1998). As expected, the tensile modulus of glass and kevlar fibre were the highest reaching 100 GPa and strain to failure is less than 4 % and hence both are considered as brittle fibres i.e. no difference between yield stress and maximum stress. The strength and stiffness of flax and glass fibres are comparable if density is taken in to consideration. This could be due to the highly ordered structure of cellulose microfibrils embedded in a lignin matrix. Hair is not a brittle fibre and its strength is one-third of the flax fibre but its strain to failure is 10 times the value of flax fibre. Smaller values of hair stress and stiffness (Table 1) compared to flax could be primarily due to two reasons. First one is structural degradation in which keratin microfibrils in hair are initially arranged in alpha helix pattern and it converts into beta pleated sheets while stretching. Second one is due to the weaker network density, in which less number of covalent bonds are present in hair compared to flax fibre. Flax has very strong three dimensional network structure due to the presence of lignin matrix in its cell wall. Surprisingly hair is considered as elastic fibre even though plot shows the response of crystalline α - helix keratin, segmental motions of amorphous matrix, α to β keratin transition, composite yielding and failure of amorphous matrix along with pull-out of crystalline macrofibrils. Both viscose and lyocell fibres show elastic and plastic regions and lyocell has high yield stress and low elongation due to its high draw ratio and crystallinity. That’s why lyocell is studied as reinforcement in composites as reported by Adusumalli et al. (2006) whereas viscose is still being used only in textile applications.