PSI - Issue 14

152 Ramesh Babu Adusumalli et al. / Procedia Structural Integrity 14 (2019) 150–157 R.B. Adusumalli / Structural Integrity Procedia 00 (2018) 000 – 000 microfibrils, the intermediate filaments (50 % helical) are having 6 % cystine (having low S-S linkages). So hair fibre can be considered as a highly anisotropic composite having alpha keratin crystalline protein embedded in amorphous keratin protein matrix resembling carbon-carbon composites. Understanding hair properties in tension and compression is of great need in cosmetic and wool industries. Hair fibres have diameters ranging from 50-110 µm and their mechanical properties are dependent on nanocomposite structures of microfibrils as well as moisture content. Aramid (Kevlar) and glass are considered as synthetic fibres. The word aramid is a portmanteau of the word aromatic polyamides, whose fibres are a broad class of synthetic fibres widely used in aerospace industry due to its many salient features like low density (1.4 g/cm 3 ), high strength, and good vibration damping characteristics. Like cellulose and protein fibres, kevlar fibres are also highly anisotropic with respect to fibre direction due to which their compression strength is one-eighth of the tensile strength as explained in Chawla (1998). Glass fibres are thin fibres (5-20 µm) made of silica and calcium oxide and are spun while the glass is still hot and molten. E-glass fibres are amorphous because it has random network of silica tetrahedra making E-glass fibre as one of the best isotropic fibres. E-glass fibres are dense (2.4 g/cm 3 ) and are used widely as thermal and electric insulation materials and also as reinforcements as reported in Chawla (2010).

Fig. 1. Cross section of (a) Pulp fibre (b) flax fibre bundle (c) hair fibre (d) viscose fibres (e) lyocell fibres (f) kevlar fibres. Note the lumen in pulp, flax fibres (a,b) and medulla in hair fibres (c).

2. Tensile testing of single fibres to measure strength and modulus

Depending on fibre modulus, two types of single fibre tensile tests are available. Low modulus fibres (e.g. viscose) are usually tested by direct gripping and high modulus fibres (e.g. carbon) are tested by using a carrier like paper frames (Fig. 2) as reported in Adusumalli et al. (2006). Since the present investigation involves cellulose (flax), protein (hair), kevlar and glass fibres (modulus <100 GPa), the paper frame set-up was adopted to maintain consistency. A universal testing machine (Zwick/Roell) equipped with a 50N load cell was used for tensile testing. Specially ordered rubber jaws were used for excellent gripping. At first any crimp in the fibre was removed manually and the fibre was fixed to the paper frames as shown in Fig. 2. Tensile tests were carried out until failure at a cross-head displacement rate of 1 mm/min. To obtain a representative set of results more than 50 single fibres of each type were tested. Tensile strength and failure strains were calculated from the respective maxima in the recorded stress-strain graph (Fig. 3). Modulus is calculated from the initial elastic portion (1-2 % strain) of the stress-strain plot using origin software.