K. G. Satyanarayana

Structure and properties of some vegetable fibres

The stress-strain curve for sisal fibres has been experimentally determined. Ultimate tensile strength (UTS), initial modulus (YM), average modulus (AM) and per cent elongation at break of fibres have been measured as function of fibre diameter, test length and test speed. UTS, YM, AM and per cent elongation lie in the range 530 to 630 MN m−2, 17 to 22 GN m−2, 9.8 to 16.5 GN m−2 and 3.64 to 5.12 respectively for fibres of diameters ranging between 100 and 300μm. No significant variation of mechanical properties with change in diameter of the fibres was observed. However, with increase in test length of the fibres, the UTS and per cent elongation are found to decrease while YM and AM increased in the test length ranging from 15 to 65 mm. With the increase in speed of testing from 1 to 50 mm min−1, YM and UTS are found to increase whereas per cent elongation and AM do not show any significant variation. At a test speed of 500 mm min−1 the UTS value decreases sharply. The above results are explained in terms of the internal structure of the fibre such as the cell structure, microfibrillar angle, defects, etc. Scanning electron microscope (SEM) studies of the fractured tips of the sisal fibres reveal that the failure of the fibre is due to the uncoiling of microfibrils accompanied by decohesion and finally tearing of cell walls. The tendency of uncoiling seems to decrease with increasing speed of testing.

Natural fibre-polymer composites

Abstract There is a growing interest in the development of new materials which enhance optimal utilization of natural resources, and particularly, of renewable resources. Natural fibres such as jute, coir and sisal belong to this category. This paper describes systematic work carried out so far on the structure-property relationship of these fibres including fracture modes. Attempts to incorporate them in polymers and characterization of these new composites, with and without subjecting them to environmental conditions, are reported. Problems arising out of processing of the composites and attempts made to minimize these problems are also described. Efforts to fabricate a few components and evaluation of their performance in actual use are presented. Suggestions for future work are also given.

Structure property studies of fibres from various parts of the coconut tree

Fibres from different structural parts of the coconut palm tree (Cocos nucifera, linn.) have been examined for properties such as size, density, electrical resistivity, ultimate tensile strength, initial modulus and percentage elongation. The stress-strain diagrams, fracture mode, microfibrillar angle as well as cellulose and lignin contents of these fibres have been determined. The observed properties have been related to the internal structure and chemical composition of the fibres. Some potential uses of these fibres have been listed.

Role of magnesium in cast aluminium alloy matrix composites

Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO4), zirconia (ZrO2), titania (TiO2), silica (SiO2), graphite, aluminium oxide (Al2O3) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.

Mechanical properties of banana fibres (Musa sepientum)

The stress—strain curve for banana fibre is determined. Properties such as the initial modulus (YM), ultimate tensile strength (UTS) and percentage elongation are evaluated as a function of fibre diameter, test length and speed of testing. It is found that YM, UTS and % elongation show little variation in their values for fibres of diameter ranging from 50 to 250 μm. The UTS and breaking strain are found to decrease with an increase in the test length while both breaking strength and breaking strain remain constant with the increase of speed of testing from 0.5 to 100 × 10−3 m and thereafter they both decrease. These observed properties are explained on the basis of the internal structure of the fibre, namely, the number of cells, spiral angle and the number of defects. Scanning electron microscopic (SEM) studies of the fractured surfaces of these fibres indiacte that the failure is due to pull-out of microfibrils accompanied by tearing of cell walls; the tendency for fibre pull-out seems to decrease with increasing speed of testing.

Fabrication and properties of natural fibre-reinforced polyester composites

Abstract An attempt has been made to find new uses for natural fibres — one renewable resource which is otherwise under-utilized. The structure and properties of the fibres, and the fabrication and physical and mechanical properties of their polyester-based composites are described. The performance of these composites is evaluated after exposure to indoor and outdoor weathering by both destructive and non-destructive testing methods. The preparation of various consumer articles such as a voltage stabilizer cover, mirror casing, a projector cover and roofing are also reported. This study demonstrates the potential of natural fibres for non-conventional applications and points out some of their limitations.

Mechanical behaviour of coir fibres under tensile load

Stress-strain curves of coir fibres have been determined. Mechanical properties including initial modulus, strength and percentage elongation of coir fibres have been evaluated as functions of retting treatment (during retting the coconut husks are soaked in saline water for a period of about six months to facilitate the extraction of fibres presumably due to a bacterial process), fibre diameter, gauge length and strain rate. No significant differences in mechanical properties were observed between retted and unretted fibres. The strength and percentage elongation seem to increase for both retted and unretted fibres up to a fibre diameter of 0.2×10−3 m whereafter they remain almost constant. On the other hand, moduli seem to decrease with increase in diameter of the fibre. The observed modulus values and percentage elongation have been related to microfibrillar angle. Observed strength values have been explained on the basis of structural changes occurring with an increase in the diameter of the fibre. Scanning electron/microscope studies have indicated that the failure of the fibre is due to the fracture of the cells themselves accompanied by the uncoiling of microfibrills. There is no appreciable variation in strength and percentage elongation with strain rates for any one diameter of the fibre. On the other hand, with increase in gauge length, a decrease in both strength and percentage elongation at break has been observed. These have been attributed to an increase of probability of defects and localized deformation and gentle necking, respectively.

An empirical evaluation of structure-property relationships in natural fibres and their fracture behaviour

An attempt is made in this paper to arrive at an empirical relationship between the structure and properties of lignocellulosic fibres through computer analysis. Significant regression equations for ultimate tensile strength (UTS) and percentage elongation of these fibres with structural parameters such as chemical composition, microfibril angle etc., have been arrived at using best fit. The results clearly indicate a narrowing down of the deviations between the observed and derived values of mechanical parameters reported earlier. This is attributed to (a) consideration of several structural parameters in the regression equation and (b) the analysis being free from any assumptions. Finally the fracture modes observed in these fibres have been classified and this is explained in terms of structure-property relationships.

Copper coating on carbon fibres and their composites with aluminium matrix

A uniform and continuous coating of copper was given to carbon fibres by cementation or electroless techniques. In both cases, when coating thicknesses were less than 0.2 μm, copper deposition was discontinuous over the fibres, and above 0.2 μm, coatings were continuous. In electroless coating, about 75% of the continuously coated fibres had a coating thickness range 0.2–0.5 μm and above this showed isolated dendrite deposits of copper. In the cementation process, about 75% of the continuously coated fibres had a coating thickness range 0.2–0.6 μm, and above this thickness, fine crystallite-type copper deposition was found over smoothly coated copper. The ultimate tensile strength of continuously electroless-coated fibres were nearer to the uncoated fibres, suggesting defect-free coating, while fibres coated by the cementation process exhibited lower ultimate tensile strength values. The tensile fracture of both electroless- and cementation-coated fibres showed delamination of the coating, suggesting poor bonding between coating and the fibre. In composites, prepared by dispersing the coated chopped fibres in a pure aluminium matrix, uniform and random distribution of the fibres were observed without appreciable fibre-metal interaction. The CuAl2 intermetallics were largely found in the matrix and only very small amounts were observed at fibre/matrix interfaces. Additions of about 2 wt% Mg to the matrix prior to the fibre dispersion did not appreciably change the distribution pattern of the fibres, but in addition to CuAl2 phase, Mg2Si phases were observed in the matrix as well as at the interface.

Structure and properties of some vegetable fibres Part 3 Talipot and palmyrah fibres

The mechanical properties of palmyrah and talipot fibres have been evaluated. Like other natural fibres, these fibres were also found to be viscoelastic in nature. The variation of the initial modulus (YM), ultimate tensile strength (UTS) and elongation (%) values were determined as a function of fibre fineness, test length and speed of testing. Change in test length from 10 to 100 mm palmyrah fibres produces a decrease in UTS from 220 to 161 MN m−2. Such variations were not observed in the case of talipot fibres except at low test length. The observed results have been explained in the light of the structural characteristics of these fibres namely chemical constituents espiral angle, number of defect centres, cell sizes, etc.