Tensile properties of jute fibres A. S. Virk, W. Hall* and J. Summerscales One hundred tensile tests were undertaken at each of five distinct fibre lengths (6, 10, 20, 30 and 50 mm) on a single batch of jute fibres from South Asia. The Young’s moduli were found to be independent of length. The ultimate stress (fracture strength) and fracture strains were found to decrease with increasing fibre length. The variation in mechanical properties at each fibre length was characterised using Weibull statistics based on a maximum likelihood estimate; referred to as point estimates. Two empirical based models (a linear and a natural logarithmic interpolation model) have been developed to estimate the fracture properties at any length between 6 and 50 mm. These two interpolation models were also developed based on maximum likelihood estimates. The point estimates were used to benchmark the performance of the two models. The natural logarithmic model was found to be superior to the linear model. Keywords: Natural fibres, Jute, Tensile properties, Composites, Weibull statistics Introduction Natural fibres can be divided into three groups, vegetable, animal or mineral fibres. Vegetable fibres can be wood (further subdivided into softwood or hardwood) or non-wood (bast, leaf or seed hair) fibres. 1 Bast fibres are those ‘obtained from the cell layers surrounding the stems of various plants’. 2 These fibres are used in textile applications and are increasingly being considered as reinforcements for polymer matrix com- posites as they are perceived to be ‘sustainable’. Initial results from a quantitative life cycle assessment have been presented by Dissanayake et. al. 3,4 Natural bast fibres are composed primarily of cellulose. Cellulose microfibrils have a potential Young’s modulus of y140 GPa 5 which is comparable to that of man made aramid (Kevlar/Twaron) fibres at y125 GPa. The bast fibres which are currently attracting the most interest are flax and hemp (in temperate climates) or jute and kenaf (in tropical climates). The widespread use of natural fibres as the reinforce- ment in polymer matrix composites has been con- strained by the wider ‘natural’ variation in their mechanical properties compared to their man made counterparts. Thus, an in depth understanding of their natural variation is necessary for these fibres to emerge as a realistic alternative to synthetic fibre reinforcements for structural composites. The fibre tensile properties are limited by the presence of critical flaws. If a fibre comprises a series of elements (or links) then the strength of that fibre is governed by the weakest link. A longer fibre contains more links than a shorter one and the probability of a critical flaw therefore increases with fibre length, resulting in longer fibres having a lower tensile strength (on average). 6 This physical model of the fibre is often referred to as the principle of ‘weak link scaling’. In contrast to these physical based models, empirical or phenomenological models can be developed from a (limited) database of experimental results. A number of authors have used the principle of weak link scaling in an attempt to fully characterise the statistical distribution of synthetic and natural fibre properties but have found limited success. 7–9 The present paper considers jute fibres: 500 fibres are characterised (five different lengths with 100 fibres at each length). The variation in mechanical properties at each fibre length is established using Weibull statistics based on a maximum likelihood estimate (MLE). Two empirical based models (a linear and a natural logarith- mic interpolation model) are then developed to estimate the fibre properties (ultimate strength and fracture strain) at any length between 6 and 50 mm. The investigation therefore aims to enhance understanding of the stochastic nature of jute fibres with reference to the authors’ focus on natural fibre reinforced polymer matrix composite structures. Physical characterisation of fibres Jute technical fibres from a single source grown in South Asia were mounted on slotted cards (based on Grafil Test 101?13) with Devcon 2 Ton epoxy adhesive. A total of 500 fibre tests were carried out (100 fibre tests at each of the five gauge lengths: 6, 10, 20, 30 and 50 mm to an accuracy of ¡1 mm at each end). Before testing, measurements of fibre ‘diameter’ were taken at 1 mm intervals along the length of the fibre using an Olympus BX60MF optical microscope (serial number: 5M04733) and analySIS image analysis software. This is necessary as the fibre diameter varies along its length. Table 1 shows the mean fibre diameters and standard deviations at each fibre length and the overall measurements for all Advanced Composites Manufacturing Centre, School of Engineering/ Reynolds Building, University of Plymouth, Plymouth PL4 8AA, UK *Corresponding author, email wayne.hall@plymouth.ac.uk ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 5 September 2008; accepted 20 September 2008 DOI 10.1179/174328408X385818 Materials Science and Technology 2009 VOL 25 NO 10 1289