The challenges of natural fibres as an engineering composite reinforcement

Glass fibres (GF) currently still represent more than 95% of the reinforcement fibres used globally in the composites industry. However, the increasing pressure on natural resources and the large amounts of energy required in GF production has led to an upsurge in interest in the reinforcement potential of natural fibres (NF). However, a certain level of reinforcement performance is still required from such fibres in order to succeed in engineering composite applications. In this context many researchers refer to the respectable level of axial modulus exhibited by some NF, which can be made to appear even more attractive by comparing modulus/density ratios (see Table 1). However, an overwhelming number of the published results based on such justifications have failed to fulfil the expectation of matching GF composite performance, and the NF show only moderate reinforcement in stiffness and very little positive (and often negative) effects on composite strength. The majority of the widely available and commercially attractive, low cost, “technical” NF present a number of challenges in terms of their characterization, processing and performance which are further compounded by the high levels of variability inherent in materials sourced directly from nature. Consequently the documentation of their properties is often presented with extremely wide ranges of possible values (see Table 1). Further points to note in Table 1 are the lower strengths and higher “diameters” of NF. In fact most technical fibres are highly non-circular and are actually composites in their own right. This presents engineers, used to the availability of consistent and accurate mechanical property data on manmade fibres, and a range of micromechanical theories and characterization tools based on fibre reinforcement having a circular cross section, with significant challenges in terms of designing reliable structures based on these natural fibre composites. This paper will review a number of these challenges and focus in detail on two areas, NF anisotropy and their non-circular cross section, which although almost universally acknowledged, have received little attention in respect of their consequences on the characterization and performance of natural fibres as a composite reinforcement