Protocol for Quantification of Defects in Natural Fibres for Composites

Natural bast-type plant fibres are attracting increasing interest for being used for structural composite applications where high quality fibres with good mechanical properties are required. A protocol for the quantification of defects in natural fibres is presented. The protocol is based on the experimental method of optical microscopy and the image analysis algorithms of the seeded region growing method and Otsu’s method. The use of the protocol is demonstrated by examining two types of differently processed flax fibres to give mean defect contents of 6.9 and 3.9%, a difference which is tested to be statistically significant. The protocol is evaluated with respect to the selection of image analysis algorithms, and Otsu’s method is found to be a more appropriate method than the alternative coefficient of variation method. The traditional way of defining defect size by area is compared to the definition of defect size by width, and it is shown that both definitions can be used to give unbiased findings for the comparison between fibre types. Finally, considerations are given with respect to true measures of defect content, number of determinations, and number of significant figures used for the descriptive statistics. 1. Introduction Natural fibres, in the form of bast fibres from plants like flax, hemp, and jute have made a noteworthy contribution to the composite industry since the 1990s [1, 2]. However, having being used mainly for semistructural applications, for example, for automotive interior panels, natural fibres are attracting increasing interest for being used also for structural applications, for example, for wind turbine rotor blades [3]. In order to be successful in these structural applications, high quality fibres with good mechanical properties are required to form a competitive materials alternative to the conventional glass and carbon fibres. A central quality parameter for natural fibres is the amount of fibre defects which is expected to affect their strength properties and their reinforcement efficiency in composites [4]. Observations of natural fibres using polarised optical microscopy reveal that the fibres contain a number of structural irregularities distributed along their lengths (Figure 1). It is believed that these irregularities are local misalignments of the cellulose microfibrils within the fibre cell wall, and, as such, they are believed to form weak points and are therefore denoted defects; other terms like kink bands and dislocations are also frequently used in the literature [5]. Previous studies have shown that the