7th International Conference on Composites Testing and Model Identification C. González, C. López, J. LLorca (Editors) IMDEA, 2015 X-RAY MICROTOMOGRAPHY OF NATURAL FIBRE COMPOSITES Arttu Miettinen1, Roberts Joffe2,3, Liva Pupure2 and Bo Madsen4 1 Department of Physics, University of Jyväskylä FI-40014 Jyväskylä, Finland Email: [email protected], web page: http://www.phys.jyu.fi 2 Luleå University of Technology, Composite Centre Sweden SE-97187 Luleå, Sweden Email: [email protected], [email protected], web page: http://www.ltu.se 3 Swerea SICOMP P.O. Box 271, S-94126 Piteå, Sweden web page: http://www.swerea.se/sicomp/ 4 Technical University of Denmark, Department of Wind Energy P.O. Box 49, DK-4000 Roskilde, Denmark Email: [email protected], web page: http://www.dtu.dk Keywords: Tomography, Flax, Natural fibres, Microstructure ABSTRACT Structural composites are usually reinforced with glass- or carbon fibres, but lately various natural fibres (e.g. flax, hemp) have been proposed as an environmentally superior alternative . They seem to exhibit mechanical performance comparable to glass fibres, especially when properties normalized to density are considered. Furthermore, a number of companies are producing natural fibre fabrics (e.g. woven and non-crimp) and pre-pregs that are well suited for manufacturing of composite structures. Normally natural fibre composites are designed based on models (e.g. micro-mechanical) developed for synthetic fibres. These models use assumptions about regular structure of fibres and yarns that are well suited for synthetic fibres. The structure of natural fibres and yarns made of natural fibres is more complex , containing irregularities on many size scales. Modelling without taking these features into account might lead to unreliable results. The microstructure of composites and geometry of reinforcement is typically studied from polished cross-sections of test samples. The method may be unsuitable for natural fibre composites as the natural fibres are soft and sensitive to moisture. Traditional polishing techniques are thus rendered inefficient for producing polished surfaces of high quality. Additionally, polishing may change the structure of the composite e.g. by filling voids or by introducing micro-cracks. An alternative non-destructive method is X-ray microtomography that creates a threedimensional map of the composite sample based on density differences between the constituents (Fig. 1a) . In the present work, the method is applied to natural fibre composites manufactured from various types of fabrics, viz. non-crimp fabric, woven fabric and unidirectional fabric obtained by filament winding of dry fibre roving. The structure of Arttu Miettinen, Roberts Joffe, Liva Pupure, Bo Madsen the composites is analysed and observed to be rather different from what is expected. Different kinds of structural features including cracks and voids are identified and characterized (Fig. 1b). Figure 1: (a) X-ray microtomographic image showing the irregular meso-structure of composite manufactured from non-crimp flax fibre fabric. (b) A single X-ray microtomographic slice through a yarn in the composite in (a) visualising cracks (black lines) and fibre bundles (bright regions). REFERENCES    P. Wambua, J. Ivens, I. Verpoest, ‘Natural fibres: can they replace glass in fibre reinforced plastics?’, Compos. Sci. Technol., 63, 2003, 1259-1264. K. Charlet, C. Baley, C. Morvan, J. P. Jernot, M. Gomina, J. Breard, ‘Characteristics of Hermes flax fibres as a function of their location in the stem and properties of the derived unidirectional composites’, Composites Part A, 38, 2007, 1912-1921. A. C. Kak & M. Slaney, Principles of computerized tomographic imaging, IEEE Press, New York, 1988.
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