7th International Conference on Composites Testing and Model Identification
C. González, C. López, J. LLorca (Editors)
IMDEA, 2015
Arttu Miettinen1, Roberts Joffe2,3, Liva Pupure2 and Bo Madsen4
Department of Physics, University of Jyväskylä
FI-40014 Jyväskylä, Finland
Email: [email protected], web page:
Luleå University of Technology, Composite Centre Sweden
SE-97187 Luleå, Sweden
Email: [email protected], [email protected], web page:
P.O. Box 271, S-94126 Piteå, Sweden
web page:
Technical University of Denmark, Department of Wind Energy
P.O. Box 49, DK-4000 Roskilde, Denmark
Email: [email protected], web page:
Keywords: Tomography, Flax, Natural fibres, Microstructure
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
[1]. 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 [2], 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) [3]. 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).
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.