Mechanical characterisation of flax-based woven fabrics and in situ measurements of tow tensile strain during the shape forming

Abstract

Forming complex shape composite parts with a good production rate/cost ratio is of particular importance for the automotive industry. The sheet forming of woven reinforcements is a promising technique, especially if complex shapes with singularities such as case corners can be obtained. Due to the more and more important recycling needs, the use of flax fibre based reinforcements may be considered for structural or semi-structural parts. During the sheet forming of a tetrahedron shape, the tows constituting the architecture of the reinforcement material are submitted to tensile strains. When using glass or carbon fibre tows, strains to failure are generally not reached. When flax-based fabrics are considered, the failure/degradation strength of the tows constituting the fabric may be reached. Even if no apparent failure is visible when observing the tows during forming, the strains measured by a mark tracking method indicate that the degradation limit of particular tows of the preform has been reached. This could lead to local lack of fibre density and to possible zones of weakness for the composite part. As a consequence, it is essential to improve the tensile performances of the tows constituting the fabric without losing their good impregnation characteristics and good ability to reach high mechanical properties for the composite part.