Complex shape forming of a flax woven fabric; analysis of the tow buckling and misalignment defect

TLDR: In this article, a flax fiber plain-weave fabric has been used to form a complex tetrahedron shape, which contains several geometric singularities required by many automotive parts such as double or triple curvature and low-curvature edges.

Abstract: With the view to minimise the impact on the environment and to produce structural parts with a good production-rate/cost-ratio, the sheet forming of woven flax based fabric was investigated in this study. A flax fibre plain-weave fabric has been used to form a complex tetrahedron shape. This shape is of particular interest as it contains several geometric singularities required by many automotive parts such as double or triple curvature and low-curvature edges. Globally, the complex tetrahedron shape was obtained, but tow buckling (out of plane bending of tows) was observed in specific zones of the shape. The main mechanism at the origin of this defect has been defined. A reduction of the tow buckle size was obtained by increasing the membrane tension. The influence of fabric architecture at the mesoscopic and macroscopic scales on the appearance of the tow buckles was demonstrated and discussed. Solutions to prevent the appearance of this defect based on the design of the fabric architecture at the tow or fabric scales were successfully proposed. As a consequence, when sheet forming of complex shapes is considered, specific fabric architectures should be chosen to prevent the appearance of the buckling defect.

Figures

Fig. 1. Architecture of reinforcement 1 at the scales of the fabric and the tow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Forming of reinforcement 1: (a) wrinkles outside of the useful zone; (b) position of the buckles and bending of the tows; (c) focus on the buckles; and (d) measurement of the bending angle on face 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Localisation of the buckle zone for initial fabric orientation of 0°. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Influence of the initial fabric orientation and the global Blank-holder pressure on the size of the buckles for reinforcement 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 12. Influence of selected Blank-holder pressure on the size of the buckles for reinforcement 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Localisation of the buckle zone for initial fabric orientation of 90°. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Complex shape forming of a flax woven fabric; analysis of the tow buckling and misalignment defect" ?

With the view to minimise the impact on the environment and to produce structural parts with a good production-rate/cost-ratio, the sheet forming of woven flax based fabric was investigated in this study. 

Q2. What are the future works in "Complex shape forming of a flax woven fabric; analysis of the tow buckling and misalignment defect" ?

To increase the possible development of the natural fibre composites to complex shape parts, this study investigated the possibility to preform, using the sheet-forming process, a highly complex shape with a triple curvature without defect from the selected reinforcements. 

Q3. What is the reason for the decrease in size of the buckles on face 3?

When the pressure of blank-holders 2, 4and 5 (small ones) was increased and the pressure of the large ones was maintained to 1 bar, the size of the buckles remained constant on edge 1 but decreased on face 3. 

Q4. What are the main factors that are considered in the case of the sheet forming process?

In the case of the sheet forming process, sliding conditions within the tool and the pressure of the blank-holders are generally considered. 

Q5. Why does the buckle size decrease on face 3?

The decrease in face 3 is probably due to the fact that the tension is raised in the tows perpendicular to the ones passing by the triple point forming the buckles. 

Q6. What is the reason for the decrease of the buckle size on edge 1?

Once again, one may suppose that the decrease of the buckle size on edge 1 is due to the rise of the tension in the tows forming the buckles. 

Q7. What did the buckles show in localised zones?

Increasing the global blank-holder pressure to tighten the whole mem-brane or increasing selected blank-holder pressures to tighten the tow exhibiting buckles showed that the size of the buckles decreased in localised zones but not in the whole buckled areas. 

Q8. What should be done to investigate the presence of defects?

A whole series of complex shapes should also be preformed to investigate the presence of defects such as buckles and misalignment and the mechanisms associated to their appearance. 

Q9. What is the reason for the decrease in buckle size on face 3?

The results presented in the previous paragraphs with the goal to reduce the occurrence or the size of the buckles show that the rise of the small blank-holder pressure, corresponding to a reduction of the fabric movement in the corner and to an increase of the tension in the tows passing by these blank-holders, slightly reduces the buckle size on face 3. 

Q10. What are the main reasons for the use of twisted yarns in textiles?

as discussed by Goutianos and Peijs [49] sufficient tensile properties of the yarns are necessary for these materials to be considered for textile manufacturing or for processes such as pultrusion or filament winding. 

Q11. What is the main reason for the occurrence of buckles?

The occurrence of buckles therefore depends on the shape to be formed because the shape may induce bending of the tows not only in their own plane but also on the architecture at the macroscopic scale of the fabric. 

Q12. What is the effect of the pressure of the blank-holders on the tows?

Differential blank-holder pressures were also applied for the 0° orientation to locally increase the tension on the tows exhibiting the buckles. 

Q13. What are the main characteristics of the mechanical approaches?

These mechanical approaches, based on the finite element method, take into account the complex mechanical behaviour of dry fabrics. 

Q14. What was the purpose of the reinforcement?

This reinforcement was originally developed to manufacture large panels with low curvature and was therefore not designed for complex shape forming. 

Q15. What is the first flax fabric used in this study?

The first flax fabric (Fig. 1) used in this study, is a plain-weave fabric with an areal weight of 280 ± 19 g/m2, manufactured by Groupe Depestele (France) from untwisted tows. 

Q16. What is the energy consumption of a traditional production of flax mats?

They showed, in the case of traditional production of flax mats, with the use of synthetic fertilizers and pesticides associated with traditional fibre extraction such as dew retting and hackling, that the energy consumption linked to the production of a flax mat is comparable to the energy consumed during the production of a glass mat. 

Q17. What is the main reason why they recommended the use of natural fibres for the reinforcement of composite?

They also recommended designing new architectures of woven fabrics from aligned fibres tows, as it is the case for the reinforcement studied in this work, instead of spun yarns. 

Q18. What were the other tests on the first reinforcement?

To investigate the influence of the process parameters on the generation of defects during the sheet forming process, other tests on the first reinforcement were conducted. 

Q19. How can the authors study the mechanical state of the preform?

It is also possible to study the mechanical state of the preform by quantifying the strain levels in different deformation modes (tension, in-plane shear).