A Solution consisting in increasing the blank holder pressure has been already men­tioned in Section 7.3.3.1 to get rid of wrinkles. However, low blank holder pressure

should be preferred to avoid to homogeneity defects caused by too high strained tows. The use of high resistance tows could also be a solution to prevent this defect. Using low blank holder pressure would also delay the appearance of tow sliding de­scribed in Section 7.3.2.2 as this defect only appears when the blank holder pressure is increased. This therefore means that compromises need to be found already at this level to prevent the appearance of these defects.

To prevent the appearance of buckles different solutions may be developed. A first solution consists in designing specific fabric architecture as it was showed in Section 7.3.2.1 that the architecture of the fabric was a critical parameter. Indeed, for the two orientations the buckles appear on the warp tows (on the face C and its opposed edge for 0°orientation and on faces A and B for the 90°orientation).

Reinforcement 1 considered in this study is not balanced since an important space is observed between the weft tows whereas it is almost nonexistent between the warp tows as shown schematically in Fig. 7.26.a. This space controls the appear­ance of buckles. Its presence between the weft tows allows the warp tows to bend out of plane. Between the warp tows the lack of space prevents the movement of the weft tows. As a consequence, a balanced fabric with no space between two consecu­tive warp and weft tows could prevent tow buckling (see Fig. 7.26.b).

This new fabric (reinforcement 3) manufactured with the same un-twisted tows was manufactured by GroupeDepestele and tested under the same process condi­tions as the previous studied fabric. The results presented in Fig. 7.5 confirm the absence of buckles.

A second type of solution was investigated to prevent buckling on the final pre­form; it focuses on the optimization of the forming process parameters so that the local tensions in the preform can be changed in the defect zones. Nevertheless, the change of the local tensions with no modification of stresses in the rest of the fabric is not easy with the geometry of the bank holders used in this study (Fig. 7.1.b).

To reach this goal, new specially designed blank holders have been elaborated to apply minimum pressure to the tows passing by the triple point. New tests are conducted on reinforcement 1 for the 0°orientation. The final preform presented in Fig. 7.28 is obtained for a blank holders’ pressure of 3bar, applied on the warp tows

on which the buckles appeared previously. No buckle is observed on the Face C and its opposed edge, unlike with the previous blank holder system.

7.5 CONCLUSIONS

The possibility of manufacturing complex shape composite parts with a good pro­duction rate is crucial for the automotive industry. The sheet forming of woven reinforcements is particularly interesting as complex shapes with double or triple curvatures with low curvature radiuses can be obtained. To limit the impact of the part on the environment, the use of flax fiber based reinforcements may be consid­ered for structural or semistructural parts. This study examines the possibility to develop composite parts with complex geometries such as a tetrahedron without defect by using flax based fabrics. An experimental approach is used to identify and quantify the defects that may take place during the sheet forming process of woven natural fiber reinforcements. Wrinkling, tow sliding, tow homogeneity defects and tow buckling are discussed. The origins of the defects are discussed, and solutions to prevent their appearance are proposed. Particularly, solutions to avoid tow buckling caused by the bending of tows during forming are developed. Specially designed flax based reinforcement architecture has been developed. However, if this fabric design has been successful for the tetrahedron shape, it may not be sufficient for other types of shapes and that is why the optimization of the process parameters to prevent occurrence of buckles from a wide range of commercial fabrics was also investigated with success.