The first tests are carried out for reinforcement 1 with an orientation 0°. The blank holders’ pressure is set to 1bar. The preform in its final state is presented on Figs. 7.6a and 7.6d. The final shape is in good agreement with the expected tetrahedron punch without large wrinkle or un-weaving on the useful zone. Only very small wrinkles can be observed on corners of the edges for both reinforcements 1 and 2. At the local scale, on faces and edges, the tow buckling defects (Figs. 7.6.b and 7.6.d) and misalignment of tows (Fig. 7.6.b) are identified. Tow buckling only takes place on the Face C and on the opposed edge (between Faces A and B) whereas misalign­ment of tows is observed on all faces.

Depending on the initial orientation of the fabric, tow buckling (Fig. 7.6.b) ap­pears on faces and on one edge of the formed tetrahedron shape. These buckles zones converge from the bottom of the useful shape up to the triple point (top of the tetrahedron) (Fig. 7.6.d) Due to this defect the thickness of the preform is not ho­mogeneous. The height of some of the buckles can reach 3 mm near the triple point. Due to this thickness in-homogeneity generated by these buckles, the preform could not be accepted for composite part manufacturing.

At the fabric scale, the buckles are the consequence of out of plane bending of the tows perpendicular to those passing by the triple point. The tows passing by the triple point (vertical ones, or weft tows for orientation 0°) are relatively tight. They seem to be much more stressed than the warp tows, perpendicular to the one pass­ing by the triple point. It can be expected that the size of the buckles depends on those tows tension. In this zone, there is no homogeneity of the tensile deformation. This is illustrated by the orientation of the tows perpendicular to the one passing by the triple point on both sides of the buckle zone (drawn Fig. 7.2.d). These tows are

curved instead of being straight, and this phenomenon is probably at the origin of the buckles. The tow misalignment is also observed for reinforcement 2 (Figs. 7.7.a and 7.7.b) globally at the same location as for reinforcement 1. However, tow buck­les do not appear on the faces of the shape. Only very small buckles can be observed on the edge opposed to Face C in the case of a 0° orientation with a low blank holder pressure of 1 bar, as observed in Fig. 7.8.

However, tow buckles do not appear on the faces of the shape. Only very small buckles can be observed on the edge opposed to Face C in the case of a 0° orienta­tion with a low blank holder pressure of 1 bar, as observed in Fig. 7.8.

If tow buckles and small wrinkles can be observed on the shapes formed using the tetrahedron punch, the causes of the defects have not yet been discussed. It is therefore proposed to discuss the issues concerning the appearance of the defects that can be encountered during sheet forming of natural fiber based woven fabrics.