Very small wrinkles may be observed in the corner basis of the tetrahedron as shown by Fig. 7.15.

FIGURE 7.15

The observed wrinkling defects have been observed for both reinforcement in the case of low blank holder pressures (lower than 1 bar). For higher pressure, the tension of the membrane become more important and a coupling effect72between in­plane shear and tension of the membrane already reported for carbon fabrics prob­ably acts to suppress the wrinkle. However, it is important to apply pressures lower than the appearance limit of the tow sliding.

Shear angle measurements have been carried out using the mark tracking meth­od to investigate the zones where the wrinkles may potentially appear, because it is commonly admitted that wrinkles appears when shear angles are too high. Figure 7.16 shows the shear angles measurements at different locations of Faces B (A to F) and A (G) for reinforcement 1 with orientation 0°.

The results presented in Fig. 7.16 show relative shear angle homogeneity on the whole two faces of the tetrahedron. The magnitude of these angles is not very high and is probably lower than the locking angle (evaluated in Section 3.3) above which wrinkles may appear. Figure 7.17 for the reinforcement 1 shows the measurements of shear angles at the corner basis at the bottom of Edge 1. The measured angles are much higher than the ones recorded on Faces B and A. This therefore explains why wrinkles can be observed on that location and not on the Faces.

Figure 7.18 shows the in-plane shear angles for corner 1 and 2 for orientation 0° for reinforcement 2 with a blank holder pressure of 5 bar. It shows that the shear angles have a tendency to rise as a function of the distance from the central line of the corner. The shear angles are also larger at the bottom of the corner. It is also in­teresting to note that the shear angles measured on reinforcements 1 and 2 are very close despite the fact that the fabric architectures are different. The shape therefore has a tendency to impose the shear angles in the corners.