As indicated in Section 7.3.2.1, the raise of the blank holder pressure may have for consequence to also increase the tension in the fabric. Close to the basis of the shape, sliding of tows may appear when high blank holder pressures are applied therefore limiting the sliding of the fabric between the blank holder and the die. This phenomenon is shown in Fig. 7.12. The tow-sliding defect cannot obviously be accepted when manufacturing a composite part because the homogeneity of the reinforcement does not remain.

This type of defect also depends in a large extent of the complexity of the shape to be formed as well as the cohesion of the woven fabric. As an example, it is in­teresting to note that the tow sliding phenomenon appears for uniform blank holder pressures higher than 2 bar for reinforcement 1whereas it only appears for pressure higher than 5 bar for reinforcement 2.

The location of the phenomenon also depends on the orientation of the fabric. For reinforcement 1 at orientation 0°, tow sliding starts at places where tows are very tight (at the basis of Face C and at the bottom of Edge 1) as shown by Fig. 7.12.

For orientation 90°, the tow sliding phenomenon takes place on the basis of the faces A and B at the bottom of the buckle line where the tows seem to be the tightest (Fig. 7.13).

For reinforcement 2, the tow-sliding phenomenon only appears when the blank holder pressure is higher than 5 bar. This therefore means that the cohesion of this reinforcement is higher than the one of reinforcement 1. However, the phenomenon also takes place for orientation 0° at the bottom of the buckle line as shown by Fig. 7.14

The appearance of the tow sliding phenomenon shows that it is particularly im­portant to well control the blank holder pressure when a complex geometry is con­cerned and that it may not be possible to apply too high pressure if the cohesion of the reinforcement does not allow it. This may be a limitation of a fabric as increasing
the blank holder pressure may be useful to get rid of tow buckles (reinforcement 2) or suppress small wrinkles by increasing the tension of the whole membrane.