7.3.2.1 TOW BUCKLING

Tow buckles mainly appear during the forming of reinforcement 1 on faces and edg­es of the tetrahedron shape. The localization of the buckles seems to be influenced by the initial positioning of the fabric, and the size of the buckles probably depends on the tension state of the tows perpendicular to the ones passing by the triple point. As a consequence, two initial positioning of the fabric have been tested as shown in Fig. 7.9 in conjunction with different blank holder pressures.

FIGURE 7.9 Initial positioning of the woven fabric.

Figure 7.10 shows that in the case of the orientation 0°, buckles only appear on edge 1 and on the middle of face 3. No buckles are observed on faces 1 and 2.

Fig. 7.10. Reinforcement 1: Localization of the buckle zone for initial fabric orientation of 0°

As the bending of the tows perpendicular to the ones passing by the triple point is the mechanisms supposed to be at the origin of the buckling defect, measurements of the bending angles on each faces has been carried out. Results are presented in Table 7.1:

TABLE 7.1 Bending Angle of the Horizontal Tows Measured on the Buckle Zone

The bending angles of the tows exhibiting buckling on the 3 faces of the shape were also measured. The values are reported in Table 7.2.

TABLE 7.2 Bending Angle of the Horizontal Tows Measured on the Buckle Zone Orientation 90° Face number 1 2 3 Bending angle (°) 138 141 143

For orientation 90° the measured bending angles are situated in the same range of values as the ones measured for the 3 faces for orientation 0°. As a consequence,

the bending of the tows (Fig. 7.6d) is not responsible for the changes of the buck­le zone location for the 2 tested orientations. The initial reinforcement orientation seems to be crucial. As a consequence, bending is not a sufficient criterion to predict the appearance of the buckles.

The reinforcement considered in this study is not balanced. The tows, used in the warp and the weft directions are similar. However, a space between the weft tows (about the width of a tow) is observed on the fabric whereas this space is not present between the warp tows. As buckles only appear on bending zones where the weft tows are vertical, (face 3 and edge 1 orientation 0° and face 1 and face 2 orientation 90°) one can conclude that the architecture of the reinforcement is a key parameter conditioning the appearance of the buckles. When the warp tows are vertical (with­out any space between them) the buckles do not appear even though the horizontal tows exhibit the same amount of bending. This suggests that the presence of the space between the weft tows is one of the parameter that controls the appearance of the buckles.

The process parameters may also play a role in the occurrence of the buckling defect. The tows showing the buckles are not tight, and the effect of increasing the blank holder pressure upon the occurrence of the tow-buckling defect has been per­formed. In a first extent, the bending angles are considered. Table 7.3 reports for the 0° orientation the values of the bending angles in the 3 faces of the tetrahedron for three uniform blank holder pressures (uniform pressure applied to the fabric around the shape).

TABLE 7.3 Evolution of the Bending Angle as a Function of the Uniform Blank Holder Pressure for Both Reinforcements Reinforcement 1 Reinforcement 2 AngleTFace Pressure bar A B C AngleTFace Pressure bar A B C 1 138±5 136±4 146±4 2 139±5 143±4 141±5 1.5 137±5 137±4 142±5 4 144±6 144±5 149±6 2.5 135±4 139±4 138±4 5 137±4 147±6 147±5

For reinforcement 1, Table 7.3 shows that the bending angle measured on Face Cslowly decreases as a function of the increasing blank-holder pressure. This is probably due an increasing deformation of the tows passing by the triple point as these ones can drag in a larger extent to the top of the shape the perpendicular tows showing the buckles. The relative similar values observed on Faces A and B are probably due to measurement dispersion and to their relative inaccuracy. For rein­forcement 2, no real tendency can be extracted from the bending angles values. It has to be noted that measurement were also performed for orientation 90° for both reinforcements and that similar conclusions can be emitted. As a consequence, the change of the blank holder pressure does not influence much the bending angle and therefore the tow orientation on the Faces and it is not really possible to control it.

However, the size, or height of the buckles may be influenced by an increasing tension of the tows. For reinforcement 2, small size buckles observed in Edge 1 dis­appear when a uniform blank holder pressure of 2 bar is applied. For reinforcement 1, the buckles remain. Table 7.4 shows the heights of the buckles, measured on edge 1 and face 3 for the 0° orientation and on faces 1 and 2 for the 90° orientation for different increasing uniform blank-holder pressures.

TABLE 7.4 Reinforcement 1: Size of the Buckles as a Function of the Uniform Blank Holder Pressure (orientation 0°) Blankholder pressure(bar) Height of buckles (mm) 1 1.5 2 2.5 Edge 1 1.3±0.2 1.1 ± 0.1 1 ± 0.1 0.8±0.2 Face C 0.7±0.1 0.8±0.1 0.9±4 0.8±0.1

Table 7.4 shows that a relative reduction of the buckle size is observed on edge 1 while increasing the blank holder pressure. This reduction is probably due to a high­er tension in the tows showing the buckles. In Face C, the size of the buckles can be considered as constant as the precision of the measurement is about ± 0.1 mm.

To locally increase the tension on the tows showing the buckles, differential blank holder pressures can be applied. The pressure of blank holders 1 and 6 was increased with the goal to raise the tension in the tows showing the buckles in edge 1 (opposite of Face C). The pressure in the other blank holders remains at 1 bar.

TABLE 7.5 Size of the Buckles as a Function of the Increasing Blank Holder 1 and 6 Pressures

Blank holder pressure(bar) Height of buckles(mm)	0.75	1.25	2
Edge 1	1.1±0.1	0.9±0.1	0.8±0.1
Face C	0.7±0.1	0.8±0.1	0.8±0.1

Table 7.5 shows that the size of the buckles decreases as it was expected by increasing the tension in blank holders 1 and 6 as the tension in the tows exhibiting the buckles is probably raised. In Face C, the size of the buckles remains constant as blank holders 1 and 6 do not influence their behavior. To reduce their size, the pres­sure of blank holders 2 and 4 was raised and the pressure in the other blank holders remains at 1 bar.

TABLE 7.6 Size of the Buckles as a Function of the Increasing Blank Holder 2 and 4 Pressures Blank holder pressure (bar) Height of buckles (mm) 0.75 1.25 2 Edge 1 0.7±0.1 0.8±0.1 0.8±0.1 Face C 1.1±0.2 0.9±0.1 0.5±0.1

Table 7.6 shows that the size of the buckles this time decreases in Face C by increasing the tension in blank holder 2 and 4 as the tension in the tows showing the buckles is raised. However, the size of the buckles in Edge 1 remains constant.

The observations performed on the two previous tests indicate that it may be difficult to decrease simultaneously the size of the buckles and therefore to stop their occurrence by only working with the blank holder pressure and this for our test configuration.

It has to be noticed that the blank-holder pressure was not raised above values of

2.5 bar as another defect (sliding of tows within the membrane) appears in this case.