7.2.1 SHEET FORMING DEVICE FOR DRY TEXTILE REINFORCEMENT

A device presented in Fig. 7.1 was especially designed to analyze the possibility to form reinforcement fabrics. Particularly, the device was developed to examine the local deformations during the forming process64. The device is the assembly of a mechanical part and an optical part. The mechanical part consists of a punch/open die system coupled with a classical blank-holder system. The punch used in this study (Fig. 7.1.b) is a tetrahedron form with 265 mm sides. Its total height is 128 mm and the base height is 20 mm. The edges and vertices possess 10 mm radius for the punch and 20 mm for the die. As the punch possesses low edges radiuses, it is expected that large shear stains take place during forming. A triangular open die (314x314x314 mm3) is used to allow for the measurement of the local strains during the process with video cameras associated to a marks tracking technique65. A piloted electric jack is used to confer the motion of the punch. Generally, the punch veloc­ity is 30 mm/min and its stroke 160 mm. The maximum depth of the punch is 160 mm. A classical multipart blank-holder system is used to prevent the appearance of wrinkling defects during the preforming tests by introducing tension on the fabric. It is composed of independent blank-holders actuated by pneumatic jacks that are able to impose and sense independently a variable pressure. The quality of the final preform may depend on several process parameters such as the dimensions, posi­tions, and the pressure applied by each of the blank-holders can be easily changed to investigate their influence on the quality of the final preform66. Before starting the test, a square piece of fabric is positioned between the die and the blank-holders. The initial positioning of the fabric is of particular importance as it partly conditions the final tow orientations within the part. However, it is not possible to establish be­fore the test their final position at the end of the forming and as a consequence their mechanical stiffness. So, for the tests presented below, it was chosen to align the warp or the weft tows with an edge of the tetrahedron (the opposite edge of Face C (Fig. 7.1.b). To avoid bending of the fabric under its own mass, a draw bead system is used to apply low tensions at the tow extremities. At the end of the performing test, the dry preform can be fixed by applying a spray of resin on its surface so that the preform can be removed from the tools and kept in its final state.

At the end of the preforming test, several analyzes at different scales can be performed. A first global analysis at the macroscopic scale concerning the final state of the preform before removing it from the tool can be performed. It consists in ana­lyzing if the shape is obtained and if the shape shows defects. Another analysis, at the mesoscopic scale, consists in analyzing the evolution of the local strains (shear, tension) during forming.

Using this device, an experimental study to analyze with the tetrahedron shape, the generation of defects (wrinkles, tow buckles, tow sliding, vacancies, etc.) can be performed. The influence of the process parameter and particularly the blank-holder pressures on the generation and the magnitude of defects is also commented and analyzed.