Investigation of the formability behaviour during stamping of tufted and un-tufted carbon preforms: towards localized reinforcement technologies

The originality of this work consists in the study of the stamping behaviour of tufted and un-tufted carbon preforms. Several preforms by varying the tufting thread orientations (0°, 90° and 0°/90°) and with different stratifications ([0°/90°,-45°/+45°] and [0°/90°,-45°/+45°]2) were manufactured in order to study their out-of-plane deformability. The stamping test was carried out using a hemispherical punch and conducted at two blank-holder pressures (0.05 and 0.2 MPa). The experimental data highlight the influence of tufting threads on the forming force, maximum material draw-in, shear angle mapping and wrinkling phenomenon during the forming process. Furthermore, the orientations of tufting thread, the number of layers and the pressure of the blank-holder significantly affected the forming behaviour: all tufted preforms present a higher forming force, lower maximum material draw-in and shear angles and notable structural defects than the un-tufted preform. The increase in the pressure of the blank-holder from 0.05 to 0.2 MPa increases all these characteristics and emphasizes the structural defects on the fibrous reinforcements. Similarly, the transition from two layers to four layers in lamination at the same blank-holder pressure is followed by an increase of the forming force required for stamping operation, reducing the material draw-in and the shear angles especially those measured at the transient zone, and causes more structural defects on all stamped tufted and un-tufted preforms. For all these reasons, two localized tufting configurations, Right Localized Tufted (RLT) and Inclined Localized Tufted (ILT), at the stamping transition area are proposed. The stamping results show that these two original configurations present a minimum punch force and a maximum material draw-in similar to those measured on the un-tufted structure. The shear angles are higher than those recorded on the conventionally tufted preforms (over the entire surface).