Approach to net-shape preforming using textile technologies. Part II: Holes

Over the last decade studies have been performed on the on-line manufacturing of bores and the integration of bolts or other mechanical fasteners into the reinforcing fibre structure. Textile technologies provide the possibility for on-line integration of bolts or other fasteners into the dry preform. Accuracy, efficiency and properties of the built notched textile structure are different for each method investigated. However, using these technologies drilling or any other mechanical post treatment can be prevented. Thus, mechanical properties are different due to non-cut fibres or changed fibre orientations. Mechanical post-treatment free manufacturing offers economical advantages if a simple preforming process can be applied. Braiding, 3D-weaving, warp- and weft-knitting, biaxial reinforced weft-knitting and Tailored Fibre Placement (TFP) technologies were evaluated relative to their potential for the integration of bolts. This study focuses on the comparison of on-line manufactured bores to those drilled mechanically. Although there are many factors which influence joint strength, such as joint geometry, fibre orientation, stacking sequence and through-the-thickness pressure, no study reports on direct manufacturing of holes. In this study, the variability and influence of on-line manufacturing is investigated with respect to diverse textile parameters. Open-hole tension and bearing-strength testing methods were applied to characterize mechanical behaviour, while stereo-microscopy was used to evaluate achieved qualities of the Fibre Reinforced Plastic (FRP). Near net-shape holes improved the mechanical performance of the drilled holes slightly for particular load cases and textile technologies. The correct integration of the spacing bolt into the textile process was found to be essential for an accurate positioning of the joints and thus the placement of the preform into the mould.