CONSOLIDATION OF BRAID-BASED CFRP STRUCTURES USING AN EXPANDABLE MANDREL

One of the most interesting preform technologies especially for longish geometries is the braiding technique. In combination with hollow and removable cores this technique can be one of the most effective for carbon fiber reinforced structures in high volume production. Among the composite applications it competes with filament winding, pultrusion and tape lay-up methods [1] These different methods are all interesting for longitudinal and hollow structures but limited by their manufacturing technology itself regarding geometry and textile structure. The braiding technology is used to manufacture near-net shaped preforms using a robot which guides the mandrel through the center of a braiding machine for overbraiding. Beside advantages like automation and high volume production, a third yarn system (0°-yarn) can be implemented during the braiding process. In this way longitudinal parts can be reinforced with biaxially-oriented ( torsional load ) and triaxiallyoriented yarns ( flexural load ). In combination with a mandrel which is suitable for the whole process chain (Braiding, Infusion, Curing, Demoulding) it becomes very interesting for high volume production. In order to implement a manufacturing process which fullfills requirements for high volume production, Bernet [2] shows that using an expandable mandrel in combination with braiding technology offers a good possibility for high volume production. An inflatable bladder offers the possibility to consolidate the impregnated preform during curing. At the end of the process the mandrel can be removed by simply deflating the bladder.by removing the pressure. The structure of a braid shows disadvantages regarding mechanical performance in comparison to e.g. wounded structures. Braided sleeves are built up by the intertwining of yarns which leads to out-ofplane waviness due to the yarns crossing each other. Previous studies [3], [4] describe that CFRP structures which are based on braids or weaves show lower performance in stiffness and strength compared to structures based on non-crimped fabrics (NCF). The advantage of using an inflatable mandrel in order to reduce time and costs for the production of hollow braided parts is shown and already used in different applications, for example tennis rackets or bicycle parts. Lehmann [5] describes the draping behaviour of biaxial braided sleeves. This behaviour explains the possibility of enlarging the impregnated preform during curing and the simplified demoulding after the curing process. This studies are focused on biaxial structures and does not consider triaxial structures. First investigations [6] show how an inflatable bladder affects the laminate structure in comparison to a CFRP structure which is manufactured on a steel mandrel. Shafts are manufactured and analysed based on laminate quality and torsional performance. All are manufactured with 3 layers of braid. To gain first experience the inflatable bladder is pressurized with 4bar during the curing process. The enlargement caused by the bladder ranges from 25.1mm to 26mm. A shaft with an inner diameter of 26mm is manufactured using a steel mandrel for comparison. The effect on the structure is visible regarding fiber volume fraction (FVF), wall thickness and reduced undulation. Micrographs show the path of the braiding rovings in which the effect of consolidation is presented. Comparing torsional stiffness, the pressurized version presents a slightly improved performance. However the FVF is different, which affects the wall thickness. The improvement cannot be explained by yarn consolidation. Another point has to be considered regarding this study [6]. The pressurized version is braided over a mandrel with smaller outer diameter which is then CONSOLIDATION OF BRAID-BASED CFRP STRUCTURES USING AN EXPANDABLE MANDREL