Modelling of fabric draping: Finite elements versus a geometrical method

Over the last years, the demand for high strength and low weight materials, such as fabric-reinforced plastics, has grown in the aeronautical industry. By decreasing the production cost with short production cycle times and increasing reproducibility by automation, fabric-reinforced plastics are becoming increasingly better economical competitors to other construction materials such as for example aluminium. A relatively fast process is the Rubber Press Forming-process (RPF) for fabric-reinforced thermoplastics. Typically, the production cycle is in the order of a few minutes and consists of infrared heating of a pre-form above melt or glass-transition temperature, hot pressing of the pre-form and cooling the pre-form below the glass-transition temperature of the thermoplastic matrix. In the process, the negatively shaped rubber upper mould presses the hot pre-form on a positively shaped steel lower mould. Both the upper and lower mould are temperature controlled (rubber more indirectly). Heating, pressing, cooling and removing the product from the press are automated. Due to the good draping characteristics of the fabric material, the processing forces are low and the tools are relatively inexpensive. However, during production shrinking and warpage may result in unacceptable dimensional change of the products. Fibre re-orientation is one of the major factors causing these distortions. Especially when producing doubly curved parts, the process of draping causes the angle between the warp and weft yarns to vary over the product with this double curvature. As a result of this re-orientation, the thermomechanical properties of the fibre reinforced composite material show a corresponding distribution, in turn leading to a distribution of residual stresses. Obviously, the resulting fibre orientations must be predicted accurately to predict the overall thermomechanical properties and distortions of the product. In the past, draping has been modelled using several methods. A widely used method is geometrical draping. Geometrical draping methods however are not capable of modelling process specific boundary conditions. To incorporate these boundary conditions FE simulations are used to predict the fibre re-orientation. Here, a fabric reinforced fluid was implemented in DIEKA, a Finite Element (FE) package used for modelling for instance the process of deep drawing in metals [1]. The FE-model is compared with the geometrical draping method and the results are discussed. Thermoplastic composite materials can be processed by Rubber Press Forming at elevated temperatures. Process specific boundary conditions are difficult to incorporate in the classical geometric drape simulation methods. Therefore, a fabric reinforced fluid model was implemented in the Finite Element package DIEKA, which is capable of modelling the boundary conditions as well. The model predictions of both types of simulations are compared for a double dome geometry, having separated contact areas, leading to different results.