Formability analyses of uni-directional and textile reinforced thermoplastics
Introduction
The re-melting capability of fibre reinforced thermoplastic laminates allows for high volume production of thin-walled complex shaped parts for automotive and aerospace industry. This paper considers the process of stamp forming a pre-heated laminate into the desired geometry. The potential of this process was already shown in the 80s by Krone and Walker [1], and Okine [2] for high-performance composites. Such composites are currently used in aerospace industry, where this process is applied to the production of stiffeners that appear in the wing and fuselage assemblies. The process also shows a high potential for use in the automotive industry due to the high production rates that can be achieved. Product examples are car doors, hoods, and car construction pillars.
Two composite materials used in aerospace industry are considered in this study. One material is the CETEX® TC1200 PEEK/AS4, which consists of a polyetheretherketone (PEEK) thermoplastic matrix and uni-directional AS4 carbon fibres. The other material is the CETEX® PPS glass fabric US style 7781. These are respectively referred to as UD/PEEK and 8HS/PPS throughout this paper. The reinforcement architecture of these materials is shown in Fig. A.1 at the meso- and micro-level. The industry encounters large formability differences between these materials. Defects such as wrinkling occur frequently. Such defects lead to a knock-down of the product’s in-service performance. The local thickness increments caused by the wrinkle also results in poorly consolidated spots elsewhere in the product and possibly damage of the matched-metal tooling.
Comparative studies of materials with different reinforcement architectures are scarcely available in the literature. De Luca et al. [3] investigated the forming behaviour of uni-directional (APC2-AS4) and textile (PEI-CETEX) reinforced thermoplastic laminates. A product was selected with a bead that consists of single and double curvature areas. Experiments with these materials were shown to have very different drapability characteristics, especially in terms of wrinkling. Formability analyses of textile reinforced laminates are conducted by many other researchers. The most recent review on this topic is given by Gereke et al. [4]. Well documented practical formability analyses were already given by Hou [5], who studied the stamp forming of glass 8HS/PEI laminates into hemispherical moulds. Here, the effects of blank size, blank holder pressure, and mould geometry on the formability were investigated experimentally. A more recent practical analysis of textile reinforcement forming was given by Allaoui et al. [6], which included an extensive numerical analyses as well. For the formability analyses of laminates with a UD fibre reinforcement, the reader is referred to the work of Mallon et al. [7], Ó Brádaigh et al. [8], and the more recent work of Larberg [9], Haanappel et al. [10], [11], and Hallander et al. [12].
Product development times can be reduced when the effects of material parameters and process variables on the forming behaviour can be predicted. Forming prediction codes become increasingly available, which are categorised as continuous, semi-discrete, and meso-FE model approaches [4]. Validation is mainly performed with relatively simple products, such as domes [13], [14], [15]. A more complex geometry referred to as the “double dome” has been and is currently being benchmarked by several research groups [16], [17], [18], [19].
In this paper, the differences in forming behaviour between UD/PEEK and 8HS/PPS are investigated in detail for a representative product geometry used in the aerospace industry. Forming experiments are conducted and analysed firstly. Several deformation mechanisms are characterised for both materials. The results are subsequently processed in the finite element forming simulation software AniForm [20], which falls in the category of continuous approaches. The underlying theory has been described by Ten Thije et al. [21]. AniForm uses an implicit solution scheme and it deals with separate constitutive relationships to describe the in-plane, interface, and bending mechanisms of the modelled plies. Forming predictions for the considered geometry are presented and compared with the results of the forming experiments.
Section snippets
Forming experiments
Forming experiments were carried out in order to investigate the formability differences between two different composite materials: carbon UD/PEEK and glass 8HS/PPS. A representative product geometry for aerospace applications was selected as shown in Fig. A.2. This stiffening part appears in a wing fixed leading edge, designed and built by Fokker Aerostructures. The product contains a couple of features, such as the flat web and the long straight flange. Two beads and the curved flange are
Material characterisation
Intra-ply shear and tool-ply friction (Fig. A.7) of the considered materials were characterised for simulation purposes, which is shown in this section. The data will be used as an input for the forming simulations, to be addressed in a later section. Moreover, the obtained material property data contribute to a better understanding of the formability differences observed in the previous section. Bending properties have not been characterised due to the absence of a mature bending test for
Forming simulations
A forming prediction tool can be used to obtain a better understanding of the deformations that appear during the forming of a particular blank design into the aimed product geometry. As a result, predicted critical spots can be anticipated on by modifying the product and process design. Modifications of product geometry, material, lay-up, and process settings may eventually lead to the production of products without any defects. This section shows the set-up of a forming simulation for the
Conclusions
The formability of two different composite materials used in aerospace industry was investigated. Quasi-isotropic laminates of PEEK with a uni-directional carbon fibre reinforcement (UD/PEEK), and PPS with a woven glass fibre reinforcement (8HS/PPS) were considered.
The UD/PEEK blanks were very sensitive to wrinkling near doubly curved areas. Many small wrinkles were observed in the web of the product, whereas the 8HS/PPS blanks deformed smoothly without defects in those areas. Both materials
Acknowledgements
The laminates used for the experiments were supplied by Ten Cate Advanced Composites. Their good support during the experimental program is gratefully acknowledged. The stamp forming experiments were conducted at the Fokker Aerostructures company and were guided by Steven Teunissen, Richard Roerink and Michael Wielandt. Preparations were done by Gijs Gijsbertse. Their help is also highly appreciated. We also thank Dirk Soeteman for his support with the photogrammetry analyses.
Furthermore, the
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