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Composites Part B: Engineering

Thermoforming simulation of multilayer composites with continuous fibres and thermoplastic matrix

Abstract

Continuous Fibre Reinforced Thermoplastics (CFRTPs) have made their way into the aerospace an automotive industries as structural components. Thermoplastic composites offer many advantages over thermoset composites such as low cycle time and recyclability.

The development of a thermoforming process is complex and expensive to achieve by trial/error. This can be favourably replaced by numerical analyses. A simulation approach for thermoforming of multilayer thermoplastic is presented. Each prepreg layer is modelled by semi-discrete shell elements. These elements consider the tension, in-plane shear and bending behaviour of the ply at different temperatures around the fusion point. The contact/friction during the forming process is taken into account using forward increment Lagrange multipliers. A lubricated friction model is implemented between the layers and for ply/tool friction. Thermal and forming simulations are presented and compared to experimental results. The computed shear angles after forming and wrinkles are in good agreement with the thermoforming experiment. It will be shown by the comparison of two simulations that the temperature field play an important role in the process success.

Introduction

Prepreg composite forming is one of the main manufacturing processes for high performance composite materials. It represents an alternative to the LCM processes (Liquid Composites Moulding) [1], [2] in which a resin is injected or infused into a textile preform. The prepregs are semi-products where the matrix is already integrated into the continuous fibre textile reinforcement. The resin can be thermoplastic or thermoset. Thermoforming CFRTP prepreg is a fast manufacturing process compared to LCM process or thermoset prepreg forming that need a long polymerisation stage [3]. The duration of the thermoplastic forming process can be in the range of 1   min. In addition the composites with thermoplastic matrix are more easily recyclable than thermoset materials. The fibre volume fraction in the final composite part obtained by thermoplastic thermoforming can be high. The forming process must be performed at a temperature close to or higher than the melt temperature of the resin in order to render the textile prepreg deformation possible.

Depending on the geometry of the final composite parts, on the prepreg characteristics (weaving, properties of the fibres, of the matrix, etc) and on manufacturing parameters (temperature, tool loads, blank holder forces, etc), double-curved shape manufacturing may be difficult and can lead to defects (wrinkling, porosities, fibre fracture, etc). Simulation software for composites forming has been developed to predict the conditions for the process feasibility and optimise the main forming parameters [4], [5], [6], [7], [8], [9], [10], [11]. The present paper presents an approach for the thermoforming of thermoplastic prepregs. For the single ply deformation, it is based on the methods that have been developed for the simulations of the forming of dry textile reinforcements in LCM processes [9], [10]. This approach was extended to the forming simulations of multilayer continuous fibre reinforcements with thermoplastic resin composites taking into account thermal and viscous effects and contact/friction between the plies.

Forming simulations need a description of the mechanical behaviour of the composite ply during forming. As this ply is generally modelled by shell finite elements (or membrane elements if the bending stiffness is neglected), the mechanical behaviour of the prepreg ply during forming is given by biaxial tensile properties [12], [13], [14], in-plane shear properties [15], [16], [17], [18], [19], [20], [21], [22], [23], [24] and bending properties [25], [26], [27]. Biaxial tensile curves are non-linear due to the undulation change of the yarns and depend on the warp–weft strain ratio k (k  = ε warp/ε weft). The bending stiffness is low because of the possible motions between the fibres, but it is important in the determination of the size of wrinkles [10]. In-plane shear is the most important deformation mode to obtain double curved shapes. In thermoplastic prepreg forming simulations, the in-plane shear properties of the ply must be measured at high temperatures because the forming is performed above (near) the melt temperature of the matrix.

Section snippets

Semi-discrete shell finite elements

The modelling of textile composites forming can be realised at different scales. The mechanical behaviour of textile composite is particular because of the possible motions between fibres (microscopic scale) or yarns (mesoscopic scale) [28], [29], [30]. Most of the forming process simulations are performed at the macroscopic scale. Several mechanical models have been proposed for textile composite reinforcements [31], [32], [33], [34], [35], [36]. However there is no widely accepted model that

Mechanical properties of the fabric layer and geometry of the tools

The following thermoforming simulations are performed on 5-harness satin/PEEK (polyetheretherketone) prepregs. The main proprieties are given in Table 1.

The forming process of CFRTP must be performed at a temperature above the melting point of the resin to make the textile prepreg deformation possible. The manufacturing of doubly curved parts requires in-plane shear deformations. Consequently the in-plane shear properties are important for forming simulation. The in-plane shear behaviour at

Conclusion

A numerical simulation of the multilayered CFRTP prepreg forming and the comparisons with experimental approach were described in this paper. The simulation takes into account thermal and viscous friction effects. A correct correlation was observed between numerical and experimental results.

It is important to predict the feasibility conditions of the prepreg composite forming through numerical simulation analysis. The numerical simulation can improve the understanding of the forming process. On

Acknowledgements

The research work reported in this paper was supported in the scope of the project CRISTAL (CRISTAL: Collaborative Project French Ministry of Industry with the partners Armines, Atmostat, Carbone Forgé, CARMA, Daher, EADS IW, Eurocopter, INSA Lyon, MBDA, Schappe Techniques, Snecma, Université de la Méditerranée and with the support of Ministère de l'Économie, de l'Industrie et de l'Emploi (DGCIS), local communities, Région Rhône Alpes, Conseils Généraux 01 et 06 and FEDER).

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