N. Hamila

Modelling the development of defects during composite reinforcements and prepreg forming.

Defects in composite materials are created during manufacture to a large extent. To avoid them as much as possible, it is important that process simulations model the onset and the development of these defects. It is then possible to determine the manufacturing conditions that lead to the absence or to the controlled presence of such defects. Three types of defects that may appear during textile composite reinforcement or prepreg forming are analysed and modelled in this paper. Wrinkling is one of the most common flaws that occur during textile composite reinforcement forming processes. The influence of the different rigidities of the textile reinforcement is studied. The concept of ‘locking angle’ is questioned. A second type of unusual behaviour of fibrous composite reinforcements that can be seen as a flaw during their forming process is the onset of peculiar ‘transition zones’ that are directly related to the bending stiffness of the fibres. The ‘transition zones’ are due to the bending stiffness of fibres. The standard continuum mechanics of Cauchy is not sufficient to model these defects. A second gradient approach is presented that allows one to account for such unusual behaviours and to master their onset and development during forming process simulations. Finally, the large slippages that may occur during a preform forming are discussed and simulated with meso finite-element models used for macroscopic forming. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

Stability of 3D Textile Composite Reinforcement Simulations: Solutions to Spurious Transverse Modes

The simulation of thick 3D composite reinforcement forming brings to light new modeling challenges. The specific anisotropic material behavior due to the possible slippage between fibers induces, among other phenomena, the development of spurious transverse modes in bending-dominated 3D simulations. To obtain coherent finite element responses, two solutions are proposed. The first one uses a simple assumed strain formulation usually prescribed to prevent volumetric locking. This solution avoids spurious transverse modes by stiffening of the hourglass modes. Nevertheless the deformation obtained by this approach still suffers from the inability of the standard continuum mechanics of Cauchy to describe fibrous material deformation. The second proposed approach is based on the introduction of a bending stiffness which both avoids the spurious transverse modes and also improves the global behavior of the element formulation by enriching the underlying continuum. To emphasize the differences between different formulations, element stiffnesses are explicitly calculated and compared.

Analysis of Defect Developments in Composite Forming

Continuous fiber-reinforced composite materials are increasingly used because they meet high performance standards associated to light weight. Recent-generation aircrafts make extensive use of composite materials for their primary structure [1]. There is also a strong interest in the automotive industry to reduce the mass of the components and consequently the fuel consumption of vehicles [2, 3]. However, this composite material potential comes with prices to pay. In particular, the good mechanical properties of the composite request that the manufacturing process is of high quality or put in another way that the manufactured part is defect-free or at least that it contains as few defects as possible.

The Bending Behaviour Characterisation of Thermoplastic Prepregs and its Influence on the Wrinkling

The bending deformation of thermoplastic prepregs is one of the key deformation modes in the thermoforming due to its crucial role in the wrinkling occurrence. The influence of temperature is of main importance because the viscous effect of resin is temperature dependent and prepregs thermoforming is usually performed closed to resin’s melting point. The currently available bending test devices are not adapted for thermoplastic prepregs since these devices can only be operated at room temperature. To solve this problem, a new cantilever test with an optical measuring performed in an environmental chamber is proposed. The bending properties of PPS-carbon satin prepregs are measured at a series of high temperatures. It’s shown that the bending stiffness of the fore-mentioned pepregs is strongly affected by the temperature and shows a non-linear bending behaviour. The measured bending properties are used to simulate a thermoforming process. The influence of bending properties on the simulation results, especially to the wrinkling is presented as well.

Locking and Stability of 3D Woven Composite Reinforcements

3D woven composite reinforcements preforming simulations are an unavoidable step of composite part processing. The present paper deals with thick composite fabric behavior modelling and issues arising during the numerical simulation of preforming. After the description of the independent deformation modes of initially orthotropic reinforcements, a physically motivated and invariant based hyperelastic strain energy density is introduced. This constitutive law is used to show the limitations of a classical finite element formulation in 3D fabric simulations. Tension locking is highlighted in bias extension tests and a reduced integration hexahedral finite element with specific physical hourglass stabilization is proposed. Instabilities due to the highly anisotropic behavior law, witnessed in bending dominated situations, are exposed and a stabilization procedure is initiated.

The Need to Use Generalized Continuum Mechanics to Model 3D Textile Composite Forming

Abstract3D textile composite reinforcements can generally be modelled as continuum media. It is shown that the classical continuum mechanics of Cauchy is insufficient to depict the mechanical behavior of textile materials. A Cauchy macroscopic model is not capable of exhibiting very low transverse shear stiffness, given the possibility of sliding between the fibers and simultaneously taking into account the individual stiffness of each fibre. A first solution is presented which consists in adding a bending stiffness to the tridimensional finite elements. Another solution is to supplement the potential of the hyperelastic model by second gradient terms. Another approach consists in implementing a shell approach specific to the fibrous medium. The developed Ahmad elements are based on the quasi-inextensibility of the fibers and the bending stiffness of each fiber.

Bias Extension Test for In-Plane Shear Properties during Forming - Use at High Temperature and Limits of the Test

The results of in-plane shear tests performed on 5-hardness satin woven carbon/PPS thermoplastic prepregs are described. The experimental analyses are based on bias-extension tests performed in an environmental chamber. The results are given for different temperatures on both side of the melting point. This range of temperature is those of the part during a thermoforming process. In another hand it is shown that second-gradient energy terms allow for an effective prediction of the onset of internal shear boundary layers which are transition zones between two different shear deformation modes. The existence of these boundary layers cannot be described by a simple first-gradient model.

Thermoforming Modelling and Simulation of Multilayer Composites with Continuous Fibre and Thermoplastic Matrix

CFRTP prepreg laminates thermoforming (Continuous Fibre Reinforcements and Thermoplastic Resin) is a fast composite manufacturing process. Furthermore the thermoplastic matrix is favourable to recycling. The development of a thermoforming process is complex and expensive to achieve by trial/error. A simulation approach for thermoforming prepregs thermoplastic is presented. This model is based on a continuous approach. A hyperelastic behaviour is associated with dry reinforcements. The hyperelastic potential is built from the contribution of three principal deformation modes that are supposed to be independent. A nonlinear viscoelastic model based on the generalization of simple rheological models is associated with the in-plane shear mode. The finite element simulation of a thermoforming example using this model is presented.

Thermoforming Simulation of Multilayer Composites with Continuous Fibre and Thermoplastic Matrix

CFRTP prepreg laminates thermoforming (Continuous Fibre Reinforcements and Thermoplastic Resin) is a fast composite manufacturing process. Furthermore the thermoplastic matrix is favourable to recycling. The development of a thermoforming process is complex and expensive to achieve by trial/error. 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.

Numerical Analysis of Non-Isothermal Viscoelastic Materials for Thermoforming of Polymer Films

In order to improve existing processes and study new industrial products, numerical simulation is a direct and effective solution to optimize the manufacturing processes and predicts experimental tests. In the present paper, the thermoforming of thin polymers films is analysed. A numerical modelling of the thermoforming process for non-isothermal viscoelastic sheet under large strains is proposed. The first step consists in heating the sheet using infrared lamps. The heterogeneous radiative heat transfer due to the shape of the tools is taken into account. The deformation of polymer sheets under this heterogeneous temperature is simulated. The temperature field is included in the viscoelactic behaviour laws of thin thermoplastic membranes under integral forms. The Lagragian formulation together with the assumption of the membrane shell theory is used. The finite element simulation of the different steps is presented. Moreover, the forming simulations are compared to experimental results, showing good agreements. Introduction Most industrial processes (thermoforming, injection moulding...) require the understanding of thermo-mechanical behaviour of polymeric sheets. Furthermore, the mastery of the deformation of polymers is an important stake. In this work, we are interested in developing models to simulate the thermoforming process of thin polymer films. Indeed, thermoforming is a common polymer forming process used in the manufacture of large thermoplastic components. The branches of industry interested in this process are those of the food packaging, the household electrical appliances, and the automobile industry. The thermoforming process [1,2] consists of fixing the sheet blank (clamping), heating the sheet up to the forming temperature, forming the sheet into a mold, making the products more rigid and retaining their final shape. The Fig. 1 gives a schematic representation of the process. Fig. 1: Thermoforming process The objective of this paper is to estimate the thickness and stress distribution of the deformed sheet. First, the approach and theoretical bases of the finite element analysis are discussed. Then the preheating simulation is described. The main goal of simulating this step is to estimate the Key Engineering Materials Online: 2013-06-13 ISSN: 1662-9795, Vols. 554-557, pp 1692-1698 doi:10.4028/www.scientific.net/KEM.554-557.1692