Progress in Computational Structures Technology
Chapter 12 P. Boisse*+, D. Soulat+ and J.L. Daniel+
*Laboratoire de Mécanique des Contacts et des Solides, LaMCoS, UMR CNRS, INSA de Lyon, Villeurbanne, France +Laboratoire de Mécanique des Systèmes et des Procédés, LMSP, UMR CNRS ENSAM, ESEM, Orléans, France Keywords: continuous fibres, fabric behaviour, specific finite element, forming, composites, thermoplastic, porosities, shell, pinching.
This chapter concerns the simulation of two forming processes for continuous fibres composites. The first part is focused on dry woven reinforcements for RTM process. The mechanical behaviour of the reinforcement is defined at the woven cell level by two biaxial tension surfaces and by the shear torque versus yarn angle curve. Finite elements made of woven cells are built based on those quantities. A forming experiment and its simulation show the importance of the mechanical behaviour of the reinforcement. The second part concerns forming of CFRTP (Continuous fibres reinforcement and thermoplastic matrix). The relative sliding of the plies during forming leads to model each ply by a set of shell finite elements. The study focuses on the reconsolidation stage that is necessary to avoid porosities at the end of the forming. In order to simulate this step where the through the through the thickness stresses have an important role, a shell with pinching element is developed. This work presents a method of simulation of this forming stage based on a mechanical approach using finite elements which take into account the specificity of the mechanical behavior of dry fabrics. these finite elements consist of woven elementary meshs. The elementary deformation energy is calculated as the sum of deformation energies of each one of these woven cells. This approach makes it possible to introduce the specific behavior of a woven cell. For instance, the tension behaviour is given by surfaces which are identified by three complementary methods (experimental tests, 3D simulations, and development of models). The shear behaviour is given by torque-angle curves obtained by a picture frame test. In the case of unbalanced fabric forming the importance of the taking into account of the behavior of woven is shown. It is also shown that shear plays an important role in wrinkle description. The second part of the paper concerns CFRTP forming. This technology based on the use of continuous fibres and thermoplastic matrix is a promising alternative to the thermoset composites. Its use increases on the new planes for example A340/500-600 and A380. The advantage lies in a shorter cycle time. Moreover storage, recycling and repairs are easier. Forming is done in only one operation on the whole of the plies. This work mainly concerns the phase of compaction necessary at the end of the cycle. One shows on micrographies the presence of porosities during working and the importance of the compaction to close again these porosities. A new finite element of hull with pinching is used to simulate this phase. An additional degree of freedom makes it possible this element to take into account the stress/strain through the thickness what is necessary to simulate the reconsolidation of the composite. A simulation on a Z reinforcement is presented for which the normal stresses through the thickness are analyzed.
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