Keywords: forming, composites, thermoplastic, porosities, shell, pinching.

Composites with continuous fibres and thermoplastic matrix (CFRTP) are a promising alternative to the use of thermoset composites. It is much increasing in the manufacturing of new planes, for instance in the A340/500-600 and A380 programs [1] [1]. Their main advantage is the shorter processing cycle. The forming is made, after heating at a temperature higher than melting, with a punch and die process usually using a rubber on the die. Finally a reconsolidation is obtained by applying a pressure on the punch. The objective is to avoid all residual porosity at the interface of the plies. This last stage is important and critical because the health requirements for loaded aeronautical parts are severe.

The proposed paper is focused on the description of the simulation of the laminate forming by shell elements. The analysis of the forming is made using one shell set per ply. There is a viscous contact between the plies and they can slide relatively.

Porosities within the thickness of the composite are source of possible fracture in the service life and they must be avoided especially for aeronautical applications. That is the reason for the re-consolidation stage of the forming process. In order to investigate the apparition of these voids and their resorption during forming and compaction stages, experiments have been done on a Z shape reinforcement. The forming process has been stopped at different level of the manufacturing and micrographies have been done in order to measure the porosities. The measures and micrographies have been done at different places of the specimen especially in a flat and in a curved part. These experiments show clearly that porosities appear during the heating and forming stages and that the reconsolidation is essential for the final quality of the part. Consequently the numerical simulations have to describe this stage. The re-consolidation has been studied in [2] and some models have been proposed for the local consolidation. These studies have shown that the re-consolidation depends on the stress state in the laminate and mainly on the normal stress in the consolidation stage. This stress is not present in classical shell theory. Some finite elements with stress/strain through the thickness have been proposed [3,4]. In the present work a new shell element is used where a through the thickness strain degree of freedom is introduced [5]. The stress through the thickness is not zero and this kind of finite element can describe correctly the compression during the re-compaction stage without change the general type of F.E. analysis.

Different tests show the effectivity of the normal stress computations in comparison with 3D analyses. It is shown that "pinching" locking can be avoided by using appropriate coupling in the constitutive operator. Simulation example of the forming and the compaction stage are shown in the case of a Z profile. They are in good agreement with forming experiments made on this profile.

1

Maison S., Thibout C., Garrigues C., Garcin J.L., Payen H., Sibois H., Coiffer C., Vautey P., Technical developments in thermoplastic composites fuselages, SAMPE journal, 34, no5, 1998, 33-39.

2

Lee W., Springer G., A model of the manufacturing process of thermoplastic matrix composites, Journal of Composite Materials, Vol. 21, p1017-1055, 1987. doi:10.1177/002199838702101103

3

Simo J.C., Fox D.D., Rifai M.S., On a stress resultant geometrically exact shell model. Part 4 : Variable thickness shells with trough-the-thickness stretching, Comp. Meth. in App. Mech. and Engng., 1990, vol. 79, p 21-70. doi:10.1016/0045-7825(90)90094-3

4

Butcher N., Ramm E., Roehl D., Three dimensional extension of non-linear shell formulation based on the enhanced assumed strain concept. Int. J. for Num. In Engng., 1994, Vol 37, p.2551-2568. doi:10.1002/nme.1620371504

5

Coquery M.H., Modélisation d'un joint de culasse multifeuille, Ph. D. Thesis, ENSAM Paris, 1999

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