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ABSTRACT
The present thesis has been developed through a studying period of six months spent in South Holland at E.S.A.-ESTEC. This work is focused on the studying of a particular composite material weave geometry, actually under developing, known as Triaxial Woven Fabric (TWF) which has having a big use in the Spatial field of communication satellite reflectors. TWF is a particular weaving developed on a plane and composed by three set of yarns that interlace at 00 –600 +600 degrees (see picture below).
Figure 1: TWF specimen.
A particular attraction of this material is that it is almost mechanically quasi-isotropic on a macroscopic scale. This feature makes it particularly suitable for thin structural elements obtained from a single ply and, at the same time, particularly attractive to designers of spacecraft antennas and the next generation of low cost deployable structures. However the strong influence of the geometrical parameters on the mechanical behaviour and the difficulties in implementing this material in analysis codes is a drawback.
The main objective of this study was to define a suitable F.E.M. model which is able to simulate the real behaviour of a TWF specimen under either mechanical and thermal loads;
the present work will start improving the TWF finite element model introduced by the previous work of Giuseppe Palermo [1], which will be considered as reference. The quality of the new model will be investigated through mechanical simulations; E1 modulus simulation test will be considered as base of comparison with the results available from the previous work. Validation of the model will be defined by comparing the results of the E1
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modulus simulation results with the results available from Literature; in the detail, the TWF Average E1 modulus value of 32.2 GPa evaluated in the Zhao Q., Hoa SV. , Oullette P. work [3] will be considered as objective to achieve. The starting finite element model is a detailed one; it accounts for all the geometrical parameters of the yarn but it is made up of only straight elements so it does not account for the real path that is followed by the free parts of the yarn in the cross regions of the fabric. Moreover the material definition is linked to Coordinate Systems whose orientation is fixed, so the material properties cannot follow the shape of the yarn itself (see Appendix A). Starting from this, a new finite element model was developed introducing improvements in the definition of the yarn geometry, the material properties and the number of elements used for meshing the surfaces of the yarn.
It will be shown in detail how the model is built. A comparison between old and new results showed an unclear behaviour which needed to be explained through additional tests.
It was concluded that solid element model must be even more refined to obtain accurate results. Since such a refinement was not possible in the scope of this work, a new Shell element model was introduced instead. Simulation of mechanical test showed good agreement with models and test results found in the literature. Finally an attempt was made to reduce the degrees of freedom of a complex TWF model with the use of superelement theory. Mechanical tests were simulated using the superelement TWF models in order to verify this technique. The results will be shown in detail and suggestions for future work will be given.
