Keywords: NCF, Stiffness Averaging Method, RVE, stitching, waviness, processing variables, FEM.

Traditional aerospace high-performance composites, based on unidirectional prepreg tape, generally provide unsurpassed in-plane specific properties, but their manufacturing costs are still very high. Indeed, the material itself is expensive, as well as the time consuming lay-up process and the equipment needed for storage and cure. Moreover, the out-of-plane properties of traditional composites are usually low due to the lack of through-thickness reinforcements.

On the other hand, textile technology, in combination with liquid moulding technique (RTM, RFI, etc.) offers economically attractive alternatives to the traditional prepreg composites. However, the amount of crimp present in the fibre yarns, although gains the integrity of the fabric, can causes a reduction of the in-plane material properties and can also induce dangerous failure mechanism. As a result, the Non-Crimp Fabrics (NCF) based composites have attracted the attention of many researchers and industries, offering lower operating cost and improved through-thickness properties with no significant drop in the in-plane performances. Due to the complex architecture of NCF materials, the local geometrical and material characteristics can significantly influence the global mechanical properties. It follows that the most efficient way to obtain an accurate global model is to first undertake the development of an accurate local model taking into account the micro-structure and the meso-structure of these innovative materials [1,2,3,5,6,8,9,10]. Nevertheless, there is still a lack of understanding about the effects of both NCF manufacturing variables and composite manufacturing parameters on the final composite performance and very few attempts to handle with this problem have been reported in literature [11,12].

This paper presents a novel finite element based approach able to take into account the complex architecture of the Non-Crimp Fabric composite materials and a model able to simulate their mechanical behaviour. By means of the Stiffness Averaging Method [4], the proposed model takes into account the processing variables (fibres and matrix components, type of stitching, type of dry preform, fibre waviness, stitching parameters, etc.) influencing the mechanical performances within a defined Representative Volume Element (RVE). The proposed approach consists in subdividing (discretization) the reinforcement system and the tows system of the RVE into distinct sets of sub-volumes respectively able to follow the path of the stitching thread and of the tows' waviness. From a numerical point of view a new material, called "NCF material", has been implemented in the research oriented finite element code B2000.

Applications to simple coupons loaded in tension are presented in order to demonstrate the capability of the developed model to take into account the processing variables. First of all, the results in terms of deformed shapes and load- displacement curves are presented. Then, the calculated longitudinal stiffness as function of the fibres waviness amplitude is reported in order to investigate the degradation of the in-plane properties due to the influence of the processing variables. Finally, the comparison between the obtained numerical results and experimental data available in literature [7] are shown in order to prove the effectiveness of the proposed approach.

In general, for all the applications, the developed tool has been found very effective and powerful in predicting the mechanical performances of Non-Crimp Fabric composite structures.

1

Pastore A.C.M., Whyte D.W., Soebruto H., Ko F.K., "Design and Analysis of Multiaxial Warp Knit Fabrics for Composites", Int. Journal of Industrial Fabrics, Vol 5 No 1, pp. 4-17, 1986.

2

Dexter H.B., Hasko G.H., "Mechanical Properties of Multiaxial Warp-Knit Composites", Composite Science and Technology, Vol. 56, pp 367-380, 1996. doi:10.1016/0266-3538(95)00107-7

3

Chou T. and Ko F.K., "Textile Structural Composites", Elsevier, 1989.

4

Bogdanovich A.E. and Pastore C.M., "Mechanics of Textile and Laminated Composites", Chapman & Hall, First Edition, 1996.

5

Kregers A.F. and Melbardis Yu. G., "Determination of the deformability of three-dimensionally reinforced composites by the stiffness averaging method", Mekhanika Polimerov, No. 1, pp. 3-8, January-February, 1978. doi:10.1007/BF00859550

6

Pastore C.M. and Gowayed Y.A., "A self-consistent fabric geometry model: modification and application of a fabric geometry model to predict the elastic properties of textile composites", Journal of Composites Technology & Research, JCTRER, Vol. 16, No. 1, January 1994, pp. 32-36. doi:10.1520/CTR10392J

7

Edgren F., "Mechanical Aspects on NCF Composites Behaviour", Licentiate Thesis, KTH Aeronautical and Vehicle Engineering, Stockholm, Sweden, 2003.

8

Drapier S., Wisnom M.R., "Finite-element investigation of the compressive strength of non-crimp-fabric based composites", Comp. Sci. Techn., 59: 1287-1297, 1999. doi:10.1016/S0266-3538(98)00165-1

9

Drapier S., Wisnom M.R., "A finite-element investigation of the interlaminar shear behaviour of non-crimp-fabric-based composites", Comp. Sci. Techn., 59: 2351-2362, 1999. doi:10.1016/S0266-3538(99)00091-3

10

Xiaoping Ruan and Tsu-Wei Chou, "Failure Behaviour of Knitted Fabric Composites", Journal of Composite Materials, Vol. 32, No. 3/1998: 198-222. doi:10.1177/002199839803200301

11

Ramakrishna, S. and Hull, D., "Tensile Behaviour of Knitted Carbon-Fiber Fabric/Epoxy Laminates, part II: Prediction of Tensile Properties". Comp. Sci. Tech., 1994, 50:249-258. doi:10.1016/0266-3538(94)90146-5

12

Ramakrishna, S. "Characterisation and Modelling of the Tensile Properties of Plain Weft knitted-fabric Reinforced Composites". Comp. Sci. Tech., 1997, 57:1-22. doi:10.1016/S0266-3538(96)00098-X

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