Keywords: optimization tools, tailored fibre placement, composites, HTP-connection beam, local buckling, stiffened panel, post-buckling, curved shells.

This paper presents two new design tools for lightweight aerospace structures. The first tool is the Tailored Fibre Placement (TFP) design tool TACO. It is used to optimize the fibre orientations of structures made of Carbon Fibre Reinforcement Plastics (CFRP). The optimization concept is explained and results are given for a horizontal tail plane connection beam of an aircraft. The second tool, iBuck, is a fast, semi-analytical local buckling and post-buckling tool for stiffened panels that are loaded in-plane. Results are presented for axially loaded panels and compared to FE-results.

Tailored Fibre Placement (TFP) is a textile process for the production of fibre reinforced structures. Using TFP the carbon fibre rovings may be placed on a base material in almost any desired orientation, thus deploying calculated optimum fibre quantities and orientations for optimal performance. In common composite structures the anisotropic material properties are usually not fully exploited. The tool TACO (Tailored Composite Design Code) was developed to optimize complex composite structures and it is embedded in the MSC PATRAN / NASTRAN environment. Using an optimization criterion in combination with FEM provides a very efficient optimization method leading to a curvilinear fibre pattern in contrast to the constant fibre direction of traditional design [1,2,3]. The application of such a procedure allows an optimized design for a specific composite light-weight-component. The tool changes the fibre orientations within a user-defined layer of a finite-element (FE) model such that the fibres are as closely aligned to the direction of the principal stresses as possible. For failure analysis the so called Simple Parabolic Criterion (SPC) was implemented, which is able to distinguish between fibre fracture and inter fibre fracture as different failure modes. TACO was applied to optimize a preliminary version of the horizontal tail plane (HTP)-connection beam of an Airbus A340 airplane for which experimental data were available. The experimental results were compared to the TACO predictions. The results for the optimized HTP-beams were also compared to the results for the conventional composite beams made of fabrics that had been tested earlier.

iBuck, a semi-analytical local buckling and post-buckling tool for stiffened panels, is presented in the second part of the paper. The panels are assumed to be representative for an aircraft fuselage and are stiffened in axial and circumferential direction. By local buckling it is meant that the skin within a bay may deflect laterally and may induce rotation of the stiffeners. The stiffeners themselves are not allowed to deflect in out-of-plane direction. In this context, semi-analytical means that the problem formulation is entirely based on the foundations of analytical continuum mechanics. However, numerical methods are used to solve the equations [4]. The skin is modelled as a Donnell type thin, weakly curved shell [5]. For both the stringer and the frame additional degrees of freedom in terms of deflection functions are introduced. It is assumed that both the stringers and the frames are rigidly connected to the skin. Longitudinal and transverse continuity must be ensured at all times, that is, the axial and transverse stretching of skin, stiffeners and doublers must be equal [6]. The structure may be loaded axially, laterally, by in-plane shear stresses or by internal pressure where the loads are increased incrementally. iBuck was implemented in the C programming language using Visual C++. Standard clapack routines from the lapack (linear algebra package) package were used to solve the equations. The FE analysis was carried out with ABAQUS 6.3.1. The structure was modelled using S4 shell and the equations were solved using the damped Newton method (*static,stabilize). Results are presented for axially loaded panels and compared to FE-results.

1

C. Mattheck, "Design in Nature", Interdisciplinary Science Reviews, 19(4), 298-314, 1994.

2

U. Kütemeier, P.J. Jörn, A.S. Herrmann, "Reverse Engineering für eine Bauteilfertigung in Tailored Fibre Placement-Bauweise", DGLR-Tagung, Munich, 17-20 November, 2003.

3

P. Mattheij, K. Gliesche, D. Feltin, "Tailored Fiber Placement-Mechanical Properties and Application", Journal of Reinforced Plastics and Composites, 17(9), 1998. doi:10.1177/073168449801700901

4

E. Riks, "An Incremental Approach to the Solution of Snapping and Buckling Problems", Int. J. Solids and Structures, 15 (1979), 529-551. doi:10.1016/0020-7683(79)90081-7

5

S. Timoshenko, "Theory of Plates and Shells", McGraw-Hill, New York, 1959.

6

E. Byklum, "A Simplified Method for Elastic Large Deflection Analysis of Plates and Stiffened Panels due to Local Buckling", Thin-Walled Structures 40, 925-953, 2002. doi:10.1016/S0263-8231(02)00042-3

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