JEC COMPOSITES MAGAZINE - Issue #58 - June - July 2010 - 39

Fibre placement
of designing variable-stiffness composites and to demonstrate the weight reduction potential in composite structures.

Design of a variable-stiffness cylinder subjected to bending
The design study included the optimization of a variablestiffness cylinder for maximum buckling load carrying capability at a given weight, while subjected to manufacturing, strength and stiffness constraints; the production of a variable-stiffness cylinder using fibre placement; a modal test and a structural bending test. A conventional 0°, 90° and ±45° laminate was optimized to serve as a baseline. The cylinders were made of 24 plies of graphite-epoxy material, with a 30 cm radius and a test length of 81 cm. The layup of the baseline shell was: [±45, 02, 902, 0, ±45, ±45, 90]S, the variable-stiffness shell was a hybrid of variablestiffness plies and constant-angle plies. The variable-stiffness plies had an angle variation in the circumferential direction, see Figure 1(a). The variation of the fibre angle within a ply was determined by a small number of design variables Ti, which fixed the fibre angle at 45° increments around the circumference, as shown in Figure 1(b). The fibre orientations in between these locations are defined by a constant curvature path, such that the fibre angle varies as:

(a) Axial stiffness distribution

(b) Axial load distribution
Fig. 2: Stiffness and load distribution around the cylinder circumference under bending

where θi is the circumferential coordinate at which the fibre angle is defined by Ti. The calculated buckling moment of the optimum variablestiffness cylinder was 805 kNm, an improvement of 18% compared with the buckling moment of the baseline cylinder, which was 678 kNm. The buckling load improvement was achieved by redistributing the load around the circumference due to the non-uniform stiffness. The layup of the variable-stiffness shell was [±45, ±ψ1, 0, 90, ±ψ3, 0, 90, ±ψ5]S, where ψ1, ψ3 and ψ5 represent three different steered ply definitions. The design variables, T0 to T4, for each of the three steered plies are given in Table 1.
Table 1: Design variables for the variable-stiffness laminate

The variation of the axial stiffness with the circumferential coordinate is given in Figure 2(a), where the stiffness of the variable-stiffness cylinder Exv is normalized with the axial stiffness of the baseline cylinder Exb. The axial stiffness on the tension side, between θ = 270° and θ = 180°, was more than 80% larger than the stiffness of the baseline cylinder, while the stiffness on the compression side, near θ = 180°, was slightly smaller than that of the baseline. The bending moment was applied to the ends

Focus
Fibre-placed, variable-stiffness composite laminates offer the possibility to change laminate stiffness gradually and to tailor the stiffness distribution so as to increase structural efficiency. Advanced fibre placement can be used to produce variable-stiffness composites, challenging designers to make full use of the production capabilities AFP has to offer. Ongoing research at the Faculty of Aerospace Engineering of the TU Delft in cooperation with Fokker Aerostructures and Boeing is aimed at generating knowledge and at developing and validating the tools required to exploit the weight reduction potential that variable-stiffness composites offer the aerospace industry.

Ply
ψ1(θ) ψ3(θ) ψ5(θ)

T0 (°)
10 10 10

T1 (°)
10 10 12

T2 (°)
10 11 10

T3 (°)
10 57 34

T4 (°)
25 62 69

No58 June - July 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #58 - June - July 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #58 - June - July 2010