Patent Application
20080048359
The above aspects and further advantages of the present invention will become more apparent when the following detailed description is read together with the accompanying drawings wherein:
a is a schematic top view of a moulding tool for a spar with a rib and two curved flanges and a fibre star indicating the fibre directions for prepregs of uni-directional fibres (fibre tape) in the way they would be arranged if one should try to produce the spar in a conventional manner.
b schematically shows the moulding tool for the spar in
In the present specification, the following terms shall be interpreted as follows:
Prepreg—“Prepreg” shall here be interpreted as being a fibre tape or a fibre fabric or a non-woven fibre cloth or any other type of fibre layer, which has been impregnated with a matrix.
Matrix—“Matrix” shall here be interpreted to be a material which, as is well known in this technical field, binds together the fibres in a composite material, or a material which is used in order to bind together the fibres in a composite material.
Fibre tape—“Fibre tape” shall here be interpreted as being a fibre layer in which the fibres are oriented substantially in one single direction, so called unidirectional fibres, and thus contains solely or mainly only parallel fibres.
Fibre fabric—“Fibre fabric” shall here be interpreted to be a fibre layer in which the fibres are oriented in two or more directions, and in which the fibres in the different directions are woven or twisted together in some way.
Stack—“Stack” or “fibre stack” shall here be interpreted to be a number of prepregs put on top of each other.
Downforming—“Downforming” shall here be interpreted to be the process of applying a pressure on a prepreg being placed on a moulding tool, said pressure being applied in a specific direction in order to make the prepreg to better follow the moulding tool.
Critical fibre direction—“Critical fibre direction” shall here be interpreted to be the fibre direction of some of the prepregs which is most parallel with a curve of the moulding tool.
The inventors have identified difficulties associated with the production of curved parts made of composite material. Different explanation models have been proposed and different ways of solving the problems have been tested. The inventors have realised that the main problem is that fibres intended to follow a concave surface tend to take the shortest trip way and want to hang stretched in the air, confer with a string of a bow, roughly speaking. When such a stretched fibre is to be formed down and pressed close towards a moulding tool it is stretched and offers resistance, and forming of e.g. a flange becomes impossible. One way of solving the problem is to cut up the fibres, but this is expensive, time consuming and decreases the strength. A purpose of the present invention is to provide a cost effective, fast and simple method which also preserves or improves the strength.
A technical solution for the forming along a curved line is to avoid fibres which get hung up. This can be achieve by carefully choosing the fibre directions beforehand so that there will be no fibres contacting two points of the moulding tool where there is a line passing in the free air between said points and outside said moulding tool. It is often necessary, due to requirements regarding strength, that the composite laminate comprises many fibre directions—e.g. [0/90/45]. So a technical solution of the problem is to simply turn the entire “fibre stack” (i.e. all fibre directions). Turning the entire [0/90/±45]-“stack” by 10 degrees gives the result [10/100/55/−35]. In this way the original stack is being kept at the same time as hanging fibres are avoided. In practice, it is of course not a prepared ready to use fibre stack that is turned, but instead already when punching the prepregs, fibre directions are chosen so that when said prepregs are put on the moulding tool their three-dimensional orientation will be such that a line of sight between two points on said moulding tool does not correspond with any fibre.
In conventional methods, the fibre stack [0/90+45/−45]s8 is the most common but also [+30/−30/90]s6 and [0/22.5/45/67.5/90]s12 has given good results when tested. The term “s” in “s8”, “s12” etc means symmetrical, i.e. the stack is mirrored in the middle and becomes symmetrical, i.e. the layers are oriented in a sequential order so that the same fibre direction is obtained on the outer layers, the same fibre direction on the second next outer layers and so on. For instance the fibre stack [0/90+45/−45]s8 is built up of the layers having the fibre directions 0, 90, +45, −45, −45, +45, 90, 0. The number after “s” indicates the number of layers (8 in the example).
Practical forming attempts have shown that the method to choose displaced fibre directions works well and that the fibres do not make resistance at downforming.
a and 1b show part of a moulding tool for a wing spar with a curved flange. The moulding tool 100 is provided with a spar rib surface 110 for the becoming spar rib, a concave flange surface 105 for the becoming concave flange, and a convex flange surface 115 for the becoming concave flange. Furthermore, there is outlined in
In
In another preferred embodiment, the prepreg is first put on a plane surface. The fibre directions are then oriented so that all fibres, when the fibre stack is being lifted to the moulding tool, will cross one and the same edge of the moulding tool only once. The inventors have realised that if a fibre 120, as expressed in
The advantage of the method described above, comprises that composite objects having a complicated geometry, e.g. certain wing spars for airplanes, may be produced in a very cost efficient way by: exploiting an automatic taping machine, a so called ATL-machine; to put the entire stack on a plane surface in one single working moment; and to form the entire fibre stack in one single working moment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05109658.4 | Oct 2005 | EP | regional |
