Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including interlaminar mechanical fastener
Reexamination Certificate
1999-10-05
2002-03-12
Weisbergen, Rich (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including interlaminar mechanical fastener
C428S292100, C428S293700
Reexamination Certificate
active
06355337
ABSTRACT:
DESCRIPTION
1. Technical Field
The invention relates to a structural element of high unidirectional rigidity, in which unidirectional fibre strands are completely embedded in sheaths and said sheaths are connected to a shell skin and wherein part of the shell skin is in the form of stringers whereby the shell skin has a skin plane and comprises a skin thickness. It also relates to a method of producing dimensioned amounts of fibre stiffness perpendicular to the skin plane of the shell skin in the case of large structural elements.
2. Background of Related Art and Summary of the Invention
Structural elements of high unidirectional rigidity are known. For example, EP 0 758 607 A2 and U.S. Pat. No. 5,735,486 disclose a wing having shear resistant wing shells made of fibre reinforced materials for use in aircraft. In the case of the wing, tensile and compressive force accommodating elements are disposed on the inner face of the wing shells. These are in the form of unidirectional fibers extending longitudinally of the wing. Mutually spaced stringers, whose fibre constituent is formed from a layered fibre structure which is connected to the layered fibre structure of the wing shell, are formed on the inner face of the wing shell longitudinally of the wing. Unidirectional fibre bundles, which are embedded in the shear resistant wing shells, are arranged between the mutually spaced stringers. They extend longitudinally of the wing and are substantially rectangular in section. The space between the stringers accommodates a plurality of fibre bundles and it is divided across its width by partition walls extending parallel to the stringers. The fibre portion of the stringers and/or of the partition walls is formed by folding at least the inner fibre layer of the torsion skin of the wing shell. The partition walls may be provided with layered sections which lie on the upper face of the fibre bundles.
The expression fibre structures is usually understood as meaning laminar structures which are formed by layering differently oriented unidirectional individual layers. A method of manufacturing conventional laminar structures is known from DE 37 39 753 A1. However, all of these structures have the disadvantages of the state of the art i.e. stresses perpendicular to the fibers, a danger that the laminations will part due to a lack of stiffness over the matrix layers, and restricted ability to dimension them due to the multi-axial stresses. In addition, it is not possible to provide a so-called fail-safe form of construction.
When using isotropic materials, an adequate degree of stiffness is needed in every direction subjected to loads. Consequently, it is known to use multi-layer laminates in order to obtain adequate degrees of stiffness in each direction. Since each of the direction related properties alters in dependence on the layer in accord with the orientation of the fibers, the tensile stresses in a cross-section of the wall are very inhomogeneous and the failure behavior is correspondingly complex. Thus, stress components may be at work which are effective in the direction of least rigidity. The rigidity of a purely longitudinally loaded unidirectional single layer is, for example, 50 to 250 times greater than in the case of transverse tensile loads. Fibre reinforcements having a unidirectional fibre orientation are, however, very much less sensitive to transverse compressive strains than to transverse tensile stresses, for which reason the rigidity is some three times greater in the case of transverse compressive loads. The transverse tensile stresses in particular can represent an unfavorable load on the boundary layer between the fibre bundles. Stresses are thereby conveyed in a direction perpendicular to the direction of orientation of the fibers. The resin layer between the fibre strands must be able to withstand these stresses. However, a corresponding problem is also presented by the stress in the boundary layer between the fibers and the resin of a transversely loaded unidirectional single layer.
Consequently, the object of the invention is to produce a structural element of the type mentioned in the first part of claim
1
which overcomes the aforementioned problems and in which considerably higher failure limits can be achieved by constructional measures than are attainable when using currently available laminar structures.
In the case of a structural element of the type mentioned in the first part of claim
1
, this object is achieved in that there is provided an inner division of the cross-section of the shell skin thereby providing the strength of the fibers and the stiffness of the fibers in a direction perpendicular to the skin plane for every direction of loading, and in that one or more parting layers and/or a joint are provided in or on an upstanding skin fold and/or within the shear resistant sheath of a fibre strand.
The invention is based on the realization that the requisite rigidity cannot be matched to the flow of forces in the structure by providing a large area textile structure consisting of differently oriented individual layers. Rather more, a partially isotropic structure would thereby be produced which, additionally, has a low degree of stiffness perpendicular to the plane of the shell skin and is inclined to become delaminated.
The special feature of the solution in accordance with the invention lies in the deliberate usage of the decoupling caused by the envisaged parting layer for the anisotropic structural design. This decoupling results in directed frictional connections in each of the spatially separated regions which can be optimally withstood by parallel disposed fibers, i.e. which can be overcome. The stress, the stiffness and the strength are all oriented in the same direction.
The crucial point of the advantage of this solution lies in the freedom obtained in this manner of being able to design an entire structure in such a manner that the course of the frictional connections is optimal in the sense of the behavior of the structure. Unfavorable loading of the fibers is avoided to the widest possible extent. In addition, premature failure is practically eliminated.
Good utilization of the high rigidity of the fibers is made possible by virtue of the principle underlying the invention. The failure behavior of the structure is determined by the fibre stiffness and no longer, as was frequently the case in the state of the art, by the secondary stiffness transverse to the orientation of the fibers. As a result of the invention, an anisotropic design is now possible for the entire structure instead of the isotropic designs using anisotropic materials that are conventional today.
It has already been proposed in DE 197 30 381 C1 and GB 2 331 282 A, to additionally equip structural elements of high unidirectional rigidity with parting layers which interrupt the stress flow between adjacent unidirectional fibre bundles. This step is very advantageous, but as a sole measure, the result thereof is that all of the force flows between the unidirectional strand elements are interrupted except in the case of a compressive loading when the boundary faces are contacted. Due to the invention however, there is, in addition, a complete decoupling action, even under compressive loads, in the sense of an optimal loading of the fibre strands.
By using the measures in accordance with the invention, the advantage is obtained that one can avoid the failure of a component, as occurs when using unidirectional individual layers in a multi-layer composite, wherein stress components transverse to the orientation of the fibre occur under load, and shear stresses are generated which can cause the undesired failure of the component at values far below the theoretical values or those measured in the fibre strands. Through the formation of a structure in accordance with the invention, a structural element is obtained in which appreciably higher failure limits are produced since the critical stress components are, to a large extent, avoided through the construction of the structu
Pabsch Arno
Piening Matthias
Sigle Christof
Deutsches Zentrum für Luft-und Raumfahrt e.V.
Salter & Michaelson
Weisbergen Rich
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