CA2609510A1

CA2609510A1 - Sandwich structure having a frequency-selective double-wall behavior

Info

Publication number: CA2609510A1
Application number: CA002609510A
Authority: CA
Inventors: Christian Thomas; Martin Wandel
Original assignee: Individual
Current assignee: Airbus Operations GmbH
Priority date: 2005-05-28
Filing date: 2006-05-24
Publication date: 2006-12-07
Also published as: JP5063595B2; DE502006002077D1; CN101228572B; DE102005024549B3; JP2008545903A; RU2007142936A; BRPI0611397A2; CN101228572A; WO2006128632A1; RU2405216C2; EP1886301A1; EP1886301B1

Abstract

The invention relates to a sandwich structure (1, 10) which comprises a core (2, 20) and cover layers (3, 4, 30, 40) disposed on both sides of the core (2, 20). Said core (2, 20) is provided with core recesses (5, 50) which are dimensioned in such a manner that the acoustic behavior of the sandwich structure (1, 10) in the area of the core recesses (5, 50) corresponds to the acoustic behavior of a double wall while at the same time maintaining the global acoustic behavior of the sandwich structure (1, 10).

Description

Airbus Deutschland GmbH 05HH39 Sandwich structure with frequency-selective double wall behavior Technical field The invention relates to a sandwich structure, compris-ing a core and outer layers arranged on both sides of the core.

Prior art Greater requirements for the acoustic comfort of pas-sengers in large-capacity aircraft demand better acous-tic properties of the cabin wall structure and conse-quently of the cabin wall elements making up the lining of the cabin. Standard material for these cabin wall elements takes the form of sandwich structures, such as for example sandwich panels with a core of a honeycomb structure.
The structure of a sandwich panel is represented by way of example in Figure 5. The sandwich panel 100 com-prises a three-layer composite construction with outer layers 300, 400 of the thickness dl and d2 and a sup-porting core 200 of the height h. A fiber-reinforced composite material is used for example as the outer layer on the upper side 300 and underside 400 of the supporting core 200. The supporting core 200 is formed by a honeycomb of paper impregnated in phenolic resin, which in its appearance resembles a bees' honeycomb.
It can be assumed that in future foams, for example of phenolic resin, will be used increasingly as the core material.

A major advantage of sandwich panels is their very high bending resistance together with low weight. In a sandwich panel, tensile and compressive stresses are transferred predominantly through the outer layers, while the core transfers the shear stresses resulting from deformation of the overall structure perpendicu-larly in relation to the surface of the panel.

In comparison with materials of a homogeneous structure and an identical or comparable weight per unit area, the acoustic properties of sandwich panels with respect to sound transmissions and sound emission are poor how-ever. For instance, the sound transmission loss above the limiting coincidence frequency that can be achieved with sandwich panels lies significantly below the value for homogeneous materials calculated on the basis of the mass law. By contrast with homogeneous wall struc-tures, in the case of sandwich panels there is a dip in the sound transmission loss in the region of the limit-ing coincidence frequency in the form of a wide-band plateau. The cause of this behavior is the shearing resistance of the core, which in this frequency range dominates the propagation velocities of the transverse waves within the sandwich panel.

Therefore, one possible way of improving the sound transmission loss of sandwich panels is to keep the propagation velocity of the transverse waves below the velocity of air-borne sound by reducing the shearing resistance of the core.

Thus, it is known from DE 100 34 990 Al and EP 1 382 439 A3 to reduce the shearing resistance of the core of a sandwich panel for example by slitting. As a result, the variation of the sound transmission loss is im-proved and the sound radiation of the sandwich panel is reduced.

Another possible way of reducing the adverse influence of the coincidence effect on the sound transmission loss is to increase the structure-borne sound insula-tion of sandwich panels.

A sandwich panel with outer layers arranged on both sides of a core is known from EP 1 061 190 Al. The core itself in this case comprises at least two core layers, the two outermost ones of which are respec-tively joined to an outer layer. The core layers are spaced apart from one another by means of spacing ele-ments. This sandwich panel represents an improvement in the sound insulation for the area of structural acoustics in comparison with single walls, and a sim-plified structural design in comparison with conven-tional double walls, such as for instance plaster board/mineral wool/plaster board walls. The design of the core layers arranged spaced apart from one another is in this case based exclusively on static aspects.
In EP 1 061 190 Al, no consideration is given to struc-tural design conditions and acoustic properties of the sandwich panel. Possible ways of avoiding the double-wall resonant frequency of double walls are not pro-posed.
It is known from Feng, L. "A modified honeycomb panel to increase sound transmission loss"; Tenth Interna-tional Congress on Sound and Vibration, Stockholm 2003;
pages 4549-4554, to increase the sound transmission loss of sandwich panels of an aluminum type of con-struction by locally detaching the adhesive bond be-tween the outer layer of aluminum and the core. As a result, on the basis of the mass law, the sandwich panel behaves acoustically in approximately the same way as a wall of a homogeneous material in the range of the limiting coincidence frequency and in the range of the double-wall resonance. A disadvantage of this is that only partial adhesive attachment of the outer lay-ers to the core of a sandwich panel is unsuitable for use in aircraft construction. In production, an addi-tional separating sheet has to be introduced in the re-gions that are not to be adhesively attached, and this sheet cannot be removed after production and conse-quently leads to an increase in the weight of the sand-wich panel. In the regions that are not adhesively at-tached, the core, separating sheet and outer layer rat-tle, which leads to the individual components becoming damaged and consequently reduces the service life of the sandwich panel.

DE-B 14 22 020 and DE-B 11 91 597 disclose a sandwich structure, comprising a core and outer layers arranged on both sides of the core, the core having regularly arranged core clearances.

Technical object of the invention The invention is therefore based on the object of de-veloping a sandwich structure with an improved sound transmission loss.

Advantages of the invention The object is achieved by a sandwich structure as claimed in claim 1, the core preferably having core clearances arranged directly under one or both outer layers, which clearances are dimensioned such that the acoustic behavior of the sandwich structure in the re-gion of the core clearances locally corresponds to the acoustic behavior of a double wall, and at the same time the global acoustic behavior of the sandwich structure is retained. By appropriate dimensioning and arrangement of the core clearances, the acoustic behav-ior of the sandwich structure according to the inven-tion in the high-frequency range corresponds to that of a double wall, and in the low-frequency range corre-sponds to that of a conventional sandwich panel. In order to achieve this acoustic behavior, the core clearances arranged in the core between the outer lay-ers are on the one hand large enough that the acoustic behavior of the sandwich structure in the region of the core clearances corresponds locally to the acoustic be-havior of a double wall, and on the other hand small enough that the global acoustic behavior of the sand-wich structure is retained. The core may in this case comprise a honeycomb structure, from which the core clearances are cut out. It is similarly conceivable for the core to consist of a cured foam, for example an expanded synthetic resin, from which the core clear-ances are cut out, or are left when the foam expands during its production. The core clearances create re-gions in which the outer layers are supported by the honeycomb structure or the foam of the core and regions in which the outer layers are not supported, or only a little. The sandwich structure may for example be a sandwich panel.

The sandwich structure according to the invention has the advantage over the prior art that in the range of the limiting coincidence frequency it behaves in a way corresponding to a conventionally constructed sandwich panel of identical dimensions, and above the limiting coincidence frequency it behaves acoustically like a double wall and not like a conventional sandwich panel.
Below the limiting coincidence frequency, the sandwich structure according to the invention behaves like a conventional sandwich panel. A further advantage of the sandwich structure according to the invention is a weight reduction in comparison with conventional sand-wich panels, resulting from the partial removal of the core. A positive side effect is the reduction of the shearing resistance of the core brought about by the core clearances, combined with the positive effects de-scribed above that this has on the sound transmission loss.

An advantageous refinement of the invention provides that the core clearances are formed in such a way that at least one outer layer has regions which are not sup-ported by the core. In this case, the core clearances may be formed in such a way that the core is partially complete, or only removed over part of its height, so as to create regions in which both outer layers are not supported by the core or regions in which only one outer layer is not supported by the core. The first natural frequency of the outer layer regions that are not supported by the core in the region of the core clearances is preferably below the limiting coincidence frequency of a sandwich structure of the same dimen-sions without core clearances, and above the double-wall resonance of the outer layers. In addition, the natural frequencies of neighboring outer layer regions that are not supported by the core are preferably dif-ferent. In order to avoid interference between neighboring core clearance regions, the ratio of the natural frequencies of neighboring core clearance re-gions preferably corresponds to a prime number or an irrational number.

Another advantageous refinement of the invention pro-vides that at least one outer layer has different thicknesses in the region of neighboring core clear-ances. The different thicknesses of the outer layer in the region of neighboring core clearances has the ef-fect of achieving different natural frequencies of the outer layer regions that are not supported by the core in the region of the core clearances.

An additional, advantageous refinement of the invention provides that the outer layers arranged on opposite sides of the core have different thicknesses. By mak-ing the outer layers arranged on the opposite sides of the core have different thicknesses, the double-wall resonance can be influenced. A suitable choice of the thicknesses of the outer layers allows the double-wall resonance to be set to the desired frequency. This can similarly be achieved by the outer layers arranged on opposite sides of the core consisting of different ma-terials.

According to a further advantageous refinement of the invention, it is provided that neighboring core clear-ances have different geometries. As an alternative or in addition, neighboring core clearances may have dif-ferent dimensions. The core clearances preferably have in plan view geometries that are simple to produce, with for example a rectangular, square, circular or triangular cross section. In this case, the core clearances may extend over the entire height of the core, or they may only take up part of the height of the core. In principle, it is also conceivable for the core clearances to be arranged in such a way that both outer layers have regions in which they are not sup-ported by the core, the core being retained continu-ously for example in the region of the middle of the height. The core is preferably reduced by the core clearances to a core lattice, at least under one outer layer. If the core clearances extend over the entire height of the core, the core lattice comprises webs connecting the outer layers to one another. In the case of all the embodiments, the core may be produced from a honeycomb structure from which the core clear-ances are cut out. It is similarly conceivable for the core to be produced from an expanded material.
Further advantageous refinements of the invention are obtained from a combination of the dependent claims and the following description of the exemplary embodiment.

Brief description of the drawings:
In the figures:

Figure 1 shows a schematic representation of the structure of a sandwich structure according to the invention in cross section, Figure 2 shows a schematic representation of various geometries of core clearances in plan view, Figure 3 shows a schematic representation of the structure of a sandwich structure according to the invention in cross section, with core clearances only arranged under one outer layer, Figure 4 shows a representation of the variation of the sound transmission loss of a sandwich structure according to the invention, of a double wall and of a conventional sandwich panel with the same dimensions and the same weight per unit area, and Figure 5 shows a schematic representation of the structure of a conventional prior-art sand-wich panel in cross section.

Detailed embodiment of the invention A sandwich structure 1, represented in Figure 1, sub-stantially comprises a core 2, two outer layers 3, 4, which are arranged on both sides of the core 2, and core clearances 5, which are arranged in the core 2 and take up the entire height h of the core 2. In the re-gion of the core clearances 5, the outer layers 3, 4 are not supported by the core 2. Depending on how they are viewed, these regions are also referred to as core clearance regions or outer layer regions in which the outer layer 3, 4 is not supported by the core 2. The dimensions al, a2 of the core clearances 5 are on the one hand dimensioned to be large enough for the acous-tic behavior of the sandwich structure 1 in the region of the core clearances 5 to correspond locally to the acoustic behavior of a double wall, and are on the other hand dimensioned to be small enough for the global acoustic behavior of the sandwich structure 1 to be retained. The core 2 in this case comprises a hon-eycomb structure, from which the core clearances 5 are cut out. In principle, it is also conceivable for the core 2 to be produced from a cured foam, for example from an expanded synthetic resin. Figure 1 shows a cross section along the line A-A through the core 2' of a sandwich structure 1 that is represented in plan view in Figure 2a), in the case of which the core clearances 5' have a rectangular geometry. The dimensions al, a2 of neighboring core clearances 5 of the same geometry differ from one another, in order to avoid interference between the natural frequencies of the regions of the outer layers 3, 4 that are not supported by the core 2.
In the case of the sandwich structures 1', 1", 1"', 111" that are represented in Figure 2, the core clear-ances 51, 511, 5111, 511" once again take up the en-tire height h of the core 21, 211, 211', 21111, so that there only remains a core lattice 6, which is shown in principle for various geometries of the core clearances 51, 511, 5111, 511" and comprises the webs 7 connect-ing the outer layers to one another. Thus, the core clearances 51 in Figure 2a) have a rectangular geometry in plan view. In Figure 2b), the core clearances 5"
have a circular geometry. Circular core clearances 5"
can be cut out particularly easily from the core 211, for example by means of a hole saw. In Figure 2b), di-agonally neighboring core clearances 5" have different dimensions. In Figure 2c), a core 211' with triangular core clearances 511' is represented. Neighboring, tri-angular core clearances 511' are arranged in different directions, have different dimensions, or both. In Figure 2d), the core clearances 511" arranged in the core 211" are dimensioned such that all that remains of the core 211" are columns 8 forming the core lat-tice 6 and connecting the outer layers to one another.
The columns 8 are preferably irregularly arranged, in order to avoid interference between the natural fre-quencies of the regions of the outer layers that are not supported by the core 211".

In the case of the sandwich structure 10 represented in Figure 3, the core clearances 50 take up only part of the height h of the core 20, so that the core 20 is partially removed only under one outer layer 30. As a result, one outer layer 30 has regions which are not supported by the core 20, whereas the other outer layer 40 is completely supported by the core 20. In princi-ple, the same geometries as are represented in Figure 2 are conceivable for the core clearances 50.
The diagram represented in Figure 4 shows the variation of the sound transmission loss TL over the frequency for a sandwich structure according to the invention, a double wall and a conventional sandwich panel. In this case, the sandwich structure according to the inven-tion, the double wall and the conventional sandwich panel have the same dimensions with respect to the dis-tance of the outer layers from one another, and the same weight per unit area.
The suitably dimensioned core clearances produce a fre-quency-dependent behavior of the sound transmission loss TL of the sandwich structure according to the in-vention, which corresponds both to that of a sandwich panel or a single wall and to that of a double wall.
In the low-frequency range, the behavior corresponds largely to that of the conventional sandwich panel or single wall. In the high-frequency range, the behavior corresponds to that of the double wall. The acoustic behavior of the sandwich structure is influenced by the core clearances in such a way that the behavior known from the prior art of the propagation velocity of the transverse waves over the frequency is eliminated. As a result, in the case of the sandwich structure accord-ing to the invention, the coincidence of the transverse waves with the surrounding air is completely avoided, and so too is the formation of a plateau-like coinci-dence dip in the sound transmission loss TL.

In order to achieve the frequency-selective double wall behavior of the sandwich structure according to the in-vention that is described and shown in Figure 4, and thereby achieve the elimination of the coincidence of the conventional sandwich panel (i.e. one which does not have any core clearances), knowledge of the limit-ing coincidence frequency fswo of a comparable conven-tional sandwich panel of identical dimensions and mate-rials for the outer layers and the supporting core is necessary for the design of the core clearance.

The dimensions of the core clearances, such as for ex-ample the dimensions al and a2 that are shown Figure 1, are subsequently to be chosen such that, below the lim-iting coincidence frequency fswo, the outer layer re-gions that are not supported by the core have a first natural frequency fe1. In this case, the sandwich structure according to the invention no longer acts globally as an overall structure but locally as a dou-ble wall. On the other hand, the clearances are chosen to be so small that, in the range of double wall reso-nance fdw0r the global behavior of a conventional sand-wich panel is still obtained, and no deterioration in the sound transmission loss occurs here. This means that the first natural frequency fel of the free outer layer regions must lie above the double wall resonance fdwo. Altogether, the following must therefore apply fawo < f < f ewo -As a result, the positive acoustic properties of the sandwich panel can be retained for the sandwich struc-ture according to the invention in the low-frequency range. At high-frequency, the sandwich structure ac-cording to the invention behaves acoustically like a double wall (Figure 4).

To sum up, an increase in the sound transmission loss is achieved together with a simultaneous weight reduc-tion.

It is particularly emphasized that the limiting coinci-dence frequencies of the individual outer layers con-sidered on their own lie significantly above the fre-quency range that is relevant for acoustic comfort for the materials that are typically used in aircraft con-struction. An occurrence of undesired cavity reso-nances in the core clearance regions in which the outer layer is not supported by the core is likewise only to be expected above the frequency range that is relevant for acoustic comfort. Furthermore, the core lattice, or the core remaining between the outer layers, acts like a compartmentalization of a double wall cavity.
This likewise has positive effects on the sound trans-mission loss. Something that has to be avoided is tun-ing of the regions of the outer layers that are not supported by the core to the same natural frequency.
In this case, the overall structure would act at the corresponding frequency like a panel made up of indi-vidual emitters. The clearances should therefore have different geometries and/or dimensions. As an alterna-tive or in addition to this, different outer layer thicknesses and materials may be chosen. Partial re-moval of the core under only one outer layer, repre-sented in Figure 3, also leads to the desired different outer layer properties.
It is easily possible for the invention to be put into mass production, since the corresponding regions of the core merely have to be removed before the core and the outer layers are joined together. The further produc-tion procedure is not adversely affected. Since no new or additional materials are required, no approval on the part of the competent authorities in this respect is required either.

Industrial applicability The invention can be industrially applied in particu-lar, but not exclusively, in the area of the production of sandwich structures for use in aircraft construc-tion.

List of designations 1 sandwich structure 2, 21, 211, 2111, 2"" core 3 outer layer 4 outer layer 5, 51, 511, 5111, 5111' c o re c l e ar anc e 6 core lattice 7 web 8 column sandwich structure core outer layer outer layer core clearance 100 sandwich panel 200 supporting core 300 outer layer 400 outer layer d thickness of the outer layer h height of the core al dimension of a core clearance a2 dimension of a core clearance

Claims

1. A sandwich structure, comprising a core and outer layers arranged on both sides of the core, the core having core clearances which are dimensioned such that the acoustic behavior of the sandwich struc-ture in the region of the core clearances corre-sponds to the acoustic behavior of a double wall, and at the same time the global acoustic behavior of the sandwich structure is retained, and which are dimensioned such that the acoustic behaviour of the sandwich structure in the low-frequency range corresponds to that of a corresponding sandwich structure of the same dimensions without core clearances and in the high-frequency range corre-sponds to that of a double wall.

2. The sandwich structure as claimed in claim 1, the core clearances being formed in such a way that at least one outer layer has regions which are not supported by the core.

3. The sandwich structure as claimed in claim 1 or 2, the first natural frequency of the outer layer re-gions that are not supported by the core in the re-gion of the core clearances being below the limit-ing coincidence frequency of a sandwich structure of the same dimensions without core clearances, and above the double-wall resonance of the outer lay-ers.

4. The sandwich structure as claimed in claim 1, 2 or 3, the natural frequencies of neighboring outer layer regions that are not supported by the core being different.

5. The sandwich structure as claimed in claim 4, the ratio of the natural frequencies of neighboring core clearance regions corresponding to a prime number or an irrational number.

6. The sandwich structure as claimed in one of the preceding claims, at least one outer layer having different thicknesses (d) in the region of neighboring core clearances.

7. The sandwich structure as claimed in one of the preceding claims, the outer layers arranged on op-posite sides of the core having different thick-nesses (d).

8. The sandwich structure as claimed in one of the preceding claims, the outer layers arranged on op-posite sides of the core consisting of different materials.

9. The sandwich structure as claimed in one of the preceding claims, the core clearances having the effect that the core is partially removed only un-der one outer layer, so that one outer layer has regions which are not supported by the core, and the other outer layer is completely supported by the core.

10. The sandwich structure as claimed in one of the preceding claims, the core being produced from a honeycomb structure.

11. The sandwich structure as claimed in one of claims 1 to 9, the core being produced from an expanded material.

12. The sandwich structure as claimed in one of the preceding claims, neighboring core clearances hav-ing different geometries.

13. The sandwich structure as claimed in one of the preceding claims, neighboring core clearances hav-ing different dimensions.

14. The sandwich structure as claimed in one of the preceding claims, the core clearances having a rec-tangular or square cross section.

15. The sandwich structure as claimed in one of claims 1 to 13, the core clearances having a circular cross section.

16. The sandwich structure as claimed in one of claims 1 to 13, the core clearances having a triangular cross section.

17. The sandwich structure as claimed in one of the preceding claims, the core clearances taking up only part of the height (h) of the core.

18. The sandwich structure as claimed in one of the preceding claims, the core being reduced to a core lattice, at least under one outer layer.

19. The sandwich structure as claimed in claim 18, the core lattice comprising webs connecting the outer layers to one another.