WO2013113866A1

WO2013113866A1 - Closed structural assembly with improved resistance to compression

Info

Publication number: WO2013113866A1
Application number: PCT/EP2013/052012
Authority: WO
Inventors: Jean-François GENESTE
Original assignee: European Aeronautic Defence And Space Company Eads France
Priority date: 2012-02-02
Filing date: 2013-02-01
Publication date: 2013-08-08
Also published as: FR2986500B1; FR2986500A1; EP2809577A1

Abstract

The invention concerns a structural assembly (100) comprising: - a closed chain defining a closed peripheral contour lying within a plane, comprising a plurality of successive structural links (200), each structural link comprising two structural branches (201a, 201b) facing one another and of convexity oriented in substantially opposing directions, linked together at each of the opposing ends (204a, 204b) thereof by a so-called end node (300), - means (400, 500) for geometrically stabilising the peripheral contour, said means being disposed in such a way as to prevent the geometric deformation of the peripheral contour in the plane under the effect of external pressure forces exerted, and said means comprising: - for each link (200), a first inextensible stabilising member (400) linking two branches (201a, 201b) of a same link, excluding the end nodes (300), - a second inextensible stabilising member (500) linking two non-adjacent end nodes of two separate links. The structural assembly is advantageously intended for the production of stratospheric balloons.

Description

Closed structural assembly with improved compression resistance

The present invention belongs to the field of structures. More particularly, the invention relates to a closed structure whose compressive strength is improved without increasing its mass. The invention finds particular application to the production of a stratospheric balloon.

It is known that the materials are generally more resistant in tension than in compression. For example, the realization of a structure, such as a hollow tube of any dimension, working in tension is easy while the realization of an identical structure working in compression is complicated. Indeed, for a given dimension, making a structure resistant to a compressive force generally leads in return to significantly increase its mass. This increase in mass penalizes the design of architectures working in compression in many fields of industrial application, in particular in the field of aeronautics.

To illustrate an example, we can cite the case of the current stratospheric balloons, that is to say those which fly in the stratosphere, layer of the terrestrial atmosphere which begins, at temperate latitudes, around 20 km of altitude . These balloons are so-called drifting balloons, that is to say that it is difficult to stabilize them at altitude during a plurality of diurnal and nocturnal cycles. This stems essentially from the fact that when one wishes to drive such balloons, their power supply being solely of solar origin, their mass equation does not converge. In other words, given the winds, the energy needed to counter them and keep a geostationary position is too large and has a mass impact too large for such balloons can remain a year in the air at their cruising altitude.

A solution to gain mass would be to replace the aerostatic gas (eg helium) contained in the balloon by vacuum. For example, for a balloon of 23,000 m ³ , this would represent a significant saving of the order of 300 kg of helium. However, the pressure at 20 km altitude being 54 hPa, the pressure force that would exert on the structure current balloon would be too important. At present, no sufficiently light structure can hold such an effort.

The implementation of structures whose compressive strength is improved without mass penalty is therefore important for industrial uses, particularly for making stratospheric balloons.

According to the invention, a structural assembly comprises:

a closed chain defining a closed peripheral contour forming part of a plane, comprising a plurality of successive structural links, each structural link comprising two structural branches facing each other, of convexity oriented in substantially opposite directions, interconnected in each their opposite ends by a so-called end node,

- Geometric stabilization means of the peripheral contour, said means being arranged so as to prevent geometric deformation of the peripheral contour in the plane under the effect of external pressure forces exerted.

The geometric stabilization means comprise:

for each structural link, a first inextensible stabilization element connecting two structural branches of the same structural link, excluding end nodes,

a second inextensible stabilization element connecting two non-adjacent end nodes of two distinct structural links.

By inextensible, we mean an element that has a zero deformation or almost zero for the efforts that will have to support the structural assembly.

The structural assembly according to the invention, thus has a geometry that allows it to withstand external pressure forces that would be exerted on it.

The structural assembly is advantageously intended for producing stratospheric balloons.

According to preferred embodiments, the invention also satisfies the following characteristics, implemented separately or in each of their technically effective combinations.

In preferred embodiments of the invention, each structural branch of a structural link comprises at least two successive segments interconnected by an intermediate node, the first stabilization element extending between two intermediate nodes of the two branches.

Advantageously, not to weigh down the structural assembly, the segments are hollow beams, preferably made of composite material.

According to an advantageous characteristic of the invention, the two structural branches of a structural link are symmetrical with respect to a straight line passing through the two end nodes of said link.

In embodiments of the invention, the first and / or second stabilizing element is a rigid element.

Rigid means an element whose shape and dimensions do not undergo substantial changes during use of the structural assembly.

Most advantageously, each end node of a structural link is connected to a non-adjacent end node of a separate structural link by a second stabilizing element.

The invention also relates to a method for producing a structural assembly comprising the steps of:

define a desired initial geometrical shape of the structural assembly,

positioning N points, corresponding to N end nodes, on the geometrical shape, N greater than or equal to 3,

for each end node, connect it to an adjacent end node by a structural link,

- For each link, connect the two structural branches of said structural link by a first stabilizing element, - connect two non-adjacent end nodes of two separate structural links by a second stabilizing element.

The invention also relates to a three-dimensional structural framework comprising a plurality of structural assemblies such as previously described in at least one embodiment and a plurality of geometric stabilization means of the structural framework in space, two structural assemblies being connected by at least one geometric stabilization means of the structural framework, each stabilization means geometric structure of the structural framework being bonded, at two opposite ends, to end nodes of each structural assembly.

In embodiments of the invention, a means for geometrically stabilizing the structural framework in space is a rigid element.

The invention also relates to a stratospheric balloon. Said stratospheric balloon comprises a structural framework as defined above in one of its embodiments and a skin stretched over the peripheral contour of each structural element as previously described in one of its embodiments. The stratospheric balloon thus produced is a closed structure, thus making it possible to contain a vacuum, and its geometry makes it possible to withstand high external pressures, of the order of at least 50 hPa, while maintaining an acceptable mass. Typically, and for comparison, for a current stratospheric balloon of 23000 m ³ at 20 km altitude, the total mass of the stratospheric balloon would be 2000 kg while a stratospheric balloon according to one embodiment of the invention , which would hold the vacuum and for a toric type geometry, would be of the order of 400 kg.

The invention will now be more precisely described in the context of preferred embodiments, which are in no way limiting, represented in FIGS. 1 to 7, in which:

FIG. 1 schematically illustrates a first embodiment of a structural assembly according to the invention,

FIG. 2 illustrates an alternative embodiment of the structural assembly according to the invention,

FIG. 3 illustrates another variant embodiment of the structural assembly according to the invention,

FIG. 4 illustrates another variant embodiment of the assembly structural element according to the invention,

FIG. 5 illustrates an enlargement of a link of the structural assembly,

FIG. 6 illustrates a perspective view of a portion of an example of a stratospheric balloon represented in the form of a torus, made from a plurality of structural assemblies,

Figure 7 illustrates a perspective view of a portion of another example of a stratospheric balloon represented as a torus, made from a plurality of structural assemblies. Exemplary embodiments of a structural assembly 100 according to the invention are illustrated in FIGS. 1 to 4.

The structural assembly 100 is inscribed in a plane and comprises:

a closed chain comprising a plurality of successive structural links 200, two adjoining links being connected by a so-called end node 300,

at least one inextensible element 500 extending between and connecting two non-adjacent end nodes 300 of two distinct links 200.

The term "adjoining" means that the objects associated with it are abutting with each other, whether these objects are in contact with one another or in close proximity to one another.

By adjacent nodes, it is meant that the nodes are linked by the same link in the closed chain.

By non-adjacent nodes, it is meant that the nodes are not linked by the same link in the closed chain.

By the term "closed chain" means a succession of links, each link is mechanically secured, with a certain clearance, to the two adjacent links.

The inextensible members 500 are referred to as the second inextensible stabilizing element and their function will be described later.

By inextensible element, we mean an element which presents a zero or even almost zero deformation for the efforts that will have to support the structural whole.

In an exemplary embodiment of the structural assembly, each end node 300 is connected to a non-adjacent end node 300 by a second stabilization element 500.

In the nonlimiting examples of FIGS. 1, 2 and 4, the closed chain has a substantially circular geometrical shape, illustrated in the figures by the dotted circle 101, and comprises six end nodes 300, called A, B, C, D, F and G, regularly positioned on this geometric shape and six structural links 200 extending between and each connecting two adjacent end nodes 300. Each end node 300 is connected to four non-adjacent end nodes by a second stabilization element.

In the example of FIG. 3, the closed chain has a substantially ellipsoidal geometric shape, illustrated in the figures by the dotted ellipse 101, and comprises twelve end nodes 300 regularly positioned on this geometric shape and twelve links 200 s extending between and each connecting two adjacent end nodes 300. Each end node 300 is connected to at least one non-adjacent end node by a second stabilization element 500.

Each structural link 200 has two structural branches

201 a, 201 b opposite, convexity oriented in substantially opposite directions, interconnected at each of their opposite ends 204a, 204b by an end node 300.

In the present description, the curvature of a branch is always defined from an outside to an inside of the chain. Thus, a branch is said to be convex 201a when said branch has a convexity turned towards the outside of the chain and a so-called concave branch 201b when said branch has a concavity turned towards the outside of the chain.

The closed string defines a closed peripheral contour inscribed in the plane.

The convex branches define a closed external peripheral contour. The concave branches define a closed peripheral contour called internal. Each structural branch 201a, 201b of a link 200 comprises at least two successive segments 202a, 202b interconnected by an intermediate node 203a, 203b.

In addition, each structural link 200 comprises an inextensible element 400 for connecting an intermediate node 203a of the convex branch 201a to an intermediate node 203b of the concave branch 201b. Each intermediate node of a convex branch is linked to an intermediate node of the concave branch by an inextensible element 400.

The inextensible element 400 is referred to as the first stabilizing element and its function will be described later.

In the non-limiting example of FIGS. 1 and 3, the convex branch 201 has, concave 201 b respectively, a link 200 comprising two successive segments 202 a, respectively 202 b, linked together by an intermediate node 203 a, called H, respectively 203b, denoted J. A first stabilizing element 400 connects the intermediate node 203a of the convex branch 201a to the intermediate node 203a of the concave branch 201b of the link 200.

In the non-limiting example of FIG. 2, the convex branch 201 has, concave 201b respectively, a link 200 comprising three successive segments 202a, respectively 202b, linked by two intermediate nodes 203a and 203b, respectively. Two first stabilization elements 400 connect the two intermediate nodes 203a of the convex branch 201a to the two intermediate nodes 203b of the concave branch 201b of the link 200.

In one embodiment of a link 200, as illustrated in FIGS. 1 and 3, to obtain a uniform distribution of the forces and to allow reproducibility of the manufacturing processes, the convex branch 201a and the concave branch 201b of a link are symmetrical with respect to a straight line passing through the two end nodes of said link.

In a particular embodiment of a link 200, as illustrated in Figure 4, each segment 202a, 202b shown in Figure 1 is itself composed of a so-called elemental link 200 '. The elementary link 200 'takes up the characteristics of the structural link 200 as described previously, that is to say having two branches elementary elements 201 a ', 201 b' each comprising at least two segments, referred to as elementary segments 202a '202b', successively linked by an intermediate elementary node 203a ', 203b'. The intermediate elementary nodes 203a ', 203b' of each elementary branch 201 a ', 201b' are connected by a first inextensible stabilization element 400 '.

In a particular embodiment of an elementary link, each elementary segment is itself composed of a link comprising two sub-branches each comprising at least two successive sub-segments linked by an intermediate node. The intermediate nodes of each sub-branch are connected by a first inextensible stabilization element.

In one embodiment of the structural assembly, to allow reproducibility of the manufacturing processes and therefore to reduce the cost of manufacture, said structural assembly has identical links in the closed chain.

The geometry of the structural assembly as described is chosen such that, when external pressure forces are exerted on said structural assembly, the components (i.e. the segments and the first and second stabilizing elements ) the constituent thus work in tension rather than in compression.

In an exemplary embodiment of a segment of a link, said segment is a cylindrical beam of any cross-section of closed ends. Preferably, to lighten the structural assembly by weight, the cylindrical beam is hollow. A hollow cylindrical beam presents a compromise between the tensile strength and the mass.

Preferably, the cross section of the beam is circular. The calculation of the thickness of the beam of circular section and the diameter of the cross section is within the reach of the skilled person.

Preferably, the beam is chosen from a composite material, for example based on carbon, which has a compromise between its Young's modulus and its density, that is to say a material having good tensile mechanical properties ( Young modulus for example between 300 and 700 GPa) without penalizing the mass of the beam. In one embodiment of the beams, said beams have a rough surface, mainly at the end nodes and intermediate nodes to better withstand the shear forces.

The first and second inextensible stabilizing elements form geometric stabilization means of the closed peripheral contour defined by the chain of the structural assembly in the plane arranged so as to prevent a geometrical deformation of the peripheral contour in the plane, and by extension of the structural assembly, under the effect of external pressure forces exerted on said structural assembly.

The first inextensible stabilizing elements, respectively second non-extensible stabilizing elements, also ensure a mechanical stability of the link, respectively of the structural assembly.

The first stabilizing elements 400 associated with a link 200 also advantageously provide a geometric stability of said link. The first stabilizing elements HJ and KM will work in tension when the structural element is subjected to external pressure forces, thereby preventing the link from deforming.

Indeed, a maximum force F _p before buckling that can support a beam is given by the formula: p Û

Where E is the Young's modulus, I the inertia of the beam and L its length. Thus, for a beam to support a maximum force F _p desired, it is possible to play, for a given material, either on the inertia or the length of said beam. An object of the invention to not penalize the bulk of the structural assembly, only a change in the length of the beam is acceptable.

For this, instead of placing a beam of length L _A B between two end nodes A and B, shown in dashed lines in FIG. 5, the invention proposes to have, for example, six beams AH, KH, KB, BM, MJ, JA successive, forming a closed link, and all having a length less than the length L _A B of the AB beam. Thus, the six beams are of one share less prone to buckling as the beam length L _A B It is clear that the greater the number of beams forming the link, the greater the buckling strength of each beam is increased. The skilled person, by his general knowledge, is able to choose the number of beams forming a link according to the resistance to external pressure forces desired.

On the other hand, it is noted that all or part of the beams do not take all of the buckling effort directed in the direction AB, but that these beams take it with a non-zero angle. As a result, the buckling force for a beam is to be multiplied by the sine of this angle, a factor of less than 1.

The second stabilizing elements 500, inter alia AC, BF, DG in FIG. 1, make it possible to prevent the structural assembly 100 from collapsing in the plane and will work in tension when the structural element 100 is subjected to external compression forces. Indeed, if, for example, a pressing force is exerted on the structural assembly 100 at the point F so that the point F tends to go inward of the circle 101 defined by the closed chain, the points G and D have a natural tendency to go out of the circle 101, which is made impossible by the second non-extensible stabilizing elements 500. The structural assembly 100 is therefore extremely resistant to external pressure forces.

The choice of the number of second non-extensible stabilization elements and their positioning between the different nodes so as to prevent, in the plane, a geometrical deformation of the peripheral contour of the structural assembly under the effect of external pressure forces exerted, is within the reach of the skilled person.

In one embodiment, the first and second inextensible stabilizing elements 400, 500 are inextensible son, thus having a negligible mass relative to the mass of a beam, making it possible not to weigh down the structural assembly.

In an exemplary embodiment, these stabilizing elements 400, 500 can be made of materials either aramid, such as a Kevlar ^® yarn which has very good mechanical properties tensile strength (tensile strength of the order of 3100 MPa and a Young's modulus between 70 and 125 GPa) and fatigue, or composite, such as a carbon wire that has a tensile strength of order of 7000 MPa and a Young's modulus of the order of 520 GPa.

In another embodiment, the first and second inextensible stabilizing members 400, 500 are rigid elements, i.e. elements whose shape and dimensions do not undergo substantial changes during use of the invention. structural ensemble. These rigid elements form spacers, that is to say that they maintain a constant spacing between the end nodes to which these rigid elements are connected.

In one embodiment of a link, to provide better resistance to shear forces and better geometric positioning in position of the structural assembly, two successive beams of a branch are mechanically connected by the first stabilizing elements 400. This embodiment also makes it possible to reduce the mass of the link, since there is no addition of junction device for the beams.

In the example where the first stabilizing elements 400 are wires, each end of the wire passes through the closed ends of two successive beams associated therewith. Preferably, the ends of the wire meet and form one. Thus, the first stabilizing elements 400 consist of two wires instead of a wire.

In one embodiment of the structural assembly, to provide better resistance to shear stresses, the ends of the beams of two adjoining links at an end node 300 are mechanically bonded by the second stabilizing members 500 .

The present invention is not limited to the examples of structural assemblies described and illustrated. Those skilled in the art are able to adapt the invention to geometric shapes of undescribed structural assemblies as well as to forms and arrangements of links that are not described.

The structural assembly of the invention is advantageously made from any initial geometrical shape 101. In a first step, an initial geometrical shape 101 desired for the structural assembly 100 is defined. This geometric shape is chosen such that it delimits a closed volume, preferably a convex volume.

In a second step, N points are positioned on the geometrical shape 101, the N points corresponding to the N end nodes, N being greater than or equal to 3.

The person skilled in the art, by his general knowledge, is able to choose the number of points on the geometrical shape according to the particular characteristics of the structural assembly (material, geometry of the segments, the links and the structural assembly and depending on the desired external pressure force resistance for said structural member.

Preferably, the N points are evenly spaced on the geometric shape.

In a third step, two adjacent nodes 300 are connected by a structural link 200.

In a fourth step, the two branches of a link are connected by a first stabilization element.

This step is performed for each link.

In a fifth step, non-adjacent end nodes of two distinct links are connected by a second stabilizing element.

This step is performed for each node.

The structural assembly is advantageously usable in the production of a stratospheric balloon, as illustrated in FIGS. 6 and 7.

In the example of Figures 6 and 7, the stratospheric balloon has a shape of a torus.

The present invention is not limited to stratospheric toroidal balloons. Those skilled in the art are able to adapt the invention to undescribed geometric shapes, such as, for example, a spherical, lenticular shape.

The stratospheric balloon 600 comprises: a structural frame 700 in a three-dimensional space comprising:

a plurality of structural assemblies 100,

a plurality of means 710 for geometric stabilization of the structural framework in space,

a skin 800 stretched over the closed peripheral contour of each structural element.

Two structural assemblies 100 are connected by at least one geometric stabilization means 710, each geometric stabilization means being connected, at two opposite ends 71 1, to end nodes 300 of each structural assembly 100.

In the example of Figures 6 and 7, for the sake of clarity, a half-torus is shown. In Figure 6, only two structural elements and three radial stabilizing elements per structural element are illustrated. In Figure 7, only the concave branches of two structural members and three radial stabilizing elements per structural member are illustrated.

The skin 800 is chosen so as to have sufficient strength not to break under external pressure forces.

In an example of skin, the skin comprises an airtight membrane and a crisscross-type wire mesh structure whose mesh is chosen so as to resist external pressure forces.

Preferably, the membrane is made of a material of the Ethylene tetrafluoroethylene (ETFE) type.

Preferably, the intersecting wire structure comprises for example at least one wire made of a material selected from the following: metal, aramid such as for example Kevlar®, carbon, among others.

In one embodiment, a geometric stabilization means of the structural framework in space is a rigid element.

In an embodiment illustrated in FIG. 6, the skin 800 is stretched over the outer peripheral contour of each structural element.

In another embodiment, illustrated in FIG. 7, the skin 800 is stretched over the inner peripheral contour of each structural element. Preferably, the membrane is made of a transparent material.

By "transparent material" is meant a material that passes solar and infrared radiation with a minority absorption. This material may in particular be made of polyethylene or polyester, which are the materials generally used to manufacture stratospheric balloons.

In one embodiment of the torus, the torus 600 further comprises a rim 900.

In an exemplary embodiment of the rim, said rim working in compression, the rim is formed by a closed chain formed of links as for the structural assembly.

The components of the structural framework (ie the segments, the first and second stabilizing elements of the stabilizing elements, the means of geometric stabilization of the framework in the space), possibly the rim if it exist, by their natures and their shapes, are chosen so that the stratospheric balloon withstands external pressure forces of the order of at least 50 hPa, the individual load depending on the geometry of the balloon and the number of structural elements put to achieve it while maintaining an acceptable mass.

In the case of the stratospheric balloon application, it may advantageously be envisaged to fill the internal volume of the vacuum core. Thus, the management of the gas storage is removed. The total weight of the ball is considerably lightened.

The above description clearly illustrates that by its different characteristics and advantages, the present invention achieves the objectives it has set for itself. In particular, it proposes a closed structure having a geometry allowing to work a maximum of its components in traction rather than in compression, without penalizing the weight of the structure.

Claims

Structural framework (700) three-dimensional characterized in that it comprises:

a plurality of structural assemblies (100) comprising:

a closed chain defining a closed peripheral contour forming part of a plane, comprising a plurality of successive structural links (200), each structural link (200) comprising two structural branches (201 a, 201 b) facing each other, convexity oriented in substantially opposite directions, interconnected at each of their opposite ends (204a, 204b) by a so-called end node (300),

means (400, 500) for geometric stabilization of the peripheral contour, said means being arranged in such a way as to prevent geometric deformation of the peripheral contour in the plane under the effect of external pressure forces exerted, and said means comprising:

for each structural link (200), a first inextensible stabilization element (400) connecting two structural branches (201 a, 201 b) of the same structural link, excluding end nodes (300),

a second inextensible stabilization element (500) connecting two non-adjacent end nodes (300) of two distinct structural links (200),

a plurality of means (710) for geometric stabilization of the structural framework in space, two structural assemblies (100) being connected by at least one geometric stabilization means (710), each geometric stabilization means of the framework structure at two opposite ends (71 1) at end nodes (300) of each structural assembly (100).

Structural framework (700) three-dimensional according to claim 1 wherein each structural branch (201 a, 201 b) of a link comprises two segments (202a, 202b) successive interconnected by a node said intermediate (203a, 203b), the first stabilizing element (400) extending between two intermediate nodes of the two structural branches (201 a, 201 b).

3 - structural framework (700) three-dimensional according to one of the preceding claims wherein the two structural branches (201 a, 201 b) of a link are symmetrical with respect to a straight line passing through the two end nodes (300) said structural link (200).

4 - Structural frame (700) three-dimensional according to one of the preceding claims wherein the first and / or second stabilizing element is a rigid element.

5 - Structural framework (700) three-dimensional according to one of the preceding claims wherein each end node (300) of a structural link (200) is connected to an end node (300) not adjacent a structural link separated by a second stabilizing element (500).

6 - Structural frame (700) according to claim 5 wherein a geometric stabilization means of the structural framework in space is a rigid element.

7 - stratospheric balloon (600) characterized in that it comprises a structural frame (700) according to one of claims 1 to 6 and a skin (800) stretched on the peripheral contour of each structural element (100).

8 - Process for producing a structural assembly (100) of a structural framework (700) according to one of claims 1 to 6 characterized in that the method comprises the steps of:

define a desired initial geometrical shape of the structural assembly,

positioning N points, corresponding to N end nodes (300), on the geometrical shape, N greater than or equal to 3,

for each end node (300), connect it to an adjacent end node (300) by a structural link (100),

for each structural link (100), connecting the two structural branches (201 a, 201 b) of said structural link by a first stabilizing element (400), connecting two non-adjacent end nodes (300) of two distinct links by a second stabilizing element (500).