Abstract:
The invention relates to a method of assembling pre-impregnated plies by local isolated polymerization of the assembly region ( 30 ) under the action of a radiation to which the resin of the prepreg is sensitive. 
     The assembly method makes it possible in particular to produce very large sized gossamer structures ( 7 ) in the form of a concertina-folded tube which is deployed by injecting pressurized gas once in space then stiffened by polymerization under the effect of the same radiation. The pylon concerned ( 7 ) thus comprises two structural elements ( 10, 20 ) secured end to end while at the same time maintaining its flexibility and firmness qualities.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to the assembly of prepregs. The assembly method according to the invention may be applied to the production of structures of large dimension necessitating the connection between different structural elements made of resin, for example “end to end”, and/or structures of complex shape necessitating the junction between two elements, for example of different shape if necessary. The invention is based on the local polymerisation by radiation of prepregs to assure an assembly. 
         [0002]    In a more specific manner, the assembly according to the invention concerns structures that are folded and, after assembly, the material maintains a flexibility suited to a compact folding. 
         [0003]    The invention concerns in particular the production of prepreg based structures, particularly ones capable of being deployed, for example through inflation, in particular for a space use, and which cannot be produced in a single piece. 
         [0004]    The invention particularly finds an application in the production on earth of assemblies of sections of tubes that maintain sufficient mechanical strength to withstand the flight environments, and/or inflation and stiffening capabilities in space. 
       STATE OF THE PRIOR ART 
       [0005]    The production of compact and ultra-light devices, suited to being deployed through inflation after being placed in orbit, has been envisaged from the start of the space era. 
         [0006]    These devices comprise a set of hollow structures, generally tubular, which consist of thin membranes folded so as to form a concertina and the deployment of which results from their filling by a pressurized gas such as nitrogen, which is stored in an adjoining tank. These structures are thus known as “gossamer structures”. 
         [0007]      FIG. 1  illustrates in schematic form the type of construction presently retained for a gossamer structure intended to be stiffened by polymerisation of a resin induced by a radiation, and particularly by ultraviolet radiation. Only a portion of the structure has been represented in the deployed state in this figure, so as to clearly show its constituent parts. 
         [0008]    A gossamer structure is shown in the form of a tube  1 , the wall of which is formed by a flexible membrane which comprises one or several thicknesses of a prepreg fabric  2  of a resin based composition, held in a sandwich between two thin polymer films  3 ,  4 . These films  3 ,  4  have the function of assuring the impermeability required to inflate the structure and avoid the impregnated fabric  2  bonding to itself when the wall is folded before deployment, which would in fact prevent the deployment of the structure. In the case of a gossamer structure, the polymers constituting the thin films  3 ,  4  must meet specifications specific to the space field: in particular, the films must be capable of withstanding extreme temperatures and have particularly good mechanical strength properties. 
         [0009]    A multilayer thermal cover  5  if necessary completes this sandwich  2 ,  3 ,  4 . 
         [0010]    One of the difficulties posed by the production of gossamer structures is linked to the aggressiveness of the space environment. Indeed, space is full of micrometeorites capable of puncturing any impermeable membrane and thereby giving rise to leaks and a de-inflation, making the deployed structures lose their shape. It is therefore necessary to stiffen them after their deployment, by mechanical, physical or chemical techniques. 
         [0011]    In particular, chemical techniques aim to induce, by temperature, ultraviolet or other radiation, etc., the polymerisation of a resin; this polymerisation may be accelerated by catalysts conveyed by the filling gas, except in the case of gossamer structures where an internal impermeable bladder is provided for. In fact, the polymerisation of a resin induced by a radiation would be one of the most interesting ways of stiffening gossamer structures, as regards the reliability of this stiffening technique, the costs of the materials used and their implementation, and the mechanical properties of the stiffened structures. 
         [0012]    Thus for example, if the resin of the film  2  is photopolymerisable, the structure  1  moreover comprises a suitable radiation source, for example an ultraviolet lamp,  6  which is arranged inside the tube  1 . The photo-polymerisable resin based composition  2  comprises in this case, apart from a reactive oligomer corresponding to the base molecule of the resin, at least one photo-initiator sensitive to the radiation concerned, in other words a compound that decomposes when it is exposed to it to produce chemical species, the role of which is to initiate the polymerisation of the resin. 
         [0013]    A preferred route is polymerisation by visible light: a particularly suitable membrane is described in document FR 2 876 983. 
         [0014]    However, the production of structures of large size poses a problem. Indeed, the dimension of the raw materials, namely the films  2 ,  3 ,  4 , available is limited. Moreover, the shaping of films in order to produce the final part requires manufacturing tools, in particular, for a gossamer structure  1 , degassing mandrels for folding in concertina shape; these tools have to remain within size and weight criteria compatible with their handling and their reliability: a too high deflection of the mandrel is advised against. 
         [0015]    To produce such structures, it thus appears necessary to assemble several structural elements, for example two portions of tube end to end. 
         [0016]    The first envisaged technique concerns the bonding of two portions. However, the use of a third material is problematic, especially in the space field: the adhesive must be fully compatible with the films  2 ,  3 ,  4 ; it must maintain the impermeability of the structure and also do this during the folding, deployment, stiffening, and the mechanical action-effects inherent in the use of the structure, and do this at the level of a circumference of the pylon  1 . In particular, a non polymerised material of prepreg ply type  2  is not easy to bond. 
         [0017]    More generally, this problem of assembly is posed for any complex structure, of the type involving formation of a sphere from flat plies, tube/flat or tube/sphere assembly, or formation of a multiple tube structure, etc. 
       DESCRIPTION OF THE INVENTION 
       [0018]    The invention proposes, under one of its aspects, making up for the drawbacks of existing assembly techniques. 
         [0019]    The invention therefore has the objective of defining a method of assembling prepreg fabrics, which makes it possible:
       to assure a sufficient mechanical strength of the bond for manipulations, close to the strength of the initial prepreg,   while at the same time maintaining the initial flexibility of the prepreg,   while being compatible with a folding,   and without deteriorating the properties of the impregnation resin of the prepreg.       
 
         [0024]    A second objective of the invention is to enable the production of parts of very large dimensions for inflatable structures, which requires that the bond also withstands the inflation forces, in other words the strength of the tube in the inflated state, before its stiffening polymerisation. 
         [0025]    In its conventional meaning, the term prepreg concerns a material in the form of a sheet of fibre fabric, impregnated by a resin, the polymerisation of which is not completed, for example a resin known as a “B-state resin” when it is thermosetting. Prepregs are then conventionally used to manufacture composites, after which the hardening is completed. 
         [0026]    The basic principle of the invention consists in using a prepreg polymerisable at least partially by radiation, in other words capable of locally polymerising without inducing evolution of the adjacent non activated resin. It is then possible to assure an assembly without use of adhesive, by irradiating the liaison zone, if appropriate also heating it. In a surprising manner, an optimisation of the geometry, the surface and the layout of the bonded bond, has made it possible to maintain the flexibility of the bond and has shown a compatibility with a folding. 
         [0027]    The invention thus concerns, under one of its aspects, a structure that comprises two prepreg elements secured to each other on a junction zone. The junction zone is characterised by the superposition of two prepregs that remain independent with the exception of discrete points where the two prepregs are welded to each other, by local cross-linking or polymerisation. 
         [0028]    Advantageously, the junction zone is composed of at least two parallel assembly strips spanning the structure, in other words for example over a width or a perimeter of the prepreg depending on the geometry of the structure, separated from each other by a zone in which the two prepregs are uniquely superimposed; each of the strips comprises discrete points of crossed polymerisation of the two prepregs. Thus, the zone between the strips keeps its flexibility and enables in particular a folding; according to the geometry of the folding, there may be a plurality of single superposition zones that cross each other, with a variable pattern. 
         [0029]    The geometry of the discrete points is preferably such that the projection on an edge of the junction zone of these discrete points forms a continuous line. In the case where several assembly strips are present, preferably, continuous lines are formed by the projection on the edge of each strip of its discrete points. Advantageously, in order to keep a large amount of flexibility in the assembly, the projection on the edge of each strip only comprises a single point. 
         [0030]    Thus, the structure according to the invention may comprise fold zones, for example to form a concertina, in particular a fold in each junction zone. 
         [0031]    For example, the structure is an inflatable tubular element, capable of being deployed in space and comprises suitable ancillaries, in particular end bases. The structure according to the invention may also comprise a protective film above two portions of prepreg, and/or a protective film on the other face. One of the films particularly may be impermeable. 
         [0032]    The prepreg according to the invention comprises advantageously an epoxy or epoxy-acrylate resin, and a photo-initiator, chosen among iron-arene complex salts of general formula (I) in which A represents an arene group, whereas X represents a non nucleophilic anion: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0033]    Under another aspect, the invention concerns an assembly method that makes it possible in particular to produce a structure as described previously. The assembly method according to the invention comprises the local superposition of two elements comprising a prepreg, then the irradiation in discrete points of the superposition zone in order to carry out a local polymerisation on a junction zone. The irradiation is adapted to the nature of the resin composing the prepreg: in particular, the structures comprise an epoxy or epoxy-acrylate resin and a photo-initiator chosen among iron-arene complex salts of formula (I), wherein the irradiation is carried out by means of a visible light. 
         [0034]    Advantageously, the method is carried out for the assembly of tubular structures, and through the intermediary of a mask. The mask comprises the geometry adapted to the desired schema of discrete junction points. 
         [0035]    According to a preferred embodiment, the assembly method is integrated in the method of forming a gossamer type structure. In particular, the assembly method is carried out after the first structural element has been coupled to its base, then folded, in order to be able to extend the tube by a second structural element. Once the assembly is formed, there may be folding of the second part of the tubular structure. 
         [0036]    Under another aspect, the invention concerns a polymerisation mask adapted to the previous method. Preferably, the mask is composed of at least two irradiation parts, advantageously in the form of strips, separated from each other by an opaque part. Each irradiation strip comprises a number of discrete points localised in a more or less random manner on its surface. Preferably, the projection on one of the sides of the strip of discrete points forms a continuous line; this projection is advantageously such that only one point is present at each time. 
         [0037]    The mask may contain several opaque strips that cross each other so as to form non polymerised zones, the pattern of which depends, for example, on a subsequent folding. 
         [0038]    The invention finally concerns a space deployment pylon, produced by the previous method with the predefined mask. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0039]    The characteristics and advantages of the invention will be better understood on reading the description that follows and by referring to the appended drawings, given by way of indication and in no way limiting. 
           [0040]      FIG. 1 , already described, schematically illustrates a gossamer structure, in deployed and opened out position. 
           [0041]      FIG. 2  shows a deployment pylon according to a preferred embodiment of the invention. 
           [0042]      FIG. 3  illustrates the assembly zone by a method according to the invention. 
           [0043]      FIG. 4  represents the assembly method according to the invention. 
           [0044]      FIG. 5  shows an assembly polymerisation mask according to a preferred embodiment of the invention. 
           [0045]      FIGS. 6A to 6F  describe a method for producing a deployment pylon according to one embodiment of the invention. 
           [0046]      FIGS. 7A to 7C  show other structures obtained by assembly according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
       [0047]    Although described hereafter for the production of an inflatable tubular structure for space use, evidently this embodiment is only illustrative. The assembly according to the invention may be applied to all cases where it is wished to produce composite structures, of very large size, planar or tubular or spherical or composed of different elements of same shape or not, folded or not. 
         [0048]    The structure concerns for example a space deployment pylon  7  that can attain more than 10 m length with an internal diameter of 160 mm, illustrated in  FIG. 2 . The pylon  7  is shown in the form of a thin wall tube  1  secured at each end to a base element  8 , usually in aluminium alloy; the pylon  7  forms a concertina along the fold zones  9  for the launch then is deployed through inflation once in space. The material of the walls of a pylon  7  of this type must withstand a tractive force of 2.4 N/mm for an internal pressure of 300 mbar. 
         [0049]    According to a preferred embodiment, the walls of the inflatable tube  1  may in particular consist of:
       an internal bladder  3 , for example in Kapton® our in Upilex®, assuring impermeability to the inflation gas, particularly nitrogen;   one or several plies  2  of glass/resin composite, two in the preferred context, which will give, after polymerisation, its stiffness to the wall;   an external membrane  4 , for example in aluminised Kapton®.       
 
         [0053]    In particular, and according to the preferred embodiment, the plies  2  of composite are prepregs fabrics composed of a material such as described in FR 2876983. In particular, an epoxy resin, for example of DER 330™ or DER 332™ type, is coupled with a cyclopentadienyl 1-methyl-naphthalene ferrocenium hexafluorophosphate photo-initiator of formula II: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0054]    A cyclopentadienyl benzene ferrocenium hexafluorophosphate (formulae III or IV) may also be recommended. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0055]    It is obviously also possible to use a composite polymerisable at least partially by ultraviolet, the resin of which is for example acrylate and the photo-initiator sensitive to UV. 
         [0056]    The formation of the pylon  7 , in particular the folding  9  according to a suitable pattern, necessitates a specific tooling, which cannot attain this length of 10 m. Thus, the pylon  7  is separated into a first structural element  10  and a second structural element  20 , which are advantageously of identical nature, by a junction zone  30 . 
         [0057]    To form the assembly according to the invention, in order to reduce the fragility zone and avoid too high increased thicknesses, it is preferable to carry out a “stepped” junction at the level of the different films of the structure and illustrated in  FIG. 3 . In particular, at the level of the end of the first element  10 , the internal bladder  13  extends beyond the composite plies  12  which themselves extend beyond the external film  14 ; naturally any other configuration would be possible, depending on the nature of the assembly and the number of plies  2 . The same inverse geometry is found on the second element  20  which will be assembled end to end with the first, so that there is a partial two by two overlapping of each of the films. 
         [0058]    The junction between the external  14 ,  24  and internal  13 ,  23  films may be formed in a conventional manner, particularly by a suitable bonding or a thermal melting. The width of the superposition of each of these films between the two structural elements  10 ,  20  is dimensioned according to known criteria, so as to carry out this bonding, for example over 10 mm. 
         [0059]    As regards the composite part  12 ,  22 , a bonded assembly poses difficulties, due to the non polymerisation of the component, and in particular due to the stresses that are concentrated there during the deployment. 
         [0060]    Usually, another assembly technique between two prepreg plies  2  is carried out by polymerisation such that the impregnation resin plays in some way the role of adhesive between the different plies: it is in this way that the stacks are formed. However, this technology is automatically accompanied by the stiffening of the assembly, and cannot be applied to a gossamer structure. 
         [0061]    According to the invention, an isolated polymerisation is used that leaves sufficient space between the polymerised points in order for the prepregs to maintain all their flexibility. This possibility is offered by the choice of an impregnation resin sensitive to a radiation, in particular to light; as a consequence, the prepreg comprises a photosensitive resin that may be cured and enables the stiffening of the structure once inflated. In particular, the prepreg comprises advantageously an epoxy or epoxy-acrylate resin, and a photo-initiator, chosen among iron-arene complex salts of general formula (I) in which A represents an arene group, whereas X represents a non nucleophilic anion: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    the preferred material comprises an epoxy resin with cyclopentadienyl benzene ferrocenium hexafluorophosphate or cyclopentadienyl 1-methyl-naphthalene ferrocenium hexafluorophosphate. 
         [0062]    The chemical nature of the prepreg thereby offers the possibility of a local cross-linking of the composite material. This local polymerisation, carried out by means of a radiation selected as a function of the prepreg, enables a cohesion between the two layers of resin, a perfectly localised “welded” bond and which does not extend to the non irradiated zones. 
         [0063]    The principle according to the invention is schematised in  FIG. 4 : the first ply  12  and the second ply  22 , manufactured in prepreg material, in other words their resin is not polymerised, are locally superimposed on a junction zone  30 ; a pressure is applied for a good contact. Above the junction, a mask  40  with a specific pattern comprising irradiation parts  42  and opaque parts  44  is locally applied. The whole is then irradiated, by light within the context of a photosensitive resin partially as described previously. According to the pattern of the mask  40 , if necessary by additional heating, there will be polymerisation of specific points  32 , and cohesion of two opposite plies, whereas between two welding points  32 , which correspond to the opaque parts  44  of the mask  40 , the two plies  12 ,  22  do not interact and form a simple superposition  34 . Advantageously, in order to avoid any start of stiffening, the mask  40  is extended by an opaque protection  46  which makes it possible to ensure that the irradiation only concerns the welding points  32 . 
         [0064]    For reasons of firmness, although isolated, the polymerisation preferably maintains a continuity at the composite junction  30 . On the other hand, to maintain its flexibility during the assembly, the points  32 , which are stiffened, must remain sufficiently small and distant from each other. In order to produce this type of assembly for a gossamer structure, the overlap zone  30  between the two composite plies  12 ,  22  is provided to be sufficiently extended so that the points  32  can be disseminated in a manner distant from each other, in other words distant by at least the size of their neighbour. 
         [0065]    It has been noted that, for a composite such as described above, polymerisation points  32  of around 4 mm diameter may easily be obtained, while at the same time maintaining their function during the inflation, and a relative flexibility of the sheets when they are sufficiently distant to each other. 
         [0066]    Given that the junction zone  30  may be relatively extended and to assure the bond, it is preferable to form a junction in two parts, between which it is possible to carry out for example a folding. In particular, according to an advantageous embodiment, the junction zone  30  may be separated into three parallel strips, a first assembly strip  30   1 , an intermediate strip  38  in which the composite films  12 ,  22  are juxtaposed and non bound, in other words maintaining all their flexibility, and a second assembly strip  30   2 . 
         [0067]    The folding of a structure may moreover be complicated. For example, for a pylon  7  illustrated in  FIG. 2 , the fold lines may form chopped off hexagons; according to other geometries, other more or less complex patterns may be envisaged. The junction zone  30  may in these cases provide for other “intermediate strips” intended to form the zones where a fold will be made, the strips  38  may cross each other in varied patterns. 
         [0068]    Moreover, if the intermediate composite layer  12 ,  22  is formed from several films, and particularly two as illustrated in  FIG. 3 , it is advantageous to carry out a stepping between the different films, with in particular a cross over: the external ply  22 A of the second element  20  is welded on its end part to the external  12 A and internal  12 B plies of the first element  10 , and the internal ply  12 B of the first element  10  is secured on its end part to the external  22 A and internal  22 B plies of the second element  20 . One of the intermediate strips  38  then comprises the superposition uniquely of the external plies  12 A,  22 A of the two structural elements  10 ,  20 . 
         [0069]    According to the preferred embodiment described above, an overlapping zone  30  of 74 mm is thus provided for between the composite plies  12 ,  22 , with a first strip  30   1  of 32 mm, an intermediate strip  38  of 10 mm, and a second strip  30   2  of 32 mm. For the provided diameter of 160 mm, each strip  30   1 ,  38 ,  30   2  has a length of 126 mm. 
         [0070]    A schema for a suitable polymerisation mask  40  is illustrated in  FIG. 5 : the irradiation strips  40   1 ,  40   2  are thus “cross-hatched” over a width of eight possible irradiation zones. Over each width, a single irradiation point  42  will enable the polymerisation, in order to maintain as far as possible the flexibility of the assembly strip  30 , the points  42  moreover being randomly distributed so as to be masked or not by light. 
         [0071]    More generally, according to a preferred embodiment of the invention, the polymerisation is carried out in a localised and discrete manner on an assembly strip  30   i  of a predefined width and of length spanning the two composite plies  12 ,  22  to be assembled. The junction zone  30  is divided into a cross-hatch of welding points  32  in the longitudinal and lateral directions. The mask  40  is patterned in such a way that the projection in the direction of the length on the edge  48  of the strip  40   i  of all the squares  42  that are polymerised form a continuous line. Advantageously, only one square  42  over the width will enable the polymerisation, in order to stiffen the structure as little as possible. 
         [0072]    In particular with a view to security, it may be advantageous to double up this configuration, with formation on the mask  40  of two irradiation parts  401 ,  40   2 , identical or symmetrical or other, separated between each other by a part  48  opaque to the activation radiation of the resin, of width depending on the use of the corresponding part  38  on the assembly, in particular sufficient to enable a folding, and 10 mm in the preferred embodiment. 
         [0073]    Moreover, as described above, apart from the opaque part  48 , the mask  40  may comprise other opaque parts forming strips where a fold is provided for. Thus, as illustrated in  FIG. 5 , a “square” fold is envisaged, and the mask  40  is conceived with crossed strips intended to form the folds P 1 , P 1 ′, P 2 , P 2 ′. A schema to form the pattern of  FIG. 2  is naturally also envisaged and part of the invention. A mask according to the invention thus comprises a plurality of irradiation strips separated by opaque strips, said opaque strips forming a predefined and optimised schema. 
         [0074]    One embodiment of a pylon  7  according to the invention is thus schematised in  FIG. 6 . 
         [0075]    In a first step ( FIG. 6A ), the different films of material (namely internal bladder  13 , composite plies  12 A,  12 B, and external layer  14 ) are wound round a mandrel  50 ; their cylindrical formation is carried out as is usual in this field, by bonding for example or by a method according to the invention. The mandrel  50  advantageously comprises perforations  52  that will enable a degassing. 
         [0076]    In a second step, the first end of the pylon  7  is formed, by integration with a first base  8 , preferably in aluminium alloy:  FIG. 6B . As is normal, an integration tool  54  is coupled to the mandrel  50 ; the films of material  12 ,  13 ,  14  are arranged so that the external  14  and internal  13  layers, intended for the securement on the base  8 , form a protuberance in relation to the composite plies  12 . A bonding of the internal bladder  13  is then carried out on the base element  8 , and the external layer  14  on the internal layer  13 , by exercising a pressure on the chosen adhesive  15  or by forming a thermal weld  16 . 
         [0077]    In a third step illustrated in  FIG. 6C , the assembly of films  12 ,  13 ,  14  is folded, as normal, and withdrawn from mandrel  50 . A vacuum may be applied via perforations  52 , so as to lay the internal layer  13  flat on the mandrel  50 . 
         [0078]    A base  8  of pylon  7  is therefore obtained, which is coupled to a concertina  10 ′ of composite structure, advantageously stowed around a square stowage tool  56 . One end of the composite structure  10 ′ remains however non folded and around the mandrel  50 . Usually, this end would be coupled to the second base of the pylon  7 . 
         [0079]    Within the scope of the invention, it is wished to form a longer pylon. An assembly of this non folded part with a second element  20  of similar nature to the first is then carried out. The second element  20  is positioned around the mandrel  50 , on the side opposite to the first, with a partial overlapping  30  between the different layers of the two structural elements  10 ,  20 :  FIG. 6D . As has been made clear above, it is preferable that this overlapping is carried out in a stepped manner. 
         [0080]    In order to form the assembly, at the level of the junction  30 , an irradiation mask  40 , for example that of  FIG. 5 , is positioned, preferably associated with a light protective layer  46  on the sides of the mask  40 , in order to be sure to only polymerise, if necessary partially, the resin on the points previously defined  32 . The junction zone  30  is irradiated through the mask  40 , for example by means of a visible light electroluminescent panel  60 . If the resin so requires, the polymerisation may be completed by a heat treatment. 
         [0081]    Advantageously, the internal and external layers  13 ,  14  are themselves also partially assembled by means of heat. 
         [0082]    An assembly is thereby obtained composed of a first structural element  10  secured at one end to a pylon base  8 , then concertina-folded  10 ′, then wound round the mandrel  50 , and a second structural element  20  wound round the mandrel  50 , wherein the first and second elements  10 ,  20  are superimposed at the level of an assembly zone  30  where they are partially joined. 
         [0083]    It is then possible to continue the folding, in a continuous manner with the folding of the first element  10 , and in particular by positioning a fold at the level of the non polymerised zone  38  between the two junction strips  301 ,  30   2 , as well as at the level of other non polymerised zones stemming from the strips P 1 , P 1 ′, P 2 , P 2 ′. 
         [0084]    In the case of a pylon  7  composed of two structural elements  10 ,  20 , in particular a pylon of ten or so metres, the exterior end of the second element is incorporated in a base  8  ( FIG. 6E ), before carrying out the folding ( FIG. 6F ). 
         [0085]    Tests have shown that this embodiment makes it possible to obtain a pylon  7  that meets the conditions required for space use as is known. Moreover, no adaptation of the material normally used has been necessary: the pylons  7  have been folded and deployed using the normal tools, in particular TADECS. No influence of the assembly zone  30  has been observed during folding, or during deployment. The behaviour of the pylon produced according to the previous embodiment has not been different to that of a “normal” single pylon, just as the profile of inflation by pressure has not been modified. Above all, no damage has been observed on the pylon  7  itself at the level of the assembly zone  30 . In fact, the bonding of the external membrane  14 ,  24  is the only visible sign that a junction between two separate structural elements  10 ,  20  has been carried out. 
         [0086]    The method according to the invention therefore completely meets requirements in the aerospace field. It enables in particular a pylon of unlimited length to be produced, without increasing the costs of the material. 
         [0087]    Naturally, it is possible to envisage the junction of several elements end to end, in order to produce an even longer structure. Moreover, although described herein with a resin at least partially photosensitive to light for an inflatable gossamer structure, other embodiments are possible: the assembly method according to the invention enables an impermeable junction, without addition of material, on prepregs, while at the same time maintaining their future flexibility and polymerisation characteristics. In particular, several steps of the method described are naturally not obligatory: the emplacement of end bases obviously only concerns the production of gossamer type deployment structures. 
         [0088]    Although described around a mandrel placed under vacuum, the presence of the latter is not indispensable. In particular, the objective of the mandrel is to facilitate the folding of the tube. This concerns the previous structure, or more generally a concertina structure. The assembly method according to the invention may naturally be used with a folding being provided for. 
         [0089]    The method is likewise not limited to assembly between two portions of tube: it is possible to weld together two flat structural elements. Which is why the bonding of external and internal plastic films, in order to produce this tubular structure, has not been described, since it is known to those skilled in the art: the only requirement is that the adhesive used is compatible with a use in space, and in particular, it must not degas under vacuum. 
         [0090]    It is however possible, as illustrated in  FIG. 7A , to use an assembly according to the invention also to enclose a tubular structure  110 : a flat prepreg  112  may be “closed” over itself and the superposition edge  113  is welded by an assembly method according to the invention. 
         [0091]      FIG. 7A  further illustrates a complex structure that may be produced by a method according to the invention: a first tubular structure  110  is coupled to a second tubular structure  120  in order to form a T (or any other angle other than 90°, or instead to form a Y); the second tubular structure  120  may also have been formed from a flat ply welded by the previous local polymerisation  123 . The assembly zone  130  is formed by a method according to the invention. Naturally, it is possible to double up this configuration, to obtain an X, or any other shape. 
         [0092]    More generally, the method according to the invention makes to possible to produce any structure, once the assembly zone, if necessary the fold zone, enables the definition of a suitable polymerisation mask. In particular, as illustrated in  FIG. 7B , it is possible to assemble flat prepreg elements  140   1 ,  140   2 , etc. by assembly zones  150  and thereby form a spherical structure  160 . The assembly of tubes  170   i  end to end by a junction zone  180  can enable, apart from the formation of a pylon, the formation of a torus type closed structure  190  illustrated in  FIG. 7C , by way of example. 
         [0093]    Even though, in the case of thin structures, polymerisation by light or UV is sufficient, for structures up to several centimetres thick, it may be preferable to use a polymerisation by electrons, with all known intermediate technologies (X-rays in particular). This falls within the scope of the assembly according to the invention. 
         [0094]    Advantageously however, the products produced according to the invention are used in particular to manufacture radars, solar panels, reflectors, sun shades, antennae, mirrors or solar sail type devices, intended to equip orbital devices such as satellites, telescopes or orbital stations, or space or planetary exploration devices such as probes or robots.