Structural element provided with applied inserts for a composite acoustic structure, and associated manufacturing method

Abstract

The invention relates to a structural element (10) for a composite acoustic structure (100), the structural element comprising at least one cellular core (20) having a network of hollow cells (21) delimited by partitions (22) that extend between two faces (23, 24) of the cellular core (20), and at least one resistive skin (30) covering one of the faces of the cellular core (20), the structural element (10) being characterised in that it comprises a plurality of applied inserts (50), each insert (50) having a tubular through-body (51) open at the ends (53, 54) thereof, and a flange (52) protruding from the associated tubular body (51), and in that the resistive skin (30) is perforated by each of the inserts (50) positioned facing all or some of the cells (21) such that, for each insert (50), the flange (52) is positioned against the resistive skin (30) on a first side, and the tubular body (51) opens out on a second side opposite the first side.

Description

DESCRIPTION STRUCTURAL ELEMENT WITH ATTACHED INSERTS FOR AN ACOUSTIC COMPOSITE STRUCTURE AND ASSOCIATED MANUFACTURING PROCESS TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates, in general, to the technical field of acoustic attenuation structures or panels.

[0002] The invention relates to the manufacture of composite panels comprising a central honeycomb core sandwiched between two skins, particularly applied to acoustic attenuation structures used to reduce noise produced in demanding environments, for example in the aeronautical field in aircraft engines as well as in gas turbines or exhaust thereof, or in the railway field. PREVIOUS STATE OF THE ART

[0003] Acoustic attenuation structures are classically made up of, on the one hand, an acoustic plate or skin called a "resistive" skin which is permeable to the acoustic waves which we wish to attenuate and, on the other hand, a solid plate or skin called a "reflective" skin, between which is placed a core forming a cellular body, for example an alveolar structure of the honeycomb type.

[0004] The acoustic skin is generally acoustically porous and perforated with a multitude of orifices allowing fluidic communication between the exterior and interior of the honeycomb core of the composite structure, thus forming an acoustic attenuation structure of the acoustic composite panel type.

[0005] As is well known, such acoustic attenuation structures form Helmholtz resonators that attenuate acoustic waves within a certain frequency range. Each cell in the honeycomb core, open at the corresponding perforation in the acoustic skin, forms a Helmholtz resonator. These acoustic attenuation structures typically have a honeycomb-type core and exhibit acoustic performance. The resulting absorption is thus limited to a relatively narrow range of frequencies depending on the shape and dimensions of each of the alveoli.

[0006] One solution for increasing the acoustic attenuation frequency range of an acoustic structure is to superimpose several structural elements, each with a honeycomb core, the cells having similar or different shapes and dimensions. In such a configuration, the composite honeycomb core attenuation structure, i.e., the honeycomb structure, can be a structure with 1 degree of freedom (SDOF structure, SDOF for "Single Degree Of Freedom"), 2 degrees of freedom (DDOF structure, DDOF for "Double Degree Of Freedom"), or, more generally, with M degrees of freedom (MDOF structure, MDOF for "Multiple Degree Of Freedom"), where M is an integer greater than 2.

[0007] When the acoustic composite structure has several degrees of freedom, it comprises several layers of superimposed cell bodies or honeycomb cores, with two adjacent stacked honeycomb core layers separated by a septum. This septum is known to consist of a microporous wall perforated with holes such that, for any two given cells in a pair of superimposed cells, each belonging to one of two distinct and adjacent stacked honeycomb core layers, the cells of the given pair communicate acoustically with each other. The septum thus resembles an intermediate resistive skin.Although such an acoustic composite structure has a greater overall thickness, this characteristic allows the volume of each cavity forming a Helmholtz resonator to be increased and consequently the frequency band of the attenuated acoustic waves to be extended to lower frequencies, for example between 500 and 1000 Hz.

[0008] Drilling through the septum is a well-known and relatively simple process for flat acoustic composite structures forming regular flat acoustic panels. However, such drilling is complex for acoustic composite structures with curved or even complex shapes.

[0009] The use of add-on pieces designed to fit into each cell of the cellular structure to improve the acoustic performance of an acoustic panel is also known. In document FR 3 082 987, for example, truncated cones are connected by bars at their long bases, which must be positioned in notches made at the ends of the cells. The truncated cones are designed to fit inside each associated cell, with each long base inscribed within a section of the cell's interior space.

[0010] However, this solution is difficult to implement, particularly regarding the control of positioning between the cone truncates and the cells, as well as the sealing between these elements. Indeed, if the geometry of the notches and the bars do not perfectly match, some bars will not be correctly positioned in the notches, resulting in play with the acoustic skin. The performance and sealing of the acoustic attenuation structure are then degraded. The manufacturing process is also more difficult to implement in the case of curved parts due to the rigidity imparted to the acoustic structure by the use of cone truncates.

[0011] Besides the complexity of the manufacturing process, the use of such a structure added to the cells of a honeycomb core of an acoustic panel substantially increases the mass of the resulting acoustic panel, which is critical in certain applications such as in aeronautics.

[0012] Furthermore, in the case of manufacturing structures with multiple degrees of freedom (DDOF and MDOF), the manufacturing processes for the perforated septum are often complex to implement within the overall manufacturing process of the acoustic structure. In particular, it is known to use a process of draping a pre-impregnated fabric over a cellular body, which is then autoclaved. The increased number of insert positioning steps with this type of manufacturing process, in combination with the other standard manufacturing steps of the acoustic composite structure, results in a significant manufacturing time and, consequently, a higher manufacturing cost.

[0013] Other solutions exist, such as defining the holes in an acoustically porous skin at the same time as the skin itself. This is particularly true when using additive manufacturing processes. However, even if such a manufacturing process for the acoustically porous skin can be implemented in parallel with a manufacturing process for a complete composite structure, such processes are particularly time-consuming and present additional constraints specific to this type of manufacturing process. DESCRIPTION OF THE INVENTION

[0014] The invention aims to remedy all or part of the drawbacks of the prior art by proposing in particular a solution enabling the obtaining of a simple composite acoustic structure which constitutes a high-performance acoustic insulator.

[0015] To this end, according to a first aspect of the invention, a structural element for an acoustic composite structure is proposed, the structural element comprising at least one honeycomb core having a network of hollow cells delimited by partitions extending between two faces of the honeycomb core, and at least one resistive skin covering one of the faces of the honeycomb core, the structural element being remarkable in that it comprises a plurality of attached inserts, each insert having a tubular body through which open at its ends and a collar projecting from the associated tubular body, and in that the resistive skin is perforated by each of the inserts positioned opposite all or part of the cells so that, for each insert, the collar is positioned against the resistive skin on one side, and the tubular body opens on a second side, opposite the first side.

[0016] Thanks to this combination of features, the design of the add-on inserts—that is, inserts separate from the resistive skin—is simplified, and their installation on the resistive skin of a structural element is straightforward, even if the associated face of the honeycomb core has a non-planar shape. Furthermore, the use of such add-on inserts does not excessively stiffen the assembly. Finally, these tubular inserts improve acoustic performance while minimizing weight compared to prior art solutions.

[0017] In one embodiment, the inserts are formed in one piece, the tubular body of each insert extending from a first end with a flange to a second end. Preferably, the second end is circumferentially beveled, meaning that the tubular body has a decrease or narrowing in its outer diameter towards, preferably all the way to, the second end. In such a configuration, the end then has a frustoconical outer sheath. This beveled end facilitates perforation of the resistive skin.

[0018] According to one embodiment, the first side of the resistive skin corresponds to the outer side of the structural element with respect to the resistive skin, opposite the alveolar core, the second side of the resistive skin being the side opposite the alveolar core.

[0019] In one embodiment, the resistive skin is formed from a multilayer composite structure. Preferably, the resistive skin comprises a pre-impregnated fabric, and even more preferably, a layer of fabric interposed between two layers of adhesive. It should be noted that the characteristics of the structural element during its manufacture (in pre-impregnated form, for example) are also the same once the structural element is manufactured. The autoclave curing process, which finalizes the manufacturing, does not alter the structure of the structural element or the acoustic composite structure in which it is integrated, if applicable.

[0020] According to one embodiment, the tubular body of each of the inserts extends between a first and a second end along an opening axis, the tubular body having, at the first end, the collar extending in a plane orthogonal to the opening axis, the tubular body being open axially at its two ends.

[0021] According to one embodiment, the tubular body of the inserts has a constant cross-section, for example cylindrical.

[0022] In one embodiment, the inserts are separate from each other. In this way, the inserts are not joined together before the perforation of the resistive skin. This minimizes the rigidity of the resulting structural element.

[0023] In one embodiment, the inserts are formed in one piece, preferably from thermoplastic material(s), preferably obtained by molding, for example by injection. Such inserts are simple to manufacture, strong, and inexpensive.

[0024] According to another aspect of the invention, it relates to a remarkable acoustic composite structure in that it comprises at least one structural element as described above.

[0025] In one embodiment, the acoustic structure can form a simple acoustic panel, particularly when a reflective skin is added to cover the other of the two faces of the honeycomb core, namely the face opposite the resistive skin to the honeycomb core. This results in a simple composite acoustic structure forming a panel, flat or not, in which the honeycomb core is interposed between the reflective and resistive skins.

[0026] According to one embodiment, the acoustic composite structure comprises a composite structure with N degrees of freedom comprising a superposition of N layers of honeycomb cores, N being greater than or equal to 2, the composite structure comprising at least one septum separating two of the adjacent stacked layers of honeycomb cores, the resistive layer perforated by the inserts forming the septum or one of the septa of the acoustic composite structure.

[0027] In the case where N equals 2, the composite structure forms a DDOF, the resistive layer perforated by the inserts forming the septum of the DDOF. This results in an acoustic composite structure whose outer resistive skin, i.e., open to the outside, can have an aerodynamic shape and which simultaneously addresses the issues of sound absorption at low and high frequencies by means of a Helmholtz resonator adapted thanks to the tubular inserts.

[0028] In another aspect, the invention also relates to a method for manufacturing a structural element for an acoustic composite structure, the structural element comprising at least one honeycomb core having a network of hollow cells delimited by partitions extending between two faces of the honeycomb core, and at least one resistive skin covering one of the faces of the honeycomb core, the structural element comprising a plurality of attached inserts, each insert having a through tubular body open at its ends and a collar projecting from the associated tubular body, the resistive skin being perforated by each of the inserts positioned opposite all or part of the cells so that, for each insert, the collar is positioned against the resistive skin on one side, and the tubular body opens on a second side, opposite the first side, the manufacturing process of the structural element being remarkable in that it comprises the following steps: manufacturing of the honeycomb core;positioning of the resistive skin so that it covers one of the faces of the alveolar core; positioning of the inserts in the resistive skin opposite all or part of the alveoli so that, for each insert positioned, the collar is positioned against the resistive skin on one side, and the tubular body opens on a second side, opposite the first side.

[0029] According to one embodiment, the manufacturing process of the structural element includes a step of coating with a preparation such as an adhesive, for example based on polymer material(s), so as to coat the first side of the resistive skin perforated by the added inserts and the upper faces of the collars of the inserts, the coating step preferably being followed by a crosslinking step, preferably again by the application of heat.

[0030] The resistive skin with the inserted inserts is thus coated with a pre-prepared film, such as an adhesive film, for example an epoxy adhesive, which is cross-linked with heat, particularly around the openings defined by the tubular body of the inserts, in order to release the adhesive from the openings. This additional layer of adhesive allows the insert collars to be embedded between two layers, the skin The adhesive combines resistive material on the one hand, and a crosslinking preparation on the other, to ensure improved retention of the inserts throughout the acoustic structure's service life. In the case of a DDOF (Diffuse Density Frontal Observatory), the adhesive also has the advantage of performing a dual function: firstly, it secures the inserts against the associated membrane, and secondly, it bonds the two layers of honeycomb cores of the DDOF structure when the membrane perforated by the inserts forms the septum separating them.

[0031] According to one embodiment, the positioning step of the inserts in the resistive skin comprises at least: a first positioning step in which a tool carries at least one punch and comes opposite a socket, the punch having a head configured to come into contact against the collar of the associated insert, and a shank, the shank passing axially through the tubular body of the associated insert along its opening axis; a second positioning step, subsequent to the first positioning step, in which the punch is moved by an axial translation along the opening axis so as to perforate the resistive skin, the head of the punch pushing the insert until its collar comes into direct contact against the resistive skin; and a third positioning step in which the punch withdraws leaving the insert in the resistive skin.

[0032] According to another aspect of the invention, it also relates to a method of manufacturing an acoustic composite structure as described above, the method of manufacturing the acoustic composite structure being characterized in that it comprises the following steps: manufacturing a structural element according to the method described above; assembly of the structural element with at least one skin, preferably a resistive or reflective skin, covering the other of the two faces of the alveolar core.

[0033] This distinct skin to which the structural element is assembled is notably a reflective skin, for example in the case of an SDOF structure or may be another reflective skin, for example if the reflective skin of the structural element is intended to form the septum of a DDOS structure.

[0034] According to one embodiment, the manufacturing process for the acoustic composite structure includes a step of superimposing at least one layer of honeycomb core with the acoustic composite structure

[0035] Preferably the reflective skin is draped in prepreg form, with an autoclave curing step preferably being carried out after assembly. BRIEF DESCRIPTION OF THE FIGURES

[0036] Other features and advantages of the invention will become apparent from the following description, with reference to the accompanying figures, which illustrate: Figure 1: a schematic cross-sectional view of part of a complex-shaped acoustic composite structure according to a first embodiment; Figure 2: a schematic cross-sectional view of part of a planar acoustic composite structure according to another embodiment, shown without the resistive and reflective skins on the end faces; Figure 3: an isometric perspective view, from below, of the acoustic composite structure of Figure 2; Figure 4: a top view of the acoustic composite structure of Figure 2; Figure 5A: A view of a first positioning step of an insert in the resistive skin of a manufacturing process for a structural element according to an embodiment; Figure 5B: A view of a second positioning step of the insert in the resistive skin of the manufacturing process for a structural element, this second step being subsequent to the first step of Figure 5A; Figure 5C: A view of a third positioning step of the insert in the resistive skin of the manufacturing process for a structural element, this third step being subsequent to the second step of Figure 5B; Figure 6A: A schematic cross-sectional view taken locally at the level of an insert of a coating step of a preparation, so as to coat the outer side of a resistive skin perforated by the inserts and the upper faces of the collars of the inserts;Figure 6B: a view similar to Figure 6A in which the preparation coating step was followed by a crosslinking step.

[0037] For clarity, identical or similar elements are identified by identical reference symbols across all figures. DETAILED DESCRIPTION OF A METHOD OF IMPLEMENTATION

[0038] Figure 1 illustrates a schematic cross-sectional view of part of an acoustic composite structure 100 according to an embodiment of the invention.

[0039] The acoustic attenuation structure 100 here comprises a two-degree-of-freedom structure, commonly referred to as "DDOF". The acoustic attenuation structure 100 comprises a superposition of two honeycomb cores 20, 20', a lower honeycomb core 20 and an upper honeycomb core 20', each having a network of hollow cells 21, 21' delimited by partitions 22, 22'. The two honeycomb cores 20, 20' are separated from each other by a septum 30'. Each of these The honeycomb cores 20, 20' are here constructed with a honeycomb-type structure, for example, of the NIDA® type. For instance, the cells 21, 21' of the different layers of cells 20, 20' may be chosen to have different thicknesses depending on the layer in order to attenuate acoustic waves of different frequency bands. The honeycomb structure of each of the honeycomb cores 20, 20' is preferably made of at least one metallic material(s), preferably metallic material(s) capable of withstanding high temperatures depending on the intended use. In the aeronautical field, for example, one or more metallic materials capable of withstanding hot discharge temperatures may be chosen. The materials used to make up the network of cells 21, 21' can of course be different, for example in thermoplastic material(s) or in synthetic material(s), for example in aramid.It should also be noted that other forms of cells 21, 21' can be used, and not only hexagonal shapes in the case of honeycombs.

[0040] The cellular structure formed by the superposition of the two alveolar cores 20, 20' is covered: on one of its two faces, with a resistive skin 31, also called an acoustic skin, permeable to the acoustic waves that we wish to attenuate and covered, and on another of its two faces, opposite to the face covered by the resistive skin 31, with a reflective skin 32, also called a solid skin, which is generally oriented opposite to a noise source, i.e. to the rear with respect to the acoustic composite structure.

[0041] The septum 30' forms an intermediate resistive skin separating the two alveolar cores 20, 20'.

[0042] The internal space of the cells is particularly important because each of the alveoli in this central cellular structure, namely each of the alveoli of the two alveolar cores 20 and 20', forms a Helmholtz resonator. The resonator is thus formed by a bottle, created by an alveolus of the nest. of bees and a neck formed by a perforation in the resistive skin. In order to optimize acoustic performance, the septum 30' is itself also perforated, and the entire thickness of the DDOF structure is thus used to attenuate acoustic waves, the cells of the two honeycomb cores 20, 20' being superimposed, a given pair of superimposed cells 21, 21' communicating with each other via an orifice 55 in the septum 30'. In this way, the septum 30' forms an intermediate resistive skin between the two honeycomb cores 20, 20'.

[0043] These orifices 55 are each delimited by an insert 50. Each insert 50 has a through-tubular body 51 open at its ends 53, 54 and a collar 52 projecting from the associated tubular body 51. The tubular body 51 being through-tubular, it allows the communication of acoustic waves through it between two cells 21, 21' of a given superimposed pair of cells in the alveolar cores 20, 20'.

[0044] The intermediate resistive skin 30, formed here by the septum 30', is perforated by each of the added inserts 50, which are positioned opposite all or part of the alveoli 21 of the lower alveolar core 20. The lower alveolar core 20 and the septum 30' together form a structural element 10. Alternatively, it should be noted that manufacturing a structural element 10 comprising the septum 30' forming the resistive skin and the upper alveolar core 20' is also possible but more complex because it implies that for each positioned insert, the flange is positioned against the resistive skin on one side, and the tubular body opens on a second side, opposite the first side. The first side of the resistive skin corresponds to the inner side, facing the alveolar core, and the second side of the resistive skin is the outer side of the structural element with respect to the resistive skin, opposite the the alveolar soul.

[0045] It should also be noted that the structural element 10 can be integrated into a structure other than a DDOF. For example, the acoustic composite structure could be a simple composite panel formed by the honeycomb core 20 sandwiched between the reflective skins 31 and resistive skins 30, 32. According to another example, the acoustic composite structure could have more than 2 degrees of freedom, for example 3. In this case, the resistive skin, pierced by the added inserts 50, can be one of the intermediate skins forming a septum, or even the outer resistive skin. Preferably, a configuration will be chosen in which the resistive skin pierced by the inserts 50 forms a septum separating two of the layers of honeycomb cores, for example to guarantee the flatness of the outer resistive layer in order to guarantee optimal aerodynamic performance.

[0046] In this embodiment, the structural element 10 thus formed is configured so that, for each insert 50: the collar 52 of said inserts 50 is positioned against the associated resistive skin 30, here the septum 30', on a first side here external to the structural element 10 with respect to the associated resistive skin 30, namely here on the side opposite to the lower alveolar core 20, and the tubular body 51 opens on a second side, opposite to the first side, in particular into one of the alveoli 21 associated with the lower alveolar core 20.

[0047] The 50 inserts are independent of each other; that is, they are not connected to each other except by the septum 30' once positioned on it. In other words, they are not connected by dedicated fixation means, unlike in the prior art.

[0048] The tubular body 51 of the inserts 50 is cylindrical and axially open at its two ends 53, 54. This allows the communication of acoustic waves through the open hollow space of the tubular body 51 delimiting the associated orifice 55.

[0049] The tubular body 51 of each of the inserts 50 extends between its two ends, namely a first and a second end 53, 54 along an opening axis A. Once an insert 50 is positioned on the resistive skin forming here the septum 30', the opening axis A is such that it is oriented at least locally perpendicular to the face of the alveolar core 20 covered by the septum 30'.

[0050] The tubular body 51 carries at the first end 53, the collar 52 which extends in plane orthogonal P to the axis of opening A. The collar 52 has the shape of a discal crown coaxially surrounding the tubular portion of the first end 53 of the tubular body 51. The inserts 50 are pressed through the resistive skin forming here septum 30' so that the collar 52 comes into direct contact against said associated skin.

[0051] To simplify manufacturing, the 50 inserts are each formed in one piece, preferably from thermoplastic materials, preferably obtained by molding, for example, by injection. Examples of thermoplastic materials that can be used include polyetheretherketone (PEEK), polyetherimide (PEI), polyphenylene sulfide (PPS), and polycarbonate (PC). Of course, other materials, alone or in combination, can be used. Similarly, other manufacturing processes can be employed, such as extrusion or additive manufacturing.

[0052] Each insert 50 may have a second end 54 that is circumferentially beveled in the direction of a narrowing in diameter of the tubular body towards the second end 54. This bevel can facilitate the positioning of the associated insert 50 during perforation of the skin, even if the second end 54 is not itself designed to perform the perforation of said skin, for example, when using a punch 60 (see Figures 5A, 5B, and 5C described below). This bevel can also be created to facilitate the manufacture of the insert 50 by forming a draft angle.

[0053] The inner diameter of the tubular body 51 is predetermined and calibrated according to the intended use of the acoustic composite structure 100, its dimensions and the frequency ranges of acoustic waves to be attenuated.

[0054] The collar 52 and the tubular body 51 are connected by a fillet oriented towards the second end 54. In the case where the insert is manufactured by molding, the fillet offers the advantage of allowing a more homogeneous cooling rate, if the insert 50 is made of metallic material(s) in particular, or of ensuring good flow in the mold and therefore good filling, if the insert 50 is made of material(s) thermoplastic(s) in particular, or more simply to ensure good demolding of the part. Once inserted through the resistive skin 30, formed here by the septum 30', the fillet comes into contact with the perforated skin into which the insert 50 is inserted and softens the angle to preserve intact the edge of the skin at the perforation.

[0055] Figures 2, 3 and 4 illustrate schematic views of part of an acoustic composite structure 100 shown without the resistive skins 32 and reflective skins 31 at the end faces, in cross-sectional, isometric perspective from below and from above respectively.

[0056] This second embodiment illustrated in figures 2, 3 and 4 differs from the first embodiment of figure 1 essentially in that it is planar, that is to say that it does not have a complex curved shape.

[0057] The manufacturing process for this type of panel will be better understood in light of the following description.

[0058] A first step in the manufacturing process consists of manufacturing the structural element 10. Initially, the honeycomb core 20 is manufactured, intended to form the lower honeycomb core 20 of the final DDOS, the honeycomb core 20 here having the shape of a honeycomb structure.

[0059] If the alveolar core has a complex shape, for example a non-planar curved shape, as illustrated in Figure 1 for example, the alveolar core 20 can then be formed at this stage of the process.

[0060] Next, the resistive skin 30 of the structural element 10 is positioned to cover one of the faces 23, 24 of the honeycomb core 20. The resistive skin is formed of a multilayer composite structure. It is draped over the associated face of the honeycomb core 20 as a pre-impregnated film composed of several layers. This multilayer pre-impregnated skin includes, in particular, at least one structural ply, for example, a fabric made of glass fibers pre-impregnated with epoxy resin. This structural ply is also interposed between two epoxy adhesive films, one of the adhesive layers of which will be in contact with one end of the partitions 22 of the alveolar network 21 of the lower alveolar core 20, on the side of the face covered by the skin.

[0061] Of course, the nature of each layer constituting the resistive skin 30 of the structural element 10 can vary. For example, the layers can be woven or non-woven. Several structural plies can be superimposed, which is particularly advantageous in terms of strength. Strength is further increased when the fibers in the different woven plies have different orientations. The fiber orientation is configured to ensure the best strength for the intended end use. The adhesive layer(s) can also vary and be woven or non-woven. The fabric of the woven plies can also be made of a material other than glass, for example, carbon fiber. One of the layers can also be a thermoplastic film or fabric.

[0062] To ensure a perfect seal of the associated resistive skin, which is particularly important for guaranteeing good sound absorption, a multi-layered skin with an airtight barrier is preferred, at least one layer being airtight and having an airflow resistance equal to or greater than approximately 50 rayls MKS, or 50 kg s -1 ■ m " 2 in SL fundamental units

[0063] Once the pre-impregnated resistive skin 30 is positioned covering one of the faces 23, 24 of the alveolar core 20, the set of inserts 50 is positioned in the resistive skin 30 opposite all or part of the alveoli 21 of the alveolar core 20 so that, in this example and for each insert 50 positioned, the collar 52 is located directly against the resistive skin 30 on the outside side of the structural element 10 with respect to the resistive skin 30, opposite the alveolar core 20, and the tubular body 51 opens into the associated alveolus 21.

[0064] This method of positioning the inserts 50 is illustrated in detail in Figures 5A, 5B, and 5C. In particular, the following are illustrated respectively: a first step in positioning an insert 50 in the resistive skin 30 of the structural element 10 in which a tool (not illustrated) carries a punch 60 and comes opposite a socket 21, the punch 60 having a head 62 configured to come into contact against the collar 52 of the insert 50, and a rod 61, the rod 61 passing axially through the tubular body 51 of the insert 50 along its opening axis A. The rod 61 extends axially under the head 62 and has a greater length than that of the insert 50 which guarantees that the rod 61 passes through the tubular body 51 of the insert 50 on both sides.The external diameter of the rod 61 is chosen so as to be substantially equal to the internal diameter of the tubular body 51 so that the friction generated between the punch 60 and the insert 50 is sufficient to keep the insert 50 fitted onto the punch during this first step, without however forcing it too much so that it remains possible to remove the insert 50; a second step of positioning the insert 50 in the resistive skin 30 of the manufacturing process of the structural element 10, this second step being subsequent to the first step. Here, the punch 60 is moved by an axial translation, along the opening axis A. The rod 61 passes axially through the tubular body 51 of the insert 50 along its opening axis A, the distal end of the rod 61 comes into contact first, i.e. before the insert 50, against the resistive skin 30 during the movement of the insert 50 by the punch 60.The distal end of the rod 61 is equipped with a point to facilitate perforation of the resistive skin 30. The punch 60 is moved along the opening axis A so as to perforate the skin 30, the head 62 of the punch pushing the insert 50 until its collar 52 comes into direct contact with the prepreg 30. During this step, the collar 52 adheres to the first layer of the prepreg reflective skin. The choice of a top layer of the prepreg skin 30 must therefore ensure the adhesion of the insert 50 and thus, when the punch 60 moves in the opposite direction to withdraw, the insert 50 remains in position while the punch 60 is withdrawn. a third positioning step of the insert 50 in the resistive skin 30 in which the punch 60 withdraws leaving the insert 50 in the skin 30 thanks to a stickiness of the resistive skin 30 greater than the friction forces holding the insert 50 on the punch 60; then the punch 60 will look for another insert 50 and then repeat the positioning steps of the insert 50, until all the inserts 60 are positioned on the structural element 10.

[0065] Note that the tool could be, for example, a robotic arm equipped at its end with a punch 60. Furthermore, the robotic arm can be configured to place one insert 50 at a time, using a single punch 60, or it can carry several punches 60 and be configured to position multiple inserts 50 synchronously. The advantage of a positioning tool mounted on a robotic arm is that it facilitates the placement of inserts 50 on a complex surface. The robotic arm could be, for example, a 6-axis robotic arm.

[0066] According to one characteristic, care is taken to ensure that the length of the insert 50, i.e., its tubular body 51, is strictly less than the depth of the associated cell 21. In this way, for each insert 50, there is a gap between the bottom of the associated cell and the second distal end 54 of the insert 50. This prevents the insert 50 from interfering with the skin that will be positioned on the other side of the alveolar core 20 before it is draped. Similarly, once the structural element is formed, after curing, this characteristic guarantees good sound absorption.

[0067] Once all the inserts 50 are positioned, a coating step is carried out using a preparation 40. The preparation can be, for example, an adhesive, such as one based on polymer materials. This step consists in particular of coating at least, on the one hand, the outer side of the resistive skin 30 perforated by the inserts 50, the side against which the lower faces of the collars 52 of the inserts 50 are in contact, and on the other hand, the upper faces of the collars 52 of the inserts 50. The coating step is followed by a crosslinking step, for example, by the application of heat.

[0068] In other words, the multilayer resistive skin 30, equipped with the inserts 50 inserted through said skin, is thus coated with a film of epoxy adhesive which is then cross-linked by applying heat around the orifices 55 of the inserts 50 in order to release them from the adhesive (see Figures 6A and 6B). The cross-linking of the adhesive hardens it, and any adhesive present in the orifices 55 of the tubular bodies 51 is locally removed, leaving these orifices 55 free. This additional layer 40 allows the collars 52 of the inserts 50 to be embedded between the additional preparation layer on the one hand, and the resistive skin 30 on the other, thus ensuring that the inserts 50 remain in place throughout the service life of the acoustic composite structure 100.A collar 52 will be chosen for each insert 50 with a minimum thickness to ensure the flatness of the skin once the inserts have been added and inserted into the multilayered skin 30 and embedded with the cross-linked preparation layer.

[0069] Once the structural element 10 is formed, it is assembled with at least one skin, such as a resistive skin 32 (see for example Figure 1) so that it covers the other of the two faces of the honeycomb core 20. In the case of a DDOF, another honeycomb core 20' and its reflective skin 31 are also assembled by being glued.

[0070] Once the composite structure is assembled, the whole is placed in an autoclave to undergo a curing stage. In this way, the different septa 30' which separate the different layers of alveolar cores 20, 20', the different layers of alveolar cores 20, 20', the rear reflective skin 32 and the front outer resistive skin 31 are assembled into a single piece before being introduced into the autoclave.

[0071] This results in a composite acoustic structure extending according to the desired shape and exhibiting optimal technical characteristics.

[0072] Naturally, the invention described above is by way of example. It is understood that a person skilled in the art is capable of carrying out different embodiments of the invention without departing from its scope.

[0073] For example, one can plan to cook the structural element alone in an autoclave and then the structural element can be integrated into an acoustic composite structure by means of another process, for example by brazing.

[0074] It is emphasized that all features, as they are apparent to a person skilled in the art from this description, the drawings and the attached claims, even if in practice they have only been described in relation to other specific features, both individually and in any combinations, may be combined with other features or groups of features disclosed herein, provided that this has not been expressly excluded or that technical circumstances render such combinations impossible or meaningless.

Claims

CLAIMS Structural element (10) for an acoustic composite structure (100), the structural element comprising at least one honeycomb core (20) having a network of hollow cells (21) delimited by partitions (22) extending between two faces (23, 24) of the honeycomb core (20), and at least one resistive skin (30, 30') covering one of the faces of the honeycomb core (20), the structural element (10) comprising a plurality of attached inserts (50), each insert (50) having a tubular body (51) open at its ends (53, 54) and a collar (52) projecting from the associated tubular body (51), the resistive skin (30, 30') being perforated by each of the inserts (50) positioned opposite all or part of the cells (21) so as that, for each insert (50), the collar (52) is positioned against the resistive skin (30, 30') on one side, and the tubular body (51) opens on a second side, opposite the first side,The structural element (10) is characterized in that the inserts (50) are formed in one piece, the tubular body (51) of each insert (50) extending between a first end (53) having the collar (52) and a second end (54), said second end (54) being circumferentially beveled. The structural element (10) according to claim 1 is characterized in that the resistive skin (30, 30') is formed of a multilayer composite structure. Structural element (10) according to claim 1 or 2, characterized in that the tubular body (51) of each of the inserts (50) extends between a first and a second end (53, 54) along an opening axis (A), the tubular body (51) having, at the first end (53), the collar extending in a plane orthogonal (P) to the opening axis (A), the tubular body (51) being axially open at its two ends (53, 54). Structural element (10) according to claim 2 or 3,characterized in that the tubular body (51) of the inserts (50) has a constant cross-section, for example cylindrical. A structural element (10) according to any one of the preceding claims, characterized in that the inserts (50) are distinct from one another. A structural element (10) according to any one of the preceding claims, characterized in that the attached inserts (50) are formed from thermoplastic material(s), preferably obtained by molding, for example by injection. An acoustic composite structure (100) characterized in that it comprises at least one structural element (10) according to any one of the preceding claims.Acoustic composite structure (100) according to the preceding claim, characterized in that it comprises N degrees of freedom formed by a superposition of N layers of honeycomb cores (20, 20'), N being greater than or equal to 2, the acoustic composite structure (100) comprising at least one septum (30') separating two of the adjacent stacked layers of honeycomb cores (20, 20'), the resistive layer (30) perforated by the inserts (50) forming the septum (30') or one of the septa (30') of the acoustic composite structure (100).Method of manufacturing a structural element (10) for an acoustic composite structure (100), the structural element comprising at least one honeycomb core (20) having a network of hollow cells (21) delimited by partitions (22) extending between two faces (23, 24) of the honeycomb core (20), and at least one resistive skin (30, 30') covering one of the faces of the honeycomb core (20), the structural element (10) comprising a plurality of attached inserts (50), each insert (50) having a tubular body (51) open at its ends (53, 54) and a collar (52) projecting from the associated tubular body (51), the resistive skin (30, 30') being perforated by each of the inserts (50) positioned opposite all or part of the cells (21) so that, for each insert (50), the collar (52) is positioned against the resistive skin (30, 30') on one side, and the tubular body (51) emerges from a second. side, opposite the first side, the manufacturing process of the structural element (10) being characterized in that it comprises the following steps: manufacturing the alveolar core (20); positioning the resistive skin (30, 30') so that it covers one of the faces (23, 24) of the alveolar core (20); positioning the inserts (50) in the resistive skin (30, 30') opposite all or part of the alveoli (21) of the alveolar core (20) so that, for each insert (50) positioned, the collar (52) is positioned against the resistive skin (30, 30') on a first side, and the tubular body opens on a second side, opposite the first side.A manufacturing method according to claim 9, characterized in that it comprises a step of coating with a preparation (40) such as an adhesive, for example based on polymer material(s), so as to coat the first side of the resistive skin (30, 30') perforated by the attached inserts (50) and the upper faces of the collars (52) of the inserts (50), the coating step preferably being followed by a crosslinking step, preferably again by the application of heat.A manufacturing method according to claim 9 or 10, characterized in that the positioning step of the inserts (50) in the resistive skin (30, 30') comprises at least: a first positioning step in which a tool carries at least one punch (60) and comes into contact with a cavity (21), the punch (60) having a head (62) configured to come into contact against the collar (52) of the associated insert (50), and a shank (61), the shank (61) passing axially through the tubular body (51) of the associated insert (50) along its opening axis (A); a second positioning step, subsequent to the first positioning step, in which the punch (60) is moved by an axial translation along the opening axis (A) so as to perforate the. resistive skin, the head (62) of the punch pushing the insert (50) until its collar (52) comes into direct contact with the resistive skin; and a third positioning step in which the punch (60) withdraws leaving the insert (50) in the resistive skin.

12. Method of manufacturing an acoustic composite structure of the type comprising N degrees of freedom formed by a superposition of N layers of honeycomb cores (20, 20'), N being greater than or equal to 2, the acoustic composite structure (100) comprising at least one septum (30') separating two of the adjacent stacked layers of honeycomb cores (20, 20'), the resistive layer (30) perforated by the inserts (50) forming the septum (30') or one of the septa (30') of the acoustic composite structure (100), the method of manufacturing the acoustic composite structure (100) being characterized in that it comprises the following steps: manufacturing a structural element (10) according to any one of claims 9 to 11; assembly of the structural element (10) with at least one skin, preferably a resistive (32) or reflective (31) skin, covering the other of the two faces of the alveolar core (20).

13. Manufacturing method according to claim 12, characterized in that it comprises a step of superimposing at least one layer of honeycomb core with the acoustic composite structure (100).

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