EP2370249A1

EP2370249A1 - Core material

Info

Publication number: EP2370249A1
Application number: EP09803799A
Authority: EP
Inventors: Laurent Mezeix; Dominique Poquillon; Christophe Bouvet; Jean-Luc Brian; Valia Fascio; Philippe Vie
Original assignee: Institut National Polytechnique de Toulouse INPT; Ateca; Universite Toulouse III Paul Sabatier
Current assignee: Institut National Polytechnique de Toulouse INPT; Universite Toulouse III Paul Sabatier
Priority date: 2008-12-03
Filing date: 2009-12-01
Publication date: 2011-10-05
Also published as: WO2010063941A1; FR2939077B1; FR2939077A1

Abstract

Core material (3) consisting of a composite used for the complete or partial filling of hollow bodies, or as core in a sandwich structure (10) by being inserted between two skins (20). This composite comprises fibres (7, 8), beads (6) and resin (5), said beads (6) being linked together via fibres (7) that are bonded thereto, whereas said resin (5) fills the space between said beads (6) and said fibres (7, 8).

Description

SOUL MATERIAL

The invention relates to the field of composite materials and more particularly that of composite materials used as core material, with structural properties, for the total or partial filling of hollow bodies or as a core in a sandwich-type structure, and it has the following advantages: object such a core material.

A core material of this type may for example be used, without limitation, for the total or partial filling of hollow structures such as for example a boat mast or wind turbine blades.

The core material according to the invention may also be used as one of the constituents of sandwich structure panels, which generally result from the combination of two skins, of small thickness, made of relatively high strength and high modulus materials. , and a much thicker soul and low density.

The skins of the sandwich structure panels may be metal, for example aluminum alloys, or composites, carbon epoxy laminates, for example. These skins are glued on a soul which ensures spacing, which generates the bending stiffness of the panel. The core materials used are mainly foams, organic or metallic, and honeycombs.

In the current state of the art, these two families of solutions have specific advantages and disadvantages. Honeycombs have cell walls, which are positioned in the thickness of the core and contribute to the rigidity of the panel especially in compression. However honeycombs do not allow to vary the dimensional and mechanical characteristics on different parts of a panel. Their height is not adjustable locally and their cells closed. One of the defects of these core materials is therefore their non-ventilation despite the existence of drainage holes that are not always sufficiently effective to ensure adequate ventilation. good evacuation of water. This additional load is penalizing and further damages the durability of the panels.

In addition, in the aeronautical field for example, the realization of pieces of complex shapes with honeycomb cores can be problematic. Indeed, for small radii of curvature, the honeycomb is difficult to machinable or can be difficult to form.

As regards the foams used, they comprise closed cells that are therefore not ventilated. Among the foams, syntactic foams comprising a resin matrix in which glass microspheres and sometimes fibers have been injected, as is the case, for example, with the foams described in US Pat. Nos. 5,837,739 and 5,846,357. , US 6,068,915 and US 2005/049329. In foams such as those described in US 5,837,739, US 5,846,357, US 6,068,915, US 6,171,688, US 2008/014413, WO 2005/092961 and US 2005/049329, the resin consists of matrix whose main purpose is to transmit the mechanical forces, while the microspheres are used as a dopant in order to modify the viscosity of the resin and lighten the material, while the fibers improve the mechanical strength of the assembly.

The present invention aims to provide a new core material, which is an alternative to existing core materials, and overcomes the disadvantages thereof, including providing superior mechanical properties.

The core material according to the present invention consists of a composite material used for the total or partial filling of hollow bodies or as a core in a sandwich type structure, and it is essentially characterized in that this composite material comprises fibers, beads and resin, said beads being bonded together through fibers secured thereto, while said resin fills the space between said beads and said fibers. The resin is not intended to constitute a matrix, it has only a vacuum filling function, the beads confer rigidity to the material, while the fibers improve the mechanical strength and make the connection between the beads.

For this purpose, the beads do not consist of microspheres with a diameter of the order of one hundred microns as in the abovementioned documents, but in beads having a diameter of preferably between 1 and 6 mm.

According to an additional feature of the core material according to the invention, it comprises an assembly of elements each consisting of a ball to which fibers are joined by welding, brazing or gluing, said elements being connected to each other by a entanglement of fibers.

The balls are not in contact with each other, they constitute a frame around which the fibers are architected. According to a particular embodiment of the core material according to the invention, the balls are hollow spheres.

The use of hollow spheres has the advantage not only of the lightness of these, but also of their mechanical properties. Indeed, the hollow spheres are currently used in various applications in the field of shock because they allow to obtain energy absorbers which have a great crushing stroke, up to 85%, and an insensitivity to the speed of shock.

The hollow spheres, as a three-dimensional reinforcement, provide the mechanical strength of the core material according to the invention.

Indeed, the hollow spheres assemblies have an open porosity which gives them good mechanical strength.

The combination of hollow spheres and entangled fibers makes it possible to increase the energy absorption capacities of the core thus manufactured, and to lighten the final material obtained.

The hollow spheres are, in a manner known per se, made of metal or of synthetic materials such as polymer, ceramic or elastomer, or mineral. The diameter of these hollow spheres is between 1 and 6 mm, and a core material according to the invention may comprise hollow spheres all of the same diameter, or not.

The diameter and the nature of the shell of the hollow spheres are sized according to the desired absorption capacity and the type of fibers used.

According to an additional characteristic of the core material according to the invention, the fibers and the balls or hollow spheres are made of different materials.

Advantageously, the fibers are carbon, glass or metal material, their diameter is less than their length, preferably they have a section corresponding to a diameter of between 5 and 200um, and a length of between 10 and 60mm .

The volume concentration of the balls or hollow spheres is between 20 and 70%, and preferably, it is of the order of 60%.

By modifying the proportion of beads and / or the density of entangled fibers, it is possible to produce density gradient cores. The advantages and characteristics of the core material according to the invention will emerge more clearly from the description which follows and which relates to the appendices, which relate in particular to comparative tests of the sandwich structure panel properties some of which comprise a plastic material. soul according to the invention. In the accompanying drawing: Figure 1 shows a schematic sectional view of a part of a part comprising a core material according to the invention. - Figure 2 shows a schematic cross-sectional view of a portion of a sandwich structure panel comprising a core material according to the invention. FIG. 3 represents curves of the mechanical performances of sandwich structure panels comprising a core material according to the invention. FIG. 4 shows the results of compression tests carried out on a sandwich structure panel comprising a core material according to the invention, and sandwich structure panels of core material of other designs. FIG. 5 shows curves illustrating the energy absorbing capacity of a sandwich structure panel comprising a core material according to the invention and core material sandwich structure panels of other designs. FIG. 6 represents curves illustrating the values of the shear modulus as a function of the deflection obtained, for two sandwich structure panels, one of which comprises a core material according to the invention. FIG. 7 represents a comparative table of a sandwich structure panel comprising a core material according to the invention and core material sandwich structure panels of other designs.

Referring to Figure 1, we can see a manufactured part 1, comprising a shell 2 forming a hollow body filled with a core material 3 according to the invention.

The core material 3 consists of the assembly of elements 4 bonded together by a resin 5, epoxy for example, each of the elements 4 consisting of a ball, or a hollow sphere, 6 on which fibers 7 are joined, the elements 4 being connected together, in addition to the resin 5, by fibers 8.

Referring now to FIG. 2, a schematic representation of a particular use of a core material according to the invention, namely a sandwich structure panel 10, can be seen.

This panel 10 comprises two outer skins 20 between which is inserted a core 3 of composite material according to the invention also composed of elements 4. In the core material 3, whether for a part 1 or a panel 10, the fibers 7 are joined to the balls or hollow spheres 6 by welding, brazing or other similar techniques, or glued by means of a binder such as not limited to a resin. According to a non-limiting embodiment, the fibers 7 are pre-impregnated by spraying a resin over their entire surface.

Preferably, the fibers 8 are not oriented, they are entangled randomly to form a mattress. They ensure the cohesion of the core material 3 and transmit the forces to the reinforcement constituted by the balls 6.

The balls 6 make it possible to give rigidity to the core 3, the use of hollow spheres instead of the beads makes it possible to lighten it, while the fibers 7 and 8 make it possible to increase the level of stress during the compression plate and thus to obtain an improvement of 1 energy absorption.

Preferably, a core material 3 according to the invention comprises hollow spheres 6 to replace the balls. Composite materials containing hollow spheres alone are already known. Their main disadvantage is that in case of shock, the hollow sphere assemblies have adhesive fracture facies. These structures burst into pieces and the energy absorption is not done by crushing the hollow spheres, but by breaking the connection between the hollow spheres.

The core material according to the invention makes it possible to overcome this disadvantage by making it possible to reinforce the cohesion between the hollow spheres, and when the material is subjected to external stresses, the fibers transmit the forces to the hollow spheres. The configuration of the core material, proposed in the patent, makes it possible to avoid making the "conventional" connections work by bonding between the balls, which locally concentrate the forces. With the core material object of the present invention, the forces are distributed over the fibers and hulls of the hollow spheres. This architecture makes it possible to increase not only the energy absorption capacities of the material but also its shear strength.

With reference to FIG. 3, the stress-strain curves obtained on four sandwich structure panels comprising a core material according to the invention, which all identically have two 0.6 mm thick carbon skins, can be seen. , whose core has a thickness of 28.8 mm and which comprises resin, entangled carbon fibers and hollow polymer spheres, the density of carbon fibers being equal to 250 kg / m3. These four panels differ only in their volume concentration of hollow spheres which is respectively 20%, 40%, 60% and 70%.

In parallel with this, FIG. 4 shows stress-strain curves obtained on sandwich structure panels of the same thickness and with identical skins, but whose cores are composed differently. Thus the panels tested are a panel whose core is honeycomb, a panel whose core is made of a polymethacrylimide foam, a panel whose core comprises hollow spheres alone, and a panel according to the invention comprising hollow spheres and architectural fibers.

In view of these two series of tests, it can be seen on the curves of FIG. 3 that, with regard to the mechanical behavior of the core for a concentration of 20%, there is no true peak, while the behavior of a panel at a concentration of 70% is close to that of a panel whose core material includes only hollow spheres, with a low plateau stress. It emerges that to obtain an intermediate behavior the volume concentration of hollow spheres must be of the order of 60% for the geometry and the nature of the tested materials.

Moreover, the curves of FIG. 4 show that a sandwich structure panel comprising a core material according to the invention has a rigidity superior to that of sandwich structure panels whose core comprises hollow spheres alone.

On the other hand it can be seen that the Young's modulus of a core material sandwich structure panel according to the invention is increased by 120% with respect to the Young's modulus of a sandwich core structure panel. base of hollow spheres, and that it is of substantially the same order of magnitude as the Young's modulus of a honeycomb core sandwich structure panel. We can also see, by comparing the curves for the core material panel according to the invention and the sandwich structure panel whose core contains hollow spheres alone, that for a deformation of 10%, the absorbed energy is multiplied by 10 for a density increase of only a factor of 2. Thus, the core material sandwich structure panel according to the invention is not penalizing in terms of mass, since the hollow spheres are known for their low density. On the contrary, the addition of hollow spheres makes it possible to lighten the core material while improving the absorption capacities thereof.

FIG. 5 shows the energy absorbing capacity of a core material sandwich structure panel according to the invention, compared with those of two sandwich structure panels, one of foam core, and the another of honeycombs.

The absorbed energy was thus calculated from the previous compression tests by integrating the area under the different stress-strain curves.

In this figure it can be seen that the sandwich structure panel according to the invention has an increase in its energy absorbing capacity with respect to the foam core material panel, and even more with respect to the panel of soul in honeycombs.

With reference to FIG. 6, we can see the evolution of the shear moduli as a function of the deflection obtained for two sandwich structure panels each comprising two skins. made of carbon having a thickness of 2 mm and a core thickness of 26 mm, the core of one being composed, according to the invention, of resin, entangled carbon fibers and hollow polymer spheres, for a mass volume of 300 kg / m ³ , while the core of the other consists solely of entangled carbon fibers and resin, for a density of 250 kg / m ³ .

We can see that for a core material sandwich structure panel according to the invention, the shear modulus increases significantly, and that the value of the shear modulus for the core material sandwich structure panel containing fibers alone. is about 30 MPa, while for the core material according to the invention the value of the module is about 140 MPa. Thus, for an increase in density that is not significant, a substantial gain in shear modulus is obtained.

In the table of Figure 7, are reported the results of flexural tests carried out on three panels of sandwich structure. Each panel has a different kind of core, namely one containing hollow spheres and fibers according to the invention, one in honeycombs, and the last in polymethacrylimide foam.

From these tests it follows that the shear modulus G obtained for a core according to the invention is approximately twice that obtained for a honeycomb core and for a polymethacrylimide foam core.

The mechanical strength of a sandwich structure panel comprising a core material according to the invention is all the more important as the fibers adhere to the surface of the balls or hollow spheres.

It has thus been found that a sandwich structure panel comprising a core material according to the invention has mechanical performance superior to that of existing sandwich structure panels. Thus, its rigidity is greater than that of sandwich structure panels whose core comprises hollow spheres alone or is made of foam, and substantially equal to that of honeycomb core sandwich structure panels, while its energy absorbing capacity is greater than that of sandwich core foam and honeycomb structure panels. From the manufacturing point of view, a core material according to the invention can be obtained according to the method which consists in carrying out the following successive steps:

- the separation of the fibers,

- addition of resin, - mixture of fibers and balls or hollow spheres,

- casting of the soul, and

- polymerization.

As regards the manufacture of a sandwich structure panel, the crosslinking of the core to the skins is carried out together with the polymerization step.

According to another embodiment, the core according to the invention can be obtained by combining successive layers of different densities, composed of different proportions of beads or fibers of different densities. Thus, without limitation, a density gradient material can be constructed.

The core material according to the invention is relatively flexible and can be molded into various shapes. It is thus possible to produce complex shapes by molding such as that shown in FIG. 1. This embodiment makes it possible to avoid any subsequent machining or forming and makes it possible to overcome the difficulties and constraints associated with these techniques.

Moreover, this material offers advantages such as its open porosity which allows a certain ventilation or the ability to adapt to complex geometries. In terms of applicability, this material, because of its constituents, is multifunctional and can be used for applications where, for example, good fire resistance and electrical conductivity properties are sought. The core material according to the invention can be used in the manufacture of sandwich structure panels intended for the field of transport in general, and aeronautics in particular, and advantageously for parts of strong curvature, a leading edge by example. It can also be used for the total or partial filling of hollow bodies in the space field, but also in the wind field for applications to wind turbine blades for example.

Claims

1) core material (3) consisting of a composite material used for the total or partial filling of hollow bodies (1) or as a core in a sandwich-type structure (10) being interposed between two skins (20), characterized in that this composite material comprises fibers (7, 8), balls (6) and resin (5), said balls (6) being bonded together through fibers (7) which are secured to them, while said resin (5) fills the space between said beads (6) and said fibers (7, 8). 2) core material (3) according to claim 1, characterized in that it comprises an assembly of elements (4) each consisting of a ball (6) to which are joined fibers (7) by welding, brazing or bonding, said elements (4) being connected to each other by a fiber entanglement (7, 8).

3) Core material (3) according to claim 1 or claim 2, characterized in that the balls (6) have a diameter of between 1 and 6 millimeters.

4) core material according to any one of the preceding claims, characterized in that the balls (6) are hollow spheres.

5) core material (3) according to any one of the preceding claims, characterized in that the fibers (7, 8) and the balls or hollow spheres (6) are made of different materials.

6) core material (3) according to any one of the preceding claims, characterized in that the fibers (7, 8) are carbon, glass or metal material, their diameter is less than their length, they have a section corresponding to a diameter of between 5 and 200um, and a length of between 10 and 60mm.

7) core material (3) according to any one of the preceding claims, characterized in that the volume concentration of the balls or hollow spheres (6) is between 20 and 70%,

8) core material (3) according to claim 7, characterized in that the volume concentration of the balls or hollow spheres (6) is of the order of 60%.

9) sandwich structure panel (10) consisting of two skins (20) of small thickness and a core interposed between said two skins (20), characterized in that the core is made of a core material (3) according to any one of the preceding claims.

10) Use of the core material (3) according to any one of claims 1 to 8, in the manufacture of parts (1; 10) in the aeronautical field, in the spatial field, or in the wind field.