METHOD FOR REPAIRING A FIBER COMPOSITE COMPONENT

Description

[0001] The invention relates to a method for repairing a damaged area of ​​a fiber composite component, which is formed or manufactured from at least one fiber layer of a fiber material of a fiber composite material and a matrix material of the fiber composite material embedding the fiber material.

[0002] Due to their strength-to-weight ratio and stiffness, fiber-reinforced composite components made from fiber-reinforced composite materials are now indispensable. Fiber-reinforced composite materials, from which such components can be manufactured, typically have two main components: a fiber material and a matrix material that embeds the fiber material. The fiber material essentially imparts its load-bearing properties to the composite component in the direction of the reinforcing fibers. Through the curing of the matrix material embedding the fiber material, the reinforcing fibers are forced into the specified load direction and thus form an integral unit with the matrix material.

[0003] Not only in large-scale or large-area fiber composite components are these built up layer by layer from several layers of fiber material, similar to a laminate, whereby the directions of the reinforcing fibers can differ from layer to layer. Due to the layered structure of a fiber laminate, an isotropic material property can be approximated in the component plane, since the direction of the reinforcing fibers now varies according to the respective fiber layers. However, anisotropic material properties can also be specifically defined.

[0004] After the fiber laminate has been formed as a fiber preform from the individual fiber layers of the fiber material, the matrix material embedding the fiber material is usually cured under temperature control and pressure, so that the fiber material and matrix material form an integral unit. Dry fiber materials can be used, which must be infused with the matrix material at a later stage. However, pre-impregnated fiber materials, so-called prepregs, are also known, in which the matrix material is already incorporated into the fiber material before the formation of the fiber laminate or fiber preform.

[0005] When fiber-reinforced composite components are used in exposed locations, such as aircraft fuselages, or in other applications, it is often necessary to repair the component if it develops a defect. Such a defect can be caused, for example, by an object impacting the composite component with a certain force. Besides damaging the reinforcing fibers of one or more layers of the composite, such a defect can also exhibit phenomena where individual fiber layers separate from each other and are no longer held together by the cured matrix material.

[0006] For example, DE 10 2011 056 088 A1 discloses a method for repairing a damaged area of ​​a fiber composite component. In this method, a scarf joint is created in the area of ​​the damaged area to first remove the damaged fiber material. Z-pins are then inserted in the scarf joint area. Repair fiber semi-finished products are then draped around these Z-pins, inserted into the scarf joint, and thus fill the damaged area of ​​the fiber composite component. The Z-pins are intended to provide maximum stability and stiffness to the repaired area.

[0007] A similar repair method is also known from US 5,868,886, in which Z-pins are driven into the existing base structure of the fiber composite component in the area of ​​the damage to be repaired using ultrasound.

[0008] When repairing such damaged areas using appropriate repair fiber semi-finished products, fiber semi-finished products are generally used that correspond to those used in the manufacture of the fiber composite component. However, it has been shown that this results in significant waviness on the component surface and, furthermore, that the repair area is usually not flush with the surface. This is due to the fact that, unlike in the manufacture of the entire fiber composite component, the repair fiber semi-finished products used at the repair area cannot be subjected to sufficient pressure to ensure that the fiber volume fraction matches that of the rest of the fiber composite component.Because the use of a pressure furnace (autoclave) allows external pressure to be applied to the surface of the fiber composite component during its manufacture, compressing the fiber layers and thereby increasing the fiber volume fraction. During repair, the repair fiber semi-finished products would have to be subjected to pressure in the same or a similar manner, which is difficult to achieve given the fiber composite structure.

[0009] Furthermore, it has been shown that a large single-layer thickness leads to the formation of pure resin or adhesive areas that do not contribute to strength and typically form areas where premature failure occurs.

[0010] WO 2017 / 081456 A1 discloses a method for repairing a multi-layered fiber composite component, wherein a repair cavity is first created in the area to be repaired. This repair cavity is created by removing material from the damaged area by scarfing.

[0011] It is therefore an object of the present invention to provide an improved method for repairing a damaged area of ​​a fiber composite component, in which the repaired damaged area has a similar strength and stiffness to the rest of the fiber composite component and, in addition, waviness on the surface of the repaired damaged area as well as thickening in the area of ​​the damaged area can be avoided.

[0012] The problem is solved according to the invention by the method of claim 1. Claim 1 proposes a method for repairing a damaged area of ​​a fiber composite component, wherein the fiber composite component is made of a fiber composite material. The fiber composite material, by definition, comprises a fiber material and a matrix material, wherein the fiber material is provided in the form of individual fiber layers. The fiber composite component is therefore produced from at least one such fiber layer of the fiber material by curing the matrix material embedding the fiber material. However, the fiber composite component will typically have a plurality of individual fiber layers arranged layer by layer on top of each other.

[0013] By design, the method initially comprises the step of removing the damaged area from the fiber composite component by creating a repair cavity within the component. According to one embodiment, this can be achieved by scarfing, where the edges of the repair cavity are chamfered towards the bottom of the cavity.

[0014] Subsequently, at least one repair fiber layer of a fiber material is inserted into the manufactured repair cavity of the fiber composite component, and then the matrix material embedding the fiber material is consolidated (e.g., cured). The matrix material can be the same matrix material used to manufacture the fiber composite component. However, it is also conceivable to use a different matrix material that, for example, cures at lower temperatures.

[0015] The repair fiber layers can be introduced, for example, by using an additional adhesive. The repair fiber layers are then glued into the repair cavity or bonded to each other. The adhesive can have the same properties as the matrix material used or different properties to adapt the repair to prevailing conditions.

[0016] Accordingly, the fiber composite component itself can be made from a first fiber composite material comprising a first fiber material and a first matrix material, while a second fiber composite material comprising a second fiber material and a second matrix material is used for the repair. The first matrix material and the second matrix material can be the same or different from each other.

[0017] According to the invention, at least one repair fiber layer is introduced into the repair cavity, the thickness of which is less than the thickness of the fiber layers of the rest of the fiber composite component. Consequently, the fiber layers of the fiber composite component made of the first fiber composite material differ from the fiber layers of the fiber material of the second fiber composite material with respect to their thickness, such that the thickness of the repair fiber layers is less than the thickness of the fiber material of the fiber composite component. Different thicknesses can be used. The repair fiber layers can all have the same thickness, only some of the repair fiber layers can have the same thickness, or all repair fiber layers can have different thicknesses.

[0018] Because the layer structure within the repair cavity no longer corresponds to the original layer structure of the fiber composite component after such a repair, the repair patch within the cavity can be adapted much more precisely and flexibly to the prevailing conditions. It has been shown that by using repair fiber layers with a thinner layer thickness than those of the fiber layers in the rest of the fiber composite component, a better match in repair stiffness can be achieved without compromising the load-bearing properties of the entire component. Significantly smaller resin and adhesive pockets now form at the edges of the repair cavity, resulting in higher repair stiffness.

[0019] Furthermore, it has been shown that surface waviness can be reduced by using repair fiber layers with a smaller thickness, which is particularly advantageous for laminar flow surfaces. The reduced waviness also increases compressive strength. By using repair fiber layers with a smaller thickness than the original fiber layers, stress concentrations at the edges of the layers (where the original is ground smooth and the repair is discrete) can be reduced. This is because the repair cavity is typically scarfed, while the repair fiber layers are stepped. The smaller layer thickness reduces stress concentrations caused by the stepped repair.

[0020] Furthermore, multiple repair fiber layers with load-optimized layer orientation can be attached to a single fiber layer of the fiber composite component, thereby improving the mechanical properties of the repair. The reduced individual layer thickness lowers the local adhesive stress and thus increases the bond strength. Damage tolerance is higher due to the larger interface area. Manufacturing-related variations in overlap lengths can be compensated for by using thinner repair fiber layers.

[0021] According to one embodiment, the repair cavity in the fiber composite component is produced by forming a scarf joint in the area of ​​the damage.

[0022] The scarf joint creates a kind of conical repair cavity that runs towards the fiber composite component, into which the individual repair layers are then inserted layer by layer.

[0023] According to one embodiment, the repair fiber layers are arranged in a stepped configuration within the repair cavity, such that the second repair fiber layer, laid on top of a first repair fiber layer, has a larger extent and at least a different orientation than the underlying first repair fiber layer. Consequently, the layer structure of repair fiber layers forms a stepped or stair-like geometry at the layer ends (layer extensions), which does not correspond to the geometry of the repair cavity. By reducing the individual layer thickness of the repair fiber layers, the negative properties of the stepped or stair-like geometry in the edge regions are reduced.

[0024] According to one embodiment, all repair fiber layers introduced into the repair cavity are designed such that they have a layer thickness that is smaller than the layer thickness of the fiber layers of the rest of the fiber composite component. Different layer thicknesses can be used.

[0025] This ensures that the number of repair fiber layers in the repair cavity is greater than the number of fiber layers of the fiber composite component that are saturated and thus affected by the production of the repair cavity.

[0026] According to a further embodiment, the at least one repair fiber layer is already pre-impregnated with a matrix material. In other words, at least one repair fiber layer, preferably all repair fiber layers whose layer thickness is less than the layer thickness of the fiber layers of the rest of the fiber composite component, is a so-called prepreg material. It is also conceivable, however, that the repair fiber layers are infused with a matrix material in an infusion process before or after being inserted.

[0027] According to one embodiment, at least one of the repair fiber layers has a layer thickness of less than 80%, preferably less than 60%, and particularly preferably less than 40%, of the layer thickness of the fiber layers of the rest of the fiber composite component. In this way, it can be achieved, in the best case, that at least 1 1 / 2, preferably at least 2 or even more, repair fiber layers are assigned to or correspond to one fiber layer of the fiber composite component, thereby improving the repair stiffness.

[0028] According to one embodiment, at least one repair fiber layer, preferably all repair fiber layers, has a layer thickness of less than 100 g / m², preferably less than 75 g / m² and particularly preferably less than 50 g / m².

[0029] In contrast, according to one embodiment, the fiber layers of the remaining fiber composite component can have a layer thickness of more than 100 g / m², preferably more than 135 g / m².

[0030] According to one embodiment, the repair fiber layers are introduced into the repair cavity such that the second repair fiber layer, laid on top of a first repair fiber layer, has a different principal fiber direction than the first repair fiber layer. Consequently, the direction of the reinforcing fibers (principal fibers) differs between two successive repair fiber layers, resulting in different principal fiber directions. Because the repair fiber layers are thinner than the fiber layers of the fiber composite component, more than one repair fiber layer is assigned to each fiber layer in the repair cavity. Therefore, in the plane of a fiber layer of the fiber composite component within the repair cavity, there are now repair fiber layers with different fiber directions.This significantly improves the strength and stiffness of the repair patch, as well as the load-bearing properties within the repair patch, with respect to a single fiber layer. However, two repair fiber layers with the same orientation can also be placed on top of each other.

[0031] According to one embodiment, the main fiber directions can differ by an angle of 45° or 90°. However, any angle can be used in principle.

[0032] According to one embodiment, one or more cover ply layers are laid on top of the repair fiber layers inserted into the repair cavity. This covers the repair fiber layers inserted into the repair cavity with the cover ply, thereby also achieving improved load distribution at the repaired area.

[0033] It can be provided that at least one of the cover fiber layers, preferably the uppermost cover fiber layer, has a larger extent than the repair cavity in at least one direction. Preferably, at least one of the cover fiber layers has a planar extent that is larger than the repair cavity on the surface of the fiber composite component. This ensures that this at least one cover fiber layer completely covers the repair cavity and, in particular, overlaps the repair cavity in the edge region. The repair fiber layer can have a larger area than the repair cavity on the surface of the fiber composite component. It can be provided that this property applies to all cover fiber layers.

[0034] Alternatively or additionally, it can also be provided that at least one cover fiber layer has a thickness equal to or smaller than that of any of the repair fiber layers in the repair cavity. Preferably, this cover fiber layer has a thickness equal to that of all repair fiber layers in the repair cavity, or the same thickness as all repair fiber layers in the repair cavity, provided that all repair fiber layers have the same thickness. This can ensure that the sealing of the repair cavity by the final cover fiber layer has a more favorable effect on statics, aerodynamics, and / or appearance. It can be provided that this property applies to all cover fiber layers.

[0035] The invention is explained in more detail using the accompanying figure as an example. It shows: Figure 1 - Schematic representation of a repaired fiber composite component.

[0036] Figure 1Figure 10 shows a fiber composite component which, for illustrative purposes, has only four fiber layers 11 of a fiber material. The four fiber layers are arranged one above the other and joined into an integral unit by curing a matrix material that embeds the fiber material of the fiber layers 11. Each of the fiber layers 11 has a principal fiber direction, which can differ from fiber layer to fiber layer, in order to achieve the best possible load-bearing properties across the entire planar plane of the fiber composite component 10 and to at least approximate the properties of isotropic and anisotropic materials.

[0037] The fiber composite component 10 now exhibited damage in the first two upper fiber layers 11a and 11b, necessitating repair. For this purpose, a repair cavity 12 was created by removing the damaged area, for example, by drilling, grinding, or milling. The first three upper fiber layers 11a, 11b, and 11c in the area of ​​the repair cavity 12 were removed to provide space for a corresponding repair patch.

[0038] The repair cavity 12 is realized in the form of a scarf joint, in which the edge areas are chamfered towards the bottom of the repair cavity.

[0039] Repair fiber layers 13 are then successively inserted into the repair cavity 12 thus formed in order to refill the repair cavity 12 with fiber material. The repair fiber layers 13 inserted into the repair cavity 12 have a layer thickness d 2 that is smaller than the layer thickness d 1 of the fiber layers 11 of the fiber composite component 10.

[0040] The layer thickness of a fiber layer or a repair fiber layer is understood to be the maximum extent of the fiber material orthogonal to the fiber layer plane of the fiber layers.

[0041] Due to the fact that the repair fiber layers have a lower layer thickness d 2 than the layer thickness d 1 of the fiber layers 11 of the fiber composite component 10, more repair fiber layers 13 can be introduced into the repair cavity 12 than fiber layers 11 of the fiber composite component 10 were removed from the fiber composite component 10 in the area of ​​the repair cavity 12.

[0042] In a schematically idealized form, in Figure 1 The repair fiber layers 13 are designed such that two repair fiber layers 13 inserted into the repair cavity 12 each correspond to a fiber layer 11 of the fiber composite component 10. If the main fiber direction between these two repair fiber layers 13, which are assigned to a fiber layer 11 of the fiber composite component 10, is varied, then, with respect to a fiber layer 11 of the fiber composite component 10 at the repaired area, the patch within the repair cavity 12 can achieve better load-bearing properties and thus compensate for possible disadvantages of the repair.

[0043] Furthermore, the reduced layer thickness d 2 compared to the fiber layers 11 of the fiber composite component 10 reduces the stair-step effect in the edge region 14 caused by the adjacent repair fiber layers 13. This results in a higher layer resolution in the edge region 14 due to the reduced layer thickness. Consequently, the volume of resin or adhesive pockets in the edge region 14 of the repair cavity 12 can be reduced, thereby also increasing the strength and stiffness of the repair patch.

[0044] In the exemplary embodiment of the Figure 1It is further shown that a repair fiber layer is also applied as a final top layer 15, which extends beyond the repair cavity 12 at its ends. The top layer 15 can also have a thickness that is less than the thickness d1 of the fiber layers 11 of the fiber composite component, thereby reducing discontinuities in the outer surface 16 of the fiber composite component. This is particularly advantageous for surfaces exposed to aerodynamic flow, as it reduces air resistance. Reference symbol list

[0045] 10 - Fiber composite component 11 - Fiber layers 12 - Repair cavity 13 - Repair fiber layers 14 - Edge area 15 - Top layer 16 - Outer surface d1 - Layer thickness of the fiber layers 11 d2 - Layer thickness of the repair fiber layers 13

Claims

1. Method for repairing a damaged area of a fiber composite component (10) which is formed from at least one fiber layer (11) of a fiber material of a fiber composite material and a matrix material of the fiber composite material embedding the fiber material, the method comprising the following steps: - removing the damaged area from the fiber composite component (10) and producing a repair cavity (12) in the fiber composite component (10), - inserting at least one repair fiber layer (13) of a fiber material into the repair cavity (12) produced in the fiber composite component (10), and - consolidating the matrix material embedding the fiber material of the repair fiber layer (13) after introducing the repair fiber layer (13) into the repair cavity (12) of the fiber composite component (10), characterized in that at least one repair fiber layer (13) is introduced into the repair cavity (12), the layer thickness of which is smaller than the layer thickness of the fiber layers (11) of the remaining fiber composite component (10).

2. Method according to claim 1, characterized in that the repair cavity (12) is produced in the fiber composite component (10) by forming a mounting in the area of the damaged area.

3. Method according to claim 2, characterized in that the repair fiber layers (13) are arranged in a staircase-shaped manner in the repair cavity (12) in such a way that the second repair fiber layer (13) placed on top of a first repair fiber layer (13) has a greater extension in at least one direction than the first repair fiber layer (13).

4. Method according to one of the preceding claims, characterized in that all repair fiber layers (13) introduced into the repair cavity (12) have a layer thickness (d2) that is smaller than the layer thickness (d1) of the fiber layers (11) of the remaining fiber composite component (10).

5. Method according to one of the preceding claims, characterized in that at least one repair fiber layer (13) is preimpregnated with the matrix material or the fiber material is infused with the matrix material before or after the at least one repair fiber layer is introduced into the repair cavity produced.

6. Method according to one of the preceding claims, characterized in that at least one repair fiber layer (13) has a layer thickness (d2) of less than 80%, preferably less than 60%, particularly preferably less than 40% of the layer thickness (d1) of the fiber layers (11) of the remaining fiber composite component (10).

7. Method according to one of the preceding claims, characterized in that at least one repair fiber layer (13) has a layer thickness (d2) of less than 100 g / m2, preferably less than 75 g / m2, particularly preferably less than 50 g / m2.

8. Method according to one of the preceding claims, characterized in that the fiber layers (11) of the remaining fiber composite component (10) have a layer thickness (d1) of more than 100 g / m2, preferably more than 135 g / m2.

9. Method according to one of the preceding claims, characterized in that the repair fiber layers (13) are introduced into the repair cavity (12) in such a way that the second repair fiber layer (13) placed on top of a first repair fiber layer (13) has a main fiber direction different from that of the first repair fiber layer (13).

10. Method according to claim 9, characterized in that the main fiber direction differs by an angle of + / - 30°, + / - 45°, + / - 60° or 90°.

11. Method according to one of the preceding claims, characterized in that one or more cover fiber layers are placed on the repair fiber layers introduced into the repair cavity.

12. Method according to claim 11, characterized in that at least one cover fiber layer has a larger extension than the repair cavity in at least one direction and / or that at least one cover fiber layer has a layer thickness that is smaller than or equal to a repair fiber layer of the repair cavity.