Verifying the positioning of a fibrous preform in a blade

Description

GENERAL TECHNICAL FIELD

[0001] The invention relates to the general field of turbomachine blades made of composite material and relates to a method for controlling the manufacture of such a blade using a preform and a mold. STATE OF THE ART

[0002] Composite materials are used to make turbomachine blades, particularly for the turbomachine fan.

[0003] As illustrated on the Figure 1 , a blade conventionally comprises a body 1 made of composite material manufactured from a preform. This preform comprises a weave of a plurality of weaving threads or fibers, which comprise warp threads and weft threads. The weave may be in a single piece obtained by three-dimensional weaving and may include in particular carbon, glass, aramid and / or ceramic fibers.

[0004] The composite material body comprises a root 11, a stilt 12 and a blade 13 having an aerodynamic profile. In the following, a blade reference frame X, Y, Z is defined: the X axis corresponds to the longitudinal direction of the blade root, the Z axis corresponds to the longitudinal direction in which the blade 13 extends from the root 11, the Y axis corresponds to the thickness of the blade. Of course, the X, Y and Z axes are orthogonal two by two, the blade reference frame being orthonormal.

[0005] The preform is cut according to the shape and dimensions of the constituent parts of the blade, and placed in a mold.

[0006] A binder comprising a thermosetting resin is then injected into the mold, in order to impregnate the entire preform. After heating the mold, the molded part is removed from the mold. Molding is, for example, a vacuum resin injection process of the RTM type (in English, " Resin Transfer Molding ”) or VARRTM (in English, “ Vacuum Resin Transfer Molding ").

[0007] During the manufacturing of the blade, a thick preform is placed in the cavity of an injection mold. 3D woven preforms are thick one-piece fabrics. In the case of a turbomachine fan blade, the one-piece preform is of variable thickness in order to be adapted to the aerodynamic profile of the blade (see Figure 1 ).

[0008] Therefore, when placing this variable thickness preform in a variable thickness injection mold, it is important to ensure that it is correctly positioned, in order to obtain a part of homogeneous material health. Indeed, if the thick area of the preform is poorly positioned and ends up in a thin area of the mold, the fiber volume rate of the part will be greatly affected and potentially non-compliant.

[0009] Thus, in the two groups of wires provided for the preform, there are structural wires, ensuring the structure of the preform, and tracer wires that are visually identifiable from the others and regularly arranged at least on the surface of the preform. These few glass wires positioned on the surface are called glass tracers or tracers in the remainder of what will be described. The latter, which represent a visible grid, make it possible to spatially locate the preform, and therefore to evaluate its correct positioning.

[0010] To check the integrity of the blade after injection, the positioning of the preform in the blade obtained after injection is checked. The check consists of measuring the height in Z along the X axis of the center of the glass tracer, called the neutral fiber, in the blade reference frame.

[0011] To do this, an X-ray tomography of the blade is used to ensure its integrity and to verify the location of the neutral fiber in relation to their theoretical position.

[0012] In particular, thanks to tomography, it is possible to reconstruct the neutral fiber in the tomographic volume and to verify its correct positioning. However, such a process only allows capturing fragments of glass tracers and not its entirety and therefore capturing fragments of the neutral fiber.

[0013] THE Figures 2a and 2b, respectively illustrate a tracer 21 partially detected in a blade 1 by X-ray tomography and 3D reconstruction, and a theoretical tracer 22 in 2b. A method for verifying the positioning of a fiber preform in a blade is described in EMERSON MONICA J ET AL: "Individual fiber segmentation from 3D X-ray computed tomography for characterizing the fiber orientation in unidirectional composite materials", COMPOSITES PART A: APPLIED SCIENCE AND MANUFACTURING, ELSEVIER, AMSTERDAM, NL, vol. 97, January 11, 2017 (2017-01-11), pages 83-92, ISSN: 1359-835X, DOI: 10.1016 / J.COMPOSITESA.2016.12.028

[0014] Since there are inaccuracies in the detection of the neutral fiber, the verification of the correct positioning of the preform and therefore of the integrity of the part is imprecise. PRESENTATION OF THE INVENTION

[0015] The invention makes it possible to overcome the aforementioned drawbacks.

[0016] To this end, the invention proposes, according to a first aspect described in claim 1, a method for verifying the positioning of a fiber preform in a blade, the blade having been obtained by injecting a resin into a mold having a blade shape and in which a preform has been placed, the blade extending in an orthonormal X, Y, Z blade reference frame, the blade comprising a blade root extending longitudinally along an X axis, a blade extending from the blade root along a Z axis, the blade having a thickness defined along a Y axis, the preform comprising glass tracers positioned on the surface of the preform, the center of the tracers defining a neutral fiber located at a height along the Z axis in the direction defined by the X axis, the method comprising the following steps: acquisition of 2D tomographic projections of the blade by means of an imaging system comprising an X-ray source, each projection being acquired according to a given orientation of the X-ray source relative to the blade; accumulation of the 2D projections according to the direction of the Y axis so as to obtain a cumulative 2D image according to the x and z directions; determination for each pixel column defined according to the direction of the z axis, of a gray level profile; processing of each of the profiles obtained so as to locate the Z position of the neutral fiber according to the direction of the X axis.

[0017] The invention, according to the first aspect, is advantageously supplemented by the following characteristics, taken alone or in any of their technically possible combinations: the projection consists of obtaining a 2D image consisting of pixels having the maximum gray level among a stack of pixels of the different sections stacked in the direction of the Y axis; the projection consists in that the 2D image is made up of pixels whose gray level is the sum of the pixels of the different sections stacked in the direction of the Y axis; the gray level profile is a Gaussian, the location of the neutral fiber being the center of the Gaussian; the gray level profile is a double Gaussian, the location of the neutral fiber being the center of the two Gaussians; the method comprises a step of filtering the location in the Z direction of the neutral fiber in the direction of the X axis; the method comprises a step of comparing the location of the neutral fiber thus obtained with a theoretical location of the neutral fiber.

[0018] The invention proposes, according to a second aspect, a method for manufacturing a turbomachine blade made of composite material, the method comprising the following steps: placing a fiber preform in a mold having a blade shape; vacuum injection of a resin into the mold comprising the preform so as to obtain the blade called injected blade; checking the positioning of the fiber preform in the injected blade by means of a method according to the invention.

[0019] According to a third aspect, which is not included in the subject matter of the claims, there is provided a blade of a turbomachine fan obtained by a method according to the second aspect of the invention.

[0020] According to a fourth which is not included in the subject matter of the claims, there is provided an aircraft comprising a fan blade according to the third aspect of the invention.

[0021] The advantages of the invention are multiple. From the point of view of time accuracy, repeatability, reproducibility and control time

[0022] The detection accuracy is improved compared to known techniques, in particular, from the point of view of accuracy, repeatability, reproducibility and control time.

[0023] The treatments implemented are fast and require few resources because one dimension is eliminated, unlike conventional tomography which implements a 3D reconstruction.

[0024] The measurement of the positioning of the neutral fiber is thus obtained with high precision. PRESENTATION OF THE FIGURES

[0025] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which, in addition to the figures 1 , 2a and 2balready discussed: there Figure 3 illustrates steps of a method of manufacturing a turbomachine blade according to the invention; Figure 4 illustrates an imaging system according to the invention; the Figure 5 illustrates steps of a method for verifying the positioning of a preform in a blade according to the invention; Figure 6 schematically illustrates pixels of an image; Figures 7 and 8 illustrate gray level profiles obtained by means of a method according to the invention; Figure 9 illustrates a 2D image obtained using the process of Figure 5 .

[0026] In all figures, similar elements have identical references. DETAILED DESCRIPTION General presentation of the process

[0027] In relation to the Figure 3 ,a method for manufacturing a turbomachine blade made of composite material, comprises the following general steps. These steps can be implemented in different ways known to those skilled in the art and will not be detailed below but only presented in a general manner (see also the introduction to this application).

[0028] A fiber preform 2 is placed in a mold 3 having a blade shape (step E1) and a resin is injected (step E2) under vacuum into the mold comprising the preform so as to obtain the blade.

[0029] Then, the method comprises a step of verification (step E3) of the blade implementing in particular an imaging system making it possible to obtain 2D projections or 2D images. It is from a processing of these 2D projections that the verification is carried out. Computed tomography imaging system

[0030] There Figure 4illustrates an imaging system 10 for acquiring 2D projections of a blade 1. X-rays, R from a source S are emitted at different angles (1, ..., i, ..., n) towards the blade 1. After passing through the blade 1, they are detected by a detector D and forming a set of 2D projections I 1 , ..., I i ,..., IN . It should be noted that there are as many 2D projections acquired as there are angles considered. All the angles cover a semicircle around the blade 1. The acquisition is therefore implemented by the detector D and above which the blade 1 is placed. This is for example a digital camera.

[0031] The 2D projections are then processed by a processing unit connected to or forming part of the imaging system. The processing unit is, for example, a processor. In the case where the processing unit is remote from the imaging system, it is connected wired or wirelessly to the imaging system. The processing unit makes it possible to implement methods for processing the 2D images obtained. Verification of the positioning of a fiber preform in a blade

[0032] The verification of the positioning of a fiber preform in the blade 1 is implemented by the processing unit of the imaging system 10 described above. In particular, this verification comprises the steps described below, in relation to the Figure 5 .

[0033] Several 2D tomographic projections of the blade 1 using the imaging system 10 are acquired (step E31). Each projection corresponds to a given orientation of the X-ray source relative to the blade. There are then N 2D images I 1 , ..., IN . Each projection consists of several grayscale pixels.

[0034] These 2D images are then accumulated (step E32) along the Y axis so as to obtain a cumulative 2D image denoted I along the directions of the X and Z axes. In each image each pixel has a gray level value. Each pixel P to P m of the cumulative image I is obtained by the average of the gray levels of the corresponding pixels, along the Y axis. These cumulative pixels have the same coordinates in X and Z. By this accumulation, one dimension is dispensed with, here the Y dimension. Indeed, to evaluate the positioning of the preform, only two coordinates are sufficient, the position information along the direction of the Y axis (in the thickness of the blade 1) turns out to be of little importance for evaluating the integrity of the blade 1.

[0035] This accumulation amounts to accumulating voxelic information in a direction.

[0036] According to one embodiment, the accumulation (step E32a) consists in that the cumulative 2D image obtained is made up of pixels having the maximum gray level among the stack of pixels of the different sections stacked in the direction of the Y axis. According to this embodiment, the consideration of the glass fiber only (material used for the neutral fiber) is maximized but can however omit some parts if the signal is too weak.

[0037] According to one embodiment, the accumulation (step E32b) consists in the fact that the cumulative 2D image obtained is made up of pixels whose gray level is the sum of the pixels of the different 2D projections stacked in the direction of the Y axis. According to this embodiment, the quantity of information taken into account is maximized, but a parasitic signal can be detected.

[0038] Then, for each pixel column defined along the z axis, a gray level profile is determined (step E33). The scale is advantageously an unsigned 16-bit scale so that there are 65536 gray levels from black to 0 to white (65535). Thus, there are as many profiles as there are columns along the direction of the Z axis.

[0039] There Figure 6 schematically illustrates an image comprising pixels (a square is a pixel). A profile consists for each column c1, c2, c3, c4, ..., cM of a variation of the gray level.

[0040] THE figures 7 and 8 show two profiles depending on whether the glass tracer includes a strand ( Figure 7 ) or two strands ( figure 8 ). The profile includes a gray level on the ordinate and a distance on the abscissa. The profile of the Figure 7 is a simple but wide Gaussian and the profile of the figure 8is a double Gaussian composed of two narrow Gaussians. These two signature possibilities are characteristic of glass fiber and are the only two cases that can be encountered. Indeed, since glass fiber is a set of twisted glass and carbon strands, it appears in X-ray tomography as two white lines when the glass strands are distributed on either side of the neutral fiber, or as a single but thick white line when the strands cross at the neutral fiber.

[0041] Then we process (step E34) each of these profiles to locate the neutral fiber. Indeed, by referring to image I of the Figure 5 , we understand that the gray level of each column makes it possible to obtain at what height along the Z axis the neutral fiber is located.

[0042] In particular, the processing consists, in the case of a profile of the type illustrated on the Figure 7, to detect the center of the Gaussian and in the case of a profile of that illustrated on the figure 8 , to detect the center of the two Gaussians. The detected values are illustrated by the thick dotted curve.

[0043] Therefore, at the end of the processing step (step E34), we obtain along the X axis the height at which the neutral fiber 22 is located.

[0044] Additionally, all the heights thus obtained can be filtered (step E35) to smooth the different values and thus filter out the outliers. Advantageously, regression filtering can be implemented. Figure 9 illustrates a cumulative 2D image of the neutral fiber after filtering. In comparison with image I of the Figure 5 we note that the curve representing the neutral fiber is less noisy.

[0045] Filtered or not, image I ( Figure 5 ) or the image If ( figure 9) are each a 2D view of the neutral fiber of better quality than that obtained with the methods of the prior art (see the Figure 2a ).

[0046] Finally, a comparison with nominal dimensions and predefined tolerances (step E36) of the location of the neutral fiber thus obtained with a theoretical location of the neutral fiber is implemented to enable the integrity of blade 1 to be assessed.

Claims

1. A method for checking the positioning of a fibrous preform in a blade, the blade having been obtained by injecting a resin into a mould having a blade shape and into which a preform has been placed, the blade extending in an orthonormal blade reference frame X, Y, Z, the blade comprising a blade root extending longitudinally along an X axis an airfoil extending from the blade root along a Z axis, the blade having a thickness defined along a Y axis, the preform comprising glass tracers positioned on the surface of the preform, the centre of the tracers defining a neutral fibre located at a height along the Z axis in the direction defined by the X axis, the method comprising the following steps: acquiring (E31) tomographic 2D projections of the blade by means of an imaging system comprising an X-ray source, the method being characterized in that, during the acquisition, each projection is acquired according to a given orientation of the X-ray source with respect to the blade; accumulating (E32, E32a, E32b) the 2D projections in the direction of the Y axis so as to obtain an accumulated 2D image in the x and z directions; the method being further characterised by comprising the following steps: determining (E33) a greyscale profile for each pixel column defined in the direction of the z axis; processing (E34) of each of the profiles obtained so as to locate the Z position of the neutral fibre in the direction of the X axis.

2. A method according to claim 1, wherein the accumulating (E32a) consists in obtaining a cumulated 2D image consisting of pixels having the maximum grey level from a stack of pixels of the different slices stacked in the direction of the Y axis.

3. Method according to claim 1, in which the projection (step E32b) consists in the fact that the cumulated 2D image is made up of pixels whose grey level is the sum of the pixels of the different slices stacked in the direction of the Y axis.

4. Method according to one of the preceding claims, in which the grey level profile (G1) is a Gaussian, the location of the neutral fibre being the centre (X1) of the Gaussian.

5. Method according to one of claims 1 to 4, in which the grey level profile (G2) is a double Gaussian, the location of the neutral fibre being the centre (X2) of the two Gaussians.

6. Method according to one of the preceding claims, comprising a step (E35) of filtering the location in the Z direction of the neutral fibre in the direction of the X axis.

7. Method according to one of the preceding claims, comprising a step (E36) of comparing (E35) the location of the neutral fibre thus obtained with a theoretical location of the neutral fibre.

8. A method of manufacturing a composite turbomachine blade, the method comprising the following steps: placing a fibrous preform in a balde-shaped mould; vacuum injection of a resin into the mould comprising the preform so as to obtain the injected blade; checking the positioning of the fibre preform in the injected vane by means of a process according to one of the preceding claims.