FR3067114A1 - METHOD AND DEVICE FOR DETECTING DEFECTIVE SITES IN FIBER PATCHES - Google Patents

Abstract

The invention relates to a method and a device for detecting defective sites (5) in a multi-directional fiber web (4), made of a fiber-based material of a fiber-reinforced composite material for the manufacture of a part. composite reinforced with fibers. The pluridirectional fiber web (4) has at least two layers of fibers in which the direction of electrically conductive reinforcing fibers is different at a certain angle from one layer to the other. The first layer of fibers (1) is contacted by two electrodes, and an electrical resistance is then measured using the two electrodes in contact. Based on the measured resistance, it is then possible to detect a defective site in a second layer of the multi-directional fiber web.

Description

® METHOD AND DEVICE FOR DETECTING FAULTY SITES IN FIBER WEBS.

FR 3,067,114 - A1 (© The invention relates to a method and a device for detecting defective sites (5) in a multidirectional fiber sheet (4), made of a material based on fibers of a composite material reinforced with fibers for the manufacture of a fiber-reinforced composite part. The multidirectional fiber web (4) has at least two layers of fibers in which the direction of electrically conductive reinforcing fibers is different from a certain angle of a layer by The first layer of fibers (1) is contacted by two electrodes, and an electrical resistance is then measured using the two electrodes in contact. Based on the resistance measured, it is then possible to detecting a defective site in a second layer of the multidirectional fiber web.

i

Method and device for detecting defective sites in sheets of fibers

The invention relates to a method and corresponding device for detecting defective sites in a multidirectional fiber web, which is made of a fiber-based material of a fiber-reinforced composite material for the manufacture of a fiber-reinforced composite part. fibers, the ply of multidirectional fibers having at least two layers of fibers in which the direction of electrically conductive reinforcing fibers is different by a certain angle, from one layer relative to the other.

Parts made of a fiber-reinforced composite material, known as fiber-reinforced composite parts, are today essential in the aeronautics and space sector. However, the use of such materials is also finding an increasingly favorable response in the automotive field. Fiber-reinforced plastics are produced, in particular critical structural elements with a minimum weight, thanks to the high specific strength and rigidity, per unit of weight. Thanks to the anisotropic properties of fiber-reinforced composite materials, resulting from the orientation of the reinforcing fibers, the parts can be precisely adapted to local loads or stresses, thus allowing optimal exploitation of the material in terms of 'a lighter construction method.

Fiber-reinforced composite materials generally comprise two essential components, namely on the one hand the fiber-based material with the reinforcing fibers, and on the other hand the matrix material. During the manufacturing process, the matrix material having been infused into the fiber material, is hardened by being subjected to pressure and temperature stress, so that the matrix material and the fiber material then form an integral unit, and the reinforcing fibers present in the fiber-based material are thus fixed according to their predetermined ordering.

Since the load-bearing properties of fiber-reinforced composite parts made of such fiber-reinforced composite materials are generally conditioned by the reinforcing fibers present in the fiber material, such fiber-reinforced composite parts fibers have, in directions outside the plane of the fibers as well as in directions outside the direction of the reinforcing fibers, reduced load resistance properties which must be compensated in another way depending on the applications. For this purpose, it is not uncommon to use or manufacture plies of fibers, most often formed of several layers of fiber-based material, the layers of individual fibers being arranged so that the direction of the fibers of reinforcing a layer of fibers differs from the direction of the reinforcing fibers of other layers of fibers. This allows the fiber-reinforced composite parts to exhibit not only a unidirectional load resistance property, but bi- or multidirectional load resistance properties, since the load-resistant reinforcing fibers have different directions in the respective fiber layers.

Thus, it is not uncommon in practice for such layers of multi or multidirectional fibers to be of such a construction that a layer of fibers has an orientation of 0 ° (nominal fiber direction), while the others fiber layers, in particular directly adjacent to this first layer of fibers, have a fiber orientation of +/- 45 ° relative to the nominal fiber direction, or a fiber direction of + / 90 ° relative to the fiber direction nominal (nominal relative to the reinforcing fibers or main fibers).

Particularly in the case of automated laying of fibers for the manufacture of multi-directional fiber webs, downstream quality control is an important element in the overall manufacturing process, since defects during the laying of fibers, such as free gaps between two strips of fibers or deviations from the orientation of the fibers, can lead to undesirable properties of the material of the subsequent part, to the point that this part can for example become unusable. Particularly in the case of large surface areas, of large size, such as, for example, wing shells, the scrapping of such a part due to defective laying of the fibers constitutes one of the factors of increased costs, so it is desirable to detect these scrap parts very early, or completely avoid them.

Materials based on laid fibers may be subject to visual inspection. However, if several layers of fiber-based material have been laid, a visual inspection generally does not make it possible to check the layers of fibers located below, which fixes the physical limits of this type of quality assurance.

From this background, the aim of the present invention is to indicate an improved device and an improved method making it possible to detect non-destructively, defective sites in sheets of multidirectional fibers, the defective sites to be detected being precisely those which are not visible and are embedded inside the layer of fibers, in layers of intermediate fibers.

The desired goal is achieved by a method for detecting defective sites in a multidirectional fiber web, made of a fiber-based material of a fiber-reinforced composite material for the manufacture of a fiber-reinforced composite part, the ply of multidirectional fibers having at least two layers of fibers in which the direction of electrically conductive reinforcing fibers is different by a certain angle a from one layer relative to the other, characterized in that a first layer of fibers is electrically contacted using a first electrode and at least a second electrode spaced from the first electrode, so that between the first and the second of the contacted electrodes is formed a measurement path in the fiber web multidirectional, so that the measurement path formed between the first and second electrodes extends at an angle a by comparison rt to the electrically conductive reinforcing fibers of the first layer of fibers, and parallel to a supposed direction of the electrically conducting reinforcing fibers of a second layer of fibers, and in that by means of an electrical resistance measuring device , an electrical resistance of the measurement path in the multidirectional fiber layer is measured using the first and second electrodes, and a defective site is detected in the second fiber layer by a data processing unit as a function of the resistance measured electrical resistance, when the measured electrical resistance exceeds a predetermined threshold value.

In accordance with the invention, a method is proposed for detecting defective sites in a multidirectional fiber web made of a material based on fibers of a fiber reinforced composite material for the manufacture of a fiber reinforced composite part. The sheet of fibers in this case has at least two layers of fibers, each having main or reinforcing fibers whose direction differs from one layer of fibers to another. The direction of the reinforcing fibers of a layer of fibers is here most often substantially parallel and oriented in the same direction. The fiber layers of the fiber layer are however superimposed so that a layer of fibers has a first direction of the electrically conductive reinforcing fibers, and said at least one other second layer of fibers has a second direction of the electrically conductive reinforcing fibers, said first direction forming a predetermined angle with said second direction. Typical angles between these directions are 45 ° or 90 °.

Fiber-based materials comprising electrically conductive reinforcing fibers can here in particular be composite materials reinforced with carbon fibers.

According to the invention, a first layer of fibers of the multidirectional fiber ply is contacted by two electrodes mutually spaced from each other, so that a measurement path is formed in the multidirectional fiber ply between the first electrode in contact and the second electrode in contact. The first layer of fibers contacted by the two electrodes is here generally one of at most two layers of external fibers of the present multidirectional fiber ply. It is obviously also possible to bring into contact with the first layer of fibers (exterior), more than two mutually spaced electrodes, and thus form more than one measurement path in the multidirectional fiber ply. In the following, however, we will continue to refer to two electrodes, the principle being able to be extended without problem to more than two electrodes.

The two electrodes are brought into contact with the first layer of fibers so that the measurement path formed between the first and the second electrode extends at a certain angle relative to the electrically conductive reinforcing fibers of the first layer of fibers, the measurement path being simultaneously oriented parallel to a supposed direction of the electrically conductive reinforcing fibers of a second layer of fibers, lying under the first layer of fibers being generally covered by the first layer of fibers, and therefore not visible.

The supposed direction of the reinforcing fibers of the second layer of fibers generally results from the assumption that the second layer of fibers has been correctly oriented with regard to the orientation of its fibers. The supposed direction of the electrically conductive reinforcing fibers of the second fiber layer can thus also be considered to be a set direction of the reinforcing fibers of the second fiber layer.

By means of an electrical resistance measuring device, an electrical resistance of the measurement path in the multidirectional fiber sheet is then measured, using the first and second electrodes, which contact the first layer of fibers. the fiber sheet. A defective site is detected by a data processing unit in the second layer of fibers not contacted by the electrodes, with regard to the measured electrical resistance, when the measured electrical resistance exceeds a predetermined threshold value.

It has been found that the electrical resistance in such a multidirectional fiber layer is proportional to the defective sites of the layer or layers of fibers located inside, so that a corresponding resistance measurement by means of electrodes placed contact with the outer fiber layers would help identify internal defective sites.

It is thus possible for the first time to detect defective sites in sheets of multidirectional fibers by non-destructive means, with in principle the possibility of dispensing with a subsequent manual control, since the process can be automated using a device adequate. It is thus conceivable to start by manufacturing the sheets of fibers and then not subject them to a corresponding detection of defective sites until after their manufacture.

According to an advantageous embodiment, a gap between the electrically conductive reinforcing fibers of the second layer of fibers is detected as a defective site by the data processing unit as a function of the electrical resistance measured, when the resistance electrical measurement exceeds a predetermined threshold value for this defective site. Such a gap occurs, for example, when the tolerances between neighboring strips of fibers may not have been respected during the laying of the second layer of fibers. However, such defective sites also appear when the fiber-based material has manufacturing defects, and that for example, at a given location, the corresponding reinforcing fibers are missing. A gap as a defective site is here characterized by missing reinforcing fibers.

According to another advantageous embodiment, a deviation from the supposed direction of the electrically conductive reinforcing fibers (set direction) of the second layer of fibers is detected as a defective site by the data processing unit. function of the measured electrical resistance, when the measured electrical resistance exceeds a predetermined threshold value for this defective site.

The greater the deviation from the supposed direction of the electrically conductive reinforcing fibers of the second layer of fibers, and the greater the electrical resistance which can be determined using the resistance measuring device, so that In addition to detecting the existence of such a deviation, it is also possible to quantify the deviation from the set direction of the electrically conductive reinforcing fibers of the second layer of fibers.

According to another advantageous embodiment, this layer is spaced apart, by a first electrode produced in the form of a roller or roller and by a second electrode produced in the form of a roller or roller, the electrodes produced in the form of roller or roller being moved on the first layer of fibers, in order to thus establish different measurement positions where the electrical resistance of the measurement path in the multidirectional fiber layer is measured. Thus, for example, two electrodes produced in the form of a roller or a roller make it possible to measure a complete construction part, which has been manufactured on a shaping tool, for example in the form of a multidirectional fiber sheet made up of several layers. fiber.

It is particularly advantageous here to move the electrodes produced in the form of a roll or roller on the first layer of fibers during the resistance measurement, so that the corresponding resistance measurement is carried out continuously during the displacement of the electrodes produced. in the form of a roller or roller.

According to another advantageous embodiment, the ply of multidirectional fibers has at least three layers of fibers, so that at least one other, third layer of fibers is provided which, in common with the first layer of fibers, encloses the second layer of fibers. This means that the second layer of fibers is sandwiched between the two layers of fibers (first layer of fibers and third layer of fibers), and is contiguous, by one of its sides, with the first layer of fibers, and by a second side , to the third layer of fibers.

The third layer of fibers here also comprises electrically conductive reinforcing fibers, having a direction which differs from that of the reinforcing fibers of the second fiber layer, the electrically conductive reinforcing fibers differing by a certain angle between the second layer of fibers. fibers and the third layer of fibers.

According to the invention, the third layer of fibers is, exactly like the first layer of fibers, contacted by at least two electrodes, so that the third layer of fibers is contacted by at least a third electrode and a fourth electrode and qu 'As a result, a measurement path is formed in the sheet of fibers, between the third and the fourth electrode, which are in contact with the third layer of fibers. The third and fourth electrodes are here brought into contact with the third layer of fibers so that the measurement path formed between the third and fourth electrodes extends at a certain angle relative to the reinforcing fibers of the third layer of fibers. , and, in addition, parallel to a supposed direction of the electrically conductive reinforcing fibers of the second layer of fibers, so that preferably the measurement path formed by the first and the second electrode is substantially identical (to contact tolerances close) to the measurement path formed by the third and fourth electrodes.

The electrical resistance measuring device then makes it possible to determine the electrical resistance in the measurement path formed by the third and the fourth electrode, a defective site being detected in the second layer of fibers by the data processing unit in function of the electrical resistance measured. In this case, it is possible to start by measuring the electrical resistance of the measurement path formed by the first and the second electrode, then measuring the resistance of the measurement path formed by the third and the fourth electrode, a defective site then being detected by the data processing unit as a function of the measured resistance of the first measurement path (first and second electrode), and of the electrical resistance of the second measurement path (third and fourth electrode). This makes it possible to increase the precision of detection of faults and in particular a finer detection and with a higher resolution, of deviations from the supposed direction (direction of reference) of the reinforcing fibers.

According to another advantageous embodiment for this purpose, the electrical resistance of the measurement path formed by the first and second, as well as third and fourth electrodes, is determined using the electrical resistance measuring device, by a measurement according to a four-wire method, which makes it possible to detect very fine differences between the electrical resistances.

Advantageously, the first layer of fibers is contacted by first and second electrodes produced in the form of a roller or roller and the third layer of fibers is contacted by third and fourth electrodes produced in the form of roller or roller, the electrodes produced in the form of a roller or roller being moved over the respective fiber layer, in order to establish different measurement positions where the electrical resistance of the measurement path in the multidirectional fiber layer is measured.

Advantageously, the first electrode produced in the form of a roller or roller contacts the first layer of fibers and the third electrode produced in the form of a roller or roller contacts the third layer of fibers so that the first electrode is located opposite the third electrode, and / or the second electrode produced in the form of a roller or roller contacts the first layer of fibers and the fourth electrode produced in the form of a roller or roller contacts the third layer of fibers so that the second electrode is located next to the fourth electrode.

In this case, it is also conceivable that the third and fourth electrodes are produced in the form of a roller or roller and thus contact the third layer of fibers, so that, in a manner similar to the first example of embodiment with regard to the first and the second electrode in the form of a roller or roller, the third and the fourth electrodes can also be displaced globally relative to the third layer of fibers and therefore relative to the layer of fibers, in order to establish different positions measurement where the electrical resistance of the measurement path in the multidirectional fiber web must be measured.

It is thus possible to contact a sheet of fibers formed from at least three layers of fibers, respectively on its outer layers (first layer of fibers and third layer of fibers), so that the first and the second electrode on the first layer of fibers and the third and fourth electrode on the third layer of fibers form a common measurement path in the fiber web, for measuring, the electrical resistance in the common measurement path using the measuring device of resistance. This is particularly advantageous when the electrodes produced in the form of rollers or rollers are in a fixed position and the sheet of fibers formed of several layers of fibers is guided and transported through the arrangement of rollers or rollers, which is used for example during the routing of plies of multidirectional fibers until the laying of the fibers. It is thus possible to continuously determine the electrical resistance of the common measurement path, and to detect corresponding defective sites in the internal fiber layer.

The object sought by the invention is also achieved with the aid of a device configured to detect defective sites in a multidirectional fiber sheet made of a fiber-based material of a fiber-reinforced composite material for the manufacture of a fiber-reinforced composite part, the multidirectional fiber web having at least two layers of fibers in which the direction of electrically conductive reinforcing fibers is different by a certain angle a from one layer relative to the other , characterized in that the device has at least two mutually spaced electrodes configured to come into contact with the first layer of fibers of the multidirectional fiber layer, so that a measurement path is formed in the multidirectional fiber layer, between the first electrode and the second electrode, in that there is provided an electric resistance meter e configured to measure an electrical resistance of the measurement path in the multidirectional fiber web by means of the first and second electrodes in contact, and in that an electronic data processing unit is provided, designed to detect a defective site in the second layer of fibers as a function of the resistance measured.

The device has for this purpose at least two electrodes spaced from each other, provided and configured to come into contact with the multidirectional fiber sheet made up of several layers of fibers, on the same common side (first layer of fibers). The device includes an electrical resistance measuring device configured to determine, inside the fiber web, the electrical resistance in the measurement path formed by the first electrode and the second electrode, which are in contact. A data processing unit is provided, capable of detecting a defective site on the basis of the electrical resistance measured. A defective site is here detected using the data processing unit, when the measured electrical resistance exceeds a predetermined and adjustable threshold value.

Advantageously, the data processing unit is configured to detect as a defective site a gap between the electrically conductive reinforcing fibers of the second layer of fibers, as a function of the electrical resistance measured by the processing processing unit. data, and / or to detect as a defective site a deviation from the supposed direction of the electrically conductive reinforcing fibers of the second layer of fibers, as a function of the electrical resistance measured by the data processing unit.

It is also conceivable here that the multidirectional fiber sheet has a third layer of fibers which, in common with the first layer of fibers, encloses the second layer of fibers, and in which the direction of the electrically conductive reinforcing fibers is different from at a certain angle a with respect to the electrically conductive reinforcing fibers of the second layer of fibers, that the device has a third electrode and at least a fourth electrode configured to contact the third layer of fibers of the multidirectional fiber ply, so that a measurement path is formed in the multidirectional fiber sheet between the third electrode and the fourth electrode, and that the electrical resistance measuring device is configured to measure an electrical resistance of the measurement path in the multidirectional fiber sheet at using the third and fourth cone electrodes tact, and that the electronic data processing unit is designed to detect a defective site in the second layer of fibers as a function of the resistance measured.

Advantageously, the electrical resistance measuring device is configured to measure the electrical resistance of the measurement path by means of the first, second, third and fourth electrodes, by a measurement according to a four-wire method.

It is also possible here to envisage that one, several or all of the electrodes are produced in the form of rollers or rollers, and designed to be displaced on the respective fiber layer contacted, in order to establish different measurement positions. where the electrical resistance of the measurement path in the multidirectional fiber sheet is measured.

It is thus possible to produce a device of the type of that mentioned above, which is characterized by being designed to implement the method explained above.

The invention will be explained in more detail, by way of example, with regard to the appended figures. These show:

Figure 1 a schematic representation of the measurement principle for the detection of gaps;

Figure 2

Figure 3

Figure 4 a schematic representation of the measurement principle for the detection of fiber orientation faults;

a schematic representation of the measuring device according to the invention;

a schematic representation of a roller produced as an electrode.

FIG. 1 schematically shows the measuring principle of the present invention, for detecting defective sites inside layers of fibers. In the exemplary embodiment of FIG. 1, a first layer of fibers 1, a second layer of fibers 2 and a third layer of fibers 3 are shown, which together form the sheet of fibers 4. The second layer of fibers is in this case arranged between the first layer of fibers and the third layer of fibers, and is thus sandwiched between the two layers of outer fibers 1 and 3.

FIG. 1 shows on this subject the reinforcing fibers of the individual layers of fibers 1 to 3. The direction of the reinforcing fibers of the first layer of fibers has an angle of + 45 °, while the reinforcing fibers of the third layer of fibers 3 have an angle of -45 °. Together, the reinforcing fibers of the first layer of fibers 1 and the reinforcing fibers of the third layer of fibers 3 form an angle of 90 ° between them. The indications in degrees are here defined relative to the nominal direction of the fibers, which is defined by the reinforcing fibers of the second layer of fibers 2 and is 0 °. The reinforcing fibers of the second layer of fibers 2 thus have an angle of 0 ° with respect to the nominal direction of the fibers, so that between the reinforcing fibers of the second layer of fibers 2 and the reinforcing fibers of the first layer of fibers 1, an angle of + 45 ° is formed, while between the reinforcing fibers of the second layer of fibers 2 and the reinforcing fibers of the third layer of fibers 3, an angle of - is defined 45 °.

As shown in the embodiment of FIG. 1, the zone 5 of the second layer of fibers 2 does not include reinforcing fibers, so that the second layer of fibers 2 has a defective site in said zone 5, at know a gap or gap. In the following, it must now be detected using the invention described below.

According to the invention, a first electrode 10 and a second electrode 11 then come into contact with the outer fiber layer 1, so that between the first electrode 10 and the second electrode 11, it is formed in the fiber sheet 4, a measurement path 12 on which the electrical resistance is determined, by a first measurement RI. During the first electrical resistance measurement RI, the first electrode 10 and the second electrode 11 are arranged on the first layer of fibers 1 in an area where the second layer of fibers 2 has corresponding reinforcing fibers. During the resistance measurement, a current flow begins by passing through the first layer of fibers 1 and then passes along the electrically conductive reinforcing fibers of the second layer of fibers, up to the second electrode 11, so that the we can expect a very low electrical resistance here. Only the contact resistances between the electrode and the first layer of fibers 1, as well as the transition resistances from the first layer of fibers to the second layer of fibers, significantly influence the electrical resistance measured.

It is quite different when the first electrode 10 and the second electrode 11 are brought into the zone of the defective site 5, so that the measurement path 12 now extends inside the zone 5 where the second layer of fibers 2 does not contain any reinforcing fibers. During such a second measurement R2, an electrical resistance is then markedly greater than in the case of the first resistance measurement RI, because a current flow produced by the electrodes 10 and 11 for the resistance measurement then no longer extends exactly along the measurement path 12, but, if necessary, passes through the first layer of fibers 1, or else turns into areas of the second layer of fibers 2 where the corresponding reinforcing fibers are presented.

Thus, as shown diagrammatically in FIG. 1, it can be based on a resistance measurement to detect whether the layer of fibers 2 situated inside the layer of fibers 4 has a defective site in the form of missing reinforcing fibers. , because the resistance of the defective sites inside the second layer of fibers is always proportional to these defective sites, provided that the measurement path 12 is oriented in accordance with the assumed nominal orientation of the reinforcing fibers of the second fiber layer 2.

FIG. 2 schematically shows the second measurement principle by which a deviation from the supposed orientation of the reinforcing fibers of the second layer of fibers is detected as defective site 6

2. Here also the first electrode 10 and the second electrode 11 come into contact with the first layer of fibers 1 of the sheet of fibers 4, the measurement path 12 thus formed having an orientation corresponding to the supposed orientation (direction of reference) of the reinforcing fibers of the second fiber layer 2. In reality, the set direction 13 deviates from the effective direction 14 (real direction) by an angle a, which must be detected as a defective site 6 (defect) .

Regarding the resistance measurement R3, it can now be seen that the measured electrical resistance of the measurement path 12 is higher than under ideal, normal conditions, so that it can be concluded that there is a difference in orientation as a defective site.

Unlike the defective site 5 where there is a lack of reinforcing fibers, it can be seen here that the electrical resistance measured is certainly higher than the normal ideal resistance, but all the same significantly lower than in the case of the absence of reinforcing fibers . As a result, we have the following relationship:

RI <R3 <R2

With the help of a threshold value, it is thus possible to deduce from the electrical resistances measured the two types of fault 5 and 6, so that it is not only possible to detect the presence or not of a defective site, but also qualify the type of it.

FIG. 3 schematically shows an exemplary embodiment of a device intended for detecting faults in accordance with the measurement principle of FIG. 1 and of FIG. 2. The device 100 comprises rollers 101, 102, 103 and 104, on the periphery of which electrodes 111, 112, 113, 114. are arranged. The rollers 101 to 104 and the 10 electrodes 111 to 114 which are arranged therein are connected, in terms of signal transmission technique, to a resistance measuring device. electrical 120, which determines the electrical resistance in the measurement path 130 formed by the electrodes 111 to 114. The measurement path 130 is here inside the fiber web 140, consisting of three layers of fibers 141, 142 and 143. The fiber layers 141 and 143 are in this case the outer fiber layers, which will be contacted by the corresponding electrode respectively, while the fiber layer 142 is clamped e by the two outer fiber layers 141 and 143, and is located inside.

Furthermore, the device comprises a data processing unit 150 which then detects or not a defective site with regard to the electrical resistance measured in the measurement path 130.

According to the device 100 of FIG. 3, the sheet of fibers 140 can be transported in the direction characterized by the arrow in the figure, thus allowing, during transport, the continuous measurement of the resistance in the measurement path 130, which varies, and continuous detection of corresponding defective sites.

The electrodes 111 to 114 are here arranged on the outside of the roller

101 to 104 respectively, so as to be mutually offset in the axial direction, thus allowing measurement over the entire width of the fiber web 140.

Such a roller is shown by way of example in FIG. 4. The roller 101 here has different electrodes 111a to 11h, the position of each individual electrode being, from a radial point of view, namely with respect to the periphery, respectively different. Thus, the electrode 111a is offset radially relative to the electrode 111b arranged next to it, so that the electrode 111a contacts the respective layer of fibers, while the electrode 111b does not contact this layer of fibers. At the axial position of the electrode 111a, it is then possible to carry out a corresponding measurement in the sheet of fibers. If the roller 101 continues to rotate, it is the next electrode 111b which comes into contact with the sheet of fibers, while the electrode 111a ceases to contact the corresponding layer of fibers, thereby allowing a corresponding measurement to be made at the level of the axial position of the electrode 111b.

It is thus possible, by rotation of the roller 101, to continuously measure the entire width of the sheet of fibers. The individual electrodes 111a to 11h are isolated from each other, so that the measurement is not influenced by any other electrodes.

It has thus been found that in the case of a measurement path with a length of approximately 6 cm, and a layer system consisting of three layers of fibers, the two outer layers having an orientation +/- 45 ° and the inner layer having a normal nominal orientation of 0 ° and extending in the direction of measurement, it was possible to measure an electrical resistance of approximately 1 Ohm in the absence of a defective site. If, on the other hand, the fiber sheet has a gap in the intermediate fiber layer, namely the absence of reinforcing fibers at this point, the resistance increases in this case to around 120 Ohm. In the case of a deviation from the normal nominal fiber orientation, a resistance between 120 Ohm and 1 Ohm is measured. In the case of a deviation of an angle of approximately 5 °, the resistance increases even in the areas where the reinforcing fibers are present, up to a value of approximately 20 Ohm.

Claims (15)

claims 1. Method for detecting defective sites (5, 6) in a multidirectional fiber web (4, 140), made of a fiber-based material of a fiber-reinforced composite material for the manufacture of a reinforced composite part of fibers, the multi-directional fiber web (4, 140) having at least two layers of fibers (141, 142, 143) in which the direction of electrically conductive reinforcing fibers is different by a certain angle a from a layer by relative to the other, characterized in that a first layer of fibers (1) is electrically contacted using a first electrode and at least a second electrode spaced from the first electrode, so that between the first and second of the contacted electrodes (111, 112, 113, 114) a measurement path (12, 130) is formed in the multidirectional fiber sheet (4, 140), so that the measurement path (12, 130) formed between the first and uth electrodes (111, 112, 113, 114) extend at an angle a with respect to the electrically conductive reinforcing fibers of the first layer of fibers, and parallel to a supposed direction of the electrically conductive reinforcing fibers of a second layer of fibers, and in that by means of an electrical resistance measuring device, an electrical resistance of the measurement path (12, 130) in the multidirectional fiber sheet (4, 140) is measured using the first and second electrodes (111, 112, 113, 114), and a defective site (5,6) is detected in the second layer of fibers by a data processing unit (150) according to the electrical resistance measured, when the measured electrical resistance exceeds a predetermined threshold value. 2. Method according to claim 1, characterized in that a gap between the electrically conductive reinforcing fibers of the second layer of fibers is detected by the data processing unit (150), as a defective site (5 , 6), as a function of the measured electrical resistance, when the measured electrical resistance exceeds a predetermined threshold value for this defective site (5, 6). 3. Method according to claim 1 or 2, characterized in that a deviation from the supposed direction of the electrically conductive reinforcing fibers of the second layer of fibers is detected as a defective site (5, 6) by l data processing unit (150) as a function of the measured electrical resistance, when the measured electrical resistance exceeds a predetermined threshold value for this defective site (5, 6). 4. Method according to one of the preceding claims, characterized in that the first layer of fibers (1) is contacted by a first electrode produced in the form of a roller or roller and by a second electrode produced in the form of roller or roller , the electrodes (111, 112, 13, 114) produced in the form of a roller or roller being displaced on the first layer of fibers (1), in order to establish different measurement positions where the electrical resistance of the path of measurement is measured measure (12, 130) in the multidirectional fiber web (4, 140). 5. Method according to claim 4, characterized in that the electrodes (111, 112, 13, 114) produced in the form of a roll or a roller are displaced on the first layer of fibers (1) during the resistance measurement (R3) . 6. Method according to one of the preceding claims, characterized in that the multidirectional fiber sheet (4, 140) has a third layer of fibers (3) which, in common with the first layer of fibers, encloses the second layer of fibers (2), and in which the direction of the electrically conductive reinforcing fibers is different by a certain angle a with respect to the electrically conductive reinforcing fibers of the second layer of fibers, in that the third layer of fibers (3) is electrically contacted by a third electrode and at least a fourth electrode spaced from the third electrode, so that a measurement path (12, 130) is formed in the multidirectional fiber web (4, 140) between the third and fourth contacted electrodes (111, 112, 113, 114), in that the measurement path (12, 130) formed between the third and fourth electrodes (111, 112, 113, 114) extends at an angle a pa r with respect to the electrically conductive reinforcing fibers of the third layer of fibers, and parallel to a supposed direction of the electrically conducting reinforcing fibers of the second layer of fibers, and in that by means of the electrical resistance measuring device , an electrical resistance of the measurement path (12, 130) in the multidirectional fiber sheet (4, 140) is measured using the third and fourth electrodes (111, 112, 113, 114), and a defective site ( 5,6) is detected in the second layer of fibers by the data processing unit (150) according to the measured electrical resistance, when the measured electrical resistance exceeds a predetermined threshold value. 7. Method according to claim 6, characterized in that the electrical resistance of the measurement path (12, 130) is determined by means of the first, second, third and fourth electrodes, using the resistance measuring device electric, by measurement using a four-wire method. 8. Method according to claim 6 or 7, characterized in that the first layer of fibers (1) is contacted by first and second electrodes (111, 112, 13, 114) produced in the form of a roller or roller and the third fiber layer (3) is contacted by third and fourth electrodes (111, 112, 113, 114) made in the form of a roller or roller (101, 102, 103, 104), the electrodes (111, 112, 113, 114) produced in the form of a roller or roller being moved over the respective fiber layer, in order to establish different measurement positions where the electrical resistance of the measurement path (12, 130) is measured in the multidirectional fiber sheet ( 4, 140). 9. Method according to claim 8, characterized in that the first electrode (10) produced in the form of a roller or roller contacts the first layer of fibers (1) and the third electrode produced in the form of roller or roller contacts the third layer of fibers (3) so that the first electrode is located opposite the third electrode, and / or in that the second electrode (11) produced in the form of a roll or roller contacts the first layer of fibers (1) and the fourth electrode made in the form of a roller or roller contacts the third layer of fibers (3) so that the second electrode (11) is located opposite the fourth electrode. 10. Device (100) for detecting defective sites (5, 6) in a multidirectional fiber web (4, 140) made of a fiber-based material of a fiber-reinforced composite material for manufacturing a part fiber-reinforced composite, the multidirectional fiber web (4, 140) having at least two layers of fibers (141, 142, 143) in which the direction of electrically conductive reinforcing fibers is different by a certain angle a from a layer with respect to the other, characterized in that the device (100) has at least two mutually spaced electrodes (111, 112, 113, 114) configured to come into contact with the first layer of fibers of the multidirectional fiber layer (4 , 140), so that a measurement path (12, 130) is formed in the multidirectional fiber web (4, 140), between the first electrode and the second electrode, in that a device is provided. resistance measurement electrical configured to measure an electrical resistance of the measurement path (12, 130) in the multidirectional fiber web (4, 140) by means of the first and second electrodes (111, 112, 113, 114) in contact, and in that 'There is provided an electronic data processing unit (150) designed to detect a defective site (5, 6) in the second layer of fibers according to the resistance measured. 11. Device (100) according to claim 10, characterized in that the data processing unit (150) is configured to detect as a defective site (5, 6) a gap between the electrically conductive reinforcing fibers of the second layer of fibers, as a function of the electrical resistance measured by the data processing unit (150), and / or to detect as a defective site (5, 6) a deviation from the estimated direction of the fibers of electrically conductive reinforcement of the second layer of fibers, as a function of the electrical resistance measured by the data processing unit (150). 12. Device (100) according to claim 10 or 11, characterized in that the multidirectional fiber sheet (4, 140) has a third layer of fibers (3) which, in common with the first layer of fibers, encloses the second fiber layer (2), and in which the direction of the electrically conductive reinforcing fibers is different by a certain angle a from the electrically conductive reinforcing fibers of the second fiber layer, in that the device (100) has a third electrode and at least a fourth electrode, configured to contact the third fiber layer of the multidirectional fiber web (4, 140), so that a measurement path (12, 130) is formed in the fiber web multidirectional (4, 140) between the third electrode and the fourth electrode, and in that the electrical resistance measuring device is configured to measure an electrical resistance of the measurement path re (12, 130) in the multidirectional fiber web (4, 140) by means of the third and fourth electrodes (III, 112, 113, 114) in contact, and the electronic data processing unit (150) is designed for detecting a defective site (5, 6) in the second layer of fibers as a function of the resistance measured. 13. Device (100) according to claim 12, characterized in that the electrical resistance measuring device is configured to measure the electrical resistance of the measurement path (12, 130) by means of the first, second, third and fourth electrodes , by measurement using a four-wire method. 14. Device (100) according to one of claims 10 to 13, characterized in that one, several or all of the electrodes (III, 112, 113, 114) are produced in the form of rollers or rollers (101, 102, 103, 104), and designed to be displaced on the respective fiber layer contacted, to establish different positions for measuring the electrical resistance of the measurement path (12, 130) in the multidirectional fiber layer (4, 140) . 15. Device (100) according to one of claims 10 to 14, characterized in that the device (100) is designed to implement the method according to one of claims 1 to 9.