DE102016208599A1 - Non-destructive method for determining the fiber orientation of carbon fiber reinforced materials - Google Patents

Abstract

A nondestructive method for determining the fiber orientation of carbon fiber reinforced materials is given. In the method, a specimen having a carbon fiber reinforced material and an eddy current test apparatus are provided. A surface area of the specimen to be scanned is defined. Subsequently, at least four surface scans of the surface area are carried out with mutually different sensor positions, so that impedance values are obtained for each area scan at a multiplicity of measuring points. Furthermore, the determined impedance values are recorded. Thereafter, an arbitrarily large partial area of the area can be selected. The individual impedance values of the measurement points located in the subarea area are averaged for each of the four area scans, and a graph is generated based on the impedance averages.

Description

A method is provided for determining the fiber orientation of carbon fiber reinforced materials. The carbon fiber reinforced materials may be, for example, so-called sheet molding compounds (SMC).

Methods for non-destructive testing of composite structures and fiber reinforced plastics are known in the art.

The publication DE 38 27 229 A1 describes, for example, a method for non-destructive testing of semi-finished products and components made of fiber-reinforced plastics by means of eddy current probes.

In the publication DE 102 34 551 B3 For example, a method is described for detecting the oxidation of carbonaceous fibers or fiber bundles in composites using the eddy current method.

Furthermore, it is known in the art that locally at selected locations of a sample body polar diagrams can be created by a sensor rotation. On the basis of these polar diagrams the fiber orientation can be recognized. The direction of highest conductivity is usually represented in the polar diagram as the maximum deflection. The sensors are usually direction-dependent differential probes.

However, by the local generation of polar diagrams, it takes a very long time to cover a larger area, e.g. a whole plate, to analyze the fiber orientation.

In addition, there is another method by which surface scans are performed in different sensor positions. In the process, so-called C-scans of a whole sample are displayed and the measured values are used to calculate the local anisotropy and directional vectors of the fibers. This can be represented by the C-scan over the entire sample surface.

However, this method has the disadvantage that the representation of a direction vector does not always correspond to reality. For example, for a 0 ° sheet and 90 ° sheet build up, the resulting directional vector is at 45 °. With respect to the directional stiffness characteristics of such a build, this fiber orientation is not correct. The same applies to CF-SMC ("SMC", Sheet Molding Compound) or CFRP molding compounds.

It is therefore an object to be solved to provide a method for determining the fiber orientation of carbon fiber reinforced materials, by means of which entire surface areas of carbon fiber reinforced specimens can be analyzed with little expenditure of time.

This object is achieved by a method according to the independent claim. Advantageous embodiments and further developments will become apparent from the dependent claims, the following description and the drawings.

According to one embodiment, in a method for determining the fiber orientation of a sample body and a test apparatus, in particular a test apparatus for determining at least one fiber-direction-dependent parameter are provided. The specimen is preferably a specimen having a fiber composite. For example, the specimen may comprise a carbon fiber reinforced material. The test apparatus is preferably an eddy current test apparatus.

In the method, a surface area of the sample body to be scanned is defined. The surface area may, for example, be a partial area of the sample body or of the surface of the sample body. Alternatively, the surface area may for example also be a complete surface side of a sample body, for example a complete surface side of a plate-shaped sample body. At least four area scans of the surface area are performed so that impedance values are obtained for each area scan at a plurality of measurement points. The impedance values each include a real part and an imaginary part. Preferably, the four area scans are performed in mutually different directions. For example, the surface scans can be performed successively with mutually different sensor positions of the eddy current testing device. Preferably, the eddy current testing device comprises two coils, wherein in each of the four surface scans the orientation of the coils or the arrangement of the coils differ from one another with respect to the scanning direction.

The determined impedance values are recorded or stored, for example separately for each of the area scans. In particular, the real parts and imaginary parts of the impedance values can be stored. This gives e.g. for each area scan, an "impedance point cloud" in which the impedance values of the individual measurement points or the real parts and imaginary parts of the impedance values of the individual measurement points are shown.

The impedance values can be recorded, for example, by a computer unit. The computer unit may be part of the eddy current testing device, for example. Alternatively, the computer unit and the eddy current testing device can also be implemented separately from one another.

After recording or storing the impedance values, an arbitrarily large partial area of the area can be selected. The partial surface area may here and below also be referred to as "region of interest" or "ROI". Advantageously, there may be an arbitrarily large time interval between performing the area scan and selecting the partial area or between recording the impedance values and selecting the partial area , By recording the plurality of impedance values, a partial area can also be defined with a large time interval.

After selecting the patch area, the individual impedance values of the measurement points lying in the selected patch area are averaged for each of the four area scans. For example, the imaginary parts and real parts of the impedance values can be separately averaged for each of the four area scans so that an imaginary part average and a real part average value are obtained per area scan. Alternatively, for example, only the imaginary parts of the impedance values may be averaged for each of the four area scans.

Furthermore, a graphical representation based on the impedance averages, in particular based on the four mean values or based on the four imaginary part averages and / or four real part averages is created. The graphical representation makes it particularly easy to make a statement about the fiber orientations in a sample body or about the mechanical properties of the sample body. The graphical representation may be, for example, a polar diagram.

With the aid of the polar diagram, it is advantageously possible to display a plurality of directions within a sub-area without the possibility of a misinterpretation.

In accordance with a further embodiment, the mean values of the imaginary parts of the determined impedance values are used to generate the graphical representation. Preferably, the eddy current testing device is compensated or calibrated before the measurement or before the surface scans are performed on a reference body, in particular on a graphite reference body. Thereby, e.g. It can be ensured that the imaginary parts of the impedance values in the CFRP material to be examined give information about the fiber orientation.

According to another embodiment, each of the four area scans includes a plurality of individual scans. For example, the individual measuring points of the individual scans can each have a distance of 0.5 mm to 1.5 mm, preferably of 1.0 mm, to the next measuring point. For example, the surface area can be scanned meander-shaped at each of the four area scans. This makes it possible to scan the surface area particularly effectively.

According to a further embodiment, the test device or the eddy current testing device has a semi-transmission sensor, in particular a direction-dependent semi-transmission sensor.

According to a further embodiment, the four surface scans are performed with the sensor positions 0 °, -45 °, 45 ° and 90 °. A sensor position of 0 ° preferably means that the two coils of the eddy current testing device are arranged along the scanning direction relative to one another. A sensor position of 90 ° preferably means that the two coils of the eddy current testing device are arranged perpendicular to the scanning direction.

According to a further embodiment, the four area scans are respectively repeated with one or more further frequencies and the impedance values determined thereby are recorded. For example, the area scans can be performed with frequencies between 1 MHz and 15 MHz.

With the method described here, in particular also with a time interval for performing the area scans, any number of arbitrarily large sub-area areas can be defined here and associated polar diagrams can be generated. This advantageously provides good information about the fiber orientations of a sample body in a short time.

Further advantages of the method described here will become apparent from the following in connection with the 1 to 3 described embodiments. Show it:

1 a schematic representation of a method for determining the fiber orientation of carbon fiber reinforced materials according to an embodiment, and

2 and 3 schematic representations for creating a polar diagram according to further embodiments.

1 shows a schematic representation of a method 100 for determining the fiber orientation of carbon fiber reinforced materials, wherein in method step A, a sample body of a carbon fiber reinforced material and an eddy current testing device are provided. In method step B, a surface area of the sample body to be scanned is defined. Subsequently, in method step C, at least four area scans of the surface area are successively performed in mutually different directions, so that impedance values are obtained for each area scan at a multiplicity of measuring points within the area area. The four surface scans are preferably carried out in each case with different sensor positions of the eddy current testing device. In method step D, which can take place, for example, simultaneously with method step C, the determined impedance values are stored. Thereafter, in method step E, an arbitrarily large partial surface area or a so-called "region of interest" within the surface area is selected. Based on the selected subarea area, in method step F, the individual impedance values of the measuring points lying in the subarea area are averaged for each of the four area scans. In method step G, a graphical representation based on the four mean values is created. The graphical representation can in particular be a polar diagram.

Furthermore, in particular also with a time interval, a further arbitrarily large subarea region of the surface region can be selected, and the individual impedance values of the measuring points lying in the further subarea region are averaged for each of the four surface scans, and a further graphical representation, in particular a further polar diagram, based on these four means. This process can be repeated as often as desired for further partial surface areas of the surface area.

In the 2 a graph is shown in which the average values of the determined impedance values of four area scans are plotted. The area scans were performed in the directions 0 °, -45 °, 45 ° and 90 °. The real parts of the impedance values are plotted on the x-axis, the imaginary parts of the impedance values on the y-axis.

The 3 shows a further graphical representation, in particular a polar diagram, which by means of in the 2 represented average values of the impedance values of the four area scans, in particular the average values of the imaginary parts of the impedance values, was generated. For the generation of the polar diagram, the values of the imaginary parts of the impedance values were taken from the 2 transferred to the polar diagram.

Alternatively or additionally, the embodiments shown in the figures may have further features according to the embodiments of the general description.

LIST OF REFERENCE NUMBERS

100

method

A-G

steps

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3827229 A1 [0003]
• DE 10234551 B3 [0004]

Claims (11)

 Non-destructive method of determining fiber orientation of carbon fiber reinforced materials, comprising the following steps: Providing a specimen comprising a carbon fiber reinforced material, Providing an eddy current testing device, Defining a surface area of the sample to be scanned, Performing at least four area scans of the surface area, each with mutually different sensor positions, so that impedance values are obtained for each area scan at a multiplicity of measuring points, Recording the determined impedance values, Selecting an arbitrarily large partial area of the area, Averaging of the individual impedance values of the measuring points lying in the partial area for each of the four area scans, and - Creation of a graphical representation based on the impedance averages.  The method of claim 1, wherein each of the four area scans comprises a plurality of individual scans.  The method of claim 1, wherein the area is scanned meandering at each of the four area scans.  Method according to one of the preceding claims, wherein the graphical representation is a polar diagram.  Method according to one of the preceding claims, wherein the mean values of the imaginary parts of the determined impedance values are used to produce the graphical representation.  The method of claim 5, wherein the eddy current testing device is compensated before performing the four surface scans on a reference body, in particular on a graphite reference body.  Method according to one of the preceding claims, wherein the surface scans are performed with four mutually different sensor positions of the eddy current testing device, wherein the eddy current testing device comprises two coils, wherein the orientation of the coils with respect to the scanning direction at each of the four sensor positions differs.  Method according to one of the preceding claims, wherein the eddy current testing device comprises a direction-dependent semi-transmission sensor.  Method according to one of the preceding claims, wherein the four surface scans are performed with the sensor positions 0 °, -45 °, 45 ° and 90 °.  Method according to one of the preceding claims, wherein the four area scans are repeated in each case with one or more further frequencies and the impedance values determined thereby are recorded.  Method according to one of the preceding claims, wherein an arbitrarily large period of time can lie between the performance of the area scans and the selection of the partial surface area or between the recording of the impedance values and the selection of the partial surface area.

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