Method for non-destructively inspecting objects made of fiber composite material, in particular of planar design

Abstract

A method for inspecting objects (5), in particular of planar design, made of fiber composite material, at least in a single layer, preferably in aviation applications, without destruction, wherein ultrasonic signals are generated in the object by means of a transmitting transducer (2), which are detected by means of a receiving transducer (3) after their propagation in the object (5), wherein the ultrasonic signals are generated as electromagnetic ultrasonic signals by means of at least one transmitting transducer (2) designed as EMUS transducer by means of an electrically conductive layer (13) arranged on the surface of the object or in the object, wherein the ultrasonic signals detected by means of at least one receiving transducer (3) designed as EMUS transducer are used for determining defects (1) of the form of delaminations, porosity fields or other inhomogeneity forms of planar design by means of an evaluation device (20), and a device for carrying out the method, comprising a transmitting transducer (2), a receiving transducer (3) and an evaluation device (20), wherein the receiving transducer (2) is designed to detect different wavelengths.

Description

Technical Field

[0001] The present invention relates to a method, preferably in aerospace applications, for non-destructive inspection of objects made of fiber composite materials, particularly those with a planar design, wherein the fiber composite material is designed at least in a single layer, wherein ultrasonic signals are generated in the object by transmitting transducers, and the ultrasonic signals are detected by receiving transducers after propagating in the object. Background Technology

[0002] In aerospace applications, but also in other industrial sectors, fiber composites in the form of carbon fiber reinforced plastic (CFK) or glass fiber reinforced plastic (GFK) have been increasingly used over the years to enable exceptionally lightweight structures. The light empty weight of aircraft minimizes fuel consumption and associated operating costs. However, these fiber composites, which have at least one layer of fibers, are more difficult to inspect than conventional metallic materials, both during manufacturing and during any recurring inspections. Currently, these fiber composites are primarily inspected using conventional ultrasonic inspection techniques, such as by acoustically coupling a transducer equipped with piezoelectric material to a planar object to be inspected, such as a fuselage or wing section of an aircraft, using a suitable carrier medium like water. However, it has been shown that the strong sound attenuation in these materials, especially at higher operating frequencies, makes inspection more difficult. This results in a poor signal-to-noise ratio, making it difficult to assess the amplitude of the signal components reflected or transmitted at defects. Summary of the Invention

[0003] The object of this invention is to improve the detection of defects, particularly in planar-designed objects made of fiber composite materials, in the form of delamination, pore fields, or other planar-designed inhomogeneities.

[0004] In the method according to the invention, an ultrasonic signal is generated as an electromagnetic ultrasonic signal by at least one transmitting transducer designed as an EMUS transducer through a conductive layer disposed on or in the surface of an object, wherein the ultrasonic signal, examined by means of at least one receiving transducer designed as an EMUS transducer, is used to determine defects in other forms of inhomogeneity, such as layering, pore field, or planar design, by means of an evaluation device.

[0005] An EMUS transducer is an electromagnetic ultrasonic transducer, also known as an EMAT transducer, where "EMAT" stands for "electromagnetic acoustic transducer." Compared to traditional ultrasonic technology, this method utilizes electromagnetic interactions to couple ultrasonic signals into or from an object. Essentially, the Lorentz force contributes, and in the case of ferromagnetic test objects, magnetostriction also contributes. In principle, such an EMUS probe or transducer consists of a specially shaped high-frequency coil and a magnetization mechanism, preferably in the form of a yoke made of permanent magnets. In the transmitting case, a pulsed current signal is applied to the high-frequency coil, inducing eddy currents in a conductive object near its surface. Due to superposition with a static magnetic field, oscillating Lorentz forces act on charge carriers, and these forces couple with the material lattice, thus serving as the source of the ultrasonic signal. In the receiving case, the entire process proceeds in reverse.

[0006] According to the invention, eddy currents are generated in a conductive layer particularly close to or forming the surface, which extends planarly over the area to be tested and is either laid out beforehand or already present for testing. Preferably, according to the invention, as a conductive layer in a fiber composite material for aerospace applications, a lightning protection grid, often present on or within the surface of a planar object under test, is used; this lightning protection grid, for example, is embedded as the top layer of the object as a copper grid. Within the scope of the invention, it has been found that this lightning protection grid, as a conductive layer, ensures sufficient acoustic coupling. This coupling effect is particularly good when the material thickness of the conductive component is greater than the penetration depth of the electromagnetic field.

[0007] The objects under inspection are planar structures, meaning their extension in the test direction is significantly greater than that in the direction perpendicular to the test direction. For analytical purposes, these objects are infinitely extended planes, in fact, slabs ranging from one to tens of square meters in size. Such slabs, for example, used in aircraft fuselages, are made of fiber composite materials, at least on the outer surface to be tested, i.e., having at least one layer of fiber composite material. The slab need not be perfectly flat, but can also be bent or flexed for the application.

[0008] Fiber composites can be constructed in single or multiple layers. Aerospace applications typically involve five to ten or more thin layers composed of one or more different fiber composites, which are interconnected, particularly by bonding or welding.

[0009] The result of the method according to the invention is the location of the defect in the x and y directions, i.e., along the surface of the object and its depth position. Hereinafter, the depth position relative to the total thickness of the layer to be measured is also specified; for example, for a 4 mm thick plate made of fiber composite material, the position of 12.5% ​​or 25% corresponds to a depth of 0.5 mm or 1 mm below the surface on which the transmitting and receiving transducers are disposed.

[0010] The method according to the invention is preferably applicable to inspecting defects that are longer in the direction of sound propagation than transverse to that direction, i.e., defects that are larger in the direction parallel to the plane of the object than in the direction of the object's thickness. In particular, the length of the non-uniformity in the direction of sound propagation used to determine the depth location is at least two to three times the wavelength of the generated ultrasonic signal.

[0011] It goes without saying that the detected ultrasonic signal does not necessarily have to be the same as the generated ultrasonic signal. During the propagation of the ultrasonic signal within the medium, the ultrasonic signal changes due to defects, and these changes can provide inferences about the corresponding anomalies.

[0012] It goes without saying that appropriate control electronics are used in order to generate ultrasonic waves via a transmitting transducer. This control electronics can be combined with or at least partially separated from the evaluation device. EMUS transducers are therefore typically operated by means of a control unit having such control electronics for controlling the transmitting transducer and by means of an evaluation device for one or more receiving transducers. These parts of the device according to the invention can operate at least partially in a common electronic unit, wherein the evaluation device may also additionally have a separate EDP device.

[0013] Compared to non-destructive inspection using conventional piezoelectric probes, inspection using EMUS transducers is particularly advantageous because it eliminates the need for coupling agents. This means that using EMUS transducers on the object under test is easier than with existing techniques, where, for example, an immersion tank is used for the object to achieve good acoustic coupling between the probe emitting the ultrasonic signal and the object. It should also be understood that air or an air gap is typically present between the EMUS transducer used according to the invention and the object, and that air or any protective gas present is not a coupling agent. According to the invention, the acoustic signal is not generated in the transmitting transducer but rather within the object under test itself through a conductive layer, although this conductive layer is composed of a fiber composite material.

[0014] Preferably, a guided wave is generated by a transmitting transducer, which can propagate well in a planar object. Compared to conventional piezoelectric probes, the EMUS transducer is particularly adept at generating and probing single-mode and selective plate wave modes.

[0015] Preferably, a Lamb wave or a guided SH wave is generated using a transmitting transducer for non-destructive inspection, particularly generating an A0 mode or an S0 mode. Excitation frequencies between 10 kHz (inclusive) and 1000 kHz (inclusive) have proven particularly effective for the fiber composite materials used. Furthermore, the excitation frequency is advantageously in the range of 50 to 500 kHz, and more particularly in the range of 200 to 220 kHz, which is especially suitable for plates with a thickness of 2 to 8 mm, preferably 4 mm, and a wavelength of 4 to 8 mm, preferably 6 mm.

[0016] Preferably, in order to determine defects in the evaluation apparatus, the local phase velocity and / or local wavelength of the received ultrasonic signal are determined at the location of the receiving transducer and used to determine the depth of the defect. The local wavelength is the wavelength of the ultrasonic signal at the object location where the receiving transducer is situated on or above the object's surface; that is, the receiving transducer and the ultrasonic signal belonging to the local wavelength are perpendicular to the object's surface. A similar situation applies to the local phase velocity. When the surface is horizontally oriented relative to the ground, the transducer is, for example, located on the top side of the object under test, whereby the ultrasonic signal is directly below it. The present invention utilizes the fact that when the ultrasonic signal of a guided wave encounters an inhomogeneity in a plate that also extends along the direction of sound propagation, the physical boundary conditions change, thereby affecting the wave mode or the propagation of the ultrasonic signal.

[0017] The method according to the invention also utilizes the fact that, when the defect extends planarly along the direction of ultrasonic wave propagation, the energy of the original ultrasonic signal is separated, and the varying ultrasonic signal propagates above and below the region of the defect. The change in plate thickness associated with the original ultrasonic signal then causes a shift in the operating point in a dispersion plot, which shows the relationship between plate thickness, wavelength, phase velocity, group velocity, and frequency. Since frequency is a conserved variable in a linear system, the phase velocity must change, which is equivalent to a change in the trace wavelength. The change in the phase velocity or wavelength of the ultrasonic signal accompanying the shift in the operating point is locally detected by the receiving transducer between the defect and the surface on or at which the receiving transducer is arranged, such that the position of the receiving transducer in the x and y directions corresponds to the position of the defect in the x and y directions. The x and y directions correspond to the planar extension of the object, wherein the x direction is preferably the measurement direction and corresponds to the preferred propagation direction of the ultrasonic signal.

[0018] As will be explained in more detail below, the location of defects is determined, for example, by scanning the object to be inspected in a grid pattern, or by a locally extended, preferably multi-channel, receiving transducer or receiving sensor array.

[0019] Furthermore, it is preferable to determine the defect depth using a material-specific correlation between the defect depth and phase velocity and / or wavelength. Particularly for the preferred A0 or S0-λ modes, the closer the delamination is to the surface and the shorter the wavelength, the stronger the wavelength or phase velocity variation of the guided ultrasonic signal. This material-specific or composite-specific correlation can be determined experimentally beforehand, or by simulation if the structure of the composite material is known.

[0020] Advantageously, to determine the defect, at least one spatial Fourier transform is performed on the detected ultrasonic signal at least a portion of the measurement path of the receiving transducer (at a specific time point). Needless to say, depending on the structure of the receiving transducer or probe, this may initially require multiple measurements along the measurement path. The length of the measurement path used for the Fourier transform of the ultrasonic signal to be evaluated is preferably at least 1 cm, more preferably at least 2 cm. An upper limit is preferably 5 cm or more preferably 4 cm. Starting from the transmitting transducer, the path scanned by the receiving transducer is thus continuously checked over correspondingly large distances Δx. This is done continuously; therefore, for example, with a transducer x-resolution of 1 cm along the measurement direction, a 6 cm long component to be inspected is checked 6 times.

[0021] The result of the Fourier transform then exists in the k-space, i.e., the wavenumber space, where the maximum value of the wavenumber amplitude is shifted in the region of the defect or anomaly that extends in a plane, because the wavelength of the guided wave changes, i.e. decreases, in that region.

[0022] If multiple spectra are determined at different time points in the measurement direction, the maximum values ​​of the wavenumber and / or phase velocity can advantageously be determined from, in particular, statistical observations of the determined spectra at different time points, or otherwise read out. In a simple case, such statistical evaluation is, for example, formed by the arithmetic mean of the squares of the absolute values ​​of the spectra, from which the wavenumber v of the maximum value can be derived, and the wavelength λ can be derived from the relation v = 1 / λ.

[0023] According to an advantageous improvement of the method according to the invention, a Lamb wave A0 mode is employed to inspect for defects at depths between 0% and, particularly, 50%, of the object's thickness. Using an EMUS transducer, the A0 mode can be excited particularly well as a flexural wave of the observed object, with no cutoff frequency below which it can no longer propagate. 0% corresponds to the surface of the object where the transducer is located.

[0024] In addition to or as a supplement to using the A0 mode of the Lamb wave for defect inspection, the S0 mode of the Lamb wave can also be used. Specifically, to inspect defects at depths between 50% and 100% of the object's thickness (but also between 0% and 50%), the S0 mode of the Lamb wave can be used first, with the A0 mode generated by the S0 mode in the defect region additionally used for evaluation. The resulting strong changes in phase velocity can be detected particularly well.

[0025] Preferably, the receiving transducer moves in the direction of ultrasonic signal propagation, and / or a linear array of one or more combined receiving transducers is used, wherein relative movement of the receiver relative to the transmitter can be eliminated by at least partially covering the measurement path in space using the linear array. In the case of excitation pulses (also known as pulse bursts), the use of multiple receiving devices arranged sequentially in the measurement direction results in virtually simultaneous multiple measurements at corresponding locations. In particular, a linear array with multiple receiving transducers arranged sequentially, each receiving transducer having one or more receiving channels, can more quickly cover a larger measurement path in space.

[0026] To target the propagation direction, a phased array transmitting transducer can also be used. Otherwise, the propagation direction preferably corresponds to the measurement or inspection direction, i.e., the direction in which one or more receiving transducers move, which should be a linear movement along the surface away from or towards the receiving transducer.

[0027] Typically, the EMUS receiving transducer moves in a prescribed manner along or against the propagation direction of the guided wave, recording an A-scan at each position. Multiple A-scans are then combined into a data matrix. A spatiotemporal Fourier transform is then performed on this data. The dispersion relation is then obtained at the operating point of the corresponding receiving transducer, and thus the desired value is obtained.

[0028] In addition to the preferred A0 and / or S0-λ modes, higher-order Lamb wave modes and guided SH wave modes are also suitable. For example, in the A0-λ mode, the closer the layer is to the surface, the shorter the wavelength. The wavelength or phase velocity variation of the guided ultrasound in the defect region is greater. This is related to the receiver position, which is therefore perpendicular to the defect location and indicates the x and y positions of the defect.

[0029] The purpose stated at the outset is also achieved by means of an apparatus for performing the methods described above or below, wherein the apparatus includes a transmitting transducer, a receiving transducer, and an evaluation device, and wherein the receiving transducer is designed to detect different wavelengths, i.e., it is capable of operating broadbandly in the wavenumber space / k-space. In contrast, the transmitting transducer is designed to specifically excite exactly one wavelength, i.e., it operates narrowly in the wavenumber space.

[0030] It goes without saying that the housing, cable, or other components used to form the probe and then use it in operation are also typically considered transmitting or receiving transducers. Transmitting or receiving transducers are therefore synonymous with probes having one or more transmitting transducers and / or one or more receiving transducers, respectively.

[0031] Different variations can be conceived for the design of a broadband-operating receiving transducer; the receiving transducer is particularly preferably designed to include a magnetizing mechanism and at least one conductor, the magnetizing mechanism preferably being formed by a yoke having at least one permanent magnet, and the conductor having only one conductor loop formed by one or more windings, the conductor loop having an inlet and a return portion that extend particularly laterally to the inspection direction during operation. Instead of a permanent magnet, an electromagnet can also be used, and the magnetic field of the electromagnet can be considered static or quasi-static if for the purpose of exciting the desired mode.

[0032] The inlet and outlet of the receiving transducer are, for example, coil-type windings of an electrical conductor in which a voltage or current is induced. The inlet can be formed by one or more printed conductors oriented parallel to each other. These printed conductors of at least one electrical conductor are preferably close or tightly packed together and in contact with each other through their insulation. The outlet is a plurality of one or more printed conductors extending parallel to such an inlet, which are correspondingly connected to leads via deflection sections. By using a conductor with exactly one conductor loop, the receiving channel and receiving transducer for detecting different wavelengths are designed to be broadband. Known EMUS receiving transducers have printed conductors designed to meander at certain intervals, and these receiving transducers are designed to detect specific wavelengths. These receiving transducers, intended for evaluating reflection and transmission effects, are designed to be narrowband, not broadband as in this invention.

[0033] In operation, i.e., when the receiving transducer is properly oriented relative to the surface of the object being measured, the inlet and outlet are parallel to the surface and arranged vertically relative to each other in this respect. Specifically, in a view along the surface, particularly in the direction of motion or sound propagation, the inlet and outlet are arranged vertically relative to each other on the perpendicular line of the surface (the surface is oriented parallel to the ground). In particular, the inlet and outlet are spaced apart from each other in the range of 1 to 5 mm, preferably from 2 to 4 mm, so that no inductive effect occurs in the portion away from the surface. Needless to say, in the edge regions of the inlet and outlet, they merge together, and in the central portion, they are spaced apart.

[0034] To form a linear array, it is provided with multiple receiving transducers, which are arranged successively or side by side and combined with each other in the inspection direction. As described above, at least one corresponding inlet and outlet section is provided as sensor elements between the magnetic poles of the corresponding yokes. They are also arranged vertically relative to each other with respect to the surface.

[0035] At least two magnetic yokes can preferably have a common ferromagnetic connector, which allows for a simplified construction of the device. Specifically, when two yokes placed side-by-side share a single magnetic pole, the resulting receiving transducers are combined with or integrally formed with each other. Accordingly, the successive or side-by-side magnetic poles to be closely arranged on the surface in this linear array construction are designed to generate a magnetic field that propagates as horizontally as possible within the object, i.e., across the planar extension of the object. Through the common ferromagnetic connector, magnetic closure occurs on the side of the receiving transducer facing away from the object surface in order to form a compact array of linear transducers.

[0036] Between the poles of at least one yoke, regardless of whether it is a receiving transducer with one or more yokes, according to an improvement of the invention, preferably at least two conductor loops forming independent receiving channels can be provided, with the lead-in and return portions of the conductor loops extending parallel to each other. Minor deviations from this parallelism, especially those caused by windings, are disregarded here. These lead-in and return portions are typically formed by their own conductors. Thus, for example, six receiving channels can be formed by three yokes formed successively. Depending on the distance between the conductor loops in the measurement direction, the linear array can then have a local resolution, for example, between 1 mm and 5 mm.

[0037] Viewed from the measurement direction, the magnetic yokes are preferably arranged sequentially. However, at least two linear arrays placed side-by-side in the measurement direction can also be used, forming a matrix of receiving transducers and covering a larger area. If necessary, to generate Lamb waves, the same number of transmitting transducers placed side-by-side can then be used. Attached Figure Description

[0038] Other advantages and details of the invention will become apparent from the following description of the accompanying drawings; schematically:

[0039] Figure 1 The wavelength variation of waves guided at the delamination sites in the composite material is shown;

[0040] Figure 2 The dispersion curves of the A0 mode are shown for plate thicknesses of 100%, 75%, and 25%.

[0041] Figure 3 A schematic inspection head facility with a transmitting transducer and a receiving transducer is shown;

[0042] Figure 4 A schematic inspection head facility with a transmitting transducer and a linear array is shown;

[0043] Figure 5 A B-scan with a defective area is shown;

[0044] Figure 6 The wavenumber shift determined using the method according to the invention is shown;

[0045] Figure 7 A portion of the device according to the invention is shown in the form of a transmitting transducer;

[0046] Figure 8 A view showing a portion of the device according to the invention in the form of a receiving transducer;

[0047] Figure 9 A partial cross-section of the receiving transducer according to the present invention is shown;

[0048] Figure 10 A side view of another receiving transducer above the object is shown;

[0049] Figure 11 A calibration curve for evaluation in the method according to the invention is shown. Detailed Implementation

[0050] The features explained below according to embodiments of the present invention may also constitute the subject matter of the invention individually or in combinations different from those shown or described. Wherever meaningful, functionally identical parts have the same reference numerals.

[0051] The method according to the present invention is based on Figure 1 The effect shown is that the local phase velocity or local wavelength in the defect 1 region exists in the form of a planarly extending inhomogeneity, currently delamination, which varies at a depth of 25% of the thickness d of the planarly formed object 5 made of fiber composite material or fiber composite material. The inhomogeneity extends planarly in the direction of the object, which is designed, for example, as an aircraft fuselage component. In the regions on both sides of the defect 1 where the thickness d is 100%, the wavelength λ is 6 mm. Above the delamination, i.e., towards the side where the receiving and transmitting transducers are to be arranged, the thickness of the object is 25%, corresponding to 75% of the thickness of the composite material below the delamination. Although a local wavelength λ of 6 mm for the ultrasonic signal is generated on the left and right sides of the defect due to these boundary conditions, this local wavelength is reduced to λ = 4.44 mm in the delamination region. This local wavelength variation can be checked using the device or method according to the invention. The guided wave is in the region of 25% of the plate thickness, i.e., at a depth of 25% of the plate thickness. Figure 1 The wavelength variation between the upper surface and defect 1 is generated by a dispersion curve at a specific frequency, which is purely exemplary at 210 kHz. In particular, the frequency of an object with a maximum total thickness of 8 mm is preferably in the range between 200 kHz and 220 kHz. Figure 2 ).

[0052] For the currently excited A0-λ mode, the closer the layer is to the surface, the smaller the wavelength, and the stronger the wavelength variation of the guided ultrasound. For the object currently used in the form of a 4mm CFK plate and the A0 mode, Figure 11 This relationship is illustrated exemplarily, wherein the detected local trace wavelength or wavelength λ = 4.44 mm produced in the region of defect 1 belongs to a depth of about 0.5 mm, that is, a layer of 12.5 at a thickness of 4 mm.

[0053] Figure 3 and 4 A schematic diagram of an apparatus having a transmitting transducer 2 and a receiving transducer 3, or a transmitting transducer 2 and a linear array of multiple receiving transducers 3.1, 3.2 to 3.n, is shown. The EMUS transducer is operated by means of a control unit 30 for controlling the transmitting transducer 2 and by means of an evaluation device 20 on the side of one or more receiving transducers. The evaluation device 20 is configured to receive ultrasonic signals and may also have separate, i.e., remotely arranged EDP devices. Arrow 4 indicates the direction of sound propagation and the measurement direction in the object 5. According to the invention, one or more receiving transducers ( Figure 3 Moving in the direction of sound propagation according to arrow 6, and at each corresponding predefined position for the desired resolution, a snapshot of the transmitting transducer 2 is detected and recorded as a so-called A-scan. An A-scan represents the process of signal amplitude changing over time at the receiver position. Multiple A-scans can be combined into a data matrix (B-scan, ...). Figure 5 This allows for the execution of multiple spatial Fourier transforms at different time points. From this, the dispersion relation is obtained at the operating point, i.e., at the receiver's location, ultimately yielding the desired value. According to... Figure 4 A variant of this method performs the same approach very similarly, but here, a combination of receiving transducers 3.1, 3.2 to 3.n, which are linear arrays, receives the potentially varying ultrasonic signal from a single snapshot. This depends on... Figure 3 The measurement point resolution shown can be slightly reduced compared to this, because the receiving coils are spaced at a fixed predetermined distance from each other, and therefore the resolution of the linear array is predetermined. The linear array can then be moved again according to arrow 6. If it is not desired to measure the same location multiple times using different receivers 3.1 to 3.n of the same linear array, for example to improve resolution, the array can be moved by the total length of the array in the propagation direction 4.

[0054] Figure 5The amplitude of the ultrasonic signal detected in a so-called B-scan is illustrated exemplarily for a near-surface defect with a depth of 12.5%, where the distance along the measurement path x between the receiving transducer 3 and the transmitting transducer 2 is plotted on the x-axis. The y-axis represents the propagation time of the detected signal. Identifiable variations in the ultrasonic signal are observed, where the area of ​​defect 1 used for evaluation is indicated by a dashed box. Defect-free areas used for evaluation before and after defect 1 along the measurement path are shown in boxes 8 with solid lines and 9 with dotted lines.

[0055] The amplitude determined for defect 1 in the wavenumber spectrum is in the relevant Figure 6 The corresponding wavenumber spectra are shown as dashed line 7, and those associated with boundary regions 8 and 9 (solid line and dashed line) are more or less superimposed as solid line and dashed line 10 and 11. From the determined wavenumbers, subsequent analysis can be conducted using… Figure 11 The relationship between the two is used to determine the depth and location of defects. This relationship is determined experimentally based on extensive testing of the material to be inspected. Figure 11 The calibration curves shown can be stored in a database for a large number of composite materials, or stored in an evaluation device that the evaluation agency can access.

[0056] For the excitation of guided A0 modes, it is well known that the force must act perpendicularly on the plate. Using the Lorentz force, a perpendicular force can then be applied to the test object; however, for this to be achieved, the magnetic field and eddy currents must be tangentially oriented to the plate. According to... Figure 7 The transmitting transducer is designed for this purpose, and its transmission transducer is in Figure 7 The image shows the contact or rest position on object 5. Dashed line 12 represents the deflection of the excited A0 mode. The conductive layer 13, in the form of a lightning protection grid, is shown as a solid line and is embedded in object 5, which is shown as a single layer. As a result, layer 13 acoustically couples with other areas of object 5.

[0057] The transmitting transducer 3 shown is controlled by control electronics 30 (not shown in detail) (see [link]). Figure 3 and 4 The system also includes multiple magnetic yokes 14, with N representing their north pole and S representing their south pole. The resulting magnetic field lines 15 extend relatively parallel to the surface 16 of the object. Conductor portions 17 extend into the drawing in the indicated direction of current flow, and other conductor portions 18 extend from the drawing in the indicated direction of current flow. Conductor portions 17 and 18 are spaced apart between the magnetic poles, such that the A0 mode is excited at a specific excitation frequency.

[0058] Like the receiving transducer, the magnetic yoke 14 can have a common ferromagnetic connector 19, and thus can be constructed more simply and compactly.

[0059] Figure 8The receiving transducer according to the invention is shown in a first position x0 (the receiving transducer is shown on the left) and a position x0+Δx∙n, wherein the receiving transducer has an introduction portion 21 and a return portion 22 between the magnetic poles of the yoke 14, which are respectively formed by multiple windings of the conductor loop (see...). Figure 9 Δx is the step size between measurements, and n is the number of steps. Using a conductive layer 13 designed as an image protection grid, the receiving transducer scans the local wavelength of the ultrasonic signal propagating in the object 5, and currently detects the defect 1 at position x0 + Δx∙n by wavelength variation. The receiving transducer 3 is a broadband design. An exemplary design of a conductor loop with an inlet 21 and a return 22 is shown in... Figure 9 As shown, the coil-type conductor loop forms an inlet section 21 with a total of ten lower winding segments and a return section 22 with ten upper winding segments. The conductor joints 23 and 24 typically lead to the RC- or RC+ elements of the evaluation device, through which the induced current is tapped and fed to further evaluation. In the present case, the total width B observed in the direction transverse to direction 4 is between 0.5 mm and 1.5 mm, particularly 1 mm; the length L of the conductor loop is between 8 and 12 mm, currently preferably 10 mm; and the total height H of the conductor loop is equal to 3 mm to avoid inductive effects in the printed conductor return section 22.

[0060] The introduction and return sections are spaced apart from each other along the perpendicular line to the surface of object 5.

[0061] Linear array form of receiver transducer combination according to Figure 10 The system comprises six independently operating channels, each formed by a conductor loop as described above. These channels CH1 to CH6, each in the form of a conductor loop, are arranged in pairs between the magnetic poles of the corresponding yoke 14. The linear array also includes ferromagnetic connectors 19 for the magnetic circuit, and the corresponding conductor loops or inlet portions 21 and return portions 22 formed between the two poles of the yoke are spaced approximately 1 mm apart. This receiver-transducer assembly corresponds to... Figure 4 The receiving transducer shown has units 3.1, 3.2 and 3.3.

[0062] In summary, using the method according to the present invention, the location and depth of aerospace-related defects in fiber composites can be detected and recorded by means of selectively guided ultrasonic modes. Mode switching effects and operating point shifts in the mode spectrum are locally recorded, and unfavorable signal-to-noise ratios, reflected and / or transmitted ultrasonic signals in existing fiber composites are avoided when evaluating amplitude. Defects with a minimum diameter of 3 mm can be identified.

Claims

1. A method for non-destructively inspecting an object (5) made of a fiber composite material, wherein the fiber composite material is designed to be at least a single layer, wherein, An ultrasonic signal is generated in the object by a transmitting transducer (2), and the ultrasonic signal is detected by a receiving transducer (3) after it propagates in the object (5). The ultrasonic signal is characterized by being generated as an electromagnetic ultrasonic signal by at least one transmitting transducer (2) designed as an EMUS transducer through a conductive layer (13) arranged on the surface of the object or in the object. The ultrasonic signal detected by the evaluation device (20) with the aid of at least one of the receiving transducers (3) designed as an EMUS transducer is used to determine defects (1) of other forms of inhomogeneity of layering, pore field or planar design. In order to determine the defect (1) in the evaluation device (20), the local phase velocity and / or local wavelength of the received ultrasonic signal are determined at the location of the receiving transducer, and the depth of the defect (1) is determined by the material-specific correlation between the depth of the defect (1) and the phase velocity and / or the wavelength.

2. The method according to claim 1, characterized in that, The method is designed for non-destructive inspection of objects made of fiber composite materials in aerospace applications (5).

3. The method according to claim 1, characterized in that, The object (5) is a two-dimensional object (5).

4. The method according to claim 1, characterized in that, The inspection was performed without coupling agent.

5. The method according to any one of claims 1 to 4, characterized in that, The guided wave is generated by the transmitting transducer (2).

6. The method according to any one of claims 1 to 4, characterized in that, In order to determine the defect (1), at least one spatial Fourier transform is performed on the detected ultrasonic signal along at least a portion of the measurement path of the receiving transducer (3).

7. The method according to claim 6, characterized in that, The maximum values ​​of wavenumber and / or phase velocity are determined from the determined spectrum at different time points.

8. The method according to any one of claims 1 to 4, characterized in that, To inspect for defects (1) at depths between 0% and 50% of the thickness of the object (5), Lamb wave A0 mode was used.

9. The method according to any one of claims 1 to 4, characterized in that, To examine the defect (1) at a depth location between 50% and 100% of the thickness of the object (5), the S0 mode of the Lamb wave is first used, wherein, for evaluation, the A0 mode generated by the S0 mode in the region of the defect (1) is additionally used.

10. The method according to any one of claims 1 to 4, characterized in that, The receiving transducer (3) moves in the direction of propagation of the ultrasonic signal and / or uses a linear array of one or more receiving transducers (3).

11. An apparatus for performing the method according to any one of claims 1 to 10, comprising a transmitting transducer (2) designed as an EMUS transducer, a receiving transducer (3) designed as an EMUS transducer, and an evaluation device (20), characterized in that, The receiving transducer (3) is designed to detect different wavelengths.

12. The apparatus according to claim 11, characterized in that, The receiving transducer (3) includes at least one magnetization mechanism and at least one conductor, the conductor having only one conductor loop formed by one or more windings, the conductor loop having an inlet and a return (21, 22).

13. The apparatus according to claim 12, characterized in that, The magnetization mechanism is formed by a magnetic yoke, which has at least one permanent magnet.

14. The apparatus according to claim 12, characterized in that, The conductor circuit has an inlet and a return section (21, 22) that extend laterally in the inspection direction during operation.

15. The apparatus according to claim 12, characterized in that, In operation, the inlet and outlet (21, 22) are parallel to the surface of the object to be tested (5) and are arranged vertically to each other in this respect.

16. The apparatus according to claim 15, characterized in that, To form a linear array, multiple receiving transducers (3.1, 3.2, 3.n) are arranged sequentially or side-by-side and combined with each other in the inspection direction.

17. The apparatus according to claim 15 or 16, characterized in that, The combined receiving transducer has at least two magnetic yokes with a common ferromagnetic connector (19).

18. The apparatus according to claim 15 or 16, characterized in that, At least two conductor loops forming independent receiving channels are arranged between the magnetic poles of at least one yoke, wherein the lead-in and return portions (21, 22) of the conductor loops extend in parallel.

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