Abstract
A method is provided in which an ultrasonic signal is generated as an electromagnetic ultrasonic signal by the at least one transmitting transducer, which is in the form of an EMUS transducer, by means of a conductive layer arranged on the surface of the object or in said object. An evaluation apparatus is used to utilize the ultrasonic signal detected by the at least one receiving transducer, which is in the form of an EMUS transducer, in order to determine a flaw in the form of a delamination, a porefield or other such two-dimensional inhomogeneities.
Claims
1. A method for nondestructively testing objects made of a fiber composite material which is in at least single-layer form, the method comprising: generating, via a transmitting transducer in the form of an EMUS transducer, an electromagnetic ultrasonic signal in the object by a conductive layer arranged on a surface of the object or in the object, detecting propagation of the ultrasonic signal in the object by a receiving transducer in the form of an EMUS transducer; determining, by an evaluation apparatus utilizing the ultrasonic signal detected by the at least one receiving transducer, a flaw in the form of a delamination, a porefield or other such two-dimensional inhomogeneities.
2. The method as claimed in claim 1, wherein the testing method is performed without coupling means.
3. The method as claimed in claim 1, wherein the transmitting transducer is used to produce a guided wave.
4. The method as claimed in claim 4, wherein the flaw is determined by virtue of a local phase velocity and/or a local wavelength of the received ultrasonic signal at a location of the receiving transducer being determined in the evaluation apparatus and used to determine the depth of the flaw.
5. The method as claimed in claim 4, wherein a material-specific correlation of a depth of the flaw with the phase velocity and/or with the wavelength is used to determine the flaw depth.
6. The method as claimed in claim 5, wherein the flaw is determined by performing at least one spatial Fourier transformation of the detected ultrasonic signal over at least part of a measuring section of the receiving transducer.
7. The method as claimed in claim 6, wherein a maximum of the wavenumber and/or the phase velocity is determined from a consideration of the ascertained spectra at different times.
8. The method as claimed in claim 1, wherein flaws at a depth of between 0% and up to 50% of a thickness of the object are tested by using the A0 mode of a Lamb wave.
9. The method as claimed in claim 1, wherein flaws are tested by initially using an S0 mode of a Lamb wave, an A0 mode that results from the S0 mode in a region of a flaw additionally being used for evaluation.
10. The method as claimed in claim 1, wherein the receiving transducer is moved in the direction of propagation of the ultrasonic signal and/or a linear array comprising one or more receiving transducers is used.
11. An apparatus for performing the method as claimed in claim 1, the apparatus comprising: a transmitting transducer, a receiving transducer, and an evaluation apparatus, wherein the receiving transducer detects various wavelengths.
12. The apparatus as claimed in claim 11, wherein the receiving transducer comprises: at least one magnetization device, and at least one conductor, which merely comprises a conductor loop, formed by one or more windings, with supply and return paths.
13. The apparatus as claimed in claim 12, wherein during operation the supply and return paths are arranged parallel to a surface of the object to be tested and above one another with reference thereto.
14. The apparatus as claimed in claim 13, further including a plurality of receiving transducers arranged in succession in a direction of testing or beside one another and are combined with one another, in order to form a linear array.
15. The apparatus as claimed in claim 13, wherein at least two magnet yokes of the combined receiving transducers have a common ferromagnetic connector.
16. The apparatus as claimed in claim 15, wherein at least two conductor loops forming independent receiving channels are arranged between the poles of at least one magnet yoke, each of the supply and return paths of said conductor loops running parallel.
17. The method as claimed in claim 1, wherein the objects are planar objects.
18. The method as claimed in claim 1, wherein the transmitting transducer is used to produce a guided wave; and the flaw is determined by virtue of a local phase velocity and/or a local wavelength of the received ultrasonic signal at a location of the receiving transducer being determined in the evaluation apparatus and used to determine the depth of the flaw.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] Reference is now made more particularly to the drawings, which illustrate the best presently known mode of carrying out the invention and wherein similar reference characters indicate the same parts throughout the views.
[0040] FIG. 1 shows a change of wavelength of a guided wave at a delamination in a composite material.
[0041] FIG. 2 shows a dispersion curve relating to an A0 mode for 100%, 75% and 25% plate thickness.
[0042] FIG. 3 shows a schematic probe arrangement having a transmitting transducer and a receiving transducer.
[0043] FIG. 4 shows a schematic probe arrangement having a transmitting transducer and a linear array.
[0044] FIG. 5 shows a B-scan containing a region provided with a flaw.
[0045] FIG. 6 shows the wavenumber shifts ascertained using the method according to the invention.
[0046] FIG. 7 shows part of an apparatus according to the invention in the form of a transmitting transducer.
[0047] FIG. 8 shows a view with part of an apparatus according to the invention in the form of a receiving transducer.
[0048] FIG. 9 shows a partial detail from a receiving transducer according to the invention.
[0049] FIG. 10 shows a further receiving transducer in a side view above an object.
[0050] FIG. 11 shows a calibration curve for the evaluation in the method according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0051] The features explained below from the exemplary embodiments according to the invention may also be the subject matter of the invention individually or in combinations other than those shown or described. Where appropriate, parts having functionally identical effects are provided with identical reference numerals.
[0052] The method according to the invention is based on the effect shown in FIG. 1 that the local phase velocity or local wavelength changes in the region of a flaw 1 in the form of a two-dimensionally extending inhomogeneity, in the present case a delamination, at a depth of 25% of a thickness d of a planar object 5 made of fiber composite material. The inhomogeneity extends two-dimensionally in the direction of the object, which is in the form of an aircraft fuselage element, for example. In the regions having a thickness d of 100% to the left and right of the flaw 1 shown, there is a wavelength λ of 6 mm. Above the delamination, i.e. toward the side on which a receiving transducer and a transmitting transducer need to be arranged, the thickness of the object is 25%; accordingly, 75% of the composite material thickness is beneath the delamination. Whereas these constraints mean that the resultant local wavelength λ of the ultrasonic signal is 6 mm to the left and right of the flaw, said wavelength is reduced to λ=4.44 mm in the region of the delamination. This local wavelength change may be detected using an apparatus according to the invention or the method according to the invention. The change of wavelength of the guided wave in the region of the 25% thickness of the plate, i.e. between the upper surface in FIG. 1 and the flaw 1, results from the dispersion curve for a specific frequency, which in the present case, purely by way of example, is 210 kHz; in particular, the frequency for objects having an overall thickness up to 8 mm is preferably in a range between 200 kHz and 220 kHz (FIG. 2).
[0053] For the A0 Lamb mode excited in the present case, the result is that the closer the delamination is to the surface, the shorter the wavelength and the sharper the change of wavelength of the guided ultrasonic wave. For the object in the form of a 4-mm CFRP plate used in the present case and an A0 mode, FIG. 11 shows this relationship by way of example, the detected local trace wavelength, i.e. the trace wavelength produced in the region of the flaw 1, or wavelength λ=4.44 mm, being associated with a depth of approximately 0.5 mm, i.e. the delamination is at 12.5% of a thickness of 4 mm.
[0054] FIGS. 3 and 4 show schematic representations of the apparatus having a transmitting transducer 2 and a receiving transducer 3 or a transmitting transducer 2 and a linear array comprising multiple receiving transducers 3.1, 3.2 to 3.n. The EMUS transducers are operated by means of a control unit 30 for actuating the transmitting transducers 2 and by means of an evaluation apparatus 20 at the receiving transducer(s). The evaluation apparatus 20 is intended to receive the ultrasonic signal and may additionally also have separately, i.e. remotely, arranged EDP means. An arrow 4 indicates the direction of sound propagation and the direction of measurement in an object 5. According to the invention, the receiving transducer(s) (FIG. 3) are moved in the direction of sound propagation according to arrow 6, and, for each accordingly predefinable position of the desired resolution, a shot from the transmitting transducer 2 is detected and a so-called A-scan is taken. The A-scans show the time characteristic of the signal amplitude at the location of the receiver. Multiple A-scans may be combined to produce a data matrix (B-scan, FIG. 5), as a result of which multiple spatial Fourier transformations may be performed at different times. From these, the dispersion relationship and thus ultimately the desired sizes are obtained at the operating point, i.e. at the location of the receiver. The method is performed quite similarly according to the variant shown in FIG. 4, albeit that here a receiving transducer combination containing receiving transducers 3.1, 3.2 to 3.n as a linear array receives the, possibly varied, ultrasonic signal from a single shot each time. Depending on the measurement point resolution of the setup shown in FIG. 3, the spatial resolution in this instance may be somewhat reduced compared to this, since the receiver coils are at a firmly predefined distance from one another and therefore the resolution of the linear array is predefined. Subsequently, the linear array may likewise again be moved according to arrow 6. If one does not wish to measure the same place repeatedly using different receivers 3.1 to 3.n of the same linear array, e.g. in order to improve the resolution, the array is moved in the direction of propagation 4 by the total length of the array in each case.
[0055] As an example, FIG. 5 shows the amplitude of the detected ultrasonic signal in the so-called B-scan for a defect close to the surface at a depth of 12.5%, the distance of the receiving transducer 3 from the transmitting transducer 2 along the measuring section x being plotted on the x axis. The y axis is the time of flight of the detected signal. A discernible variation occurs in the ultrasonic signal, the region of the flaw 1 that is used for the evaluation being shown by a dashed box. Flaw-free regions before and after the flaw 1 along the measuring section that are used for the evaluation are shown by a box 8 having a solid line and by a box 9 having a dash-dot line.
[0056] The amplitudes in the wavenumber spectrum that are ascertained for the flaw 1 are shown accordingly as a dashed curve 7 in the associated FIG. 6; the wavenumber spectra associated with the framed regions 8 and 9 (solid and dash-dot lines) are shown more or less above one another as solid and dash-dot lines 10 and 11. It is then possible to use the relationship in FIG. 11, which has been determined experimentally as an example on the basis of a large number of experiments for the materials to be examined, to determine the depth of the flaw from the ascertained wavenumber. The calibration curve shown in FIG. 11 may be stored for a multiplicity of composite materials in a database which the evaluation device is able to access, or which is stored in the evaluation apparatus.
[0057] Excitation of a guided mode A0 is known to require the forces to act vertically on the plate; using the Lorentz force, a vertical force may then act on the test body, for which purpose the magnetic field and the eddy current are oriented at a tangent to the plate, however. This is what the transmitting transducer shown in FIG. 7 is designed for, which in this FIG. 7 is shown in a position bearing against or a position resting on the object 5. The dashed curve 12 shows the deflection of the excited A0 mode. A conductive layer 13 in the form of a lightning protection mesh is shown as a solid line and is embedded in the object 5, which is shown as a single layer. As a result, the layer 13 is acoustically coupled to the further regions of the object 5.
[0058] The transmitting transducer 3 shown, which is actuated by control electronics 30 (cf. FIGS. 3 and 4), which are not shown further, additionally comprises a plurality of magnet yokes 14, the north and south poles of which are denoted by N and S. The resultant magnetic field lines 15 run comparatively parallel to the surface 16 of the object. Conductor pieces 17 run into the plane of the figure with indicated directions of current flow, and further conductor pieces 18 run out of the plane of the figure with indicated directions of current flow. Conductor pieces 17 and 18 are spaced apart between the magnet poles in such a way that an A0 mode is excited at a specific excitation frequency.
[0059] As in the case of the receiving transducer, the magnet yokes 14 may have a common ferromagnetic connector 19 and may thus be constructed in a simpler and more compact manner.
[0060] FIG. 8 shows a receiving transducer according to the invention at a first position x0 (left-hand representation of the receiving transducer) and at a position x0+Δx.Math.n, the receiving transducer having a supply path 21 and a return path 22 between each of the poles of the magnet yoke 14, said paths each being formed by a plurality of windings of a conductor loop (cf. FIG. 9). Δx is the step size between individual measurements, and n is the number of steps. Using the conductive layer 13 in the form of an image protection mesh, the receiving transducer samples the local wavelength of the ultrasonic signal propagated in the object 5 and detects the flaw 1, in the present case at the location x0+Δx.Math.n, by means of the change in the wavelength. The receiving transducer 3 is of wideband design. An exemplary form of a conductor loop having a supply path 21 and a return path 22 is shown in FIG. 9, a conductor loop of coil-type design forming the supply path 21 with a total of ten lower winding sections and forming the return path 22 with 10 upper winding sections. The connections 23 and 24 of the conductor usually lead to RC− and RC+ elements of the evaluation apparatus, via which the induced currents are then tapped off and supplied to further evaluation. In the present case, the total width B viewed in the direction at right angles to the direction 4 is between 0.5 mm and 1.5 mm, in particular 1 mm, the length L of the conductor loop is between 8 and 12 mm, in the present case preferably 10 mm, and the overall height of the conductor loop is H equals 3 mm in order to avoid induction effects in the conductor line return path 22.
[0061] With reference to a vertical onto the surface of the object 5, the supply and return paths are situated above one another in a spaced-apart manner.
[0062] A receiving transducer combination in the form of a linear array as shown in FIG. 10 is provided with a number of six channels operating independently of one another that are each formed by conductor loops as described previously. These channels CH1 to CH6 in the form of individual conductor loops are each arranged in twos between poles of respective magnet yokes 14. The linear array likewise has a ferromagnetic connector 19 for the magnetic circuit, and the respective conductor loops or supply and return paths 21 and 22 formed between two poles of a magnet yoke are spaced apart by around 1 mm. Such a receiving transducer combination corresponds to a receiving transducer having units 3.1, 3.2 and 3.3 as shown in FIG. 4.
[0063] In summary, the method according to the invention may be used to detect the position of defects relevant to aviation in fiber composite materials by using selectively guided ultrasonic wave modes and to record the depth. Mode conversion effects and operating point shifts in the mode spectrum are recorded locally and the adverse signal-to-noise ratios in the fiber composite materials present are avoided in the amplitude assessment of reflected and/or transmitted ultrasonic signals. Defects up to a minimum diameter of 3 mm may be identified in this case.
