POLE SHOE FOR MAGNETIZING DEVICE AND ITS USE

Description

FIELD OF APPLICATION AND STATE OF THE ART

[0001] The invention relates to a pole piece for use on a magnetization device for magnetizing a test volume of a test material in a testing device for flux leakage testing of ferromagnetic test material with a substantially circular-cylindrical surface for detecting defects, in particular for testing ferromagnetic pipes. Furthermore, the invention relates to a testing device comprising at least one such pole piece. Such a testing device comprises a test head with at least one test probe for scanning the surface of the test material to detect magnetic stray fields caused by defects. The magnetization device is designed to generate magnetization of the test material in the region of the test head.

[0002] Test material with a "substantially" circular-cylindrical surface nominally has a mathematically exact circular-cylindrical surface or lateral surface. "Substantially circular-cylindrical" means that deviations from the exact circular cylindrical shape may occur within the scope of manufacturing tolerances, but that the surface is nevertheless largely circular-cylindrical. In particular, largely circular-cylindrical surfaces with diameter variations of up to a maximum of 2% of the diameter, including ovality, are considered "substantially circular-cylindrical."

[0003] The test fixture is designed so that the test fixture and the test piece move relative to each other parallel to a passage axis during testing. For this purpose, the test fixture can be stationary while the test piece moves through it. Alternatively, the test fixture can be moved axially (parallel to the passage axis) while the test piece does not move in the axial direction. In either case, the test piece can be tested over a long length, if necessary continuously.

[0004] Magnetic flux leakage methods are an important component in the non-destructive testing of semi-finished and finished parts for defects, both during the manufacturing process and during the cyclical inspection of finished parts. Magnetic flux leakage methods are less sensitive to some adverse material properties, such as surface roughness or scale deposits in hot-rolled products, than, for example, eddy current or ultrasonic testing.

[0005] This results in a better ratio between the useful signal and the interference signal (N / S ratio), which enables more reliable error detection.

[0006] In a test device for detecting defects using flux leakage measurement, a test volume of the test piece is magnetized using a magnetization device and scanned using at least one magnetic-field-sensitive test probe (flux leakage probe) to detect stray magnetic fields caused by the defects. Magnetization can be achieved using a direct magnetic field (DC) or an alternating magnetic field (AC). During the test, a relative movement occurs between the probe and the surface of the test piece in a scanning direction along a test track. During scanning, the probe is held at a relatively small but finite test distance from the surface of the test piece.

[0007] In so-called rotating systems, the test fixture is designed so that the test head and magnetization device rotate around the throughput axis. The test object then preferably does not rotate around its longitudinal center axis. Relative rotation can also be achieved by rotating the test object around its longitudinal center axis, in which case the test head and magnetization device do not rotate.

[0008] When testing for defects in ferromagnetic semi-finished products using the DC flux leakage method, specifically with a rotating system, the material to be tested (test item) is placed between two magnetic poles of an electromagnet. The magnetic conductivity of the test material (µM>>1) results in a magnetization in the material that can be between 0.5 T and 2 T, for example. The sensor elements in the test head measure the magnetization in air (µL=1) a few tenths of a millimeter to approximately 2 mm above the surface of the test item. In defective material, the magnetic flux in the material is disrupted and escapes from the material. This changes the magnitude and direction of the magnetization at the location of the test head, allowing the sensor elements to detect a signal.The magnitude of the signal depends on the ratio of the two magnetic conductivities (operating point), the shape and position of the defects (internal or external defects, oblique or longitudinal defects) and the homogeneity of the magnetization in the material.

[0009] A magnetization device has pole shoes that are configured to introduce the magnetizing field into the test material and to extract it from the test material. These are usually interchangeable, dimension-dependent components that allow the testing device to be adapted to different test material diameters. A pole shoe has a pole shoe body made of magnetically conductive, in particular ferromagnetic, material, on which a field exit surface is formed that is adapted to the surface of the test material. The pole shoe is designed such that it can wrap around the test material along an axial length measured parallel to the direction of travel (pole shoe length) and over a wrap angle measured in the circumferential direction of the test material. During testing, an air gap with an air gap thickness measured radially to the pass axis remains between the field exit surface and the surface of the test material.

[0010] DE 10 2014 212 499 A1 of the applicant describes in connection with the Fig. 1 For example, a test device with a rotating subsystem comprising a rotating head with a ring yoke rotating around the test object, which is part of the magnetization device and has pole pieces aligned radially to the test object surface at diametrically opposite locations, to which magnetization windings are attached. The field exit surfaces facing the test object are concavely curved in a circular cylindrical manner, ideally resulting in an air gap thickness that is uniform in the axial and circumferential directions. The test head has a probe array with a row of relatively small magnetic field-sensitive test probes oriented parallel to the direction of travel, which enable high spatial resolution. The application further discloses a special evaluation to enable high-resolution flux leakage tests for defects of various types, with particular attention being paid to difficult-to-detect oblique defects.

[0011] DE 38 41 747 A1 discloses a magnetizing device for non-destructive material testing of workpieces using the magnetic pole method for surface defects, in particular longitudinal defects. The magnetizing device is equipped with one or more current-carrying coils that generate a magnetic field in the workpiece. Similar to a stator or rotor in electric motors or generators, the device comprises a group of components consisting of ferromagnetic pole cores arranged circumferentially or in a star shape relative to their longitudinal axis, as well as coils applied to these coils. These coils encompass an internally located workpiece or are encompassed by an externally located workpiece. This group of components generates a preferably circumferential magnetic field acting on the workpiece surface without contact. TASK AND SOLUTION

[0012] The invention is based on the object of enabling a flux leakage test that offers even better reproducibility of test results, in particular also for oblique defects.

[0013] To achieve this object, the invention provides a pole shoe having the features of claim 1 and a testing device equipped with at least one such pole shoe having the features of claim 6. Advantageous further developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference.

[0014] The inventors have recognized that the geometry of pole pieces known from the prior art has systematic weaknesses that can be particularly detrimental when detecting skew errors. The invention proposes significant deviations from the previous design principles to remedy these deficiencies.

[0015] In particular, it was recognized that the reproducibility of measurements for angular defects can be improved if the magnetic field at the probe position is sufficiently homogenized, or even better than before. This homogenization of the magnetic field, i.e., the greatest possible uniformity of the magnetic flux density in magnitude and direction at the probe position, can be achieved by changing the pole piece geometry. Firstly, the wrap can be varied in the axial direction. Alternatively or additionally, the field exit surface of the probe can be designed such that the air gap thickness varies in the axial direction during testing.

[0016] According to one formulation, this is achieved in a generic pole piece, for example, by the field exit surface of the pole piece having a non-concave-cylindrical shape which deviates from a concave-cylindrical reference surface in such a way that a radial distance of the field exit surface from an axis of curvature of the reference surface varies in the axial direction of the pole piece. This variation must lie outside the small variations that are usually caused by manufacturing tolerances. The axial direction of the pole piece is the direction that runs parallel to the direction of travel or to the test fixture axis when a pole piece is mounted ready for use in a test fixture. This approach can be understood as follows. Conventionally, pole pieces are designed so that the field exit surface has a concave-cylindrical shape.The field exit surface then coincides over its entire surface with a concave-cylindrical reference surface, the axis of curvature of which should coincide as closely as possible with the passage axis or with the test fixture axis of the test fixture or with the central longitudinal axis of the test material passing through. In this conventional configuration, the radial distance of the field exit surface from this axis of curvature is uniform in the axial direction across the entire field exit surface and is greater than the outer radius of the test material by the air gap thickness. The radial distance of the field exit surface from the axis of curvature of the reference surface can also be referred to as the pole shoe radius. When designing pole shoes, this is calculated so that a pole shoe provides sufficiently good magnetization for test material with a specific outer diameter range. The air gap thicknesses are generally in the range of 5 mm to 25 mm.

[0017] It has proven expedient to abandon this conventional method of designing the pole shoe geometry. Instead, the field exit surface of the pole shoe should be given a non-concave-cylindrical shape, which deviates from a concave-cylindrical reference surface in such a way that the radial distance of the field exit surface from an axis of curvature of the reference surface, i.e., the pole shoe radius, varies in the axial direction of the pole shoe. This results in a corresponding air gap variation in the axial direction during testing, i.e., a variation in the air gap thickness depending on the axial position of a location parallel to the scan direction. This can favorably influence the field distribution of the magnetization in order to improve the homogeneity of the magnetization field at the location of the probe for a given probe position compared to conventional solutions.

[0018] Alternatively or additionally, the invention proposes that the wrap angle vary in the axial direction of the pole piece. Here, too, the variation is well outside of manufacturing tolerances.

[0019] Preferably, both measures are combined, so that in preferred embodiments, both an air gap variation or a variation of the pole piece radius and a wrap variation are present. This makes the positive effects less dependent on the speed of the relative rotation and, in the case of pipes, on the wall thickness.

[0020] In some developments, the radial distance of the field exit surface from the curvature axis of the reference surface varies in the axial direction of the test head such that the radial distance is greater in a central region between the axial end regions of the field exit surface than in the axial end regions. Thus, the air gap is larger in the central region (greater air gap thickness) than in the axial end regions.

[0021] Alternatively or additionally, it can be provided that the wrap angle in a central region between the axial end regions of the field exit surface is smaller than in the axial end regions

[0022] Preferably, both measures are combined. If the wrap or wrap angle is greater in the axial end regions than in the central region, a relative increase in magnetization occurs in the axial end regions compared to a pole piece with uniform wrap in the axial direction, which counteracts the observed decrease in magnetization in conventional pole pieces. Varying the air gap by increasing the air gap in the center of the pole piece compared to the axial ends results in a reduction in the magnetic field in the center compared to conventional approaches because the air gap is larger there than at the axial ends. This allows the magnetization in the axial central region to be reduced compared to conventional solutions.Together with the tendency to increase the magnetization at the axial ends due to the larger wrap angles there, the combination of wrap variation and air gap variation results in a significantly better homogeneity of the magnetic field over a longer length of the test material, especially at the location of the test head.

[0023] The extent of the air gap variation or the variation of the pole piece radius and / or the extent of the wrap variation can be optimized based on tests and / or magnetic field calculations or simulations. The inventors have discovered that it can be advantageous and sufficient for most applications if the wrap angle varies between a minimum wrap angle U min and a maximum wrap angle U max and if the wrap variation is greater than 5° and / or less than 35°. The term "wrap variation" (UV) refers to the difference between the maximum wrap angle and the minimum wrap angle, i.e. UV = U max - U min . As a rule, the homogenizing effect of the wrap variation is not sufficient if the wrap variation is significantly below 5°.However, with a wrap variation significantly above 35°, the wrap angle may become too small in the area of ​​minimum wrap, so that dynamic effects can counteract the desired effect.

[0024] For a better understanding, the following should be explained. Simulations by the inventors have shown that, in addition to the static magnetization of the material by two pole pieces, magnetization effects due to induced currents must also be taken into account in a rotating magnetization system. When the magnetic field transfers from a pole piece to the test material with simultaneous relative rotation between the pole piece and the test material, electric currents are induced in areas of the test material with changing magnetic fields. The greater the magnetic field change and the shorter the time in which the magnetic field must change, the stronger the currents. These currents generate magnetic fields that counteract the original magnetic field and can lead to a distortion or a shift in the magnetization. The time the magnetic field has to change in the test material depends on the rotation speed and on the wrap.The rotation speed is a process parameter that can be influenced by appropriate settings during the test. The wrap, on the other hand, is a parameter that must be set at the pole piece. The unwanted counter-effects caused by induced currents mentioned here can be limited if the differences between maximum and minimum wrap are not too large.

[0025] Particularly favorable ranges for varying the air gap and the pole piece radius were also identified through complex simulations. According to these simulations, it appears advantageous if the radial distance of the field exit surface from the reference surface's curvature axis varies between a minimum radius RFA min and a maximum radius RFA max, and if the radius difference ΔRFA = RFA max - RFA min is greater than 2 mm and / or less than 40 mm, and / or if the radius difference is between 5% and 40% of the minimum radius RFA min.

[0026] If these conditions are met, the desired homogenization effects can be achieved in the majority of practically relevant cases without having to fear undesirable side effects, such as insufficient magnetization with an excessively large air gap thickness.

[0027] The radius difference ΔRFA corresponds to the air gap variation during testing.

[0028] According to a further development, a further contribution to good homogenization of the magnetic field at the probe location can be achieved by having the field exit surface have a maximum wrap angle of more than 90°, whereby the maximum wrap angle can preferably be in the range of 100° to 160°. This takes into account the realization that, in principle, it appears beneficial for homogenization to make the wrap angle as large as possible. On the other hand, space is required to accommodate probes. Good design solutions are possible within the specified angle ranges.

[0029] The course of the wrap in the axial direction can be such that the wrap varies continuously with the axial position. However, this is not mandatory. In some cases, there are sections with a constant wrap angle that transition into a section with a smaller wrap angle. Transitions between areas of larger and smaller wrap angles can be continuous or smooth. However, this is not mandatory. The transitions can also be stepped, so that sections of different, possibly constant, wrap angles follow one another. There can be inclined transition areas between sections of different wrap angles. The same applies to the design of the field exit surface with regard to the desired air gap variation.

[0030] The invention also relates to a testing device of the type mentioned above. The magnetization device has pole pieces aligned radially to the test specimen surface for introducing the magnetic field and dissipating it from the test specimen. By using at least one pole piece according to the claimed invention, the homogeneity of the magnetization field at the location of the test head can be improved.

[0031] Preferred testing devices comprise a rotating head that can rotate around the test specimen and supports the magnetization device. This device has pole pieces aligned radially to the test specimen surface at diametrically opposite locations, with field exit surfaces facing the test specimen. The magnetization device preferably has exactly two pole pieces that are diametrically opposite each other, rotationally symmetrical to the traversing axis. The pole pieces can be connected via a ring yoke to create an effective magnetic connection.

[0032] A relative rotation between the magnetization device / test head on the one hand and the test object on the other hand can also be realized in a test device without a rotating head by rotating the test object around its longitudinal center axis.

[0033] To optimize the influence of the relative rotation between the probe / magnetization device and the test piece on the spatial distribution of the magnetization field in the test piece, it has proven advantageous to position the probe not symmetrically between the pole pieces, but asymmetrically in the circumferential direction. An asymmetry angle between an axial section plane perpendicular to a diametrical connecting line of the pole pieces and the probe can, for example, range from 5° to 15°.

[0034] Although a test head can be equipped with a single test probe, preferred test heads comprise multiple test probes, enabling more efficient testing. The test head preferably has a probe array with a plurality of magnetic-field-sensitive test probes arranged next to one another in an axial direction parallel to the scan direction, thereby defining an effective test head length. The effective test head length is also referred to below for simplicity as the "test head length" and corresponds to the length of the magnetic-field-sensitive region of the test head in the axial direction. Distributing many relatively small-area test probes along this length of the test head results in high spatial resolution combined with high testing efficiency, since many adjacent, preferably overlapping, test tracks of the individual test probes are scanned simultaneously.Preferred variants and their evaluations are described, for example, in the above-mentioned DE 10 2014 212 499 A1, the disclosure content of which is incorporated into the description.

[0035] In order to ensure that the magnetic field in the test object is as homogeneous as possible, even in the area of ​​the axial ends of the test head or in the areas scanned by the test probes arranged in these end areas, the pole piece preferably projects beyond the axial ends of the test head at both axial ends, so that there is an axial projection on both sides. The pole piece length should therefore be greater than the test head length. On the other hand, the pole piece should not be too long in the axial direction. Although this could improve the homogeneity of the magnetic field in the area of ​​the test head while keeping the length of the test head unchanged, a lot of energy would be used to magnetize areas that are not being tested. A good compromise has been found to be a ratio L / PKL between the axial pole piece length L and the (effective) test head length PKL in the range of 1.5 to 3.0, preferably below 2.0.

[0036] The invention can be used for testing pipes and also solid round bars. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Further advantages and aspects of the invention emerge from the claims and from the description of embodiments of the invention, which are explained below with reference to the figures. Fig. 1 shows a schematic diagram of a test device for flux leakage testing of ferromagnetic tubes, which is equipped with conventional pole pieces; Fig. 2A bis 2C show different types of errors; Fig. 3 illustrates some geometry parameters for characterizing the test geometry; Fig. 4 shows schematically a pole piece with a cylindrical field exit surface according to the state of the art; Fig. 5A bis 5C illustrate in different ways the field distribution of the tangential field in a pole piece with a cylindrical field exit surface; Fig. 6 bis 9 schematically illustrate some magnetization-influenced problems regarding defect detection; Fig. 10 shows schematically the shape of a pole piece according to an embodiment of the invention from behind; Fig. 11 shows schematically a radial section through two pole shoes arranged in test configuration according to an embodiment of the invention; Fig. 12 shows schematically a pole piece with a non-circular cylindrical field exit surface according to an embodiment of the invention; Fig. 13 bis 15 illustrate how a wrap variation and an air gap variation, both individually and in combination, affect the homogenization (evenness) of the magnetization; Fig. 16A, 16B shows the field distribution of the tangential field on a pole piece according to the state of the art ( Fig 16A ) compared to the field distribution of the tangential field according to an embodiment with wrap variation and air gap variation ( Fig. 16B ). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] Embodiments of the claimed invention are explained below using a testing device 100 for flux leakage testing of ferromagnetic test material 110 in the form of hot-rolled ferromagnetic tubes in a continuous process. In the testing device, a test volume of the test material is magnetized by means of a magnetization device 200 and scanned with the aid of at least one magnetic-field-sensitive test probe (flux leakage probe) housed in a test head 120 to detect magnetic stray fields caused by the defects. A relative movement takes place between the test head or the test probe and the surface 115 of the test material in a scanning direction.

[0039] The testing device 100 is designed for the detection of defects, imperfections, or discontinuities of various types and can, for example, reliably detect rolling defects on both the inner surface 116 (internal defects) and the outer surface 115 (external defects). Both longitudinal defects (defects with a main extension direction parallel to the longitudinal axis of the tube) and transverse defects (defects with a main extension direction in the circumferential direction or perpendicular to the longitudinal axis of the tube) and oblique defects (transverse to the longitudinal direction and the circumferential direction) can be reliably detected and characterized. Fig. 2A bis 2C show schematically different types of errors.

[0040] The test device 100 is designed, among other things, for the test material to pass through the test device 100 parallel to a passage axis 130 during testing, and for the test head 120 and the magnetization device 200 to rotate about the passage axis 130. The passage axis corresponds to the test device axis. This results in helical test tracks. In one embodiment, two subsystems are integrated in a multi-test block. A rotating subsystem is provided for longitudinal defect testing, among other things, the basic principle of which is explained using Fig. 1 is explained. For the transverse flaw detection, a stationary subsystem (not shown) with an annular arrangement of several sensor arrays distributed around the circumference of the arrangement is provided. The subsystems are arranged one behind the other in the direction of flow through the pipe, whereby the sequence can be arbitrary. In other embodiments not shown in detail, a single system may be sufficient, e.g., a single rotating system.

[0041] The rotating subsystem (cf. Fig. 1 ) has a rotating head 240, which, during testing operation, rotates around the throughput axis 130 or a rotating head rotation axis coaxial therewith, which corresponds to the test device axis. The rotating head comprises a magnetization device 200 with an annular yoke 210 rotating around the test specimen 110, which has pole pieces 250 aligned radially to the test specimen surface at diametrically opposite locations. Magnetization windings 255 are attached to the pole pieces. When current is applied to these, a magnetic flux or a magnetic field MF (DC field) is generated inside the test specimen 110, the field lines of which run more or less in the circumferential direction of the test specimen, i.e., perpendicular to the longitudinal direction of the tube.

[0042] On the rotating head 240, probe heads 120 are arranged in the circumferential direction between the pole pieces 250, each containing a probe array, each probe array comprising a plurality of individual probes (test probes). The series of probes extends over a probe length in the axial direction AX or parallel to the passage axis 130. Examples of possible probe arrangements are shown in DE 10 2014 212 499 A1, the disclosure of which is incorporated by reference into the content of this description.

[0043] During the test, the ring yoke 210, together with the pole pieces 250 and the test heads 120, rotates at speeds between approximately 60 and approximately 1200 rpm, depending on the type of probe. The pipe 110 to be tested is simultaneously transported forward in the direction of travel 130 at a test speed (for example, up to 3 m / s or more). The test heads 120 grind against the pipe surface, scanning it continuously along a helical path. The probes of the probe array are arranged within the test heads at a small test distance from the surface 115 of the test material, which can, for example, be in the range of 0.2 mm to 2 mm.

[0044] Because the magnetic field lines FL run essentially in the circumferential direction within the pipe material, this test is particularly sensitive to longitudinal defects LF-A on the outside of the pipe 115 and longitudinal defects LF-I on the inside of the pipe 116, which maximally disturb the magnetic flux in the circumferential direction and thus generate strong stray flux fields (cf. Fig. 2A ). Also oblique defects SF-A, SF-I ( Fig. 2C ) can be detected, albeit with lower sensitivity. Transverse defects QF-A, QF-I running in the circumferential direction (cf. Fig. 2B ), however, are not detected with sufficient signal strength.

[0045] To illustrate the test geometry and some relevant parameters, Fig. 3 a cross-section perpendicular to the passage axis 130 through the plane of the pole shoes 250 with the tube arranged therebetween. The tube is ideally moved coaxially to the passage axis 130 through the test device 100 so that its longitudinal center axis coincides with the passage axis 130. The schematically illustrated pole shoes 250 correspond to the state of the art; such a pole shoe is shown in the oblique perspective of Fig. 4 shown schematically.

[0046] Each of the pole pieces 250 is made of a magnetically conductive, preferably ferromagnetic material and has a pole piece body 252, on which a field exit surface 255 is formed, adapted to the surface of the test piece. This surface has the shape of a rectangular section of the outer surface of a circular cylinder. For magnetization, the field lines FL of the magnetizing magnetic field emerge from the field exit surface, bridge a material-free space or air gap LS, and penetrate the test piece 110 on the opposite side via the outer surface 115.

[0047] The pole shoes 250 are designed rotationally symmetrically to the through-passage axis 130. Each pole shoe wraps around the circular-cylindrical outer surface of the test piece over a wrap angle U, which corresponds to the angle measured in the circumferential direction between the outer edges of the pole shoes as seen in the circumferential direction. The wrap angle here is 90° in each case. Each pole shoe extends in the axial direction (parallel to the through-passage axis) over a certain axial length L (pole shoe length L) (see Fig. 4 ).

[0048] The concave-cylindrical shape of the field exit surface 255 is designed so that the radial distance between the outer surface 115 (outer tube surface) of the test piece and the field exit surface 255 is as constant as possible over the entire wrap angle U and the entire length L of each pole piece. The air-filled space between the field exit surface and the test piece surface is usually referred to as the "air gap" LS. The pole piece geometry is designed for a certain diameter range around a nominal diameter. If the test piece has the nominal diameter, the air gap thickness is largely constant in the circumferential direction. Deviations from the nominal diameter result in a slight variation in the air gap thickness in the circumferential direction, but not in the axial direction.

[0049] The essentially constant air gap thickness is achieved by adapting the shape of the field exit surface to the test material surface and, for this purpose, is concave-cylindrical and coincides with a concave-cylindrical reference surface REF, whose axis of curvature KA coincides with the pass-through axis 130 when the test device is correctly adjusted. The air gap thickness, i.e. the radial thickness of the air gap LS, is thus the difference between the radius of curvature of the field exit surface 255 (corresponding to the radial distance RFA of the field exit surface 255 from the axis of curvature KA of the reference surface REF) and the outer radius RA of the pipe or the test material. The clear width measured through the pass-through axis between the facing field exit surfaces is also referred to as the pole shoe diameter PSD. With correctly adjusted pole shoes, half of the pole shoe diameter PSD thus corresponds to the radius of the reference surface REF.

[0050] Using finite element simulations (FEM simulations), the extent and orientation of the magnetization within the test material in the area magnetized by the pole pieces were investigated. It was shown that the shape of the state-of-the-art pole pieces ( Fig. 3 oder 4 ) results in the tube's magnetization being higher in the axial center between the pole pieces than at the (axial) edges of the pole piece. Furthermore, the magnetic field lines bulge at the axial edges of the pole piece, meaning the magnetic field lines are no longer parallel to each other there. The only positions where the magnetic field lines are parallel are in the center between the pole pieces, but only when the rotational speed between the pole piece and the test piece is zero (no relative rotation).

[0051] For further illustration, the Figuren 5A und 5B Schematic top views of a tube whose longitudinal center axis is oriented in the axial direction of the test fixture. The conditions for the tangential field at the surface of the test material are shown. The tangential field is the component of the magnetic field that runs in the circumferential direction within the test material. For explanation: The actual tangential field in cylindrical coordinates is the magnitude of the component B phi . In order to describe the bulging at the edge with arrows, the component B z is also required, i.e. the arrows represent the angle between B phi and B z.

[0052] Figur 5A shows the course of the tangential field with arrows. The arrows have unit length and indicate the orientation of the tangential field. Figur 5B represents lines of equal flux density of the tangential field, with the flux density being highest within the inner regions. The diagram in Figur 5C schematically depicts the strength of the tangential field TF (in T (Tesla)) and the angle WK between it and the axial direction as a function of the axial location along the probe length. The dotted curve TF shows the magnitude of the tangential field, which is strongest in the center. The solid line represents the angle WK between the circumferential direction and the orientation of the tangential field component. The absolute magnitude of the angle increases from the center to the axial ends, with angular deviations in different directions.

[0053] In general, the magnetic field lines should be as perpendicular to the defect as possible to maximize the defect signal during flux leakage testing. As the angle between the defect orientation and the magnetic field lines decreases, the defect signal also decreases. This is one reason for the generally reduced sensitivity for angular defects. For longitudinal defects, the path of the tangential field, i.e., the field whose field lines run tangentially, is crucial.

[0054] If internal defects are to be detected, it is desirable to ensure a sufficiently high tangential field on the inside of the pipe as well. However, the relative rotation between the test specimen and the magnetization device results in two effects that should be considered. Firstly, the skin effect shifts the tangential field toward the outside of the pipe, and secondly, the maxima of the tangential field on the inside and outside of the pipe shift in opposite directions.

[0055] In order to ensure good detection of internal defects, it has therefore proven advantageous to position the probe asymmetrically between the pole pieces, rather than symmetrically. For illustration, Fig. 3 an axial section plane 215, which is perpendicular to a diametrical connecting line between the pole pieces. The test head can, for example, be arranged offset in the circumferential direction by an asymmetry angle of 5° to 15° relative to this section plane. For illustration, Figur 5B a preferred position of the test head PK is shown (see also Fig. 11 ). In addition, the rotation speed should not be too high.

[0056] Some aspects of the problems of the prior art identified by the inventors are described below with reference to Fig. 6 bis 9 explained. The PK probe has a probe array with a straight row of individual, small-area probe elements or test probes PS, which are evenly distributed over the length of the PKL probe (see Fig. 6 , applies to all Fig. 6 bis 9 ). The test tracks that can be scanned overlap each other, so that the test probes together define an effective test head length, which is referred to as the test head length PKL.

[0057] Both the lower magnetization mentioned above and the field lines in the axial end regions of the pole pieces that are no longer parallel lead to a lower error signal, especially if a defect is detected by a probe element located at one end region of the test head PK.

[0058] Fig. 6 This is illustrated by a schematic diagram that plots the normalized sensitivity S of a sensor, probe element, or test probe PS as a function of the axial position PAX relative to the probe PK or relative to the probe length PKL (measured in the axial direction). The arrows TF represent the orientation of the tangential field (tangential component of the magnetic field at the test specimen surface), and the arrows LF represent the orientation of the longitudinal defect in the inhomogeneous magnetic field. Curve S represents the sensitivity, which varies over the probe length. The error signal therefore depends on the position at which the defect is detected by the probe. The drop in the error signal could be compensated for by adjusting for an adjustment error (dashed line KOMP).

[0059] However, the situation is different with oblique faults SF, i.e. faults that run obliquely to a longitudinal fault and a transverse fault, for example at an angle between 10° and 30° or 40° relative to a longitudinal fault (cf. Fig. 7 ). Due to the tilt, the defects at one end of the probe are now at a different angle to the field lines than at the opposite axial end. This results in different error signals (or different sensitivities) depending on whether a defect is detected at the beginning or at the end of a probe. Because of the tilt of the defect, it is at one end of the probe (in Fig. 7 left) is almost perpendicular to the field lines, meaning that here the defect generates an almost maximum error signal, while at the opposite end, the same oblique defect SF generates only a significantly smaller error signal because it forms a smaller angle with the field lines. By adjusting the angle, the error signal can be increased at both ends of the probe (line COMP). However, adjusting for a longitudinal defect cannot compensate for the signal drop caused by the oblique position. This results in relatively poor error reproducibility.

[0060] Based on these observations, an optimization approach was developed according to which, among other things, the reproducibility for skew defects can be improved if the magnetic field at the probe position is homogenized (better than previously) over the length of the probe. In a homogeneous magnetic field, the angle between the skew defect and the field lines is always the same over the length of the probe, ie, regardless of which end of the probe PK the skew defect SF is detected at. For illustration, Fig. 8 a longitudinal defect LF in a homogeneous magnetic field. With an ideally optimized pole piece geometry, the field lines are parallel across the entire length of the probe, and the longitudinal defect is always at a 90° angle to them, resulting in a consistently maximum error signal.

[0061] Fig. 9 For comparison, this figure illustrates the effect on skew defects (SF) in a homogeneous magnetic field. With an ideally optimized pole piece, the angle between the skew defect (SF) and the field lines (FL) is less than 90°, but constant, across the entire probe length. This results in a smaller, but constant, error signal compared to the longitudinal defect.

[0062] According to the inventors' findings, improved homogenization of the magnetic field in the pipe or in another test object at the position of the test head can be achieved by targeted changes to the pole shoe geometry. Firstly, the thickness of the air gap can be varied by giving the field exit surface of the pole shoe a non-concave-cylindrical shape that deviates from a concave-cylindrical reference surface in such a way that the radial distance of the field exit surface from the curvature axis of the reference surface (corresponding to the scan axis) varies appropriately in the axial direction of the test head. Alternatively or additionally, the wrap can be varied so that the wrap angle becomes a variable function in the axial direction of the pole shoe.

[0063] The test device can otherwise be constructed as in the context of Fig. 1 described, but with asymmetrically arranged test head PK.

[0064] To illustrate a possible design of a pole shoe, Fig. 10 The geometry of a pole piece 250 from behind, i.e., from the radial outside, together with the wrapped tube, which is the test piece 110. The wrap variation can be seen, with a relatively large wrap at the axial edges of the pole piece and a smaller wrap in the middle. Fig. 11 shows a schematic longitudinal section along a radial plane containing the passage axis 130, together with the looped tube and the position of the test head PK. In the radial plane (drawing plane), a diametrical connecting line runs between the pole pieces, perpendicular to which lies the radial section plane 215. This illustration clearly shows the air gap variation, with the air gap LS being larger in the central region between the axial ends than in the axial end regions. Fig. 12 schematically shows an embodiment of a pole piece 250 whose field exit surface 255 is designed using the principles of the claimed invention.

[0065] The wrap varies in the example of Fig. 10 between the minimum wrap U min in the middle section MB and the maximum wrap U max in the two end sections EB. The difference between the minimum value and the maximum value is referred to here as the wrap variation UV, thus: UV = U max - U min . In the example case, the wrap variation UV results from the fact that the longitudinal edges in the middle section MB are set back symmetrically to the central axis by an amount UV1 or UV2 compared to the longitudinal edges in the end sections EB, where UV = UV1 + UV2.In the example, the wrap in the two end regions EB is constant over a certain axial length (for example, between 10% and 20% of the total length L of the pole piece), then decreases continuously towards the center along a transition region and is then constant again in the middle region MB, whereby the axial length of the middle region MB with the minimum wrap angle is between 40% and 60% of the total length L of the pole piece in the axial direction. The shape of the pole piece 250 is mirror-symmetrical with respect to a plane of symmetry SE running parallel to the axial direction. Instead of the inclined transition region, steps can also be provided, or a continuous transition without corners or steps between the region of maximum circumferential angle and the region of minimum circumferential angle. The pole piece shape does not have to be symmetrical. For example, UV1 and UV2 can be unequal, at least in sections.

[0066] In the example, the PK probe is positioned centrally between the axial ends and extends across the entire central region of minimum wrap, up to the respective transition sections, but not into the end sections. It is also possible for the axial ends of the probe to extend into the end sections, but preferably not beyond the axial ends of the test shoe.

[0067] Preferably, the pole piece extends beyond the axial ends of the probe at both axial ends, creating an axial projection on both sides. This contributes to homogenization in the area of ​​the axial ends of the probe. Generally, the greater the projection, the better the homogeneity. However, the pole piece length L should not be greater than necessary, as this would, among other things, increase the energy required for magnetization and could lead to space constraints. In preferred embodiments, the ratio between the pole piece length L and the probe piece length PKS is in the range of 1.5 to 3.0, preferably below 2.5.

[0068] The extent of the wrap variation UV should be significantly above typical manufacturing tolerances, for example, 5° or more. On the other hand, the wrap variation should also be moderate, for example, no more than 35°, to ensure that secondary effects do not become disruptive.

[0069] Both the maximum and minimum values ​​of the wrap angle are significantly more than 90°. The maximum value can, for example, be in the range of 100° to 160°.

[0070] The axial variation of the air gap LS should also be well above the manufacturing tolerances and should be neither too large nor too small. In the test configuration of Fig. 11 An air gap LS exists between the field exit surface 255 of a pole piece 250 and the test piece surface 115, the thickness of which, measured in the radial direction, varies between a minimum value LS min in the two axial end regions EB and a maximum value LS max in the central region MB. The difference between the extreme values ​​results in an air gap variation LV = LS max - LS min .

[0071] In the two axial end regions EB, the field exit surface 255 is circularly cylindrically curved and coincides with a circular-cylindrical reference surface whose axis of curvature KA coincides with the direction of travel 130, whereby the radius (radius) of this reference surface has the value RFA min. In the central region MB, the field exit surface is also circularly cylindrically curved over a certain length. It coincides with a circular-cylindrical reference surface whose radius (radius of curvature) is RFA max. Between the end sections and the relatively long central region MB, there are transition sections in the form of frustoconical inclined surfaces whose radius continuously increases or decreases in the axial direction.

[0072] The field exit surface 255 is thus rotationally symmetrical with respect to the curvature axis KA of the individual sections MB and EB, as well as the transition sections, in the sense that the field exit surface can be described as a section of a rotationally symmetric surface. However, the field exit surface has a shape that deviates significantly from the circular cylindrical shape of state-of-the-art pole pieces.

[0073] The geometry of the non-circular cylindrical shape of the field exit surface 255 can also be described analogously to a cylindricity error. The end regions with the minimum radius of curvature RFA min define an inner cylinder in such a way that they can be described as partial surfaces of an inner cylinder. The central region with the maximum radius of curvature RFA max defines an outer cylinder in such a way that the sections with the maximum radius of curvature RFA max lie on this outer cylinder. The course of the field exit surface thus extends from the inner cylinder to the outer cylinder and has sections that lie between these limiting cylinders. The shape deviation from a circular cylindrical surface can be quantified by the radius difference between the two radii RFA max and RFA min. Relative to the minimum radius RFA min, the air gap variation (or the radius difference) should neither be too small nor too large.Preferably, the air gap variation is between 5% and 40% of the minimum radius RFA min .

[0074] In Fig. 11 It is also clearly visible that the probe PK, which extends in the axial direction over a probe length PKL, is not arranged symmetrically in the circumferential direction between the diametrically opposed pole pieces, but is offset by an asymmetry angle AW relative to the axial section plane 215. This counteracts dynamic effects resulting from the relative rotations between the magnetization device or the probe and the test object during the test. It has been shown that asymmetry angles AW in the range of 5° to 15° bring particularly significant improvements in test sensitivity, especially for oblique defects and / or in the detection of internal defects.

[0075] Based on the Fig. 13, 14 und 15 It is explained how the measures of wrap variation and air gap variation, both individually and in combination, affect the homogenization of the magnetic field generated by the pole piece. The dependence of the magnetization M on the axial position PAX in the area of ​​the test head PK is shown. The solid line ZYL shows the magnetization curve for a conventional pole piece with a circular cylindrical field exit surface, the dashed line the corresponding curve for a pole piece according to Fig. 10 bis 12 .

[0076] Fig. 13 illustrates the effect of wrap variation UV. Increasing the wrap in the axial end regions compared to the central section and / or reducing the wrap in the central region relative to the axial end regions leads to a relative increase in magnetization in the end regions while leaving the magnetization in the central region essentially unchanged, so that the non-uniformity of the magnetization can be reduced over the axial length.

[0077] Fig. 14 illustrates that by increasing the air gap in the center region relative to the axial end regions, the magnetization in the center region decreases relative to the end regions compared to a conventional pole piece with a circular cylindrical field exit surface. It is evident that even the air gap variation LV alone leads to a homogenization of the magnetization in the axial direction.

[0078] Fig. 15 schematically illustrates the effect of the combinations of wrap variations and air gap variations. While the magnetization in the central region MB is reduced by increasing the air gap, the magnetization in the end regions EB is relatively increased due to the increase in the wrap angle there, so that an essentially uniform magnetization results over the entire length of the test head PK.

[0079] The effect can be particularly clearly seen by observing the course of the orientation of the tangential field. Fig. 16A shows the conditions described in the introduction for a circular cylindrical field exit surface (ZYL) according to the state of the art, in particular the bulges of the field lines in the axial end areas of the pole piece. Fig. 16BIn comparison, this shows the field distribution for a pole piece N-ZYL with a non-circular cylindrical field exit surface according to an embodiment of the claimed invention. It can be seen that the area of ​​homogeneous magnetization in the center has significantly increased toward the axial edges, and that the bulging at the axial edges is less pronounced than in the prior art. This demonstrates the effectiveness of the measures according to the invention.

Claims

1. Pole shoe (250) for use on a magnetizing device (200) for magnetizing a test volume of a test material (110) in a testing apparatus (100) for leakage flux testing of ferromagnetic test material with a substantially circular-cylindrical surface (115) for determining defects, in particular for testing ferromagnetic pipes, wherein the testing apparatus comprises a testing head (120, PK) with at least one testing probe (PS) for scanning the surface of the test material for detecting magnetic leakage fields caused by defects and the magnetizing device (200) is designed to magnetize the test material in the region of the testing head (120, PK), wherein the testing apparatus is designed for the testing apparatus and the test material to perform a relative movement parallel to a passage axis (130) in a test mode; wherein the pole shoe (250) has a pole shoe body (252) which consists of magnetically permeable material and on which a field exit surface (255) adapted to the surface (115) of the test material (110) is formed in such a way that, in a test configuration, the pole shoe (250) wraps around the test material (110) along an axial length (L) of the pole shoe measured parallel to the passage direction (130) over a wraparound angle (U) and an air gap (LS) remains between the field exit surface (255) and the surface of the test material, characterized in that the field exit surface (255) of the pole shoe (250) can be described as a detail of a rotationally symmetrical surface and has a non-concave cylindrical shape which differs from a concave cylindrical reference area (REF) in such a way that a radial distance (RFA) of the field exit surface from a curvature axis (KA) of the reference surface varies in the axial direction of the pole shoe, wherein the radial distance (RFA) of the field exit surface (255) from the curvature axis (KA) of the reference surface varies in the axial direction of the testing head (120, PK) in such a way that the radial distance (RFA) in a central region (MB) between axial end regions (EB) of the field exit surface is greater than in the axial end regions, and / or in that the wraparound angle (U) varies in the axial direction of the pole shoe, wherein the wraparound angle (U) in a central region (MB) between axial end regions (EB) of the field exit surface (255) is smaller than in the axial end regions (EB), wherein, in the two axial end regions (EB), the field exit surface (255) is curved in a circular-cylindrical manner and coincides with the circular-cylindrical reference surface in each case.

2. Pole shoe according to Claim 1, characterized in that that the wraparound angle (U) varies between a minimum wraparound angle Umin and a maximum wraparound angle Umax and a wraparound variation UV = Umax - Umin is greater than 5° and / or less than 35°.

3. Pole shoe according to either of the preceding claims, characterized in that the radial distance RFA of the field exit surface (255) from the curvature axis (KA) of the reference surface varies between a minimum distance RFAmin and a maximum distance RFAmax and a radii difference ΔRFA = RFAmax - RFAmin is greater than 2 mm and / or less than 40 mm and / or in that the radii difference ΔRFA is between 5% and 40% of the minimum radius RFAmin.

4. Pole shoe according to any of the preceding claims, characterized in that the field exit surface (255) has a maximum wraparound angle Umax of more than 90°, wherein the maximum wraparound angle Umax is preferably in the range of from 100° to 160°, in particular in the range of from 115° to 140°.

5. Use of a pole shoe according to any of the preceding claims in a magnetizing device of a testing apparatus (100) for leakage flux testing of ferromagnetic test material (110) with a substantially circular-cylindrical surface (115), in particular of ferromagnetic pipes, for determining defects, wherein the testing apparatus is designed for the testing apparatus (100) and the test material (110) to perform a relative movement parallel to a passage axis (130) in a test mode, the testing apparatus comprising: at least one testing head (120, PK) with at least one testing probe (PS) for scanning the surface of the test material (110) for detecting magnetic leakage fields caused by defects; the magnetizing device (200) for magnetizing a test volume of the test material (110) in the region of the testing head (120, PK), wherein the magnetizing device (200) has pole shoes (250) oriented radially with respect to the test material surface, wherein the pole shoe (250) has a pole shoe body (252) which consists of magnetically permeable material and on which a field exit surface (255) adapted to the surface of the test material (110) is formed in such a way that, in a test configuration, the pole shoe (250) wraps around the test material (110) along an axial length (L) measured parallel to the passage direction (130) over a wraparound angle (U) measured in the circumferential direction of the test material and an air gap (LS) remains between the field exit surface (255) and the surface of the test material.

6. Use according to Claim 5, characterized by a rotating head (240) which can be rotated around the test material (110) and has a magnetizing device (200) which has, at diametrically opposite points, pole shoes (250) which are oriented radially with respect to the surface (115) of the test material and have field exit surfaces (255) facing the test material (110).

7. Use according to Claim 5 or 6, characterized in that the pole shoes (250) are diametrically opposite to each other rotationally symmetrically with respect to the passage axis (130) and the testing head (PK) is arranged asymmetrically between the pole shoes (250) in the circumferential direction, wherein preferably an asymmetry angle (AW) measured in the circumferential direction between an axial sectional plane (215) situated perpendicular to a diametric connecting line of the pole shoes (250) and the testing head (PK) is in the range of from 5° to 15°.

8. Use according to Claim 5, 6 or 7, characterized in that the testing head (PK) has a probe array (SA) with a plurality of magnetic field-sensitive testing probes (PS), which are arranged side by side in an axial direction parallel to the passage direction (130) and define an effective testing head length (PKL).

9. Use according to any of Claims 5 to 8, characterized in that the pole shoe (250) extends beyond the axial ends of the testing head (PK) at both axial ends, so that there is a two-sided axial projection, wherein preferably a ratio between the axial pole shoe length (L) and the testing head length (PKL) is in the range of from 1.5 to 3.0, preferably below 2.5.