Abstract:
A device is disclosed for material testing on a test object having at least electrically conducting and ferromagnetic material parts. The test object has at least one technical surface on which at least one electromagnetic ultrasonic transducer (EMUS) is rolled. The at least one transducer includes at least one permanent magnet or an electromagnet and at least one eddy current coil. The at least one eddy current coil has at least one electrical strip conductor which is disposed at or parallel to a surface area of a rolling member which can be rolled on the technical surface of the test object, with the surface area rolling along with the rolling member during rolling.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for testing material on a test object having at least electrically conducting and ferromagnetic material parts, the test object having at least one technical surface with at least one electromagnetic ultrasonic transducer (EMUS) provided with at least one magnet and at least one eddy current coil. 
     2. Description of the Prior Art 
     Electromagnetic ultrasonic transducers are used in a known manner for the purpose of non-destructive material testing and measurement of test objects comprising electrically conducting materials which moreover possess ferromagnetic properties. 
     Basically electromagnetic ultrasonic transducers can be differentiated into two types: on the one hand, those with which produce so-called horizontally polarized shear waves which are able to propagate inside the test object predominantly parallel to the coupling-in surface; and on the other hand, ultrasonic transducers for generating in the test object so-called freely propagating ultrasonic waves preferably propagating inside the test object perpendicular to the coupling-in surface. In both instances, excitation of ultrasonic waves inside a test object results from the occurrence of magnetostriction and Lorenz forces inside the test object material, which can be generated by the presence of a temporally largely constant magnetic field overlapping with an electromagnetic alternating field generated by an electro-magnetic alternating current. 
     A typical setup for exciting ultrasonic waves according to the so-called EMUS principle is shown in  FIGS. 8   a  and  8   b . Common EMUS transducers  3  comprise a permanent magnet  1  and an eddy current coil  2 , which are designed as one unit for joint handling. Usually the eddy current coil  2  is designed as a rectangular flat coil or a spiral flat coil each of which have an electrically conductive strip and is attached to a magnetic pole side of the permanent magnet  1  in such a manner that a permanent magnetic field passes vertically through the coil  2 . If the aforementioned EMUS transducer  3  is placed on an electrically conducting ferromagnetic test object  4 , the permanent magnetic field overlaps inside the test object with an eddy current field generated by the eddy current coil, on the one hand, generating magnetostrictive effects due to the overlapping of the magnetic field components of the eddy current field with the permanent magnetic field entering vertically through the surface of the test object and, on the other hand generating the Lorenz forces due to the eddy currents induced in the test object, which then generate pressure waves occurring normally in relation to the surface of the test object as well as radially polarized shear waves capable of propagating as ultrasonic waves inside the test object. Both types of ultrasonic waves, that is the ultrasonic waves propagating normally in relation to the surface of the test object and ultrasonic waves propagating in longitudinal direction to the surface of the test object due to radially polarized shear waves are suited according to the state of the art for testing faults, for example detecting cracks inside the test object, as well as for measuring the thickness of the wall of the test object. 
     Since in use eddy current coils are very sensitive to outside mechanical influences, the eddy current coils must principally be protected against mechanical wear, which is difficult in particular due to the fact that in ferromagnetic test objects the eddy current coil located between the permanent magnet and the test object is pressed onto the surface of the test object by the magnetic forces of attraction and is therefore subject to considerable fretting. 
     In this context, German Patent 35 11 076 A1 describes a test pig for electromagnetic testing of the walls of steel pipes, such as, for example as part of nondestructive testing of wall weaknesses due to rusting of the pipe walls. A pig, which is described in detail therein, is provided with electromagnets, which are distributed uniformly around the circumference, each comprising two measuring heads which are axially aligned to each other, a yoke connecting the measuring heads and a magnetizing coil on the measuring heads, with the field of each electromagnet running parallel to the center axis of the pipe. For ultrasonic measurement, an eddy current coil, to which are applied strong and very rapidly rising current pulses, is disposed directly at least on one of the poles, and the measuring heads. The pipes of pipelines are provided with circumferential seams at the adjoining parts of two adjacent pipe pieces. When the above briefly described test pig runs over the seams during continuous inspection, the circumferential seams subject the electromagnetic transducer to impacts which, moreover, are markedly intensified by the magnetic forces prevailing between the electromagnets and the wall of the pipes. The previously described fretting and the additional impacts to the electromagnetic ultrasonic transducer, in particular to the eddy current coil, lead to a short lifetime of the EMUS transducer, which needs to be addressed. 
     Although fretting can be reduced by decreasing the magnetic forces of attraction prevailing between the EMUS transducer and the to-be-inspected test object, for example by decreasing the magnetic field induction, this measurement would also immediately lead to distinctly diminishing the EMUS transducer&#39;s efficiency, that is force density induced to generate ultrasound inside the test object reduces in the same way, due to which the detection sensitivity in receiving scattered or reflected ultrasonic waves diminishes to the same extent. 
     Japanese Patent 111 33 003 describes a device for inspecting material using ultrasound which is suited in particular for inspecting the material of pipes. According to claim  4  therein, the device comprises single permanent magnets which are arranged to form a ring of segments with an outer and an inner circumferential edge. The adjacent permanent magnets have opposite magnetic poles at the outer and inner circumferential edge. Disposed in windings on the outer circumferential edge of this ring is an electrical strip conductor of at least one eddy current coil. The device is introduced in operation into a pipe that the outer circumferential edge with the strip conductors slides along the inner wall of the pipe, leading to corresponding fretting on the strip conductors. 
     U.S. Pat. No. 4,898,034 describes a device for testing hot materials, such as metals and ceramics, using ultrasound. An embodiment uses an agent made of zircon which is in contact with the hot material to be examined. Furthermore, a coupling medium (borax) is in contact with the hot material and the zircon agent. The zircon agent and the coupling medium receives ultrasonic waves propagating from the hot material through the coupling medium and the zircon agent. In the embodiment shown in FIG. 1 of U.S. Pat. No. 4,898,034, the zircon agent is designed as a ring with an outer and an inner circumferential edge. In operation, the outer circumferential edge of the ring is rolled over the hot material to be examined. A lever, which is attached to the rotational axis of the zircon ring, holds the ultrasound transmitter constantly as shown in downward perpendicular position. In this manner the ultrasound transmitter including the eddy current coil attached to it is pressed against the inner circumferential edge of the ring, leading once again to fretting of the ultrasound transmitter. 
     SUMMARY OF THE INVENTION 
     The present invention is a device for material testing of a test object having at least electrically conducting and ferromagnetic material parts based on electromagnetic ultrasonic excitation and using an electromagnetic ultrasonic transducer array (EMUS) so that eddy current coils required for generating eddy currents are not subject to any or minimum fretting. Furthermore, conducting material testing on the test object continuously is possible. 
     Contrary to the usual electromagnetic ultrasonic transducer arrays which are provided with permanent magnets or electromagnets and at least one eddy current coil and in which the eddy current coil is moved in a sliding manner in order to inspect the material at the surface of a test object and therefore are subject to slip friction wear, the electromagnetic ultrasonic transducer according to the present invention provides a new eddy current coil design which is combined with a rolling member which is rolled over the surface of a test object. The electromagnetic ultrasonic transducer, hereinafter EMUS transducer, according to the present invention is subject to less wear compared to standard versions. The rolling friction forces occurring in the EMUS transducer according to the present invention are substantially less than the slip friction forces which considerably increases the lifetime of the EMUS transducer according to the present invention. 
     If a prior art EMUS transducer is moved by slipping over an uneven surface of a test object in a slipping process, the eddy current coil therein is subject to increased wear due to the unevenness of the surface of the test object, such as, for example, due to bulging at the welding seams. With the EMUS transducer according to the present invention, surface unevenness is simply rolled over without lasting impairment of the eddy current coil. 
     Another advantage of the EMUS transducer according to the present invention is conducting material inspection continuously as will be described in detail in the following. 
     Thus a device for testing material on a test object which comprises at least electrically conducting and ferromagnetic material parts and which possesses at least one technical surface having an electromagnetic ultrasonic transducer provided with a magnet which is permanent or an electromagnet and at least one eddy current coil according to the invention includes at least one eddy current coil having at least one electrical strip conductor which is disposed at or parallel to a surface area of a rolling member which is disposed on the technical surface of the test object which can be rolled over. 
     In a particularly preferred embodiment, the rolling member, which preferably is a disk, reel, wheel or ball, is combined with the permanent magnet or electromagnet in such a manner that the rolling member, the permanent magnet or electromagnet as well as the at least one eddy current coil which is attached on the rolling member or connected to the rolling member, is moved uniformly in relation to the test object. 
     Another preferred embodiment provides for separate handling of the at least one permanent magnet or electromagnet and the combination of rolling member and eddy current coil. Further details to the preferred embodiments are described in the following with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is made more apparent in the following by way of example using preferred embodiments with reference to the accompanying drawings without the intention of limiting the scope or spirit of the invention. 
         FIG. 1  shows a schematic representation of EMUS transducers having a permanent magnet and an eddy current coil with elliptically shaped strip conductor loops, attached to the circumferential edge of a rolling member; 
         FIG. 2  shows a schematic representation of an EMUS transducer having a permanent magnet and an eddy current coil with a strip conductor windings at the circumferential edge of a rolling member; 
         FIG. 3  shows a schematic representation of an EMUS transducer having a permanent magnet and two ferromagnetic return paths; 
         FIGS. 4 and 5  show a view of an EMUS transducer having two permanent magnets and an eddy current coil; 
         FIGS. 6 and 7  show a representation of an EMUS transducer having an electromagnet and a separate eddy current coil; and 
         FIGS. 8   a  and  b  show a prior art EMUS transducer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The left representation in  FIG. 1  shows a front view and the right representation shows a lateral view, of the EMUS transducer according to the present invention, which due to its principle of construction is also referred to as an EMUS wheel. The EMUS transducer is provided with a rolling member  5  which in the preferred embodiment is designed to be a ring or reel which is hollow inside and has an outer circumferential edge  51 . The rolling member  5  has a center axis of rotation A about which the rolling member  5  rolls relative to the technical surface  6  of the test object  4 . An eddy current coil  2  is wound along the circumferential edge  51  of the rolling member  5  as shown in the left representation. The eddy current coil  2  comprises a through-going electrical conductor including elliptical strip conductor loops  52  which are wrapped along the circumferential edge  51  of the rolling member  5  in such a manner that the entire circumferential edge  51  of rolling member  5  is covered by the loops  52 . It is obvious that when the current is applied to the strip conductors  52 , two immediately adjacent strip conductor loops  52  have current flowing in opposite directions. The alternative strip conductors  52 ′ are wound on the circumferential so that two strip conductors wound immediately adjacent to each other extend in the same direction. The strip conductor  52  and  52 ′ are each suited for effectively coupling in ultrasonic waves into the test object  4 . 
     Each EMUS transducer shown in  FIG. 1  is provided with a permanent magnet  7  to introduce a temporally constant magnetic field into the test object. The permanent magnet  7  is attached to the axis of rotation A in such an asymmetrical manner that a magnetic pole, preferably the magnetic north pole N is disposed maximally close to the circumferential edge  51  of the rolling member  5 . When the rolling member rolls along the technical surface  6  of the object  4 , the magnetic north pole N of the permanent magnet  7  is drawn to the ferromagnetic test object  4  and, due to its rotational mobility, about the axis of rotation A always stays facing the test object  4 , so that the magnetic north pole is always directed downward. Thus the permanent magnet  7  generates a magnetic field whose magnetic field lines are always oriented perpendicular to the technical surface  6  of the test object  4 . 
     If the eddy current coil  2  is fed with pulsed current, eddy currents are induced in the test object which interact with the magnetic flow oriented normally to the technical surface  6 . Ultrasonic waves with circular polarization are generated in test object  4  by developing Lorenz forces. The ultrasonic waves propagate essentially perpendicular to the technical surface  6  inside the test object  4 . 
     The eddy current coil  2  also functions as a reception coil for the ultrasonic waves reflected back inside the test object  4 . 
     As an alternative to the strip conductors of the eddy current coil  2  depicted in  FIG. 1 ,  FIG. 2  shows a variant of the EMUS transducer in which the eddy current coil  2  has electrical windings  53  which are each disposed around the circumferential edge  51  of the rolling member  5 . The design of the strip conductors  53  of the current coil  2  is shown in the left representation of  FIG. 2 . Due to the alternative embodiment of the strip conductors  53  according to the preferred embodiment in  FIG. 2 , ultrasonic waves with linear polarization are generated in the test object  4 . The ultrasonic waves however are due to the same excitation principle by Lorenz forces occurring as in the preferred embodiment according to  FIG. 1 . 
     In both preceding embodiments of  FIGS. 1 and 2 , the rolling member  5  is preferably not a metallic material. The rolling member  5  can, of course, also be made of a ferromagnetic and electrically conductive material. In this case, however, care must be taken that the strip conductors  52  or  53  of the eddy current coil  2  are electrically insulated from the rolling member  5 . It is also expedient, for further reduction of the rolling friction occurring between the rolling member  5  and the technical surface  6 , to provide a protective coat (not depicted) to protect the eddy current coils  52  or  53 . 
     In contrast to the preceding preferred embodiments of  FIGS. 1 and 2  in which a temporally constant magnetic field is oriented perpendicular to the technical surface  6  of the test object  4  and is coupled into the test object  4 , the preferred embodiment of an EMUS transducer designed according to the invention depicted in  FIG. 3  causes a magnetic field to be coupled into the technical surface  6 , which is oriented tangentially to the technical surface of test object  4 .  FIG. 3  shows again in the left representation, a front view and in the right representation, a lateral view of the EMUS transducer  3 . In the preferred embodiment, the strip conductors of the eddy current coil  2  are wound around the surface of a cylindrical or rod-shaped permanent magnet  7 . Attached at the opposite N and S magnetic poles of the permanent magnet  7  are two disk rolling members  5  composed of ferromagnetic material, which is preferably a ferrosteel and which project radially outward from the axis of rotation of the permanent magnet  7  including the eddy current  2 . The disk rolling members  5  each act as a yoke which conducts the magnetic field lines so that a magnetic circuit including the ferromagnetic rolling members  5  and the test object is closed. Due to the magnetic return path, a magnetic field is coupled tangentially to the technical surface  6  inside test object  4 . The eddy currents excited by the eddy current coils  2  generate inside the test object  4  a secondary alternating magnetic field which overlaps with the constant magnetic field of the permanent magnet  7 . The ultrasonic waves are excited by the developing magnetostrictive effect and, like in the case of the embodiment according to  FIG. 2 , have a linear polarization. The disk rolling members  5 , which enclose the permanent magnet  7  on both sides, have two functions. On the one hand the rolling members  5  act as a magnetic yoke and on the other hand they permit the ultrasonic transducers to roll over the technical surface  6  of the test object  4 , with the eddy current coils  2  always assume a constant distance from the technical surface  6 , due to which the strip conductors are subject to no mechanical wear from rolling friction. 
       FIGS. 4 and 5  show two further preferred embodiments of an EMUS transducer  3  according to the invention. These embodiments are provided with two permanent magnets  7  and  7 ′ and an eddy current coil  2 . The only difference in the designs of the two embodiments is in the eddy current coils  2 . The permanent magnets  7  and  7 ′ are attached with their opposing magnetic north poles N to the ferromagnetic rolling member  5 , which preferably is a ring or a wheel. Due to the opposite magnetic north poles N, a displacement of the magnetic field lines occurs in such a manner that they are coupled, via the ferromagnetic ring unit rolling member  5 , perpendicular to the technical surface  6  of the test object  4 . The ferromagnetic rolling member  5  acts simultaneously as a concentrator of the magnetic field by which the magnetic field at the contact points between the rolling member  5  and the technical surface  6  is coupled into the test object  4  in a concentrated manner. Moreover, the ultrasonic-wave excitation principle is the same as in the preferred embodiments in  FIGS. 1 and 2 . 
     In order to improve closure of the magnetic circuit in the preferred embodiments shown in  FIGS. 4 and 5 , a ferromagnetic end piece may be provided on the front magnetic south poles, which like the rolling member  5  comes into contact with the technical surface  6  of the test object  4 . 
     In some material testing applications using permanent magnets can be obviated, as for example material with testing on sheet metals. In this case electromagnets are preferable.  FIGS. 6 and 7  show preferred embodiments each with separate arrangement between the electromagnet  7  and the eddy current coils  2 . The yoke-shaped electromagnet array  7  has two magnetic poles N and S which each can be placed on the technical surface  6  of the test object  4  to feed a tangential magnetic field. Provided in the area of the tangential magnetic field is a rolling member  5  at whose circumferential edge an eddy current coils  2  are provided. In the example of the  FIG. 6 , the rolling member  5  is located on a top side of the test object facing away from the electromagnet array  7 . In the example according to  FIG. 7 , both the electromagnet array  7  and the rolling member  5  are located on a common technical surface  6  of the test object  4 . The excitation principle of the ultrasonic waves inside the test object  4  is identical to that according to the preferred embodiment in  FIG. 3 . The tangentially running magnetic field which is fed by the electromagnet  7  into the test object  4  interacts with the eddy currents and the alternating magnetic field in such a manner that, due to the occurrence of magnetostrictive effects, linear polarized ultrasonic waves are generated. Of course, eddy current coils  2 , designed as rolling members  5 , can be provided in the area of the tangential magnetic field. As in the preferred embodiments shown in  FIGS. 6 and 7 , since no magnetic attraction forces act between the rolling member  5  and the technical surface  6  of the test object  4 , wear of the EMUS transducer is minimal. 
     Rolling the rolling member  5  along the circumferential edge on which the eddy current coils are disposed uniformly allows conducting continuous inspection in contrast to the hitherto used locally discrete EMUS testing arrangements. The invention, also referred to as EMUS wheel, is fundamentally suitable for an application to different fields such as for measuring the wall thickness and fault inspection of sheet metals, rails, pipes and pipelines as well as railroad wheels, oil containers or the outer walls of ships and other security containers. The EMUS transducer can also be combined with transport systems, for example so-called pig systems used in long-distant pipelines and the like to perform inspection. 
     LIST OF REFERENCES 
     
         
           1  permanent magnet 
           2  eddy current coil 
           3  EMUS transducer 
           4  test object 
           5  rolling member 
           6  technical surface 
           7  permanent magnet