Abstract:
An array of magnets designed of flexible components and materials can be easily shaped to fit to the contour of various curved surfaces and structures. EMATs that incorporate these magnets, in addition to being flexible, may be smaller in volume than the conventional EMAT magnets and therefore easier to apply to complex structures where access may be restricted. Also, flexible multiple-pole magnet arrays can be easily and economically fabricated in various shapes and configurations, thereby increasing versatility, utility and cost effectiveness in comparison to the rigid, conventional magnet designs.

Description:
This application claims the benefit of the filing date of U.S. Provisional Application for Patent Ser. No. 60/590,636 filed Jul. 23, 2004, which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   Electromagnetic acoustic transducers (EMATs) comprising flexible magnets that conform to the surface of the object to which they are applied, thereby providing superior performance at reduced cost of fabrication compared to conventional EMAT designs that are composed of rigid and expensive components. 
   BACKGROUND 
   Electromagnetic acoustic transducers (EMATs) are electrical devices that can transmit and receive sound waves in electrically conducting materials without requiring contact with the material. Since sound waves reflect from defects such as cracks and voids, EMATs are typically used as inspection devices. The characteristics of the sound waves transmitted from and received by EMATs, including frequency, intensity, mode and beam shape are determined primarily by the EMAT design and electrical excitation of the EMAT components. 
   EMATs offer several advantages when compared to piezoelectric transducers. EMATs do not require any fluid coupling, unlike piezoelectric transducers in which the sound is produced in the probe and transferred to the material through a coupling medium such as oil or water. EMATs can inspect at greater speeds and therefore provide greater throughput when they are used in automated inspection systems. Since EMATs generate sound waves immediately below the surface of the material being tested, they provide greater accuracy, reliability and repeatability for applications in which the material is contaminated, rough, heated to elevated temperatures or moving at high speeds. Since fabrication of EMATs can be very precise, the EMAT or its components can be interchanged with minimal variation in characteristics or performance. The simple construction of EMATs provides a nearly unlimited variety of designs to facilitate shaping, steering and focusing beams to achieve the desired acoustic effects. 
   EMATs are typically composed of two fundamental components: magnets and coils of insulated electrical conductors. Either permanent magnets or electromagnets (magnets) are used to produce magnetic fields that penetrate the surface of the material component being tested. Coils composed of electrical conductors, commonly referred to as RF coils, are placed between the magnets and the test material. These RF coils are used to induce high frequency magnetic fields in the test material. Interaction between the fields from the magnet and the fields from the RF coils produce forces within the atomic or molecular lattice of the test material. The forces vary in intensity and direction with time at frequencies equal to those of the current in the RF coils. The oscillating forces produce acoustic or sound waves that normally propagate within the test material and away from the EMAT in two opposing directions. 
   Illustrated in  FIG. 1  is an EMAT configuration that is used to generate vertically polarized shear (SV) waves, Lamb waves and surface waves, which are also referred to as Raleigh waves. A magnet  1  produces a magnetic field  2  perpendicular to the metal part under test, or the test material  3 . A meander radio frequency (RF) coil  4  illustrated by but not limited to a meander coil composed of insulated electrical conductors is energized by an alternating power source  5 , and results in alternating current  6  which flows in the RF coil  4  between its terminals. The alternating current  6  produces alternating fields  7 , which encircle the eddy currents  8  and penetrate the surface of the test material  3 . The penetrating alternating fields  7  induce alternating eddy currents  8  in and near the surface of the test material  3 . Alternating magnetic fields  9 , which encircle the eddy currents  8 , are also generated in the test material  3 . The alternating fields  7  from the eddy currents  8  interact with the alternating magnetic fields  9  from the magnet  1  to produce Lorentz forces  10 , in the test material  3  and under each RF coil  4 . These Lorentz forces  10  result in sound waves, such as horizontally polarized shear waves, which are ultrasonic acoustic or sound waves commonly known in the art as SH waves  11 , which propagate from the EMAT in opposite directions in the test material  3 . 
   Illustrated in  FIG. 2  is an EMAT which uses a magnet array  12  such as an array of permanent magnets and an encircling RF coil  4  to generate SH waves  11 . Part of the RF coil  4  is under the magnet array  12 , and also in close proximity to the test material  3 . When an alternating power source  5  is applied to the RF coil  4 , eddy currents  8  and the associated alternating magnetic fields  9  are induced in the test material  3 . Interaction of the magnetic fields  2  from the magnet array  12  and the alternating fields  7  from the eddy currents  8  produce Lorentz forces  10  in the test material  3 , which are near the surface and also parallel to the surface of the test material  3 . These Lorentz forces  10  result in SH waves  11  that propagate in opposite directions in the test material  3 . 
   Illustrated in  FIG. 3  is an EMAT, which uses a magnet  1  such as an electromagnet and RF coils  4  to produce SH waves  11  in some ferromagnetic materials  14  that exhibit the property of magnetostriction. A magnet coil  13  composed of insulated, electrical conductors is wound around a core of ferromagnetic material  14 . When the magnet coil  13  is excited by electrical power source  15 , a transient current  16  flows between the terminals of the magnet coil  13 . The transient current  16  in turn generates a tangential magnetic field  17 , a part of which penetrates the surface of the test material  3 . The tangential magnetic field  17  induces transient eddy currents  18 , which flow under and around the poles of the magnet  1 . 
   RF coil  4  is excited by alternating current  6  at frequencies that are greater than the component frequencies of the transient current  16  of the magnet coil  13 . Alternating current  6  in the RF coil  4  induces alternating eddy currents  8  and associated magnetic fields  9  in the test material  3 . When the test material  3  exhibits the physical property of magnetostriction, the vector summation of the resultant magnetic fields  9  induced by the RF coil  4  and the tangential magnetic fields  17  induced by the magnet  1 , cause expansion and contraction of the test material  3 . Alternating expansion and contraction of the test material results in propagation of SH waves  11  from the EMAT in two directions. 
   SUMMARY 
   An array of magnets designed of flexible components and materials can be easily shaped to fit to the contour of various curved surfaces and structures. EMATs that incorporate these magnets, in addition to being flexible, may be smaller in volume than the conventional EMAT magnets and therefore easier to apply to complex structures where access may be restricted. Also, the flexible magnet arrays can be easily and economically fabricated in various shapes and configurations, thereby increasing versatility, utility and cost effectiveness in comparison to the rigid, conventional magnet designs. 
   An electromagnetic acoustic transducer is provided, adapted to conform to the surface of a non-planar test substrate. 
   In certain embodiments, the electromagnetic acoustic transducer comprises an array of magnets conformable to the non-planar test substrate surface, wherein the magnets contain magnetic poles and interconnecting segments. 
   In one embodiment, the array of magnets comprises a flexible compound containing particles of ferromagnetic material, wherein electrical conductors are disposed between the magnetic poles capable of generating magnetic fields perpendicular to the faces of each magnetic pole when conducting current. 
   In another embodiment, the array of magnets comprises a flexible compound containing particles of permanent magnet material, wherein the magnetic poles are optionally magnetized to provide static magnetic fields perpendicular to the face of each magnetic pole. 
   A method of interrogating a test substrate having a non-planar surface is provided using the electromagnetic acoustic transducer comprising: 
   conforming the electromagnetic acoustic transducer to the surface of the test substrate in monitoring proximity to the surface, 
   generating a sound wave by interaction of fields from the electromagnetic acoustic transducer magnet and electrical conductor, and, 
   detecting at least one characteristic of the sound wave reflected by the test substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an EMAT comprising a permanent magnet and an RF coil for generation and detection of SH waves, Lamb waves and surface waves in electrically conducting materials. 
       FIG. 2  illustrates an EMAT comprising an array of permanent magnets and an RF coil for generation and detection of horizontally polarized shear waves. 
       FIG. 3  illustrates an EMAT comprising an electromagnet and meander RF coil for generation of horizontally polarized SH waves in ferromagnetic materials that exhibit magnetostriction. 
       FIG. 4  illustrates a flexible EMAT that is adapted for generation and detection of SH waves in non-planar electrically conducting materials. 
       FIG. 5  illustrates a flexible multiple-pole magnet array comprising mechanically and magnetically linked magnetic pole pieces and distributed windings of electrical conductors comprising an RF coil. 
       FIG. 6  illustrates a flexible RF coil placed in close proximity to the magnetic pole faces of the flexible multiple-pole magnet array. 
       FIG. 6A  illustrates the eddy currents and magnetic fields associated with one magnetic pole face in the array of  FIG. 6 . 
       FIG. 7  illustrates a flexible RF coil embedded in the magnetic pole faces of an array of flexible magnets. 
       FIG. 7A  illustrates a cross-section of the embedded RF coil conductor along line A–A′ of  FIG.7 . 
   

   DETAILED DESCRIPTION 
   Electromagnetic acoustic transducers (EMATs) can be easily shaped during or after fabrication so that the EMATs can be used to interrogate components and structures having curved surfaces without substantial loss of signal response to defects or properties of these components and structures that could otherwise be caused by poor compliance of the EMAT to the surface of the test material. The EMATs comprise primarily two component parts: magnets and electrical conductors which provide RF signals such as RF coils. The magnets may be comprised of one or more cores of ferromagnetic material and electrical conductors. 
   An EMAT is disclosed which comprises a magnet or a flexible multiple-pole magnet array which contains materials designed, fabricated and integrated with electrical conductors which provide RF signals such as RF coils. The EMAT can be easily shaped during or after fabrication so that it can be used to interrogate components and structures having curved surfaces. This substantially reduces the loss of signal response to the defects or properties of these components and structures caused by poor compliance and decrease in proximity of the EMAT to the surface of the test material or substrate. 
   The flexible multiple-pole magnet array may be formed in rows wherein each row has a radius of curvature about a point or points so as to provide focusing of the generated SH waves in a test material component. The array of magnets may have variation in the distance between adjacent magnetic poles that is a function of the radial distance from the focal point. This variation in the array of magnets causes a change in the vertical width of the SH wave. In other embodiments, two or more arrays of magnets may be arranged in tandem with each having a different radial distance between magnetic poles so that they will have approximately the same SH wave angle and focal point when operated within a prescribed range of frequencies. In a further embodiment, the array of magnets may have high frequency (RF) conductors embedded in grooves which lie across the magnetic pole faces and are collinear with radial projections from the focal point. 
   A flexible multiple-pole magnet array may comprise an array of magnets and magnetic poles fabricated at least in part from a flexible material such as silicone rubber containing particles of ferromagnetic material such as iron, or permanent magnet material such as neodymium iron boron. 
   Electrical conductors may have a shape, width and thickness such that they can be installed between the magnet poles and energized with an electrical current to provide alternating magnetic polarity between adjacent magnetic poles. In other embodiments, the electrical conductors may have a shape, width and thickness such that they can be installed between the magnet poles in multiple layers, connected in series and energized electrically to provide alternating magnetic polarity between adjacent poles. 
   Illustrated in  FIG. 4  is a conformable-flexible multiple-pole magnet array  19 , which may be used with other electrical components known in the art to form an EMAT that generates SH waves in a curved metallic component, for example but not for limitation, such as a steel pipe  20 . The magnets  1  contain magnetic poles  21  and interconnecting links or segments, both of which can be comprised of either ferromagnetic or non-ferromagnetic material. The flexible multiple-pole magnet array  19  may be fabricated and assembled so that it conforms to the curvature of the material structure to which the EMAT will be applied to perform the desired test. 
   One method of fabricating the flexible multiple-pole magnet array is to mold a conformable-flexible compound, for example but not for limitation, such as silicone rubber, impregnated or filled with particles of ferromagnetic material  14 , for example but not for limitation, such as iron. In this embodiment, at least one RF coil  4  comprising insulated electrical conductors is installed between the poles  21  to generate magnetic fields  2  that are perpendicular to the faces of each magnetic pole  21  when the RF coil  4  is energized by electrical currents. 
   In another embodiment, the conformable-flexible compound is impregnated with permanent magnet material  14 , for example but not for limitation, such as neodymium iron boron. In this embodiment, the magnetic poles  21  may be magnetized prior to use to provide static magnetic fields  2  that are perpendicular to the faces of each magnetic pole  21 . 
   Illustrated in  FIG. 5  is a planar view of a flexible, multiple-pole magnet array  19  that may be used with other electrical components for generation of SH waves  11 . It is comprised in part of an array of north (N) and south (S) magnetic poles  21 , which are connected mechanically and magnetically by linkages of magnetic material (not shown). One such embodiment uses a flexible hydrocarbon containing material, for example but not for limitation, an elastomer such as silicone rubber that is impregnated with particles of ferromagnetic materials or permanent magnetic materials, such as iron or neodymium iron boron compounds respectively. This mixture may be molded into flexible multiple-pole magnet arrays  19  containing one or more magnetic poles  21  in a variety of configurations, which provide enhancements in EMAT performance, including increased SH wave  11  intensity, SH wave  11  steering and focusing. 
   The flexible multiple-pole magnet array  19 , may comprise layers of insulated conductor  22  and second insulated conductor  23 , which may be woven between the magnetic poles  21  so that they provide magnetization in a direction that has a predominant magnetic field vector component perpendicular to the magnetic pole  21  face and the surface of the test material  3 . The insulated conductor layer  22  and second insulated conductor layer  23  may be placed between the magnetic poles  21  in a pattern that produces opposite polarity in adjacent magnetic poles  21  when the insulated conductor layer  22  and second insulated conductor layer  23  are energized by a current source  27 . When the flexible multiple-pole magnet array is used as a permanent magnet array, the insulated conductor layer  22  and second insulated conductor layer  23  may be absent, or removed, to provide increased flexibility and conformity to the test material  3  surface. 
   Assembly of the magnet may include the insertion of insulated conductor layer  22  between the poles, followed by the insertion of a second insulated conductor layer  23 , part of which overlays insulated conductor layer  22 . When insulated conductor layer  22  and second insulated conductor layer  23  are connected electrically at junction  24 , the flexible multiple-pole magnet array&#39;s  19  interior magnetic poles  21  are effectively encircled by two interwoven insulated conductors (insulated conductor layer  22  and second insulated conductor layer  23 ) that carry electrical current in the same direction when energized at terminals  25  and  26  by current source  27  which in one embodiment is a direct current source. Additional pairs of conductor layers similar to insulated conductor layer  22  and second insulated conductor layer  23  may be installed over insulated conductor layer  22  and second insulated conductor layer  23  and connected in series or in parallel with said layers to provide increased magnetizing current and increased magnetic field normal to each magnetic pole  21  face. 
   The array of magnetic poles  21  may be shaped and positioned so that they collectively produce a focusing SH wave  11  at an approximate radial distance  28 , as indicated in  FIG. 5 . The width  29  of each magnetic pole  21  may be a function of its radial distance  28  from the focal point  32 , increasing in proportion to the radial distance  28  from the center of the magnet  1 . The distance  30  between magnetic poles  21  in conjunction with the excitation frequency of the RF coil  4  determines the angle of the SH wave  11  with respect to the normal direction to the test material  3  surface. A decrease in distance  30 , or a decrease in RF excitation frequency within the functional range, results in an increase in the angle of the SH wave  11  with respect to the surface of the test material  3 , that is, the test substrate. 
   A variation in the distance  30  between adjacent magnetic poles  21  as a function of radial distance  28  causes a change in the vertical width of the SH wave  11 . For example, a decrease in distance between two magnetic poles  21  that is proportional to the radial distance  28  to the magnetic pole  21  pair can result in a decrease in the vertical width of the SH wave  11  and greater resolution in detecting defects. Similarly, two or more flexible multiple-pole magnet arrays  19 , each having a different radial distance  28  between magnetic poles  21 , may be arranged in tandem so that they will have approximately the same focal point  32  when operated within the prescribed range of frequencies. 
   The RF coils  4  illustrated in  FIG. 6  are comprised of electrical conductors attached to a flexible substrate  31  of electrically insulating material. The RF coils  4  are attached to the magnetic pole  21  faces, so that they are in close proximity to the test material  3 . When the alternating voltage of an alternating power source  5  is applied to the RF coils  4 , Lorentz forces  10  are applied to the test material  3  at an instant in time when the voltage is positive in the directions as indicated in  FIG. 6  and  FIG. 6A . The Lorentz forces  10  are in diametrically opposite directions between upper and lower adjacent magnetic poles  21  in each column of magnetic poles  21 . This is attributed to the opposing polarity of adjacent magnetic poles  21 . As, induced eddy currents  8  with associated magnetic fields  9  reverse direction under adjacent columns of magnetic poles  21 , the Lorenz forces  10  are in the same direction in a given row of magnetic poles  21 . These alternating forces add to produce an SH wave  11  traveling toward the focal point  32 . 
   The magnetic poles  21  of the multiple-pole electromagnet array  19  may provide for an increase in the electromagnetic coupling of the RF conductors  33  to the test material. This electromagnetic coupling can be further increased by embedding RF conductors  33  within the magnetic poles  21  of the ferromagnetic material  14 , as illustrated in  FIG. 7 . As shown in  FIG. 7A , the embedded RF conductors  33  and the magnetic poles  21  may be closer to the test material  3 , thereby increasing the quantity of alternating magnetic field  9  that penetrates the test material  3 . The amplitude of the induced eddy currents  8  that are induced by the alternating fields  7  is increased, which in turn increases the intensity of Lorentz forces  10  and the resultant SH wave  11  in the test material. 
   It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as described herein. It should be understood that the embodiments described above are not only in the alternative, but can be combined.