Abstract:
Systems, methods, and devices for an Electromagnetic Acoustic Transducer (EMAT) comprising: a ceramic wear surface disposed about a first aperture of an EMAT tip portion, a transducing coil means disposed within a volume of the EMAT tip portion; where at least one magnet and an insulative surface may be interposed between the ceramic wear surface and the insulative structure. Optionally, a signal transmitting means may be configured to conduct a signal from the transmitting coil means via a second aperture of the tip.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of International Application No. PCT/US2012/024167 filed Feb. 7, 2012, which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/462,805 filed Feb. 7, 2011, the disclosures of which are hereby incorporated by reference in their entirety herein for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to electromagnetic acoustic resonance inspection techniques, and specifically to Electromagnetic Acoustic Transducers (EMATs) used in nondestructive testing. 
       BACKGROUND 
       [0003]    EMATs can be placed in close proximity to items under test. These items under test may be in motion relative to the EMATs. Shredded pieces of these items, i.e., fines, may end up in the test ends of the EMAT and migrate toward the coil, causing the coil to be inoperable. 
       SUMMARY 
       [0004]    Embodiments of the invention include an Electromagnetic Acoustic Transducer (EMAT) comprising: a tip, having a hollow interior, the tip comprising a front surface having a first aperture; a coil, where the coil may be disposed within the hollow interior of the tip and where the coil may be substantially coplanar with the front surface of the tip; and a ceramic wear surface, where the ceramic wear surface may be comprised of an electrically benign ceramic, e.g., yttrium-reinforced zirconium; and where the ceramic wear surface may be disposed about the first aperture of the tip and may be substantially coplanar with the front surface of the tip. In some embodiments, the EMAT may further comprise an epoxy, where the epoxy may be disposed about the coil and within the hollow interior of the tip, and where the epoxy may couple the ceramic wear surface to the front surface of the tip. In other embodiments, the EMAT may further comprise an insulative disk, where the tip further comprises a back surface having a second aperture, and where the insulative disk may be disposed over the second aperture of the tip. Additionally, the EMAT may comprise an epoxy, where the epoxy is disposed about the coil and within the hollow interior of the tip, and where the epoxy may couple the ceramic wear surface to the front surface of the tip and may couple the insulative disk, which may be comprised of a high dielectric constant, to the back surface of the tip. In some embodiments this EMAT further comprises at least one magnet, where the insulative disk may be disposed between the at least one magnet and the coil. 
         [0005]    Methods of this invention may include a method of non-destructive testing of a material comprising: disposing a first Electromagnetic Acoustic Transducer (EMAT) lineally opposite a second EMAT, where the material under test may be interposed between the first EMAT and the second EMAT; transmitting a signal from the first EMAT; receiving the signal by the second EMAT; and transmitting the received signal for processing; where the opposed surfaces of the first EMAT and the second EMAT closest to the material under test may be in motion relative to the first EMAT and the second EMAT and/or may each be comprised of a ceramic wear surface, e.g., yttrium-reinforced zirconium. 
         [0006]    Embodiments of the invention may include an electromagnetic acoustic transducer (EMAT) comprising: a ceramic wear surface disposed about a first aperture of an EMAT tip portion, a transducing coil means disposed within a volume of the EMAT tip portion; where at least one magnet and an insulative surface may be interposed between the ceramic wear surface and the insulative structure. In some embodiments the EMAT may include a signal transmitting means configured to conduct a signal from the transmitting coil means via a second aperture of the tip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, and in which: 
           [0008]      FIG. 1  depicts an exemplary EMAT in an exploded view; and 
           [0009]      FIG. 2  depicts an exemplary transmitting EMAT sensor and an exemplary receiving EMAT sensor pair lineally disposed opposite one another, where a wire is shown interposed between the tips of each. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The methods described herein are intended to address several issues that have presented problems for the implementation of EMATs used in the industrial or in the high volume component inspection environment. While these methods have particular application in EMATs that are used in the application of the electromagnetic acoustic resonance and specifically the techniques used in the continuous wave methods of the acoustic resonance inspection systems, the application of these methods can be favorably applied to conventional pulsed EMAT implementations. The wear problem became most problematic when the transducers designs became smaller and smaller. The applications mainly comprise of small items such as small diameter tubing, fasteners, or stamped or forged components including, engine valves presenting problems for the application of EMATs. They must be small in size to accommodate tight radiuses and to allow for their placement very near the surface of the article under test and the close proximity of the transducer to each other. While not in contact with the surface, their near surface proximity often traverses components with sharp hardened edges and contour transitions rapidly fatigues traditional transducer wear face materials and wear coatings. 
         [0011]    A low dielectric constant, extremely hard, non-metallic material such as a thin machined ceramic material as a tile, for example, may be selected from: silicon carbide, sapphire or other industrial ceramic. This tile may be placed directly onto the coil face of the transducer. This entire assembly may then be cemented in a hard epoxy within the transducer tip. Preferably, the hard wear face is backfilled with a void-filling hard compound in order to ensure that the structural rigidity required to support the brittle wear plate is present. Depending upon the transducer application, a field focusing pole piece may be used to reduce the transducer frontal area to enable coverage of small diameter or tight radius contours. The coil may be backed with a high dielectric constant insulative tile material including mica or perovskite, reducing direct or induced electromagnetic coupling into the magnet assembly. This reduces the unwanted eddy current losses generated by the coil coupling into the magnet assembly. 
         [0012]      FIG. 1  depicts an exploded view of an exemplary transducer assembly  100  exclusive of its cable assembly and its ferritic steel cable anchor.  FIG. 1  shows an exemplary transducer embodiment with a ceramic wear plate  107  that may be imbedded into the housing tip  106 , where the housing tip  106  comprises a first aperture  108  and a second aperture  109 . This may be backed by a pancake coil  105  wound of either regular single filament magnet wire or multifilament magnet wire, sometimes called litz wire particularly useful at higher frequencies, and a field reducing magnet  103  faced with an insulative tile  104 . The thin insulative tile  104  may be of a high dielectric constant and may protect the coil  105  from contact with an electrically conductive magnet  103 , 102  backing the coil. For example, a 0.005″ titanate-based ceramic with a dielectric constant of 8,000 to 12,000 may be used. The pancake coil  105  may be constructed of a poly-coated copper wire, e.g., for testing titanium, having an exemplary range of approximately 1.5 mm to 3 mm in diameter. For example, the coil  105  may be ninety turns of thirty-three gauge wire. The coil  105  may have a responsive frequency in the range of 10-15 MHz. The field reducing magnet  103  is then stacked onto a larger, more powerful, magnet structure  102  contained within a housing  101  and integrated tip assembly  106 . The field focusing/reducing magnet  103  may be a neodymium, iron, boron magnet having exemplary field strengths of 42 to 52 Mega Gauss Oersteds, and the larger magnet  102  depicted in  FIG. 1  may be of the same material having a high field strength value such as 55 MGOE. 
         [0013]    Epoxy sealing compounds for both the tip section and housing body are not depicted. A hard epoxy sealing compound may be disposed on the backside of the ceramic wear plate  107  to provide durability. In some embodiments, the ceramic wear plate  107  may be serviced, e.g., the EMAT may be resurfaced and a new ceramic wear plate may be affixed, at least once. The wear plate  107  may be comprised of a thin machined, electrically benign industrial ceramic or natural mineral tile that is affixed to the housing  101  and coil-magnet assembly. This may greatly improve transducer life in high volume industrial applications. 
         [0014]      FIG. 2  depicts an exemplary embodiment  200  comprising an exemplary transmitting EMAT sensor  210  and an exemplary receiving EMAT sensor  220  pair disposed about a wire  230 . The wire may be fixed or have a direction of motion  240  relative to the EMAT sensors  210 ,  220 . The EMAT sensors  210 ,  220  may be composed of the materials depicted in the exploded view of  FIG. 1 . The transmitting EMAT sensor  210  may excite electrical surface eddy currents in the material being tested, e.g., a wire  230 , and convert them into mechanical sound waves via the Lorenz force to bring about a stable acoustic resonance. The receiving EMAT sensor  220  may detect the resonance in the material being tested, and present the modulated signal for processing. This process creates carefully bounded, acoustic fields in the areas directly under the sensor faces. The material being tested may continuously move past the EMAT sensors  210 ,  220  at high feed rates, e.g., up to 2,500 feet per minute. The material to be tested may also be stationary, e.g., engine components, fasteners, and medical devices. While not exclusive to this application, this invention has particular importance for use in high frequency electromagnetic acoustic resonance applications. A ceramic wear plate  107  affixed, e.g., by a hard epoxy, to the tip assembly  106  of the transmitting EMAT sensor  220  protects the coil ( FIG. 1 ,  105 ) from damage, e.g., caused by pieces of the material being tested that are magnetically attracted to the magnet structure ( FIG. 1 ,  102 ) contained within a housing  101 . A ceramic wear plate may also be affixed to the receiving EMAT sensor  220 . Both of the EMAT sensors may be positioned at a distance proximate to, but not in direct contact with, the material to be measured, e.g., one five-thousandth to one fifty-thousandth of an inch from the material to be measured. The ceramic wear plate  107  may be thin, e.g., up to one five-thousandth of an inch, electrically inert, and harder than the material to be measured, e.g., having a Rockwell hardness in the range of 77 to 81, and preferably a Rockwell hardness 75 and above. The materials need to be hard, making them puncture and wear tolerant: tough, giving them fracture resistance as opposed to being brittle. The ceramic wear plate  107  may be made of any electrically inert, machinable ceramic material having low dielectric constant, e.g., yttrium-reinforced zirconium, industrial diamonds, silicon carbide, alumina, or sapphire. 
         [0015]    It is contemplated that various combinations and/or sub-combinations of the specific features and aspects of the above embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments may be combined with or substituted for one another in order to form varying modes of the disclosed invention. Further, it is intended that the scope of the present invention herein disclosed by way of examples should not be limited by the particular disclosed embodiments described above.