Abstract:
An ultrasonic probe and a method for the nondestructive testing of a test specimen are described, which are activatable individually or in groups as phased array for the emission or reception of ultrasonic plate waves in a predefinable propagation direction in the test specimen wall to be tested. At least one ultrasonic transducer segment with at least two segment parts emits an ultrasonic plate wave field into the test specimen and which are activatable jointly and simultaneously as a phased array. The at least two segment parts are arranged along a common plane so that the ultrasonic wave fields provided from or received by the at least two segments mutually overlap and each have a main propagation direction which encloses an acute angle α in a projection of the plane.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Reference is made to German Patent Application Serial No. 10 2011 018 954.8, entitled “Ultrasonic Probe and Method for the Nondestructive Testing of a Planar Test Specimen,” filed Apr. 29, 2011, which application is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an ultrasonic probe and a method for the nondestructive testing of a planar test specimen, such as a pipe wall, a slab, or a plate, which provides a plurality of ultrasonic transducer segments, which are activatable individually or in groups by means of phased array technology for the emission of ultrasonic plate waves having a predefinable propagation direction in the test specimen to be tested. 
         [0004]    2. Description of the Prior Art 
         [0005]    Ultrasonic probes of the above-mentioned species are used, for example, in the nondestructive testing of extended test objects, in order to detect cracks or other types of material defects in test specimens or test specimen walls. In particular in the examination of weld seams, the ultrasonic waves are incident at an inclined angle relative to the weld seam as much as possible, in order to detect weld seam defects either in a reflection, diffraction, or transmission measurement in this manner. 
         [0006]    In addition to the incidence of the ultrasonic signals in a test specimen wall to be studied at a permanently predefined incidence angle, due to which it is necessary to move the ultrasonic probe used for this purpose relative to the weld seam to register the entire weld seam geometry, ultrasonic probes are known and are in use which, based on the phased array technology, allow a variation of the incidence angle of the ultrasonic waves during the testing. Using this electronic pivoting technique, during which the main propagation direction of the ultrasonic wave field which can be coupled into the test specimen is pivoted, probe movements in the direction of the weld seam can be omitted or minimized, whereby nondestructive ultrasonic examinations on test specimens can be performed more rapidly and therefore more cost-effectively. 
         [0007]    Ultrasonic probes, which are also referred to as phased array probes, which are suitable for the use of the phased array technology, consist of a plurality of individually activatable ultrasonic transducer segments, which generate ultrasonic waves. The individual segments, which generate ultrasonic waves are combined to form a phased array probe, are differently activated in the case of transmission or differently read out in the case of reception. In most cases, the differing activation of the individual segments is performed by individual amplitude and/or phase configuration or by a time-delayed activation, in order to achieve an angular incidence or focusing of the ultrasonic wave field, which results through superposition of the individual fields originating from the ultrasonic transducer segments and which is emitted as a whole by the phased array probe. 
         [0008]    In spatially delimited test specimens to be studied, for example, plates or plate-shaped test specimens, so-called ultrasonic plate waves can form, whose propagation direction is oriented parallel to the plate surface and whose ultrasonic wavelength is of the magnitude of the plate thickness. In addition to classical piezoelectric angle probes, electromagnetic ultrasonic transducers can also be used for the excitation and reception of such ultrasonic plate waves, during which a selective excitation of selected plate wave modes and also plate waves having an SH polarization are possible. The latter relates to shear waves, which are polarized parallel to the wall surface. Therefore, these waves can also be referred to as “horizontally polarized shear waves” or, for the case of plate waves, as “horizontally polarized plate waves”. 
         [0009]    In order to set a specific incidence direction in a voluminously extended test specimen, the known phased array technology can be used. Segmented ultrasonic probes are used, which can be activated in a phase-oriented manner using suitable electronics. This approach can also be used for the excitation of ultrasonic plate wave modes. It is thus possible, if the emission direction of the segments is aligned perpendicularly to the linear arrangement of the segments, to rotate the emission direction of the ultrasonic summation signal through a phase-oriented activation in the plate plane. See P. Wilcox, M. Lowe, and P. Cawley: Lamb and SH Wave Transducer Arrays for the Inspection of Large Areas of Thick Plates, in Review of Progress in QNDE 2000, 19, pages 1049-1056. However, it is problematic that the pivot range of the ultrasonic wave field is limited by the aperture width of each individual ultrasonic transducer segment. 
         [0010]    The publication DE 10 2004 063 482 B3 describes an arrangement for the incidence of US shear waves into a tubular or slab-shaped ferromagnetic test specimen for the crack testing thereof. Both the emission and also the reception of the US shear waves are performed with the aid of an HF coil arrangement, which is attached to a probe, which is pre-magnetized in the area of the HF coil arrangement. 
         [0011]    DE 10 2004 053 584 A1 discloses nondestructive material testing utilizing of ultrasound, in which EMUS transducers are used to generate ultrasonic waves which can be emitted into the test specimen perpendicular to the test specimen surface. 
         [0012]    Finally, DE 10 2008 002 394 A1 describes a universal ultrasonic probe for the emission of ultrasonic waves which propagate within a test specimen parallel to the test specimen surface, predominantly for studying welding melt zones. The probe is ring-shaped and has segmented separate regions for the emission and the reception of ultrasonic waves. 
       SUMMARY OF THE INVENTION 
       [0013]    The invention is a refinement of an ultrasonic probe for the nondestructive testing of a planar test specimen, such as a pipe wall, a slab, or a plate, having an ultrasonic probe. The probe has a plurality of ultrasonic transducer segments, which are activatable individually or in groups in a phased array at least for the emission of ultrasonic plate waves having a predefinable propagation direction in the test specimen to be tested. The pivot range, within which the ultrasonic plate waves can be generated having predefinable propagation direction, is enlarged. The enlargement of the pivot range is to be implementable using the simplest possible technical means, which are cost-effective to implement. 
         [0014]    A method according to the invention for the nondestructive testing of a planar test specimen, such as a pipe wall, a slab, or a plate, is described. Features which advantageously refine the invention are the subject matter of the further description, in particular with reference to the exemplary embodiments. 
         [0015]    According to the invention, an ultrasonic probe for the nondestructive testing of a planar test specimen, such as a pipe wall, a slab, or a plate, includes a plurality of ultrasonic transducer segments, which are activatable individually or in groups by means of a phased array at least for the emission of ultrasonic plate waves having a predefinable propagation direction in the test specimen to be tested. The at least one ultrasonic transducer segment has at least two segment parts, from each of which an ultrasonic plate wave field can be emitted into the planar test specimen and which are activatable jointly, that is, simultaneously, by as a phased array. The at least two segment parts are arranged along a common plane so that the ultrasonic wave fields assignable to the at least two segments mutually overlap and each have a main propagation direction, which enclose an acute angle α in a projection on the plane. 
         [0016]    The invention differs from the previous implementation of individual ultrasonic transducer segments in that the ultrasonic transducer segments, which are each implemented as a unit, are split into at least two halves which are separated into at least two spatially separated units, each of which generate or originate an ultrasonic wave field. Both halves of an ultrasonic transducer segment, that is, both segment parts, are disposed in a common plane and enclose an acute angle relative to one another, so that the ultrasonic plate wave fields originating from both segment parts each have a main propagation direction. The main propagation directions enclose an acute angle relative to one another. The ultrasonic plate wave fields generated by both segment parts are superimposed to form an ultrasonic plate wave field having a divergence angle, which is greater than a divergence angle or aperture angle of an ultrasonic wave field which originated from a non-divided, prior art ultrasonic transducer segment. 
         [0017]    Through the measure according to the invention of the division of an ultrasonic transducer segment into at least two parts, which are not arranged in parallel but rather at an acute angle to one another, the pivot range of the phased array ultrasonic probe for emitting ultrasonic plate waves can be enlarged, to expand the spectrum of application of ultrasonic probes. 
         [0018]    The invention is applicable to piezoelectric and also electromagnetic ultrasonic transducers. A division of the prior art ultrasonic transducer segment into two parts to implement the invention is also not limiting. The emission divergence of an ultrasonic transducer segment may be enlarged further by the ultrasonic transducer segment being divided into three, four, or more segment parts, which preferably have an identical structural design and in which each two adjacent segment parts are disposed in a plane enclosing an acute angle α. The angle α fundamentally may be varied so that 0°&lt;α&lt;0°, but preferably α is &lt;60°, particularly preferably  α  is less &lt;15°. 
         [0019]    A preferred embodiment divides the ultrasonic transducer segment into two segment parts of equal size, that is, into a segment part pair, which jointly enclose the acute angle α according to the invention. 
         [0020]    A preferred embodiment of an ultrasonic probe according to the invention provides a line-shaped arrangement of a plurality of segment part pairs, whose assignable transmission and/or reception apertures face toward the same half space. The individual segment part pairs are preferably activated in the same manner as the prior art individual phased arrays so that both halves of a segment part pair are activated simultaneously, that is, at the same time. In contrast respective adjacently arranged segment part pairs are activated, for example, with a phase delay or time delay based on the prior art phased array. As a result, a total ultrasonic plate wave field is emitted by the ultrasonic probe implemented according to the invention, that is., by the sum of all segment part pairs arranged adjacent to one another, providing a superposition field composed of all individual wave fields of the segment part pairs which propagates at a predefinable emission angle or based on a predefinable focal area. 
         [0021]    Depending on the arrangement and implementation of the ultrasonic probe, ultrasonic plate wave fields can be both generated or received according to the invention. 
         [0022]    The method of operation on the device according to the invention is distinguished by at least two separate ultrasonic transducer fields, which come into spatial superposition with one another, being emitted and/or received by at least one ultrasonic transducer segment. A main propagation direction is assignable to each segment, which encloses an acute angle α relative to one another. 
         [0023]    The above-described at least one transducer segment is to be understood according to the invention as the “occurrence location” for the formation or occurrence of at least two separate ultrasonic wave fields, which are spatially superposed with one another. The method according to the invention can be physically implemented in an ultrasonic transducer segment which is formed in one, two, or multiple parts. It is essential that, from the location of an ultrasonic transducer segment within a plurality of arranged ultrasonic transducer segments, from which an ultrasonic plate wave field having predefinable propagation direction can be generated based on prior art phased arrays. At least two separate ultrasonic plate wave fields originate, which come into spatial superposition, each having an assignable main propagation direction, which in turn encloses an acute angle α relative to one another. To implement this requirement using the simplest possible technical means, the above separation of an ultrasonic transducer segment into at least two halves is utilized. Alternative measures, which are more technically complex to implement, however, are also applicable. 
         [0024]    With the method according to the invention, the emission divergence of the ultrasonic plate wave fields emitted from each “occurrence location” is enlarged in comparison to the prior art implementation of ultrasonic phased array probes. In particular, the emission divergence of the overall ultrasonic plate wave field emitted from all “occurrence locations” is also enlarged. In this way the pivot range, within which the main propagation direction of the total ultrasonic plate wave field can be varied, can also be enlarged according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         [0025]    The invention will be described for exemplary purposes hereafter without restriction of the invention on the basis of exemplary embodiments with reference to the drawings. In the figures: 
           [0026]      FIGS. 1   a  and  b  show an illustration of a prior art ultrasonic probe, which is known per se, for generating ultrasonic plate waves and a view of an ultrasonic transducer segment; 
           [0027]      FIGS. 2   a  and  b  show an arrangement according to the invention of two segment part pairs or a plurality of segment pairs; 
           [0028]      FIG. 3  shows an embodiment of a segment part pair based on an electromagnetic ultrasonic transducer; and 
           [0029]      FIGS. 4   a  and  b  show an alternative embodiment of a segment part pair of the invention based on an electromagnetic ultrasonic transducer. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 1   a  shows a schematic view to illustrate a prior art ultrasonic probe  1  known per se, which has four ultrasonic transducer segments S 1 , S 2 , S 3 , and S 4 . The four ultrasonic transducer segments S 1  to S 4  are implemented to generate ultrasonic plate waves  2 , which are capable of propagating in the exemplary embodiment shown within a test specimen wall  3 , which is coincident with the plane of the drawing of  FIG. 1   a.    
         [0031]    The individual ultrasonic transducer segments S 1  to S 4  are activated with a phase delay or time delay in a suitable manner by means of phased array technology, in order to obtain an ultrasonic plate wave field  2 ′ having a main propagation direction which is pivoted through pivot angle α′ relative to the uninfluenced propagation direction  4  of the individual ultrasonic transducer segments S 1  to S 4 . 
         [0032]    Each of the individual ultrasonic transducer segments S 1  to S 4  visible in  FIG. 1   a  has a transducer body W, shown in  FIG. 1   b.  A frontal outlet/reception aperture A has an aperture width D, which typically corresponds to the transducer width. The ultrasonic transducer segment W can be implemented both in the form of a piezo transducer or as an electromagnetic ultrasonic transducer (EMUS transducer). An ultrasonic plate wave field  2 ′, which has an aperture angle w′, which predefines the emission divergence of the ultrasonic transducer segment W, is emitted or generated within a test specimen wall frontally via the outlet aperture A. Ultrasonic plate waves  2 ″ which are generated within the test specimen for example by reflection are detected by the ultrasonic transducer segments S 1  to S 4 . Reflections of ultrasonic waves inside a test specimen take place at locations of discontinuities, like cracks, inhomogeneity of material, test specimen walls etc. 
         [0033]    The emission divergence w′ of the ultrasonic transducer segment W is bounded, inter alia, by the ultrasonic wave frequency and the aperture width D. At the same time, the maximum possible pivot range of an ultrasonic probe composed of a plurality of ultrasonic transducer segments, which are operated as a phased array, is also predefined by the delimitation of the divergence range. In order to widen the pivot range of the phased array ultrasonic plate wave probe and therefore make it accessible for a broader application spectrum, according to the invention the ultrasonic transducer segment implemented as a unified component shown in  FIG. 1   b  is split into at least two halves. The halves are not arranged in parallel to form an acute angle relative to one another. This is illustrated schematically in  FIG. 2   a , which shows two segment parts W 1  and W 2  of an ultrasonic transducer segment, which are implemented structurally identical to one another and enclose an acute angle α defined by their segment longitudinal extensions WS 1  and WS 2 . The two segment parts W 1  and W 2  each have an outlet aperture A 1  and A 2 , which respectively correspond in sum to the outlet aperture of an undivided ultrasonic transducer segment according to  FIG. 1   b . Both segment parts W 1  and W 2  are arranged located adjacent to one another in a common plane, which corresponds to the plane of the drawing in  FIG. 2   a , and enclose with their respective segment longitudinal extensions WS 1  and WS 2  the acute angle α, which fundamentally varies in an angle ranging between 0° and 90°. However, the angle preferably ranges so that α&lt;60°, particularly preferably &lt;15°. The selection of the angle α is particularly performed in such a manner that the ultrasonic plate wave fields  2 ′, which are emitted from both segment parts W 1  and W 2 , mutually overlap, so that the superposition of both wave fields occurs at a minimum distance a from the outlet apertures A 1  and A 2 . 
         [0034]    Through the embodiment according to the invention of the at least two segment parts W 1  and W 2 , the divergence range of the total ultrasonic plate wave field of overlapping segment parts W 1  and W 2 , is also enlarged in comparison to a single prior art ultrasonic transducer segment according to  FIG. 1   b,  which is known per se. The maximum pivot range of an ultrasonic probe having a plurality of divided ultrasonic transducer segments is also enlarged at the same time. This is schematically shown in  FIG. 2   b .  FIG. 2   b  shows four segment part pairs P 1 , P 2 , P 3 , P 4 , which each correspond to one ultrasonic transducer segment S 1 , S 2 , S 3 , S 4  in the ultrasonic probe  1  shown in  FIG. 1 . The four segment part pairs P 1  to P 4  shown in  FIG. 2   b  each have two segment parts W 1  and W 2  arranged tilted relative to one another according to the view illustrated in  FIG. 2   a , and generate a total ultrasonic plate wave field from superposition of all individual ultrasonic plate wave fields. The total divergence of the four segment part pairs P 1  to P 4  is greater than that which is generated by the four ultrasonic transducer segments S 1  to S 4 , so that the total pivot range, which is settable using phased arrays, can also be enlarged. 
         [0035]    The segment part pairs P 1  to P 4  which are shown in  FIG. 2   b  are arranged directly adjacent to one another in a common plane, which corresponds to the plane of the drawing in  FIG. 2   b  with outlet apertures being located in a common plane. 
         [0036]    The above explanations, which are referred to as ultrasonic transducer segments, are only capable of emitting ultrasonic plate wave fields. Of course, the individual segment part pairs P 1  to P 4  can also be used at the same time or alternatively only exclusively for receiving ultrasonic plate wave fields. 
         [0037]    As already described, the two segment parts W 1  and W 2  according to  FIG. 2   a  may be either implemented as piezo transducers or as electromagnetic ultrasonic transducers. An embodiment of two segment parts WS 1  and WS 2  based on an electromagnetic ultrasonic transducer principle is explained with reference to  FIG. 3 . Each individual segment part WS 1  and WS 2  has a permanent magnet array, which includes a plurality of individual bar magnets joined directly to one another in a stack. Each bar magnet has alternating magnetic poles in the stack direction SR 1  and SR 2 . Both segment parts WS 1  and WS 2  are arranged to be tilted relative to one another with respect to their two stack directions SR 1  and SR 2  and enclose the acute angle α. In addition, each segment part WS 1  and WS 2  includes an HF coil including HF 1  and HF 2 , which are both connected in series and are therefore activatable uniformly, that is, simultaneously, to generate or receive ultrasonic plate waves. Through the mutually tilted arrangement of both permanent magnet arrays, the main emission directions H 1  and H 2  of both segment parts WS 1  and WS 2 , along which the ultrasonic plate waves are emitted, enclose the angle α, which enlarges the emission divergence of the illustrated segment part pair P. 
         [0038]    A further exemplary embodiment of the implementation of an electromagnetic ultrasonic transducer is illustrated in  FIG. 4   a . In this case, each segment part comprises a meandering HF coil system SP 1  and SP 2  including a plurality of coil loops SP 1  and SP 2  arranged along a meandering direction M 1  and M 2 . The meandering directions M 1  and M 2  enclose the acute angle α. Both meandering coils SP 1  and SP 2  are also connected in series and are therefore activatable simultaneously. 
         [0039]    In addition, a permanent magnet generates a stationary or quasi-stationary magnetic field M which is oriented parallel to the surface of a test specimen wall at the location or in the region of the meandering HF coil arrangements SP 1  and SP 2 . In the case of the exemplary embodiment illustrated in  FIG. 4   a , the magnetic field lines of the permanent magnetic field M are oriented substantially perpendicular to M 1  and M 2 , and enclose an angle of 90°±α/2 relative to the meandering directions M 1  and M 2 . 
         [0040]    In contrast thereto, the permanent magnet field M in the exemplary embodiment according to  FIG. 4   b , which also has two meandering HF coil arrangements SP 1  and SP 2  connected in series, is aligned substantially parallel to the meandering directions M 1  and M 2 . In particular, the magnetic field lines enclose an angle of ±α/2 relative to the meandering directions M 1  and M 2 . Through such an arrangement, Lamb wave modes are preferably excited in the test specimen wall, while in contrast the exemplary embodiment according to  FIG. 4   a  preferably generates SH wave modes. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  ultrasonic probe 
           2  ultrasonic plate waves 
           2 ′ ultrasonic plate wave field 
           3  test specimen wall 
           4  original propagation direction 
         WS 1  and WS 2  segment part 
         W ultrasonic transducer segment 
         D aperture width, transducer width 
         A aperture 
         w′ aperture angle, divergence 
         SP 1  and SP 2  meandering HF coil arrangement 
         M 1  and M 2  meandering direction 
         H 1  and H 2  main sound emission direction 
         SR 1  and SR 2  stack direction 
         P 1 , . . . , P 4  segment part pair 
         P segment part pair