CA1189947A - Non-destructive, non-contact ultrasonic material testing method and apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

There is disclosed a method for non-contact, non-destructive testing of a -test body of ferromagnetic and/or electrically-conductive material with ultrasound waves, including the steps of producing in a near-surface region of the test body a low-frequency alterna-ting magnetic bias field having flux lines generally parallel to a surface of the test body, producing high frequency alternating magnetic excitation fields in the near-surface region generally parallel to the surface during a time interval when the bias field is at a quasi-static maximum, adjacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultrasound waves are gen-erated in the test body and detecting high frequency alternating magnetic fields in the near-surface region during the same time interval when the bias field is at a quasi-static maximum and producing a signal therefrom representative of the ultrasound waves.

Description

NON-DESTRUCTIVE NON-CONTAC~ ULTRASONIC MATERIAL

TESTING METHOD AND APPARATUS
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The present invention relates to methods and apparatus for nondestructive non-contact testing of materials with ultrasonic waves.
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Ultrasonic transducers are known which produce ultrasonic (US) waves in electrically-conductive materials by electrodynamics or (in the case of ferromagnetic materials) magnetostrictive methods. such transducers generally employ a high frequency coil in combination with a static pro magnetization (static bra field) having flux lines directed either parallel to or normal to a surface of the body of material to be tested. In some caves (such as for production of shear-horizontal (SO) waves by magnetostrictive excitation and forces normal to the surface of the test body by electrodynamics excitation), a bias field having flux lines extending over relatively large distances is of interest.
Until now, permanent magnets or electromagnets with DO supply or pulsed direct voltage were usually employed for producing horizontal magnetic fields, i.e. fields with flux lines extending generally parallel to the surface of the -test body. examples of such arrangements are given, inter alias in the following :

~,~

. . .. .

Lot 7 1. RUB. Thompson, "Non contact Transducers", 1973 IJltrasonics Symposium Proceeding, It New York, 1973.

2. RYE. Beissner, electromagnetic Acoustic transducers : A
survey of the state-of-the-art, Nondestructive Testing oration Analysis Center (NTIAC), San Antonio, Texas January 1976.

3. I Visual, RUB Thompson, "Periodic magnet non-contact electromagnetic acoustic wave -transducer - theory and application", 1977 Ultrasonics Symposium Proceedings, IEEE, New York, cat 77 CASEY.

4. H. Shim, A Bar, "Improved design or non-contacting electromagnetic acoustic transducers', 1977 Ultrasonics Symposium Proceedings, IRE New York, cat ~77 . CHIHUAHUAS.

5. RUB Thompson, "A model for the electromagnetic generation of ultrasonic guides waves in ferromagnetic metal I ' polycrystal~", It Trans. Sines Ultrasonics, Volt 25, Noah, Jan. 1978.

- 6. RUB. Thompsoltl, "New configurations for the electromagnetic generation of EYE waves in ferromagnetic materials", 1978 Ultrasonic Symposium Proceedings, lye, New York, Cat.
~j~78 Cal 344~ o

7. W. Moor, W. Repplinger, 'IElektrodynamische beruhrungslose Anregun,g wrier Ultra~challwelle" Materialprufung, 20 (1978) -

8. US. Patent No. 3~850,028 to Robert B. Thompson et Al issued November 26, 1974.

9. US. Patent No. 3,58~5,213, to James R. okay et at, issued June 8, 1981.

10. Us Patent No. 3,460,063, to James R. Luke et at, issued August 5, 1969.

11 . US. Patent No. 3, 786,672, to Martin R. Gaerttner, issued January 22, 1974.

'7

12. West German Auslegeschri~t No. 26 55 804, laid open June 15, 197~.

13~ W. Thinner, I. Alrpeter, "Determination of residual stresses using micro magnetic pyrometers 1982. Proceeding Germany-United States Workshop on Research and Development to New Procedures in NO Springer Verlag Berlin, 1982.
However, static magnetic fields parallel to the surface are difficult to produce when the ratio of thickness of test body to pole distance is high, and lead to reduced efficiency.
he magnetic fields then penetrate deeply into the material. The field intensity in the near-surface region of the test body is therefore relatively low. however 9 for ultrasonic excitation, only the field parallel to the surface in the near-surface region -is of interest; for a given magnetization power the magnetic field intensity is higher than for the same DC-power.
Moreover, transducers with static magnetic bias field, particularly transducers with permanent magnets, are difficult to move over the surface of a ferromagnetic test body in the event of strong fields and consequently render manipulation of the test heads (containing the bias magnet and high frequency transducer) difficult. A further drawback is that the test body ma possibly become magnetized.
It is also known from US. Patent No. ~,918,295 to Joachim Herbert, issued November 11, 1975, to employ high frequency transmitter coils with an electromagnet driven by a low-frequenc~ ARC. source to produce ultrasound waves or non-destructive testing. The high frequency windings are continuously energized to generate ultrasound waves in the test piece, while the low frequency magnet coil is continuously energized to amplitude-modulate the ultrasound waves with the low frequency magnetic fields. Received acoustic signals are separated from internal noise or external interference by detection of the amplitude modulation.

3 g 7 or excitation of sartorial ultrasonic waves in ferromagnetic and conductive test bodies, the invention provides producing a horizontal magnetic bias field in the near-surface of a test body by using a time-variable magnetic field of relatively low-frequencyO
By varying the magnetic field, the flux lines of the magnetic bias field are urged into the near-surface (skin effect). By using alternating magnetic polarities the test head can thus be moved easily over the surface of the test body, even in the even-t of high surface field intensities.
Furthermore, the test head volume can be reduced without reducing the surface field intensity of the bias field.
The time-variable magnetic field can be produced with air coils, directly magnetically coupled to the body to be tested, or with an electromagnet having a yoke core. or small magnetization losses, the magnet joke must be composed of magnetically conductive sheets which are electrically insulated with respect to one another. The sheet thickness is determined according to the rules ox ARC. transformers.
or receiving and transmitting ultrasonic waves, it it necessary according to the invention to synchronize transmission and reception with magnetization. Transmission and reception thus occur in electrodynamics transducer when the surface field intensity it at a maximum.
With magne-tostric-tive transducers, transmission and reception occur when the decrease of the magnetostriction with the magnetization field intensity is greatest. This condition is valid for most ferritic steel materials. Synchronization can be carried out with the aid of the energizing current of the bias field magnet. The period of time necessary for transmission and reception (path of sound) determines the maximum frequency of magnetization. It is endeavored to 39~'7 transmit and receive with a field which is as constant as possible quasi static This means that, during ultrasonic testing, -the change in the field intensity (induction) proceeds relatively slowly.
More particularly, the present invention pro-vises a method for non-contact, non-destructive testing of a test bray of ferromagnetic and/or electrically-conductive material with ultrasound waves, comprising the steps of producing in a near-surface region of -the test body a low-frequency alternating magnetic bias field having flux lines generally parallel to a surface of the test body, producing high frequency alternating magnetic excitation fields in the near-surface region generally parallel to the surface during a time interval when the bias field is at a quasi static maximum, ad-jacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultrasound waves are generated ion the test body, and detecting high frequency alternating magnetic fields in -the near-surfaee region during the same time interval when the bias field is at a quasis-tatie maximum and producing a signal therefrom representative of the ultrasound waves.
One embodiment of the invention provides, as a method for non contact non-destructive testing of a test body of ferromagnetic and/or electrically-conductive material with ultrasound waves, comprising the steps of producing in a near-surfaee region of the test body a low-frequeney alternating magnetic bias field having flux lines generally parallel to a surface of the test body, producing high frequency alternating magnetic excitation fields in the near-surfaee region generally parallel to the surface during a time interval.
when the bias field is at a quasi static maximum, ad-junta excitation fields having opposing polarity and I
..... . . . . ..

having flux lines lying in mutually parallel directions, whereby ultrasound waves are generated in the -test body, and detecting high frequency alternating magnetic fields in the near-surface region during the same -time interval when the bias -field is at a quasi static maximum and producing a signal -therefrom representative of the ultrasound waves.
In this method, the flux lines of the excite-lion fields lie generally parallel -to the flux lines of the bias field, such that dynamic forces are elect trodynamically generated in -the test body in a direction normal to the surface of the -test body, which launch longitudinal waves, Raleigh waves and Lamb waves, and dynamic forces are magnetostric-tively generated in the test body in a direction parallel to the surface, which launch transversal waves, Raleigh waves and Lamb waves.
The invention also provides apparatus for non-contact, nondestructive -testing of a -test body ox elect trically-conductive and/or ferromagnetic material with ultrasound waves, comprising means for producing in a near-surface region of the test-body a low frequency alternating magnetic bias field having flux lines generally parallel to a surface of the test body, and means, synchronized with the bias field means, for producing high frequency alternating magnetic excitation fields in the near-surface region generally parallel to the surface during a time interval when the bias field is at a quasi-static maximum, adjacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultra-sound waves are generated in the test body, and detect-in high frequency alternating magnetic fields in -the nursers region during a time interval when -the bias field is at a quasi static maximum, and producing an output signal therefrom representative of -the ultrasound pa ... I.. . .... . , , .. i waves.

In -the accompanying drawings:

Figure 1 shows a schematic representation of a transducer arrangement for producing US waves in a -test body;

Figure 2 is a cross-sectional view -taken along line II-II of Figure l;

Figure 3 shows the flux lines of a static mug-netic field in a body of conductive material;

Figure 4 shows the flux lines of an alterna-tying magnetic field in a body of conductive material;

Figure 5 is a time diagram illustrating -the manner of synchronizing high frequency coil energi~.ation with the low frequency bias field in accordance with the invention;

Figure 6 shows schematically in perspective view an arrangement for ultrasonic testing of a test body in accordance with the invention;

Figure 7 is a time diagram illustrating the producing of the s-tart pulses synchronized with -the bias field;

Figure 8 is a time diagram illustrating the time relations between the picking up of ultrasound and Barkhausen-noise;

5b .. ..

.. . . . . .. . . . . . . . ..

inures pa and 9b chow schematically a high frequency coil contraction useful in practicing the present invention.

Figure I show a preferred US -test head in accordance with the invention.

The preferred embodiments are described below with reference to the drawings.

Figure 1 illiterate a test body 10 oriented relative -to a coordinate system Zeus, such that its upper surface lies in an x-y plane. A meander type coil 12 is situated in an zoo plane above the upper surface of the test body, and a magnetic bias field Boy is produced in the upper surface of the test body in the y-direction by a magnetic yoke not shown (see jig. I). Coil 12 is energized with a high frequency current.
Figure 2 shows a section of test body 10 taken along line II-II of figure 1. the elongate coil portions of coil 12 lie in the y-direction, producing high frequency flux lines in the x-direction in the near-surface region of the test body.
The high frequency magnetic field in turn induces eddy currents on -the near surface region ox the conductive test body in a pattern generally resembling -the configuration of coil 12. The period 14 of the coil 12 is the transducer wavelength and corresponds to the sinusoidal variation of an ultrasonic field along the x-direction.
In the presence of a bias magnetic field having flux lines in the direction (Boy in Fig 1) magnetostrictive forces are thus applied to the metallic lattice of a ferromagnetic test body which produce shear horizontal (So) ultrasound waves in the test body. In this case, -the ultrasound waves are polarized (have components of displacement) in only the y-direction. Ike waves propagate in the test body in the x-and/or directions.

It the magnetic bias field is instead oriented in the x direction box in Fig 1) produced by a magnetic yoke not shown (Lee Fig I) dynamic forces in the x-direction are magnetostrictively generated in a ferromagnetic test body, and dynamic forces in the Z direction are electrodynamically generated by Laurent forces acting on the eddy currents in the test body.
Most prior arrangements proposed for the non-contact generation of US waves employ a static magnetic bias field in combination with a high frequency excitation coil. The static bias field may be generated by permanent magnet or by an electromagnet energized by a DO or pulsed DO source.
Figure 3 illustrates diagrammatically the static flux lines produced when such a magnet 16 is placed near a test body 10.
The field penetrates relatively deeply into the test body and only a small portion of its strength in the x direction is in the near-surface region (of thickness "t").
In contrast, figure 4 illustrate the flux lines of an electromagnet driven by a low frequency ARC. source. The time-var~ing yield has a large proportion ox its strength in the x-direction in -the near surface region, due to skin effect. Since the near-~urface flux ox the magnetic bias field in the x- (or y-) direction is that which interacts with -the high frequency field to produce US waves, it will be recognized that greater efficiency can be achieved with a time-varying bias field. Moreover, the magnet can be more easily moved over the surface ox a ferromagnetic text body, and magnetization of the test body is avoided.
The time variable magnetic bias field may be produced by air coils situated to be directly magnetically coupled to the test body. Preferably, however, the bias field is produced by an electromagnet having Q laminated yoke core.
The laminations are of magnetically conductive sheets which are electrically insulated from one another. the sheet thickness is determined accordingly -to the rules of ARC.
transformers.

of or -transmitting (exciting) and receiving (detecting) Us waves with a non-contact test head in accordance with the invention, it is necessary to synchronize excitation and detection of the US waves with the detection time-varying bias yield strength. Thus, US wave excitation occurs with electrodynamics transducers when the near-surface bias field intensity is at a maximum.
With magnetostrictive transducers 9 excitation and detection occur when the decrease of the magnetostriction with the magnetization field intensity is greatest. This requirement is valid for most ferritic steel materials.
Synchronization can be carried out with the aid of the energizing current ox the magnet. The period ox time necessary for transmission and reception (path of sound) determines the maximum frequency of magnetization. It is endeavored to transmit and receive with a field which is as constant as possible (quasi static). This means that, during ultrasonic testing, the change in the field intensity (induction) proceeds relatively slowly. Quasi static means that -the time interval should be smaller than 10% of the period duration of the AC-magnetization current. hi time interval can be sufficiently long to transmit and detect more than one ultrasonic pulse.
Synchronization of excitation and detection it shown diagrammatically in Figure pa. the high frequency coil it energized when the bias yield intensity it in the region ox its maximum. If interval is small relative to the period of the alternating bias yield, -the bias yield strength will remain nearly unchanged during the excitation or detection.
the -time interwove may be long enough to transmit and detect more than one US-pul~e as shown in Fig. 5b~ The first starting point to is synchronized with the magnetization as mentioned above, the following starting points to in are given by the maximal achievable repetition ratio Figure 6 illustrates an arrangement in accordance with the invention for the ultrasonic twitting of a ferromag-netic test body 30 of thickness D. An electroma~let 32 having a laminated yoke core 34 ox electrically-insulated magnetically-conductive sheets is wound with a coil 36. An ARC. source 38 energizes coil 36 to continue produce an alternating bias field in the near-surface region of test body 30, One flux line of the alternating bias field Jo lying generally parallel to the surface of body 30 is illustrated.
An excitation coil 40 (which ma be of any of a number of forms) lies above and generally parallel to the upper surface of the test body 30. Coil 40 is energized by a high frequency source 42 synchronized with the ARC. power source 38 by suitable trigger circuitry 44.
he ARC. source 38 produces a synchronization pulse (Fig. 7b) which is synchronous with the maximum of the sinus wave produced by the same generator (Fig. pa). With the aid of the synchronization pulse and edge triggered mono~lops a tart pulse is produced (jig. 7c) which triggers the high frequency source 42. This can be a tone burst or a pulse generator.
A detection coil 46 it likewise situated near the surface ox body 30 adjacent the nursers region containing the bias yield I Interaction ox US waves in the test body with the bias field By energizes detection coil 46. the output signal V prom detection coil 46 to representative of the detected US waves, which generally ha another shape and smaller energy than the transmitting signal and results from the reflection ox the transmitted US signal at Dakota and geometrical ox lades.
With the arrangement of figure 6, the flux lines of bias yield I are parallel to the elongate portions of coil 40 (hence, normal to the flux lines produced by coil 40 in the near-surface region of test body I resulting in propagation of So wave in the test body which have their direction of movement parallel to the upper body surface and their direction of propagation in the x plan. The SO wave are reflected by defect or by geometrical obstacles (edges, Buckley, eta) and detected at coil 46, producing an output signal.
If alternating current it used for energizing the bias field magnet, the described arrangements for ultrasonic excitation are suitable a-t the same time for pickup magnetic-inductive ~arkhausen poises in ferromagnetic materials (see reference no. 13 above).
he physical mechanism which produces Barkhausen noise occurs mainly near the coercive field strength of the test material; this it near the zero crossing point of the AC
magnetization current. jig. shows diagrammatically the detected US-signal during time interval and the Barkhausen-noise during the Nero crossing points of the AC-current for the magnetization.
It will be understood that the excitation repetition frequency it limited by the US path length, which in turn depends upon the geometry of the text body and the direction in which US wave are propagated in the test body. But the hysteresis losses increase with the magnetization frequency The inventor have wound that an AC source ox 10 En - 1000 Ho it suitable for energizing the bias magnet.
It is further noted that the text body need not have a strictly planar surface. Pro example, the test body may be a curved plate or pipe wall with a relatively large radius of curvature compared to the ultrasonic wavelength (for example, 6mm US-wavelength, 100 em inner radius).
In such case, the bias magnet yoke and excitation/
detection coils are preferably adapted to the shape of the test body surface (e.g., curved); the bias field flux lines and excitation field flux lines are thus considered herein as being "parallel" to the surface configuration of the test body even where such surface it not strictly planar.

inures pa, b show by way ox non-limiting example an excitation detection coil configuration contemplated a being useful within the scope ox the present invention. Such a configuration is known from German Patent DE-AS 26 55 804.
In figure 10, an ultrasonic test head preferred in accordance with the invention for excitation and detection of SO waves in thin 9 curved test bodies it shown.
At the inner surface of a pipe, a section of which it indicated at 479 a bias electromagnet with an u-formed yore 48 consisting ox laminated sheets 49 is oriented in the z direction (e.g. the direction of the pipe axis). my means of two magnetizing coils 50 a low frequency magnetic field in the near surface region parallel to the inner surface of the pipe it produced between the two pole shoes 51~ The bias magnet yoke, especially the pole shoes 51 thereof, is adapted to the inner curvature of the pipe 47. A meander-type coil 52, alto adapted to the pipe curvature, is situated between the pole shoes 51. The elongate coil portions ox coil 52 lie in the z-direction7 producing high frequency flux lines in the near-~ur~ace region parallel to the inner surface and perpendicular to the direction If the magnetizing coils 50 and the high frequency coils 52 are energized as described above, an ultrasonic wave propagate with polarization parallel to the pipe surfaces in the 0,r-plane as guided Shimmied in circumferential direction 53 or as a bulk Shove on a zig-zag-path 54 in the pipe wall. the Chavez are reflected by defects in the pipe wall or wall thickness reduction of the pipe wall and detected by a coil similar to coil 52 producing an output signal.
German Patent DE-AS 26 55 804 describes a scheme for electrically optimizing the properties of Miss. In this context, "electrically optimum" means that, by adjusting certain parameters (e.g. the wire gauges used, transient times, wide-band character for multiplexing modes) a maximum signal-to-noise ratio it ox twined O

It is within the scope of the present invention to further improve upon the operating characteristics of such Emits by employing an alternating bias field with which excitation and detection of US waves it synchronized. EMIT coil arrangements in which an alter-noting bias field may be advantageously employed are disclosed in German Patent 26 55 804, published May 10, 1979.
It is preferred that high frequency transducer coils have a configuration as shown in Fits. pa and 9b, wherein a coil winding D wound around a web STY n-times in grooves G in a transducer body of nonconductive material, and then wound n-times around -the adjacent web so that a common direction of current flow prevails in a groove and, in the adjacent groove, the opposite direction prevails. This type of winding makes i-t posy sidle to construct a transducer with greater efficiency than the simple meander type coil shown in Fig. 1.
The foregoing preferred embodiments are given to illustrate the various ways in which the invention may be employed. Those skilled in the art will recognize other arrangements within the spirit and scope of the invention defined by the following claims.

Claims (9)

1. A method for non-contact, non-destructive testing of a test body of ferromagnetic and/or electrically-conductive material with ultrasound waves, comprising the steps of :
a) producing in a near-surface region of the test body a low-frequency alternating magnetic bias field having flux lines generally parallel to a surface of the test body;
b) producing high frequency alternating magnetic excitation fields in said near-surface region generally parallel to said surface during a time interval when the bias field is at a quasi-static maximum, adjacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultrasound waves are generated in the test body; and c) detecting high frequency alternating magnetic fields in said near-surface region during the same time interval when the bias field is at a quasi-static maximum and producing a signal therefrom representative of said ultrasound waves.

2. The method of claim 1, wherein the flux lines of said excitation fields lie generally parallel to the flux lines of said bias field, such that dynamic forces are electrodynamically generated in the test body in a direction normal to the surface of the test body, which launch longitudinal waves, Rayleigh waves and Lamb waves, and dynamic forces are magnetostrictively generated in the test body in a direction parallel to said surface, which launch transversal waves, Rayleigh waves and Lamb waves.

3. The method of claim 1, wherein the flux lines of said excitation fields lie normal to the flux lines of said bias field, whereby ultrasound waves magnetostrictively generated in the test body are polarized parallel to said surface and perpendicular to their propagation direction.

4. A method for non-contact, non-destructive testing of a test body of ferromagnetic and/or electrically-conductive material with ultrasound waves, comprising the steps of :

a) producing in a near-surface region of the test body a low-frequency alternating magnetic bias field having flux lines generally parallel to a surface of the test body;

b) producing high frequency-alternating magnetic exitation fields in said near surface region in a plane generally parallel to said bias field flux lines and parallel to said surface during a time interval when the bias field is at a quasi-static maximum, adjacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultrasound waves are generated in the test body; and c) detecting high frequency alternating magnetic fields in said near-surface region during the same time interval when the bias field is near coercive field strength, and producing a signal therefrom representative of Barkhausen noise.

5. Apparatus for non-contact, non-destructive testing of a test body of electrically-conductive and/or ferromagnetic material with ultrasound waves, comprising :

means for producing in a near-surface region of the test-body a low frequency alternating magnetic bias field having flux lines generally parallel to a surface of the test body ; and means, synchronized with said bias field means, for a) producing high frequency alternating magnetic excitation fields in said near-surface region generally parallel to said surface during a time interval when the bias field is at a quasi-static maximum, adjacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultrasound waves are generated in the test body; and b) detecting high frequency alternating magnetic fields in said near-surface region during a time interval when the bias field is at a quasi-static maximum, and producing an output signal therefrom representative of said ultrasound waves.

6. The apparatus according to claim 5, wherein said bias field means comprises an electromagnet having a laminated yoke core, and a coil winding about said core.

7. The apparatus according to claim 6, wherein said bias field means further comprises a low frequency signal source coupled for energizing said coil winding.

8. The apparatus according to claim 5, wherein said synchronized means includes at least one transducer having:

a non-conductive transducer body with a plurality of elongated and mutually parallel webs separated by grooves, and coil windings wound a plurality of times around the webs such that current in portions of the windings lying in each groove flows in a uniform direction and that current in portions of windings lying in adjacent grooves flows in opposing directions.

9. The apparatus according to claim 5, wherein said synchronized means further includes a high frequency signal generator, coupled to a first said transducer and to said bias field producing means, for energizing said transducer coil windings when said bias field is at a quasi-static maximum.