Note: Descriptions are shown in the official language in which they were submitted.
NON-DESTRUCTIVE NON-CONTAC~ ULTRASONIC MATERIAL
TESTING METHOD AND APPARATUS
.
The present invention relates to methods and
apparatus for nondestructive non-contact testing of
materials with ultrasonic waves.
.
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.
