Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for the
non-~estructive testing of ferromagnetic material, preferably
of cylindrical test pieces such as tubes or bars, as well as
to an apparatus for effecting the method.
The non-destructive testing of materials can be
effected by means of ultrasonics, where piezoelectric trans-
ducers are used emitting ultrasonic waves that are passed
into the sub~ect by means of a coupling medium, e.g. water.
In addition, there is an electrodynamic generation of ultra
sonics in electroconductive ferromagnetic workpieces. By
the electrodynamic system ultrasonic waves are generated by
use of a coil which generates high-frequency pulsed eddy cur-
rents into the workpiece. A magnetic field acting in this
region at the same time, together with the eddy currents
- leads to the generation of Lorentz forces acting on the grid
system of the workpiece and causes sound waves. This prin-
ciple illustrating the generation is reversible for the
receptio~. The testing of workpieces of ferromagnetic mate-
rials, e.g. tubes or bars, can also take place in a stray-
flux rotary system where a magnetic flux is generated in the
test piece, and flaws in the test piece can lead to stray
fields being detected by means of magnetic field probes.
~s to prior art, reference is made to DE.OS 25 59
125, published August 15, 1976 to Institutul National Pentru
Creatie Stiintifica si Techmca - INCREST, DE.PS 26 21 684
issued July 12, 1979 to ~oesch Werke AG, Sumitomo Search No.
17, May 1977, pages 73 to 79.
Stray flux measurings are carried out in order to
locate longitudinal flaws or holes in cylindrically-shaped
test pieces. However, this technique makes it impossible or
hardly possible to supply data on sensed longitudinal flaws
and measured data on the dimensions of tubes or bars such as
wall thickness. ~herefore, in order to ascertain these
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values one will normally fall back on ultrasonic-rotation
systems, which are not only essentially more expensive but
also due to the required ultrasonic coupling via e.g. a water
course, of a rather complicated construction.
The present invention provides a method and an
apparatus for simultaneously sensing longitudinal flaws,
holes, transverse flaws as well as for determining dimensions
by taking advantage of the magnetic field generated by the
magnet rotating around the test piece.
In accordance with the present invention the mag-
netic field generated by the magnet is used simultaneously
for the electro-dynamic excitation of ultrasonics and for
measuring the stray flux. Consequently the method according
to the invention is a combination of conventional testing
techniques of which each can be separately and optimally used
for the detection of longitudinal flaws and holes or, trans-
verse flaws and the measuring of dimensions. Thereby it is
especially emphasized that for the testing no coupling med-
ium is required so that in particular the cost-saving pro-
perties of the stray flux-rotation systems are retained.
The method according to the invention, i.e. the
combination stray flux measuring and electrodynamic excita-
tion techniques can be applied by an apparatus where the
test piece is surrounded by a rotating magnet having two
pole shoes; at least one magnetic field probe is provided
on the circumferential surface of the test piece rotating
synchronously to the magnet, and eddy current exciters'
and reception probes are arranged in the ranges of the pole
shoes facing the test piece and/or within the range of the
magnetic field probe facing the test piece. In other words,
some conventional features of an apparatus referring to stray
flux-rotation testing systems are integrated into the concept
according to the invention. Essentially these are rotating
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pole shoes of a magnet generating a magnetic flux in the test
piece. If the field strength is increased to a value where
the test piece material reaches magnetic saturation, then
defects such as cracks or cross section reductions will lead
to relatively strong stray fields that can be detected by
means of magnetic field probes rotating synchronously with
the magnet around the test piece. In a preferred embodiment
two groups of magnetic field probes are provided, which being
disposed 90 to the pole shoes, synchronously rotate around
the test piece. By an appropriate measuring of the stray flux
and utilization of the described arrangement of pole shoes
and magnetic field probes, outer defects up to 10% of the wall
thickness can be pexfectly determined.
In order to enable the measuring of dimensions and
the location of cross flaws, an electrodynamic excitation of
ultrasonic signals takes place according to the present in-
vention. The magnetic field required for this is generated
by the rotating magnet yoke having two pole shoes, so that
those elements can be used which are already required for
the stray flux measuring. Thus, one uses especially those
magnetic fields generated by the rotating magnet and appear-
ing vertical and parallel relative to the surface of the
test piece. The parallel fields appear within the range of
the magnetic field probes being arranged in a region of 90
to the pole shoes, whereas the vertical ones appear in the
region of the pole shoes. The ultrasonic waves generated
together with the magnetic field and the eddy current pulses
can be transverse of surface waves, depending upon the arr-
angement of the eddy current exciting probes and the recep-
tion probes. The transverse waves are then used to determine
dimension values, thus e.g. wall thickness, and the surface
waves for the detection of cross flaws.
In an especially preferred embodiment of the inven-
tion, for the generation of transverse waves the eddy current
exciting sondes and the reception probes are arranged in the
free ends of the pole shoes facing the test piece, wherein a
further embodiment several exciting and receiving probes are
arranged side by side so as to be able to scan several traces
at the same time.
An optimum excitation of surface waves takes place
in the range of the magnetic field probes arranged in regions
of 90 with respect to the pole shoe s, so that in a further
embodiment the eddy current exciting probes and the reception
probes simultaneously are arranged on the magnetic field
probes. In a preferred embodiment of the invention -these
probes are flat coils of which the windings are meander-like.
Thus, for an optimum utilization of the transmitted measured
values, the main axes of the meander wire arranged windings
can run parallel or almost parallel to each other or can
describe a right angle or substantially a right angle with
respect to each other.
A main advantage of the method according to the in-
vention is that by a combination of the conventional stray flux
method and the conventional electrodynamic excitation of
ultrasonics, without using any coupling medium one can measure
not only wall thicknesses but also make locations of transverse
and longitudinal flaws or holes, where for generating the re-
quired magnetic fields that of the rotating magnet is utili-
zed.
The present invention will be further described by
3~ way of the accompanying drawings in which:-
Figure 1 is a sectional view of a rotation testingarrangement in accordance with one embodiment of the present
invention; - -
, Figure 2~illu~trates an arrangeme~t of eddy currentexciting probes an~ reception prcbes; and
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Figures 3 and 4 show preferred arrangements of mag-
netic field prohes and eddy current exciting and receiving
probes, arranged on a common support.
Fig. 1 ~trictly schematically presents a section of
a rotation testing unit, including essential elements of stray
flux rotation testingsystems known per se, as they are also
described in literature, like e.g. ASME, September 18 to 23,
1977, Houston, Texas, Nondestructive Inspection of Oil Country
Tubular Goods, pages 1 to 13; The Sumitomo Search No. 17, May
1977, pages 73 to 79; Materials Evaluation, July 1977, pages
52 to 56.
Such stray flux rotation systems essentially com-
prise a magnet having two pole shoes 12 and 14, which con-
15^ cen-trically s~rround and rotate about a tube 16. A magnetic
- flux is thus generated in the test piece 16 due to the mag-
netic field between the pole shoes 12 and 14. If the field
strength of the magnet is increased to a value causing mag-
netic saturation in the material of the test piece 16, then
in case of cross sectional reductions caused by defects such
as longitudinal flaws of holes, relatively strong stray fields
will appear over these defects. These stray fields are then
detected by means of ma~netic field probes 18 and 20, which
arranged in regions 90 to the pole shoes 12 and 14, are
synchronously rotating around the test piece 16 at the fre-
quency w. The magnetic field probes 18 and 20 are eachsupported by carriers 22 and 24 respectively. By means of
such stray flux measurings one can exactly detect external
flaws up to 5% of the wall thickness and internal flaws up
to 10% of the wall thickness.
By means of these stray flux measurings, however,
one cannot find transverse flaws and measuring data on e.g.
the dimensions of tubes or bars, such as wall thicknesses
of tubes. According to the present inven~ion, however,
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these are detected in the same rotation system by means of
the conventional electrodynamic excitation method for ultra-
sonics. For that purpose it is necessary that a maynetic
field appears parallel or vertical to the surface of the test
piece -together with an eddy current pulse generated in the
surface of the test piece, in order to generate different
force pulses on account of the appearing Lorentz forces on
the surface of the test piece resulting in ultrasonic waves.
Depending on the magnetic field direction, transverse, longi-
tudinal or surface waves can be generated as ultrasonics.For measuring the wall thickness one uses generally trans-
verse waves whereas the detection of transverse flaws takes
place by means of surface waves.
The electromagnet or permanent magnet having the
- pole shoes 12 and 14, while rotating now generates a magnetic
~ield, extending underneath from the pole shoes vertically
to the surface of the test piece and staggered through 90,
thus within the range of the magnetic field probes 18 and 20
parallel to the surface. This means that in one operation
with the stray flux measuring one can also effect an electro-
dynamic excitation of ultrasonic signal waves by means of
the rotating magnet.
In order to be able to measure the wall thickness
of the test piece 16, eddy current exciting probes and re-
ceiving probes are arranged in the free ends of the pole
shoes 12 and 14 facing the test piece. In Fig. 1 these
probes 26, 28 or, 30, 32 respectively, are schematically
shown. Consequently the eddy current pulses generated by
the probe 26 or 30 are sensed by the eddy current receiving
probe 28 or 32, respectively bein~ arranged in the same pGle-
shoes 12 or 14, respectively in order to be evaluated in
conventional electronic systems (not shown). In this con-
nection reference is made to l'Ultrasonic Testing of Half-
wrough~ Material by Using Electrodynamic Instrument Trans-
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Eormers", Hoesch Huettenwerke AG, Dortmund, January 25, 1980
or the German Patent Specification No. 26 21 684 issued July
12, 1979 to Hoesch Werke AG and the German Disclosure No.
29 24 819 published January 15, 1981 to M.A.N. Masc~inenfabrik
Augsburg-Nurnberg.
By the arrangement of the eddy current excitïng
probes and reception probes 26, 28 or 30, 32, respectively
within the ranges of the pole shoes 12 and 14 wall thicknesses
at an accuracy of + 5/100 mm can be determined.
The optimum excitation of surface waves takes place
within the range of the magnetic field probes 18 and 20 so
that consequently at the same place there are also arranged
eddy current exciting probes and receiving probes 34, 36 or
- 38, 40, respectively. These probes are preferably placed in
the holdiny device 22 or 24, respectively serving as supports
for the magnetic field probes 18 and 20. For an optimum
detection of measuring data, the eddy current exciting pro-
bes and receiving probes 34, 36 or 38, 40, respectively
should be flat coils of which the windings are meander-sha-
ped, as shown in Figures 3 and 4. By the wave extending in
the surface area, portions thereof are reflected to the
receiver preferably if these are positioned diagonally to
the direction of propagation and in the surface area. There-
by different sensor arrangements can be achieved. According
to Fig. 3 the main axes of the meander-like configured coils
34 and 36 extend parallel to each other and are arranged next
to the magnetic field probes 18. According to Fig. 3 several
magnetic field probes are simultaneously arranged side by
side in order to simultaneously scan a larger surface area
of the test piece 16. The magnetic field probe 18 as well
as the meander-shaped coils 34 and 36 are carried by the same
support 22.
Fig. 4 is a top view of the holding device 24,
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where several magnetic field probes 20 are provided side by
side. The eddy current exciting or receiving probe 38 or
40, respectively is likewise of meander-like configuration,
where the axes of them are defining an angle of preferably
90 degrees relative to each other.
By the special arrangement of the eddy current
exciting or receiving probes 34, 36 or 38, 40, respectively
defects close to the surface, can be detected, of which the
extension is smaller than 10% of the respective wall thick-
ness. In the sensing of transverse notches it has been found
that defects of 7% of the wall thickness can still be clearly
detected.
In order to engage a larger surface area of the
test piece 16 during a rotation also in regard to the eddy
current exciting and receiving probes 26, 28 or 30 and 32,
respectively arranged in the pole shoes 12 and 14, arrange
several probes can be disposed side by side, as shown in
Fig. 2.
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