Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"ULTRASONIC FLAW DETECTING METHOD"
BACKGROUND_OF~ _INVENTION
The present invention in general relates to maintenance
technology of equipment such as steam drums, heat transfer
tubes of a steam generator, pressure tubes used in a
pressure tube type nuclear reactor, and equipment and piping
such as pressure vessels and pipes for use in a nuclear
reactor and an ordinary plant. More particularly, it
relates to an ultrasonic flaw detecting method for detecting
a surface flaw of the above-mentioned equipment and piping
and for quantitatively measuring the depth of the surface
flaw.
Ultrasonic flaw inspection has conventionally been
carried out for maintenance management of the above-
mentioned plant equipment and piping as an object to be
inspected. Specifically, surface flaws of the inspecting
object are inspected by a ultrasonic angle beam method in
which, as shown in Fig. 5, a probe ~ scans the surface of
the inspecting object 1 while emitting ultrasonic waves onto
the surface of the inspecting object 1 at a predetermined
oblique angle. The ultrasonic waves enter into the
inspecting object 1 at a predetermined angle of refraction,
. When no surface flaw exists in the inspecting object 1,
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the ultrasonic angle beame is repeatedly reflected at the
surface and the bottom of the inspecting object I to be
propagated forward within the inspecting object l. When a
surface flaw 3 exists, however, the ultrasonic angle beam
which is reflected on the bottom of the inspecting object
meets a corner of the surface flaw, i.e. an intersection of
the side wall of the flaw 3 and the surface of the
inspecting object l. As a result, an echo appears at the
corner and runs back to the point of incidence by following
the same path. The echo is detected and converted into an
electrical signal, and is observed as "flaw corner echo"
having the maxinum amplitude at a position W1 of the
distance axis (axis of abscissas) on the screen of an
oscilloscope 4 which is electrically connected to the probe
2. By observing the flaw corner echo, the surface flaw 3 of
the inspecting object l is detected according to the
ultrasonic angle beam method.
In order to quantitatively measure the flaw depth of the
surface flaw 3 ("H" in Fig. 5), there maY be employed a tip
echo method. When the probe 2 moves to the position 2' in
Fig. 5 during the scanning, "flaw tip echo" which appears
at the pointed end of the flaw 3 is observed at a position
W2 on the screen of the oscilloscope 4. The position W1 of
the flaw corner echo and the position W2 of the flaw tip
echo observed on the same osilloscope screen correspond to
the propagation distance of the ultrasonic waves from the
point of the emission of the ultrasonic angle beam and the
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point of the reception of the echo by the probe 2 and by the
probe 2', respectively. According to the tip echo method,
the flaw depth H of the surface flaw 3 is obtained by first
reading the positions of the flaw corner echo W1 and the
flaw tip echo W2 observed on the distance axis of the
oscilloscope screen, calculating the difference in distance
(Wt - W2), and then substituting the obtained difference and
the angle of refraction ~ into the following equation (1):
H = ~W~ - W2) X COS ~ ............. (1)
However, in the case where a quantitative measurement of
the surface flaw depth of the inspecting cbject is actually
made by the tip echo method, a parasitic echo appears on the
oscilloscope screen in addition to the above-mentioned flaw
corner echo and flaw tip echo. This parasitic echo is
produced by ultrasonic waves which are emitted from the
probe 2 or 2' onto the inspecting object 1 and run as
"surface waves" on the surface of the inspecting object,
without entering into the inspecting object (see Fig. ~).
This parasitic echo is almost indistinguishable from the
flaw tip echo and often causes troubles in the measurement.
For example, in the case of a flaw whose flaw depth is 2 to
3mm present in a pressure tube of 4.3mm in thickness used in
a pressure tube type nuclear reactor, a parasitic echo
appears near the flaw tip echo. Such a parasitic echo is so
indistinguishable from the flaw tip echo that it causes even
a skilled operator to mistake one for the other. As a
result, the measurement error is as great as about l.5mm,
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thereby impairing the accuracy of measurement. In the case
of a flaw depth of 3 to 4mm, a parasitic echo which overlaps
and succeeds the flaw tip echo appears. In this case, it is
impossible to quantitatively measure the surface flaw depth
of the inspecting object. Thus, the tip echo method is not
always practically applicable to the quantitative
measurement of the surface flaw with respect to a thin
material whose thicknss is about 5 to 6mm or below.
SUMMARY OF THE IN~ENTION
The present invention has been made in view of the
foregoing, and has an object to provide an ultrasonic flaw
detecting method in which the depth of surface flaws of a
thin material whose thickness is about 5 to 6mm or below can
quantitatively be measured by the tip echo method easily and
accurately.
The ultrasonic flaw detecting method according to the
present invention includes the steps of detecting a surface
flaw of an inspecting object which has a uniform thickness
by causing a probe to scan while emitting ultrasonic waves
at a predetermined oblique angle onto the surface of the
inspecting object and quantitatively measuring the depth of
the surface flaw by a tip echo method. The above object can
be achieved by abutting an ultrasonic wave shielding member
against the surface of the inspecting object in an area
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between the point of incidence of the ultrasonic waves and
the surface flaw.
Any material which can absorb and thereby shield surface
waves of the ultrasonic waves may generally be used as the
above-mentioned ultrasonic wave shielding member. A rubber
material is preferably used as the ultrasonic wave shielding
member for the reason that it is possible to ensure a stable
contact with the inspecting object even in the case where
the inspecting object is a pipe having a bent portion. For
the same reason, it is preferable that the material be
abutted against the surface of the inspecting object by
pressing.
According to the present invention, the surface flaw of
the inspecting object can be detected by the ultasonic angle
beam method, and the flaw depth of the surface flaw can be
quantitatively measured by the tip echo method in the same
manner as the conventional measuring system.
Particularly in the present invention, the surface waves
which cause the parasitic disturbing echo can be absorbed
and shielded by abutting the ultrasonic wave shielding
member against the surface of the inspecting object in the
area between the point of incidence of the ultrasonic waves
and the surface flaw. In Fig. 5, the flaw depth of the
surface flaw does not exceed the thickness of the inspecting
object, and the inspecting object is presumed to be a flat
plate having a uniform thickness of T. In such a case, the
ultrasonic wave sielding member may be positioned and
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abutted againsl the surface of the In.spectlng object, in
prlnciple from the flbove-mentioned equation (1), withln a
range extending from the point of incidence of the
ultrasonic waves to a point which is away as much as T x tan
8 from the point of incidence of the ultrasonic waves in the
direction of scanning with the probe 2. The position of the
ultrasonic wave shielding member within the above-decsribed
range will cause no problem in measuring the flaw tip echo
unless the surface flaw is much extensive.
Further, in order to ensure the measurement of the flaw
tip echo, it is preferable to position the ultrasonic wave
shielding member as close to the point of incidence of the
ultrasonic waves as possible.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1 and 2 are explanatorY views showing an
embodiment of a measuring system according to the present
invention;
Fig. 3 shows an oscilloscopic waveform indicating the
surface flaw of the inspecting object recorded by the
measuring system shown in Figs. 1 and 2;
Fig. 4 shows an oscilloscopic waveform with the
ultrasonic wave shielding member eliminated from the
measuring system shown in Figs. 1 and 2; and
Fig. 5 is an explanatory view showing an example of a
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conventional measuring system.
PR~EERR~P_EMBODIMENTS OF ~HE IN~ENTION
The present invention will be described hereinbelow with
reference to the drawings and embodiments of the invention.
Figs. 1 and 2 show an example of a measuring system to
be used when a method according to the present invention is
employed to measure a surface flaw 3 casued along the axis
of an inspecting object 1 which is a pressure tube for use
in a pressure tube type nuclear reactor and which is 118mm
in bore and 4.3mm in thickness. The measuring system
comprises a horizontal flat support plate 5 which is
disposed within the inspecting object 1 so as to be
coaxially rotatable therewith. A proe ~ is fixed by a probe
fixing means 6 at a predetermined eccentric position on the
support plate 5 so as to emit the ultrasonic waves on to the
inner surface of the inspecting object 1 at a predetermined
oblique angle. Thus, the emission of the ultrasonic waves
and the detection of the ultrasonic echoes can be carried
out while the support plate 5 is rotated, and the scanning
with the probe 2 can be conducted. An ultrasonic wave
shielding member 7 made of a plate-shaped rubber such as a
rubber wiper is also fixed by a fixing means 8 on the
support plate 5. The tip portion of the ultrasonic wave
shielding member 7 is abutted against the inner surface of
the inspecting object 1 at a position slightly moved in the
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direction of scann~ng relatlve to the point of Incidence of
the ultrasonic waves. Further, water serving as a couplant
is filled inside the pressure tube of the inspecting object
l, i.e., between the inspecting object l and the probe 2.
The probe 2 is electrically connected to an oscilloscope 4
which is externally provided to record on the screen thereof
echo signals detected by the probe 2.
Using the measuring system thus constructed, the surface
flaw 3 which has been caused along the axis of the
inspecting object l is detected by the above-described
ultrasonic angle beam method and the surface flaw depth is
quantitatively measured by the tip echo method, under the
condition that the support plate 5 is rotated
counterclockwise while the probe 2 and the oscilloscope 4
are operated.
The echo images recorded on the screen of the
oscilloscope 4 are shown in Fig. 3, and the echo images
measured without using the ultrasonic wave shielding member
7 are shown in Fig. 4. These echo images have been
obtained as the results of the above detection and
measurement.
It is apparent from the comparison between the data
shown in Figs. 3 and 4 that, according to the ultrasonic
flaw detecting method of the present invention, the
parasitic echoes have efficiently been eliminated.
When a high-resolution ultrasonic flaw detector is used
togehter with a high-frequency wave (about lO MHz), high-
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resolutlon and focused type probe, lt is posslble to confinethe measurement error withln about ~0.5mm for the inspection
of a pressure tube used in a pressure tube type nuclear
reactor whose thickness is 4.3mm.
During the rotation of the support plate S in the above
measurement, the ultrasonic wave shielding member 7 may
possibly be detached from the surface of the inspecting
object 1 due to the rising of the ultrasonic wave shielding
member 7, or unsmooth rotation of the support plate 5 may
possibly occur. In order to prevent such inconveniences, it
is preferable to interpose a spring or the like member (not
shown) between the ultrasonic wave shielding member 7 and
its fixing means 8 to thereby properly press the ultrasonic
wave shielding member 7 against the surface of the
inspecting object 1.
From the foregoing description, it is understood that,
according to the present invention, the surface wave which
forms a parasitic echo is effectively absorbed and thereby
shielded by the ultrasonic wave shielding member abutted
against the surface of the inspecting object in the area
between the point of incidence of the ultrasonic waves and
the surface flaw. Accordingly, in the inspection of, for
example, a pressure tube of 4.3mm in thickness which is for
use in a pressure tube type nuclear reactor, it is possible
to confine the measurement error within about l0.5mm in
contrast to the conventional measurement error of about
I1.5mm, when the flaw depth of the surface flaw whose depth
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is 2 to 3mm is measured. On the other hand, when the flaw
depth of 3 to 4mm Is measured in the above-mentloned
inspection, even the skllled operator has not been able to
distinguish the flaw tip echo from the parasitic echo.
According to the method of the present invention, however,
the measurement of such flaw depth of the surface flaw can
be easily performed by an operator having an ordinary skill
in the ultrasonic flaw detection technology, and the
measurement error can be confined within about +0.5mm.
Further, the quantitative measurement of the surface flaw
depth can easily and accurately be conducted even with
respect to an inspecting object made of a thin material of
about 5 to 6mm in thickness or an inspecting object which is
not flat but tubular.
Therefore, by the method of the present invention, a
quantitative inspection can be accomplished with respect to
the extent of progress of abnormality such as a leak due to
the breakage of plant equipment or piping before such
abnormality becomes apparent. This provides an advantage in
that a better maintenance management of the plant equipment,
piping and the like can be performed. Accordingly, the
present invention can greatly contribue to the improvement
of safety especially in a nuclear reactor plant which
demands extremely higher safety than an ordinary plant
does.
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