Eddy-current flaw detector probe

Details Eddy-current flaw detector probe Eddy-current flaw detector probe

2000-02-29

2002-12-31

Snow, Walter E. (Department: 2862)

Electricity: measuring and testing

Magnetic

With means to create magnetic field to test material

C324S262000

Reexamination Certificate

active

06501267

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an eddy current testing probe which is usable in nondestructive testing.
BACKGROUND ART
Eddy current testing probes have been developed for use in nondestructive testing, which are performed, for example, during manufacturing of steel or nonferrous products or maintenance of heat exchanger tubes in various plants. Basically, testing of a specimen by using an eddy current testing probe is performed by generating an eddy current at a surface of the specimen by an excitation coil, and monitoring change in impedance of a detection coil due to an influence of the eddy current to detect a flaw. When a flaw exists at the surface of the specimen, the flaw affects the eddy current generated at the surface of the specimen. When the eddy current changes, an influence of the change in the eddy current appears in the impedance of the detection coil. Therefore, the flaw in the specimen can be detected by monitoring the change in the impedance of the detection coil.
FIGS.
7
(
a
) to
7
(
d
) are schematic diagrams illustrating the constructions of various types of conventional eddy current testing probes, where the eddy current testing probes can be sorted into the respective types based on their modes of detection of the change in the impedance and their constructions of the excitation coils and the detection coils.
Based on the modes of detection of the change in the impedance, the eddy current testing probes can be sorted into an absolute type and a differential type. The eddy current testing probe of the absolute type detects the flaw in the specimen by a detection coil, as illustrated in FIGS.
7
(
a
) and
7
(
b
), and the eddy current testing probe of the differential type detects the flaw in the specimen based on a difference between amounts of impedance generated in a pair of detection coils, as illustrated in FIGS.
7
(
c
) and
7
(
d
).
Based on the constructions of the excitation coils and the detection coils, the eddy current testing probes can be sorted into a selfinduction type as illustrated in FIGS.
7
(
a
) and
7
(
c
), and a mutual-induction type as illustrated in FIGS.
7
(
b
) and
7
(
d
). In the eddy current testing probe of the selfinduction type, a single coil functions as both of an excitation coil and a detection coil, where the excitation coil generates an eddy current, and the detection coil detects impedance. In the eddy current testing probe of the mutual-induction type, an excitation coil (primary coil) and a detection coil (secondary coil) are provided separately.
As described above, the eddy current testing probes can be sorted into the four types as illustrated in FIGS.
7
(
a
) to
7
(
d
), according to their modes of detection of the change in the impedance of the detection coil and their constructions of the excitation coils and the detection coils.
The basic constructions and operations of the eddy current testing probes illustrated in FIGS.
7
(
a
) to
7
(
d
) are explained below.
In the eddy current testing probe of the absolute and selfinduction type as illustrated in FIG.
7
(
a
), an excitation and detection coil
59
which functions both of the excitation coil and the detection coil is arranged to face a specimen (a planar object to be tested)
10
. An oscillator (not shown) and an instrument (not shown) for monitoring the impedance are connected to the excitation and detection coil
59
. The oscillator is provided for supplying an alternating current to the excitation and detection coil
59
.
In order to detect a flaw in the specimen
10
by using the eddy current testing probe having the above construction, first, an alternating current from the oscillator is supplied to the excitation and detection coil
59
for generating an alternating magnetic field as illustrated by the arrows F
1
and F
2
, so that an eddy current is generated at the surface of the specimen
10
. Thus, impedance corresponding to the eddy current is generated in the excitation and detection coil
59
. If a flaw exists at the surface of the specimen
10
, the eddy current changes, and thus the impedance of the excitation and detection coil
59
also changes. Therefore, the flaw in the specimen
10
can be detected by monitoring the impedance of the excitation and detection coil
59
.
In the eddy current testing probe of the absolute and mutual-induction type as illustrated in FIG.
7
(
b
), a detection coil
51
and an excitation coil
52
are arranged so that the detection coil
51
and the excitation coil
52
face the specimen
10
, and are adjacent to each other. An instrument (not shown) for monitoring the impedance is connected to the detection coil
51
, and an oscillator (not shown) is connected to the excitation coil
52
.
In the above construction, an alternating magnetic field as illustrated by the arrows F
3
and F
4
is generated by the excitation coil
52
so that an eddy current is generated at the surface of the specimen
10
. Then, impedance generated by the eddy current in the detection coil
51
is monitored to detect the flaw.
In the eddy current testing probe of the differential and selfinduction type as illustrated in FIG.
7
(
c
), the excitation and detection coils
59
a
and
59
b
forming a pair are arranged at the same distance from the specimen
10
to face the specimen
10
. An oscillator (not shown) is connected to each of the excitation and detection coils
59
a
and
59
b
for supplying an alternating current to the excitation and detection coils
59
a,
59
b
. In addition, an instrument (not shown) for monitoring a difference between amounts of the impedance generated in the excitation and detection coils
59
a
and
59
b
is connected to both the excitation and detection coils
59
a
and
59
b.
When detecting a flaw in the specimen
10
by using the eddy current testing probe having the above construction, first, an alternating current from the oscillator is supplied to the excitation and detection coils
59
a
and
59
b
to generate an alternating magnetic field as illustrated by the arrows F
5
and F
6
by the excitation and detection coil
59
a
and an alternating magnetic field as illustrated by the arrows F
7
and F
8
by the excitation and detection coil
59
b,
so that eddy currents are generated at the surface of the specimen
10
. At this time, impedance is generated in each of the excitation and detection coils
59
a
and
59
b
corresponding to the eddy current. When no flaw exists at the surface of the specimen
10
, the state of the surface is uniform, and the distribution of the eddy current generated at the surface of the specimen
10
is also uniform. Therefore, amounts of the impedance generated in the respective excitation and detection coils
59
a
and
59
b
are identical. On the other hand, if a flaw exists at the surface of the specimen
10
, the distribution of the eddy current generated at the surface of the specimen
10
is not uniform due to the existence of the flaw. Therefore, amounts of the impedance generated in the respective excitation and detection coils
59
a
and
59
b
become different. Thus, the flaw can be detected by monitoring a difference between the amounts of the impedance generated in the excitation and detection coils
59
a
and
59
b.
In the eddy current testing probe of the differential and mutual-induction type as illustrated in FIG.
7
(
d
), an excitation coil
52
and a pair of detection coils
51
a
and
51
b
are arranged to face the specimen
10
, where the pair of detection coils
51
a
and
51
b
is located nearer to the specimen
10
than the excitation coil
52
. An oscillator (not shown) is connected to the excitation coil
52
, and an instrument (not shown) for monitoring a difference between amounts of the impedance generated in the detection coils
51
a
and
51
b
is connected to the detection coils
51
a
and
51
b.
The detection coils
51
a
and
51
b
are arranged at the same distance from the specimen
10
so that the detection coils
51
a
and
51
b
are symmetrically located with respect to a centerline of the excitati

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