DEVELOPMENT OF EDDY CURRENT PROBE FOR THICKWALLED PLATES AND QUANTITATIVE EVALUATION OF
CRACKS
T. Takagi, T. Uchimoto, K. Sato1 and H. Huang
Institute of Fluid Science
Tohoku University,
Katahira 2-1-1, Aoba-ku, Sendai 980-8577, JAPAN
ABSTRACT. This paper demonstrates the crack detection of thick-walled non-magnetic metal plates
by eddy current testing, which used to be difficult because of the skin effect generally. For the
purpose, this paper proposes a novel eddy current testing probe for cracks in thick-walled plates and
evaluates the capability of the present probe. The probe was designed, based on the numerical
computation using 3D fast eddy current code. The advantages of the present probe are strong eddy
current on the back of specimens and small decay of eddy current in the thickness direction. Through
experiments, we confirmed that this probe can detect the back artificial defect on INCONEL718
specimen with thickness of 7.0mm and 304 Stainless steel specimen with thickness of 8.0mm.
INTRODUCTION
Eddy current testing (ECT) includes some advantages such as high sensitivity
against surface cracks, fast and non-contact testing, simple easy signal data acquisition and
processing. However, its greatest drawback is the skin effect; eddy currents decay in the
thickness direction of targets, which limits the application of ECT to thin-walled materials.
For the last decade, ECT technique made a great progress through its application to
steam generator tubes in nuclear power plants [1, 2]. Special stress should be laid on that
ECT allows quantitative evaluation of detected cracks together with numerical simulation
of eddy current (EC) signals [3-5]. It is expected that, overcoming the impediment due to
the skin effect, ECT should be applied to welded parts in thick-walled austenite stainless
steels and nickel based alloys, where conventional ultra-sonic testing has problems in
detectability of cracks.
This paper proposes a novel eddy current testing probe for thick-walled plated,
aiming at extension of application of ECT. For the purpose, geometry and dimensions of
the probe are optimized with use of eddy current analysis based on the reduced magnetic
vector potential method with discretisation by edge based finite elements [6-9]. Capability
of the fabricated probe is evaluated through experiments using INCONEL 718 specimen
and 304 stainless steel specimen with welded parts.
1
Present affiliation: HITACHI, Ltd., Minami-Ohi 6-26-2, Shinagawa-ku, Tokyo 140-8573, JAPAN
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/$20.00
397
DESIGN OF ECT PROBE FOR THICK-WALLED PLATES
Basic Concept of Present Probe
The skin effect makes it difficult to detect and evaluate flaws in the opposite side of
scan surface of thick-walled plates; enough amount of eddy current can not reach around
flaws in the back side due to the decay of eddy currents, and large eddy currents in the
vicinity of the probe can be a large noise source reflecting states of the scan surface which
is not interesting. The above two problems can not be fixed simultaneously, only by tuning
the test frequency.
Here, two coils carrying current opposite to each other shown in Figure 1 are
considered as a exciter to satisfy two conditions: (1) to generate strong eddy current
opposite to scan surface, and (2) to reduce decay of eddy current in the thickness direction.
A differential pick-up coil is arranged between two exciting coils. The arrangement and
dimensions of each coils are determined by eddy current computation so as to meet the
above two conditions.
Determination of Arrangement and Dimension of Coils
Parameter survey was conducted using eddy current analysis based on the reduced
magnetic vector potential method and edge based finite elements [5, 6]. Taking a target of
an INCONEL plate with the thickness of 7 mm, optimal arrangement and dimensions of
each coil were investigated. Frequency of the exciter was fixed to 10 kHz, which
corresponds to 5.5 mm of the skin depth for INCONEL 718. The current density of
exciting coils and the lift-off are 1.0 MA/m2 and 0.2 mm, respectively, which was also
fixed in the survey. As for each coil constituting the exciter, three independent parameters
should be determined as is shown in Figure 2(a).
At first, focusing on three parameters of the exciting coils, width W, height H and
inner radius IR, sensitivity analyses were carried out as shown in Figure 2(b). The base
values for W, H and IR are 3.0 mm, 3.0 mm, 3.0 mm, respectively, and the target
parameter is amount of eddy current density at the field point located on the back side of
the plate shown in Figure 2(a). Figure 2(b) depicts that the width W has the largest
dependence on the eddy current at the field point on the back side. Here, 6.0 mm of the
width is adopted, taking the consideration of constraint of space.
Kck-u
two
Flux
FIGURE 1. Schematic drawing of the developed eddy current probe.
398
Fixing the width to 6.0 mm, the dependence of H and IR on the back eddy current
was examined based on the contour plot of the back eddy current against H and IR as
shown in Figure 2(c). One can observe that the steepest change of the back eddy current
lies on the line H = IR, and the design point for the exciting coils should be in the vicinity
of the line H = IR for efficient excitation. Finally, the dimensions of each exciting coil, W,
H and IR are determined as 6.0 mm, 12.0 mm and 10.0 mm, respectively. The number of
turns is 1995.
The ratio of the back eddy current Ib to the surface eddy current Is and is sensitive
against the distance between two exciting coils D. Figure 3 shows the relation between D
and the ratio of /, and Ib together with the relation between D and Ib. This figure implies
that, in increasing D, the ratio of Ib to Is successfully increases, though Ib decreases. The
tradeoff between the eddy current ratio and the back eddy current decided 12 mm of the
distance between the exciting coils.
The differential pickup coil is installed between the two exciting coils. To increase
the sensitivity of the pickup coil, diameters and heights of two coils are 12 mm and 6 mm,
respectively, filling up the space between the exciting coils. Each number of turns is 1,300.
Figure 4 shows the eddy current distributions on the longitudinal plane of
symmetry of the probe. On the point of symmetry of the probe, the ratio of Ib to Is of the
amplitude, real parts and imaginary parts are 0.72, 0.75 and 0.35, respectively. It should be
noted that the real part component is dominant and it can be concluded that large ratio of Ib
to Is is attained by the exciter of the present probe.
Width
Inner Radius
7mm
Inner Diameter /
Point of Measurement
(a) Parameters for design of exciting coils.
8.E+04
O.E+00
0
2
4
6
Width, Height, Inner Radius[mm]
(b) Relation among width, height and inner radius.
399
(c) Relation between height and inner diameter.
FIGURE 2. Parameter survey to determine dimensions of exciting coils.
- Surface EC -* Back EC -*- Back/Surface
l.OE+06
0.8
8.0E+05
0.6,
4.0E+05
O
0.2-1
2.0E+05
O.OE+00
0
5
10
15
Distance between Exciting Coils [mm]
FIGURE 3. Dependence of distance between exciting coils on eddy current distribution.
22mm
,12mm»
~J^
U25CNB
851193,
714258
577352
FIGURE 4. Computed distribution of eddy current in INCONEL 718 plate with the thickness of 7.0 mm.
Test frequency and lift-off are 10 kHz and 0.2 mm, respectively.
400
EVALUATION OF THE DEVELOPED PROBE
Experimental Setup for Evaluation of the Probe
Figure 5 shows the whole system of eddy current testing employed for evaluation
of the present probe. It consists of 2-D stage, stage controller, EC instrument ASSORTPC2 and PC for data acquisition and stage control.
Experiment with use of INCONEL 718 Specimen
Figure 6 shows INCONEL 718 specimen of 7.0 mm in the thickness used for
evaluation of the present probe. It simulates the INCONEL 718 piping of Japanese HII-A
launch vehicle. It has a welded portion and its bead was removed. Three half-elliptic
simulated cracks A, B and C are introduced along the edge of the weld line. The length and
width of all cracks are 10 mm and 0.2 mm, respectively. The depths of cracks A, B and C
are 1.00 mm, 0.50 mm and 0.25 mm, respectively. Preliminary measurement using a
conventional pancake coil of 112 turns, 1.0 mm in bore, 2.0 mm in diameter and 0.5 mm in
height was carried out, in advance. It was designed for inspection of INCONEL 600 tubes
with thickness of 1.25 mm and the smallest detectable crack is outer defect of 60% in
depth. The probe could not detect any crack from the crack-free face of the INCONEL 718
specimen.
The test frequency and liftoff were set to 10 kHz and 0.2 mm, respectively, which
is the same as set in the design procedure before. Overview of scan with the probe is
shown in Figure 7 and scan pitches for X direction and Y direction were both 0.5 mm.
Phase of complex voltage output by the EC instrument was tuned so that y component of
voltage indicates crack information remarkably.
Figure 8(a) shows a result of 2-D scan of the cracks B of 0.25 mm in depth. In this
case, the probe scanned the crack opening surface. Four peaks of y-component of the
complex probe voltage Vy were observed around the defect. This is because the probe is
anti-symmetric regarding to the two exciting coils, and the differential coil is installed on
the plane of anti-symmetry. Figure 8(b) shows a result of 2-D scan of the cracks C of 0.5
mm in depth. In this case, the probe scanned the opposite face from the crack opening
surface. The present probe successfully observed four peaks around the defect and detected
the defect though conventional pancake probe did not detect it.
Experiment with use of 304 Stainless Steel Specimen
Figure 9 shows the 304 stainless steel specimen of 8.0 mm in thickness. It also has
a welded portion, however, there remains its bead of 0.5 mm in maximum height. Two
simulated rectangular cracks A and B of 10 mm in length and 0.2 mm in width are
introduced; the crack A is located along the edge of the weld line and its depth is 1.6 mm,
and the crack B is located perpendicular to the weld line and its depth is 4.0 mm.
Figure 10 shows the results of 2-D scan of the cracks A and B. The scans were
carried out from the opposite side from the crack opening face. Because of the presence of
the bead on the scan surface, the liftoff and test frequency are set to 0.5 mm and 1 kHz,
respectively. By tuning the phase of the probe voltage, the probe successfully
distinguished signals of a back defects A and B from ones due to the bead as is shown in
FGURE 10, which supports for the robustness of the probe against noises of welded parts.
401
A/D
X-Y
FIGURE 5. Experimental setup for ECT with use of the present probe.
FIGURE 6. INCONEL 718 test piece. Conductivity and relative permeability are 0.83 x 106S/m and 1.001,
respectively.
Coils
™^
~ ~
_ ___^™
A
^
FIGURE 7. Overview of Developed Probe.
402
..
^ M
Coils
Line A
10.4
j 03
10.2
icy.
jo
I-at
1 -0x2
I-&3
SJ
10*0
0,0
Signal I>|¥1
SJ Mi ISJ
Signal Vy [V]
(a) Crack C
(b) Crack B
FIGURE 8. 2-D maps of EC signals Vy at 10 kHz. (a) Crack C of 0.25 mm in depth is scanned from the
crack opening face, and (b) Crack B of 0.5 mm in depth is scanned from the opposite side of crack opening
face.
FIGURE 9. 304 stainless steel specimen. Conductivity and relative permeability are 1.54 x 106S/m and
1.00, respectively.
0J
414
0
5
X[mm
10 IS
X[mm]
Signal vy[V]
Signal Vx [V]
(a) Crack A
(b) Crack B
FIGURE 10. 2-D maps of EC signals Vy at 1 kHz. (a) Crack A of 1.6 mm in depth is scanned from the
opposite side from crack opening face, and (b) Crack B of 4.0 mm in depth is scanned from the opposite side
from crack opening face.
403
CONCLUSION
This paper proposed and developed a novel ECT probe for thick-walled plates. The
probe consists of two exciting coils carrying current opposite to each other and a
differential pick-up coil arranged between the exciting coils. This arrangement enables
strong eddy current opposite to scan surface and weak decay of eddy current in the
thickness direction. The arrangement and size of coils of the probe was decided through
the numerical computation using 3D fast eddy current code so that the probe can induce
eddy current with appropriate distribution. The following conclusions are drawn from
experiment with use of INCONEL718 specimen with thickness of 7.0mm and 304 stainless steel
specimen with thickness of 8.0mm.
1. It was confirmed that the present probe can detect a back artificial defect with depth of
0.5mm on the INCONEL718 specimen, though a conventional pancake probe did not
detect it.
2. The probe successfully distinguished signals of a back artificial defect with depth of
1.6mm from ones due to the bead, which supports for the robustness of the probe
against noises of welded parts.
ACKNOWLEDGEMENTS
This work is partially supported by Grant-in-Aid for Specially Promoted Research
(COE)(2) (11CE2003) by the Ministry of Education, Culture, Sports, Science and
Technology, JAPAN. We are grateful to T. Satoh of Institute of Fluid Science, Tohoku
University for his technical assistance. We also thank International Institute of
Universality for providing 304 stainless steel specimen.
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