A LAMB WAVE STUDY ON THERMAL DAMAGE IN A DEGRADED
PLATE
I. K. Park1, Y. Cho2, and J. L. Rose3
!
R. I. of NDE Technology, Seoul National University of Technology, 172,
Gongneung-dong, Nowon-gu, Seoul, 139-743, Korea
2
School of Mechanical and Automotive Engineering, Inje University,
Kimhae City, Kyongsangnam-Do, South Korea
3
Ultrasonic Lab, Department of Engineering Science and Mechanics,
Penn State University, University Park, PA 16802, USA
ABSTRACT. The feasibility of Lamb waves for monitoring thermally degraded materials is explored. It
turns out that the use of Lamb waves leads to a promising nondestructive technique for the purpose of
microstructure evaluation and material characterization of such materials. This is because Lamb modes
can interact with entire area of a plate-like specimen while a conventional point-by-point technique is
confined to just local investigations. Consequently, Lamb modes' data can show a better sensitivity and
provide us with various features for thermal damage evaluation, compared to ones of local inspection,
which results in the enhancement of experimental reliability. 2.25Cr-lMo steel specimens for various
degradation levels were prepared by isothermal aging heat treatment at 650°and evaluated by the present
technique to investigate the influence of the thermal damage to the Lamb wave feature based on the
modal energy loss ratio.
INTRODUCTION
Recently, the typical material degradation found in the atomic or turbine power
plant is due to high temperature creep and aging [1,2]. However, it is not always possible
or practical to evaluate well-prepared specimens of identical condition to ones in use for
laboratory test. In this sense, development of an efficient and reliable technique to monitor
material degradation condition has been of great concern [3]. It is well known that various
ultrasonic waves have been used for material inspection and condition monitoring [8].
This study aims at investigating the potential of Lamb waves for the
characterization of thermally damaged materials. Furthermore, the experimental data based
on the Lamb wave technique is correlated with the ones of conventional, visual material
characterization by TEM (Transmission Electron Microscopy) so that the feasibility of the
present approach is verified. In this study, it is proposed to apply ultrasonic Lamb waves to
evaluate degradation of thermally damaged materials. 2.25Cr-1.0Mo steel, which is widely
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/S20.00
1379
used in various fields, including the power industry due to its resistance against the
environment of high temperature and corrosion.
EXPERIMENTAL DETAILS
Specimens
The test material is 2.25Cr-lMo steel used as a turbine rotor material for a hightemperature and high-pressure power plant. The reason why this material was chosen for
this study is that there is much demand for this alloy in various industries because of its
unique characteristics like corrosion resistance and the suitability for use in a high
temperature environment. The chemical composition (in wt%) of the material is given in
Table 1. Table 2 shows the accelerated aging time at 650°C for equivalent microstructure
served at 538°C. This is to simulate the microstructures of long term served materials at
elevated temperature because of the difficulty to sample the aged materials on site [7]. All
specimens were given homogenous treatment to obtain uniform substructure. Surface
roughness of the specimens were maintained within 1 fm rms. The sheet type specimen of
90mm in length, 24mm in width and 10.6 mm thickness was used for measuring ultrasonic
characteristics. Especially for Lamb wave test, the thickness was selected as 2.4 mm.
Table 3 shows the mechanical properties of test materials.
Experimental Setup for Bulk Wave Immersion Test
Figure 1 is a schematic diagram of the experimental setup for measuring
attenuation by the immersion test. Broadband immersion type transducers of 0.5 inch
diameter with 25 and 50 MHz center frequency were used along with a ultrasonic C-scan
system as shown in Figure 1. Analog signals were digitized using a Lecroy 9374M digital
storage oscilloscope (DSO) with a sampling rate of I GHz. Data processing was performed
by the Pentium PC and MATLAB software routines. The pulse echo technique was used
for attenuation coefficient measurements. Ultrasonic waveforms are to be viewed on the
CRT of an ultrasonic probe and oscilloscope. The attenuation values were analyzed in
frequency domain through an FFT of the waveforms averaged 1,000 times in the
oscilloscope.
Experimental Setup for Lamb Wave Test
Figure 2 shows a schematic diagram of the experimental setup for the Lamb wave
pitch-catch test. A couple of 1 MHz commercial type contact longitudinal transducers
supplied by Japan Probe Co., along with the Ritec tone burst system is used to generate
and receive the Lamb wave signals. The RF waveforms of each specimen were obtained
by varying propagation distance between the two transducers. Mode identification was
TABLE 1. Chemical composition of 2.25CrMo steel (wt. %).
Element
Composition
Element
Composition
C
0.138
P
0.014
Si
0.142
Mn
0.46
S
0.004
Cr
2.27
Mo
0.97
Fe
Bal.
1380
carried out with the as-received specimen for the modes, S0 and A1 at fd =2.4 MHz mm
and the results are given in Table 4.
carried out with the as-received specimen for the modes, SO and Al at f d =2.4 MHz mm
and the results are given in Table 4.
TABLE 2. Accelerated aging time at 650
for equivalent microstructure served at 538 .
TABLE 2. Accelerated aging time at 650°C for equivalent microstructure served at 538°C.
Time served
As80,000 170,000
260,000
at 538 (hr)
received
Time
served
AsAging
time
170,000
260,000
80,000 3,100
0
1,500
4,800
received
at 538°C(hr)
at 650
(hr)
Aging time
4,800
0
1,500
3,100
at 650°C(hr)
TABLE 3. Mechanical properties of test materials.
TABLE 3. Mechanical properties of test materials.
Tensile
Yield
Mechanical
Elongation
Strength
Strength
Properties
(%)
2
)
(MPa)
(kgf/mm
Yield
Tensile
Mechanical
Elongation
Strength
Strength
Value
49
64.3
24
Properties
(%)
(kgtfmm2)
(MPa)
Value
3-Axial Controller
Hitachi, miscope20
49
64.3
Pulser/Receiver
24
Hardness
(Hv)
Hardness
203.8
(Hv)
203.8
Digital Oscilloscope
Digital Oscilloscope
RF signal
Lecroy 9374M
Lecroy 9374M
ASCII File
Zig
Probe
Data analyzer
Pentium III
Water
Zig
Specimen
FIGURE 1. A schematic diagram of experimental setup.
FIGURE 1. A schematic diagram of experimental setup.
1381
FIGURE
Lamb wave
wave pitch-catch
pitch-catch test.
test.
FIGURE 2.
2. Experimental
Experimental setup
setup for
for Lamb
Presented
below
is is
thethephase
Presented
below
phasevelocity
velocitydispersion
dispersioncurves
curvesfor
forthe
theas-received
as-receivedspecimen.
specimen.
15
A2
Cph[mm/¥
ì sec]
S2
10
A1
S1
S0
5
A0
0
0
0
0 .5
0.5
1
1
1 .5
1.5
2
2 .5
3
f·d
z·m m ] 3
2 [M H2.5
3 .5
3.5
4
4
4 .5
4.5
5
5
FIGURE 3. The phase velocity dispersion curves for the as-received specimen.
FIGURE 3. The phase velocity dispersion curves for the as-received specimen.
TABLE 4. Group velocity comparisons for S0 and A1 at fd=2.4 MHz mm.
TABLE 4. Group velocity comparisons for SO and Al at fd=2.4 MHz mm.
Lamb modes
Lamb modes
Group
velocity
Group velocity
S0 (theory)
SO (theory)
1.8
1.8
S0 (exp.)
SO1.915
(exp.)
1.915
A1(theory)
A 1 (theory)
3.6
3.6
A1(exp.)
Al(exp.)
3.724
3.724
RESULTS AND DISSCUSSION
RESULTS AND DISSCUSSION
Microstructure Observation by SEM & TEM and Conventional Bulk Wave Tests
Microstructure Observation by SEM & TEM and Conventional Bulk Wave Tests
In order to investigate the change of carbide morphology and carbide to degradation
In order to investigate the change of carbide morphology and carbide to
steps, we observed a change of microstructure with increasing degradation time with a
degradation steps, we observed a change of microstructure with increasing degradation
field emission scanning electron microscope (FESEM) and transmission electron
time with a field emission scanning electron microscope (FESEM) and transmission
microscope (TEM) for the 2.25CrMo steel material. Figure 4 shows the results of the TEM
electron microscope (TEM) for the 2.25CrMo steel material. Figure 4 shows the results of
micrographs
morphology
of the carbides
agingwith
timeaging
in thetime
as-received
and artificially
the TEM micrographs
morphology
of thewith
carbides
in the as-received
and
aged specimens. Carbides became coarsened and spheroidized as the aging time was
1382
artificially aged specimens. Carbides became coarsened and spheroidized as the aging time
was increased. Micro acicular carbide decreased in number and in 1,500 hours it was not
observed. Grain boundary carbide coarsened and grew into a union.
Figure 5 shows the dependence of the Vickers hardness value on aging time. We
can tell that the downfall of the hardness value grew saturated as the degradation time
passed. The value of hardness decreases more rapidly in a short aging time and the change
becomes slower in longer aging time. Since these change of hardness values is related to
the material degradation extent, we can predict destructive measurement indirectly and
also identify the possibility of an evaluation of material degradation as well.
Material Condition Monitoring Based on Modal Energy Loss
Even though the data from the bulk wave immersion tests shows a feasibility for
material condition monitoring as presented in Figure 6, the deviation in experimental data
doesn't seem to be significant enough to be a reliable feature considering inevitable
experimental uncertainties and error sources. In addition, they appear to have some
problem in consistency. The Lamb wave technique is motivated by such findings. For the
Lamb wave tests, the variation in the modal energy loss ratio is monitored with respect to
aging time change expecting the feature can allow us to tell about a material condition
during its degradation. This is because such a change of microstructure of the alloy as the
appearance of the carbide precipitation may cause energy loss.
(a)
(b)
(d)
(c)
FIGURE 4. TEM micrographs showing the morphology of the carbide
(a) as-received (without thermal aging) (b) 920 hr aging time (c) 1800 hr aging time (d) 3700 hr aging time
1383
Vickers Hardness ( Hv )
280
280
260
260
I
240
^ 240
O
220
200
180
18
160
0
10000
10000
20000
20000
30000
30000
40000
40000
50000
60000
50000
60000
D
e g ra d a tio n TTime
im e (( hhours
o u rs ) )
Degradation
FIGURE5.5. Effect
Effectof
of degradation
degradation time
time on
FIGURE
on Vickers
Vickers of
of 2.25Cr-1.0Mo
2.25Cr-1.0Mosteel
steelhardness
hardnessatateach
eachprobe
probedistance.
distance.
Attenuation Coefficient [dB/mm]
1 .5
1 .2
0 .9
•
a s -re c e iv e d
as-received
1200hr
• 1200hr
1800hr
A 1800hr
3100hr
V
3100hr
4800hr
+ 4800hr
0 .6
0 .3
0
25
30
25
30
35
40
F re q u e35n c y [M H z40]
45
45
50
50
Frequency [MHz]
FIGURE 6. Attenuation coefficient measured by bulk wave.
FIGURE 6. Attenuation coefficient measured by bulk wave.
Unlike a bulk wave test, modal energy loss of Lamb waves can not simply be
Unlike
a bulk
wave
test, modal
energy
lossofoftheir
Lamb
waves natures.
can not Rather,
simply itbe
measured
in terms
of an
amplitude
decrease
because
dispersive
measured
terms
of an aamplitude
decrease
of surrounded
their dispersive
natures.ofRather,
is a betterinway
to define
modal energy
term because
as the area
by envelope
the RF it
iswaveform
a better way
to define
a modal
energy
as theofarea
surrounded
by envelope
of the
RF
since
the modal
energy
is theterm
function
amplitude
as well
as duration
time.
waveform
sinceRF
thewaveforms
modal energy
the function
as wellsetup
as duration
Two different
wereiscaptured
basedofonamplitude
the pitch-catch
varying time.
the
Two
different
RF transmitter
waveformsand
were
captured
based the
on the
pitch-catch
setup
the
distance
between
receiver
by adding
time-delay
barrier
withvarying
15.3 mm
distance
between
and Then,
receiver
adding
the time-delay
barrier
15.3and
mm
length as
shown transmitter
in Figure 7.
thebyareas
of the
two envelopes
are with
defined
determined
as A1 in
andFigure
A2, respectively
thisareas
study.of the two envelopes are defined and
length
as shown
7. Then,inthe
Figureas8Al
andand
9 represent
the correlation
determined
A2, respectively
in thisbetween
study. aging time and the energy loss ratio
A2/A1Figure
experimentally
obtained atthef=1.0
MHz for
S0 andaging
A1 modes,
respectively.
is
8 and 9 represent
correlation
between
time and
the energyIt loss
notedA2/A1
that the
energy loss ratio
of A1
decrease
from 0.7
to 0.1 with It
ratio
experimentally
obtained
at mode
f=1.0 MHz
for remarkably
SO and Al modes,
respectively.
aging
increase
indicating
factdecrease
that 2.25Cr-1.0Mo
a very
isrespect
noted to
that
the time
energy
loss ratio
of Al the
mode
remarkablyalloy
frombecomes
0.7 to 0.1
with
lossy material
to thermal
respect
to agingdue
time
increasedamage.
indicating the fact that 2.25Cr-1.0Mo alloy becomes a very
The result
ofto
thethermal
S0 mode
also tends to descend with an increase of aging time but its
lossy material
due
damage.
sensitivity
to be
as good
as thetoone
of the with
A1 mode.
Compared
to the
bulk
The doesn’t
result ofseem
the SO
mode
also tends
descend
an increase
of aging
time
but
attenuation
dataseem
by the
Figure
6, it isCompared
proved that
modal
itswave
sensitivity
doesn't
to beimmersion
as good astestthegiven
one ofinthe
Al mode.
to the
bulk
energy
loss ratio of
theby
Lamb
modestest
cangiven
be a more
reliable
feature
to
wave
attenuation
data
the wave
immersion
in Figure
6, and
it issensitive
proved that
modal
classify
the
level
of
material
degradation.
A
further
detailed
study
on
the
difference
in
energy loss ratio of the Lamb wave modes can be a more reliable and sensitive feature to
sensitivity
the ofenergy
lossdegradation.
feature of Lamb
modes
may be
required
on thein
classify
the oflevel
material
A further
detailed
study
on thebased
difference
correlation
between
their
wave
structures
and
microstructures
of
the
alloy.
sensitivity of the energy loss feature of Lamb modes may be required based on the
correlation between their wave structures and microstructures of the alloy.
1384
0 .2
0m m
1 5 .3 m m
Amplitude [V]
0 .1
0 .0
-0 .1
-0 .2
0
10
20
Tim e [jis]
30
40
50
15.3mm
T im e [ µ s ]
FIGURE7.7. The
The variation
variation of
of RF-signal
RF-signal envelope
envelope of
FIGURE
of SO
S0 mode
mode atatf=1.0
f=1.0MHz
MHzdue
duetotothe
theincrease
increaseofof15.3
15.3
mmininpropagation
propagation distance.
distance.
mm
-Area (A2/A1)
1 .0 2
A re a (A 2 /A 1 )
1 .0 0
Area rate
£ 0.98
2 0 .9 8
0 .9 6
0 .9 4
0 .9 2
0
500
1000 1500 2000
0
500
1000
2500
1 Aging
500
2time
000
2500
[hr.]
A g in g tim e [h r.]
3000
3000
3500
3500
4000
4000
FIGURE 8. The variation of SO energy loss at f=lMHz with respect to aging time.
FIGURE 8. The variation of S0 energy loss at f=1MHz with respect to aging time.
0 .8
0.7
A re a (A 2 /A 1 )
0 .7
0.6
0 .6
0.5
Area rate
S
<
0 .5
0.4
0 .4
0.3
0 .3
0.2
00.1
.2
0 .1
0
500
1000
1500
500
1000
1500
2000
2500
A g i n g tim e [hr.]
2000
2500
3000
3000
3500
3500
4000
4000
A g in g t im e [ h r . ]
FIGURE 9. The variation of Al energy loss at f=lMHz with respect to aging time.
FIGURE 9. The variation of A1 energy loss at f=1MHz with respect to aging time.
1385
CONCLUSIONS
The feasibility of material degradation evaluation by Lamb waves is explored
along with the results of TEM for microstructure change of a 2.25Cr-1.0Mo steel subjected
to thermal damage. Due to carbide precipitation increase and spheroidization near grain
boundaries of the microstructure during thermal degradation, the material becomes lossy
resulting in the promising feature of the Lamb wave modal energy loss ratio for its
condition monitoring. Lamb waves can be successfully applied to material condition
monitoring, as long as a proper mode selection is achieved.
ACKNOWLEDGEMENTS
This work is supported by the research fund of Seoul National University of
Technology.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
Yamashita, M., Viswanathan, U. K., Yamamoto, I. and Kobayashi, T., "Serviceinduced Changes in the Microstructure and Mechanical Properties of a Cr-Mo-Ni-V
Turbine Steel", ISIJ International, Vol. 37, NO. 11, pp.1133-1138, (1997).
Viswanathan, R. and Bruemmer, S. M., "In-Service Degradation of Toughness of
Steam Turbine Rotors", Transactions of the ASME, Vol. 107, pp. 316-324, (1985).
Hur, S. K., Hong, K. T. and Do, T. M., "The measurement of Degradation in CreepRuptured Cr-Mo-V steels by NDE Methods", Proc. of the 2nd Conference on
Mechanical Behaviors of Materials, Seoul, Korea, pp. 17-24, (1988).
Yokono, Y., Katoh, Y. and Nisho, K., "Characteristics of Surface SH Wave Probe
and its Application for Detecting Surface and Subsurface Flaws", Proc. of the 8th
APCNDT,(1995).
Park, I. K., Kim, H. M., "Experimental Vrification on the Detectability and
Quantitative Evaluation of Surface Flaws using Surface SH-wave Ultrasonic
Method; Selection of Optimal Testing Condition and Detection of Thin Plates",
Proceeding of KSNT, Seoul, pp. 100-112, (1999).
Park, I. K., Park, U. S. and Kim, H. M., "Nondestructive Evaluation for Degraded
2.25Cr-lMo Steel though Surface SH-wave", Proceeding of KSME, Vol. A, pp. 280285, (2000).
Adbel, A. M., Corbett, J. M. and Talpin, D. M., Met. SCI., 16, p. 90, (1982).
Cho, Y., Hongerholt, D. D. and Rose, J. L., "Lamb wave Scattering Analysis for
Reflector Characterization," IEEE Trans. Ultrasonic., Ferro. Freq. Cont, Vol. 44,
No. 1, pp. 44-52, (1997).
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