K. A. TIWARI, R. RAISUTIS: INVESTIGATION OF THE 3D DISPLACEMENT CHARACTERISTICS FOR A MACRO-FIBER ...
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INVESTIGATION OF THE 3D DISPLACEMENT
CHARACTERISTICS FOR A MACRO-FIBER COMPOSITE
TRANSDUCER (MFC-P1)
RAZISKAVA 3D KARAKTERISTIK ODMIKA PRETVORNIKA IZ
KOMPOZITA Z MAKROVLAKNI (MFC-P1)
Kumar Anubhav Tiwari, Renaldas Raisutis
1Ultrasound Institute, Kaunas University of Technology, K. Barsausko St. 59-A420 51423 Kaunas, Lithuania
k.tiwari@ktu.lt
Prejem rokopisa – received: 2017-10-04; sprejem za objavo – accepted for publication: 2017-10-26
doi:10.17222/mit.2017.166
The accurate three-dimensional (3D) displacement profile of a contact-type ultrasonic transducer ensures that the transducer is
free of defects during its manufacturing or installation. It also ensures the applicability of the transducer in structural health
monitoring (SHM) and non-destructive testing (NDT). Since its invention in 1996 by NASA, the macro-fiber composite (MFC)
transducer was positively accepted by researchers due to its lightweight, flexibility, durability and reliability. As it can be
embedded quite easily on the structure under inspection, there are different ways to which it can be used for the detection of
defects in the composite materials using guided Lamb waves. The objective of the presented work was to investigate the
operational performance of an unloaded macro fiber composite (MFC) transducer of P1-type by estimating its 3D displacement
components. The 3D spatial displacements of vibrating MFC were investigated using a Polytec 3D scanning laser vibrometer
(PSV-500-3D-HV) in order to determine the directions/planes along which the ultrasonic guide waves would be generated most
effectively. The behaviour of the MFC transducer of P1 type based on the displacement characteristics confirmed that it works
in d33 (elongation) mode, as specified in the manufacturer’s specifications.
Keywords: macro fiber composite, ultrasonic NDT, spatial displacements, Lamb waves, 3D-scanning laser vibrometer
Natan~en tridimenzionalni profil (3D) premikov kontaknega ultrazvo~nega pretvornika omogo~a, da je le-ta brez napak med
izdelavo ali namestitvijo. To namre~ zagotavlja uporabnost pretvornika za strukturno zdravstveno opazovanje (angl. SHM) in
neporu{no testiranje materialov (angl. NDT). NASA je leta 1996 izumila ultrazvo~ni kompozitni pretvornik na osnovi makro-
vlaken (MFC; angl.: Macro-Fiber Composite). Raziskovalci so potem pretvornik kmalu pozitivno sprejeli zaradi njegove majhne
mase, prilagodljivosti, trajnosti in zanesljivosti. Obstajajo razli~ni na~ini njegove uporabe za odkrivanje napak v kompozitnih
materialih z uporabo kontroliranih Lambovih valov, ker ga je zelo lahko namestiti na preiskovano strukturo. Predmet raziskave
predstavljene v tem ~lanku, so lastnosti obratovanja neobremenjenega MFC-pretvornika tipa P1 z oceno njegovih 3D-premikov.
Prostorske premike vibrirajo~ega MFC so avtorji preiskovali s 3D-laserskim skenirnim vibrometrom Polytec (PSV-500-3D-HV)
in s tem dolo~ili smeri in ravnine, vzdol` katerih so vodeni ultrazvo~ni valovi najbolj u~inkovito generirani. Potrdili so, da le-ta
deluje v na~inu d33 (raztezek) in da so karakteristike obna{anja MFC pretvornika tipa P1 v skladu s proizvajal~evo specifi-
kacijo.
Klju~ne besede: kompozit iz makrovlaken, ultrazvo~na neporu{na preiskava (NDT), prostorski premiki, Lambovi valovi,
3D-laserski skenirni vibrometer
1 INTRODUCTION
The compact size and low weight, flat geometrical
profile, ability to work as an actuator in the transmission
mode as well as a sensor in the reception mode makes
the macro fiber composite (MFC) the best available
choice for non-destructive testing (NDT) and structure
health monitoring (SHM) of the structures.1–4 One of the
best features of the MFC transducer is that MFC is an
efficient transmitter/receiver for generating/receiving the
A0 and S0 modes of guided Lamb waves.5–6 Hence, it
can be easily combined with the available contact and
non-contact ultrasonic techniques for the inspection of
structures.7 Embedded MFCs are widely used in
aerospace for generating ultrasound waves, harvesting
energy, and also for the detection of defects due to
impact.8 The aerodynamic performance of flexible wings
and the overall efficiency of the structure can be effec-
tively improved by the ability of MFCs to control the
aerodynamic shaping of the airfoils and the twisting
motion of the wings.9–10 The fiber volume fraction of
MFC is 0.824, which is more than AFC. This is also one
of the reasons for having the high performance and
stiffness of the MFC. The actuation performance of the
MFC is not only better than the AFC (1.5 times), but also
better than most widely used piezoceramic actuators.9–10
The general characteristics and features of MFC-2814-
P1 are briefly shown in Table 1.
The displacement characteristics are an effective way
of analysing transducer behaviour and spatial
displacements. Any perturbation in the displacement
characteristics would lead to generating non-standard
beams of in-plane and out-of-plane components during
the generation of ultrasonic guided waves (GW).11
Hence, the defected transducers can be omitted from the
Materiali in tehnologije / Materials and technology 52 (2018) 2, 235–239 235
UDK 620.1/.2:681.586.48:620.179.1 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(2)235(2018)
experimental analysis or applications in SHM tasks. The
prior estimation of the displacement characteristics of
the transducer is also applicable in the design of a
transducer array or to manufacture the new transducers
with specific characteristics. By knowing the spatial
displacement of the transducer, the analytical modelling
of the transducers can also be improved.12
Table 1: General characteristics of MFC-P1-M2814 (from Smart
Material Corporation)
Features Typical value
Active (length × width) 28 mm × 14 mm
Overall (length × width) 38 mm × 20 mm
Capacitance 1.15 nF
Free strain 0.155 %
Blocking force 195 N
Operating voltage -500 V to +1500 V
Operating bandwidth as a sensor 0 Hz to 1 MHz
Operating bandwidth as an actuator 0 Hz to 700 kHz
Maximum operational tensile strain < 0.45 %
Linear-elastic tensile strain limit 0.1 %
The objective of the presented work was to inves-
tigate the spatial displacement characteristics of the
MFC transducer and its applicability in ultrasonic NDT
and SHM by guided waves. First, the impedance and
phase characteristics of the unloaded MFC-P1 type
transducer were measured using the impedance meter in
order to find the resonant frequency of the transducer.
Later, the 3D spatial displacements were measured using
a 3D scanning laser vibrometer.
2 EXPERIMENTAL PART
For the experimental analysis, MFC-P1-2814 (S.n.
08J10079l) manufactured by Smart Materials1 was
investigated. The entire experimental process was con-
ducted in two steps. In the first step, the impedance and
phase characteristics of the MFC were measured using
an impedance analyzer (Wayne Kerr 6500B). The sche-
matic diagram and frequency-dependent characteristics
are shown in Figures 1 and 2, respectively. The resonant
and anti-resonant frequencies were measured at 41.38
kHz and 43.63 kHz, respectively. The measured phase
jump frequency of 42.58 kHz is also close to these
frequencies and therefore, the excitation frequency was
chosen as 42 kHz for the subsequent analysis.
In order to analyze the spatial displacements and per-
formance of the d33 (elongation) mode, the 3D dis-
placements (in-plane and out of plane components) are
measured at various spatial points of the unloaded
MFC-P1-2814 using the Polytec PSV-500-3D-HV
scanning laser vibrometer. A schematic of the experi-
mental set-up is shown in Figure 3. The PSV-500-3D-
HV laser vibrometer consists of three laser sensor heads.
The scanning head (PSV-I-500) with high precision is
called as the top head, which contains the full HD
camera (20× zoom) for visualization, alignments and
video triangulation and the geometry scan unit (PSV-
G-500). There are two scanning heads (PSV-I-520),
which do not contain the video camera and geometry
scan unit and are denoted as left/right scanning heads.
The front-end (PSV-F-500-V) unit has three digital
broadband decoders and a signal generator. The high-
frequency Doppler signal coming from the scanning
head is transmitted to the decoder and the velocity
information is extracted. The junction box (PSV-E-530)
works as an interface between the scanning heads and
K. A. TIWARI, R. RAISUTIS: INVESTIGATION OF THE 3D DISPLACEMENT CHARACTERISTICS FOR A MACRO-FIBER ...
236 Materiali in tehnologije / Materials and technology 52 (2018) 2, 235–239
Figure 2: Frequency response of MFC-P1: a) impedance characteristics and b) phase characteristics
Figure 1: Experimental schematic diagram for impedance and phase
measurements of the Macro Fiber Composite transducer (MFC-P1-
2814) using the impedance analyzer (Wayne Kerr 6500B)
the front-end unit. The measurement data is transferred
to a PC using an ethernet interface.
The three laser heads must be aligned to each other
and to the measurement object (MFC-P1) before con-
ducting the measurements. There are two ways in which
positioning of the laser beams on scan points defined on
the surface of the vibrating MFC is performed. The first
method is the 2D alignment (standard alignment), which
is used to position the laser beam, and the scan points
can be defined on the live video image. The second
method is the 3D alignment in which the angles of
spatially scanned mirrors for a given point of the spatial
scan are calculated using 3D coordinates. The software
itself offers a correction for the measurement coordinates
on the live video image and it positions the laser beam to
cover all the measurement points using the 3D alignment
and finds the spot of the reflected laser beam. Finally,
this position will be used instead of the position cal-
culated using the 2D alignment. The achieved accuracy
of the laser positions for the top, left and right laser in
the experiment was reported as 0.1 mm, 0.2 mm and 0.2
mm, respectively.
Since the active size of the MFC is 28 mm × 14 mm,
17 scanning points are selected along the length with a
separation distance of 1.75 mm and the same number of
points are selected along the width with a separation
distance of 0.875 mm by the data-analysis software of
the 3D vibrometer (Polytec). Hence, the total number of
scanned points was 289. The MFC is excited by applying
a burst-type sinusoidal signal of 120 cycles, a peak-to-
peak amplitude of 10V and an operating frequency of
42 kHz. The sampling frequency was 1.25 MHz with a
sampling interval of 0.8 μs. Although the 3D displace-
ments are measured at all points, the symmetry of MFC
structure means it is only possible to consider the points
along the edges (17 points along the length and 17 points
along the width). The defined directions of in-plane
components (X and Y-components) and out-of-plane
component (Z-component) of displacements along the
chosen points of interest are shown in Figure 4. Later,
the positive and negative values of the 3D displacements
along the length and width of the vibrating unloaded
MFC transducer were estimated in order to plot the dis-
placement characteristics.
3 RESULTS AND DISCUSSION
To obtain the correct results, the experiment is
performed three times. In each case the variation of peak
values from the mean value was observed as less than
1 %. The positive and negative values of the spatial
displacements measured along the length (L) and width
(W) of MFC-P1 type transducer are presented in Fig-
ure 5. It can be easily observed that a change in the
amplitudes of the Y-displacement component is much
higher along the length of MFC, while the X-displace-
ment component is higher along the width.
It has also been observed from Figure 5a to 5d that
the range of the Y displacement is much higher (-167 nm
to +167 nm) compared to the X displacement (-35 nm to
+35 nm) and the Z-displacement (-100 nm to +100 nm).
This leads to the confirmation that the longitudinal dis-
placement of the MFC-P1 is much higher and it operates
in elongation or d33 mode. This unique feature is quite
useful for testing composite materials by mounting on
the surface or embedding the MFC or MFC arrays into
the structure of the object. The Z-displacement (corres-
ponds to the dominant component of the A0 mode) has
the same displacement profile as the Y-displacement
(corresponds to the dominant component of the S0
mode) but has a lower amplitude of displacements as
described in Figure 5e and Figure 5f.
There are two types of fundamental Lamb wave
modes (the S0 and A0) that can be generated at low fre-
quencies.8 The average displacement of the S0 (symme-
tric mode) and A0 (anti-symmetric mode) over the thic-
kness of plate exists in the longitudinal (in the direction
of wave propagation) and transverse (out-of-plane)
directions, respectively.13 Therefore, in-plane Y displace-
K. A. TIWARI, R. RAISUTIS: INVESTIGATION OF THE 3D DISPLACEMENT CHARACTERISTICS FOR A MACRO-FIBER ...
Materiali in tehnologije / Materials and technology 52 (2018) 2, 235–239 237
Figure 4: The measured points of interest along the defined directions
of the X, Y and Z-displacement components using PSV-500 3D laser
vibrometer
Figure 3: Experimental set-up for 3D displacement measurement
using Polytec PSV-500-3D-HV laser vibrometer
ments as shown in Figures 5c, 5d indicate the perfor-
mance to generate and receive the S0 mode. The
out-of-plane Z displacements as shown in Figures 5e, 5f
indicate performance to generate and receive the A0
mode. Apart from the fundamental Lamb wave modes
(the A0 and S0), the fundamental shear horizontal mode
(SH0) is another kind of guided wave that is completely
non-dispersive and does not possess the out-of-plane
displacements.3,14
As the particle motion of the SH0 mode is in-plane, it
propagates in the perpendicular direction to the direction
of the S0 mode propagation. However, the performance
to generate the shear displacements of the SH0 mode
should be estimated by both the X and Y displacements
(Figures 5a to 5d), respectively. It should also be noted
that the system of the laser vibrometer is very sensitive
to errors and the presence of noise due to a misalignment
of the scanning heads, therefore the averaging of 64 was
applied and the ambient temperature of 23 °C was
maintained throughout the measurement process.
In the next step, interesting results were observed by
comparing the displacements at the center of MFC and at
the corner point on the longitudinal axis. These two
points are shown in Figure 6 as scanned point No. 6 and
scanned point No. 9, respectively. The envelope of the
3D displacements at these points was obtained by
applying the Hilbert transform (HT) on each of the
signals15. The results are shown in Figures 7a to 7f). The
time interval is chosen in order to pick up the maximum
displacements. After analysing the results, the outcomes
can be described as follows.
There is not much difference observed between the
maximum X-displacements (10 nm–12.2 nm) in both
cases as shown in Figure 7a and Figure 7d. Moreover,
the X-displacement is the minimum among all three dis-
placements. Therefore, the applicability of MFC trans-
ducers to generate and pick up the SH0 mode in NDT is
not effective.
It can be observed from Figures 7b and 7c that the
maximum value of the Z-displacement (out-of-plane) is
slightly higher than the Y-displacement (along with the
K. A. TIWARI, R. RAISUTIS: INVESTIGATION OF THE 3D DISPLACEMENT CHARACTERISTICS FOR A MACRO-FIBER ...
238 Materiali in tehnologije / Materials and technology 52 (2018) 2, 235–239
Figure 6: Showing the center point (scanned point 6) and corner point
(scanned point 9) on the longitudinal axis of MFC-P1
Figure 5: Positive and negative 3D (X, Y and Z) displacements along
the length and width, measured at edges of MFC-P1: a) X displace-
ments along the length and b) along the width, c) Y displacements
along the length and d) along the width, e) Z displacements along the
length and f) along the width (1- measured positive displacements, 2-
measured negative displacements)
Figure 7: 3D displacements at center (scanning point-6): a), b), c) and
corner point (scanned point-9) d), e), f) on the longitudinal axis of
MFC-P1
direction of propagation) at the center of MFC. The
difference between maximum Z and Y-displacements, in
this case, was observed as less than 6 nm.
The rapid increment in the maximum value of the
Y-displacement was observed at the corner point
(166.7 mm) as compared to the center point (21.5 mm)
as shown in Figure 7b and Figure 7e. On another hand,
the X- and Z-displacements were observed as 12.3 nm
and 33.5 nm, respectively, at the corner point, as shown
in Figure 7d and Figure 7f, respectively. The rapid
change in the maximum Y-displacements from the center
to the corner of MFC reconfirms its ability for the
effective generation/reception of the S0 mode.
4 CONCLUSIONS
In this paper a macro fiber composite transducer
MFC-P1-2814 (Sr. No 08J10079l) has been analyzed to
estimate its feasibility to be used for NDT and SHM
tasks. The impedance and phase characteristics of MFC
have been measured using the impedance analyzer
(Wayne Kerr 6500B) in order to estimate the resonant
and anti-resonant frequency values. Later, values of 3D
spatial displacements of the unloaded MFC-P1 operating
in the d33 (elongation) mode were measured using a
PSV-500-3D-HV (Polytec) scanning laser vibrometer. It
should be noted that the measurements were performed
three times and high-accuracy results were achieved with
less than 1 % variance from the mean value. It was con-
firmed that the amplitude of in-plane longitudinal dis-
placement (Y-component) is the highest among the other
X and Z-components. The measured maximum value of
the Y displacements is 167 nm. The displacement cha-
racteristics at the center point and at the corner of MFC
its longitudinal axis were also compared. The maximum
X and Z-displacements in both cases did not show a
significant difference. On another hand, the maximum
Y-displacement showed a significant increment from its
value 21.5 nm at the center to 166.7 nm at the corner
point that in turn again confirms its operation in elon-
gation mode.
It was also noticed that two different sets of measure-
ments produce the same behaviour that also confirms the
existence of systematic errors rather than stochastic ones.
The overall displacement characteristics of MFC-P1
confirm that MFC-P1 utilizes the d33 effect and effec-
tively operates in the elongation mode, as described by
the manufacturer.1 Therefore, investigating the 3D dis-
placement characteristics of the transducers in the gene-
ration of GW modes is an efficient technique in order to
verify the behaviour of the transducer for its conformity
in the application of low-frequency ultrasonic NDT and
SHM. The transducer, whose 3-dimensional characte-
ristics are not similar to its behaviour, must be omitted
and should not be used for further analysis. Moreover,
the 3D displacement profile can positively contribute to
the development of analytical and numerical models to
predict the directivity patterns of a transducer.
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