STRUCTURAL HEALTH MONITORING OF AEROSPACE STRUCTURES …
Transcript of STRUCTURAL HEALTH MONITORING OF AEROSPACE STRUCTURES …
Structural Health Monitoring of Aerospace Structures with Sol-Gel Spray Sensors
A. Ouahabi 1, a, M. Thomas 1, b, M. Kobayashi 2, c and C.-K. Jen 2,d 1Department of Mechanical Engineering, École de technologie supérieure,
1100 Notre-Dame Street West, Montréal, Québec, H3C 1K3, Canada 2Industrial Materials Institute, National Research Council Canada,
75 de Mortagne Blvd, Boucherville, Québec, J4B 6Y4, Canada [email protected], [email protected],
[email protected], [email protected]
Keywords: health monitoring, sensors, detection, localization
Abstract. A new approach is proposed for conducting structural health monitoring, based on newly
developed piezoceramic sensors. They are fabricated by a sol-gel spray technique. The potential
application of these sensors may be broad. These sensors have been evaluated for structural health
monitoring studies. The purpose of the present study aims the detection and the localization of
defects by the means of these new piezoceramic sensors. Nine sensors were integrated onto a
metallic plate with moving masses. The plate was excited by an impact at a specific location and the
vibratory signals from sensors were recorded simultaneously. The analysis of signals obtained from
nine locations was correlated with a numerical simulation in order to identify at each time the
location of the mass.
Introduction
Non-destructive testing (NDT) of materials are commonly performed to identify, characterize,
assess voids, defects and damage in metals, metal alloys, composites and other materials [1].
Furthermore, the increasing demand to improve the performance, reduce downtime, increase
reliability and extend the life of transportation vehicles, structures and engineering systems, requires
the use of systems that have integrated capabilities with built-in sensors that perceive and process
in-service information and take actions to accomplish desired operations and tasks [2]. Piezoelectric
ceramic sensors and actuators are commonly used as key candidates for smart materials and
structures. They have been used as structural vibration actuators, structural health monitoring
sensors, non-destructive evaluation probes for materials and structures, etc [3]. Thick (≥ 40 µm)
piezoelectric ceramic films can be made by the technologies of jet printing [4], dipping [5], tape
casting [6], hydrothermal method [7], etc.
Here, an alternative sol-gel spray technique is used. They take form of thick film layers with
substrates having different shapes. An important characteristic of these sensors is that they can be
coated directly onto desired sensing locations of planar or curved structures. These sensors present a
considerable advantage because their fabrication focuses on the use of hand held tools. It is possible
to obtain a thickness ranging between 25 and 150 µm and a diameter which can be less than
centimeter. Their operating temperature ranges from -100°C to 440°C [3]. For example, they can be
applied as sensors onto thin metallic and flexible membranes or around cylindrical surfaces. The
purpose of the present study aims the detection and the localization of defects by the means of new
piezoceramic sensors in the form of thin layers of piezoelectric materials coated directly onto the
structures.
Fabrication and Characterization
The fabrication process was first developed at Queen’s [8]. The piezoelectric particles are dispersed
in the sol-gel solution to produce a thick piezoelectric film [3, 9]. The spray can be carried out by an
air gun at room temperature and it is simple and inexpensive. The ball-milled sub-micron
Key Engineering Materials Vol. 347 (2007) pp. 505-510online at http://www.scientific.net© (2007) Trans Tech Publications, Switzerland
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piezoelectric lead-zirconate-titanate (PZT) or bismuth titanate (BIT) powders were dispersed into
PZT sol-gel solution. The PZT and BIT powders were chosen because of their high piezoelectric
constant and high Curie temperature (675°C), respectively. The final PZT/PZT or BIT/PZT mixture
(paint) was then sprayed directly onto selected metallic substrates, such as stainless steel through an
airbrush. With this sol-gel spray technique, the films can be produced at desired locations using a
paper shadow mask. After spraying the coating, thermal treatments such as drying, firing and/or
annealing were normally carried out using a heat gun. In special cases in this study a furnace would
be used and mentioned specifically. Multiple coatings were made in order to reach desired film
thicknesses. The film thickness is between 40 and 200 µm. Piezoelectric films were then electrically
poled using a corona discharging technique. The corona poling method was chosen because it could
pole the piezoelectric film over a large area with complex geometries. Finally, silver paste, platinum
paste or silver paint spray method was used to form the top electrodes at room temperature. The
measured relative dielectric constant of the PZT/PZT film and BIT/PZT film was about 320 and 80,
respectively. The d33 measured by an optical interferometer was 30 10-12
m/V for PZT/PZT and 10
10-12
m/V for BIT/PZT. The thickness mode electromechanical coupling constant measured was 0.2
for PZT/PZT and that for BIT/PZT was lower. Fig. 1 presents the flow chart of the fabrication
process of the thick film described above.
Fig. 1: Flow chart of the fabrication process of piezoelectric thick film.
The numerical and experimental study of integration of this kind of sensors has been developed
for the defective and no defective plate in order to compare between them, and to extract a
conclusion concerning the effectiveness of these sensors for the detection and the localization of the
defects.
Experimental Modal Analysis
Nine of piezoceramic PZT/PZT sensors were deposited onto a steel plate (100 ×100 × 0.5 mm) as
shown in Fig. 2. The thickness of theses piezoceramics is 40 µm thick. The electrodes have been
mounted on each piezoceramic layer by using a silver-pen. The wires were glued with silver epoxy.
The plate is mounted in clamped configuration. A small moving mass of 3 grams is bonded to the
plate at different locations to simulate a defect in the plate. At each location of the mass, an
experimental modal analysis has been conducted by using the impact technique and compared with
the results obtained without mass. The vibratory response was simultaneously collected at each
Piezoelectric Powder with High Curie Temperature
Mixing with Solution of High Dielectric Constant
Air Spray
Drying, Firing and Annealing By Heat Gun or Torch
Desired Thickness
Yes
Corona Poling
Fabrication of Top Electrode
No
Damage Assessment of Structures VII506
location with the piezoceramic sensors and the impact was measured with force sensor. The first
five natural frequencies were analysed. Fig. 3 presents one of the frequency response functions
(FRF). As expected, the effect of added mass can be easily detected from the decrease of natural
frequencies. However, it can be noticed that the sensitivity of the decrease is not the same at each
natural frequency. The variation of each natural frequency accordingly with the position of the
added mass will be demonstrated in the latter section.
Fig. 2: Clamped plate under impact excitation (Hammer) and classification of sensors.
0.00 400.00 Hz
0.00
0.36
Amplitu
de
( g/N)
0.00
1.00
Amplitu
de
F FRF w ithout defectF FRF w ith defect
Fig. 3: FRF of defective (mass at location 1) and no defective plate.
We have noticed a hardening effect in time of the piezoceramic layers that have produced an
increase from a small amount (up to 10%), of the natural frequencies of the plate. Also, comparative
tests with an accelerometer show that this sol-gel spray sensor is able to be used as sensor because it
delivers the same type of signal without amplifier.
1
2
3
4
5
6
7
8
9
Key Engineering Materials Vol. 347 507
Numerical Modal Analysis
The cantilever plate has been discretized into 28×28 finite elements, as shown in Fig. 4, and the
effect of piezoceramic layers has been considered. The first five natural frequencies of the plate
were numerically computed without mass and with the mass located at various positions (i) (i =
1,…9). Figure 5 shows the corresponding mode shapes (ANSYS©). It was revealed that the mode
shapes are not affected by the position of the added mass.
Fig. 4: FEM modelization of plate with piezoceramic layers.
Detection and Localization of Defect
Accordingly to the location of the mass (0 means without defect), Fig. 5 shows clearly a decrease of
natural frequencies. That decrease is more sensitive to some specific frequencies accordingly with
the position of maximum amplitude of modes, as it is put in evidence by arrows). By investigating
simultaneously all the natural frequencies that are most affected when the added mass is located at a
specific position, and by analysing the location of maximum amplitude for each considered mode, it
is now possible to locate the possible positions of defect. Table 1 shows the process of
identification. By considering more modes, the process of identification will be more accurate. The
variation of frequencies when the mass was located close to the clamp was not enough sensitive to
conclude.
Location of
mass
Affected
frequencies
nodes of maximum amplitude of
modes
Identification of mass
position
1 1st
2nd
1, 2 and 3
1 and 3
1 or 3
2 1st
4th
1, 2 and 3
1, 2, 3 and 5
1, 2 or 3
3 1st
2nd
1, 2 and 3
1 and 3
1 or 3
4 3rd
5th
2, 4, 5 and 6
1, 3, 4, and 6
4 or 6
5 3rd
4th
2, 4, 5 and 6
1, 2, 3 and 5
2 or 5
6 3rd
5th
2, 4, 5 and 6
1, 3, 4, and 6
4 or 6
Table 1: Identification of possible locations of defect.
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Mode 1
Mode 2
Mode 3
Mode 4
Variation de la 4th frequency
305
310
315
320
325
330
335
0 1 2 3 4 5 6 7 8 9 10
Position of the added mass
Fréquency [Hz]
Variation of the 1st frequency
38
39
40
41
42
43
0 1 2 3 4 5 6 7 8 9 10
Position of the added mass
Frequency [Hz]
Variation of the 2nd frequency
94
96
98
100
102
104
106
0 1 2 3 4 5 6 7 8 9 10
Position of the added mass
Frequency [Hz]
Variation of the 3rd frequency
235
240
245
250
255
260
265
0 1 2 3 4 5 6 7 8 9 10
Position of the added mass
Fréquency [Hz]
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Mode 5
Fig. 5: Variation of natural frequencies with added mass.
Conclusions
New integrated thick piezoceramic sensors were used for the detection and the localization of
defects. They seem effective for potential structural health monitoring. Their advantage lies in the
fabrication that focused on the use of the handheld and readily accessible equipment to perform sol-
gel spray technique. The merits of these sensors are mainly in their miniature and lightweight (film
thickness ≥ 40 µm), flexible, non-destructive test and smart sensing site. An application of these
sensors in order to detect a light added mass was successful, by analysing the decrease of frequency
produced by the mass. Furthermore, an analysis of the more affected frequencies and their
corresponding modes allowed for the identification of possible locations of the mass.
Acknowledgements
The authors are grateful to the support of the Industrial Materials Institute (IMI), National Research
Council of Canada (NRC), and the Consortium for Research and Innovation in Aerospace in
Quebec (CRIAQ).
References
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[9] M. Kobayashi, T. R. Olding, M. Sayer and C. K. Jen: Ultrasonics Vol. 39 (2002), p. 675
Variation of the 5th frequency
355
360
365
370
375
380
0 1 2 3 4 5 6 7 8 9 10
Position of the added mass
Fréquency [Hz]
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