This paper reports recent developments on high-temperature, multi-element integrated ultrasonic transducers (IUTs). The multi-element IUTs are fabricated from a sol-gel route, where piezoelectric films are deposited, poled and machined into an array of 16 elements. Electrical wiring and insulation are also integrated into a practical, simple high-temperature assembly. These multi-element IUTs show a high potential for structural health monitoring at high temperatures (in the 200-500°C range): they can withstand thermal cycling and shocks, they can be integrated to complex geometries, and they have broadband and suitable operating frequency characteristics with a minimal footprint (no backing needed). The specifics of multi-element transducers, including the phased array approach, for structural health monitoring are discussed.
A naturally cracked aircraft stabilizer former has been examined. By using surface shear horizontal diagnostic acoustic
waves and a multi-point detection approach, a fretting crack, 0.2-inch long, 0.03-inch deep and at 0.06-inch to a rivet
hole has been clearly identified. The proposed approach provides a simple way to interpret sensor output without
imposing demanding transducer performance requirements.
Implementation of an integratable ultrasonic sensor network with associated cable connection for high temperature
monitoring applications is demonstrated through application of a three-element ultrasonic sensor network for
temperature measurement in a turbine stator assembly. The sensor network is composed of a piezoelectric composite
film deposited on a titanium substrate with a sol-gel technique and three top electrodes deposited on the piezoelectric
film. The sensor network is glued onto a selected area of the stator assembly in such a way that three subareas with
different wall thicknesses are probed individually by each of the sensing elements. The ultrasonically instrumented stator
assembly is first heated in a furnace to different temperatures. At each temperature and for each probed location the
transit time of ultrasonic waves through assembly wall thickness is measured. Then a relationship between transit time
and wall temperature is established. In a subsequent experiment, the stator assembly is heated up to 200 °C and then let
cool down while the transit time in the assembly wall is being measured continuously. By using the transit time versus
temperature relationship obtained earlier, the heating and cooling rates at the three probed locations are determined and
then compared.
KEYWORDS: Ultrasonics, Aluminum, Composites, Structural health monitoring, Nondestructive evaluation, Electrodes, Transducers, Wave plates, Temperature metrology, Signal to noise ratio
Flexible ultrasonic transducers (FUTs) which have the on-site installation capability are presented for the non-destructive
evaluation (NDE) and structural health monitoring (SHM) purposes. These FUTs consist of 75 μm thick titanium membrane, thick (> 70 μm) thick piezoelectric lead-zirconate-titanate (PZT) composite (PZT-c) films and thin (< 5 μm) thick top electrodes. The PZT-c films are made by a sol-gel spray technique. Such FUT has been glued onto a steel pipe of 101 mm in diameter and 4.5 mm in wall thickness and operated up to 200°C. The glue served as high temperature
ultrasonic couplant between the FUT and the external surface of the pipe. The estimated pipe thickness measurement accuracy at 200°C is 34 μm. FUTs also were glued onto the end edge of 2 mm thick aluminum (Al) plates to generate
and receive predominantly symmetrical and shear-horizontal (SH) plate acoustic waves (PAWs) to detect simulated line defects at temperature up to 100°C. FUTs glued onto a graphite/epoxy (Gr/Ep) composite are also used for the detection of artificial disbonds. An induction type non-contact method for the evaluation of Al plates and Gr/Ep composites using FUTs is also demonstrated.
Two types of ultrasonic sensors are presented for structural health monitoring (SHM) and non-destructive testing (NDT)
of graphite/epoxy (Gr/Ep) composites of thickness ranging from 1mm to 27.9mm. These piezoelectric film based sensors
are fabricated using a sol-gel spray technique. The center operation frequency of these sensors ranged from 1.3MHz to
10.5MHz. For the first sensor type, piezoelectric films of thickness greater than 60μm were deposited directly onto
planar and curved Gr/Ep composites surfaces as integrated sensors. Ultrasonic signals propagating in a distance of more
than 300mm have been obtained. Anisotropy of 0° and 90° cross ply Gr/Ep composite was measured. For the second
sensor type, piezoelectric films were coated onto a 50µm thick polyimide membrane as flexible sensors that could be
attached to a host composite structure with planar or curved surfaces. The flexibility of such FUTs is achieved due to the
thin polymide, porous PZT/PZT ceramics and electrodes. An induction type non-contact method for the interrogation of
the Gr/Ep composites using integrated sensors is also presented. Such non-contact technique may be desired for NDT of
rotating composite components.
Miniature and light weight thick piezoelectric films (>40μm) integrated ultrasonic transducers (IUTs) for bulk
longitudinal (L) and shear (S) and plate acoustic waves (PAW) propagation are presented. The unique and distinct
advantages of these IUTs are that they can be fabricated, using sol-gel based technique, directly onto sample with
complex structures including curved surfaces and require no couplant for operation. Using novel mode conversion
methods, the L wave generated by IUTs can be converted to S, symmetric, anti-symmetric and shear-horizontal (SH)
PAW. The experimental results agreed well with those obtained by a finite difference based method which solves the 3D
visco-elastic wave equations. These IUTs can operate at temperatures at least up to 150°C, at center frequencies ranging
from 1 to 20 MHz, and provide damage detection range of tens of centimeters in metallic structures. An inductive
coupled technique is used to achieve non-contact measurements with these IUTs.
Flexible ultrasonic array transducers which can be attached to the desired structures or materials for nondestructive
testing and structural health monitoring applications at room and elevated temperatures are developed. These flexible
ultrasonic transducers (UTs) arrays consist of a thin polyimide membrane with a bottom electrode or stainless steel foil,
a piezoelectric lead-zirconate-titanate (PZT) composite film and top electrodes. The flexibility is realized owing to the
porosity of piezoelectric film and the thinness of substrate and electrodes. Top and bottom electrode materials are silver
paste, silver paint or electroless plated nickel alloys. The UT array is configured by the several top electrodes. The
flexible UT has been successfully tested at 150°C and also immersed into water as immersion ultrasonic probe operated
in the pulse-echo mode with good signal to noise ratio.
Piezoelectric films sprayed onto metal substrates together with interdigital transducer electrodes form the integrated
Rayleigh surface acoustic wave (RAW) transducers to excite and detect RAW. Using integrated longitudinal (L) wave
ultrasonic transducers (UTs) and mode conversion from L waves to shear waves symmetrical, anti-symmetrical and
shear horizontal types of guided plate acoustic waves have been generated and received in aluminum alloy plates. These
transducers can be operated in pulse-echo mode for in-situ non-destructive testing (NDT) and/or health monitoring
purposes in a distance of hundreds of mini-meters at 150°C. Examples of using such waves for NDT of defects are also
demonstrated.
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