Conventional non-destructive inspection of a pipe by ultrasonic wave has difficulty with inspection efficiency because it is a technique to apply by using longitudinal wave or transverse wave which propagates to the thickness direction of a pipe for smaller area than an ultrasonic sensor. However, a guide wave is provided with a characteristic of long-range propagation to the axis direction of a pipe, so it is possible to detect a lot of defects through wide range of a pipe at once. At present, there is a technique to generate a guide wave by a piezoelectric element (PZT). Such transducer has some difficulties to use in industrial application, which is required high viscosity couplant. Therefore we tried to develop a guide wave inspection system to use an electromagnetic ultrasonic transducer (EMAT) which doesn’t require any couplant. First, we could confirm that guide wave can be transmitted and received in aluminum pipe by a shear horizontal polarized-EMAT, and we have confirmed the most suitable transmission and reception EMAT-specification and the most suitable drive condition to generate for L, T and F-mode guide wave. Finally, we have evaluated the detective performance using the developed system.
The conventional non-destructive inspection of a pipe by an ultrasonic wave has low inspection efficiency because it is a technique that uses a longitudinal wave or transverse wave which propagates in the thickness direction of a pipe with a smaller area than the size of the ultrasonic sensor. However, a guide wave is provided with the characteristic of long range propagation in the axis direction of a pipe, so it is possible to detect many defects over a large pipe area at once. At present, there is a technique to generate a guide wave using a piezoelectric element (PZT). Such a transducer has some difficulties in industrial applications, which requires a high viscosity couplant. Therefore, we tried to develop a guide wave inspection system that uses an electromagnetic ultrasonic transducer (EMAT) which does not require any couplant. First, we confirmed that a guide wave can be transmitted and received in an aluminum pipe by a polarized shear horizontal transverse wave-EMAT, and we have confirmed the most suitable transmission and reception EMAT’s specification and the most suitable drive conditions to generate the L, T and F-mode guide waves. Finally, the detection performance has been evaluated by the developed system.
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.
A damage detection capability based on a flexible ultrasonic transducer (FUT) array bonded onto a planar and a curved
surface is presented. The FUT array was fabricated on a 75 μm titanium substrate using sol-gel spray technique. Room
temperature curable adhesive is used as the bonding agent and ultrasonic couplant between the transducer and the test
article. The bonding agent was successfully tested for aircraft environmental temperatures between -80 °C and 100 °C.
For a planar test article, selected FUT arrays were able to detect fasteners damage within a planar distance of 176 mm,
when used in the pulse-echo mode. Such results illustrate the effectiveness of the developed FUT transducer as compared
to commercial 10MHz ultrasonic transducer (UT). These FUT arrays were further demonstrated on a curved test article.
Pulse-echo measurements confirmed the reflected echoes from the specimen. Such measurement was not possible with
commercial UTs due to the curved nature of the test article and its accessibility, thus demonstrating the suitability and
superiority of the developed flexible ultrasonic transducer capability.
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.
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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