K.-T. Wu

Applications of Flexible Ultrasonic Transducer Array for Defect Detection at 150 °C

In this study, the feasibility of using a one dimensional 16-element flexible ultrasonic transducer (FUT) array for nondestructive testing at 150 °C is demonstrated. The FUT arrays were made by a sol-gel sprayed piezoelectric film technology; a PZT composite film was sprayed on a titanium foil of 75 μm thickness. Since the FUT array is flexible, it was attached to a steel pipe with an outer diameter of 89 mm and a wall thickness of 6.5 mm at 150 °C. Using the ultrasonic pulse-echo mode, pipe thickness measurements could be performed. Moreover, using the ultrasonic pulse-echo and pitch-catch modes of each element of FUT array, the defect detection was performed on an Al alloy block of 30 mm thickness with a side-drilled hole (SDH) of ϕ3 mm at 150 °C. In addition, a post-processing algorithm based on the total focusing method was used to process the full matrix of these A-scan signals of each single transmitter and multi-receivers, and then the phase-array image was obtained to indicate this defect- SDH. Both results show the capability of FUT array being operated at 150 °C for the corrosion and defect detections.

In situ ice and structure thickness monitoring using integrated and flexible ultrasonic transducers

Two types of ultrasonic sensors are presented for in situ capability development of ice detection and structure thickness measurement. These piezoelectric film based sensors have been fabricated by a sol–gel spray technique for aircraft environments and for temperatures ranging from −80 to 100 °C. In one sensor type, piezoelectric films of thickness greater than 40 µm are deposited directly onto the interior of a 1.3 mm thick aluminum (Al) alloy control surface (stabilizer) of an aircraft wing structure as integrated ultrasonic transducers (UTs). In the other sensor type, piezoelectric films are coated onto a 50 µm thick polyimide membrane as flexible UTs. These were subsequently glued onto similar locations at the same control surfaces. In situ monitoring of stabilizer outer skin thickness was performed. Ice build-up ranging from a fraction of 1 mm to less than 1.5 mm was also detected on a 3 mm thick Al plate. Measurements using these ultrasonic sensors agreed well with those obtained by a micrometer. Tradeoffs of these two approaches are presented.

Ultrasonic probes simultaneously producing one longitudinal and two shear waves and their potential applications

Ultrasonic probes which can simultaneously generate and receive two orthogonal polarized shear (S<inf>⊥</inf> and S<inf>//</inf>) and longitudinal (L) waves in screw shape have been presented. Thick piezoelectric films are directly fabricated onto the heads of such probes as L wave integrated ultrasonic transducers. S<inf>⊥</inf> and S<inf>//</inf> waves are obtained using mode conversion method. Application of such probes for on-line polymer injection molding process diagnostics is demonstrated.

Non-Contact Local and Global Damage Detection with Integrated Ultrasonic Transducers

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.

Structural health monitoring of composites using integrated ultrasonic transducers

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 transducers for structural health monitoring of metals and composites

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.

Integrated surface and plate acoustic wave sensors for health monitoring

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.

Flexible ultrasonic array sensors for health monitoring

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.

5H-6 Fabrication and Characterization of Thick Film Piezoelectric Ultrasonic Transducers

Lead-zirconate-titanate fine powders mixed with chemical solutions have been used to fabricate 50 mum thick films. The fabrication methods include sol-gel spray, tape casting and screen printing. The substrates were 75 mum thick stainless steel membrane. The sol-gel spray technique included multiple coatings and each coating involved spray and heat treatment. Each layer was between 5 and 15 mum thick. For tape casting and screen printing, only one layer of 50 mum was directly cast and printed onto the substrate. Then films were treated in the furnace up to 650 degC for 5 to 30 minutes in this study. These films became piezoelectric through a corona poling technique. The relative dielectric constant of the sol-gel sprayed films was around 130 and those of the tape casting and screen printing ones were lower than 100. The capability of these films as piezoelectric flexible ultrasonic transducers was confirmed in the pulse-echo mode at several MHz for non-destructive testing at room temperature and 150 degC

Engine Oil Condition Monitoring Using High Temperature Integrated Ultrasonic Transducers

High temperature integrated ultrasonic transducers (IUTs) made of thick piezoelectric composite films have been coated directly onto the lubricant oil supply and sump lines of a modified CF700 turbojet engine. These thick piezoelectric films are fabricated using a sol-gel spray technology. The center frequencies of these IUTs are in the range of 10 to 12 MHz. The top electrodes, electrical wires, conducting adhesive bond, connectors and cables have been tested successfully for temperatures of up to 500°C. By operating these IUT in transmission mode, the amplitude and velocity of transmitted ultrasonic waves across the flow channel of the lubricant oil in supply and sump lines were measured during engine operation. The results have shown that the strength of the ultrasonic waves is sensitive to the presence of air bubbles in the oil and that the ultrasound velocity is linearly dependent on oil temperature. Based on the sensitivity of ultrasound velocity to oil temperature, a method for real-time monitoring of engine oil degradation is proposed.Copyright