Anthony Gachagan

Characterization of air-coupled transducers

This paper describes a theoretical and experimental study for determination of the through-air system impulse response and insertion loss with different air-coupled ultrasonic transducers. Wide-band piezopolymer transducers (PVDF) are employed in both transmission and reception modes and their behavior assessed by means of mathematical modeling and experiment. Specifically, a linear systems approach, modified to include the influence of attenuation in the propagation medium, was used to design suitable PVDF transducers for wide-band operation in air. Suitable devices were then manufactured for determination of the transmission and reception response characteristics of piezocomposite and electrostatic transducers when operating in the air environment. A range of transducers was evaluated, including 1-3 connectivity composites of different ceramic volume fraction and mechanical matching conditions, in addition to electrostatic devices of varying design. To complement the investigation, relative performances for narrow-band operation are also presented under transmission and transmit-receive conditions. Despite the obvious measurement difficulties, good agreement between theory and experiment was observed and the methodology is shown to provide a convenient and robust procedure for comparison of through-air transducers operating in the frequency range 50 KHz to 2 MHz. Although highly resonant, the most effective composite transducers under consideration demonstrate an improvement in two-way insertion loss of 22.4 dB and 11.5 dB over a corresponding electrostatic pair, under narrow-band and wide-band operation, respectively.

An evaluation of 1-3 connectivity composite transducers for air coupled ultrasonic applications

A combination of theoretical modeling and experimental analysis is used to predict and assess the performance of 1–3 composite transducers for operation in an air‐coupled environment. Specifically, finite element analysis, supported by linear systems modeling, is employed to evaluate transmission and reception characteristics over the complete volume fraction range, for operating frequencies in the region of 500 kHz, and the theory confirmed using novel experimental techniques. The theoretical approach is then extended to assess the influence of matching and backing on transducer sensitivity and bandwidth. Throughout, the influence of air propagation is included in the simulation approach and highlighted where this factor is significant. The work is expected to provide useful design guidelines for the practical ultrasonic engineer.

Surface-bonded and embedded optical fibers as ultrasonic sensors

The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2-3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.

Air-coupled piezoelectric detection of laser-generated ultrasound

A pulsed laser has been used to generate ultrasonic transients in samples of metal and fiber-reinforced polymer composite material. These have been detected using an air-coupled piezoelectric transducer. It is demonstrated that such a transduction system can be used for longitudinal waves in bulk material, Rayleigh waves at solid surfaces and Lamb waves in thin plates. >

Surface-bonded optical fibre sensors for the inspection of CFRP plates using ultrasonic Lamb waves

Surface-bonded single-mode optical fibre sensors have been used to monitor the interaction of ultrasonic Lamb waves with defects in carbon fibre composite plates. Lamb waves were initiated using Perspex-coupled piezoelectric transducers. The defects investigated comprised holes, regions of impact damage and delaminations. Holes could be identified by analysing direct reflections and impact damage by back-wall echo amplitude. Large delaminations gave a poor direct reflection. Evidence was found for mode conversion at centre plane delaminations.

Generation and reception of ultrasonic guided waves in composite plates using conformable piezoelectric transmitters and optical-fiber detectors

A condition monitoring nondestructive evaluation (NDE) system, combining the generation of ultrasonic Lamb waves in thin composite plates and their subsequent detection using an embedded optical fiber system is described. The acoustic source is of low profile with respect to the composite plate thickness, surface conformable, and able to efficiently launch a known Lamb wave mode, at operating frequencies between 100 and 500 kHz, over typical propagation distances of 100 to 500 mm. It incorporates both piezocomposite technology and interdigital design techniques to generate the fundamental symmetrical Lamb wave mode in both metallic and carbon-fiber composite plates. Linear systems and finite element modeling techniques have been used to evaluate the operation of the transducer structure, and this is supplemented by experimental verification of the simulated data. An optical fiber, either bonded to the surface or embedded across the length of the composite plate samples, is used to detect the propagating ultrasonic Lamb waves. Single mode silica fiber has been used in conjunction with a portable 633 nm Mach-Zehnder interferometer for signal demodulation and subsequent data acquisition. This hybrid system is shown to generate and detect the fundamental symmetrical Lamb wave (s/sub 0/) in both carbon-fiber and glass-fiber reinforced composite plates. Importantly, the system signal-to-noise ratio (SNR) associated with the acoustic source compares favorably with s/sub 0/ Lamb wave generation using a conventional transducer and angled perspex wedge arrangement.

Review of high-power ultrasound-industrial applications and measurement methods

Applications involving high-power ultrasound are expanding rapidly as ultrasonic intensification opportunities are identified in new fields. This is facilitated through new technological developments and an evolution of current systems to tackle challenging problems. It is therefore important to continually update both the scientific and commercial communities on current system performance and limitations. To achieve this objective, this paper addresses two key aspects of high-power ultrasonic systems. In the first part, the review of high-power applications focuses on industrial applications and documents the developing technology from its early cleaning applications through to the advanced sonochemistry, cutting, and water treatment applications used today. The second part provides a comprehensive overview of measurement techniques used in conjunction with high-power ultrasonic systems. This is an important and evolving field which enables design and process engineers to optimize the behavior and/or operation of key metrics of system performance, such as field distribution or cavitation intensity.

A flexible piezoelectric transducer design for efficient generation and reception of ultrasonic Lamb waves

This paper describes the development of a flexible piezoelectric transducer for the generation and detection of ultrasonic symmetrical Lamb waves in plate-like structures. This piezoplatelet transducer structure comprises an array of miniature piezoceramic plates embedded within a soft setting polymer filler material, combining the efficiency of the active piezoceramic phase with a degree of flexibility, which is a function of the platelet/polymer dimensions. For many condition-monitoring applications, the generation of ultrasonic Lamb waves is often appropriate, and this was achieved by incorporating interdigital design techniques via the transducer electrode pattern. The performance of the piezoplatelet transducer structure was evaluated using a combination of linear systems and finite-element modeling, substantiated by experimental results. Importantly, the transducer is shown to operate as an ensemble of platelets, each operating in the thickness mode and well decoupled from neighboring piezoelectric elements. Using this transducer configuration, an unimodal s/sub 1/ Lamb wave, at 1.45 MHz, has been generated and detected in a 3-mm thick steel plate. Furthermore, a propagation distance of almost 1 m was recorded for s/sub 0/ Lamb wave generation/detection in a fiber-reinforced composite plate.

Transient supramolecular reconfiguration of peptide nanostructures using ultrasound

Ultrasound, i.e. high frequency oscillating pressure waves, is commonly used to overcome kinetic barriers associated with dissolution, assembly and gelation. We demonstrate that ultrasound energy may also be used to achieve transient reorganization of supramolecular nanostructures, which revert back to the original state when sound is switched off. Aromatic peptide amphiphiles, Fmoc-FL and -YL were used to study the transient acoustic response. These systems showed temporary supramolecular transitions that were sequence dependent. The changes observed were due to an altered balance between H-bonding and π-stacking, giving rise in changes in chiral organisation of peptide building blocks. Transient reconfiguration was visualized by TEM and changes in supramolecular interactions characterized by fluorescence, FT-IR and CD. Remarkably, significant differences are observed when compared to thermal heating, which relates to the oscillating and directional characteristics of ultrasound when delivering heat to a system.

Polymer composite material characterisation using a laser/air-transducer system

A wide bandwidth 1-3 connectivity piezocomposite air transducer has been used to detect laser-generated ultrasound in a variety of composite materials. Through thickness waveforms in various carbon fibre reinforced polymer (CFRP) composite plates will be presented, as well as a selection of Lamb waves. Signals were also obtained in samples of pultruded glass fibre reinforced composite of different thickness. Using the laser/air-transducer system, images were obtained of machined defects and delaminations by conventional C-scanning methods, and tomographic reconstruction techniques.