Stephen P. Kelly

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.

Characterization and assessment of an integrated matching layer for air-coupled ultrasonic applications

A novel ultrasonic matching layer for improving coupling between piezoelectric transducers and an air load is presented and the results of a theoretical and experimental program of work are provided. A combination of a porous material that has very low acoustic impedance with a low-density rubber material forms the basis of the approach. These matching layers were first analyzed experimentally using scanning electron and optical microscopy to determine the microscopic structure. Air-coupled resonance measurements were then performed to reveal the acoustic parameters of the individual layers that were identified within this multilayered structure. These data were then incorporated into a conventional linear model, and this has been verified and used to study performance and produce designs. Close correlation between experiment and theory is demonstrated. The most efficient designs have been implemented in a pitch/catch air-coupled system, and an improvement in received signal amplitude of 30 dB was achieved when compared with the unmatched case.

10D-1 On-Line Ultrasonic Inspection at Elevated Temperatures

An ability to perform on-line inspection on live plant without stopping an industrial process would be extremely valuable across a wide range of sectors including: power generation, petrochemical, offshore and steel. Frequently, plant operating temperatures are higher than existing contact inspection technologies can withstand and there is a need to have regular scheduled outages to allow cooling to occur before inspections can take place. There is significant interest therefore in the development of high temperature inspection capabilities to reduce the need for these outages. On-line monitoring would also be important as this would give the confidence needed to enable plant to operate safely beyond initial design life.

An air-coupled ultrasonic matching layer employing half wavelength cavity resonance

The concept of a multi-phase, integrated layer for improved mechanical matching between a piezoelectric transducer and an air load is explored, both theoretically and experimentally. A combination of optical and scanning electron microscopy is used to identify the internal structure of a layer comprising silicone rubber and a semi-porous membrane filter. This data is then inserted in a linear model for prediction of through transmission response and the simulations compared with experimental measurements for extrapolation of the necessary materials elastic constants. Using these properties, it is possible to design a matching layer for a given application and results are presented that confirm the success of the approach.

Ultrasmart: Developments in Ultrasonic Flaw Detection and Monitoring for High Temperature Plant Applications

Plant in the power generation, petrochemical and metals processing industries is subject to increasingly onerous operational and regulatory requirements. Where plant that operates at high temperature is involved, the costs associated with shutdown for planned or unplanned inspection to meet these requirements can be particularly high. The ability to perform condition monitoring or flaw detection at on-line plant temperatures would enable plant to remain in operation for longer periods, reduce the risk of damage from thermal cycling associated with periodic shutdowns and allow shutdowns to be completed more quickly. The associated minimizing of the loss of revenue caused by frequent and lengthy shutdowns is a highly attractive proposition to the plant operators. This paper reports on progress in the Ultrasmart Project, which is being undertaken by a consortium of UK companies and aims to address the problems associated with performing ultrasonic inspection on pressure vessels and piping at temperatures exceeding ∼350°C. A brief review of the state of current industry capabilities is given and then details of the developments investigated within Ultrasmart are reported and discussed. These include: • Liquid cooled transducers and automated scanning mechanisms suitable for deployment on components with surface temperatures up to ∼500°C. • Permanently mounted piezoelectric transducers suitable for long term flaw growth or component thickness monitoring at temperatures up to ∼750°C. • Techniques, procedures and protocols necessary to achieve reliable and quantifiable inspection capability at high temperatures. • Use of a novel non-resonant thin film Aluminum Nitride (AlN) transducer for in-situ component monitoring.© 2007 ASME

Advances in air coupled NDE for rapid scanning applications

Results obtained during experiments with a state of the art NDE system are presented. These results demonstrate the sensitivity that can be achieved with modem piezoceramic composite transducers. One set of experiments was concerned with the through transmission inspection of composite sections. These included thick honeycomb structures such as those used in the aircraft industry. The work involved a study of the influence of multiple reflections as a function of transducer airgap separation. Also investigated were the generation and detection of shear waves, Rayleigh waves and Lamb waves. The Lamb wave technique has been used to demonstrate the feasibility of producing real-time images of defects in thin plates of various materials including carbon fibre and aluminium

Real-time through transmission inspection of aircraft composites using air-coupled ultrasonic arrays

The design, development and initial testing of a fully operational, through transmission ultrasonic array system to perform air-coupled, non-destructive testing (NDT) of advanced composite materials within the aerospace industry is presented. The system offers greater test flexibility and significantly improves the achievable scan rate, when compared with existing water jet systems, without loss of resolution or flaw detection integrity. A focused piezocomposite transducer array system incorporating narrowband, low noise electronics has been designed and developed in order to meet the strict requirements of the aerospace industry. Air-coupled C scans of defective samples, typical of those encountered in the industry, have been obtained using a prototype test system. These are presented and compared with water coupled scans of the same samples.

Micromachined unimorphs and bimorphs

A frequency doubled copper vapour laser (CVL) operating at an ultraviolet wavelength of 271 nm has been used to cut miniature cantilevers in laminated composites incorporating at least one layer of the ferroelectric polymer polyvinylidene fluoride (PVDF). Devices have been fabricated from PVDF with thickness values in the range 9 /spl mu/m to 38 /spl mu/m. Laser vibrometer measurements show that the fundamental resonance frequencies of these devices correspond almost exactly with values obtained from mechanical beam theory. Ultrasonic transducers consisting of bimorph and unimorph arrays are currently being designed with the aid of a finite element model, which is not discussed.

Development of a Manual Air‐Coupled Ultrasonic Inspection Instrument for Use on Aeronautical Structures Under In‐Service Conditions

This paper describes the development programme for a portable non‐contact inspection system, suitable for in‐service inspection of aeronautical structures. The instrument is powered from a re‐chargeable battery pack which provides over six hours of continuous usage. The transducer front end is a lightweight, hand‐held assembly in which a pair of piezocomposite transducers are housed. Angular adjustment of the transducers enables the instrument to operate into a variety of materials, with metallic and composite materials of primary interest. Importantly, a single channel oscilloscope has been incorporated to provide visual interpretation of the processed received signal. Defect detection is demonstrated in a number of aerospace samples, with the resultant C‐scans presented where appropriate.