David W. Schindel

The design and characterization of micromachined air-coupled capacitance transducers

Air-coupled capacitance transducers have been manufactured using anisotropically etched silicon backplates and commercially available dielectric films (Kapton and Mylar). The small backplate pits which result from etching, provide well ordered and highly uniform air layers between the backplate surface and thin dielectric film. Such uniformity allows the transducers to be manufactured with reproducible characteristics (a property difficult to achieve through conventional manufacturing). Impulse response studies in generation and detection, have indicated well-damped, wideband behavior, with bandwidths extending from <100 kHz to 2.3 MHz (at the -6 dB points). These bandwidths are investigated as a function of excitation pulse width, applied bias potential, and dielectric film thickness. An estimate of detection sensitivity is also provided by comparison with a calibrated laser interferometer.<<ETX>>

The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging.

A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-fibre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements.

Applications of micromachined capacitance transducers in air-coupled ultrasonics and nondestructive evaluation

Micromachined capacitance transducers have been investigated in practical situations where air-coupled ultrasound has current application. Their /spl sim/2 MHz bandwidths lead to good performance in pulse-echo and through transmission operation at solid surfaces. Applications in surface profiling and distance measurement have been investigated. For the transduction of waves within solid media, the devices are shown to be capable of detecting longitudinal and shear waves at solid surfaces, as well as Rayleigh and Lamb waves. Examples are also given of fully noncontact, through transmission of carbon fibre reinforced polymer plates.<<ETX>>

Air-coupled ultrasonic NDE of bonded aluminum lap joints

This paper describes an investigation into the feasibility of using wideband air-coupled ultrasonic transducers for evaluating the integrity of bonded aluminum lap joints. Micromachined capacitance transducers are used to generate and detect ultrasonic Lamb waves in aluminum lap-joints through an intervening air layer. The transmission of Lamb waves through the bond is affected by the bonds local state, such that scanning a pair of air-transducers allows images to be produced either with the transducers on opposite sides of the plate, or on the same side. It is shown that defects in the bond-line can be detected and imaged by such a non-contract system. Confirmation of defect positions are obtained by comparing the results of the air-coupled scans to a conventional immersion ultrasonic C-scan.

Through-thickness characterization of solids by wideband air-coupled ultrasound

Abstract A non-contact inspection system employing wideband air-coupled ultrasound has been used for the characterization of thin solid material. Generation and detection of pulsed ultrasound were accomplished on opposite sides of the sample (in the through-thickness mode) using micromachined air-coupled capacitance transducers. The resulting bandwidth allowed absolute velocities and thickness changes to be estimated with reasonable accuracy ( ⩾ 1 %) for polymers, wood products, glass, and carbon-fibre reinforced polymer composites. An appreciable increase in sensitivity is shown to result for toneburst operation when the frequency matches the samples through-thickness resonance, and this was employed for the imaging of defects in composite materials.

Air-coupled Lamb wave tomography

An entirely air-coupled inspection system using a pair of micromachined silicon capacitance transducers has been used to image defects in thin plates of different materials (0.7 mm to 2.22 mm thick) using air-coupled Lamb wave tomography. A filtered back projection algorithm was used in a form of difference tomography to reconstruct images of defects up to 10 mm diameter machined in aluminium and perspex (Plexiglas) plates, as well as in samples of carbon fiber reinforced polymer (CFRP). The technique was able to resolve non-central defects as well as multiple flaws within the scan area. This flexible tomographic system was able to produce images of the change in a variety of different acoustic variables from only one set of experimental data, with success dependent on the size, shape, and location of the defect in the scan area.

Measurement of elastic constants in composite materials using air‐coupled ultrasonic bulk waves

This paper presents numerical simulations and experimental measurements of longitudinal and shear bulk waves in composite materials, generated and detected with an air‐coupled ultrasonic system. The transfer function of an anisotropic absorbing layered plate immersed in air is computed with the transfer matrix method, and the waveforms produced by transmission through a plate are simulated from the convolution of a reference waveform and the impulse response of the plate. The calculated waveforms compare well to those recorded experimentally, confirming that the waveforms recorded experimentally have the correct characteristics. It is demonstrated that it is possible to identify longitudinal and shear modes. Results are presented showing how this data may be used to recover the stiffness matrix of composite materials, using the angular variation of velocity of both modes.

Micromachined ultrasonic capacitance transducers for immersion applications

Investigations into the characteristics of water-coupled ultrasonic capacitance transducers have been undertaken for a range of transducer configurations. The radiated fields have been scanned in water using a miniature hydrophone detector, and the results compared to theory based on a plane piston approach. Micromachined backplates in conjunction with thin Mylar and mica membranes have been investigated, together with aperture modifications such as an annulus and Fresnel zone plate. The measured results agree well with theory, thus demonstrating that wideband predictable performance (>8 MHz) is obtainable with such transducers. Additionally, pulse-echo C-scans of a Plexiglas plate containing an artificial defect have been undertaken with the capacitance transducer, in order to demonstrate that the capacitance transducer has sufficient sensitivity to allow routine nondestructive testing within immersion applications.

Capacitive and piezoelectric air-coupled transducers for resonant ultrasonic inspection

Both micromachined air-coupled capacitance transducers and piezoelectric air-coupled devices have been used to form images of artificial graphite foil defects in a fibre-reinforced thermoplastic composite plate. This has been achieved using toneburst excitation at the fundamental through-thickness resonant frequency of the plate. Images are presented for both types of transducer system, and the results discussed. It is found that capacitance transducers have distinct advantages in materials characterization due to their wide bandwidth. These advantages include the ability to vary the excitation frequency in order to match to higher order resonant modes of the plate.

Ultrasonic measurements in polymeric materials using air‐coupled capacitance transducers

Wide bandwidth capacitance devices have been designed which have a high transduction efficiency in air at frequencies into the MHz range. They are sufficiently sensitive to allow transmission through solid samples, including polymers and fiber‐reinforced composites. Examples are presented of experiments showing how such transducers may be used to measure material properties and to perform ultrasonic imaging.