Lee A. J. Davis

Coded waveforms for optimised air-coupled ultrasonic nondestructive evaluation

This paper investigates various types of coded waveforms that could be used for air-coupled ultrasound, using a pulse compression approach to signal processing. These are needed because of the low signal-to-noise ratios that are found in many air-coupled ultrasonic nondestructive evaluation measurements, due to the large acoustic mismatch between air and many solid materials. The various waveforms, including both swept-frequency signals and those with binary modulation, are described, and their performance in the presence of noise is compared. It is shown that the optimum choice of modulation signal depends on the bandwidth available and the type of measurement being made.

The dynamic excitation of a granular chain for biomedical ultrasound applications: contact mechanics finite element analysis and validation

There has been recent interest in the transmission of acoustic signals along granular chains of spherical beads to produce waveforms of relevance to biomedical ultrasound applications. Hertzian contact between adjacent beads can introduce different harmonic content into the signal as it propagates. This transduction mechanism has the potential to be of use in both diagnostic and therapeutic ultrasound applications, and is the object of the study presented here. Although discrete dynamics models of this behaviour exist, a more comprehensive solution must be sought if changes in shape and deformation of individual beads are to be considered. Thus, the finite element method was used to investigate the dynamics of a granular chain of six, 1 mm diameter chrome steel spherical beads excited at one end using a sinusoidal displacement signal at 73 kHz. Output from this model was compared with the solution provided by the discrete dynamics model, and good overall agreement obtained. In addition, it was able to resolve the complex dynamics of the granular chain, including the multiple collisions which occur. It was demonstrated that under dynamic excitation conditions, the inability of discrete mechanics models to account for elastic deformation of the beads when these lose contact, could lead to discrepancies with experimental observations.

Properties of an electrostatic transducer

A thin and flexible transducer design for use in air is described. It is fabricated from three or more layers of material to form an electrostatic device with many interesting properties. The new acoustic source has an excellent high-frequency response and can be used as an acoustic radiator over a wide range of frequencies, in both the audible and ultrasonic frequency ranges. The frequency response and directivity are both modeled and compared to theory, where it is demonstrated that the device operates in a manner consistent with a plane piston source.

Ultrasonic propagation in finite-length granular chains

A narrowband ultrasound source has been used to generate solitary wave impulses in finite-length chains of spheres. Once the input signal is of sufficient amplitude, both harmonics and sub-harmonics of the input frequency can be generated as non-linear normal modes of the system, allowing a train of impulses to be established from a sinusoidal input. The characteristics of the response have been studied as a function of the physical properties of the chain, the input waveform and the level of static pre-compression. The results agree with the predictions of a theoretical model, based on a set of discrete dynamic equations for the spheres for finite-length chains. Impulses are only created for very small pre-compression forces of the order of 0.01N, where strongly non-linear behaviour is expected.

Through-transmission imaging of solids in air using ultrasonic gas-jet waveguides

An ultrasonic waveguide has been produced in air by using a gas jet. This uses the fact that a lower acoustic velocity can be produced within the jet, relative to the air surrounding it. The lower velocity is achieved by mixing carbon dioxide with air within the jet at a concentration that is a compromise between lower acoustic velocity and increasing attenuation. Using a capacitance transducer placed within the flowing gas, it is shown that improvements in the beam width can result when the gas jet is used. Air-coupled images of solid samples have been produced in through transmission, which demonstrate that an improved lateral resolution can result when a comparison is made to images from conventional air-coupled testing.

The study of chain-like materials for use in biomedical ultrasound

Wave propagation in chain-like materials has been studied previously at low frequencies. The present study has generated these waves at higher frequencies with components >200 kHz, using chains of 1 mm diameter spheres. Resonant ultrasonic horns at 73 kHz have been used as sources of narrowband excitation, which transform into a train of broadband impulses that have the characteristics of solitary waves. These have potential applications in biomedical ultrasound as high amplitude, wide bandwidth impulses.

P4M-4 Cross-Coupling in Sealed cMUT Arrays for Immersion Applications

Capacitive micromachined ultrasonic transducers (cMUTs) are becoming increasingly important as a future technology in imaging arrays, and are being proposed for applications such as biomedical ultrasound and high frequency sonar. This paper will present the results of a wide-ranging investigation into the nature and extent of mechanical cross-coupling in sealed cMUT arrays. In particular, the work has characterised the operation of small sealed multi-cavity devices specifically designed for immersion-based applications. An extensive modelling program has been undertaken using finite element via the PZFlex finite element code, in an attempt to understand the precise nature of cross-coupling within a single transducer element, comprising multiple sealed cMUT cavities. The results show that there is evidence of cross coupled signals propagated through the medium in the small sealed device, which increases under immersion applications. An additional waterproofing layer is then added to the cell for immersion purposes. The performance of the devices is compared for coatings including Parylene C and PDMS. The influence of this layer has been analysed theoretically, with the objective to minimise the influence on cMUT operation. The work is then extended to show experimental characterisation of such cMUT devices, configured in the form of 2D arrays. The cross-coupling present in the different array configurations is measured using a Polytec laser vibrometer.

Ultrasonic NDE of thick polyurethane flexible riser stiffener material

Abstract Ultrasonic signals at frequencies below 1 MHz have been used for the NDE of thick polymer samples. Coded signals such as chirps and bipolar Golay codes, together with pulse compression and signal processing, have been used to penetrate into thick sections of attenuating polypropylene riser stiffener material, using piezocomposite transducers to provide the required bandwidth. It is shown that this approach can be used to detect manufacturing defects such as air bubbles in flexible riser stiffeners, which might compromise their performance in the offshore oil and gas environments.

High-resolution acoustic imaging at low frequencies using 3D-printed metamaterials

An acoustic metamaterial has been constructed using 3D printing. It contained an array of air-filled channels, whose size and shape could be varied within the design and manufacture process. In this paper we analyze both numerically and experimentally the properties of this polymer metamaterial structure, and demonstrate its use for the imaging of a sample with sub-wavelength dimensions in the audible frequency range.

The generation of impulses from narrow bandwidth signals using resonant spherical chains

An ultrasonic horn operating at 73 kHz has been used to excite one end of a chain of steel spheres. The signal transmitted along the chain was measured at the far end using a laser vibrometer. Various chain lengths, ranging from 2-10 spheres, have been studied. It was found that a set of solitary wave impulses were generated when a high input amplitude and a minimal pre-compression force was used. Both harmonics and sub-harmonics of the input frequency could be observed. Theoretical models were developed, based on the relevant equations of motion, which modelled the end conditions properly, and their predictions confirmed the experimental observations. Solitary wave impulses were generated only when certain numbers of spheres were used, corresponding to the establishment of resonances in the form of nonlinear normal modes (NNMs). It was found that longer chains led to wider impulse bandwidths. Increased pre-compression tended to damp out this phenomenon, but gave a weakly non-linear state where the time delay of propagation along the chain changed with applied static force. It was thus established that a gated sinusoidal input could be transformed into a set of impulses, of interest in biomedical applications.