Table of contents

The Anglo-French Physical Acoustics Conference (AFPAC) had its 10th annual meeting in Villa Clythia, Fréjus, France, from 19–21 January 2011. This series of meetings is a collaboration between the Physical Acoustics Group (PAG) of the Institute of Physics and the Groupe d'Acoustique Physique, Sous-marine et UltraSonore (GAPSUS) of the Société Française d'Acoustique.

The conference has its loyal supporters whom we wish to thank. It is their loyalty that has made this conference a success. AFPAC alternates between the UK and France and its format has been designed to ensure that it remains a friendly meeting of very high scientific quality, offering a broad spectrum of subjects, welcoming young researchers and PhD students and giving them the opportunity to give their first presentations in an 'international' conference, but with limited pressure.

For the third consecutive year AFPAC is followed by the publication of its proceedings in the form of 18 peer-reviewed papers, which cover the most recent research developments in the field of Physical Acoustics in the UK and France.

Alain Lhémery CEA, France Nader Saffari UCL, United Kingdom

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The development of nanometre sized ultrasonic transducers is important in both biological and industrial applications. The small size can be important in its own right or necessary in order to generate acoustic waves with nanometric wavelengths. Potential applications of nanotransducers range from embedded sensors through to sub optical wavelength acoustic imaging. In this paper we discuss the design and fabrication of nanoscale ultrasonic transducers. The transducers rely on optical and mechanical resonances, they can be used to generate and detect high frequency ultrasound in a sample. The mechanical and optical performance of the devices have been extensively modelled using both analytical techniques and finite element modelling. This allows the fine tuning of the design parameters to ensure optimised performance for the experimental configuration. The devices can be fabricated in a number of ways, we present one method for building these types of devices, a 'top down' approach where plate structures are built up and patterned using standard photolithographic techniques. This method produces nanoscale devices in one dimension only (the others being a few microns) but produces excellent devices for testing in situ and for comparison to the models as they are easy to handle and measure. Approaches for reducing the other dimensions to the nanoscale will also be considered.

We introduce a highly sensitive laser ultrasonic detection MEMs transducer and an efficient laser ultrasound generation MEMs transducer. The detection transducer consists of a series of cantilevers with the same dimensions; any two cantilevers next to each other are separated by a solid with the same width as finger's. When ultrasound is incident upon this transducer, as long as there is a vibration component perpendicular to each finger's top surface and with a frequency the same as the finger's first resonance in the ultrasound, each finger will resonate upon the ultrasound. The moving fingers and the still solid separations form an optical phase grating, and therefore the ultrasound can be readout by a detection laser remotely. Because the ultrasound amplitude is amplified many times by the transducer's resonance before detection, the sensitivity of this transducer is much higher than that of traditional transducers. The generation transducer consists of a micro-disk seated upon a micro-stem. When a suitably focused laser pulse illuminated on the center of the disk, a certain order of flapping motion of the disk is mainly actuated, while other orders are just slightly, or not excited. This flapping motion couples a very narrow bandwidth of longitudinal wave propagating along the axis of the stem and into a sample. Because all absorbed optical energy is concentrated into this narrowband ultrasound, its amplitude is much higher than that of normal thermoelastic generation. It is possible to use these MEMs generation and detection transducers to form a simple but highly efficient laser ultrasound generation and detection system in the near future.

The term 'orientation imaging microscopy' describes generic techniques for imaging the orientation of crystalline structures of heterogeneous media. Typically, scanning electron microscope techniques are used, such as electron backscatter diffraction or transmission electron microscopy. We have developed an acoustic technique that can perform equivalent measurements. The key to enabling a practical acoustic method is spatially resolved acoustic microscopy (SRAS), a robust laser ultrasonic technique for quantitatively determining the surface acoustic wave velocity at a high spatial resolution. Here we present quantitative determination of crystallographic orientation of large nickel grains by comparing the measured SAW velocity in multiple propagation directions, with a data base of calculated velocity surfaces for all orientations. As well as discussing the method of determining the crystallographic orientation details are also presented of the recent advances in the capabilities of the latest generation of instrumentation, including significant increase in the data acquisitions rate and the reduction in size and complexity of the SRAS instrument itself.

Acoustic materials can be used as external coating to reduce radiated noise or target strength of immersed structures, or to improve performance of sonar systems. In order to optimise the design and the use of such coatings, it is necessary to have a good estimate of their intrinsic properties, in particular the complex sound velocity inside the materials. The type of material studied here consists in a viscoelastic slab containing a given proportion of micro-voids. Determination of the complex longitudinal velocity is not an easy task, and direct methods or experimental set-ups do not give sufficient results. The method presented here consists in determining the complex longitudinal velocity by solving an inverse problem, from acoustic measurements of the reflection and transmission coefficients of a test panel in a water tank. Best results, including attenuation, are obtained by over-determining the problem, using all data available. Results are consistent to the known physical behaviour of such materials, regarding influence of frequence and hydrostatic pressure.

The work focuses on the derivation of a lossy wave equation for ultrasound absorption in media with the power law of absorption, which plays an important role in bio-medical applications. Possible forms of nonlinear lossy wave equations are also discussed. The properties of the non-linear pressure fields resulting from the implementation of different non-linearity models are investigated. This part of the study is illustrated with the numerical computations of the related linear and nonlinear equations.

The strength and stability of adhesive bonded structures are related to polymer curing, when crosslinking occurs and leads to adhesive strength, stiffness and durability. Depending on the resin and curing agent used, cure time can vary from minutes to weeks. Methods based on dynamic mechanical analysis (DMA) or calorimetric techniques (DSC, DTA) are valuable for evaluating mechanical properties of adhesives, but are devoted specifically to the polymers alone, and not in situ in adhesive bonds. In this contribution, we have monitored - during crosslinking - the Young's modulus of a slow-curing DGEBA - PAMAM adhesive system, with two non-destructive, in situ, methods used for the characterisation of the adhesive in a bonded system. The first method is based on measurements obtained from strain gauges mounted on one bonded adherend. The second method uses an ultrasound technique based on the through-transmission. Both methods suggest the same curing kinetics.

Wood is a biological growth medium. It is orthotropic with longitudinal, radial and tangential axes. Furthermore, standing trees adapt themselves to environmental growth conditions, and their material properties vary with age. These changes result in variations that are much more complex than anisotropy. Studying wood quality and intraspecific variability is useful for clonal selection and for the genetic improvement of plantations. In this study, two logs of Picea abies underwent transmission tomography. The mean diameter was 16 cm (26-year-old tree) and the moisture content was 22%. The effect of the presence of bark and artificial defects was investigated. The tomographic device was specifically built for tree imaging. The imaging process was automatic with 900 ultrasonic acquisitions in 40 minutes (emission at 55 kHz with 5 periods of square wave form). The main conclusions were: speed near the bark is higher than in the centre because of the existence of juvenile wood combined with the moisture content gradient (moisture content lower near the bark). Likewise, damping near the bark is lower than in the centre. A significant relationship was observed between slowness and attenuation (R² = 0.50); when the speed increased, damping decreased. No clear effect of the presence of bark was shown on the tomographic images. The bark was thin (3 to 5 mm thick) compared to the wavelength (26 mm). The 10, 20 and 50 mm artificial holes were clearly visible on the tomographic images. However, quantitative tomography does not enable the precise location of defects.

In this paper the effect of interaction on the expansion of a bubble in a regular monodisperse cluster is investigated. By a geometric construction a two-dimensional ordinary differential equation with an exact expression for first-order bubble interactions is derived for an n-bubble model. An approximate equation is derived for the rapid expansion of the bubble which can be solved yielding an analytic expression for the collapse of a bubble which undergoes inertial cavitation. It is then demonstrated that the maximum volume of a bubble in a cluster is considerably less than that of a single bubble. This result is of significance as typically the dispersion relationship, the wave speed and the co-efficient of attenuation are calculated using a single bubble model and summed for the total number of bubbles to yield the void fraction. Furthermore it is shown that the maximum radius of a bubble in the cluster is considerably smaller than that of a single bubble, yet the duration of the collapse phase is only weakly dependent on the number of bubbles. Hence, it is conjectured that the likelihood of fragmentation due to Rayleigh–Taylor instability is reduced. The results from the analysis are in good agreement with full numerical simulations of multi-bubble dynamics, as well as experimental observations

Ultrasonic telemetry techniques consist in locating various immersed structures (for instance, components in the main vessel of fast breeder reactors). The interactions beam-targets give rise to different scattering phenomena: tip diffraction of boundaries and edges of the different parts, specular reflection, and corner effect. In order to conceive and design such imaging techniques, simulation tool needs to account for these effects. Classical methods have been studied for such problems. The diffraction coefficients based on the Geometrical Theory of Diffraction (GTD) fail in the transition regions adjacent to shadow and reflection boundaries. The uniform diffraction theory provides continuous solutions in these regions, but with more sophisticated formulation. Another simple approximation based on the integral equation, widely used for scattering problems, is the so-called Kirchhoff approximation. The Kirchhoff approximation has good performance in the specular reflection zone but fails at predicting amplitude of diffracted waves by edges. A refinement of the Kirchhoff approximation which is based on the Physical Theory of Diffraction (PTD) and combines GTD and Kirchhoff edge diffraction coefficients has been studied. This refined Kirchhoff approximation provides a simple formulation and correct results for all scattered directions, which will be illustrated in the case of a rigid halfplane or wedge.

The efficacy of high intensity focused ultrasound (HIFU) for the non-invasive treatment of cancer has been demonstrated for a range of different cancers including those of the liver, kidney, prostate and breast. As a non-invasive focused therapy, HIFU offers considerable advantages over other techniques such as chemotherapy and surgical resection, in terms of invasiveness and risk of harmful side effects. Despite its advantages, however, there are a number of significant challenges currently hindering its widespread clinical application. One of these challenges is the need to transmit sufficient energy through the ribcage to induce tissue necrosis at the required foci whilst minimising the formation of side lobes. Multielement random arrays are currently showing great promise in overcoming the limitations of single-element transducers. Nevertheless, successfully treating a patient for liver tumours requires a thorough understanding of the way in which the ultrasonic pressure field from a HIFU array is scattered by the ribcage. A mesh of quadratic pressure patches was generated using CT scan data for ribs nine to twelve on the right side. A boundary element approach based on a Generalised Minimal Residual (GMRES) implementation of the Burton-Miller formulation was used, in conjunction with phase conjugation techniques to focus the field of a 256-element random HIFU array past the ribs at both intercostal and transcostal treatment locations. This method has the advantage of accounting for full effects of scattering and diffraction in three dimensions under continuous wave excitation.

Nondestructive testing of aerospace structures often requires their immobilization or disassembly. Structural health monitoring (SHM) can overcome these problems; the use of guided elastic waves (GW) in SHM is of great interest, because they propagate long distance in the structure thickness. Structures being stiffened, optimally positioning sensors implies to determine the number of stiffeners a wave can go through while remaining detectable. Here, the diffraction of GW by a stiffener bonded to a plate is considered. Elastic and geometric invariances along stiffener axis lead to 2D computations involving the three components of wave particle displacement, whatever its incidence on stiffener. A hybrid model is developed combining the semi-analytical finite element method for GW propagation and a finite element method (FE) for the stiffener diffraction. Optimal hybridization is obtained thanks to the development of transparent boundaries of the FE domain. Such boundaries have been obtained for GW normally incident onto a scattering feature, thanks to Fraser's bi-orthogonality relation, which unfortunately does not hold for oblique incidence. A numerical approach is developed to numerically approximate it, then, to derive boundary conditions in the wanted form. Their use minimizes the size of the FE domain and avoids any artificial reflection. They provide a mean for projecting diffracted fields on modes reflected on or transmitted through the stiffener; corresponding coefficients are obtained as functions of the direction of incidence.

Acoustic surface waves are widely used to sense and map the properties of the propagation media. In order to characterise local space-time waves, methods such as Gabor analysis are powerful. Nevertheless, knowing which wave is observed, extracting its full bandwidth contribution from the others and to map it in the signal domain is also of great interest. In the Fourier domain, the acoustic energy of a wave is concentrated along the wave-number frequency (k-ω) dispersion curve, a way to extract one wave from others is to filter the signals by mean of k-ω band-pass area that keeps only the selected surface wave. The objective of the present paper is to propose 2D Finite Impulse Response (FIR) filters based on an arbitrary area shape designed to extract selected waves. FIR filtering is based on convolving the impulse response of the filter with the signals. Impulse responses derived from using k-ω elliptical areas (E-FIR) are presented. The E-FIR filters are successfully tested on three experimental space-time signals corresponding to the propagation of Lamb waves measured by standard transducers on a cylindrical shell, by laser Doppler on a plate and generated by a circular pulse and observed by shearography on a rectangular plate.

Shed-pipe grouting technology, an effective advanced supporting method, is often used in the excavation of soft strata. Steel floral pipes are one of the key load-carrying components of shed-pipe grouting supporting structures. Guided waves are a very attractive methodology to inspect multi-hole steel floral pipes as they offer long range inspection capability, mode and frequency tuning, and cost effectiveness. In this contribution, preliminary experiments are described for the inspection of steel floral pipes using a low frequency longitudinal guided wave mode, L(0,2). The relation between the number of grouting holes and the peak-to-peak amplitude of the first end-reflected signal was obtained. The effect of the grouting holes in steel floral pipes on the propagation velocity of the L(0,2) mode at 30 kHz was analyzed. Experimental results indicate that the typical grouting holes in steel floral pipe have no significant effect on the propagation of this mode. As a result, low frequency longitudinal guided wave modes have potential for the non-destructive long range inspection of multi-hole steel floral pipes. Furthermore, the propagation velocity of the investigated L(0,2) mode at 30 kHz decreases linearly with the increase of the number of grouting holes in a steel floral pipe. It is also noticeable that the effect of the grouting holes cumulates along with the increase in the number of grouting holes and subsequent increase in reflection times of longitudinal guided waves in the steel floral pipe. The application potential of the low frequency longitudinal guided wave technique for the inspection of embedded steel floral pipes is discussed.

Electromagnetic acoustic transducers (EMAT) generate forces which are sources of elastic waves in a part without contact with it. Depending on the EMAT design, normal or tangential forces can be generated. Thanks to these capabilities, EMATs constitute an interesting alternative to piezoelectric devices in many configurations of non-destructive examination. In this paper, only shear horizontal (SH) guided waves radiated by an EMAT in a plate made of ferromagnetic material are modelled. These waves are particularly interesting to use for testing welded structures: SH waves may propagate in a weld without scattering and mode conversion phenomena. All the forces generated by an EMAT (Lorentz's, magnetosctrictive and magnetic forces) being exponentially decreasing with depth, they are rewritten as series of moments, then, approximated as equivalent surface stresses. Surface stresses are then taken into account as terms of source of elastic waves. Specific features of EMAT can eventually be exploited to derive simple analytic expressions of the applied stress, leading to closed-form solutions for the modal amplitude of SH guided waves. Using these solutions, which are calculated at no computing cost, it becomes easy to study the influence of typical EMAT parameters on the modal amplitude of waves generated in the plate.

We study two canonical problems of interest in non-destructive evaluation, diffraction of a plane elastic wave by a thin crack and diffraction of a plane elastic wave by a wedge, both in the high-frequency regime. In applications this regime is usually treated using the so-called Kirchhoff approximation. It is very easy to implement but there are situations when it is known to give distorted results. We discuss an easy correction procedure, which is applicable not only in geometrical regions but inside penumbrae as well. It involves a version of the Physical Theory of Diffraction that relies on the Geometrical Theory of Diffraction rather than the full solution of the corresponding canonical problem.

Acoustic emission (AE) is a non-destructive testing method used in various industries (aerospace, petrochemical and pressure-vessel industries in general, power generation, civil engineering, mechanical engineering, etc...) for the examination of large structures subjected to various stresses (e.g. mechanical loading).The energy released by a defect under stress (the AE phenomenon) can propagate as guided waves in thin structures or as surface Rayleigh waves in thick ones. Sensors (possibly permanently) are positioned at various locations on the structure under examination and are assumed to be sensitive to these waves. Then, post-processing tools typically based on signal processing and triangulation algorithms can be used to inverse these data, allowing one to estimate the position of the defect from which emanates the waves measured. The French Atomic Energy Commission is engaged in the development of tools for simulating AE examinations. These tools are based on specific models for the AE sources, for the propagation of guided or Rayleigh waves and for the behaviour of AE sensors. Here, the coupling of a fracture mechanics based model for AE source and surface/guided wave propagation models is achieved through an integral formulation relying on the elastodynamic reciprocity principle. As a first approximation, a simple piston-like model is used to predict the sensitivity of AE sensors. Predictions computed by our simulation tool are compared to results from the literature for validation purpose.

In this paper we address the issue of a creeping wave by investigating a two-dimensional configuration which involves a surface-braking crack in a traction free planar back-wall. We assume that this back-wall is irradiated by a shear Gaussian beam which is incident at a critical angle and hits the wall away from the crack edge. We show that the generated wave - known in the engineering literature as "creeping", because it propagates near the surface - consists of non-geometrical waves, the longitudinal and transverse components of the head wave and a longitudinal Goodier-Bishop type wave whose amplitude grows linearly with the distance to its propagation path. The "creeping" wave interacts with the crack edge but the resulting field cannot be simulated using the Kirchhoff Approximation, because this does not embrace the higher order effects. We show that the diffracted field can be simulated using a modification of the Uniform Geometrical Theory of Diffraction which involves a relatively simple limiting procedure.

The acoustic modality yields non destructive testing techniques of choice for in-depth investigation. Given a precise model of acoustic wave propagation in materials of possibly complex structures, acoustical imaging amounts to the so-called acoustic wave inversion. A less ambitious approach consists in processing pulse-echo data (typically, A- or B-scans) to detect localised echoes with the maximum temporal (and lateral) precision. This is a resolution enhancement problem, and more precisely a sparse deconvolution problem which is naturally addressed in the inversion framework. The paper focuses on the main sparse deconvolution methods and algorithms, with a view to apply them to ultrasonic non-destructive testing.