Table of contents

Proceedings of the Anglo-French Physical Acoustics Conference (AFPAC)

Preface

The 17^th annual Anglo-French Physical Acoustics Conference (AFPAC) was held at the Selsdon Park Hotel is Surrey UK from 17^th to 19^th of January 2018. Over the years AFPAC has established a unique character where a most diverse range of topics covering the many application of acoustics and ultrasonics are discussed in a friendly environment with almost a residential workshop feel. The scientific standards are always high and PhD students make up a large proportion of the presentations. This 17^th annual meeting was no exception. There were 42 presentations including 4 from invited speakers. These invited speakers included Andrea Colombi from Imperial College who talked about seismic waves and metamaterials. James Windmill of Strathclyde University presented work on how the study of insects is being used to develop new sensors. Emmanuel Moulin from Université de Valenciennes talked about coda waves in Structural Health monitoring (SHM) and Régis Marchiano from Université Pierre et Marie Curie in Paris presented work from a project on inverse problems using data collected by large arrays of microphones. Other subjects covered included acoustofluidics, acoustic tweezers and particle manipulation, biomedical ultrasound applications such as histotripsy, lithotripsy and HIFU, NDE and SHM for plate-like structures, corrosion and polycrystalline materials. There were also presentations on acoustically driven bubbles. A truly eclectic mix for those who are fascinated by all things acoustic!

These proceedings contain a collections of papers submitted by the participants at the 2018 AFPAC. Submission of manuscripts has always been done on a voluntary basis and we have continued to maintain a high standard of reviewing for those who choose to have their work published here.

Nader Saffari

Alain Lhemery

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

Carbon fiber reinforced polymer (CFRP) plates and honeycomb composite sandwich structures (HCSS) are widely used in the aerospace industry as they exhibit excellent strength-to-weight ratio, stiffness, toughness, corrosion resistance, etc. Nevertheless, defects, such as face sheet delamination or core-sheet debonding, may appear due to the impact forces or thermo-mechanical aging and can degrade these properties. Structural health monitoring (SHM) based on the use of guided elastic waves (GW) is regarded as a promising solution to detect such defects, and consequently to reduce maintenance costs and to extend structure service time. GWs propagate over large distances while being sensitive to structural inhomogeneities. Here, a SHM system prototype is proposed. It relies on a sparse grid of piezoelectric transducers distributed over the structure, used for both actuating and sensing GWs. Defect imaging is performed by means of the correlation based algorithm, the so-called Excitelet. It computes correlation coefficients between the theoretical and experimental GW signals for each pixel on the image representing the region of interest of the structure. The theoretical signals for CFRP are computed using a model based on a 2D semi-analytical finite element formulation. Analytical prediction of theoretical signals for the HCSS being intractable, a homogenization model is applied to the honeycomb core to replace it by an equivalent orthotropic plate preserving the same modelling approach. Defect imaging results are presented for both structures, namely a CFRP plate and a HCSS. The resolution of the corresponding images can be related to the wavelength of the inspecting mode.

Compromised interface bonds in structural components can lead to premature loss of structural integrity. Such defects are very difficult to detect using traditional acoustic inspection techniques. We assess the ability of linear ultrasonic Non-Destructive Testing (NDT) methods to detect or differentiate between weak-kissing bonds from healthy ones, by using a set of contaminated and non-contaminated bonds. An Ultrasonic Array Transducer (UAT) enables increased informational content, hence they have been used to detect and differentiate compromised bonds. Considerable research has been made into detecting weak bonds, but a practical, cost effective and industrially applicable solution remains elusive. The key problem is the signal changes are small, therefore we have developed an iterative testing methodology, in which the experimental variation in each of the following steps is significantly reduced: Sample Preparation; Data Acquisition; Imaging[1, 2]; Feature Analysis[3, 4]; Characterization/Classification; Interpretation. Our research addresses the issues related to UAT measurement techniques, specifically Full Matrix Capture (FMC)[1, 2] and Total Focusing Method (TFM)[1, 2], Scattering Matrix(derived) [3, 4] extraction in time domain, and other related or derived methods. Here the optimization of bondline imaging, resulting in higher quality and coherence of the imaging, is presented and discussed.

Adhesive bonding technology has now gained much attention in many industries as a very versatile assembling technique. However, to be used for structural joining and critical application, high reliability is needed. Thus, efficient non-destructive control strategy should be proposed to evaluate the nominal bonding quality but also possible progressive in-service degradations. Promising results have been presented in the literature using ultrasound-based methods. While linear ultrasound is efficient to detect decohesion or voids in a structure, it is barely sensitive to bond strength. In this work, we present a method to generate high amplitude plane wave, which may produce nonlinear phenomenon that are able to reveal kissing bonds or other types of adhesion defects. In this purpose a method based on a chaotic cavity transducer has been developed to generate high energy plane wave. Then the method is evaluated on metallic bonds with bonding defects. Combined with the pulse inversion technique, nonlinear phenomenon can be measured in the form of harmonic generation in the defect zone. We use this method to image a defect created by spraying PTFE on one adherent prior to bonding.

NDE examination of industrial structures requires the modelling of specimen geometry echoes generated by the surfaces (entry, backwall...) of inspected blocks. For that purpose, the study of plane elastic wave diffraction by a wedge is of great interest since surfaces of complex industrial specimen often include dihedral corners. There exist various approaches for modelling the plane elastic wave diffraction by a wedge but for the moment, the theoretical and numerical aspects of these methods have only been developed for wedge angles lower than π. Croisille and Lebeau [1] have introduced a resolution method called the Spectral Functions method in the case of an immersed elastic wedge of angle less than π. Kamotski and Lebeau [2] have then proven existence and uniqueness of the solution derived from this method to the diffraction problem of stress-free wedges embedded in an elastic medium. The advantages of this method are its validity for wedge angles greater than π and its adaptability to more complex cases. The methodology of Croisille and Lebeau [1] has been first extended by the authors of the current communication to the simpler case of an immersed soft wedge [3]. The outline of their methodology is presented here and an application to the case of longitudinal incident and scattered waves in the case of the acoustic limit of the elastic code is presented.

A numerical framework is presented for the solution of 2D and 3D internal acoustics problems using a high-order accurate fully staggered formulation on curvilinear domains. Optimised compact finite difference schemes previously obtained in our previous paper are used for spatial discretisation, while a free parameter linear multistep method is used for temporal discretisation. The resulting scheme does not require any numerical filtering, and several benchmark cases are provided which demonstrate the significantly reduced phase velocity errors, and greater resolving efficiency compared to existing methods. Curvilinear domains are generated with the CRDT algorithm by Driscoll, with an 8th order accurate ODE solver. The governing equations for the curvilinear problem are based on a novel transformation of the decoupled velocity pressure wave equations, with simplifications made to reduce the need to interpolate derivatives at undefined locations which occur on staggered grids. The resulting transformed equations are valid only for orthogonal grids, but are computationally efficient and do not result in loss of accuracy or stability due to grid skewness. Finally, a potential application is shown, demonstrating the solution of a generated acoustic field within a crucible of liquid aluminium by a top loaded electromagnetic induction coil. Generated pressure fields agree with results shown in previous work, and demonstrate the potential use of this contactless electromagnetic excitation method as an alternative to the immersed sonotrode for the ultrasonic treatment of alloys.

Non-destructive monitoring of a material's state during its physico-chemical transformation is of interest for several industrial fields including food processing and industrial manufacturing. Recent research trends have focused on the monitoring of elastic properties (Young's modulus, shear modulus) of sol-gel products. The use of ultrasound to provide reliable information about physico-chemical properties is becoming increasingly popular. In fact, ultrasonic techniques have the main advantage of being rapid and non-invasive methods that allow parameters such as product composition, structure and physical state to be obtained. Yet, classical techniques are limited to the characterization of the medium along the propagation path using first wave packets. In this paper, an alternative technique based on studying the reverberated signals is used, by analogy with those classically used in room acoustics. These complex signals contain useful quantitative and qualitative information about medium properties and are sensitive to structural changes. In previous works, this method has shown its capability to characterize materials. Compared to classical techniques, it has the advantage of studying a medium as its whole structure. Using this method, the determination of sol-gel phase transition of Salol is presented. Measurements were performed using an aluminum mould and nine piezoelectric (PZT) patches randomly distributed on the mould rear face. One of them is used as a source and the others are connected to an eight-channel oscilloscope and are used as receivers. The mean reverberation time over four receivers has been studied and its evolution can be linked to a sol-gel phase transition.

Boiling histotripsy employs a number of millisecond-long High Intensity Focused Ultrasound (HIFU) pulses with high acoustic peak pressures at the HIFU focus to mechanically fractionate soft tissue. Studies have shown the mechanisms underpinning this tissue fractionation process; however, the question of how HIFU exposure conditions affect lesion formation still remains unclear. In the present work, a high-speed camera and a passive cavitation detection (PCD) system were used to investigate the dynamics of bubbles induced and the corresponding mechanical damage generated in optically transparent tissue-mimicking phantoms with two different boiling histotripsy exposure conditions (1. P₊ = 85.4 MPa, P_- = – 15.6 MPa; 2. P₊ = 71.5 MPa, P_- = – 13.4 MPa at focus). Our results clearly showed that there is a positive relationship between the size of a boiling bubble and the lesion dimension. At P₊ = 85.4 MPa and P_- = – 15.6 MPa, a relatively larger boiling bubble was, for instance, produced at the focus in the gel phantom followed by the presence of a wider cavitation cluster progressing toward the HIFU transducer, resulting in the formation of a larger lesion compared to that with P₊ = 71.5 MPa and P_- = – 13.4 MPa.