The application of ultrasonic NDT techniques in tribology

The use of ultrasonic reflection is emerging as a technique for studying tribological contacts. Ultrasonic waves can be transmitted non-destructively through machine components and their behaviour at an interface describes the characteristics of that contact. This paper is a review of the current state of understanding of the mechanisms of ultrasonic reflection at interfaces, and how this has been used to investigate the processes of dry rough surface contact and lubricated contact. The review extends to cover how ultrasound has been used to study the tribological function of certain engineering machine elements

Adhesive joint evaluation by ultrasonic interface and lamb waves

Some results on the application of interface and Lamb waves for the study of curing of thin adhesive layers were summarized. In the case of thick substrates (thickness much more than the wave length) the interface waves can be used. In this case the experimental data can be inverted and the shear modulus of the adhesive film may be explicitly found based on the measured interface wave velocity. It is shown that interface waves can be used for the study of curing of structural adhesives as a function of different temperatures and other experimental conditions. The kinetics of curing was studied. In the case of thin substrates the wave phenomena are much more complicated. It is shown that for successful measurements proper selection of experimental conditions is very important. This can be done based on theoretical estimations. For correctly selected experimental conditions the Lamb waves may be a sensitive probe of adhesive bond quality and may be used or cure monitoring

Damage detection in multi-layered plates using ultrasonic guided waves

This thesis investigates ultrasonic guided waves (GW) in multi-layered plates with the focus on higher order modes. The aim is to develop techniques for hybrid structures such as of adhesive bonds and composite pressure vessels (COPV) which are widely used in automotive and aerospace industries and are still challenging to inspect non-destructively. To be able to analyse GW, numerical methods and precise material properties are required. For this purpose, an efficient semi-analytical approach, the Scaled Boundary Finite Element Method, is used. The material properties are inferred by a GW-based optimisation procedure and a sensitivity study is performed to demonstrate the influence of properties on GW. Then, an interesting feature, called mode repulsion, is investigated with respect to weak and strong adhesive bonds. The results show that the coupling between two layers influences the distance between coupled modes in a mode repulsion region, thus allowing for the characterisation of adhesive bonds. At next, wave-damage interaction is studied in the hybrid structure as of the COPV. Results show that the wave energy can be concentrated in a certain layer enabling damage localisation within different layers. Further investigations are carried out on the hybrid plate with an impact-induced damage. Two well-known wavenumber mapping techniques, which allow to quantify the damage in three dimensions, are implemented and their comparison is done for the first time.In dieser Arbeit werden geführte Ultraschallwellen (GUW) in mehrschichtigen Platten untersucht, wobei der Schwerpunkt auf Moden höherer Ordnung liegt. Ziel ist die Entwicklung von zerstörungsfreien Prüfmethoden für hybride Strukturen wie Klebeverbindungen und Komposit-Druckbehälter, die in Automobil-, Luft- und Raumfahrtindustrie weit verbreitet sind und deren Prüfung immer noch eine Herausforderung darstellt. Für die GUW-Analyse werden numerische Methoden und präzise Materialeigenschaften benötigt. Hier wird die Scaled Boundary Finite Element Methode verwendet und die Materialeigenschaften werden durch ein GUW-basiertes Optimierungsverfahren hergeleitet. Um den Einfluss der Eigenschaften auf GUW zu analysieren wird eine Sensitivitätsstudie durchgeführt. Anschließend wird vermiedene Kreuzung in Bezug auf schwache und starke Klebeverbindungen untersucht. Die Ergebnisse zeigen, dass die Kopplungsstärke den Abstand zwischen gekoppelten Moden in einer vermiedenen Kreuzung beeinflusst und somit eine Charakterisierung von Klebeverbindungen ermöglicht. Als Nächstes wird dieWechselwirkung zwischen GUW und Schaden in einer Hybridstruktur wie beim Komposit-Druckbehälter untersucht. Die Ergebnisse zeigen, dass die Wellenenergie in einer bestimmten Schicht konzentriert werden kann. Damit wird die Schadenslokalisierung in dieser Schicht ermöglicht. Weiter wird die Quantifizierung der Impakt-Schäden in der Hybridplatte angestrebt, die durch Erstellung einer Wellenzahlkarte erfolgt

Laser beam shaping for enhanced Zero-Group Velocity Lamb modes generation

Optimization of Lamb modes induced by laser can be achieved by adjusting the spatial source distribution to the mode wavelength ($\lambda$). The excitability of Zero-Group Velocity (ZGV) resonances in isotropic plates is investigated both theoretically and experimentally for axially symmetric sources. Optimal parameters and amplitude gains are derived analytically for spot and annular sources of either Gaussian or rectangular energy profiles. For a Gaussian spot source, the optimal radius is found to be $\lambda_{ZGV}/\pi$. Annular sources increase the amplitude by at least a factor of 3 compared to the optimal Gaussian source. Rectangular energy profiles provide higher gain than Gaussian ones. These predictions are confirmed by semi-analytical simulation of the thermoelastic generation of Lamb waves, including the effect of material attenuation. Experimentally, Gaussian ring sources of controlled width and radius are produced with an axicon-lens system. Measured optimal geometric parameters obtained for Gaussian and annular beams are in good agreement with theoretical predictions. A ZGV resonance amplification factor of 2.1 is obtained with the Gaussian ring. Such source should facilitate the inspection of highly attenuating plates made of low ablation threshold materials like composites.Comment: 11 pages, 12 figure

Modelling the Directional Response of Fabry-Pérot Ultrasound Sensors

Fabry-Pérot ultrasound sensors offer an alternative to traditional piezoelectric sensors for clinical and metrological applications, for example, measuring high intensity focused-ultrasound (HIFU) fields. In this thesis, a model of the frequency dependent directional response was developed based on the partial-wave method, treating the sensor as a multi-layered elastic structure. An open-source MATLAB toolbox called ElasticMatrix was developed to model acoustic and elastic-wave propagation in multi-layered structures with anisotropic material properties. The toolbox uses an object-oriented framework, giving it a simple scripting interface and allowing it to be expanded easily. The toolbox is capable of calculating and plotting reflection and transmission coefficients, slowness profiles, dispersion curves and displacement and stress fields. An additional MATLAB class is included to model the frequency-dependent directional response of Fabry-Pérot ultrasound sensors. The model was validated, tested and compared with directional response measurements made on two glass-etalon sensors: an air-backed cover-slip sensor with well-known acoustic properties, and an all-hard-dielectric sensor. Features of the directional response were investigated and attributed to the critical angles of the substrate backing, and Lamb- and Rayleigh-modes propagating in the sensor. The directional response of two sensors with Parylene C (a commonly used soft-polymer) were also investigated: a sensor with a Parylene C spacer, and a glass-etalon sensor with a thick Parylene C coating. X-ray diffraction and transmission electron microscope measurements indicated Parylene has a crystal structure and impedance measurements indicated that Parylene is acoustically anisotropic. Using the measured impedance values, the modelled and measured directivity had improved agreement compared with isotropic values based on the phase-speeds of guided modes. The developed modelling tools allow detailed analysis of the physical mechanisms affecting the frequency-dependent directional response of planar Fabry-Pérot sensors. This knowledge can be used to inform future sensor design, to aid with material selection, and for deconvolution of the sensor response from acoustic measurements

Tomography applied to Lamb wave contact scanning nondestructive evaluation

The aging world-wide aviation fleet requires methods for accurately predicting the presence of structural flaws that compromise airworthiness in aircraft structures. Nondestructive Evaluation (NDE) provides the means to assess these structures quickly, quantitatively, and noninvasively. Ultrasonic guided waves, Lamb waves, are useful for evaluating the plate and shell structures common in aerospace applications. The amplitude and time-of-flight of Lamb waves depend on the material properties and thickness of a medium, and so they can be used to detect any areas of differing thickness or material properties which indicate flaws. By scanning sending and receiving transducers over an aircraft, large sections can be evaluated after a single pass. However, while this technique enables the detection of areas of structural deterioration, it does not allow for the quantification of the extent of that deterioration. Tomographic reconstruction with Lamb waves allows for the accurate reconstruction of the variation of quantities of interest, such as thickness, throughout the investigated region, and it presents the data as a quantitative map. The location, shape, and extent of any flaw region can then be easily extracted from this Tomographic image. Two Lamb wave tomography techniques using Parallel Projection tomography (PPT) and Cross Borehole tomography (CBT), are shown to accurately reconstruct flaws of interest to the aircraft industry. A comparison of the quality of reconstruction and practicality is then made between these two methods, and their limitations are discussed and shown experimentally. Higher order plate theory is used to derive analytical solutions for the scattering of the lowest order symmetric Lamb wave from a circular inclusion, and these solutions are used to explain the scattering effects seen in the Tomographic reconstructions. Finally, the means by which this scattering theory can be used to develop Lamb wave Tomographic algorithms that are more generally applicable in-the-field, is presented