In about 30 years of experiments and development, impulse drying is now considered as a well known technology and a good candidate in the constant effort to save energy in the paper industry. The drying section is indeed the most expensive section in the process of paper production. However, this potential technology has a major disadvantage, stopping its implementation in the industry. Paper, which is a porous material with a variable compressibility, experienced a sudden release of energy at the nip opening during impulse drying. Under these conditions of high intensity process (both in temperature and pressure), the fiber mat has a tendency to delaminate. This web disruption is a critical issue against impulse drying. This thesis comes up with a new approach to the problem. These last years, the technology itself has been addressed in this issue and many improvements have been reached in terms of energy release (heat transfer control, material coating ). The novel idea is then to investigate the inner structure of the paper once it has been coated with starch to a large extent (up to 10 or 20% of the relative basis weight). Starch is known for its large use in industry, but also its capability to expand under high temperature. Hence, both relative strength and bulking effects are investigated in this thesis, using numerous experiments with variable temperatures and pressures, along with ultrasonic testing and image analysis. We have the opportunity to appreciate the phenomenon of heat transfer and mass transport in the coated medium, while reaching promising results in terms of strength and bulk. These are finally investigated using scanning electron microscopy as a first step toward a pore expansion model for starch imbued handsheets.

Recent theoretical and experimental studies in a wide range of applications (ultrasonics, underwater acoustics, seismicoe) have demonstrated that Green's functions (impulse responses) can be extracted from cross-correlation of diffuse fields using only passive sensors. The technique, whose validity is supported by a physical argument based on time-reversal invariance, effectively uses a correlation process between the point source and points located in the focal zone. Indeed, the coherent noise source distributions can be considered as a timereversal mirror and the cross-correlation operations gives the field measured at one receiver after refocusing on the other receiver. Passive-only reconstruction of coherent Lamb waves (80-200 kHz) in an aluminum plate and thickness comparable to aircraft fuselage and wing panels will be presented. In particular, the influence of the noise source characteristics (location, frequency spectrum) on the signal-to-noise ratio the emerging coherent waveform will be investigated using a scanning laser Doppler velocimeter. This study suggests the potential for a structural health monitoring method for aircraft panels based on passive ultrasound imaging reconstructed from diffuse fields.

A routine non-destructive test method based on Phased Array Ultrasonic Testing (PAUT) has been developed and applied for the inspection of the first 11 T dipole prototype magnet half shell welds, and the test results are compared with the radiography and visual inspection results of the same welds. A manual scanner and alignment system have been developed and built to facilitate the inspection of the 5.5 m long welds, and to assure reproducibility of the PAUT results. Through the comparison of distance readings and signal amplitude for different focus lengths, a focal law with focus at 25 mm sound path has been selected for the routine inspection of the 15 mm thick austenitic stainless steel 11 T dipole welds. The defocusing properties (beam spread) due to the cylindrical geometry of the half shells and the sound path distance to the area of interest were taken into account. Dedicated sensitivity calibration weld samples with artificial defects (side-drilled-holes) have been designed and produced from 11 T dipole prototype austenitic stainless steel half shell welds. These provide representative calibration for the strongly attenuating and scattering austenitic stainless steel weld material. One scan with two phased array probes aligned parallel to the weld in 2 mm distance from the weld cap edge, and one scan with the probes aligned parallel to the weld in 12 mm distance from the weld cap edge are sufficient to show if the inspected welds fulfil the requirements of weld quality level B according to ISO 5817. The standard test duration for the two scans of the two 5.5 m long horizontal welds of the 11 T dipole magnets is about one day, provided that no defects are found that need to be characterized in more detail.

Entre otras aplicaciones, el Grupo de Tratamiento de Señal (GTS) de la Universidad Politécnica de Valencia ha trabajado tradicionalmente en el campo de los ensayos no destructivos por ultrasonidos. el GTS ha desarrollado sofisticados algoritmos de procesado de señal para la detección y caracterización en materiales dispersivos. El procedimiento de inspección ultrasónica de materiales implica la obtención de un registro o señal mediante un equipo emisor-receptor de ultrasonidos. Este registro será posteriormente procesado para obterner información del material. La extracción de características puede realizarse calculando parámetros temporales y/o frecuenciales de los registros en cuestión. Estos parámetros contienen información que está relacionada con propiedades físicas de los materiales y que podrán ser empleados en definitiva para clasificar y caracterizar el material. El objetivo de la tesis es el de ampliar el conjunto de parémetros que se han venido usando de manera tradicional en el grupo. Los nuevos parámetros proponen un cambio de perspectiva mediante el uso de técnicas basadas en el tratamiento digital de imagen. Se presenta una colección de descriptores geométricos y de forma que serán aplicados a las imágenes obtenidas a partir de los diagramas tiempo-frecuencia. Los citados parámetros se aplicarán junto con los parámetros que componen el modelo tradicional, o versiones modificadas de éstos, a la caracterización ultrasónica de materiales dispersivos blandos. En la tesis se presenta un modelo matemático afín a los tipos de materiales que han sido evaluados en este trabajo. Se define así un esquema que modela en el dominio de la frecuencia el registro de señal procedente de la inspección ultrasónica del material. Se proponen y estudian además diferentes métodos con el objetivo de llevar a cabo el análisis espectral de la señal ultrasónica y se discuten diferentes alternativas para la obtención y mejora de los diagramas tiempo-frecuencia. / Gómez García, S. (2010). Descriptores Geométricos y de Forma: Aplicación a la caracterización ultrasónica de materiales [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/8722 / Palancia

The focus in this work is on the use of ultrasonic testing as a method for inspecting components manufactured through additive manufacturing (AM) processes. The research is rooted in the need for effective non-destructive testingtechniques that can adapt to the unique challenges posed by AM-produced materials, including complex defect geometries and surface conditions. Ultrasonic testing is a versatile form of non-destructive testing, offering theability to detect internal flaws, such as voids, cracks, and inclusions, with highprecision and in real-time. Unlike many competing methods, ultrasonic testing works on most types of materials. Ultrasonic testing has been applied forinspection purposes for a long time. Now with emerging manufacturing methods, there is a need for evaluation techniques to keep up with this development.New data processing algorithms open up possibilities of extracting more information from the acquired signal. The thesis provides a review of UT’s capabilities in detecting and classifyingdefects within AM components, with a particular emphasis on the subtletiesintroduced by the layer-by-layer construction method inherent to AM technologies. The work advances development and validation of simulation modelsaimed at predicting the ultrasonic response from manufactured defects. Thesemodels are crucial for understanding the interaction between ultrasound wavesand material anomalies, offering insights into the potential for enhanced defectdetection strategies. The research also explores the practical case of integrating UT into the quality assurance processes by relying on mathematical simulation rather than experimental data. The findings suggest avenues for the refinement of creation of inspection procedure, including the the use of meta-models to cheaply acquire worst-case scenario defects, to better accommodate the specificities of AM materials. / Den här avhandlingen handlar om ultraljudsprovning av additivt tillverkade metalkomponenter. Ultraljud är en av flera metoder som används för att detektera defekter i komponenter utan att förstöra komponenten i processen. Samlingsnamnet för dessa metoder är oförstörande provning. Oförstörande provning är en viktig pusselbit i samhällets säkerhet då det möjliggör identifiering och utvärdering av potentiella defekter i material, vilket förebygger olyckor och strukturella fel. Vidare är det en viktig del inom hållbar utveckling genom att maximera nyttjandet av komponenter då dessa inte behöver ersättas med ett överdrivet försiktigt underhållsintervall. Ultraljudsprovning fungerar genomatt ultraljud introduceras in i en komponent, varefter en givare registrerar ekon som uppstår när ljudvågorna interagerar med eventuella defekter. Detta möjliggör både lokalisering och storleksbedömning av defekter, t.ex. sprickor, bindfel eller porer. Resultaten av den här avhandlingen syftar till att ge förutsättningar för att förbättra tolkningen av dessa signaler, dels genom att experimentellt validera simuleringsmodeller, samt tillämpning av dessa modeller för att utveckla en metamodell för att prediktera amplitudsvaret från en mängd defekter inom en viss parameterrymd. Med verktyg som detta kan billigare inspektionsprocedurer möjliggöras genom att man till större del kan förlita sig på simulering av signaler snarare än att skaffa stort underlag med experimentell data. / <p>Paper A, B and C are not included in the eletronic version. Paper C is under submission.</p>

Die Ansteuerung von Ultraschall-Phased-Arrays basiert auf zwei Vereinfachungen. Zum einen wird davon ausgegangen, dass das Schwingungsverhalten aller Einzelelemente synchron und gleichmäßig ist. Zum anderen erfolgt die Berechnung der Ansteuerungszeiten, den sogenannten Focal Laws, auf dem Ansatz der geometrischen Akustik. Beide Aspekte werden in dieser Arbeit ausführlich analysiert und neue Vorschläge für eine optimale Ansteuerung ausgearbeitet und simulativ sowie messtechnisch bewertet. Das individuelle Schwingungsverhalten der Einzelelemente von Phased Arrays liegt in einer Vielzahl von Toleranzen der Einzelkomponenten und der Fertigungsschritte begründet. Für die quantitative Bewertung kann die mechanische Auslenkung mittels Laser-Doppler-Vibrometrie lokal erfasst und evaluiert werden. Die Kompensation individueller Latenzzeiten im Ansprechverhalten führt dann unmittelbar zu einem höheren Summensignal im anvisierten Fokuspunkt. Um den Einfluss der tatsächlichen Aperturgeometrie von Einzelelementen auf die Qualität des Summensignals des Phased Arrays zu untersuchen, wurde außerdem eine Simulationsumgebung (4D-CEFIT-PSS) entwickelt. Damit ist die Wellenausbreitung unter Berücksichtigung aller wellenphysikalischer Effekte im Halbraum möglich. Somit wurden verschiedene Aperturmodelle erstellt und sowohl im Zeit- als auch im Frequenzbereich analysiert. Die wellenphysikalische Simulation ermöglicht die Berechnung modifizierter Focal Laws, die immer zu einer Verbesserung des Summensignals im Vergleich zum Ansatz mit geometrischer Akustik führen. / Ultrasonic linear phased array probes consist of several single elements. By exciting each element at a certain time wave fronts can be tilted, focused or both combined. This is accomplished by a set of delays which is called "focal law". Hence, the shape and the quality of the resulting wave front depends significantly on focal law calculation. This state-of-the-art method is based on two simplifications: firstly on the assumption that each single element has identical vibration behaviour, and secondly on the simple geometrical approximation of the signal propagation time. In this work both aspects will be investigate in detail. For characterization of the individual vibration behaviour the most important transducer parts and theirs acoustical properties will be presented. The theoretical view on the inner structure is completed by two measuring methods: scanning acoustic microscopy as well as computed tomography. Furthermore, the effective mechanical displacement of the transducer interface will be analyzed by Laser Doppler vibrometry. Hence, the individual vibration behaviour of the single elements can be compensated which yields an optimized superposition. To investigate the second assumption the 4D-CEFIT-PSS simulation environment has been developed. The combination of CEFIT (cylindric elasto dynamic finite integration technique) and PSS (point source synthesis) considers all effects of wave physics. A comprehensive parametric study shows the effects of geometrical aperture size concerning resulting signals in decided focal points. The differences of wave propagation in the time and frequency domain will be pointed out. Concluding, focal laws were calculated with the geometrical and the simulation based approach. The resulting wave propagation is simulated for selected focal points. The results were compared both ways, qualitatively and quantitatively. Hereby the difference between both methods was distinguishable. The outcome is a method for modified focal law calculation. Both, the consideration of the individual vibration characteristics and the application of the new focal laws result in higher signal-to-noise ratios for linear phased arrays.

Cold rolled carbon steel 1018C is widely used in pressurized fuel pipelines. For those structures, stress corrosion cracking (SCC) can pose a significant problem because cracks initiate late in the lifetime and often unexpectedly, but grow fast once they get started. To ensure a safe operation, it is crucial that any damage can be detected before the structural stability is reduced by large cracks. In the early stages of SCC, microstructural changes occur which increase the acoustic nonlinearity of the material. Therefore, an initially monochromatic Rayleigh wave is distorted and measurable higher harmonics are generated. Different levels of stress corrosion cracking is induced in five specimens. For each specimen, nonlinear ultrasonic measurements are performed before and after inducing the damage. For the measurements, oil coupled wedge transducers are used to generate and detect tone burst Rayleigh wave signals. The amplitudes of the received fundamental and second harmonic waves are measured at varying propagation distances to obtain a measure for the acoustic nonlinearity of the material. The results show a damage-dependent increase in nonlinearity for early stages of damage, indicating the suitability for this nonlinear ultrasonic method to detect stress corrosion cracking before structural failure.

This study investigates wave propagation in an elastic half-space with quadratic nonlinearity due to a line load on the surface. The consideration of this problem is one of the well known Lamb problems. Even since Lamb's original solution, numerous investigators have obtained solutions to many different variants of the Lamb problem. However, most of the solutions existing in the current literature are limited to wave propagation in a linear elastic half-space. In this work, the Lamb problem in an elastic half-space with quadratic nonlinearity is considered. For this, the problem is first formulated as a hyperbolic system of conservation laws, which is then solved numerically using a semi-discrete central scheme. The numerical method is implemented using the package CentPack. The accuracy of the numerical method is first studied by comparing the numerical solution with the analytical solution for a half-space with linear response (the original Lamb's problem). The numerical results for the half-space with quadratic nonlinearity are than studied using signal-processing tools such as the fast Fourier transform (FFT) in order to analyze and interpret any nonlinear effects. This in particular gives the possibility to evaluate the excitation of higher order harmonics whose amplitude is used to infer material properties. To quantify and compare the nonlinearity of different materials, two parameters are introduced; these parameters are similar to the acoustical nonlinearity parameter for plane waves.

The in-situ examination of historical structures for diagnostic and monitoring purposes is a troublesome work that necessitates the use of non-destructive investigation (NDT) techniques. The methods of quantitative infrared thermography (QIRT) and ultrasonic testing have distinct importance in this regard. The key concern of the study was developing the in-situ use of QIRT for assessment of stone masonry wall sections having different sublayer(s) and failures. For that purpose, the non-destructive in-situ survey composed of QIRT and ultrasonic testing was conducted on a 16th century monument, Cenabi Ahmet PaSa Camisi, suffering from structural cracks, dampness problems and materials deterioration. The combined use of these two methods allowed to define the thermal inertia characteristics of structural cracks in relation to their depth. The temperature evolution in time during the controlled heating and cooling process was deployed for the cracks/defects inspection. The superficial and deep cracks were found to have different thermal responses to exposed conditions which made them easily distinguishable by QIRT analyses. The depth of cracks was precisely estimated by the in-situ ultrasonic testing data taken in the indirect transmission mode. The inherently good thermal resistivity of the wall structure was found to have failed due to entrapped moisture resulting from incompatible recent plaster repairs. The IRT survey allowed to detect the wall surfaces with different sublayer configurations due to their different thermal inertia characteristics. The knowledge and experience gained on the experimental set-ups and analytic methods were useful for the improvement of in-situ applications of QIRT and ultrasonic testing.

This research deals with the second harmonic generation of Lamb waves in nonlinear elastic, homogeneous, isotropic plates. These waves find current applications in the field of ultrasonic, nondestructive testing and evaluation of materials. The second harmonic Lamb wave generation is investigated analytically in order to provide information on suitable excitation modes maximizing the second harmonic amplitude. Using an existing solution for the problem of second harmonic generation in wave guides, the solution is explained for the plate and examined as to the symmetry properties of the second harmonic wave, since published results are contradictory. It is shown that the cross-modal generation of a symmetric secondary mode by an antisymmetric primary mode is possible. Modes showing internal resonance, whose conditions are nonzero power flux from the primary wave and phase velocity matching, are shown to be most useful for measurements. In addition, group velocity matching is required. A material-independent analysis of the linear Lamb mode theory provides mode types satisfying all three requirements. Using the example of an aluminum plate, the found internally resonant modes are evaluated with regard to the rate of second harmonic generation and practical issues such as excitability and ease of measurement. Pros and cons of each mode type are presented.