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Master of Science in Engineering Science and Mechanics
Master of Science in Engineering Science and Mechanics
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ItemExperimental study on the nonlinear mixing of ultrasonic waves in concrete using array technique(Georgia Institute of Technology, 20230821) Weiss, Fiona JacquelineThis research develops a procedure that combines array technology with noncollinear ultrasonic wave mixing to detect and scan internal microscale damage in a concrete prism specimen. By mixing two wave fronts of incident shear waves generated by two ultrasonic transducer arrays, one can exploit the underlying mechanics of nonlinear wave mixing to create a longitudinal mixed wave and measure the magnitude of this nonlinear wave at a frequency that is the sum of the fundamental frequencies. The frequency of the incident waves is chosen such that it is low enough to propagate without being scattered by the in herently inhomogeneous concrete microstructure, while the resulting nonlinear phenomena are still sensitive to damage much smaller than the wavelength of the incident waves. The arrays enable beam steering, making it possible to scan for damage along an arc. Overall, scanning and imaging at different locations in a large volume throughout the specimen’s thickness is accomplished by manually adjusting the placement of the two arrays to move the mixing zone any desired, internal depth, while beam steering is used to scan at different locations of the same depth close to each other. The effectiveness of the proposed technique is demonstrated by characterizing different types of damage embedded at known locations in a concrete prism specimen. The results of this thesis are in accordance with previous research and show that beam steering along an arc to scan for damage in the concrete specimen is in fact possible.

ItemNonlinear Ultrasonic Techniques for the Quantification of Thermal Damage in Carbon/Carbon Composite Material(Georgia Institute of Technology, 20220829) Gmeiner, FabianThis research applies nonlinear ultrasonic (NLU) techniques for the quantitative characterization of Carbon/Carbon (C/C) composite friction materials. The characterization focuses on identifying thermal and oxidative damage induced by heat cycles during the lifespan of the material. Carbon/Carbon friction materials are subject to high service temperatures in air and thus susceptible to rapid oxidation damage [1], leading to a reduction in their mechanical properties and possibly shortening their lifespan. The nondestructive evaluation (NDE) of C/C composites could be used to quantify the damage state and help predict possible premature component failure.

ItemDeep Learning in Ultrasonic Wave Inversion for Thin Coatings(Georgia Institute of Technology, 20220114) Schmitz, MaximilianThis research focuses on the use of machine and deep learning to nondestructively characterize the quality of a coating in a layered system in terms of thickness and uniformness. The coating's parameters (thickness and uniformness) are evaluated using dispersion curves. To obtain these dispersion curves in the first step, this work uses a computational finite element analysis (FEA) model to obtain dispersion data for coated specimen with different thicknesses. The simulated FEA data is then processed via a twodimensional Fourier transform to obtain a frequency  wave number relation. This representation is a dispersion curve. Dispersion curves are characteristic and depend on the coating thickness and uniformness. Simulated dispersion curves for various coatings are obtained. They are then further processed and a feature representation for each dispersion curve is extracted. Those extracted features are then fed into machine learning classifiers which allow a thickness classification. The abovedescribed machine learning procedure is limited to classifying the thickness of a uniform coating. To classify if a nonuniformess is present, deep learning steps in. To obtain information about if a coating is uniform, two convolutional neural network architectures are used. Both networks are evaluated and tested and recommendations on their use are given.

ItemMODELING OF ACOUSTIC NONLINEARITY CHANGE IN CONCRETE DUE TO MICROCRACKS PARTIALLY FILLED WITH ALCALI SILICA REACTION (ASR) GEL(Georgia Institute of Technology, 20200812) Smajic, DenisAlkali silica reaction (ASR) is a complex, multiscale chemomechanical process which can lead to the expansion and damage of concrete structures by forming microcracks, gradually reducing the structure’s mechanical properties such as strength and stiffness. This work investigates the effect of the ASR induced gel present within a microcrack on the material’s nonlinear elastic constants. Therefore, significant changes such as the existence of lubrication need to be considered in addition to the contact force when deriving the stress state. The lubrication force depends on the viscosity and transport properties of the ASR gel and each of these needs to be modeled respectively. The characteristics of this force lead to the fact that nonlinearity due to microinhomogeneities is altered over time compared to the pure elastic contact force which is repulsive and tries to open the microcrack. Firstly, the contact force is approximated by a Hertzian contact between asperities, characterized by spherical tops located at different heights whereas the introduced lubrication force is derived in terms of the distance between irregularities and the gel volume present within a microcrack. As a second step, appropriate asperity distribution functions are used to derive the internal stress state within the crack which is then superimposed with the far field stress and the additional acoustic stress in order to express the nonlinear stressstrain relationship including the overall elastic constants of the solid. The higher order elastic constants are much more sensitive to microcracks than the Young’s Modulus and Poisson’s ratio which are altered only slightly due to the presence of microcracks. Nonlinear ultrasonic measurement techniques are capable of measuring second order waves which typically occur in nonlinear materials and relating these to the acoustic nonlinearity parameter . The direct dependence of on the quadratic and cubic nonlinearity parameters allows the derivation of a nonlinearity parameter relationship to the ASR gel volume present in a microcrack.

ItemProposal and analysis of a noncollinear wave mixing technique for the detection of microcracks using phased arrays(Georgia Institute of Technology, 20200812) May, Max PhilippIn this thesis, a new ultrasonic measurement setup for the detection of microscale damage in cement based materials is proposed and analyzed. The idea is to use the nonlinear phenomenon of wave mixing to characterize third order elastic behavior of a material. By wave mixing, the nonlinear interaction between two ultrasound waves creating a third resonant wave inside a material is meant. The amplitude of this third wave is dependent on the third order nonlinear constants of a material, which in turn are influenced by microscale damage inside the material structure. Furthermore, a nonlinear technique is especially needed in the case of attenuative materials, which prevent the use of higher frequencies to get good resolution with linear approaches. Recent measurement results using a wave mixing technique are designed in a fixed setup to evalute a single point inside the material or scan line regions by moving the ultrasonic equipment. This limitation originates from the use of common ultrasonic transducer elements generating an ultrasound wave at a fixed angle by using the refraction of wedges. As a remedy to this limitation, phased arrays can be used as a source for the incident waves instead. Thus, having the advantage to change the beam angle without changing the physical measurement configuration. Two phased arrays are used on one side of the specimen to generate incident shear waves. If the the beams intersect inside the material at a point fulfilling certain mixing conditions, a longitudinal resonant wave is generated which amplitude is proportional to the amount of nonlinearity inherent to the mixing volume. This resonant wave can be measured by a receiver on the other side of the material to asses, e.g., microcrack density in the mixing volume. \par In this thesis, necessary and sufficient conditions for this noncollinear shear wave mixing are derived. Furthermore, the basic laws for beam steering with phased arrays are explained and the mixing volume is modeled. Finally, advantages and limitations of this measurement technique as well as design specifics for the practical construction of a measurement setup are discussed.

ItemEvaluation of simulated plain dents in x52 pipeline steel using nonlinear Rayleigh waves(Georgia Institute of Technology, 20191004) Pfeifer, DenisThis research investigates the microstructural damage in X52 pipeline steel caused by a plain dent using nonlinear ultrasound. Therefore, the plastic deformation of a plain dent is simulated by loading X52 steel specimens on a tensile testing machine to different strain levels. A Rayleigh wave with fundamental frequency omega is then excited using a wedgecoupled transducer. As this wave propagates along the specimen's surface a second harmonic wave with frequency 2*omega is generated. The the slope of amplitude ratio A2/A1^2 measured over the propagation distance yields the relative acoustic nonlinearity parameter beta'. The relative acoustic nonlinearity parameter increases with increasing plastic deformation due to the increasing signs of material damage, such as dislocations.Thus, the nonlinear ultrasound measurements are sensitive to microstructural changes caused by plastic deformation in X52 pipeline steel. Comparing the results of round specimens to flat specimens lead to the conclusion that the comparatively high variability in the measurements is not caused by the round geometry of the pipeline but by the inherent variability due to manufacturing processes and localized effects in the material during the plastic deformation.

ItemInvestigation of microstructural changes in CR9MO1 due to thermal aging using SHG and NRUS methods(Georgia Institute of Technology, 20190903) Fahse, Daniel NiklasThis research improves the experimental setup for noncontact nonlinear resonance ultrasound spectroscopy (NRUS) developed by Maier et al. and enhances the post processing to increase the operatorindependence and lower the influence of environmental changes on the measurement by modifications in the measurement procedure and the post processing. Furthermore, the influence of ambient temperature is theoretically described. This theoretical approach is valuated on data collected with a temperature measurement added to the NRUS measurement setup. This approach could simplify the measurement procedure in the future by using the temperature measurement. The improved measurement setup is then used to study thermal aging by measuring the nonclassical hysteretic nonlinearity parameter alpha of the ferritic martensitic steel Cr9Mo1 at increasing thermal aging time. The results are compared to other monitoring techniques such as hardness, Young’s modulus and quadratic nonlinearity parameter beta. alpha and beta show surprisingly analogous functional dependency on thermal aging. To gain more insight about the microstructural effects and how they effect both nonlinearity parameters, the results are also compared to an in the same way monitored thermal aging process of a 174PH precipitate hardening stainless steel. The study is concluded with a hypothesis which can explain the observed functional dependency on aging time.

ItemNumerical study on the excitationdependent nonlinear behavior of distributed microcracks(Georgia Institute of Technology, 20190903) Goletz, MariusSince it is well known that microcracks generate higher harmonics in propagating monochromatic waves, there exist dfferent approaches to model this phenomenon. Recent research from Hoffmann et al. combined a Bilinear Stiffness model and a Rough Surface Contact model for a more complete description of the nonlinear behavior of materials containing distributed microcracks. This model intends to be utilized to simulate second harmonic generation in dependence of characteristic crack parameters, e.g. crack radius and number of cracks. The materials considered in this context are Nanostructured ferritic alloys (NFA). This research presents a numerical approach to determine this dependency, since other methods like the pertubation method fail due to the strong nonlinear effects occurring in such materials. In the first instance, the problem is formulated as a hyperbolic system of conservation laws, before it is implemented and solved with a semidiscrete central scheme. The numerical results are then studied using the signalprocessing tool fast Fourier transform (FFT) in order to analyse and interpret the nonlinear effects. To ensure on the one hand, that numerical algorithm works properly and on the other hand, to understand and interpret the results, the problem is approached step by step. After validating the numerical scheme for a linear problem, only the quadratic part of the combined model is examined for varying crack parameters. In a next step, the full model is investigated for different crack parameters. This procedure allows to better understand the evolution and physical interpretation of the strong nonlinear effect observed when the full model is considered.

ItemMeasurements of acoustic nonlinearity in coldrolled and heattreated 304 austenitic stainless steel using nonleaner ultrasound(Georgia Institute of Technology, 20190430) Park, SangyunIn energy industry, austenitic stainless steels have been used in a number of different applications. Their excellent mechanical properties, good formability, and high corrosion resistance make them a popular candidate material. Among other, SS 304 is the most common stainless steel. However, it has one disadvantage that the material is susceptible to sensitization. The sensitization occurs when the material is exposed to a certain temperature even for a short duration, where chromium and carbon forms chromium carbide precipitates along the grain boundaries. The formation of the precipitates causes chromium depletion and ultimately leads to intergranular stress corrosion cracking (IGSCC). The heat affected zone (HAZ) of weld in 304 SS structures is usually the most probable locations for sensitization. The IGSCC is of great interest because most cracking in high temperature pipes, such as water reactor pipes, initiates in HAZ due to the sensitization. Nondestructive evaluation methods based on nonlinear ultrasound such as the second harmonic generation technique are highly sensitive to material damage at microstructure level. The second harmonic generation technique measures the change of the second harmonic frequency in the initially monochromatic wave, and relates property changes in the tested materials to the measured nonlinearity parameter, β. This paper evaluates microstructures of cold rolled and heat treated stainless steel 304 by using nonlinearity parameter, β. Using longitudinal waves, the change of microstructural properties of cold rolled 304 SS is first evaluated. Next, combined effect of cold rolled and heattreated stainless steel is evaluatedusing the same technique. The results are then compared with those from previous works [1] [2] [3] [4] [18] [19], which have examined each effect individually. The result shows that the cold work causes drastic increases in nonlinearity of 304 SS due to the plastic deformation and formation of dislocations. Also, the cold rolling affects the sensitization in different ways depending on the percent cold rolled of 304 SS. The heat treatment at 675 °C induces two different effects in this material: sensitization and recrystallization. With different percent of cold rolling, the combined effect of cold rolling and heat treatment result differently on the nonlinearity of 304 SS, depending on domination of either sensitization or recrystallization.

ItemExcitationdependent nonlinear behavior of distributed microcracks(Georgia Institute of Technology, 20180824) Hoffmann, Kathrin DorisIt is well known that microcracks generate strong higher harmonics in propagating monochromatic waves. There is a large amount of literature on modeling this phenomenon, but most of these existing papers only describe one specific mechanism. For example, Zhao et al. assumes that the crack faces are either open under tension or closed under compression, and in the latter case they may slide against each other. On the other hand, the Nazarov and Sutin model assumes microcracks as an elastic contact of two rough surfaces, which are never completely separated by an external load. All these mechanisms depend on the level of excitation. In this research, a micromechanical model for the acoustic nonlinearity generation of microcracks is developed by combining the bilinear stiffness model and the rough surface contact model to describe the excitationdependent nonlinear behavior of distributed microcracks. It is shown that the first and second harmonic amplitudes have the relationship: A2 ~ A1^n; with n dependent on the amplitude of excitation, and 2 >= n >= 1 for nonadhesive crack surfaces. Nanostructured ferritic alloys (NFA) are considered as an example. These materials exhibit outstanding hightemperature properties, irradiation tolerance and thermal stability, making them a leading candidate for advanced nuclear fission and fusion applications. One characteristic property of mechanically processed NFAs is their layerlike structure, with a large number of microcracks aligned in a specific direction. Nonlinear ultrasound measurements (acoustic nonlinearity, beta) with longitudinal waves are used to characterize this material. The results show that these measurement techniques are sensitive to the orientation of the cracks. The model developed in this research is then used to interpret these experimental measurements and used to characterize the microcracks in a NFA specimen.