Title: Study of cooperative dislocation phenomena in solids by the acoustic emis- sion technique Author: Michal KNAPEK Department: Department of Physics of Materials Supervisor: doc. RNDr. František Chmelík, CSc., Department of Physics of Materials Abstract: Plastic deformation of micron-scale crystalline materials differs con- siderably from bulk specimens, as it is characterized by random strain bursts. Three categories of metallic samples were investigated in this thesis: micron- scale copper micropillars with varied geometries, submillimeter-scale aluminum microwires, and aluminum and aluminum-magnesium salt-replicated foams. Very precise fabrication methods and sensitive measurement set-ups consisting of uni- axial compression and tensile tests with concurrent acoustic emission (AE) record- ing were developed. These fine methods allowed for investigations of effects re- lated to plastic deformation at micrometer scales, i.e. the dislocation dynamics associated with the stress drops. Size effects in plastic deformation, as well as clear correlations between the stress drops and the AE events, were found in mi- crosamples, confirming that dislocation avalanches are indeed responsible for the stochastic character of deformation processes also at microscales. Open-cell pure aluminum and aluminum-magnesium...

The objective of this thesis is to study the mechanical properties of magnesium-based composite (AX41) reinforced by short Saffil fibers. Two type of samples have been investigated: fiber plane parallel respective perpendicular to the loading axis. In both case compression tests were performed in temperature range from 23řC to 200řC. Deformation test were completed by acoustic emission and neutron diffraction measurement. Both methods provide information about the ongoing deformation mechanisms. Microstructure of deformed sample was investigated by SEM and EBSD methods in order to confirm the ND and AE results. The internal strain field in the material was predicted with numerical FEM and compared with the observed experimental values.

La glace, le quartz, l'oxyde de magnésium ou encore les bonbons Wint-o-green (produits par Life Savers) sont quelques-uns des matériaux connus pour émettre de la lumière lors de leur fracture. Ce phénomène, appelé fractoluminescence, révèle que la fracture met en jeu des processus plus complexes que la seule création de surfaces. Ces émissions lumineuses permettent-elles d'analyser l'endommagement des matériaux, au même titre que les émissions acoustiques ? Né de l'union imprévue entre mécanique de la fracture et physique des particules, un montage expérimental a été développé pour étudier les émissions lumineuses et acoustiques lors de la fracture de scintillateurs inorganiques — matériaux couramment utilisés comme détecteurs de particules. Trois types de scintillateurs inorganiques ont été étudiés : le CdWO4, le ZnWO4, et le BGO. Il a été démontré que le CdWO4 et le ZnWO4 émettent de la lumière lors de leur fracture à pression ambiante. Des tests plus poussés ont été menés sur le BGO. Le phénomène de fractoluminescence dans le BGO a été étudié dans le cas de la fracture lente et de la fracture rapide, dans différentes conditions de pression : pression ambiante et basse pression (10−5 mbar). Dans chaque cas, des émissions lumineuses ont été observées mais elles dépendent fortement de l'environnement et des conditions de chargement de l'échantillon. Lors de la fracture lente sous vide, les émissions de lumière semblent globalement corrélées en temps à l'avancée de la fissure et aux émissions acoustiques, souvent employées en mécanique de la fracture comme indicateurs de l'endommagement. La résolution temporelle de la voie lumineuse à la nanoseconde a montré l'existence de groupes de photons pendant la fracture, à des échelles de temps inaccessibles par les méthodes acoustiques classiques. Le montage expérimental permet aussi de mesurer la quantité de lumière émise. L'énergie lumineuse lors de la fracture lente sous vide semble être proportionnelle à la surface créée. Lors de la fracture rapide sous vide, nous avons établi que 1.7 × 10−3 % de l'énergie de fracture est convertie en lumière contre au minimum 3 × 10−3 % a` pression ambiante. Cette différence montre une nouvelle fois l'importance de l'environnement et offre des pistes pour l'identification du mécanisme d'émission lumineuse / Materials such as ice, quartz, MgO or the Wint-o-green candy from Lifesavers are known to emit light during their fracture. This phenomenon, called fractoluminescence, shows that fracture is a much more complex process than the simple creation of surfaces. Do the light emissions allow to analyze material failure as the acoustic emissions ? An experimental setup is born out of the union between fracture mechanics and particle physics. It enables to study the light and acoustic emissions during fracture of inorganic scintillators — materials generally used as particle detectors. CdWO4, ZnWO4 and BGO have been tested. We demonstrated that CdWO4 and ZnWO4 emit light during their fracture at ambient pressure. More intensive tests were performed on BGO. Light emissions were studied for slow and fast crack propagation, at ambient pressure and under vacuum (10−5 mbar). In all cases, light emissions have been observed but they strongly depend on the environment and on the loading conditions. During slow crack propagation in vacuum, the light emission seems to be globally correlated in time to the crack propagation and to the acoustic emissions, which are often used in fracture mechanics as indicators of mechanical failure. The extreme time resolution of the light channel, down to the nanoseconds, shows clusters of photons during fracture, at time scales unreachable by usual acoustic methods. The experimental setup also allows to quantify the amount of light energy emitted. Light energy during slow fracture in vacuum seems to be proportional to the surface created by the crack propagation. During fast fracture in vacuum, 1.7 × 10−3 % of fracture energy is converted into light while at least 3 × 10−3 % at ambient pressure. This difference demonstrates, once again, how important the environment is, and offers leads for identifying mechanisms of light emission

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The main content of this master's thesis is a determination of the fatigue properties by using non-destructive testing methods – the acoustic emission technique and the X-ray diffraction. The reactor steel, that is named GOST 15Ch2NMFA, is used as a model material and is used to manufacture VVER-1000 reactor pressure vessels. The supplied material was made of two different steel melting, designated as H and S. For the basic measurement of the fatigue properties the stress-cycle asymmetry R = -1 was used. Some of the samples were used for approximate determination of the fatigue strength stress-cycle asymmetry R = 0. During the loading process the resonance frequency of the loading machine and acoustic emission signals were recorded. The changes in microstructure were estimated from the record of acoustic emissions during the whole fatigue life. The change of the microstructure was experimentally observed by X-ray diffraction analysis. The mechanism of the fatigue can be described by the observation of the microstructural processes as well as the relevant bifurcation with sufficient sensitivity. The results of the used methods indicate a decay and re-growth of mosaic blocks during the whole loading process. Therefore, it is important to find a correlation of these two non-destructive methods, or to use other non-destructive testing methods in the future.

This PhD thesis deals with the experimental study of more precise rolling contact fatigue damage detection using acoustic emission method. A series of experiments was carried out on two representatives bearing steels and the analysis of sensitivity for the presence of contact damage was performed on selected parameters of acoustic emission. The extent of damage was classified into four classes and signal parameters the most characterizing the development of damage were correlated with the extent of damage. It was also verified the influence of lubricants on acoustic emission signals. The results have an impact on the implementation of more precise rolling contact fatigue tests and evaluation of parameters of acoustic emission signal. On the basis of experiments was established methodology for more precise RCF testing method using acoustic emission on test-rig AXMAT II.

Lithium ion batteries (LIBs) are widely used for storing and converting chemical energy into electrical energy. During battery operation, lithium ions move between electrode materials, enabling energy storage. However, aging mechanisms like lithium plating can negatively impact battery performance and lifetime. Lithium plating occurs when lithium ions are reduced to metallic lithium on the graphite electrode. The undesired Li plating in LIBs leads to dendrite formation that may puncture the separator, causing internal short-circuit and ultimately thermal runaway. This study aims to investigate the internal processes of LIBs during charge and discharge. Two analysis methods are employed: online electrochemical mass spectrometry (OEMS) and acoustic emission monitoring (AEM). OEMS is a gas analysis technique that combines electrochemical measurements with mass spectrometry to provide real-time testing of cells. OEMS allows identifying and quantifying gas evolution/consumption of chemical species. AE is a diagnostic tool, offering monitoring the health of LIBs through detection and characterisation of stress waves produced by parasitic mechano-electrochemical events. The results indicates that the formation of SEI thin film layer, generated gases like hydrogen and ethylene, while consuming carbon dioxide. During induced lithium plating, hydrogen and carbon dioxide were consumed, and ethylene gas was produced, due to new SEI film formation process. The acoustic emission analysis indicated that lithium plating was an active process, whereas SEI formation was less AE active. Further research is needed to understand the relationships and significance of these processes for battery performance and safety. Overall, this study highlighted the importance of investigating aging mechanisms in LIBs to enhance their performance and longevity. By combining OEMS and AE, it was possible to analyse the batteries behaviour during cycling. The evolution of gas and acoustic signals provided insights into the reactions and processes occurring inside the battery during cycling.

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A partial discharge (PD) is the dissipation of energy caused by the buildup of localized electric field intensity. In high voltage devices such as transformers, this buildup of charge and its release can be symptomatic of problems associated with aging, such as floating components and insulation breakdown. This is why PD detection is used in power systems to monitor the state of health of high voltage transformers. If such problems are not detected and repaired, the strength and frequency of PDs increases and eventually leads to the catastrophic failure of the transformer, which can cause external equipment damage, fires and loss of revenue due to an unscheduled outage. Reliable online PD detection is a critical need for power companies to improve personnel safety and decrease the potential for loss of service. The PD phenomenon is manifested in a variety of physically observable signals including electric and acoustic pulses and is currently detected using a host of exterior measurement techniques. These techniques include electrical lead tapping and piezoelectric transducer (PZT) based acoustic detection. Many modern systems use a combination of these techniques because electrical detection is an older and proven technology and acoustic detection allows for the source to be located when several sensors are mounted to the exterior of the tank. However, if an acoustic sensor could be placed inside the tank, not only would acoustic detection be easier due to the increased signal amplitude and elimination of multipath interference, but positioning could also be performed with more accuracy in a shorter time. This thesis presents a fiber optic acoustic sensing system design that can be used to detect and locate PD sources within a high voltage transformer. The system is based on an optical acoustic (OA) sensor that is capable of surviving the harsh environment of the transformer interior while not compromising the transformer's functionality, which allows for online detection and positioning. This thesis presents the theoretical functionality and experimental validation of a band-limited OA sensor with a usable range of 100-300 kHz, which is consistent with the frequency content of an acoustic pulse caused by a PD event. It also presents a positioning system using the time difference of arrival (TDOA) of the acoustic pulse with respect to four sensors that is capable of reporting the three-dimensional position of a PD to within ±5cm on any axis. / Master of Science