Abstract Barkhausen noise measurement is an intriguing non-destructive testing method suitable for ferromagnetic materials. It is based on the stochastic movements of magnetic domain walls when the tested sample is placed in an external varying magnetic field. Barkhausen noise is typically utilised so that some features are calculated from the signal and then compared with the studied material properties. Typical features are, for example, the root-mean-square value (RMS), peak height, width and position. Better utilisation of the method, however, requires quantitative predictions of material properties. The aim of this thesis is to study and select a suitable methodology for the quantitative prediction of material properties based on Barkhausen noise measurement. The prediction considered is divided into four steps: feature generation, feature selection, model identification and model validation. In feature generation, a large set of features is calculated with different mathematical procedures. This feature set is explored in the feature selection step to find the most significant features in terms of predictions. A model with the selected features is identified and some independent data are usually used for model validation. This thesis presents the developed procedures required in feature generation and the results of the studies using different feature selection strategies and modelling techniques. The studied feature selection methods are forward selection, simulated annealing and genetic algorithms. In addition, two-step algorithms are investigated where a pre-selection step is used before the actual selection. The modelling techniques used are multivariable linear regression, partial least squares regression, principal component regression and artificial neural networks. The studies also consider the use and effect of different objective functions. The results of the studies show that the proposed modelling scheme can be used for the prediction task. The models identified mainly include reasonable terms and the prediction accuracy is fairly good considering the challenge. However, the application of Barkhausen noise measurement is very case-dependent and thus conflicts may occur. Furthermore, the changes in unmeasured material properties may lead to the unexpected behaviour of some features. The results show that linear models are adequate for capturing the major interactions between material properties and Barkhausen noise but indicate that the use of neural networks would lead to better model performance. The results also show that genetic algorithms give better selection results but at the expense of the computational cost. / Tiivistelmä Barkhausen-kohina-mittaus on ferromagneettisille materiaaleille soveltuva materiaalia rikkomaton testausmenetelmä. Mittaus perustuu magneettisten alueiden välisten rajapintojen stokastisiin liikkeisiin, kun testattava kappale asetetaan vaihtuvaan magneettikenttään. Tyypillisesti Barkhausen-kohina-mittaussignaalista lasketaan piirteitä, joita sitten verrataan tutkittaviin materiaaliominaisuuksiin. Usein käytettyjä piirteitä ovat signaalin keskineliön neliöjuuri (RMS-arvo) sekä piikin korkeus, leveys ja paikka. Menetelmää voidaan soveltaa paremmin, jos tutkittavia materiaaliominaisuuksia voidaan ennustaa kvantitatiivisesti. Tämän tutkimuksen tavoitteena on tutkia ja valita menetelmiä, jotka soveltuvat materiaaliominaisuuksien kvantitatiiviseen ennustamiseen Barkhausen-kohina-mittauksen perusteella. Ennustusmallit luodaan neljässä vaiheessa: piirteiden laskenta, piirteiden valinta, mallin identifiointi ja mallin validointi. Piirteiden laskennassa yhdistellään erilaisia matemaattisia laskutoimituksia, joista tuloksena saadaan suuri joukko erilaisia piirteitä. Tästä joukosta valitaan ennustukseen soveltuvimmat piirteiden valinta -vaiheessa. Tämän jälkeen ennustusmalli identifioidaan ja viimeisessä vaiheessa sen toimivuus todennetaan riippumattomalla testausaineistolla. Väitöskirjassa esitetään piirteiden laskentaan kehitettyjä algoritmeja sekä mallinnustuloksia käytettäessä erilaisia piirteiden valintamenetelmiä ja mallinnustekniikoita. Tutkitut valintamenetelmät ovat eteenpäin valinta, taaksepäin eliminointi, simuloitu jäähtyminen ja geneettiset algoritmit. Väitöskirjassa esitellään myös kaksivaiheisia valintamenettelyjä, joissa ennen varsinaista piirteiden valintaa suoritetaan esivalinta. Käytetyt mallinnustekniikat ovat monimuuttujaregressio, osittainen pienimmän neliösumman regressio, pääkomponenttiregressio ja neuroverkot. Tarkasteluissa huomioidaan myös erilaisten kustannusfunktioiden vaikutukset. Esitetyt tulokset osoittavat, että käytetyt menetelmät soveltuvat materiaaliominaisuuksien kvantitatiiviseen ennustamiseen. Identifioidut mallit sisältävät pääasiassa perusteltavia termejä ja mallinnustarkkuus on tyydyttävä. Barkhausen-kohina-mittaus on kuitenkin erittäin tapauskohtainen ja täten ristiriitoja kirjallisuuden kanssa voidaan joskus havaita. Näihin ristiriitoihin vaikuttavat myös ei-mitattavat muutokset materiaaliominaisuuksissa. Esitetyt tulokset osoittavat, että lineaariset mallit kykenevät ennustamaan suurimmat vuorovaikutukset materiaaliominaisuuksien ja Barkhausen-kohinan välillä. Tulokset kuitenkin viittaavat siihen, että neuroverkoilla päästäisiin vielä parempiin mallinnustuloksiin. Tulokset osoittavat myös, että geneettiset algoritmit toimivat piirteiden valinnassa paremmin kuin muut tutkitut menetelmät.

Este trabalho apresenta um método para o reconhecimento e a detecção automática dos diferentes valores ou graus de deformação plástica em Ensaios Não Destrutivos empregando o Ruído Magnético de Barkhausen. O método é baseado no uso de uma Rede Neural Probabilística que permite o diagnóstico automático dos diferentes valores de deformação plástica, conteúdo de carbono, estas medidas são procedentes das medições das amostras de placas de aço AISI 1006, 1050 e 1070, esta base de dados foi feita pelo grupo de pesquisadores do Laboratório de Dinâmica e Instrumentação LADIN da Escola Politécnica da USP, departamento da Mecânica. Os excelentes resultados da rede neural probabilística de detectar automaticamente os valores de deformação mostram a efetividade do desempenho da rede neural probabilística que tem um desempenho superior aos métodos não destrutivos tradicionais e que realmente esta nova tecnologia é uma excelente solução para o diagnóstico. / This work presents a method for automatic detection and recognition of different levels or degrees of plastic deformation in Non-Destructive Testing using the Magnetic Barkhausen Noise. The method is based on using a Probabilistic Neural Network that allows the automatic diagnosis of the different values of plastic deformation and carbon content. The measurements corresponds to samples of steel plates AISI 1006, 1050 and 1070, this database was made by the group of researchers from the Laboratory of Dynamics and Instrumentation LADIN the Polytechnic School of USP, Department of Mechanical Engineering. The results show the effectiveness of the probabilistic neural network to automatically detect plastic deformation levels as well as carbon content level. This method has a superior performance in comparison to traditional nondestructive methods.

Neste trabalho se estuda a aplicação de um método de Ensaio Não Destrutivo, baseado no Ruído Magnético de Barkhausen (RMB), na medição de falhas superficiais em aços. O RMB é gerado por abruptas mudanças na magnetização de materiais quando submetidos a campos magnéticos variáveis. Essas mudanças são afetadas pela microestrutura e a presença e distribuição de tensões elásticas (compressão e tração). Inicialmente apresenta-se um estudo de medições de tensões produzidas por flexão, e perfis de dureza, via RMB. Analisa-se a influência de parâmetros de medição e analise dos sinais, na qualidade do diagnostico. Analisou-se a sensibilidade dos sinais de RMB na medição de tensão a parâmetros tais como direção de laminação da chapa, intensidade e frequência de campo magnético de excitação. Nos estudos da aplicação do RMB em medições de dureza, se usaram amostras de ensaio Jominy, com variação contínua de dureza. Utilizou-se dois tipos de materiais e foram feitas medidas de dureza HRC, RMB e análise metalográfica. O comportamento de diferentes parâmetros dos sinais de RMB se correlacionou com as medições de Dureza e as micrografias obtidas. As amplitudes do RMB mostraram ser inversamente proporcionais aos níveis de dureza. Também se apresenta uma nova tecnologia para inspeção de superfícies, baseado no conceito do Barkhausen Contínuo. São apresentadas análises de parâmetros de medição e de configuração de sonda, na inspeção sob três situações: presença de defeitos volumétricos, deformação plástica, e tensões mecânicas aplicadas. Estudou-se a influência de parâmetros como, amplitude do campo aplicado, velocidade de varredura, posicionamento e características do sensor, no nível de detecção do dano. Métodos de processamento de sinais específicos foram desenvolvidos. Os resultados mostraram, que para cada um dos casos estudados, é possível detectar a posição e nível do dano produzido. Essa nova tecnologia aumenta o espectro de soluções de Ensaios Não Destrutivos para problemas não contemplados pelos métodos existentes. / This work studies the application of a non-destructive testing method, based on the Magnetic Barkhausen Noise (MBN). MBN is generated by abrupt changes in magnetization of ferromagnetic materials subjected to variable magnetic fields. These changes are affected by the presence and distribution of elastic stresses (compression and traction) in the material and by its microstructure. At first, the present study describes MBN measurements of stresses produced by bending and MBN measurements of hardness profiles, in steel samples. The influence of both, measurement parameters and signal analysis, in the quality of fault diagnosis is also analysed. In this context, regarding stress measurements, the MBN signal sensitivity to parameters like surface lamination direction, intensity and frequency of magnetic excitation field is studied. On the other hand, application of MBN to hardness measurements used Jominy essay samples presenting continuous hardness variation. In this case, samples of two different materials were employed and hardness measurements were obtained by HRC, MBN as well as by metallographic analysis techniques. Behaviour of different MBN signal parameters were correlated with obtained hardness measurements and micrography. MBN amplitudes were shown to be inversely proportional to hardness levels. Finally, the work describes a new surface inspection technology, based on the concept of Continuous Barkhausen. In this context, the study analyses probe configuration and measurement parameters, concerning inspection of surface fault from three different origins: volumetric, plastic deformation, applied mechanical stress. The influence of parameters like applied field amplitude, probe velocity, sensor position and characteristics, in detecting damage level was also evaluated. In particular, specific signal processing methods were developed. Results showed that, for each studied case, it is possible to detect damage position and level. This new technology increases the range of NDT essay solutions for problems not yet contemplated by existing methods.

Quando os materiais ferromagnéticos se submetem a campos magnéticos variáveis, experimentam uma mudança na indução que acontece de forma discreta constituindo os saltos de Barkhausen. Estes saltos são conseqüência da interação entre as paredes de domínio se movimentando e os pontos de ancoragem que atuam como barreiras à sua movimentação. Se durante a aplicação do campo variável, coloca-se uma bobina sensora na superfície da amostra, a mudança no fluxo magnético induz uma seqüência de pulsos de tensão elétrica que conformam o sinal de Ruído Magnético de Barkhausen (RMB). Nos últimos anos tem sido publicados trabalhos que mostram que o RMB depende da microestrutura, do teor de carbono, da dureza, do estado de tensões, da deformação, da condição superficial e da anisotropia, fazendo possível utilizá-lo como um método de ensaio não destrutivo para o monitoramento microestrutural de materiais. Esta tese mostra a aplicabilidade do RMB para avaliar a variação microestrutural em juntas soldadas de aço estrutural ASTM A36. A soldagem realizou-se em chapas de 6 mm de espessura com preparação de chanfro em V, com um e dois passes, e com tratamento posterior de alívio de tensões. Fizeram-se a caracterização de microestruturas e microdurezas numa seção transversal da junta soldada. Numa linha na superficie da chapa transversal ao cordão de solda, mediram-se as tensões residuais por difração de raios-X e nas superfícies das chapas, em várias linhas transversais ao cordão de solda, obtiveram-se sinais de RMB. Os sinais de RMB se analisaram no domínio do tempo pelo valor máximo, o valor acima de 80% do máximo, o rms, o rms do envelope, a kurtosis, o skewness e a posição de pico. Analisaram-se no domínio da freqüência o espectro de freqüência na banda completa e em diferentes bandas. Fizeram-se também análises em tempo-escala através da transformada wavelet e wavelet packet, a fim de procurar uma separação do efeito da microestrutura e das tensões residuais. Todos os parâmetros analisados foram grafados em função da distância ao centro do cordão. Obtiveram-se também mapas superficiais com os valores dos parâmetros analisados. Na caracterização microestrutural foi possível identificar tamanhos de zona afetada pelo calor (ZAC) diferentes nas amostras de um e dois passes. Verificou-se também que as microdurezas mudaram através da junta soldada, sendo que a maior dureza se apresentou no limite de fusão e a menor dureza se apresentou no fim da ZAC. Em relação ao RMB constatou-se que no limite de fusão tem-se o menor valor do parâmetro, enquanto que no fim da ZAC tem-se o maior valor do parâmetro. Os mapas superficiais permitiram identificar a região correspondente à solda. Todas as análises feitas mostraram a forte influência da mudança microestrutural. Dos resultados obtidos conclui-se que o RMB pode ser usado como método de inspeção não destrutivo para monitorar juntas soldadas reais de aços carbono. / Ferromagnetic materials submitted to variable magnetic fields experience discontinuous changes in the induction, being the sudden magnetic flux changes referred as Magnetic Barkhausen Noise (MBN). Fast movements of magnetic domain walls and their interaction with the pinning sites result in the MBN. The corresponding signals are detected as voltage pulses when a pick-up coil is placed in the surface of ferromagnetic materials. In this context, several studies have been recently reported showing the dependence of the MBN on the microstucture, hardness, carbon quantity, residual and applied stress, strain, deformation, surface condition and anisotropy. These features make the MBN an important nondestructive testing technique for material microstructural monitoring. This work shows results for the evaluation of ASTM A36 carbon steel welded plates by MBN. V-groove shape welded samples were made in a 6 mm thickness plates, with one-pass and two-pass, and post-weld heat treating for stress relief. Microstructural characterization and microhardness were made on a transversal section of welds. The residual stress were measured by X-ray diffraction. The MBN were measure on surface plates. MBN signals were analyzed on the time domain through several techniques: the maximum value, threshold of 80% above of maximum, the root mean square (rms), the root mean square of the profile, the kurtosis, the skewness and the profile peak position. The spectra in the entire band and in different width bands were analyzed on the frequency domain. To separate the microstructure effect from that of the residual stress, the wavelet and wavelet packet transform, on the timescale domain, were used. All parameters were plotted in function of the distance to center bead. The microstructural characterization identified different HAZ sizes for one-pass and two-pass. In addition, changes on microhardness throughout welded zones were verified. Greater hardness occurred in the fusion boundary whereas lower microhardness happened in the HAZ end. With MBN monitoring were verified that the MBN lower value happened in the fusion boundary, whereas the MBN bigger value happened in the HAZ end. The surface maps identified the weld region. The study showed that changes in the microstructure influence all analyzed parameters. Consequently, we can conclude that the MBN is a potentially important tool to be used in nondestructive testing for carbon steel weld monitoring.

Este trabalho apresenta os avanços no desenvolvimento de um novo método de ensaio não-destrutivo magnético. O método está baseado na técnica do Ruído Magnético de Barkhausen (RMB), particularmente em uma variante denominada Barkhausen Contínuo. O RMB é gerado devido à ação de um campo magnético variável magnetizante que produz mudanças abruptas e irreversíveis na estrutura magnética do material. Essas mudanças são influenciadas pela microestrutura e a distribuição de tensões dentro do mesmo. As medições podem ser usadas para construir uma distribuição bidimensional do RMB ao redor de um ponto fixo, cujo resultado irá refletir o nível de anisotropia magnética, usualmente indicando um eixo de fácil magnetização, parâmetro relevante dado que o comportamento da grande maioria de materiais ferromagnéticos de engenharia é usualmente anisotrópico. As mudanças no eixo de fácil magnetização podem indicar a presença de anomalias mecânicas ou abnormalidades no processo de fabricação e no caso da aplicação de uma tensão externa, podem refletir a magnitude e a direção da mesma. O presente trabalho descreve uma metodologia que faz uso de um campo magnético rotacional para obter sinais RMB relacionados ao angulo de giro, possibilitando a identificação da direção do eixo de fácil magnetização, ultrapassando as capacidades oferecidas pela técnica RMB convencional mediante o fornecimento de informação em tempo real, que permite a obtenção de um conjunto de parâmetros que quantificam a anisotropia magnética de uma amostra. A técnica foi usada para a detecção do eixo de fácil magnetização e o nível de anisotropia magnética em materiais diferentes devida aos efeitos do processo de fabricação. Posteriormente foi demonstrado que a técnica é capaz de monitorar a evolução da tensão uniaxial aplicada, obtendo curvas de calibração, sensíveis ao sentido de aplicação da tensão. Para o caso pouco estudado de amostras submetidas a tensões biaxiais, o uso da técnica do Barkhausen contínuo rotacional fez possível de verificar que as características morfológicas das medições de anisotropia magnética obtidas, guardam relação com a direção das tensões principais. Foi realizada uma avaliação do método aplicado para a medição dinâmica de anisotropia magnética em juntas soldadas, indicando estados de tensão e características microestruturais coerentes com as esperadas. O método tem a possibilidade de ser implementado para medições anisotropia magnética em alta resolução/alta velocidade. / This works presents the current advances on the development of a new method of magnetic non-destructive testing. The method is based on the magnetic Barkhausen noise (MBN), more specifically in one branch known as Continuous Barkhausen. MBN is produced due to the effect of a variable magnetic field, which causes abrupt and irreversible changes to the magnetic structure of the material. These changes are influenced by the microstructure of the material and the stress distributions within. Measurements can be used to construct a bi dimensional MBN distribution around a fixed point, which in turn will be a reflect of its magnetic anisotropy level, usually characterized by an easy axis of magnetization, an important parameter given that more often than not, the behavior of most engineering ferromagnetic materials is anisotropic. Variations of the easy axis could be the indication of mechanical anomalies or abnormalities that appear as a result of the fabrication process. If there is an external stress applied to the sample, it can provide information about its magnitude and direction. The present work describes a methodology which uses a precise rotating magnetic field in order to obtain MBN signals related to a given magnetization angle, making possible the finding of the easy axis, exceeding the limits of conventional MBN measurements by providing real time data which in turn will allow to infer a set of parameters that quantify the magnetic anisotropy of the sample. The proposed technique was successfully used to find both the easy axis and a quantitative level of magnetic anisotropy between different materials, consequence of the fabrication process. Subsequently, it was shown that the technique was able to perform a monitoring of the evolution of both uniaxial and biaxial applied stress, obtaining linear relationships (uniaxial case), sensitive to the direction of loading. In not so much studied case of biaxial loading, the use of the continuous rotational Barkhausen method made possible to observe that the morphologic characteristics of the magnetic anisotropy measurements bear a close resemblance to the direction of the principal stress field. An evaluation of the technique as a tool for the dynamic measurement of magnetic anisotropy on welded joints, indicating stress states and microstructural features coherent with the ones expected in this scenario. The method has the possibility of being implemented as a technique for high speed/high resolution measurements of magnetic anisotropy.

O presente trabalho visa apresentar informações que viabilizem a fabricação de componentes de rolamentos com limites superiores de resistência à fadiga, com a intenção de proporcionar uma vida útil maior ao rolamento, e, também, a otimização da fabricação das peças, dispensando em alguns casos o processo final e usual de retificação, o que implica uma redução de custos. As motivações deste estudo são a geração de altas tensões residuais compressivas no torneamento duro-seco a altas velocidades, a aplicação do RMB como técnica de medição das tensões residuais em aço 100CrMn6 e a otimização da qualidade/custos na fabricação de rolamentos. Para atingir essas metas, será estudada a influência dos parâmetros da usinagem (velocidade de corte (Vc), avanço (a), penetração (p) e desgaste da ferramenta (VB)) na geração das tensões residuais superficiais (sR) e sobre as forças de torneamento (Fc, Fa, Fp), em anéis internos de rolamentos cônicos de aço DIN 100CrMn6 endurecidos termicamente, procurando-se estabelecer uma correlação entre o desgaste da ferramenta e os referidos esforços e tensões residuais. Na etapa experimental, será utilizado um planejamento fatorial completo para executar os ensaios e medições projetadas. No ensaio de torneamento, serão medidas as forças de corte utilizando-se, para isso, um dinamômetro piezoelétrico. Em seguida, serão efetuadas medições de rugosidade. Também será realizado o estudo da microestrutura através da análise metalográfica. As tensões residuais serão medidas utilizando-se o método do furo-cego com extensometria elétrica, e as medidas das alterações microestruturais superficiais serão realizadas pela técnica do Ruído Magnético de Barkhausen (RMB). Serão correlacionados os valores de tensão residual obtidos pela técnica do furo com o RMB. / The present work presents information for the manufacturing of ball bearings components with higher limits of resistance to fatigue, enabling a longer useful life and also the optimization of the production of the pieces, making the usual and final process of grinding unnecessary, which would result in a costs reduction. The motivations of this study are the generation of compressive high residual stress in the hard-dry turning at high speeds, the application of the MBN as technique of measurement of the residual stress in steel 100CrMn6, and the optimization of the quality/costs in the manufacturing of ball bearings. In order to achieve these purpose, the work will study the influence of the machining parameters (cutting speed (Vc), feed rate (a), depth of cut (p), and wear of tool (VB)) in the generation of the superficial residual tensions and also the turning forces (Fc, Fa, Fp), in internal conical rings of ball bearings of steel 100CrMn6 thermally hardened, seeking to find a correlation between the stress of the tool and the referred efforts. In the experimental phase, a complete factorial planning will be used to perform the tests and measurements projected. In the turning tests, the cutting forces will be measured using a piezoelectric dynamometer. After that, measurements of superficial roughness will be performed. Also, the study of the microstructure will be made through the metallographic analysis. The residual tensions will be measured using the strain gage hole-drilling method and the measurement of the superficial microestrutural alterations will be carried out using the technical of the magnetic Barkhausen noise (MBN). The values of residual tension obtained by the hole-drilling will be correlated with the RMB.

Quando os materiais ferromagnéticos se submetem a campos magnéticos variáveis, experimentam uma mudança na indução que acontece de forma discreta constituindo os saltos de Barkhausen. Estes saltos são conseqüência da interação entre as paredes de domínio se movimentando e os pontos de ancoragem que atuam como barreiras à sua movimentação. Se durante a aplicação do campo variável, coloca-se uma bobina sensora na superfície da amostra, a mudança no fluxo magnético induz uma seqüência de pulsos de tensão elétrica que conformam o sinal de Ruído Magnético de Barkhausen (RMB). Nos últimos anos tem sido publicados trabalhos que mostram que o RMB depende da microestrutura, do teor de carbono, da dureza, do estado de tensões, da deformação, da condição superficial e da anisotropia, fazendo possível utilizá-lo como um método de ensaio não destrutivo para o monitoramento microestrutural de materiais. Esta tese mostra a aplicabilidade do RMB para avaliar a variação microestrutural em juntas soldadas de aço estrutural ASTM A36. A soldagem realizou-se em chapas de 6 mm de espessura com preparação de chanfro em V, com um e dois passes, e com tratamento posterior de alívio de tensões. Fizeram-se a caracterização de microestruturas e microdurezas numa seção transversal da junta soldada. Numa linha na superficie da chapa transversal ao cordão de solda, mediram-se as tensões residuais por difração de raios-X e nas superfícies das chapas, em várias linhas transversais ao cordão de solda, obtiveram-se sinais de RMB. Os sinais de RMB se analisaram no domínio do tempo pelo valor máximo, o valor acima de 80% do máximo, o rms, o rms do envelope, a kurtosis, o skewness e a posição de pico. Analisaram-se no domínio da freqüência o espectro de freqüência na banda completa e em diferentes bandas. Fizeram-se também análises em tempo-escala através da transformada wavelet e wavelet packet, a fim de procurar uma separação do efeito da microestrutura e das tensões residuais. Todos os parâmetros analisados foram grafados em função da distância ao centro do cordão. Obtiveram-se também mapas superficiais com os valores dos parâmetros analisados. Na caracterização microestrutural foi possível identificar tamanhos de zona afetada pelo calor (ZAC) diferentes nas amostras de um e dois passes. Verificou-se também que as microdurezas mudaram através da junta soldada, sendo que a maior dureza se apresentou no limite de fusão e a menor dureza se apresentou no fim da ZAC. Em relação ao RMB constatou-se que no limite de fusão tem-se o menor valor do parâmetro, enquanto que no fim da ZAC tem-se o maior valor do parâmetro. Os mapas superficiais permitiram identificar a região correspondente à solda. Todas as análises feitas mostraram a forte influência da mudança microestrutural. Dos resultados obtidos conclui-se que o RMB pode ser usado como método de inspeção não destrutivo para monitorar juntas soldadas reais de aços carbono. / Ferromagnetic materials submitted to variable magnetic fields experience discontinuous changes in the induction, being the sudden magnetic flux changes referred as Magnetic Barkhausen Noise (MBN). Fast movements of magnetic domain walls and their interaction with the pinning sites result in the MBN. The corresponding signals are detected as voltage pulses when a pick-up coil is placed in the surface of ferromagnetic materials. In this context, several studies have been recently reported showing the dependence of the MBN on the microstucture, hardness, carbon quantity, residual and applied stress, strain, deformation, surface condition and anisotropy. These features make the MBN an important nondestructive testing technique for material microstructural monitoring. This work shows results for the evaluation of ASTM A36 carbon steel welded plates by MBN. V-groove shape welded samples were made in a 6 mm thickness plates, with one-pass and two-pass, and post-weld heat treating for stress relief. Microstructural characterization and microhardness were made on a transversal section of welds. The residual stress were measured by X-ray diffraction. The MBN were measure on surface plates. MBN signals were analyzed on the time domain through several techniques: the maximum value, threshold of 80% above of maximum, the root mean square (rms), the root mean square of the profile, the kurtosis, the skewness and the profile peak position. The spectra in the entire band and in different width bands were analyzed on the frequency domain. To separate the microstructure effect from that of the residual stress, the wavelet and wavelet packet transform, on the timescale domain, were used. All parameters were plotted in function of the distance to center bead. The microstructural characterization identified different HAZ sizes for one-pass and two-pass. In addition, changes on microhardness throughout welded zones were verified. Greater hardness occurred in the fusion boundary whereas lower microhardness happened in the HAZ end. With MBN monitoring were verified that the MBN lower value happened in the fusion boundary, whereas the MBN bigger value happened in the HAZ end. The surface maps identified the weld region. The study showed that changes in the microstructure influence all analyzed parameters. Consequently, we can conclude that the MBN is a potentially important tool to be used in nondestructive testing for carbon steel weld monitoring.

No presente trabalho se estuda a correlação de diferentes parâmetros de sinais de Ruído Magnético de Barkhausen (RMB) e de fluxo magnético com o comportamento elastoplástico em chapas de aço carbono, visando à aplicação dos resultados como técnica de Ensaio Não-Destrutivo. Comprovou-se que o comportamento de parâmetros como a energia (RMBenegia), o valor quadrático médio(RMBrms) e a envolvente dos sinais de RMB com os diferentes estados elastoplásticos do material resultou fortemente dependente da anisotropia de laminação, quantidade de Carbono e fases, gerados no processo de produção das chapas. Como novidade introduziu-se o parâmetro chamado de Área em baixo da curva da distribuição de pulsos do sinal de Barkhausen. A evolução deste parâmetro com a deformação identificou com boa aproximação a região de transição elasto-plástica do material. Harmônicos não lineares (HNL) do fluxo magnético apresentaram uma melhor correlação com a anisotropia do que com os estados elasto-plásticos do material, mas com alta dependência ao acoplamento sonda-amostra. Enquanto que o RMB apresentou elevados níveis de reprodutibilidade. Propriedades mecânicas e comportamentos característicos de ensaios de tração em regime discreto e contínuo foram reproduzidos e identificados com um alto grau de aproximação pelo RMB e os HNL. Outros métodos magnéticos e não magnéticos usados neste trabalho não apresentaram níveis altos de sensibilidade às mudanças microestruturais devido às deformações elasto-plásticas no material, no entanto o RMB e os HNL identificaram tais mudanças microestruturais dando margem a novas soluções nos campos de ensaio e inspeção não-destrutivos. / The present work studies the relationship between different parameters of the Magnetic Barkhausen Noise (MBN) and non-linear harmonics of the magnetic flux signals (NLH) with the elastic-plastic behavior in carbon steel plates, in an effort to use the results as a non-destructive testing technique. MBN parameters such as the envelope of the MBN signal, and the scalar parameters MBNenergy, MBNrms were found to be highly dependent on the lamination processes, carbon content and phases originated during the fabrication of the samples. A new parameter entitled area under the MBN jump distribution was defined. The behavior of this parameter as strain evolved provided a good approximation of the elastic-plastic transition region. Non-linear harmonics offered a good correlation with both anisotropy and the elasticplastic states of the material, nevertheless depending heavily on the sensor coupling with the sample surface, while at the same time MBN provided good reproducibility. The characteristic behavior as well as the mechanical properties obtained from discrete and continuous tensile strength tests were reproduced by both MBN and HNL. Other magnetic and non-magnetic methods did not show the same level of sensibility to microstructure changes due to elastic-plastic strains when compared to the results obtained using MBN and HNL, giving room to new solutions in the nondestructive inspection and testing fields.

In the present thesis, we have explored multiple aspects concerning the stochasticity of magnetic domain wall motion during magnetization reversal, all of which originated from our initial study of magnetic field sensing using planar Hall effect. Magnetic field sensors occupy a very important and indispensable position in modern technology. They can be found everywhere, from cellphones to automobiles, electric motors to computer hard disks. At present there are several emerging areas of technology, including biotechnology, which require magnetic field sensors which are at the same time simple to use, highly sensitive, robust under environmental conditions and sufficiently low cost to be deployed on a large scale. Magnetic field sensing using planar Hall effect is one such feasible technology, which we have explored in the course of the thesis. The work was subsequently expanded to cover some fundamental aspects of the stochasticity of domain wall motion, studied with planar Hall effect, which forms the main body of work in the present study. In Chapter 1, we give an introduction to the phenomenology of planar Hall effect, which is the most important measurement technique used for all the subsequent studies. Some early calculations, which had first led to the understanding of anisotropic magnetoresistance and planar Hall effect as being caused by spin-orbit interaction are discussed. In Chapter 2, we discuss briefly the experimental techniques used in the present study for sample growth and fabrication, structural and magnetic characterization, and measurement. We discuss pulsed laser ablation, which is the main technique used for our sample growth. Particular emphasis is given to the instrumentation that was carried out in-house for MOKE and low field magnetotransport (AMR and PHE) measurement. This includes an attempt at domain wall imaging through MOKE microscopy. Some of the standard equipments used for this work, such as the SQUID magnetometer and the acsusceptometer are also discussed in detail. In Chapter 3 we discuss our work on planar Hall sensors that led to the fabrication of a device with a very simple architecture, having transfer characteristics of 650V/A.T in a range of _2Oe. The sensing material was permalloy (Ni81Fe19), and the value had been obtained without using an exchange biased pinning layer. Field trials showed that the devices were capable of geomagnetic field sensing, as well as vehicle detection by sensing the anomaly in Earth's magnetic field caused by their motion. Its estimated detection threshold of 2.5nT made it well suited for several other applications needing high sensitivity in a small area, the most prominent of them being the detection of macromolecules of bio-medical significance. Chapter 4: The work on Barkhausen noise was prompted by reproducibility problems faced during the sensor construction, both between devices as well as within the same device. Study of the stochastic properties led us to the conclusion that the devices could be grouped into two classes: one where the magnetization reversal occurred in a single step, and the other where it took a 0staircase0 like path with multiple steps. This led us to simulations of Barkhausen noise using nucleation models like the RFIM whence it became apparent that the two different groups of samples could be mapped into two regimes of the RFIM distinguished by their magnetization reversal mode. In the RFIM, the nature of the hysteresis loop depends on the degree of disorder, with a crossover happening from single-step switching to multi-step switching at a critical disorder level. Appropriate changes also appear in the Barkhausen noise statistics due to this disorder-induced crossover. By studying the Barkhausen noise statistics for our permalloy samples and comparing them with simulations of the RFIM, we found nearly exact correspondence between the two experimental groups with the two classes resulting from crossing the critical disorder. What remained was to quantify the 0disorder0 level of our samples, which was done through XRD, residual resistivity and a study of electron-electron interaction effects in the resistivity. All these studies led to the conclusion that the samples reversing in multiple steps were more 0defective0 than the other group, at par with the model predictions. This completed the picture with respect to the modeling of the noise. In experiments, it was found that a high rate of film deposition yielded less 0defective0 samples, which severed as an important input for the sensor construction. These results can be viewed from a somewhat broader perspective if we consider the present scenario in the experimental study of Barkhausen noise, or crackling noise in general. Two classes of models exist for such phenomena: front propagation models and nucleation models. Both appear to be very successful when it comes to experiments with bulk materials, while the comparison with experiments on thin films is rather disappointing. It is still not clear whether the models are at fault or the experiments themselves. Through our study, we could demonstrate that there can be considerable variation in the Barkhausen noise character of the same material deposited in the same way, and what was important was the degree of order at the microscopic level. This may be a relevant factor when experimental papers report non-universality of Barkhausen noise in thin films, which can now be interpreted as either insufficient defects or a sample area too small for the study. Chapter 5: Defects in a sample are not the only cause for stochastic behavior during magnetization. In most cases, random thermal 0events0 are also an important factor determining the path to magnetization reversal, which was also true for our permalloy samples. We studied the distribution of the external fields at which magnetization reversal took place in our samples, and tried to explain it in terms of the popular Neel-Brown model of thermal excitation over the anisotropy barrier. The analysis showed that even though the coercivity behaved 0correctly0 in terms of the model predictions, the behavior of the distribution width was anomalous. Such anomalies were common in the literature on switching field distributions, but there seemed to be no unified explanation, with different authors coming up with their own 0exotic0 explanations. We decided to investigate the simplest situations that could result in such a behavior, and through some model-based calculations, came to the conclusion that one of the causes of the anomalies could be the different magnitudes of barrier heights/anisotropy fields experienced by the magnetic domain wall when the reversal occurs along different paths. Though an exact match for the behavior of the distribution width could not be obtained, the extended Neel-Brown model was able to produce qualitative agreement. Chapter 6 contains a study of some interesting 0geometrical0 effects on Barkhausen noise of iron thin films. By rotating the applied magnetic field out-of plane, we could observe the same single-step to multi-step crossover in hysteresis loop nature that was brought about by varying disorder in Chapter 4. We could explain this through simulations of a random anisotropy Ising model, which, apart from exhibiting the usual disorder induced crossover, showed a transition from sub-critical to critical hysteresis loops when the external field direction was rotated away form the average anisotropy direction. Once again, simulation and experiment showed very good agreement in terms of the qualitative behavior. In the second part of this chapter, a study of exchange biased Fe-FeMn system was carried out, where it was observed that the reversal mode has been changed from domain wall motion to coherent rotation. Barkhausen noise was also suppressed. Though many single-domain models existed for this type of reversal, our system was not found to be strictly compatible with them. The disagreement was with regard to the nature of the hysteresis, which, if present, had to be a single step process for a single domain model. The disagreement was naturally attributed to interaction with the nearby magnetic moments, to verify which, simulations were done with a simplified micromagnetic code, which produced excellent agreement with experiment. In Chapter 7, we have studied the temporal properties of Barkhausen avalanches, to compare the duration distributions with simulation. We had used a permalloy sample that was sub-critical according to avalanche size distributions, and our measurement was based on magneto-optic Kerr effect. We measured duration distributions which showed a similar manifestation of finite-size effects as were shown by the size distributions. The power law exponent was calculated, which was deemed 0reasonable0 upon comparison simulations of the sub-critical RFIM. Appendix A contains a study of high-field magnetoresistance of permalloy, which shows that the dominant contribution to magnetoresistance is the suppression of electron-magnon scattering. An interesting correlation is observed between the magnetization of samples and an exchange stiffness parameter d1, that was extracted from magnetoresistance measurements. Here we also re-visit our earlier observation of permalloy thin films possessing a resistance minimum at low temperature. The origin of this minimum is attributed to electron-electron interaction. Appendix B contains the source codes for most of the important programs used for simulation and data analysis. The programs are written in MATLAB and FORTRAN 95. LabView programs used for data acquisition and analysis are not included due to space requirements to display their graphical source codes. Appendix C discusses the studies on a disordered rare-earth oxide LaMnO3. The re-entrant glassy phase is characterized with ac susceptibility and magnetization measurements to extract information about the nature of interactions between the magnetic 0macrospins0 in the system. Appendix D deals with electron scattering experiments performed with spinpolarized electrons (SPLEED) from clean metal surfaces in UHV. A study of the scattering cross sections as a function of energy and scattering angle provides information about spin-orbit and exchange interactions of the electrons with the surface atoms, and can answer important questions pertaining to the electronic and magnetic structure of surfaces. In the course of this study, planar Hall effect is seen to emerge as a powerful tool to study the magnetic state of a thin film, so that it is interesting to apply it to thin films of other materials such as oxides, where magnetization noise studies are next to nonexistent. What also emerged is that there is still a lot of richness present in the details of supposedly well-understood magnetization phenomena, some of which we have explored in this thesis in the context of stochastic magnetization processes.

Neste trabalho foram investigadas as propriedades magnéticas (curva de histerese e ruído Barkhausen) de alguns aços (ABNT 1045, 1050 e 1548) cujas superfícies foram endurecidas por indução. O objetivo era de desenvolver um ensaio não-destrutivo para determinar a espessura da camada endurecida, assim evitando a necessidade de realizar medidas (destrutivas) de microdureza. Medidas da susceptibilidade magnética mostraram um pico (ferrita) em torno de 800 A/m e outro (martensita) em torno de 3500 A/m. Assim procuramos correlacionar a amplitude do pico de ferrita com a espessura da camada dura na superfície. Para esta finalidade foram desenvolvidos sensores para medir ruído Barkhausen e um filtro para o processamento do sinal. Foi possível correlacionar medidas de microdureza com algumas características observadas nas medições magnéticas, como, por exemplo, a amplitude do sinal de ruído Barkhausen e o campo magnético do pico do sinal Barkhausen. Deste modo, encontrou-se uma relação entre a amplitude do ruído Barkhausen e a espessura da camada superficial. Seria desejável complementar essas medidas com outras e sugestões para uma continuação do trabalho são detalhadas. / Submitted by Marcelo Teixeira (mvteixeira@ucs.br) on 2014-06-03T20:29:34Z No. of bitstreams: 1 Dissertacao Gustavo Rodrigues da Silva.pdf: 8142476 bytes, checksum: 29c4748e15d0fbcc852d788b9f1fa011 (MD5) / Made available in DSpace on 2014-06-03T20:29:34Z (GMT). No. of bitstreams: 1 Dissertacao Gustavo Rodrigues da Silva.pdf: 8142476 bytes, checksum: 29c4748e15d0fbcc852d788b9f1fa011 (MD5) / This work investigates the magnetic properties (hysteresis loop and Barkhausen noise) in some steels (ABNT 1045, 1050 and 1548) whose surfaces had been hardened by induction. The goal was to develop a non-destructive test procedure to determine the thickness of the hardened layer, thus avoiding the need to perform (destructive) measurements of the microhardness. Measurements of the magnetic susceptibility showed a peak (ferrite) around 800 A / m and other (martensite) at around 3500 A / m. Therefore, a correlation was sought between the peak amplitude of ferrite with the thickness of hard layer on the surface. For this purpose sensors were developed to measure Barkhausen noise as well as a filter for signal processing. It was possible to correlate measurements of microhardness with some features observed in magnetic measurements, for example, the amplitude of the Barkhausen noise and magnetic field of peak Barkhausen signal. Thus, a relationship was found between the amplitude of the Barkhausen noise and the thickness of the surface layer. It would be desirable to supplement these with other measurements and suggestions for further work are detailed.