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In an attempt to find a way of improving the damage tolerance of composite bonded repairs to metallic aircraft structures, the effect of using conventional crack growth retardation techniques in conjunction with bonded repairs was experimentally investigated. Hence, an experimental test program was set up to determine whether fatigue crack growth under bonded repairs is retarded further by giving the crack to be repaired a crack growth retardation treatment prior to repair patch application. In addition, it was set up to determine the influence of a bonded repair on the effectiveness of a crack growth retardation method. Centrally cracked aluminium plates were used. Stop drilling followed by cold hole expansion and the application of single overloads were selected as retardation treatments. Two patch materials were considered; boron/epoxy and Glare 2. Further test variables were the aluminium alloy and the plate thickness. Fatigue testing was carried out under constant amplitude loading and baseline results were determined first. In addition to optically monitoring the crack growth, local and global out-of-plane deformations were visualised with holographic interferometry and shadow moire??. Furthermore, the stress intensity factors under the repair patch were examined with strain gauges and measurement of the central crack opening displacement. Disbonds and fracture surfaces were studied after residual strength tests. The crack growth results obtained showed that retardation treatments decrease crack growth rates under a repair patch and that the effectiveness of a retardation treatment is increased by the patch. Although identical crack growth rates were observed under boron/epoxy and Glare 2 patches, the reinitiation period after the retardation treatment lasted longer when Glare 2 patches were applied. Analytical predictions of the extent of retardation based on existing models showed that the conjunctive effect of retardation treatments and bonded repairs was underestimated. A sustained reduction in crack growth rates was observed under bonded repairs with a prior overload retardation treatment. It was concluded that the damage tolerance of bonded repairs is increased by the application of a crack growth retardation treatment because the crack growth is retarded further. These findings indicate that the range of cracks in aircraft for which bonded repairs can be considered is expanded and that economic benefits can be obtained.

Made available in DSpace on 2014-06-11T19:27:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-06-25Bitstream added on 2014-06-13T18:55:38Z : No. of bitstreams: 1 oliveira_lg_me_guara.pdf: 3165585 bytes, checksum: b01429d4faacda4b94f4a66fa205c617 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Atualmente a crescente necessidade de aumento da carga por eixo e do número de passageiros transportados pelos trens, fez com que as ferrovias em todo o mundo iniciassem pesquisas voltadas para o desenvolvimento de novos materiais e processamentos térmicos. O aumento da confiabilidade dos sistemas ferroviários é uma exigência do mercado, o que é definido como sendo a garantia de que não ocorra falhas nos componentes no período entre as inspeções de rotina, ou seja, que um defeito possa ser detectado e reparado antes que atinja um tamanho crítico que leve a ruptura do componente. Por se tratar de um equipamento fundamental nos trens, os eixos ferroviários são alvos de grande parte das recentes pesquisas na área de fadiga, desenvolvimento de novos materiais e segurança dos intervalos de inspeção, uma vez que sua falha, certamente ocasionará um descarrilamento do vagão ou locomotiva. O comportamento em fadiga dos materiais empregados na fabricação de eixos ferroviários vem sendo estudado através de ensaios de propagação de trinca por fadiga, a fim de se obter dados experimentais para definição de intervalos de inspeção. Neste trabalho, foram estudados os aços SAE-1050, comumente utilizado na fabricação de eixos ferroviários, e o SAE- 4130, que recentemente vem sendo empregado para este fim. Estes materiais foram processados por dois diferentes tipos de tratamentos térmicos, sendo um composto por normalização e alívio de tensões, e o outro por normalização, têmpera e revenimento. O comportamento mecânico dos aços foi avaliado através de ensaios de tração, charpy em diversas temperaturas e tenacidade à fratura ( IC K ). O comportamento em fadiga, por sua vez, foi avaliado através de ensaios de propagação de trinca por fadiga, realizados em corpos de prova tração compacto (CT). Para a determinação da influência da razão... / Currently the growing need of increase in axle load and numbers of passengers transported by the trains, prompted the railroads all over the world began researches about the development of new materials and heat treatments. The increase of the reliability for railway systems is a requirement of the market, and can be defined as the warranty that doesn't occur failures in the components between the inspection intervals, in other words, that a defect can be detected and repaired before it reaches a critical size to take the rupture of the component. Axles are one of the most important components in railway vehicle, thus are present in the great part of the recent researches into fatigue, new materials and safe inspection intervals, since its flaw, certainly will cause a derailment of the wagon or locomotive. The fatigue behavior of the employed materials in the production of railway axles has been studied with aid of fatigue crack growth tests, in order to obtain experimental data for definition of inspection intervals. The present work, studied the steels SAE-1050, commonly used in the production of railway axles, and SAE-4130, that recently has been used for this application. These materials were submitted to different heat treatments: normalizing and stress relief, and normalizing, quenching and tempering. The mechanical properties of the steels were evaluated through tensile test, impact test in different temperatures and fracture toughness test. The fatigue behavior was evaluated through of fatigue crack growth tests in compact tension specimens (CT). The influence of the load ratio R , on fatigue crack growth was evaluated using the ratio of 0,1 and 0,3. The results showed that the steel SAE-4130, normalized, quenched and tempered, achieved the best mechanical properties and the smallest fatigue crack growth rate, being therefore, the best option as... (Complete abstract click electronic acccess below)

Este projeto faz parte de uma Rede Interdisciplinar de Pesquisa em Superligas no Brasil (Projeto Pró-Engenharias - CAPES), que tem como objetivo o desenvolvimento técnico e científico do Brasil nesta classe de materiais. A rede é composta por especialistas de estabelecida competência em assuntos fundamentais para o desenvolvimento de materiais estruturais, tais como: fusão em vácuo, solidificação direcional, relações orientação-propriedade, tratamentos térmicos, fluência, fadiga em alta temperatura, oxidação e corrosão a quente. Assim, na parte que compete a este pesquisador, foram estudadas as propriedades de fadiga em altas temperaturas da liga MAR-M247 e de uma nova liga, a liga MAR-M247 modificada pela substituição do Ta pelo Nb, e sendo doravante denominada MAR-M247(Nb). Foram desenvolvidos estudos correlacionando parâmetros intrínsecos do material, como composição química e microestrutura adicionalmente a parâmetros extrínsecos, tais como: temperatura, estudo de oxidação, propriedades de fadiga, ensaios de tração em temperatura ambiente, 250, 500, 700, 800, 900 e 1000ºC, conforme as normas AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) E 8M-00 e ASTM E 21-00. Em relação aos ensaios de fadiga foram realizados ensaios de nucleação de trincas , conforme a norma ASTM E466-07 e os resultados foram apresentados na forma de curvas S-N e analisados estatisticamente segunda a norma ASTM E739-10. Para auxiliar na definição dos micromecanismos de fadiga, as superfícies de fratura foram analisadas utilizando o microscópio eletrônico de varredura (MEV) e microscópio ótico estereoscópico (MO). Foi realizado ensaio de oxidação para verificar a susceptibilidade a oxidação das ligas, sendo este um dos fatores que afetam a vida em fadiga em alta temperatura. / This project is part of an interdisciplinary network of Research in Super League in Brazil (Project Pro-Engineering - CAPES), which aims to technical and scientific development of Brazil in this class of materials. The network is composed of experts of established competence in matters fundamental to the development of structural materials, such as: vacuum melting, directional solidification, relationships property-oriented, heat treatment, creep, fatigue in high temperature oxidation and hot corrosion. Thus, at the racing this researcher, were studied the fatigue properties at high temperatures MAR-M247 alloy and a new league, which in reality is the MAR-M247 alloy modified by replacing the Ta by Nb, and being hereinafter referred to as MAR-M247 (Nb). Were designed studies correlating parameters intrinsic material, such as chemical composition and microstructure, with extrinsic parameters such as: temperature and type of loading, with the properties fatigue. Thus, tensile tests at room temperature environment, 250, 500, 700 and 900ºC, and tests for compression at temperatures of 700 and 900ºC were performed to determine the elastic behavior and elastoplastic material, according to the ASTM E-8M 00, ASTM E 21-00 and ASTM E209-00. Were also conducted testing of fatigue crack nucleation, as ASTM E466-07. The results were presented at form of SN curves and statistically analyzed the second ASTM E739-10. To assist in the definition of micromechanics of fatigue, the fracture surfaces were examined in a scanning electron microscope (SEM) and stereoscopic optical microscope (OM). Was oxidation test performed to check susceptibility to oxidation of the alloy, which is the factor affecting the fatigue life.

A large-area electronics (LAE) strain sensor, termed soft elastomeric capacitor (SEC), has shown great promise in fatigue crack monitoring. The SEC is capable to monitor strain changes over a large structural surface and undergo large deformations under cracking. Previous tests verified that the SEC can detect and localize fatigue cracks under low-cycle fatigue loading. In this paper, we further investigate the SEC's capability for monitoring high-cycle fatigue cracks, which are commonly seen in steel bridges. The peak-to-peak amplitude (pk-pk amplitude) of the SEC measurement is proposed as an indicator of crack growth. This technique is is robust and insensitive to long-term capacitance drift. To overcome the difficulty of identifying the pk-pk amplitude in time series due to high signal-to-noise ratio, a signal processing method is established. This method converts the measured SEC capacitance and applied load to power spectral densities (PSD) in the frequency domain, such that the pk-pk amplitudes of the measurements can be accurately extracted. Finally, the performance of this method is validated using a fatigue test of a compact steel specimen equipped with a SEC. Results show that the crack growth under high-cycle fatigue loading can be successfully monitored using the proposed signal processing method.

In this dissertation, a novel fast modeling technique called peri-ultrasound that can model both linear and nonlinear ultrasonic behavior of materials is developed and implemented. Nonlinear ultrasonic response can detect even very small material non- linearity. Quantification of the material nonlinearity at the early stages of damage is important to avoid catastrophic failure and reduce repair costs. The developed model uses the nonlocal continuum-based peridynamic theory which was found to be a good simulation tool for handling crack propagation modeling, in particular when multiple cracks grow simultaneously. The developed peri-ultrasound modeling tool has been used to model the ultrasonic response at the interface of two materials in presence of an interface crack. Also, the stress wave propagation in a half-space (or half-plane for a 2-dimensional problem) with boundary loading is investigated using peri-ultrasound modeling. In another simulation, well-established two-dimensional Lamb's problem is investigated where the results are verified against available analytical solution. Also, the interaction between the surface wave and a surface breaking crack is studied.

Ligas de alumínio que contém lítio são conhecidas por serem materiais atraentes devido à sua baixa densidade, alta elasticidade, alta resistencia mecânica e boa resistência à corrosão. A degradação do material devido à fadiga e corrosão são os dois principais fatores que contribuem para o envelhecimento de uma aeronave, e a previsão de corrosão e fadiga sob corrosão são muito importantes para sua integridade estrutural. A ocorrencia de pites de corrosão pode reduzir significativamente a iniciação da trinca de fadiga e diminuir a variação da intensidade de tensão limite destas ligas em até 50%. Com o objetivo de analisar novas ligas Al que estão no mercado, este trabalho compara o comportamento mecânico, de fadiga de baixo ciclo e eletroquímico das ligas de aluminio 2525 T3 e 7050 T7451 utilizadas pela EMBRAER, com o das ligas T851 2198 e 7081 T73511. Assim, ensaios de nucleação de trinca por fadiga foram realizados ao ar e em espécimes pré-condicionadas por 15 dias em névoa salina 5%, para a obtenção de curvas S-N. Curvas de polarização potenciodinâmicas foram obtidas para a investigação eletroquímica do comportamento destas ligas em um ambiente corrosivo. Também foram feitos ensaios de resistência à tração e dureza. Os resultados obtidos da série 2XXX mostram que as ligas são menos susceptíveis à corrosão por pites por apresentarem valores de Epite mais positivos que as ligas da série 7XXX. Porém, seus valores de jcorr também são mais positivos, o que aumenta a velocidade de corrosão das ligas, uma vez iniciado o processo. Tal fato se deve à presença de precipitados e elementos de liga que agem de modo a elevar o potencial para valores mais altos. Análises metalográficas por microscopia ótica e de varredura foram realizadas para se determinar os precipitados presentes em cada uma das ligas estudadas. O comportamento em fadiga das ligas pré-condicionadas em ambiente salino foi inferior em relação ao comportamento das ligas mantidas ao ar. Os pites funcionaram como concentradores de tensão, aumentando a ocorrência de nucleação de trincas de fadiga. A resistência à fadiga ao ar das ligas estudadas foi ligeiramente inferior ao das ligas bases. Entretanto, as quatro curvas obtidas para os espécimes pré-condicionados apresentaram-se superpostas. Isso indica que a ação dos pites rege o mecanismo de nucleação das trincas, enquanto que em ar, adiciona-se a ação do acabamento superficial. / Lithium-containing aluminum alloys are known to be attractive materials because of their low densities, high elastic modulus, high strengths and good corrosion resistance. Material degradation due to fatigue and corrosion are two major factors that contribute to the aging of an aircraft, and the prediction of corrosion and corrosionfatigue are very important for the structural integrity of aircrafts. The presence of corrosion pits can significantly shorten the fatigue crack initiation life and decrease the threshold stress intensity of an alloy by as much as 50%. The purpose of this work was to compare mechanical, fatigue and electrochemical behavior of the new \'AL\' alloys, 2198 T851 and 7081 T73511 that are on the market, with those used by EMBRAER, named 2525 T3 and 7050 T7451. The comparisons also considered fatigue resistance in pre-corrosion confining in salt environment. Therefore, air and 15-day-confined 5% salt environment specimens were submitted to crack nucleation tests to obtain S-N curves. Potentiodynamic polarization curves were also plotted to study the electrochemical behavior of this alloys in a corrosion environment. Tensile and hardness tests were performed. The obtained results for the 2XXX series showed that these alloys are less susceptible to pitting corrosion due to more positives Epite compared to 7XXX series. However, their jcorr values are also more positive, increasing the corrosion speed once it has begun. This happens due to the presence of some precipitates and a few elements that react in a way to increase the potential to higher values. Optical and electronic microscopy analyses were performed to determine the nature of precipitates of each alloy. The fatigue behavior of the salty-pre-confined environment specimens was much inferior compared to air test specimens according to each corrosion environment. Pits work as tension concentration, increasing fatigue crack nucleation. Air fatigue resistance of the new alloys was slightly inferior than the base alloys, but when the tests under pre-confined environment were conducted, the four curves overlapped. This implies that the pit action governs crack nucleation mechanism, while in air, surface finishing is an additional factor.

The constants modernization in the fuels used request improvements in the combustioneffectiveness, as a consequence the material for components as the cylinder heads mustenhance their properties. Seven different compositions of compacted graphite iron (CGI)are analysed with the aim of characterized and select the most suitable material for thecylinder head service condition. The present master thesis report focuses on the impactof the matrix- either pearlitic or ferritic- and alloying elements such as molybdenum andnickel in the fatigue crack propagation ratedadNand fracture toughness (kIc).Tests to determined the fatigue crack growth rate, according to the ASTM standardE647 and fracture toughness (ASTM E399) were conducted. The equipment utilized was aservo hydraulic machine, for the fatigue crack propagation rate test and a electromechanicalmachine for the fracture toughness. Moreover, for measure the crack length a portablemicroscope camera and a camera connected to a DIC(digital image correlation) softwarewas used. The interpretation of the data obtained from the tests were done by TEMA- aDIC software- and MATLAB.The test results are analysed describing the influence of the composition and the microstructurehave over the mechanical properties achieved. Furthermore, an analyse forrelating the graphite average length and the hardness with the fatigue crack growth rateand the fracture toughness of the materials is performed.

This thesis deals with the theoretical and practical part of the phenomenon of acoustic emission. The thesis examined the theoretical foundations of this phenomenon and also described the equipment that was used in the laboratory of VUT in Brno to perform measurements. Then measurements and their analysis were described. Several possible errors that occurred during the work on the thesis were also described. Error description is the basis for error-free laboratory testing and for improving the quality of laboratory testing.

<div>An important challenge in predicting fatigue and creep crack growth is describing crack growth rates under transient conditions. Transient conditions occur when similitude is violated at the crack tip due to the applied loads or material behavior. Crack growth models like the Paris law, valid for homogeneous materials under constant-amplitude cyclic loading or sustained loading, no longer apply. Transient crack growth rates are strongly influenced by changes in plastic deformation at the crack tip. Activation of time-dependent damage and viscoplastic deformation at high temperatures further complicates the problem.</div><div><br></div><div>This thesis advances knowledge and predictive capabilities for transient creep and fatigue crack growth in metals, with specific applications to two technologically-relevant nickel-base superalloys. Finite element computations of crack growth following overloads and in multilayered materials are conducted. Crack extension is an outcome of the boundary value problem through an irreversible cohesive zone model and its interaction with plasticity and viscoplasticity in the bulk material.</div><div><br></div><div>First, fatigue crack growth in rate-independent materials is analyzed. The plasticity formulation considers both plastic strain and gradients of plastic strain, which produce hardening beyond that predicted by classical plasticity models. The computations demonstrate that hardening due to plastic strain gradients plays a significant role in transient fatigue crack growth following overloads. Fatigue crack growth transients associated with material inhomogeneity are studied through the case of a crack growing toward interfaces between plastically dissimilar materials. Interactions between the interface strength and the yield strength mismatch are found to govern crack growth rates near the interface. Hardening due to plastic strain gradients is important for finding the critical conditions associated with crack bifurcation at an interface and penetration through an interlayer.</div><div><br></div><div>Subsequently, crack growth in rate-dependent materials is analyzed. For materials characterized by power-law viscoplasticity, fatigue crack growth rates following overloads are found to depend strongly on the material rate sensitivity. The computations predict a transition from acceleration- to retardation-dominated post-overload crack growth as the rate sensitivity decreases. The predicted post-overload crack growth rates show good agreement with high-temperature experimentally-measured trends for Alloy 617, a solid solution strengthened nickel-base superalloy proposed for use in next-generation nuclear power plants. The results demonstrate why Alloy 617 behaves in a relatively brittle manner following overloads despite being characterized as a creep-ductile material. Crack growth is also studied in materials where rate dependence is captured through time-dependent damage and dislocation storage and dynamic recovery processes. This approach is relevant for high-strength creep-brittle materials, in which the viscoplastic zone grows with the advancing crack. The computations predict crack growth retardation for several loading waveforms containing overloads. The amount of retardation depends strongly on the overload ratio and subsequent unloading ahead of the crack tip. The predicted post-overload crack extension shows good agreement with high-temperature experimentally-measured trends for Alloy 718, a precipitation-hardened nickel-base superalloy used in turbine engines and power generation applications. The results demonstrate why Alloy 718 behaves in a ductile manner following overloads, despite being characterized as a creep-brittle material.</div>