This research investigates the effects of beam diffraction from a source of finite width on nonlinear Rayleigh wave propagation in an elastic half space. Previous work has shown that nonlinear ultrasonic Rayleigh waves can be used to evaluate material damage due to plastic deformation and fatigue. In this measurement, a relatively small wedge transducer is employed to launch Rayleigh surface waves in the specimen and the first and second harmonic amplitudes are measured in the far field as a function of propagation distance. In order to obtain a reliable set of measurement data, one needs to make numerous points in a wide range of distance, which can be impractical in many cases. This research investigates model is employed and the computation results are compared with experiment ones. This research will make measurements on 7075-T651 aluminum as a specimen, compensate the diffraction effects and then, will compare the feasibility of the method proposed in this research in the results of normalized second harmonic amplitude vs propagation distance.

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This thesis describes a series of fretting fatigue experiments carried out under closely controlled conditions of partial slip. These experiments confirm the existence of a size effect whereby the fretting fatigue life of an aluminium alloy is shown to vary with contact size. The configuration chosen, of cylindrical fretting pads contacting a plane specimen is amenable to classical stress analysis and the surface tractions between the contacting bodies are derived. The effects of tension in the specimen, finite specimen thickness, differing elastic constants, and surface roughness are all investigated and incorporated into the analysis where appropriate. A technique is then developed to calculate stress intensity factors for plane cracks growing under the contact load at an arbitrary angle to the free surface. The analysis is then applied to the experimental results and three possible explanations for the size effect are proposed, based on statistical effects, crack arrest, and crack initiation. These are examined in the light of the experimental evidence and it is proposed that the variation of fatigue life with contact size is due to an increase in the amount of fretting damage above a threshold level for crack initiation. A composite parameter is chosen to characterise the severity of fretting conditions and this is shown to describe the experimental results accurately. Finally, the use of this parameter in design calculations is discussed.

Orientador: Herman Jacobus Cornelis / Coorientador: Celso Pinto Morais Pereira / Banca: Carlos Eduardo Silva de Amorim / Banca: Maria Odila Hilario Cioffi / Banca: Carlos de Moura Neto / Banca: Olivério Moreira de Macedo Silva / Resumo: Este estudo objetiva analisar a influência da espessura de uma camada intermediária de níquel químico sob cromo duro eletrodepositado no aço de alta resistência, ABNT 4340 (utilizado em trens de pouso de aeronaves), com dureza de 39 HRc e 52 HRc. A análise é realizada por meio de dados obtidos nos ensaios de fadiga por flexão rotativa, das amostras revestidas com cromo duro, com e sem camada intermediária de níquel químico. As espessuras com melhor desempenho foram submetidas a ensaio de fadiga axial, em corpos de prova com e sem entalhe. Os resultados demonstram que a aplicação da camada de níquel químico é benéfica, em todas as espessuras analisadas, porém, há espessura que seu desempenho é ainda melhor. O tema proposto surgiu do interesse da ELEB - Embraer Liebherr Equipamentos do Brasil, em buscar por novas alternativas ou melhorias ao atual revestimento de cromo duro convencional. / Abstract: The aim of this study is to analyze the effect of the thickness of an intermediate layer of chemical nickel under chromium electroplating on a high resistance steel ABNT 4340 (used in aircrafts landing gears), with hardness of 39 and 52 HRc. The analysis is done through data obtained from rotating bending fatigue tests of samples coated with hard chromium, with and without intermediate layer of chemical nickel. The thickness with better performance are submitted to axial fatigue tests, with and without notch. The results demonstrate that the application of a layer of chemical nickel is beneficial, in all of the analyzed thickness although some thicknesses present even better performance. The proposed work is due to the interest of the Brazilian Company of Aeronautics, Division of Equipments - EMBRAER/ELEB - in searching for new alternatives or improvements to the current coating with conventional hard chromium. / Doutor