A large number of axially loaded specimens of 2024-T3 aluminum sheet alloy were fatigued to failure in alternating tension. The stress ratio was kept constant at .05 and maximum stresses were chosen to cause failure in the upper part of the S-logN curve where failure is by progressive hardening. The specimens were prepared in three ways, unpolished in which the rough edges were rounded, chemically polished in which the central section was polished chemically, and mechanically and chemically polished in which the central section was rubbed with emery paper then polished chemically. Macroscopic and microscopic examinations of chemically polished specimens tested at a maximum stress of 47.5 ksi were also made. Statistical analysis was used to fit experimental frequency distributions to the life values at each stress level, to determine the effect of the different polishing methods and to determine the effect of a light oil coating. Of the two tried, the Lognormal and the Weibull, the Weibull distribution provided a better fit and was easier to use than the Lognormal. For the three different polishing groups, the mechanically and chemically polished specimens gave the longest lives at all stress levels followed by the chemically polished then the unpolished specimens. Also both groups which received a final chemical polish showed S-logN curves with a much rounder knee than did the unpolished group. Coating the specimens with a light non-corroding oil resulted in all cases in an increase in life and in more scatter in the life values. The increases ranged from 20% to 62%, At the knee of the S-logN curve, all groups showed a bimodal. distribution in the life values. This was felt to be the result of a change in the failure mechanism such as the one postulated by Wood. Examination of the fracture surface and of small cracks which formed near the edges indicated that the failure occurred in two stages; nucleation and growth of a small non-distorting fatigue crack followed by transition to a rapidly propagating ductile crack. Observations of the failures showed that it took approximately 1000 cycles from the appearance of the first small crack to when the ductile crack had propagated through the section. Microscopic examination showed that the fatigue cracks were intercrystalline and grew on the surface along slip marking which formed ahead of the tip of the crack. The only effect the crystal structure had was in the direction of the slip band formation. There was no indication that the mechanism by which the cracks initiated was different than the one by which they propagated. The cracks initiated at the surface and grew through to the other side. Once they reached the other side a ductile crack formed and final failure soon followed. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Thesis (M.S. in Aeronautical Engineering and Aeronautics and Astronautics Engineer's Degree)--Naval Postgraduate School, December 1990. / Thesis Advisor: Gorman, Michael R. Second Reader: Dutta, Indranath. "December 1990." Description based on title screen as viewed on April 2, 2010. DTIC Identifier(s): Aluminum, fatigue (mechanics), cracking (fracturing), acoustic emissions, wings, fixed wing aircraft, reconnaissance aircraft, flight simulation, 7075 aluminum, theses, Naval aircraft, stress waves, crack propagation, acoustic detection, fatigue life, E-2C aircraft, safety factor, mechanical properties. Author(s) subject terms: aircraft fatigue, againg aircraft, crack growth, 7075 alumiunum, spectrum loading, acoustic emission, plate waves. Includes bibliographical references (p. 69-71). Also available in print.

Orientador: Itamar Ferreira / Dissertação (mestrado profissional) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica, Faculdade de Engenharia Eletrica e de Computação e Instituição de Quimica / Made available in DSpace on 2018-08-14T20:41:02Z (GMT). No. of bitstreams: 1 Mattos_JoaoJoseIfarraguirrede_M.pdf: 6966915 bytes, checksum: 6a6ed6b801e5d2e44c4e448a4c71a74e (MD5) Previous issue date: 2009 / Resumo: O crescente uso das ligas alumínio-silício fundido na indústria automotiva deve-se principalmente a redução de peso, consumo de combustível e níveis de emissões. Isto inclui a liga de alumínio-silício EN AlSi7Mg0,6, equivalente a liga ASTM A357.0, que é empregada em cabeçotes de motores Diesel. É importante conhecer o impacto na integridade e confiabilidade deste componente na presença de defeitos intrínsecos dos processos de fundição convencional como a de molde permanente. Tais defeitos, como porosidades e filmes de óxidos, quando localizados na superfície, ou próxima a esta, são nucleadores de trincas de fadiga. Neste estudo é analisado o impacto na resistência à fadiga e nos micromecanismos de fratura pelo ensaio de corpos-de-prova fresados com dimensões de 7x14x60mm, retirados do cabeçote da linha de produção e submetidos a ensaio de flexão em três pontos. As superfícies de fratura dos espécimes foram analisadas em microscópio eletrônico de varredura (MEV) a fim de caracterizar os micromecanismos de fratura e o local de iniciação da trinca de fadiga. A resistência à fadiga média, para 1 milhão de ciclos, foi determinada como sendo ao redor de 140MPa. Foi observado na superfície de fratura dos espécimes testados, um claro contraste entre o micromecanismo da zona de fadiga (estrias) e zona final de fratura (alvéolos), sendo que o fator predominante para nuclear trincas de fadiga foram poros próximos da superfície. / Abstract: The increase usage of casting aluminum-silicon alloys in the automotive industry is due to reduce weight, fuel consumption, and emissions level. This includes the aluminum-silicon cast alloy EN AlSiMg0.6 (ASTM A357.0) which is used to make Diesel engine cylinder head. It is important to know the impact on the integrity and reliability of this component in the presence of intrinsical defects of conventional casting parts produced on permanent mold process. Such defects, as porosity and oxide film, when locate on the surface or subsurface of casting parts, could be a fatigue crack initiators. In this paper is analyzed the impact on the fatigue strength and micromechanisms of fracture, by using 7x14x60mm specimens machined from cylinder head drew from production assembly line, and submitted to three point bending tests. Fracture surface of the specimens were observed by SEM to characterize the micromechanisms and the initiation fracture local. The average fatigue strength, based on 1 million of cycles, is about 140MPa. It was observed on the fracture surface of fatigue test specimens, a clear contrast between the micromechanisms of fatigue zone (striations) and the final fracture zone (dimples) and fatigue crack initiation occurs at the porosities near the surface. / Mestrado / Materiais / Mestre em Engenharia Automobilistica

Aluminum single crystals were cyclically deformed in single-slip at small strain amplitudes at 77 K to presaturation. The observed mechanical behavior is consistent with other recent work. The dislocation substructure was analyzed in detail. The structure can be described as consisting of dense bundles or veins of dislocation dipoles, separated by lower dislocation density regions where debris is evident. This debris was determined to be principally relatively short dipole segments. Screw dislocations with the same Burgers vector span the channels. Dislocations were essentially all of the same Burgers vector. In-situ cyclic deformation experiments were successfully performed by the X-Y technique where thin foils are stressed in alternating perpendicular directions. Screw dislocations span the channel and easily move and reverse direction with shear reversal. Our experiments indicate that loops frequently expand from the dipole bundles into the channels and the edge component is absorbed by nearby bundles leaving screw segments behind. Dipole "flipping" was not observed and these edges are relatively difficult to mobilize. There is no obvious evidence for internal backstresses that assist plastic deformation on reversal of the applied shear. / Graduation date: 1996

A linear analysis of a commercial aluminum lattice dome was performed with the loads prescribed by the American National Standards Institute. The pressure snow loads were transformed into a distributed load along the dome members. The analysis of the dome was performed with the structural analysis program STAAD-III. The dome was modeled as a space frame with the joint member stiffness range from pinned-end to four times the member stiffness. The internal forces in the pinned-end and framed-end models were compared to the specifications by the Aluminum Association. The pinned-end model under the unbalanced snow load were found to exceed the allowable. The maximum deflection occurred when the joint stiffness was one-half of the member stiffness. The maximum axial force occurred when the joint stiffness was four times the member stiffness. The maximum moment at the midpoint of the member occurs when the member ends are pinned. / Master of Science

Interrogating metals with ultrasonic waves can be used to evaluate their microstructural and mechanical properties. These techniques analyze ultrasonic wave features in order to make inferences on the medium of interest. Current research is being conducted to determine higher order elastic properties and characterize material degradation of 7075-T651 aluminum with ultrasonics. This thesis topic will use acoustoelasticity, the stress dependency of acoustic velocity, to accomplish these goals. Acoustoelasticity is a manifestation of the inherent nonlinearity in the interatomic binding energy, which appears mathematically as higher order elastic terms in the stress strain constitutive relation. The acoustoelasticity will be determined for longitudinal and shear waves propagating through a sample under uni-axial stress. Experimentally, specific techniques and tooling will be designed to insure accurate measurements of acoustic wave velocity as a function of stress. Using acoustoelasticity the third order elastic constants of 7075-T651 aluminum will be determined. Further, Al samples will be fatigue damaged and acoustoelasticity and third order elastic constants will be mapped versus damage. Literature will be used to verify measured values of acoustoelasticity as well as provide theoretical models for acoustoelastic dependence on damage.