Fatigue crack growth behavior of AA7050 T7451 aluminum alloy under mode II loading condition in two orientations was investigated. Compact shear specimens were prepared in TL and LT directions. A loading frame for mode II type of loading was manufactured. Using the loading frame and the specimen, KIIC values and mode II fatigue crack growth rates were calculated. Fractographic analysis of the fracture surfaces of both mode II fracture toughness test specimens and mode II fatigue crack growth test specimens were done to examine the effects of mode II load. KIIC values were measured between 1.3 and 1.5 times the KIC values for this alloy. As for mode II fatigue crack growth rates, TL orientation shows the highest mode II fatigue crack growth resistance.

The effect of different aging treatments on fatigue crack growth behavior of AA6013 aluminum alloy was investigated. C(T) (Compact Tension) specimens were prepared in L-T and T-L direction for fatigue crack growth tests. Samples were in T651 as received, T42 which is solution heat treated at 538 &ordm / C for 90 minutes, water quenched and aged in room temperature for 96 hours, and one group of samples were overaged at 245 &ordm / C for 12 hours after T42 condition was achieved. Hardness and conductivity measurements were achieved for all conditions after the heat treatments. Fatigue crack growth tests were performed at as received condition T651, T42 and 245 &ordm / C aged samples in laboratory air with sinusoidal loading of stress ratio R=0.1 and at a frequency of 1 Hz. The highest fatigue crack growth resistance is observed for T651 T-L and 245 &ordm / C overaged L-T condition.

Orientador: Itamar Ferreira / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-10T13:08:50Z (GMT). No. of bitstreams: 1 Sandor_LeonardoTaborda_D.pdf: 12861330 bytes, checksum: 648e60efeced3118f4ec8272c46bddcc (MD5) Previous issue date: 2008 / Resumo: Este trabalho propõe uma metodologia para avaliar pontualmente as variações de tenacidade à fratura e taxa de crescimento de trinca de fadiga ao longo da camada cementada de um aço SAE 4320. A pequena espessura dessas camadas impede a retirada de corpos de prova nas dimensões especificadas pelas normas de ensaios de tenacidade à fratura. Assim, para simular uma camada cementada retirou-se amostras de aços SAE 43xx variando-se apenas teor de carbono de 0,20 a 1,00%. Os corpos de prova após eletrodeposição de cobre foram tratados termicamente numa carga de cementação, têmpera e revenimento para serem submetidos aos efeitos térmicos sem absorção de carbono. Os resultados da análise microestrutural, dos ensaios de microdureza, de tração e de tenacidade à fratura e de taxa de crescimento de trinca de fadiga foram agrupados em um único gráfico e comparados com o perfil de cementação de peças de aço SAE 4320 tratadas nas mesmas condições. Foi confirmado que as propriedades de fadiga variam de forma inversamente proporcional à microdureza (HV1) e que a previsão do comportamento de uma trinca numa camada cementada pode ser feita por meio de equação ou diagrama que relacionam a microdureza HV1 com a tenacidade à fratura ou taxa de crescimento de trinca de fadiga (KIC ou da/dN x ??) / Abstract: The purpose of this work is to propose a methodology for evaluating the crack growth rate and the fracture toughness along the SAE 4320 steel carburized layer. Due to the small thickness of those layers, it is impossible to machine specimens from those layers in accordance with standards. For simulating the microstructures of the carburized layer in order to get samples for tensile and the fracture toughness testing, specimens of SAE 43xx, from 0.20 %C to 1.00 %C, steels melted in vacuum induction melting, hot rolled and have been machined, assuming the local influence just the variation of the content of carbon and considering that the contents of the other alloy elements are essentially constant. The specimens after electroplated copper layer were heat treated in an industrial load of carburizing, quenching, and tempering for they be submitted to the thermal effects without absorption of carbon. The results of the microstructure analysis and microhardness, crack growth rate, and the fracture toughness tests were placed in a single graph and compared with the profile of the carburizing of the steel SAE 4320 heat treated in the same conditions. It was confirmed that the crack growth rate and the fracture toughness varies inversely proportional to the microhardness (HV1) and that the forecast of the behavior of a crack in a carburized layer can be made through equation or it designs that relate the microhardness (HV1) with the fracture toughness (KIC or CTODC) and crack growth rate (da/dN X ??) / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Rolling contact fatigue is a problem encountered with many machine elements.In the current report a numerical study has been performed in order to predictthe crack path and crack propagation cycles of a surface initiated rolling contactfatigue crack. The implementation of the contact problem is based on theasperity point load mechanism for rolling contact fatigue. The practical studiedproblem is gear contact. Different loading types and models are studied andcompared to an experimental spall profile. Good agreement has been observedconsidering short crack lengths with a distributed loading model using normalloads on the asperity and for the cylindrical contact and a tangential load on theasperity. Several different crack propagation criteria have been implemented inorder to verify the validity of the dominant mode I crack propagation assumption.Some general characteristics of rolling contact fatigue cracks have beenhighlighted. A quantitative parameter study of the implemented model hasbeen performed. / Utmattning med rullande kontakter är ett ofta förekommande problem för många maskinelement. I den aktuella rapporten utfördes en numerisk studieför att förutsäga sprickvägen hos utmattningssprickor som initierats i ytan vidrullande kontakter. Implementeringen av kontaktproblemet bygger på asperitpunktlastmekanismen för rullande kontakter. Studien av kontaktproblemetär tillämpad till kugghjul. Olika belastningstyper och modeller studeradesoch jämfördes med profilen hos en experimentell spall. Bra överensstämmelseobserverades för korta spricklängder när en modell med fördelad belastninganvänds för en belastningstyp där en normalbelastning agerar på asperiten ochvid cylindriska kontakten och en tangentialbelastning införs på asperiten. Olikakriterier för spricktillväxt implementerades för att verifiera giltigheten av antagandetatt mode I spricktillväxt är dominant. Några generella kännetecken avutmattningssprickor med rullande kontakter framhävdes. En kvantitativ parameterstudie för den implementerade modellen utfördes.

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

The mechanical behavior of a high pressure die cast AM60B Mg alloy is studied. Constant load amplitude fatigue tests were conducted at room, elevated and cold temperatures, with a stress ratio of R=0.1, and frequency of 30 Hz. The objective was to identify the possible effects of temperature on fatigue life cycle. In addition, fatigue crack propagation tests were conducted to ascertain the fatigue response of the alloy and determine its fatigue crack growth rate as a function of the applied stress ratio, experimentally, analytically and computationally, using Walker’s model. The results demonstrated that temperature had a significant influence on the fatigue life, and that the life increased at cold temperature but decreased at elevated temperature as compared to that evaluated at room temperature. In this study, the limit for applicability of LEFM was established for AM60B magnesium alloy. In addition, fatigue crack propagation test results were used to evaluate the coefficients of the Paris model.

The relationship between crack path and test method was examined by comparing the performance of adhesive-adherend combinations (six) in quasi-static fracture, mixed-mode fatigue, and creep crack growth. Crack paths in creep and quasi-static fracture were similar due to similar crack-tip plastic zone sizes in the epoxy adhesive even though the crack growth rates in creep were much smaller. Under condensed moisture and mixed-mode, creep and threshold fatigue tests produced interfacial failure. Under room-temperature dry environment, near threshold mixed-mode fatigue was interfacial, but was not in creep or quasi-static fracture. Smaller plastic zone size and crack path proximity to the interface that followed increased the sensitivity of near threshold, mixed-mode fatigue to surface properties. Therefore, the interfacial or cohesive failure of an adhesive system, which may judge the quality of the bond, can be a function of the test being conducted and may not be an absolute indicator of joint quality.

The relationship between crack path and test method was examined by comparing the performance of adhesive-adherend combinations (six) in quasi-static fracture, mixed-mode fatigue, and creep crack growth. Crack paths in creep and quasi-static fracture were similar due to similar crack-tip plastic zone sizes in the epoxy adhesive even though the crack growth rates in creep were much smaller. Under condensed moisture and mixed-mode, creep and threshold fatigue tests produced interfacial failure. Under room-temperature dry environment, near threshold mixed-mode fatigue was interfacial, but was not in creep or quasi-static fracture. Smaller plastic zone size and crack path proximity to the interface that followed increased the sensitivity of near threshold, mixed-mode fatigue to surface properties. Therefore, the interfacial or cohesive failure of an adhesive system, which may judge the quality of the bond, can be a function of the test being conducted and may not be an absolute indicator of joint quality.

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

The effect of retrogression and reaging heat treatment on corrosion fatigue crack growth behavior on AA7050 T73651 aluminum alloy is investigated. CT (Compact Tension) specimens are prepared in LS direction for fatigue crack growth tests . Samples are solution heat treated at 477 &deg / C and aged at 120 &deg / C for 24 h (T6 condition). After that, samples are retrogressed at 200 &deg / C for times of 1, 5, 30, 55 and 80 minutes in a circulating oil bath. Then, samples are re-aged at 120 &deg / C for 24 h (T6 condition). Hardness measurements are taken at different retrogression times and at the end of the heat treatment. Fatigue crack growth tests are performed at as received condition and at different retrogression times with sinusoidal loading of R=0.1 and f=1 in both laboratory air and corrosive environment of 3.5% NaCl solution. The highest fatigue crack growth resistance is observed for 30 min. and 5 min. retrogression for laboratory air and corrosive environment respectively. It is concluded that RRA can successfully be used to improve fatigue performance of this alloy.