A B S T R A C TThe fatigue crack growth properties of friction stir welded joints of 2024-T3 aluminium alloy have been studied under constant load amplitude (increasing-K), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold K values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non-conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non-conservative crack growth rate predictions next to K C instability. At threshold K values non-conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered. a = crack length AA2024-T3 = high-strength aluminium alloy grade d = slot aperture da/dN = crack growth rate E = plane-stress Young's modulus E = plane-strain Young's modulus EL = elongation at fracture FCGR = fatigue crack growth rate(s) FSW = friction stir welding h(x,a) = weight function HAZ = heat-affected zone K C = critical stress intensity factor K MAX = maximum applied stress intensity in fatigue K Ir = residual stress intensity factor in mode I of crack opening 526
Jerônimo Bernardi e Eliezer Dias Francisco pela ajuda na realização do trabalho experimental e inestimável apoio técnico;A todos os funcionários, amigos que conquistei e que me auxiliaram e a todos os professores do Departamento de Engenharia de Materiais EESC -USP; A Prof.Dr. Isa Spinelli pela amizade e valiosas correções realizadas neste trabalho; À Embraer, em especial ao Eng. MSc. Luiz Edno Pereira, pelo auxílio prestado na discussão deste trabalho e pela cessão do material para a confecção dos corpos de prova utilizados neste estudo;Aos amigos e companheiros de trabalho
This study investigates the influence of quenching/tempering temperatures on the fracture toughness and fatigue life of SAE 5160 steel, considering different phosphorus contents. Quenching and tempering treatments were applied to samples removed from different bars of commercial SAE 5160 steel with different P content. Three different austenitizing temperatures for quenching: 850, 900 and 1000 °C and a constant holding time of 15 minutes were used. The oil temperature for quenching was kept at 66 °C and the tempering conditions were 470, 500 and 530 °C with the necessary time for a final hardness of 45 ± 3 HRC. Therefore, the heat treatments cycles were applied to specimens containing low (0.012 wt. (%)), medium (0.017 wt. (%)) and high (above 0.025 wt. (%)) phosphorus contents, in order to observe the effects of this element on the susceptibility of these steels to enhance quench and tempering embrittlements. The Charpy tests results showed that the phosphorus content analyzed in this work has caused embrittlement, even in the bars with the lowest P content, leading to intergranular fracture. However, if the nucleation life is taken into consideration, this embrittlement has no effect on the nucleation fatigue life of the component.
This paper discusses the relationship between striation spacing, i.e., the microscopic crack propagation rate, as measured in postmortem fractographic inspection of fatigue fractured surfaces, and the macroscopic crack propagation rate, i.e., da/dN, as monitored during fatigue crack growth tests. Compact tensile specimens C(T) in prevalent plane-strain conditions were extracted in LT orientation from the center of a 2-in. thick rolled plate of a SAE-AMS 7475-T7351 Al alloy. Testpieces were fatigue tested according to ASTM-E647 standard, at room temperature in a servo-hydraulic closed-loop MTS testing machine operating with the unloading elastic compliance technique. da/dN-K data points were collected in the Paris' law validity region, with crack growth rates typically ranging from 0.18 to 2.02 µm/cycle. Topographical survey was conducted on the test specimen fracture surfaces in a scanning electronic microscope in order to determine striation spacing created during the fatigue test. Macro-and microcrack growth rates were compared and good correlation have been obtained for the data within the range of K assessed in the study. Results of crack growth rates have been quantitatively evaluated in terms of fatigue life estimation.
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