This study aims to investigate the flow and fracture behavior of aluminum alloy 6082-T6 (AA6082-T6) at different strain rates and triaxialities. Two groups of Charpy impact tests were carried out to further investigate its dynamic impact fracture property. A series of tensile tests and numerical simulations based on finite element analysis (FEA) were performed. Experimental data on smooth specimens under various strain rates ranging from 0.0001~3400 s-1 shows that AA6082-T6 is rather insensitive to strain rates in general. However, clear rate sensitivity was observed in the range of 0.001~1 s-1 while such a characteristic is counteracted by the adiabatic heating of specimens under high strain rates. A Johnson-Cook constitutive model was proposed based on tensile tests at different strain rates. In this study, the average stress triaxiality and equivalent plastic strain at facture obtained from numerical simulations were used for the calibration of J-C fracture model. Both of the J-C constitutive model and fracture model were employed in numerical simulations and the results was compared with experimental results. The calibrated J-C fracture model exhibits higher accuracy than the J-C fracture model obtained by the common method in predicting the fracture behavior of AA6082-T6. Finally, the Scanning Electron Microscope (SEM) of fractured specimens with different initial stress triaxialities were analyzed. The magnified fractographs indicate that high initial stress triaxiality likely results in dimple fracture.
This paper investigates the energy absorption and dissipation pattern in train-to-train collisions. For this purpose, simplified scaled rail vehicle models were designed based on the energy-absorbing characteristics of honeycombbased structures under static loading, and scaled tests were performed under dynamic loading. In this study, a oneeighth-scale model of train-to-train collisions for a three-car set was tested, and the corresponding displacement-time and velocity-time curves were obtained. The extents of energy absorption and dissipation were also calculated. Also, finite element simulations were conducted to simulate one-eighth-scale train collisions for three-car, five-car and eightcar sets. The finite element numerical simulation of scaled train collisions produced results that were consistent with experiment results. The simulation results also indicated that train sets do not significantly affect energy absorption and dissipation pattern. The energy-absorbing structure at the front of a train plays a major role in the collisions. Moving cars absorb slightly more energy than stationary cars in both the scaled test and finite element simulations.
Ni-rich Ni-Ti alloys, such as 60NiTi, show a higher hardness and dimensional stability than equiatomic or near-equiatomic NiTinol ones. These make them suitable to be employed in structural applications. Laser powder bed fusion technique (LPBF) was used in this research to print parts from a Ni-rich Ni-Ti powder mixture consisting of about 63 wt.% Ni and 37 wt.% Ti. The microstructure of the obtained parts, processed using different parameters, showed inhomogeneity, contained undesirable Ni-rich and Ti-rich regions and a significant amount of cracks. To eliminate these defects, homogenize the microstructure and to obtain the required phases, we applied hot isostatic pressing (HIP) to the printed samples with a selected set of parameters. After HIP at 180 MPa and 1050 °C for 4 h, we observed the formation of a homogeneous microstructure containing mainly NiTi and Ni3Ti phases in the printed samples. However, cracks still persisted in the microstructure of these HIP treated samples. Applying another round of HIP treatment at a temperature just slightly above the melting start temperature of the samples, treated by the elementary HIP procedure, could successfully eliminate the cracks in the microstructure of samples and also increase the crystallinity of the existing phases.
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