Powder metallurgy is an economical processing route for shaping powders directly into a final component form. Sintered steels, powder metallurgy products, have been studied and developed for many years, but their applications remain limited, partially due to a limited understanding of their temperature dependent performance. The objectives of the present work are to study the mechanical properties of a sintered steel (Fe - 0.82%C - 1.9%Cu) and investigate the mechanisms of fatigue and fracture of it at elevated temperatures, including (1) tensile and fracture behavior, (2) mechanisms of fracture toughness, (3) long and small fatigue crack propagation behavior at elevated temperatures and different R values ( R = σmin/σmax), and (4) crack closure effects and max fatigue propagation mecha...[ Read more ]
Powder metallurgy is an economical processing route for shaping powders directly into a final component form. Sintered steels, powder metallurgy products, have been studied and developed for many years, but their applications remain limited, partially due to a limited understanding of their temperature dependent performance. The objectives of the present work are to study the mechanical properties of a sintered steel (Fe - 0.82%C - 1.9%Cu) and investigate the mechanisms of fatigue and fracture of it at elevated temperatures, including (1) tensile and fracture behavior, (2) mechanisms of fracture toughness, (3) long and small fatigue crack propagation behavior at elevated temperatures and different R values ( R = σmin/σmax), and (4) crack closure effects and max fatigue propagation mechanisms at elevated temperatures.
The tensile elastic-plastic behavior of sintered steel is well described by the Ramberg-Osgood equation which provides a reliable model for the fracture toughness analysis. The results of fracture toughness tests showed KIC decreases with temperature, different behavior than is found in fully-dense materials. Three-dimensional finite element analysis showed that location of the maximum tensile stress, which depends on pore's arrangement near a crack tip, might occur at a finite distance from the crack front. The temperature-dependent fracture toughness of the sintered steel is explained by the critical stress model. Fatigue test of the sintered steel demonstrated the effects of temperature and load ratio, and indicated that the threshold value δKth for crack propagation increases with test temperatures, decreases with R. Theoretical evaluation of the closure effects induced by asperity indicated that the asperity-induced crack closure can be partially account for the observed load ratio effects on crack propagation. Kmax -δKeff however, was evidenced to be the possible determining mechanism for crack growth by analyzing the experimental data. The temperature effect on fatigue crack growth in sintered steel not only arise from their intrinsic mechanical properties, but also are affected by the microcracks toughening near the threshold regime at elevated temperature.
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