Crack growth rate curves are the fundamental material property for metal structures under fatigue loading. Although there are many crack growth rate curves available in the literature, few of them showed the capabilit...Crack growth rate curves are the fundamental material property for metal structures under fatigue loading. Although there are many crack growth rate curves available in the literature, few of them showed the capability to explain various special phenomena observed in tests. A modified constitutive relation recently proposed by McEvily and his co-workers showed very promising capability. This modified constitutive relation is further generalized by (1) introducing an unstable fracture condition; (2) defining a virtual strength to replace the yield stress; and (3) defining an overload and underload parameter. The performances of this general constitutive relation for fatigue crack growth is extensively studied and it is found that this general constitutive relation is able to explain various phenomena observed with particular strong capability on load sequence effect.展开更多
This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue （HCF） properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress am...This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue （HCF） properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they fimcfion as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.展开更多
Ni-based superalloys are largely used in the aerospace industry as critical components for turbine engines due to their excellent mechanical properties and fatigue resistance at high temperatures. A hypothesis to expl...Ni-based superalloys are largely used in the aerospace industry as critical components for turbine engines due to their excellent mechanical properties and fatigue resistance at high temperatures. A hypothesis to explain this atypical characteristic among metals is the presence of a cross-slip mechanism. Previous work on the role of thermal activation on cubic slip has shown strain accommodation in two sets of slip planes, which resembled the activation of {100} cubic slip systems along of the octahedral slip planes {111} in Ni-based superalloys under high strain and temperature, exhibiting a more homogeneous strain distribution and less strain localization. Following those previous literature evaluations of initial conditions that can potentially activate cubic-slip planes and provide the level of accommodation and strain homogenization within the grain, this paper presents some experimental procedures and results of Ni-based superalloy (IN-718) tested at 500°C under operational loading condition, without and after being submitted to an overload and overtemperature. The experiments have shown that a pre-condition of 1% strain at 700°C would increase the fatigue life of the IN-718 at 500°C by four times when compared to pristine tested samples. The present results bring up the potential of improving this material fatigue performance, opening the need to further investigate the microstructure as the precondition is applied.展开更多
Due to their superior combination of heat resistance, high temperature corrosion resistance, toughness and strength, nickel-based superalloys have become of extensive use in the aerospace industry. This research aims ...Due to their superior combination of heat resistance, high temperature corrosion resistance, toughness and strength, nickel-based superalloys have become of extensive use in the aerospace industry. This research aims to explain why the fatigue life of Inconel-718 in preconditioned samples had larger fatigue lives than pristine samples. The hypothesis is that preconditioning at 700°C and 1.0% strain could lead to thermal activation of the {100} cubic slip plane alongside the {111} octahedral slip plane, potentially improving fatigue life. Using SEM and EBSD imaging, the microstructure of Inconel-718 samples were characterized before and after preconditioning. The directions of the slip bands that formed following the preconditioning were determined. The result was that the existence of both the cubic and octahedral slip systems was confirmed, leading to the thermal activation hypothesized. The existence of both slip planes was considered to be the reason behind the improved fatigue life due to better strain accommodation within the microstructure. It is suggested that focuses for future research includes conducting in-situ observation of slip activation and the application of preconditioning as a manufacturing method.展开更多
When a metal is subject to cyclic loading with cyclic stress values much lower than its tensile strength,fatigue occurs due to accumulative and irreversible damages developed in the microstructure,causing service fail...When a metal is subject to cyclic loading with cyclic stress values much lower than its tensile strength,fatigue occurs due to accumulative and irreversible damages developed in the microstructure,causing service failure of the metal workpiece or even fatal disaster in the worst cases.To develop materials with higher fatigue limit and longer fatigue life relies on reducing or suppressing展开更多
文摘Crack growth rate curves are the fundamental material property for metal structures under fatigue loading. Although there are many crack growth rate curves available in the literature, few of them showed the capability to explain various special phenomena observed in tests. A modified constitutive relation recently proposed by McEvily and his co-workers showed very promising capability. This modified constitutive relation is further generalized by (1) introducing an unstable fracture condition; (2) defining a virtual strength to replace the yield stress; and (3) defining an overload and underload parameter. The performances of this general constitutive relation for fatigue crack growth is extensively studied and it is found that this general constitutive relation is able to explain various phenomena observed with particular strong capability on load sequence effect.
基金supported by the National Natural Science Foundation of China(Nos.51375033 and 51405006)
文摘This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue （HCF） properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they fimcfion as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.
文摘Ni-based superalloys are largely used in the aerospace industry as critical components for turbine engines due to their excellent mechanical properties and fatigue resistance at high temperatures. A hypothesis to explain this atypical characteristic among metals is the presence of a cross-slip mechanism. Previous work on the role of thermal activation on cubic slip has shown strain accommodation in two sets of slip planes, which resembled the activation of {100} cubic slip systems along of the octahedral slip planes {111} in Ni-based superalloys under high strain and temperature, exhibiting a more homogeneous strain distribution and less strain localization. Following those previous literature evaluations of initial conditions that can potentially activate cubic-slip planes and provide the level of accommodation and strain homogenization within the grain, this paper presents some experimental procedures and results of Ni-based superalloy (IN-718) tested at 500°C under operational loading condition, without and after being submitted to an overload and overtemperature. The experiments have shown that a pre-condition of 1% strain at 700°C would increase the fatigue life of the IN-718 at 500°C by four times when compared to pristine tested samples. The present results bring up the potential of improving this material fatigue performance, opening the need to further investigate the microstructure as the precondition is applied.
文摘Due to their superior combination of heat resistance, high temperature corrosion resistance, toughness and strength, nickel-based superalloys have become of extensive use in the aerospace industry. This research aims to explain why the fatigue life of Inconel-718 in preconditioned samples had larger fatigue lives than pristine samples. The hypothesis is that preconditioning at 700°C and 1.0% strain could lead to thermal activation of the {100} cubic slip plane alongside the {111} octahedral slip plane, potentially improving fatigue life. Using SEM and EBSD imaging, the microstructure of Inconel-718 samples were characterized before and after preconditioning. The directions of the slip bands that formed following the preconditioning were determined. The result was that the existence of both the cubic and octahedral slip systems was confirmed, leading to the thermal activation hypothesized. The existence of both slip planes was considered to be the reason behind the improved fatigue life due to better strain accommodation within the microstructure. It is suggested that focuses for future research includes conducting in-situ observation of slip activation and the application of preconditioning as a manufacturing method.
文摘When a metal is subject to cyclic loading with cyclic stress values much lower than its tensile strength,fatigue occurs due to accumulative and irreversible damages developed in the microstructure,causing service failure of the metal workpiece or even fatal disaster in the worst cases.To develop materials with higher fatigue limit and longer fatigue life relies on reducing or suppressing
