This paper first describes the importance of using location specific S-N curves for fatigue damage assessment of existing steel structures. It discusses the existing concepts and methods for developing S-N curves usin...This paper first describes the importance of using location specific S-N curves for fatigue damage assessment of existing steel structures. It discusses the existing concepts and methods for developing S-N curves using empirical formulae and monotonic strength parameters, such as the ultimate tensile strength and hardness. It also discusses relationships among these monotonic parameters. Then it presents formulae for developing hardness-based full range S-N curves for medium strength steels. The formulae are verified using experimental data obtained from both monotonic and cyclic testing. Finally, it describes the advantages of these hardness-based formulae for developing location specific S-N curves as hardness testing is a non-destructive test which can be carried out on specific locations in structures.展开更多
A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 1010 cycles and associated fracture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 ...A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 1010 cycles and associated fracture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy(OM) and scanning electron microscopy(SEM) were used for microstructure characteristic and fracture surface analysis. The S—N curves obtained show that fatigue failure occurs in the range of 105 ?1010 cycles, and the asymptote of S—N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 108 cycles for R=0.7. Surface roughness (the maximum roughness is no more than 3 μm) has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α2-Ti3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.展开更多
文摘This paper first describes the importance of using location specific S-N curves for fatigue damage assessment of existing steel structures. It discusses the existing concepts and methods for developing S-N curves using empirical formulae and monotonic strength parameters, such as the ultimate tensile strength and hardness. It also discusses relationships among these monotonic parameters. Then it presents formulae for developing hardness-based full range S-N curves for medium strength steels. The formulae are verified using experimental data obtained from both monotonic and cyclic testing. Finally, it describes the advantages of these hardness-based formulae for developing location specific S-N curves as hardness testing is a non-destructive test which can be carried out on specific locations in structures.
基金Project(50775182) support by the National Natural Science Foundation of ChinaProject(2006E133) supported by the Natural Science Foundation of Shaanxi Province, China
文摘A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 1010 cycles and associated fracture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy(OM) and scanning electron microscopy(SEM) were used for microstructure characteristic and fracture surface analysis. The S—N curves obtained show that fatigue failure occurs in the range of 105 ?1010 cycles, and the asymptote of S—N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 108 cycles for R=0.7. Surface roughness (the maximum roughness is no more than 3 μm) has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α2-Ti3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.
