<正>The fatigue behavior of a high strength steel 300 M in the gigacycle regime was investigated. Fully reversed tension - compression fatigue tests at ambient temperature were performed using an ultrasonic fatigue system operating at 20 kHz.The staircase test method was employed to obtain accurate values of the mean fatigue strength corresponding to fixed numbers of cycles up to 10~9.These results were compared to the curve which is estimated by the data tested in the mid-long life regime on conventional servo hydraulic test machine at 20 Hz.Results indicate that the fatigue strength determined from ultrasonic fatigue testing is lightly higher than conventional testing in the range of 10~6-10~7 cycles.It is obvious that nucleations of fractures tend to occur below the surface, if fractures happen after more than 10~7 cycles.All the fractured specimens fails from internal SiO_2 inclusions or smaller carbides and carbide clusters.
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.
