Fatigue Crack Initiation Mechanisms in Polycrystalline Iron

摘要

The results of an investigation of fatigue crack initiation in polycrystalline iron as a function of testing frequency (0.01-1000Hz) are given. Three distinct types of fatigue crack initiation mechanisms were identified. At low frequencies, 0.5Hz, cracks were observed to nucleate at intrusions, extrusions and grain boundaries. However over the frequency range studied the cracks at the intrusions and extrusions were not associated with the final fatigue failure. The results suggest that in pure iron frequencies 0.01Hz are required for fatigue failure to be initiated at intrusions and extrusions. At frequencies 0.01Hz at high strain amplitudes, cracks nucleated at the intersection of grain boundaries with the free surface as a result of the incompatable deformation of surface grains. The subsequent propagation of these cracks resulted in fatigue failure. Tests performed at low strain amplitude at 5Hz and 1000Hz initiated surface grain boundary cracking without any significant associated change of grain shape. The results indicate that fagitue failure is initiated at grain boundaries.