Observations of the Fatigue Fracture Surface of Metals with an Electron Microscope

Abstract

In our previous paper were shown the electron micro-fractographies of aluminum alloys which were fatigued under actual service loadings, and statement was made that the striations of the fracture surface corresponded to the loading cycles as one to one, and their spacings to the stress peak values.
In this paper are shown the fatigue fractographies of other metals, low carbon steel and manganese bronze. The applied loadings were of bending on their circular beam test pieces. The loading patterns were two level amplitude variations, square wave loadings and two superimposed sinusoidal ones.
Though the striations of the carbon steel test pieces were irregular, only partial correspondence of the striations and loadings could be discerned. It was confirmed that the fatigue crack propagation length in one loading cycle of the square wave increased and its fatigue lives became short as the time was prolonged during which the constant stress was maintained. The striations of manganese bronze were of two types, one was regular which was discerned in α phase grain, f.c.c., and the other was irregular in β phase, b.c.c.. There was good correspondence of the striations and the loadings on this material.
On these fracture surfaces rows of pressed marks were discerned, which were produced by pressure against the fracture surface on the counter side and sliding over each other if slightly after the crack grew. These rows of pressed marks also corresponded well with the loading cycles.