Abstract
The mechanism of fatigue crack initiation was evaluated on 500-nm-thick freestanding copper (Cu) films. We conducted fatigue experiments on notched specimens at constant maximum stress under in situ optical microscopy observation, and nanoscale fatigue damages were observed by FESEM. Fatigue damage like intrusions and extrusions formed along a twin boundary near the notch root at an early stage of fatigue life. The intrusion then penetrated the film in the thickness direction or a fatigue crack was initiated. New intrusion/extrusion-like damage then formed ahead of the crack tip, and the fatigue crack propagated via the damage, leading to the long crack. We defined the crack initiation life as the number of cycles just before the formation of the new fatigue damage. The crack initiation life increased with decreasing the maximum stress.
