Initiation and Early Growth of Fatigue Cracks

Experimental evidence concerning the mechanism of initiation and early growth of short cracks are studied using FESEM, FIB and STEM observations of the surface of fatigued specimens of several f.c.c. metals. The true shape of persistent slip markings with high resolution is obtained on FIB cuts and using lamella milled on the surface. Thick extrusions and parallel thin crack-like intrusions are typical feature before crack initiates from the tip of the intrusion. STEM also reveals the relation of the surface relief and internal structure of the material showing in copper ladder-like dislocation structure. The shape of the persistent slip markings is confronted with the predictions of the recent theoretical models. Study of the short crack growth and its kinetics shows high density of initiated surface cracks and proves the acceleration of the crack growth rate due to linkage of principal crack with initiated surface cracks. Fatigue life is determined by the growth of short cracks and short crack growth law is equivalent to the Manson-Coffin law.