MICROCRACK PROPAGATION IN FATIGUED FCC MONOCRYSTALS .2. CRACK FACES AND CRACK FRONT SHAPES

Nickel monocrystals were cyclically deformed at constant plastic strain amplitudes up to different fatigue stages (cf. part I). The crack faces and crack front shapes of microcracks which propagate along persistent slip bands were estimated by removing material layers parallel to the specimen surface. The crack faces are similar to a "folded curtain" with deflections from the primary into the cross-slip plane which increase with the crack depth. The ratio c/l of the crack depth c and crack length l increases with increasing number of cycles; this means that the cracks propagate faster in depth than in length. The shapes of the crack fronts in the interior are trochoidal in most cases and not simple semielliptic. From the changes in frequency of crack lengths in different specimen depths, crack depth distribution curves were calculated which correspond well to the depth distribution curves that were determined from cut internal planes in part I.