Fatigue damage accumulation in nickel base superalloys.

Tangentially oriented Waspaloy specimens taken from the
bore section of a turbine disc were subjected to low cycle
fatigue in the form of cyclic four point bend tests.
Substantial differences in grain size and structure occurred
across the section of the disc. Fatigue damage occurred
primarily as a result of persistent slip band formation,
cell formation, Orowan looping and precipitate shearing. A
small number of grains exhibited dislocation cells, normally
associated with higher stacking fault energy materials. The
number and width of persistent slip bands increased with the
number of cycles, but varied both between grains and stress
levels. It is proposed that initially looped precipitates
were subsequently sheared. Crack initiation was observed as
early as 1/12 of fatigue lifetime, predominantly from
inclusions. A stage-I/stage-II crack propagation transition
occurred at approximate 4Kl values of 30MPa.m1/ 2 , when the
plastic zone size was comparable to the material grain size.
Results imply that short crack growth requires prior slip
band formation.