PREDICTION OF THE COMPETITION BETWEEN SURFACE AND INTERNAL FATIGUE-CRACK INITIATION IN PM ALLOYS

Low cycle fatigue (LCF) testing of powder metallurgy (PM) superalloys has determined that LCF cracks initiate from defects such as pores and ceramic inclusions located either at the surface or internally. The r atio of surface-to-internal crack initiation sites depends upon experi mental conditions. In the current work, a probabilistic model is prese nted that describes the competition between these two failure mechanis ms from a single experimental parameter, p. The model predicts the siz e distribution of defects which initiate LCF cracks, as well as the pr oportion of specimens which fail as a result of internally located def ects. Model predictions were found to correlate well with experimental LCF data obtained from a N18 PM Ni-base superalloy. The model further enables evaluation of a ''size effect'', as it predicts that internal crack initiation is favoured for larger components/specimens.