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The lifetime to initiate an engineering crack in a component is usually predicted
from Coffin-Manson curves using smooth specimens for low-cycle fatigue.
However this procedure has to be examined carefully when cracks initiate from
defects, such as inclusions or pores that are inherited from the manufacturing
process.
A special methodology has been developed for defects occurring in turbine discs
made from powder metallurgy nickel base superalloys [1-3]. A probabilistic life
prediction model was so built combining measurements of the distribution of
defect size and a simple fracture mechanics approach to assess small crack
growth, as done by Murakami [4] for high cycle fatigue. A key issue is to know
whether defects should be considered as cracks of equivalent area. The
investigation carried out in [1] has shown that this approximation was effective in
describing fatigue life under small scale yielding.