Lifetime modelling of fatigue crack initiation from casting defects

Abstract : 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.