A PROBABILISTIC MODEL FOR PREDICTION OF LCF SURFACE CRACK INITIATION IN PM ALLOYS

The Low Cycle Fatigue (LCF) life of PM Ni-base superalloys is commonly reduced by surface crack initiation at ceramic inclusions. For this r eason, a probabilistic model has been developed that predicts the size of surface crack initiation sites from the inclusion size distributio n. For the experimental correlation of the model two sets of alloys we re examined: a ''standard'' (i.e. as-received) alloy, and a second mat erial of identical composition to which a known distribution of cerami c inclusions was incorporated (or seeded). Model predictions were foun d to be in excellent agreement with the results obtained from the seed ed materials in which the defect size distribution is larger and bette r characterized, and were satisfactory for the unseeded material in wh ich two types of surface defects (pores and ceramic inclusions) initia te LCF cracks. The results of these experiments were employed in LCF s imulations of both test specimens and full scale components. These ind icated that differences exist between the site preference for LCF crac k initiation in small test specimens and large scale components due to a scale effect. Such results demonstrate the utility of seeding exper iments for generation of LCF test data used in component design.