This paper will present a probabilistic approach developed in order to assess the fatigue life of aircraft engine parts (turbine disks) obtained by powder metallurgy technique. First of all, the main issues will be pointed out and the theoretical principles of the method will be described. Then the design implications and the experimental correlation will be emphasized. The scale effect is a major concern for the fatigue life assessment of a powder metallurgy part. It no longer allows the designer to evaluate the life of a massive part directly from experimental results based on small specimen fatigue tests as is done in the classical methodology. In order to describe this scale effect correctly, incubation sites (inhomogeneities like ceramic inclusions) must be characterized. The size of these inhomogeneities and their positions in the part appeared to be the most relevant parameters. Hence the methodology developed at SNECMA integrates the scale effect scatter through a binomial probability distribution as well as a temperature and stress-dependent life evaluation for each inhomogeneity size and position. The life calculation of a part implies an analysis of its whole volume and surface. An iterative process determines the number of cycles corresponding to a global reliability level requirement for the part. The complete methodology is then validated by comparing the calculated initiation distribution with experimental results on small specimens and test disks.

Issue Section:
Gas Turbines: Structures and Dynamics
Topics:
Aircraft engines, Fatigue life, Powder metallurgy, Disks, Ceramics, Cycles, Design, Electromagnetic scattering, Fatigue testing, Reliability, Statistical distributions, Stress, Temperature, Turbines
1.
Domas, P. A., 1992, “Substantiating Powder Metal Life Methodologies for Engines,” AGARD Report 790, “Impact of Materials Defects on Engine Structure Integrity,” pp. 11–1 to 11–8.
2.
Lautridou, J. C., de Bussac, A., and Soniak, F., 1993, “A Probabilistic Model for Fatigue Life Prediction of P.M. Ni Base Super Alloys Containing Inclusions,” Proceedings of the 5th Int. Conf. on Fatigue and Fatigue Thresholds, Montreal, Canada.
3.
McCartney, L. N., 1980, “Can Safety Factors Be Reduced Safely When Designing Against Fatigue?” Fatigue Engng. Mater, Struct. II, pp. 387–400.
4.
Pineau, A., 1990, “Super Alloys Discs Durability and Damage Tolerance in Relation to Inclusions,” in: High Temper. Mater. for Power Engng., E. Bachelet et al., eds., pp. 913–934.
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 by The American Society of Mechanical Engineers
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