FCP Report No. 14

A Micro-mechanistic Life Prediction Model Suitable for Isothermal and Thermo-mechanical Fatigue


Richard W. Neu


A life prediction model is developed for crack nucleation and early crack growth based on fatigue, environment (oxidation), and creep damage. The model handles arbitrary strain-temperature phasings (i.e., in-phase and out-of-phase thermo-mechanical fatigue, isothermal fatigue, and others including non-proportional phasings). It works for a broad range of temperatures, strain rates, and strain ranges. An oxide growth model is presented which describes oxide growth as a repeated micro-rupture process. Using this model, an oxidation damage expression is derived with constants directly linked to micro-mechanistic measurements in oxide growth. A creep damage expression, which is stress based, is coupled with a unified constitutive equation. A set of interrupted tests were performed to provide valuable damage progression information. Tests are performed in a helium atmosphere to isolate creep damage from oxidation damage. Fatigue life predictions compare favorably with experiments in 1070 steel for a wide range of test conditions and strain-temperature phasings.

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