Variation of transitional functions in multiscale fatigue crack growth of superalloys

Abstract Multiscale fatigue crack growth model (MFCGM) can provide better explanation for the multiscale effects on the process of superalloy fatigue. Transitional functions that account for material, loading and geometry effects are incorporated in the MFCGM and play vital role in the proposed model. They are the reflection of the combined microscopic and macroscopic effects. Though validity of transitional functions has been proved in former research, there are still a few critical issues to be clarified. The biggest concern is the assumption of increasing or decreasing trend of transitional curves. It is undeniable that the general trends of these curves agree with the physical phenomena of material degradation, loading restriction and size effects. However, coefficients in transitional functions remain to be specified. Under current circumstance, it is almost impossible to identify these scale parameters. Nevertheless, it is still feasible to employ appropriate coefficients and vary each of the three transitional functions. Variation of transitional functions correspondingly leads to the change of fatigue life of superalloys. Experimental fatigue data of 2024-T3 Al sheets is adopted to modify transitional functions related to material, loading and geometry. Thus the most reasonable and appropriate set of transitional functions are determined. It is found that transitional coefficients ζ η and λ play different role in fatigue crack growth behaviors. Particularly emphasized is the geometric coefficient λ. A set of transitional coefficients are determined such that fatigue crack growth of 2024-T3 Al sheets can be best described and explained. The proposed approach potentially offers the possible reference for engineering safety designs.

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