Elastic Anisotropy Effect of Crystals on Polycrystal Fatigue Crack Initiation

Abstract

Fatigue bands have been observed in both monocrystalline and polycrystalline metals. Extrusions and intrusions at the free surface of fatigued specimens are favorable sites for fatigue crack nucleation. Previous studies (Lin and Ito, 1969; Lin, 1992) mainly concerned the fatigue crack initiation in aluminum and its alloys. The elastic anisotropy of individual crystals of these metals is insignificant and was accordingly neglected. However, the anisotropy of the elastic constants of some other metallic crystals, such as titanium and some intermetallic compounds, is not negligible. In this paper, the effect of crystal anisotropy is considered by using Eshelby’s equivalent inclusion method. The polycrystal analyzed is Ni3 Al intermetallic compound. The plastic shear strain distributions and the cumulative surface plastic strain in the fatigue band versus the number of loading cycles were calculated, and the effect of crystal anisotropy on the growth of the extrusions was examined.