This is owing to the elongated grain structure in the direction of the original cold working. Anisotropy caused by cold working gives increased S– N fatigue resistance when loaded in the direction of working compared to that when loaded in the transverse direction. Fine grains reduce localized strains along slip bands reducing the amount of irreversible slip and provide more grain boundaries to aid in transcrystalline crack arrest and deflection, and thus reduce fatigue crack growth rates. Fine grain size generally provides better S– N fatigue resistance than coarser grains except at elevated temperatures where creep/fatigue interaction exists. Some generalities can be formulated for these microstructural aspects. This includes chemistry, heat treatment, cold working, grain size, anisotropy, inclusions, voids/porosity, delaminations, and other discontinuities or imperfections. Metal fatigue is significantly influenced by microstructure. Stephens, in Encyclopedia of Materials: Science and Technology, 2001 9 Microstructure Effects
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