The essential feature of “fatigue” failure in metals is a spreading crack. Studies of fatigue failure in single crystals of metal indicate that such cracks frequently start by slip along planes of atomic weakness. The study of fatigue failure of metals has shown certain limitations of the mathematical theory of elasticity when applied to actual non-homogeneous metals. The importance of the ability of a metal to resist occasional slight plastic action without starting a fatigue crack is emphasized. The relation between atomic strength and the strength of a multi-grained piece of metal is briefly discussed and a “crack-interference” hypothesis of fatigue failure suggested. The phenomenon of spreading cracks in metal under steady load at elevated temperatures is noted, and its significance discussed.

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