Abstract
Based upon a hypothesis of surface anisotropy of an otherwise isotropic material, a criterion of the yielding of a ductile metal is suggested which embraces the several well known criteria as special cases. This criterion for a system of principal stresses X, Y, Z (= 0) takes the form 
= constant; or 
= constant, which for component shear and direct stresses, q and f respectively, becomes
= constant, where f/q must fall in the range √2 to 2.
The proposed criterion is considered to be applicable in character to fatigue of a ductile metal, and it is applied to derive, from direct and shear stress fatigue test results, limiting fatigue stresses for any combination of shear stress and direct stress in phase, which is the most severe condition. Analysis indicates that the presence of static shear stress may increase the ability of a steel to withstand reversing direct stress. Apparently an opposite effect occurs when direct stress fatigue test results conform to the Goodman or modified Goodman diagrams.
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