Biaxial stress fatigue life prediction by the local strain method

The prediction of fatigue crack initiation life under biaxial stress (or under plane strain) conditions is studied by the local strain method in this paper. The cyclic stress-strain equation as applied to the first principal direction under biaxial stresses is that derived from the uniaxial cyclic stress-strain equation based on theories of elasticity and plasticity. The elastic component of the first principal stress-strain equation is obtained from the generalized Hooke's law modification, and the plastic component is obtained from the Ilyushin plastic deformation theory modification. The equation is succinct in expression and easy in application. Moreover, it is shown that the first principal stress/strain should be converted to an equivalent uniaxial stress/strain when damage calculations are carried out using Mansion-Coffin type equations. This method of biaxial stress fatigue life prediction is demonstrated via a specimen test and an engineering application.