Fatigue Life Estimation of Unnotched Carbon Steel Specimens under Combined Bending and Torsion

Abstract

As a method to estimate the fatigue life of unnotched ductile specimens under combined bending and torsion, an ellipse quadrant expressed by the following equation was proposed.
(σ/σ_N)²+(τ/τ_N)²=1
where semi-axes σ_N and τ_N are bending and torsional stresses under which failure would occur at N cycles. It was assumed that any combined stress (σ, τ) satisfying the above equation would induce failure at the same N cycles.
Fatigue tests of unnotched steel specimens under monofrequency in-phase combined bending and torsion were carried out, and a good correspondence was found between the experimental fatigue lives and those calculated by the ellipse equation.