A Cumulative Damage Rule Based on the Knee Point of the S-N Curve

Making use of a set of isodamage lines, a fresh concept for predicting the fatigue life, under variable amplitude loading, is presented. It is assumed that all isodamage lines converge at the knee point of a S-log N diagram. The life predictions based on this concept are closer to the experimental data than given by Miner’s linear damage rule. The concept is verified with the step-test data obtained on unnotched C-35 steel specimens in rotary bending. Several other reported experimental data in rotary bending tests also confirm the reliability of this simple damage rule.