FATIGUE LIFE UNDER COMBINED STRESS.

One of the most common loading conditions in a machine is cyclic bending combined with cyclic or steady-state torsion. Under these conditions, the usual methods for determining fatigue life do not apply, and textbooks provide no guidance on how to approach the problem. The method described in this article fills that void by tying together the classical theories required to predict fatigue life under any combination of stresses. The procedure applies to any stress state - uniaxial, biaxial, and triaxial - and utilizes the simple S-N curve relationships for life prediction.