Fatigue of Small Helical Extension Springs Subjected to Static Torsion and Alternating Lateral Deflection

Multiaxial fatigue studies of peened helical extension springs made of music wire were performed in a specially designed testing machine. The test regime imposed static torsion and alternating laterial deflection, with the spring ends held parallel. The maximum stresses, which were primarily due to bending, occurred in the first turn out of the specimen grip at an end of the coil axis normal to the direction of lateral motion. The stresses in the middle turn ranged between 88-94 percent of those in the ends turns. The practical fatigue limit was about 100,000-300,000 cycles, with the principal tensile stress averaging 50 percent of initial wire tensile strength and maximum shear stress averaging 28 percent of initial wire tensile strength. For static stresses up to 32 percent of tensile strength, the alternating stress at the fatigue limit was essentially invariant, as predicted by Sines’ criterion for multiaxial fatigue.