Results of very high cycle fatigue tests on helical compression springs

This report presents first results of very high cycle fatigue tests on helical compression springs which respond to external compressive forces with torsional stresses. The results of these investigation can add an important contribution to the experience of fatigue behaviour in the very high cycle regime. Most investigations performed on that field deal with specimens under tensile or rotating bending load. The springs tested were manufactured of Si–Cr-alloyed valve spring wire with a wire diameter between 2 and 5 mm, shot-peened and preset. Compared to the fatigue limits evaluated in fatigue tests on these springs up to 107 cycles substantial decreases in fatigue strength are to be observed if the fatigue tests are continued up to 108 cycles or even more. It is obvious that nucleations of fractures tend to occur below the surface, if fractures happen after more than 107 cycles. Investigations of broken springs by scanning electron microscope show a typical appearance of fracture initiation sites without non-metallic inclusions at the nucleations of fracture.