Difference between High- and Low-Stress Fatigue

Abstract

RECENT papers by Porter and Levy¹ and Benham and Ford² have suggested that the conventional S/N curve is composed of two separate curves, a discontinuity occurring at some particular stress-level. The former authors concluded from a large number of rotating bending tests on copper that the discontinuity occurred at about ± 21,500 lb./sq. in.; for lower stresses fatigue cracks originating in slip bands; for higher stresses cracks developing from L-shaped nuclei. Kemsley³ also found that with copper specimens tested at ± 15,000 lb./sq. in. (giving fracture after long endurances) cracks formed in slip bands and were transcrystalline, while at ± 25,000 lb./sq. in. (giving fracture with low endurances) slip bands were virtually absent from the specimen and cracks appeared to be inter crystalline. Benham and Ford², who tested mild steel in direct stress, found for large stress amplitudes that the specimen gauge-length extended permanently each cycle and a ‘necked-out’ fracture resulted. They termed this phenomenon ‘cyclic creep’ and such failures occurred while the stresses corresponded to the upper portion of the composite S/N diagram. For stresses in the lower portion of the S/N diagram typical fatigue fractures occurred and they suggested that, if the fatigue mechanism could be suppressed as the cyclic stress decreased, the upper curve might flatten out to a ‘cyclic creep limit’.