Effect of Yield Phenomenon on Fatigue Damage in Commercially Pure Iron Thin Wires

概要

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The effect of the specimen size on tensile and fatigue properties was studied by using thin wires with the grain size of 22μm and the diameter, Dmin, of 800μm, 700μm and 500μm (these are named Specimen-800, 700 and 500, respectivly). For tension test results, the yield strength (proof stress) strongly depended on the specimen diameter. All Specimen-800 and Specimen-700 and a part of Specimen-500 show the clear yield phenomena. However, most of Specimen-500 did not show yielding. For all specimens fractured under cyclic loading, the necking was occurred at the final stage of the fatigue process, and the initiation and the propagation of fatigue cracks were not observed. Two kinds of failure processes were observed. One was ratcheting failure and the other was delayed yielding failure. After the delayed yielding, specimens showed the process similar to ratcheting failure. It was concluded that these size effect are caused by the difference in the number of grains at the minimum cross section. Regardless of diameters of specimens, the scatter in fatigue life was very large compared with that of bulk specimen. It was concluded that the main cause of the large scatter in fatigue life was attributed to the scatter in the number of cycles to start ratcheting deformation (delayed yielding).