Abstract
Alternating two-load level torsional fatigue tests were performed using 3.5 Ni-Cr-Mo-V steel to investigate the influence of overloads on the fatigue damage in the crack initiation and propagation stages. The base block consisted of strain cycling of Δγp = 2% as the overloads and constant stress cycling of τH = 260 MPa, higher than the fatigue limit, or τL = 220 MPa, lower than the fatigue limit, as the baseline loading. Constant stress and strain cycling were also performed. It was found that torsional fatigue cracks tended to branch off under constant stress cycling, but did not branch off under constant strain cycling. Overloads always reduced the crack initiation life and the fatigue life. In the torsional fatigue, small-size cracks were easily initiated, but they propagated slowly in comparison with push-pull fatigue.
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