Abstract
Progress in instrumentation in fatigue studies in the range 107 - 109 cycles revealed new paradigm: crack growth is not a main portion of life and initiation mechanisms are of primary importance with internal fatigue crack origin. Characteristic features of fatigue in this regime is strong evidence that Wohler-type S–N or Coffin–Manson diagrams can exhibit a second lower fatigue limit - multi-stage (duplex) fatigue life diagrams. An important finding in connection with these observations was that, in the transition from HCF to gigacycle regimes, the origins of fatigue failure changed from surface to interior “fish-eye” fracture. This experimental and theoretical study is focused on fatigue crack initiation and early slow crack growth as life-controlling factors of gigacycle fatigue. Nonlocal model of damage kinetics based on the description of collective behaviour of defects (slip bands, microshears, microcracks) allowed the interpretation of fatigue damage-failure transition as specific type of criticality in out-of-equilibrium system “solid with defects”.
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