Self-similar aspects of fracture of metals in gigacycle fatigue loading

M. Bannikov1, V. Oborin1, O. Naimark1*
1ICMM UB RAS, 614013, Russia, Perm, street Academ. Koroleva, 1
Mechanisms of destruction of aviation motors advanced materials (aluminum alloy AMg6, pure titanium Grade-4 and titanium alloy TiAl6V4) are studied in terms of high- and gigacycle fatigue, including the influence of preliminary dynamic loading on the fatigue life. The theoretical understanding of the kinetics of fatigue crack growth in gigacycle loading conditions on the basis of incomplete self-similarity was shown. Samples of titanium grade Grade-4 were studied in three states: Original (average grain size of 25 um) and two submicrocrystalline (average grain size 150-200 nm) (UMP-1) and (UMP-2), obtained by equal-channel angular pressing in various conditions. Fatigue tests were carried out on Shimadzu USF-2000 ultrasonic fatigue testing machine in gigacycle fatigue regime and servohydraulic test machine Biss bi-00-100 in high cycle fatigue regime. Prestraining of AMg6 alloy was carried out on a split Hopkinson (Kolsky) bar. The increase of the fatigue strength of titanium alloys with decreasing grain size and significant decreasing in fatigue strength in gigacycle fatigue regime for preliminary deformed specimens for aluminum alloy were observed. Stages of initiation and propagation of fatigue cracks in the high-and gigacycle fatigue regimes were analyzed by quantitative profilometry of fracture surface. The description of the transition from particulate to the macroscopic destruction, including the initiation and growth of fatigue cracks is the basis for assessing the resource of materials in terms of fatigue. The scale invariance of the fracture surface morphology which allowed explaining the self-similar nature of crack growth under high- and gigacycle fatigue was established.
Received: 03 June 2015   Revised: 05 October 2015   Accepted: 10 November 2015
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