Issue34

A. Shanyavskiy, Frattura ed Integrità Strutturale, 34 (2015) 199-207; DOI: 10.3221/IGF-ESIS.34.21 199 Focussed on Crack Paths Crack path for run-out specimens in fatigue tests: is it belonging to high- or very-high-cycle fatigue regime? A. Shanyavskiy State Center for Civil Aviation Flight Safety, Airport Sheremetievo-1, PO Box 54, Moscow region, Chimkinskiy State, 141426, Russia shananta@mailfrom.ru A BSTRACT . Fatigue tests run-out specimens up to 10 6 – 5x10 7 load cycles are used to determine the stress level named “fatigue limit”. Nevertheless, it is not clear what kind of fatigue cracking takes or will take place in these specimens. To discuss this problem, fatigue tests of titanium alloy VT3-1 specimens have been performed under tension with different values of R-ratio and under rotating-bending after various thermo-mechanical treatments (tempering, surface hardening and their combinations). Well-known S-N curves in High-Cycle- Fatigue regime have been plotted with run-out specimens usually used for “fatigue limit” determination. Then, after fatigue tests, run-out specimens have been tensed up to their failure, and fracture surface analyses have been performed for all tested specimens. It is found that run-out specimens in all combinations of treatments, for different R-ratio, have fracture surfaces for crack path in Very-High-Cycle-Fatigue regime. Based on this result, all S-N curves have been reconstructed in duplex curves for High- and Very-High-Cycle-Fatigue regime without using knowledge about “fatigue limit”. Detailed fracture surfaces analyses have been developed, and crack paths have been compared for various combinations of materials and surface states. K EYWORDS . Fatigue limit; Run-out specimens; Fractography; Subsurface crack path; Duplex S-N curves. I NTRODUCTION etals fatigue occurs in the Low- and High-Cycle-Fatigue (HCF) regime with crack origination on the surface. In the case of HCF, first of all sliding in different grains within the surface layer can be noted and, then, on the second stage after critical stress-state reached by the one of the sliding plane, volume rotation occurs that directs to free surface creation [1-3]. The same mechanism plays a dominant role in the Low-Cycle-Fatigue (LCF) regime, not only for crack origination. Metal in these two regimes has such a behavior as an opened synergetical system, and the surface layer accumulates most of all energy which goes in a metal volume during cyclic loading. As is well-known, fatigue tests are performed up to stress level lim  , with many specimens have not failure at durability 10 7 – 10 8 load cycles, and this stress level (for not failed specimens, named “run-out”) is named “fatigue limit”. Nevertheless, for lower stress levels and long lifetime (more than 10 8 load cycles), fatigue cracking exists for different materials, and area of crack origination appears at subsurface [4, 5]. Very-High-Cycle-Fatigue (VHCF) regime for metals has been discovered in tests with high frequency. Between VHCF and HCF regimes, a transition region exists, where fatigue crack origination on- and subsurface occurs with different probability under the same stress level. On the border of HCF area, probability of crack origination on the surface is approximately 100%, whereas probability of subsurface crack origination is equal to about zero. M