Crack path for run-out specimens in fatigue tests: is it belonging to high- or very-high-cycle fatigue regime?

Fatigue tests run-out specimens up to 106 – 5x107 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.