Fatigue crack initiation and growth on an extruded titanium alloy in gigacycle regime: comparison between tension and torsion loadings

This paper is focused on the analysis of fatigue crack initiation and growth mechanisms in defect free VT3-1 titanium alloy (similar to Ti6Al4V) in VHCF regime under tensile and torsion loadings. Fully reversed fatigue tests were carried out between 107 and 109 cycles at 20 kHz under constant amplitude loadings (no pulse-pause). SEM observations of the specimens fracture surfaces were carried out in order to compare the crack initiation mechanisms and the different crack growth stages under different loadings. It has been shown that subsurface crack initiation may appear under tension (as usual in gigacycle regime) but in our experiments no inclusion was observed in the “fish-eye”. Furthermore, subsurface crack initiation was observed under torsion loading too, despite the maximum shear stress location at the specimen surface. Under this loading, a “fish-eye” was observed too, but again without any inclusion in its center. The suspected microstructural reasons responsible for subsurface crack initiations under torsion loading could not be observed due to significant destruction of the fracture patterns (crack lips friction under mixed mode). The results of the fatigue tests show a principal difference in crack initiation and early crack growth stage between tensile and torsion loadings. Torsion crack initiates on a plane of maximum shear stress (like in HCF regime) while tensile crack is on a plane of maximum normal stress. Sequences of changes in fracture surface roughness is the same for tensile and torsion loadings