Issue 35

A. Nikitin et alii, Frattura ed Integrità Strutturale, 35 (2016) 213-222; DOI: 10.3221/IGF-ESIS.35.25 220 10 Hz. The authors also observed a surface crack along the specimen’s axis which reached a length of about 700 µm in maximum shear stress plane before bifurcation onto the maximum normal stress plane. (a) (b) Figure 8 : Surface crack initiation in forged VT3-1 under torsion loading (a) overview of torsion crack with initiation site marked by arrow and (b) detailed crack initiation view. The microstructure of TiA6V titanium alloy [18] is globular and coarse that is different from present Ti-6Al-4Mo titanium alloy microstructure. Thus, it is difficult to distinguish the role of microstructure and loading frequency on duration of crack growth in shear plane, but it is clear, that this growth stage should be limited by the material capability to accumulate plastic deformation. It has been shown that in the case of HCF a torsion crack first growth in depth of material and after starts to branch at the surface without growing in depth [18]. In the present work on VT3-1 titanium alloy crack tip position versus number of cycles was not monitored and crack shape was reconstructed based on the fracture surface pattern. In the case of surface crack initiation a sort of crack branching threshold or critical crack length was found, Fig.9. It seems that at a certain crack length internal crack branching is also possible in the case of VHCF loading that is different from HCF. A slight difference in the fracture surface color in zones before and after branching is due to different roughness that is usually associated with difference in stress intensity factors or crack growth rate. (a) (b) Figure 9 : Fracture surfaces of specimens (a) S=; Nf= (b) S=; Nf=; with clear border of fatigue crack branching. Unfortunately, a reconstruction of the very first stage of torsion crack growth based on fracture pattern is problematic due to destruction of the fracture surface near the initiation site by friction between crack lips during Mode II stage. Finally, a surface torsion crack in VHCF is similar to HCF growth i.e. it initiates on the maximum shear stress plane and