PSI - Issue 39

Ilia Nikitin et al. / Procedia Structural Integrity 39 (2022) 599–607 Author name / Structural Integrity Procedia 00 (2019) 000–000

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a clear X-type crack was observed, but single 45 degree orientated main crack is more common for VHCF torsion.

Fig. 4. Lateral surface of extruded (left) and forged (right) titanium alloy specimens for torsion tests.

The analysis of the fracture surfaces shows two types of crack initiation under pure torsion loading (fig. 5): (1) surface and (2) subsurface crack nucleation. The subsurface crack initiation is not usual for torsion loading because normally the maximum stress is reaching at the specimen surface. The subsurface crack initiation under torsion loading is due to some sorts of microstructural inhomogeneities. The distance of subsurface crack initiation site from the specimen lateral surface is vary from 200 to 700 micrometers. The fracture surface near to the crack nucleation site is usually destroyed by fretting. In both cases the fracture surface is quite developed and shows several cracks growing in mutually perpendicular planes.

Fig. 5. The fracture surface with internal (left) and surface (right) crack initiation under pure VHCF torsion load [10].

The details of initial stage of torsion crack growth are shown on the figure 6. The crack is initiated due to shear stress amplitude provided the Mode II crack opening. At the initial stage the shear motion is important while the normal opening is not significant. As a results, a numerous of spherical particles are forming under the fretting conditions.