Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 346 concentrations) and excluded press forming defects. A cyclically loaded airscrew shaft can exchange energy with the environment through its free spline surface; thereby, at a standard stress level, the PS experiences a surface damage that is more extensive than is the damage creative—after a longer running time—of a crack without the energy exchange (most likely under the surface of the most severely loaded zone). The other stress raising areas—at the pinhole edges or spline- material defects—are the sites of locally overstressed material, equally susceptible to cracking in their own locations zones. a) b) c) Figure 6: (a) and (b) the crack origin zones (general view) of the fracture surfaces of two different PS; cracks nucleated at a pinhole edge (pointed to by arrows A and at the fretting zones of splines (pointed to by arrows B; (c) magnified view a portion of fracture surface showing the mesoscopic lines of fatigue fracture (pointed out by several arrows). Owing to a torque effect, the crack growth direction turned upward (see at the bottom part the group of fatigue lines crossing the lines of the other group)—to the external surface of the PS. R ELATION OF THE N P /N F RATIO TO THE INSPECTION PERIODICITY OF OPERATING PS e mentioned above, that, , using a ratio N p /N f of 5 to 10% was the only way to calculate the crack growth duration in an PS as long as no data on the actual time to crack initiation were available. We applied such a ratio to the cases of cracks, initiated at various stress raisers—fretting zones, acute edges, and material defects—but never grown to their critical size. W