PSI - Issue 41

Aleksandr Inozemtsev et al. / Procedia Structural Integrity 41 (2022) 510–517 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

514

5

Fig. 4. The S-N curve in the case of dynamic preloading and in the absence of preloading.

For approximation , the formula was used: σ α =

or σ = С α

(1)

The values of the constants were determined by minimizing the difference between the experimental data after preloading and approximation. C = 5744 MPa, α = 0.206. The obtained dependence approximates experimental data well in the range from 10 7 to 10 8 cycles before destruction. The roughness of fracture surfaces was measured by a high-resolution New-View 5010 interferometer-profiler (providing x2000 magnification) and then analysed based on the assumption of fractal geometry of the fracture surface profile, which is due to a correlated behaviour of multiscale defect structures within the process zone pz L preceding the crack growth. The result of fatigue tests is the fractured specimens of two types. The specimens broken during the fatigue test are of the first type. The specimens of the second type demonstrated a pronounced variation in the resonance frequency associated with the fatigue crack nucleation. The fracture surfaces of the specimens of the first and second type were uncovered by cooling with liquid nitrogen and breaking them at the narrowest section. It is assumed that the fracture surface of the fatigue test specimens under conditions of gigacycle loading has already been formed during the fatigue tests and occupies the largest part of the fracture surface, which is determined by a change in the resonance frequency. In cylindrical specimens under high cycle loading in the range of 10 6 -10 7 the fatigue crack is usually initiated at the surface (Fig. 5a). In the specimens subject to cyclic loads in the range exceeding 10 8 cycles, the crack formation starts from inside of the specimen. The fracture surface in this case displays a fatigue zone known as a “fish - eye”, which is a distinguishing feature of such fatigue regimes. The New-View scanning of the zones of fatigue crack growth (Fig. 5) allows us to obtain a series of one dimensional profiles of the surface relief in the radial direction. Within each “window”, about 12 one -dimensional profiles (Fig. 6a) were analyzed to ensure representativeness of the data along the defect-induced relief with vertical resolution of 0.1 nm and horizontal resolution of ~ 0.5 µm.