Issue 35

M.V. Bannikov et alii, Frattura ed Integrità Strutturale, 35 (2016) 50-56; DOI: 10.3221/IGF-ESIS.35.06 52 life of titanium alloy Ti6Al4V in gigacycle regime corresponds to the data of Bathias [5]. The dependence of material microstructure on its fatigue strength during gigacycle fatigue agrees with data observed in [6]. The SMC-1 titanium Ti- Grade 4 with equilibrium grain boundaries exhibits highest fatigue properties compared to the SMC-2 state with non- equilibrium grain boundaries and to the polycrystalline state (grain size of about 25 μm). Figure 3: Fatigue curve data for investigated materials: σ – applied mean stress, Nf –number of cycles to failure. 1 – Ti6Al4V Bathias [6]; 2 – Ti6Al4V; 3 – Ti Grade-4 initial state; 4 – Ti Grade-4 SMC-1 state; 5 – Ti Grade-4 SMC-2 state. Mechanisms of initiation and propagation of fatigue cracks were investigated by means of qualitative and quantitative analysis of the morphology of fracture surfaces. The results of observation reported in [7] show that during stress cycles several fine subgrains having different crystal orientations are formed in a thin layer (thickness is 400 nm) around non- metallic inclusion. The following mechanism of crack initiation under long cyclic loading was proposed: a fine granular layer caused by the intensive polygonization is gradually formed around the interior inclusion. The number of microdamage centers in this layer gradually increases and some of them coalesce. When damage spread over the entire fine granular layer the crack is finally formed around the interior inclusion. After the crack has grown to a critical size, it propagates in accordance with the Paris law kinetics:     m da C K dN , (1) where da/dN is the fatigue crack growth rate, C and m are empiric constants depending on the material, K is the stress intensity factor. Qualitative analysis was carried out by using optical and electron microscopy of the surface morphology. Q UALITY AND QUANTITATIVE ANALYSIS OF FRACTURE SURFACES estruction in gigacycle fatigue regime usually forms a characteristic type of fracture surface - "fish-eye"[6-7]. Qualitative analysis by optical microscope allows us to separate zones with different reflectivity. First zone with a radius of 150 µm from the source of crack is very dark, then light and smooth area is followed, which is then replaced by darker area (fig.4). By substituting the radius of the borders in the formula (2) for the stress intensity factor of radial inner cracks    2 / [ / (2 )] K a F D a , (2) where F is normalization function [8], D is diameter of the sample, these zones can be associated with the stages of crack nucleation and propagation. Radius a = 150 microns corresponds to the threshold value of the stress intensity factor ΔK th for this material at which the crack begins to grow. In the area between borders of 1 and 2, the crack grows steadily by D