Issue 35

M.V. Bannikov et alii, Frattura ed Integrità Strutturale, 35 (2016) 50-56; DOI: 10.3221/IGF-ESIS.35.06 54 The influence of the window size (image resolution) on the value of Hurst exponent was investigated by analysing the fracture surface with different resolution from 2.5 μm and 0.1 μm per pixel respectively. Starting from the resolution 0.3 μm by 1 pixel and larger, the value of the Hurst exponent remains practically constant (fig.6). In that case all calculation was carried out on images with 0.5 microns per pixel resolution. Figure 6: Value of Hurst exponent versus resolution of image. The correlation function (3) calculated from the profiles for both zones has two linear portions with a break on the scale, which corresponds to change in the mechanisms of fracture surface topography. Whereas functions (2) calculated separately for zones 1 and 2 reveal only one linear portion for each zone (Fig. 7). The gigacycle fatigue cracks for titanium Grade 4 were originated near the surface (70-150 μm) and the crack hotspot generally could not be detected based on the optical image. The roughness pattern analyzed according to the New View data allows one to differentiate between the zones of crack origination (size ~ 100 μm) characterized by roughness invariance and the rest of the crack propagation roughness zone. Figure 7: Correlation function built on profiles: а) inside zone 1; b) inside zone 2. In samples of pure titanium Grade-4 under gigacycle fatigue regime of loading the crack was initiated at a depth of 70-150 µm below the surface. Whereas characteristic feature of the fracture surface of such images unlike the alloy Ti6Al4V is the lack of an optical image of any zone boundaries (Fig. 8,a). However, at the height map obtained using profilometer New View 5010 (Fig.8,b) we can observe the characteristic crack initiation zone, which roughness is different from the rest of the zone of crack propagation, its radius is about to 50 microns.