Issue 49

O. Naimark, Frattura ed Integrità Strutturale, 49 (2019) 272-281; DOI: 10.3221/IGF-ESIS.49.27 279 case is associated with the H L length. This analysis allows the natural conclusion that variety of crack paths and characteristic lengths are related to duality of singularities caused by asymptotic self-similar solution for stress distribution at the crack tip and intermediate singular (blow-up) damage kinetics in the process zone. Mentioned scenario can be illustrated by the morphological images of crack nucleation and propagation in the conditions of Very High Cycle fatigue, Fig.5 [29]. Figure 5. Fracture surface of Ti6Al4V alloy in Very High Cycle fatigue regime with the images of characteristic areas of fracture surface [30]. Different mechanisms related to above mentioned scenario were supported studing the scaling (spatial invariance) properties of characteristic area of fracture surface rughness. The surface roughness was analysed by nterferometer- profiler New View 5010 to establish quantitative characteristics of the fracture surface in terms of the scaling Hurst exponent. Three distinct zones with strongly different exponent were observed (Fig. 5): the area 1 of the size ~ 100-300 μm corresponds to the zone of damage localization (associated with c L ); the area 2 representing the image of crack advance in the presence of two singularities, given by the solutions (4) and (8), with the following re-subjection of crack kinetics (area 3) to the stress intensity fator. D ISCUSSION he existence of two singularities related to the stress field at the crack tip and blow-up kinetics of damage localization is considered as the physical basis for the interpretation of the Theory of Critical Distances. The free energy metastability of solid with defects and corresponding free energy release explain the conception of the Finite Fracture Mechanics in the presence of the finite amplitude energy barrier. The variety of crack paths is analyzed as duality of inherently linked two types of singularities related to the singularity of multiscale damage kinetics under crack nucleation and singularity of stress field at the crack tip as the classical framework of fracture mechanics. The singularity of multiscale damage kinetics is a natural precursor of crack nucleation that could provide in some cases the totally independent scenario of fracture from the stress singularity at the crack tips. The influence of two singularities with the nature of intermediate asymptotical solutions for stress at the crack tip and damage localization kinetics over the set of spatial scales represents two attractors, which provides the variety of crack paths for corresponding loading conditions and the existence of characteristic scales that could be associated with the critical distance lengths. T