Issue34

V. Oborin et alii, Frattura ed Integrità Strutturale, 34 (2015) 422-426; DOI: 10.3221/IGF-ESIS.34.47 423 the understanding of fundamental aspects of multi-scale damage evolution in large range of load intensity for the prediction of the kinetics of nucleation and propagation of cracks in the damaged material with the purpose to optimize the material structure and finding the materials with low sensitivity to random dynamic loads under high- and gigacycle fatigue conditions. In this paper, aluminum-magnesium alloy (AlMg6) samples (Fig. 1) are tested. K INETIC EQUATION FOR FATIGUE - CRACK GROWTH he universal character of kinetic law establishing a relationship between the growth rate dl/dN of a fatigue cracks and a change in the stress intensity coefficient ΔK has been extensively studied both experimentally and theoretically. The power laws originally established by Paris [2] (and presently referred to as the Paris law) reflect the self-similar nature of fatigue crack kinetics. This law is related to a nonlinear character of damage evolution in the vicinity of the crack tip (called the “process zone”):   m dl A K dN   (1) where A and m are the experimentally determined constants. For a broad class of materials and wide range of crack propagation velocities under high cycle fatigue conditions, the exponent is typically close to m = 2–4. The self-similar aspects of the fatigue crack growth were studied by Barenblatt, Ritchie [3,4] using the assumption concerning intermediate self-similarity of fatigue crack kinetics to introduce the following variables for the representation of the crack growth rate a = dl/dN (where l is the crack length and N is the number of cycles): a 1 = ΔK is the stress intensity factor; a 2 = E is the Young modulus; a 3 = l sc is the scale related to the correlated behavior in the ensemble of defects on the scale a 4 = L pz associated with the process zone. 3D New View roughness data within the crack process zone (Fig.2) supported the existence of mentioned characteristic scales: the scale of process zone L pz and correlation length l s c that is the scale when correlated behavior of defect induced roughness has started. Using the Π-theorem and taking into account the dimensions of variables [dl/dN] = L , [ΔK]=FL –3/2 , [l sc ]=[L pz ] = L , and [E] = FL –2 , the kinetic equation for the crack growth: Ф( , , , ) sc pz dl dN K E l L   , (2) can be written as: 1 , pz sc sc sc L dl K dN l l E l           . (3) Estimation of the values ( ) 1 sc K E l   and / 1 pz sc L l  allowed one to suggest an intermediate-asymptotic character of the crack growth kinetics for Eq. (3) in the following form: sc pz sc L d l K dN l E l                , (4) where / sc l l l   . Introducing the parameter   / pz sc C L l   , we can reduce the scaling relation (4) to the following form analogous to the Paris law: C sc d l K dN E l          , (5) T