Issue34

G. Lesiuk et alii, Frattura ed Integrità Strutturale, 34 (2015) 290-299; DOI: 10.3221/IGF-ESIS.34.31 296    ( ; ') ij n - non-dimensional angular distribution functions of HRR strain singularities,  y ’ – cyclic yield stress in MPa, (  y ’ ≈2  y ’), I n ’ – non-dimensional exponent of n’,  - radial coordinate ahead of a crack tip,  - angular coordinate ahead of a crack tip. The cyclic plastic zone (for mode I loading) with Huber-Mises-Hencky (HMH) yielding criterion for a plane stress can be expressed (according to the [9, 10, 11, 12]) as:                    2 2 2 3 ( ) 1 sin cos 2 ' 8 (1 ') y I K n (8) In this case, the total plastic energy dissipated in a cyclic plastic zone  p (according to [9]) can be expressed as:              2 1 2 ' A 1 ' 1 ' ' 8 (1 ') y I p n K n n n EI (9) where:                     2 2 1 0 3 A 1 sin cos ( , ') ( , ') . 2 eq eq n n d (10) Fatigue crack will grow if the specific energy values  p ’ (the specific fracture energy referred to the unit of area (J/m) is reached. The authors [9] have been described the fatigue crack growth rate as:   ' W p da dN c , (11) using the formula (1) or (2) [9] we can obtain:         1 max ' A (1 ') 2 ' ' 7 th n n da K K dN I E f f . (12) In addition to the cyclical properties, the critical value of the stress intensity factor K fc and its variability (degradation) depending on the degree of microstructural degradation processes should be also considered. In the literature, K fc is often replaced by a critical stress intensity factor K c (for mode I loading). Therefore, after reduction and assumption that:     ' 1 , , A n f const I , (13) a new kinetic equation of fatigue crack growth rate for puddled steel is proposed:                     max 4 (1 ') 2 1 ' ' th c n da K K K dN K K E f f . (14) The fracture toughness of investigated S-steel was estimated using J-integral as a critical value of fracture toughness K c converted from J-integral using formula: