Issue34

G. Lesiuk et alii, Frattura ed Integrità Strutturale, 34 (2015) 290-299; DOI: 10.3221/IGF-ESIS.34.31 295 and it was periodically corrected via moving microscope (with camera). The stress ratio R=0.1 was kept and testing frequency was established on the level 12.5 Hz. During the experiment, the force, displacement and crack opening displacement (COD) signals were registered. Two types of diagrams were constructed –on the bases of magnitude of the stress intensity factor  K and (according to the [7])  t – CTOD (crack tip opening displacement). The method for estimating the value of CTOD has been described in the work [7]. The CTOD was obtained by the formula:      2 y t K E . (2) Fatigue crack growth rate model for puddled steel From the experimental results it can be concluded that the most sensitive markers of degradation are the impact resistance toughness and low cycle fatigue properties. The relationship between impact resistance toughness and fatigue crack growth conditions with the structural degradation nature of puddled steels was demonstrated below by the following equation proposed by the authors:         3 ΚCV dα Α 5 5 35 = ΔΚ - ΔΚ th 3 2 dΝ Ε ΚCV σ pl (3) In the proposed model: A means non-dimensional constant (A=2457 for post-operating state, A=1871 for normalized state), KCV means the impact energy value (Charpy test value) in J/cm 2 , KCV 35 - required minimal energy value (35 J/cm 2 ) for the modern steel, E represents Young modulus in MPa,  pl – yield tensile strength in MPa. In several studies (i.e. [7, 8, 9]), the relationship between low-cycle fatigue properties and the kinetics of fatigue crack growth in metals has been proposed. The base of consideration is always the dissipated energy during the cyclic loading. According to Ramberg-Osgood equation and Coffin-Manson (1), the specific energy till fracture has been proposed in the work [9] as:                 1/ ' ' 4 ' ' ' ' 4 1 ' ' n f n Wc f f f n k (4)     4 ' ' 1 ' Wc f f n (5) where: Wc – specific energy till fracture in J/m 3 , n’ – cyclic strain hardening exponent, k’ – cyclic strength coefficient in MPa. According to the works [9, 10, 11, 12], the tensor components of the stress and the strain can be described as follows (under a small scale yielding assumption, HRR [10] singularity crack tip field):                         2 2 ' '/ (1 ') ' ( ; ') y ij ' ' y I ij n n n K n I r (6)                         2 2 ' 1/ (1 ') ' ' y p ( ; ') ij ' ' y I ij n n K n E I r , (7) where:    ( ; ') ij n - non-dimensional angular distribution functions of HRR stress singularities,