Issue34

G. Lesiuk et alii, Frattura ed Integrità Strutturale, 34 (2015) 290-299; DOI: 10.3221/IGF-ESIS.34.31 294  t /2 – total strain amplitude,  f ’ - fatigue ductility coeficients,  f ’ – cyclic fatigue strength coeficient in MPa, 2N f – number of reversals to failure, c – fatigue ductility exponent, b – fatigue strength exponent. For the puddled steel in the post-operating steel  f ’=0.0169,  f ’=442 MPa, and exponents c=-0.3632 and b=-0.03651. For steel after heat treatment, these parameters were following;  f =0.01626,  f =378 MPa, c=-0.3165, b=-0.02702. The obtained results show much worse cyclical properties of the puddled steel in the after-operating state in comparison to the normalized state. The significant change with the transition number of cycled 2N t (marked dashed line in Fig. 5). The typical hysteresis loops (  =0.4%) for as-received state (P) and after normalisation state (N) have been shown in Fig. 6. It can be noted the higher ductility of the material for the normalized state. The yield point in the first cycles of the loading is noticeable (Fig. 6b). It is probably caused by the presence of degradation processes. Figure 5: ( a) Coffin-Manson curve for puddled steel in post-operating state, (b) Coffin-Manson curve for puddled steel in normalized state for puddled steel [6]. Figure 6 : Hysteresis loops (  =0.4%), a) for as-received state (P) and b) after normalization state (N). F ATIGUE CRACK GROWTH RATE AND ANALYTICAL DESCRIPTION n order to obtain the kinetic fatigue fracture diagram (KFFD), the CT specimens were tested (in accordance with the American Standard ASTME 647). The characteristic dimensions of the specimen are following; W = 48 mm and thickness t = 10-15 mm, a/W=0.275. The fatigue crack length was measured with the compliance variation method I