Issue34

G. Lesiuk et alii, Frattura ed Integrità Strutturale, 34 (2015) 290-299; DOI: 10.3221/IGF-ESIS.34.31 297       2 1 I I E J K . (15) For investigated steel, K c was estimated at the level of K c =134,4 MPa√m for as-received state, and for normalized state, fracture toughness is close to K c =147 MPa√m. Based on experimental results, the constant  was estimated at the level of  = 3. The different values of the  for the post-operating and normalized states is postulated. This fact can be explained by the dependence (10) on the A 1 value, so the plastic deformation ahead of the crack tip. In connection with microstructural degradation processes and dislocation motion blocking system, the lower plastic deformation ahead of the crack tip in the post-operating state (in accordance with the results of the LCF) should be expected. The assumption (13) needs further investigation for different type of the puddled steel, on the other hand, the experimental dependence provides the constant value  . The experimental data for S – steel in post-operating and normalized states with the prediction in the near-threshold regime fitting using model (3) and (14) have been presented in Fig. 7. For the steels B and W the experimental data were not so comprehensive – particularly in low-cycle fatigue investigation. Therefore, in case of B and W steels, the fatigue fracture diagrams have been constructed using a method described in [7], where the force, deformation and energy approach have been discussed . All the results with exponent model data fitting are shown in Fig. 8. It is noted, that for the puddled steel the typical experimental data scatter. The differences in the kinetics of fatigue fracture are most reflected in the near-threshold regime. It should be underlined that the fatigue crack growth rate curves for W-steel are prepared just from 3 specimens – due to the material limitations. Figure 7 : Fatigue crack growth rate for puddled steel (S) in post-operating state and normalized state, experimental data with proposed models fitting. CONCLUSIONS he paper presents the investigation results of microstructural and mechanical properties for an old steel from the 19 th century. All tested materials were identified for the puddled steel – commonly used at the turn of the 19 th and 20 th century. In all materials, the microstructural degradation processes were also identified. These processes mainly consist in precipitation of carbides and nitrides inside ferrite grains, precipitation of carbides at ferrite grain boundaries and degeneration of pearlite areas. The most sensitive parameters (in terms of microstructural degradation processes) estimated in simply engineering test is the impact resistance value. The degradation processes progress can be T