13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- stresses and, considering fatigue crack propagation, stress intensity factor range (e.g. ΔK = Kmax-Kmin) is the main parameter used to characterize the stress conditions at the crack tip. Both K and ΔK usefulness is confirmed only considering an homogeneous and linear-elastic body: obviously, a crack tip plastic zone is always present, but, if its radius is negligible, the K parameter is still valid. Under monotonic loading, plastic zone size is usually estimated as follows: 2 1 2 y y K r π σ ⎛ ⎞ = ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ (plane stress conditions) (1) 2 1 6 y y K r π σ ⎛ ⎞ = ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ (plane strain conditions) (2) Considering, as a first approximation, the investigated DCIs as homogeneous materials, the plastic zones size corresponding to the Kmax value under plane strain conditions ( max pzK r ), calculated for the first and the last measured ΔK values (ΔK1 and ΔK2 respectively], can be summarized in Table 4, 5 and 6 ( σy are considered equal to 220, 320 and 430 MPa, for the ferritic, ferritic-pearlitic and pearlitic investigated DCIs respectively). It is necessary to underline that the considered ΔK1 and ΔK2 are not the ΔK values corresponding respectively to the threshold and final fracture conditions, but they only can be considered as a first evaluation. Table 4. DCI EN GJS350-22: evaluation of the plastic zone size (plane strain conditions). R (Kmin/Kmax) ΔK1 [MPa√m] max pzK r [mm] ΔK2 [MPa√m] max pzK r [mm] 0.1 9 0.109 32 1.368 0.5 7 0.214 20 1.754 0.75 4.5 0.355 11 2.120 Table 5. DCI EN GJS500-7: evaluation of the plastic zone size (plane strain conditions). R (Kmin/Kmax) ΔK1 [MPa√m] max pzK r [mm] ΔK2 [MPa√m] max pzK r [mm] 0.1 9 0.052 32 0.655 0.5 8 0.133 27 1.511 0.75 5 0.207 14 1.625 Table 6. DCI EN GJS700-2: evaluation of the plastic zone size (plane strain conditions). R (Kmin/Kmax) ΔK1 [MPa√m] max pzK r [mm] ΔK2 [MPa√m] max pzK r [mm] 0.1 9 0.030 22 0.179 0.5 6 0.040 13 0.203 0.75 5.5 0.145 10 0.481 Considering Figures 1-3, graphite nodules maximum diameters (dmax) in the investigated DCIs are respectively equal to: - about 60-70 μm for DCI EN GJS350-22; - about 40-50 μm for DCI EN GJS500-7 (but it is necessary to remember that nodules are surrounded by ferritic shields that are sometimes interconnected, up to 300 μm of diameter, Figure 2); - about 45-55 μm for DCI EN GJS700-2. Comparing dmax and max pzK r values, considering that the graphite volume fraction is not negligible
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