Issue 13

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 13 (2010) 3-16; DOI: 10.3221/IGF-ESIS.13.01 8 Figure 13 : Stress ratio and microstructure influence on fatigue crack propagation resistance of ferritic-pearlitic DCI (obtained controlling the chemical composition). Figure 14 : DCI fatigue crack propagation resistance in ferritic- pearlitic and austempered DCIs. For all the investigated ductile cast irons, fatigue crack growth rates da/dN increases with the stress ratio, for the same  K values. This behaviour is due to crack closure effect that can be crack tip plasticity, oxide forming and/or fracture surface roughness induced [23, 24]. Roughness surface analysis and scanning electron microscope (SEM) fracture surface investigation [11, 13] show a low influence of the oxide forming and fracture surface roughness induced crack closure effect. Considering lower R values (e.g. R = 0.1) or lower  K values (near threshold), fatigue crack propagation is not influenced by matrix microstructure. Focusing ferritic-pearlitic DCIs obtained controlling the chemical composition, the best behavior is shown by GJS500-7 (50% ferrite – 50% perlite): for the same loading conditions, this DCI is characterized by lower crack growth rate values, especially for higher  K and/or R values (Fig. 13) . Comparing GJS500-7 with the ferritic-pearlitic DCI obtained annealing the pearlitic DCI, the different phases distribution implies differences in fatigue crack propagation resistance. GJS500-7 fatigue crack propagation resistance is higher than the ferritic-pearlitic DCI obtained annealing the pearlitic DCI, and is analogous to the resistance offered by the austempered DCI, for all the investigated loading conditions. SEM crack profile analysis Considering ferritic DCI, ferritic matrix-graphite nodules interfaces are not necessary a preferential propagation path: in fact, crack could propagate both nearby graphite nodules (Fig. 15) corresponding to the matrix-nodules interface (Fig. 16 and 17). However, the consequent debonding is characterized by the presence of residual graphite on ferritic fracture surface (Fig. 17). Some secondary cracks are also observed: they could initiate both at matrix-nodules interfaces (Fig. 18) and in ferritic matrix (Fig. 19, 20). These secondary cracks are characterized by a really reduced propagation path (100 – 200  m max.). Graphite nodules are also characterized by the presence of a “secondary damage”, as really short secondary cracks inside Nearby of the cracks, graphite nodules do not show any secondary damage, neither as cracks inside nodule or as cracks at nodules-matrix interface. Focusing fully pearlitic DCI, fatigue crack – graphite nodules interactions could imply both a nodule disgregation (Fig. 21) and (more frequently) a pearlitic matrix – nodules “clear” debonding, withoud residual graphite on pearlitic fracture surface (Fig. 22, 23) . Figure 15 : Ferritic DCI (R = 0.1,  K = 10 MPa√m). Figure 16 : Ferritic DCI (R = 0.5,  K = 10 MPa√m). Figure 17 : Ferritic DCI (R = 0.5,  K = 12 MPa√m). Figure 18 : Ferritic DCI (R = 0.75,  K = 9 MPa√m). 10 10 -10 10 -9 10 -8 10 -7 10 -6 100% F GJS350-22 R = 0.1 R = 0.5 R = 0.75 50% F + 50% P GJS500-7 R = 0.1 R = 0.5 R = 0.75 100% P GJS700-2 R = 0.1 R = 0.5 R = 0.75 da/dN [m/cycle]  K [MPa m 1/2 ] 3 50 10 10 -10 10 -9 10 -8 10 -7 10 -6 50% F - 50% P (GJS500-7) R = 0.1 R = 0.5 R = 0.75 50% F - 50% P (annealed GJS700-2 ) R = 0.1 R = 0.5 R = 0.75 Austempered GGG 70BA R = 0.1 R = 0.5 R = 0.75 da/dN [m/cycle]  K [MPa m 1/2 ] 3 50