numero25

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108; DOI: 10.3221/IGF-ESIS.25.15 103 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 Figure 1 : Stress –strain behaviour for carbon steel, grey iron and ferritic and pearlitic DCIs [1]. Figure 2 : Ferritic-pearlitic DCIs. Microstructure and stress ratio influence on fatigue crack propagation [8]. Considering the fatigue resistance, and considering both the initiation and propagation of micro- and macro- cracks, the role played by graphite nodules is not univocally determined. Different mechanisms are proposed for the graphite nodules [3-7]: - “rigid spheres” not bonded to the matrix and acting like voids under tension; - “crack-arresters”, due to their peculiar shape that minimizes the stress intensification at the crack tip; - “crack closure effect raisers”, due to the role they play at the lower values of the applied K min . Other research activities allowed to conclude that graphite nodule cannot be regarded as voids with no strength and that they don’t cause micro-notch stress concentration by itself [8]. It has been proposed [9]that the role played by the graphite nodules in DCIs fatigue crack propagation is more complex, suggesting the presence of a mechanical properties gradient inside the graphite nodules, probably due to the different graphite nodules solidification and growth mechanisms. Considering the fracture mechanics principles, stress intensity factor (“K”) is used to quantify the stress state ("stress intensity") near the crack tip caused by a remote load or residual stresses and, considering fatigue crack propagation, stress intensity factor amplitude (e.g.  K = K max -K min ) is the main parameter used to characterize the stress conditions at the crack tip. Both K and  K 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 y y 1 K r 2            (plane stress conditions) (1) 2 y y 1 K r 6            (plane strain conditions) (2) Considering a fatigue crack propagation problem, Eqs. (1) and (2) represent the crack tip plastic zone corresponding to the upward excursion of the load cycle (up to K(t) = K max ). Fatigue crack propagation is characterized by the presence of a “reversed” or “cyclic” plastic zone, r rpz (four times lower than the monotonic value corresponding to K max ): the tensile load reduction from the  max , and the presence of the surrounding elastic body, imply a compression condition at the crack tip. Considering that, for R = 0.1, applied  K value ranges between 9 and 32 MPa  m (Fig. 2), assuming the investigated pearlitic DCI as an homogeneous material, and according to relationships (1) and (2), crack tip plastic zone ( max pzK r , for K = K max ) and reversed plastic zone radii range respectively: Crack tip plastic zone radius, max pzK r : - between 0.099 and 1.258 mm (plane stress conditions); - between 0.033 and 0.419 mm (plane strain conditions).