Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 462-468; DOI: 10.3221/IGF-ESIS.30.56 465 Numerical procedure Stress-Strain curve of the investigated DCI was described by a linear elastic stage characterized by E=160GPa and Poisson ratio υ=0.30 and by an isotropic plasticity model where the yield function was:     VM ys F where  VM is the equivalent Von Mises stress, and ys  is the follows isotropic hardening function:   0 ys ys h pe       In the isotropic hardening function, 0 ys  is the yield stress (for the investigated material 0 ys  =430MPa), and   h pe   is the hardening function. For the investigated material the hardening function was assumed constant ( h  = 0.08GPa). All the material parameters were evaluated by calibration. The calibration was performed according material parameters simulating the mechanical behavior of an uniaxial GS700 specimen. Tetrahedral elements were used, characterized by dimensions greater than critical volume of material (about 200  m) that include nodules and pearlitic grains of the investigate DCI. FEM analysis represents a good approximation at mesoscale of DCI mechanical behavior, but it does not take into account the interaction between structural components like graphite nodules and metallic grains. As a consequence, no stress intensity factor due to interaction of different phases was considered in the FEM analysis. Triaxiality values were evaluated in terms of principal stress by means the Eq. (1) in all investigated nodules positions. Analyses were carried out by simulation of cross-head testing machine displacement. Defining TF as a stress Triaxiality Factor parameter:         1 2 3 2 2 2 1 2 2 3 3 1 1 3 1 2 TF                   (1) where, σ i , i=1, 2, 3 – Principal stresses Cross head displacement values were used in order to simulate the specimen stress field by using a model which takes in account the stiffness of testing machine. E XPERIMENTAL RESULTS AND COMMENTS DM observations onsidering metallographically prepared but not chemically etched specimens, it is possible to observe that the stress increase implies the development of slip bands, mainly generating in correspondence to the nodule equator. The slip bands become more and more evident with the local deformation. Fig. 5 and 6 show the behaviour of two nodules that are characterized by different triaxiality factor TF and Von Mises stress evolutions. 0 0.2 0.4 0.6 0.8 1 1.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Triaxiality Crosshead Displacement [mm] Nodule 1 Nodule 2 0 100 200 300 400 500 600 700 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 σ VM [MPa] Crosshead Displacement [mm] Nodule 1 Nodule 2 Figure 5 : Triaxiality factor evolution with the crosshead displacement. Figure 6 : Equivalent Von Mises stress evolution with the crosshead displacement. C