13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Table 3. DCI EN GJS700-2 chemical composition (100% pearlite). C Si Mn S P Cu Mo Ni Cr Mg Sn 3.59 2.65 0.19 0.012 0.028 0.04 0.004 0.029 0.061 0.060 0.098 Fatigue crack propagation tests were performed in laboratory conditions according to ASTM E647 standard [14], using 10 mm thick CT (Compact Type) specimens and considering three different stress ratio values (e.g. R=Pmin/Pmax = 0.1; 0.5; 0.75). Tests were performed using a computer controlled servohydraulic machine in constant load amplitude conditions, considering a 20 Hz loading frequency, a sinusoidal loading waveform. Crack length measurements were performed by means of a compliance method using a double cantilever mouth gage and controlled using an optical microscope (x40). In order to investigate the fatigue crack propagation micromechanisms, in [19, 20] the following procedures were applied: - Scanning electron microscope (SEM) observations of the crack path during fatigue crack propagation test (cracks propagate from left to right); - “Traditional” SEM fracture surface analysis (cracks propagate from left to right); - 3D fracture surface reconstruction performed after SEM analysis; - Light optical microscope (LOM) transversal crack paths analysis. Microstructure and stress ratio influence on fatigue crack propagation resistance in ferritic-pearlitic DCIs is summarized in Figure 4. 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] ΔK [MPa m1/2] 3 50 Figure 4. Loading conditions influence on fatigue crack propagation in ferritic-pearlitic DCIs. 3. LEFM considerations Considering the linear elastic 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
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