Effect of microstructure on resonant vibration fatigue fracture of ferritic spheroidal graphite cast iron

The effect of graphite nodule size, counts and eutectic cell wall inclusions on the fatigue fracture behaviour under resonant vibration of spheroidal graphite (SG) cast iron was investigated in this study. Experimental results indicated that the variations of graphite nodule size and counts slightly influence resonant frequency. The D-N curves (deflection amplitude vs. number of vibration cycles) of all test materials show similar manner. The deflection amplitude maintains a constant value for a certain period of vibration cycles (region I) and then decreases significantly with increasing the vibration cycles (region II). The feature of fatigue fracture behaviour under resonant vibration of ferritic SG cast iron can be concisely divided into four steps, namely (1) crack initiation, (2) crack linking, (3) the major cracks formation, and (4) deepening of the major cracks in the through-thickness direction. The initial three steps correspond to the region I period of the D-N curves, and the fourth step occurs in the region II period. Increasing the graphite nodule counts and refining the microstructure increase the region I period, and improve the resonant vibration fatigue life. During fatigue fracture process under resonant vibration, cracks mainly initiate from graphite nodules and some from eutectic cell walls. The eutectic cell wall inclusions are more crowded in SF specimen than that in PF specimen. So, the crack path of SF specimen is more tortuosity. The feature of fracture surface is mostly the brittle fracture.