パーライト基地球状黒鉛鋳鉄の表面疲労き裂発生・進展特性

抄録

Rotating bending fatigue tests were carried out on mainly pearlitic spheroidal graphite cast iron (pearlitic ductile iron, PDI) of smooth specimens at room temperature. Initiation and propagation behavior of surface microcracks was discussed by means of detailed successive observations of the surface and the fracture surface. The results were compared with the results of ferritic spheroidal graphite cast iron (FDI) which had been reported by the authors. The surface crack initiation was governed mainly by microshrinkages existing just under the surface in both PDI and FDI materials. A low level of negative correlation between the crack initiation life N_i and the maximum stress intensity factor of defect front K_Imax by evaluating from the area of microshrinkage as the crack initiation site was found in both PDI and FDI, but the values and the variation of K_Imax in PDI were larger than those in FDI. Surface cracks propagated with the branching and the coalescence during the fatigue process, but the crack propagating path in the pearlite base was comparatively more linear than that of the ferrite base in FDI. It was found to exhibit a decrease in the crack propagation rate da/dN during the range of small cracks in the low stress level ; on the other hand, in the high stress level the da/dN was found to be a remarkable repetition of increase and decrease in the small crack region. This behavior of the da/dN in the small crack region was similar in both PDI and FDI.