Abstract
Rotating bending fatigue tests were carried out on smooth specimens of ferrite-base spheroidal cast iron at room temperature. Initiation and growth behaviors of surface cracks as well as the statistical properties of lives of initiation and propagation were discussed by detailed observation of the specimen surface and fracture surface. Surface fatigue crack initiation was mainly governed by microshrinkage existing just under the surface. A negative correlation between the crack initiation life Ni and the square root of the projected area (√(area)) of the microshrinkage as the initiation sites was found, but the correlation factor between them was very low. Surface cracks propagated with a great deal of branching and coalescence during the fatigue process, and it was found to exhibit the remarkable repetition of increase and decrease in the crack prppagation rate da/dN during the range of small cracks. The distributions of the initiation life Ni, the propagation life Np and the fatigue life Nf in the main cracks which propagated to 2a=3000μm were well represented by a three-parameter Weibull distribution, and the dependence of the three parameters on the stress amplitude were discussed and compared with the results of other researchers and other findings of the authors.
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