| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 85 === AbstractThis study investigated the factors that affect high-
cycle fatigue (HCF) strength of austempered ductile iron (ADI).
These factors include the cast section size, heat treatment
conditions, loading mode, notch effect, and highly stressed
volume. Specimens were prepared in different notch geometries
with stress concentration factors ranging from 1 to 2.64. HCF
tests were conducted in both rotating-bending and uniaxial
cyclic loading to determine the stress-life (S-N) curve for each
specific condition. Fractography and microstructure were
analyzed by scanning electron microscopy (SEM) to identify the
fracture origins.Results show that the fatigue strength is
higher in small casting than that in large casting. The
decrease in HCF strength in large casting is attributed to the
increasing graphite nodule sizes, the deterioration in nodule
shape, and the increasing number of microshrinkage pores due to
the slower solidification rate. The fatigue strength of ADI is
higher when austempered at 360℃in comparison to that
austempered at 300℃. This could be attributed to the higher
toughness and retained austenite content present at ADI
austempered at 360℃. The fatigue strength in a two step
treatment is similar to that in austempering at 360℃. On the
effect of loading mode, the fatigue strength in rotating-bending
is higher than that in axial loading because of the difference
in highly stressed volume. The fatigue limit is decreased with
an increase in the highly stressed volume. The degree of
decrease in fatigue strength under different notch geometries
can also be related to the effect of highly stressed volume. A
prediction equation for the fatigue limit of ADI was introduced
in terms of impact toughness and highly stressed volume.
Factography analysis results indicated that the fatigue cracks
mostly initiated from large graphite nodules, irregular graphite
nodules, and microshrinkage pores.
|
|---|
