Page 274 - Biaxial Multiaxial Fatigue and Fracture

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Table 3. Lower bounds of uniaxial fatigue limits, -1, at R = -1 predicted for smooth nodular
cast iron specimens.

Vickers Graphite nodule Microshrinkage cavity
micro- Number of
Material hardness specimens Jarea,, GI 6,
Owl
of matrix Pm MPa lun MPa

obtained by inserting the value of owl for 5 specimens into aw in Eq.(l I), since 3-6 specimens
were used to determine each fatigue limit in the experiments. It is seen that the curve predicted
for the cavities is the true lower bound, though the agreement with the curve for graphite nodule
appears to be better. A small change in G,,does not significantly affect the predicted
results for awl, since awl is proportional to the -1/6 power of Jarearnax, see Eq.(4). Since, in
FCD700, the ferrite envelopes surrounding the graphite nodules are thin and their effect is
negligible, the value of &,, only for graphite nodules was used for the prediction in this
study. However, this approximation is not always valid, for example, for JIS FCD600 nodular
cast iron [24] having a typical bull's eye structure in which the ferrite regions are large in
comparison to the graphite nodules. A detailed discussion of how to apply the prediction
method to such a microstructure is available in reference [24].

Relation of smooth and hole-containing specimens with respect to non-detrimental defects

It is known that the pure torsional fatigue strength is insensitive to small defects or notches. For
example, in the case of annealed 0.37 % carbon steel, a hole of 100 pm in diameter which
caused about 20 YO reduction in fatigue strength under uniaxial loading, under pure torsional
loading was non-detrimental. Gough and Pollard [33] proposed the following empirical
criterion, called ellipse quadrant, to fit the data for smooth specimens of ductile metallic
materials
(ET+(%)
2
=1

where oWo is the smooth specimens uniaxid fatigue limit, rWo is the smooth specimens torsional
fatigue limit, oa0 is the axial or bending stress amplitude, and r,o is the torsional shear stress
amplitude at the fatigue limit under combined loading. The experimental fatigue limits of
smooth specimens of annealed 0.37 YO carbon steel were 145 MPa for and 235 MPa for -0.