Page 277 - Biaxial Multiaxial Fatigue and Fracture

The Multiaxial Fatigue Strength of Specimens Containing Small Defects 261

FCD400 cast iron specimen just below the pure torsional fatigue limit. As seen in Fig. 15%
many of the cracks initiated at graphite nodules were in direction of about f 45" to specimen
axis. In the same specimen, as shown in Fig. 15b, in the 0" or f 90" shear planes mode I1 non-
propagating cracks were observed in the matrix remote from nodules. In smooth FCD700
specimens, no mode I1 non-propagating crack was observed at the torsional fatigue limit. In the
case of FCD400, the fatigue limit of the matrix structure as predicted from Eq.( 17) is 182 MPa,
a value very close to the experimental value, 175 MPa. This stress appears to be sufficiently
high to nucleate mode I1 cracks by slip repetitions in the matrix structure, and may be why the
coexistence of both mode I and mode I1 non-propagating cracks was observed. In the case of
FCD700, it appears that the reduction in torsional fatigue strength due to graphite nodules is so
large that mode I1 non-propagating cracks cannot develop in the matrix, see Fig.14b. In addition
Fig. 14 indicates that these nodules or cavities can reduce the bending fatigue limit much more
than the torsional fatigue limit.

CONCLUSIONS

1. A criterion for fatigue failure under multiaxial loading conditions for specimens containing
defects has been presented.
2. With the aid ofthis criterion, a unified method for the prediction of the fatigue limit has been
developed. A feature of the method is that predictions can be made without the need for fatigue
testing.
3. Good agreement has been found between the predictions based upon this method and the
experimental data obtained in fatigue tests which were carried out under either uniaxial,
torsional or combined loading conditions for a variety of materials which included annealed
medium carbon steel, quenched and tempered Cr-Mo steel, femtic and pearlitic nodular cast
irons and a high strength brass.
4. When the defect is quite small, the fatigue strength is determined by a competition between
the fatigue strengths of the defect-containing part and a defect-free part.
5. There is a critical size below which defects are not deleterious. However this critical size is
dependent upon the combined stress ratio.

ACKNOWLEDGEMENT

The author wishes to thank Prof. A. J. McEvily of University of Connecticut for fruitful
discussions and for correcting the English manuscript of this paper.

REFERENCES

I. Kitagawa, H. and Takahashi, S. (1976). Applicability of Fracture Mechanics to Very Small
Cracks or the Cracks in the Early Stage, In: Proc. 2nd Znt. Con$ on Mechanical Behavior
ofMaterials, pp. 627-63 1, Am. SOC. Metals, Metals Park, Ohio.
2. Murakami, Y. and Endo, M. (1994). Effects of Defects, Inclusions and Inhomogeneities on
Fatigue Strength, Znt. J. Fatigue 16, 163-182.