Page 134 - Biaxial Multiaxial Fatigue and Fracture

Long-Life Multiaxial Fatigue of a Nodular Gmphite Cast Iron 119

As previously discussed, the driving force for a crack emanating from a small defect in equi-
biaxial tension is significantly less than that for tension or torsion. It was expected that this
would produce a correspondingly higher fatigue limit. For the nodular cast iron tested here, this
was not observed. Cast irons represent materials with very complex microstructures. The
relatively strong pearlite phase is interspersed with graphite nodules that are very weak in
fatigue. With cracks free to propagate in any direction, it is assumed that cracks change
directions as needed as they link up closely spaced weak regions of the microstructure. In the
future, it would be of interest to perform this type of testing for a more homogeneous material
with small and widely spaced defects.

CONCLUSIONS

Long-life fatigue tests of nodular graphite cast iron have been performed under uniaxial
tension, torsion and nearly equi-biaxial tension (h = 02 / 01 = 0.98). Test data for torsion
loading in the long but finite life regime was significantly below the uniaxial fatigue data, and
the fatigue limit was approximately 78% of the uniaxial fatigue limit. The nodular iron had
nearly the same fatigue limit as under both biaxial and uniaxial loading.
Fatigue cracks in biaxial fatigue were significantly more tortuous than in either torsion or
uniaxial fatigue due to the nearly equal driving force in all directions. A fatigue limit relation
previous developed for this material under tension and torsion loading with a variety of mean
stress levels is slightly modified to include biaxial loading. In the future, tests involving plane
strain would be of interest both because it is common in engineering design and because it is
somewhere between uniaxial loading and qui-biaxial tension.
Most critical plane damage parameters have been developed for ductile materials that fail by
shear crack development. Several critical plane fatigue parameters suitable for tensile damage
materials are discussed. These have primarily been developed for uniaxial load cases. Their
application to proportional loading is possible, but further development is required before they
can be applied to general non-proportional loading.

ACKNOWLEDGEMENTS
The authors are grateful to H. Laukkanen from VTT Industrial Systems for his careful
assistance in executing the fatigue tests. Experimental work was partially supported through
the project FadeKjutdesign, which is funded by the Nordic Industrial Fund, WZrtsila
Technology, Metso Corp., Componenta Cast Components, Componenta Pistons and VlT
Industrial Systems.

REFERENCES
I. Socie, D. F. and Marquis, G. B. (2000) Multiaxial Fatigue, SAE, Warrendale, PA.
2. Findley, W. N. (1959), A theory for the effect of mean stress on fatigue of metals under
combined torsion and axial load or bending, J. Engng Industry, pp. 301-306.
3. Findley, W. N. and Mathur, P. N. ,(1956) Modified theories of fatigue under combined
stress, Proc. SOC. Exp.. Stress Anal, 14, pp. 35-46.