Page 234 - Biaxial Multiaxial Fatigue and Fracture

218 A. VARVANI-FARAHANI

responsible for the largest physical damage on the critical planes, where the debonding of
materials occurs. The proposed damage parameter via its stress range term increases with the
additional hardening caused by out-of-phase tests whereas critical plane parameters that
include only strain terms do not change when there is strain path dependent hardening. The
parameter also contains no empirical constants for weighting of normal and shear energy
components, instead, the normal and shear energy components are normalized by the axial and
shear fatigue properties. These weighting factors make the damage parameter dimensionless as
the fatigue damage by definition has no unit.
The proposed fatigue damage parameter seems to show some promises in correlating fatigue
results conducted under random fatigue loading (in-progress). The random fatigue using the
proposed parameter will be examined in a close future.

CONCLUSIONS
A fatigue damage parameter has been developed to assess the fatigue lives under in-phase and
out-of-phase biaxial constant amplitude fatigue stressing where the stresses are at different
frequencies. The normal and shear stress and strain ranges, which are included in this
parameter, have been calculated from the largest stress and strain Mohr’s circles. The total
damage accumulation has been calculated from the summation of the normal and shear
energies in a block loading history on the basis of a cycle-by-cycle analysis. The parameter
contains no empirical constants for weighting of the normal and shear energy components;
instead, the normal and shear energy components are normalized by the axial and shear fatigue
properties. The proposed fatigue parameter has successfully correlated biaxial fatigue lives of
thin-walled EN24 steel tubular specimens tested under in-phase and out-of-phase biaxial
fatigue stressing where stresses were at different frequencies and included mean values.
Fatigue life correlation has varied from 1.2 to 2.0 for 104<Nf<105 and from 1.0 to 1.2 for
1da~10~.

ACKNOWLEDGEMENTS

The financial support of Natural Science and Engineering Research Council (NSERC) of
Canada (NO. 1-51 -561 37) is greatly appreciated.

REFERENCES

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cycle fatigue and life prediction, ASTM STP 770. American Society for Testing and
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4. Jordan, E.H. (1982), Pressure vessel and piping design technology-A decade of
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