Page 355 - Biaxial Multiaxial Fatigue and Fracture

Variability in Fatigue Lives: An Effect of the Elastic Anisompy of Grains? 339

of +I- 24 % for iron. This variability is very high as compared with the width of the distribution
of the Schmid factor in FCC crystal.
The spatial distribution of the maximum resolved shear stress on slips systems was
calculated by the FEM, and compared with the distribution of the Tresca equivalent stress on
the one hand and with the distribution of the Schmid factor in the model on the other hand. It
can be concluded from these calculations that the importance for fatigue crack nucleation of
this load percolation network depends on the elastic anisotropy of the material on the one hand,
and on the number of primary slip systems on the other hand. If the number of primary slip
systems and the elastic anisotropy of grains are high, the nucleation process should be
dominated by the self-organization of the stress and strain heterogeneity within the polycrystal.
On the contrary, if the number of primary slip system and the elastic anisotropy of grains are
low, the nucleation process should be dominated by the crystalline orientation of grains.
When the distribution of T,,,~ is dominated by the load percolation effect, some effects of
that network may arise in multiaxial fatigue.
It was shown, that with a similar mean value a,,,,> of the maximum resolved shear stress
on slip systems in a polycrystal, two different loading conditions are not equivalent in terms of
the nucleation of micro-cracks, since the maximum bounds for ‘tmnx can be different. For
example, these calculations show that with a similar mean value amax>. maximum bound
the
for T,,,, is higher in torsion as compared with tension. The grains with the highest value of rmnx
are located around the intersections between the heavily loaded links associated with each
principal direction. These grains are sparse but overstressed.
The role in fatigue of the self-organized spatial distribution of stress and strain in the
polycrystal. which is described in this paper, should also be important for crack coalescence
during subsequent crack growth, since it controls the number of damaged grains per unit
surface and their mutual distance.

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